twi (welding titanium)
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WELDING TITANIUMA DESIGNERS AND USERS HANDBOOK
£ 25.00
TIGTHE
T I T A N I U M
INFO RMATIO NG R O U P
WORL D CENTRE FOR
MATERIALS JOINING
TECHNOLOGY
Electronic copyright in this document as follows: Copyright TWI and the Titanium Information Group,
- Guide to best practice -
1999
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TITANIUM INFORM ATION GROUP
The TITANIUM INFORMATION G ROUP, (TIG) is an a ss ocia tio n o f Europ ea n sup pliers, d es ign en gineers, a nd
fabricators of titanium formed with the intention of promoting the use of titanium.
The a im o f the G roup is to influence t he initial select ion o f ma terials so tha t titanium is given the c onsiderat ion
merited by its uniq ue combination of physica l and m echa nica l properties, outsta nding resista nce to co rrosion andcost effectiveness in a wide range o f dem anding applications.
Regular publications and literature available from the Group present detailed and up-to-date technical and
com mercial information to m at erials engineers, plant a nd eq uipment des igners and buyers, and provide t he answ ers
to everyday questions about cost, availability, fabrication and use of titanium and its alloys.
Membe rs of the Group a re available to give presenta tions abo ut titanium o n either general or specific to pics to
com pa nies o r at sem inars. A list o f mem ber comp anies of TIG appe ars on pa ges 32 a nd 33 Cop ies o f the TIG
vide o, ‘Titanium Tod ay’ and the da ta diskett e ‘Titanium a nd Its Alloys’ are ava ilable fo r use in ed ucat iona l and
training establishments to provide an introduction to the metal its applications and properties.
Further informat ion o n TIG publica tions ca n be fo und o n the we b: w ww. titaniuminfogroup.co. uk
TWI
TWI is ba sed at Abingto n, nea r Cambridg e, UK and is o ne of Europe ’s largest indepe ndent contra ct resea rch and
techno logy o rganisations. O ver 400 staff, w ith a uniq ue blend of te chnica l bac kgrounds, international experience
and language skills, work with industry world-wide to apply joining technology effectively. Some 2500 member
co mp anies in over 50 co untries be nefit from TWI service s.
TWI’s know ho w a nd expertise co vers:
Engineering - des ign, st ructura l integ rity, fract ure, NDT.
Mat erials - stee ls, no n-ferrous a lloys including titanium, plas tics, co mpo sites, ce ramics.
Welding a nd joining- arc, e lectron bea m, las er, resistanc e a nd friction w elding, braz ing, so lde ring, a dhes ive
bond ing, fast ening.
Surfacing - arc cladding, friction, high velocity oxyfuel, laser, arc spraying.
Cutting - flam e, plasm a, wa ter jet, las er.
Manufac turing - project m ana gem ent, production/ma nufacturing e ngineering, d ecision support, ma nufacturing
systems, health and safety, q uality assurance.
Indust ry’s o bjec tives a re TWI’s o bject ives
Reduce cos ts.
Ma rket effe ctively.
Continuously improve quality and reliability.
Innovate.
TWI, Gran ta Pa rk, G reat Abingto n, Ca mbridge , CB1 6AL, UK
Tel: + 44 (0)1223 891162 Fax: + 44 (0)1223 892588Web: htt p: //w w w.tw i.co. uk em a il: tw i@t w i.co. uk
The da ta and othe r informat ion co nta ined he rein are de rived from a variety o f
so urces w hich TIG a nd TWI believe a re reliable. Be ca use it is not po ssible to
ant icipate spec ific uses a nd op erating co nditions TIG a nd TWI urge you t o
consult with the sales or technical service personnel as appropriate of the
individua l com pa nies.
M ay 1999
Written b y: Lee S Smith, Philip Threa dg ill and Micha el Gitt os TWI
Editor: Da vid Pea co ck Titanium Meta ls Corporation
Dat a ava ilab le in literature a vailab le from TWI, TIMETa nd o ther m emb ers o f TIG a nd
TWI is inco rpora te d in this publica tion .
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INTRODUCTION
The high strength, low w eight and o utsta nding corrosion resista nce po sses sed by tita nium and titanium a lloys ha ve
led to a wide and diversified range of successful applications in aerospace, chemical plant, power generation, oil
and ga s extrac tion, med ical, spo rts, and othe r industries. There is a com mo n q uestion w hich links a ll of the se
app licat ions, a nd tha t is how be st to join titanium pa rts toget her, or to other ma terials to prod uce the final compo nent
or structure. The variety o f titanium a lloys, and the va stly greater number o f engineering me ta ls a nd m at erials
requires that there should be a versatile selection of joining processes for titanium if the metal is to be capable of
use in the w ide st range o f applica tions. Although me chanical fastening, ad hesives, a nd o ther techniq ues have their
place, we lding cont inues to be the mo st impo rtant p rocess for joining titanium. Welding of t ita nium by various
processes is widely practised, and service performance of welds is proven with an extensive and continuously
extending record of achievements. Newer methods adaptable for titanium are further advancing the science,
tec hnology and ec ono mics of we lding. Applica tion of this techno logy to the de sign, manufact ure and app lica tion of
titanium is as relevant to first time use rs as to com mitted custo mers. For ma ny applica tions, choo sing the welding
process is as important a step in design as the specification of the alloy.
This ha ndb oo k, t he s ixth in a se ries, is prod uced jointly by t he Titanium Info rma tion G roup a nd TWI World
Centre fo r Mat erials Joining Techno logy. The a im of this ed ition rema ins a s w ith its p redece sso rs, to bring t oge ther
key elements of w ide ly dispersed d at a into a single source boo k. Use o f this hand boo k w ill ena ble tho se respo nsible
to select welding processes that will be appropriate to the titanium alloy, the component, and the application. Inthis way the most demanding goals for reliability, maintainability and safety can be achieved, together with the
lowest overall cost for components and systems of the highest performance and integrity.
1
CONTENTS
Introduction 1
Why use Tita nium 2
Prop erties of Titanium a nd its Alloys 3
Joining Tita nium a nd its a lloys 4
TIG w eld ing 5MIG w elding 6
Plasma and fluxed welding 7
Las er and EB w elding 8
Resistance welding 9
Frictio n we lding proc ess es 10
Diffusion bonding 13
Forge we lding proce sses 14
Brazing and soldering 15
Adhes ive bond ing a nd m echa nica l fast ening 16
Joining titanium to ot her meta ls 17
Workshop prac tice 18
Ope n air welding 19
Preparat ion of the joint for w elding 20
Welding te chnique s 21
Evaluat ion o f w eld q uality 25
Visua l inspe ct ion 26
Repair of defects 28
Distortion a nd stress relief 29
The do ’s a nd d on’ts of w elding 30
Standa rds a nd specifications 31
For furthe r help 32
TIG me mb ers 33
Titanium a l loy (grad e 5) fa br ica ted structure for high
specification military application
Keyhole plasm a w elding
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WHY USE TITANIUM
In all fields of engineering, designers, fabricators and
end users are ready to consider titanium for an ever
wide ning range o f ap plications. The m eta l and its a lloys
are no longer seen a s ‘exotic’. Outd ated and misguided
notions a bout c ost , a vailability, a nd fa brica tion are less
likely than ever to prejudice engineers who can see for
themselves all the excellent benefits which titanium
offers. This brochure has be en co mpiled to show how
the m eta l’ s reputat ion for being difficult to we ld, is bot h
mislead ing and inap prop ria te . Titanium alloy s joined by
any one of a wide range of welding processes are
routinely at work in applications as widely differing as
aero engines, offshore platform pipework, implants for
the human body and ultra lightweight roofing. Practical
and competitive welding processes ensure there are
t o d a y f e w o t h e r m a t e r i a l s t h a t c a n a p p r o a c h ,eco nom ica lly or te chnica lly, t he pe rforma nce p rovide d
by tita nium.
Titanium is as st rong as st ee l, yet 45%light er. Titanium
alloys will work continuously at temperatures up to
600°C, resisting creep and oxida tion, and d ow n to liq uid
ni trogen temperatures wi thout loss of toughness .
Titanium w ill survive ind efinitely w ithout co rros ion in
sea wa ter, a nd m os t ch lor ide env ironments . The
metallurgical characteristics which give titanium its
favourable properties can be reproduced, by selection
of an appropriate practice, in welded joints for mosttita nium alloys . The o xide film, w hich is the ba sis of the
metal’s corrosion resistance forms equally over welds
and heat a ffected z ones as over parent metal, and other
than in a few very harsh environments, weldments
perform identically to parent metal in corrosion resistant
service.
The w ide range of ava ilable tita nium a lloys ena bles
de signers to s elect ma terials and forms closely tailored
to the nee ds of the ap plica tion. The versatility o f tw o
basic compositions is such however that they continue
to satisfy the majority of applications, and this level of
com mo n use rema ins a ma jor facto r in the cost effective
production, procurement and application of titanium.
The tw o c om pos itions a re com mercially pure titanium,
(ASTM G rade 2), selected for ba sic corrosion resistance
with strength in the range 350 - 450MPa, and high
st reng th tita nium a llo y Ti-6Al-4V, (900 - 11 00MPa ).
Welding cons uma bles are rea dily ava ila ble for these
grades . Although there are other weldable a l loys ,
consumables for these may need to be obtained from
spe cialist so urces . The full range of titanium a lloys
reaches from high d uctility com mercially pure titanium
used where ex t reme formab i l i ty i s essent i a l , in
2
a p p l i c a t i o n s s u c h a s p l a t e h e a t e x c h a n g e r s a n d
architectural cladd ing and roofing, to fully heat treat able
alloys w ith strength a bove 1500MPa . Corrosion resistant
alloys are capable of withstanding attack in the most
ag gressive sour oil and g as e nvironments o r geo therma l
brines at temperatures above 250°C. High strength
oxidation and creep resistant alloys see service in aero
engines at temperatures up to 600°C. Suitable welding
p r o c e s s e s a r e e s s e n t i a l f o r t h e a p p l i c a t i o n a n d
performa nce of titanium to be op timised in most o f these
uses.
Improving the understand ing o f w elding titanium and
the p reservat ion of its properties a fter joining a re de sign
step s t ow ards increase d flexibility in ma terials selection
and use, resulting in improved q uality and performanceof products and processes. In this way, the technical
superiority o f titanium w ill be co nfirmed for even m ore
engineering applications than at present, to the mutual
benefit of the titanium industry and its customers.
What is the cost? This q uestion freq uently come s
first. The price pe r kilo o f titanium is no guide to the c os t
o f a p r o p e r l y d e s i g n e d c o m p o n e n t , o r p i e c e o f
eq uipme nt. First cost is in any event o nly one p art o f the
f u l l c o s t e q u a t i o n . M a i n t e n a n c e , d o w n t i m e a n d
replacement costs which may be a very significant
element in plant designed for long and reliable servicelife a re ano ther. In this area , w elding plays a significa nt
role , ensuring that the performance of a t i tanium
fabrica tion mat ches tha t of the m eta l overall. Add itional
costs of e nergy a ssociated with operating unnecessarily
hea vy or thermally inefficient eq uipme nt m ay be a third
penalty o n life cycle costs. Titanium is freq uently spe cified
for its ability to cut costs through reliable and efficient
performance . Tita nium w elded tube is fo r example
installed in steam turbine condensers and welded pipe
in nuclear power plant service water applications with
40 year performance gua rantees a ga inst co rrosion failure.
One ma nufact urer offers a 100 year wa rranty o n its me tal
supplied for a rchitec tural applica tions.
It is not possible in this guide to give an absolute
cost or an a ccurate compa rison o f cost for the different
we lding process es d escribed. The eq uipme nt ca pa bilities
and cost s tructures of equal ly competent welding
cont racto rs freq uently results in a range o f prices being
offered for the same basic job. Some processes are,
how ever, intrinsically m ore expensive tha n o thers. Alwa ys
seek advice from an appropriate welding specialist or
contractor before attempting to develop a budget or
notional cost for a welding project.
Light, strong titanium should be considered for applications wherever weight or space are factors or corrosion/erosion is a problem.
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PROPERTIES OF TITANIUM AND ITS ALLOYS
3
A convenient and widely used system for specif ic
identifica tion of t he various g rade s o f com mercially pure
titanium and titanium alloys used for engineering and
co rrosion resist ing app lica tions is provide d by ASTM
which cover all the forms supplied in titanium and its
alloys:-
B 265 - Strip Sheet and Plate
B 337 - Sea mless a nd Welded Pipe
B 33 8 - Sea mless an d Welde d Tube
B 348 - Bars a nd B illets
B 363 - Sea mless a nd Welde d Fittings
B 367 - Castings
B 381 - Forgings
B 861 - Seamless Pipe (to replace B337)
B 862 - Welde d Pipe
B 863 -Wire
F 67 - Una lloyed Tita nium for Surgica l App lica tions
F 136 - Ti-6Al-4V fo r Surg ica l App lica tio ns
Grades 1 ,2 ,3 ,4 a re commerc ia l l y pure ( a lpha)
titanium, used prima rily for co rros ion resista nce. Strength
and hardness increase , a nd d uctility reduces w ith grade
number. Grade 2 is the most widely used specification
in all product forms. G rade 1 is specified w hen superior
formability is required. Grades 3 and 4 are used where
higher levels of s trength a re neces sary.
Grades 7, 11, 16, 17, also alpha alloys, contain
palladium (Pd) and provide superior corrosion resistance
in pa rticular to reducing ac id chloride s. Grad es 26 and
27 are similarly also alpha alloys, and contain .1%
ruthenium (Ru) to provide enha nced co rros ion resista nce
in red ucing e nvironments . The m echa nica l prope rties of
grades 7, 16 and 26 are identical to those of Grade 2.
The mecha nica l properties of grad es 11, 17 and 27 are
similarly ide ntical to tho se o f Grad e 1.
Grade 12 (alpha) also offers superior corrosion
resista nce t o c om mercially pure titanium, but is stronge r
and retains useful levels of strength up to 300°C.
Grade 5 is the ‘workhorse’ alpha-beta alloy of the
titanium range. It is also spec ified with red uced o xygen
content (ELI) for enhanced toughness (Grade 23), and
with addition of .05%palladium for added corrosion
resistance, (Grade 24) and with palladium and nickel
(Grade 25). Current interest in this alloy for marineappl icat ions is focused upon Grade 23 with .05%
pallad ium or Gra de 29 w ith .1 %ruthenium. Restrictions
on fabricability may limit availability in certain products.
Grade 9, (near alpha) has good fabricability and
med ium levels of strength. Grade 18 (Grad e 9 + .05%
Pd) and Grade 28, (Grad e 9 + .1%Ru) offer enhanced
corrosion resistance.
Bet a -C a nd TIMETAL® 21S are high st rength highly
co rros ion resista nt be ta alloys in the ASTM rang e. They
are respe c t ive ly Grad e 19, and Grad e 21. (Thecounterpart of Grade 19 with .05%Pd is Grade 20).
Grade 32 (Navy alloy) has go od we lda bility to gether w ith
high toughness and resis tance to s tress corrosion
c r a c k i n g i n m a r i n e e n v i r o n m e n t s . G r a d e 21 ,
(TIMETAL® 21S) and G rade 32 , (TIMETAL® 5111) a re a lso
ava ilable w ith the ad dition of .05%pallad ium.
Weldm ent s in ASTM gra d e 2 a re no rma lly
characterised by increase d strength, a ccompa nied by a
reduction of ductil i ty and fracture toughness. Any
strengthening induced by cold work will be lost in the
joint region . Weldme nts in Ti-6Al-4V typica lly exhibit near-
matching strengths to the base metal, but have lower
ductility. The t oughnes s o f the we ld zo ne is superior to
alpha-beta processed material, showing similar values
to a lpha-beta processed parent alloys. Som e examples
of actua l we ld properties are given for process es
de scribed in the text, but you are strongly ad vised t o
consult with your we lding specialist in cases whe re weld
performance is critical in your design.
Designation Commerically Medium High HighestPure Titanium Strength Strength StrengthAlloys Alloys
Alloy Type Alpha Alpha-Beta Alpha-Beta Beta
0.2% Proof Stress MPa 345 - 480 480 - 550 725 - 1000 1100 - 1400
Tensile Strength MPa 480 - 620 600 - 650 830 - 1100 1200 - 1500
Elongation % 20 - 25 15 -20 8 - 15 6 - 12
Tensile Modulus GPa 103 104 110 - 120 69 - 110
Torosion Modulus GPa 45 43 40 - 48 38 - 45
Hardness HV 160 - 220 200 - 280 300 - 400 360 - 450
Density kg/1 4.51 4.48 - 4.51 4.43 - 4.60 4.81 - 4.93
Thermal Expansion 10-6/ºC 8.9 8.3 8.9 7.2 - 9.5
Conductivity W/mK 22 8.0 6.7 6.3 - 7.6
Specific Heat J/kg/ºC 525 544 565 490 - 524
Typical mecha nical prope rties a nd phys ical prope rties o f titanium and titanium a lloys (100MPa = ap prox. 15 ksi)
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WELDABILITYMost titanium a lloys ca n be fusion w elded a nd a ll alloys
can be joined by solid state processes (see table).
Indeed , Welds in titanium a re substa ntially immune to
ma ny of the w eld cracking problems that ca use trouble
with ferrous alloy fabrications. Despite this and other
beneficial characteristics, some engineers still believe
that titanium is difficult to weld, possibly due to its
pa rticular requireme nts w ith rega rd to ga s shielding, or
beca use it has no rmally been ha ndled o nly by specialist
fabrica to rs. Tita nium is act ually ea sy to we ld by m ost
processes, as are most of its more common alloys.
Embrittlement through contamination with air and
carbonaceous materials poses the biggest threat to
successful fusion welding titanium, so the area to be
welded must be clean and protected by inert ga s w hile
hot. The mea ns to protect the weldment with inert ga s
are commercially available and easy to implement.
4
JOIN ING TITANIUM AND ITS ALLOYS
Welding cons uma bles are read ily a vailable fo r the
comm on t itanium grades and specifications for w elding
wire a re provided in AWS Spe cifica tion A5.16. Permissible
filler meta l, no rmally identical to the pa rent met al, ma y
be s pec ified a s in ASTM B 8 62.
The w elda bility of tita nium a lloy s is usually as se sse d
on the ba sis o f the toughness a nd d uctility of the weld
metal. Commercially pure grades are considered very
eas y to fa bricate and are ordinarily used in the as -welded
cond ition. Titanium a lloys show reduced we ld met al
ductility a nd toughnes s. The t able below highlights the
we lda bility of the c om mo n ASTM titanium grad es a nd
ot her alloys . Tec hnical consulta tion sho uld be s oug ht
prior to de signing o r fabrica ting any o f the t ita nium a lloys,
if there is any likelihood of problems arising from
unfamiliarity w ith the ma terials co ncerned.
ASTM Grades Weldability Comments
1,2,3,4,7,11,12,13 Excellent Commercially pure and low alloy grades
14,15,16,17,26,27 w ith minor additions of Pd, Ru, Mo etc
9,18,28 Excellent Ti-3A1-2.5V grades
5,23,24,29 Fair-good Ti-6A1-4V grades
21 Excellent Beta alloy
6,6ELI Good-excellent Ti-5A1-2.5Sn
Welda bility of the c om mo n ASTM grad es
Alloy Weldability Comments
Ti-6A1-2Sn-4Zr-2Mo Fair-good C ommon aerospace alpha
&beta grade
Ti-6A1-2Sn-4Z r-6Mo Limit ed Aero space alpha &bet a grade
Beta III Excellent Beta alloy
Ti-15V-3A1-3Sn-3Cr Excellent Beta alloy
Welda bility of se lect ed no n-ASTM Ti alloys
Welding of a titanium fuel ta nk for the reco rd breaking
Breitling O rbiter III balloon (Bunting Titan ium)Fab rica tion of a large titanium pressure vesse l
(Bunt ing Tita nium)
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5
TUNGSTEN INERT GAS (TIG) WELDINGTungsten inert ga s w elding is a lso know n a s t ungsten
a rc we lding and g a s-tungs te n arc w elding (GTAW) a nd
is currently the mo st c om mo nly ap plied joining proce ss
for titanium and its alloys. Titanium is o ne o f the ea siestof me ta ls to w eld by the TIG process; the w eld poo l is
fluid a nd its com bination of low density and high surface
tension ena bles go od cont rol of the w eld surface p rofile
and penetrat ion, even when unsupported. An arc
between the tungsten alloy electrode and workpiece
obt ains fusion o f the joint region, w hile a n inert ga s (the
torch gas) sustains the arc and protects the tungsten
e l e c t r o d e a n d m o l t e n m e t a l f r o m a t m o s p h e r i c
cont am ination. The inert g as is t ypica lly a rgon, but a
mixture of helium and argon can be used to increase
penetration o r speed. Welds can be mad e a utogenously
( i .e . wi thout f i l ler addi t ion) or wi th addi t ion of a
co nsuma ble wire into t he a rc. The TIG proc es s is fullycapable of operating in all welding positions and is the
only process that is routinely used for orbital welding.
Specialist orbital welding equipment is commercially
available for a wide range o f compo nent diameters and
often has the a dd ed a dvanta ge of incorporating the inert
ga s tra iling shield nece ssa ry for tita nium fa brica tion.
ARC WELDING PROCESSES
Higher prod uctivity variants o f the TIG proce ss ha ve
been a pplied to t ita nium. Ho t w ire TIG enables a g reate r
fil l rate to be achieved, improving productivity on
multipa ss w elds req uired for heavier sec tion thickness es.Act ivate d TIG (A-TIG) ac hieves d ee pe r pe net rat ion
through the use o f a sp ecial flux sprayed onto the joint
surface s prior to we lding. The latte r proce ss ha s ha d
particular success for welding stainless steels, but its
pot ential applica tion to titanium joints ha s yet to be fully
exploited.
TIG (GTAW)
Advantages
• Manua l or mechanised process
• All posit ion capabilty• Capable of producing high quality welds
• Significant industrial experience
• No w eld spa tter
Disadvantages
• Lo w p ro d uc tivit y
• Tungsten inclusions if electrode touches
weld pool
Example welding paramet ers (1mm = .04inch) Manua l TIG w elding of a titanium vesse l (Bunting Titanium)
Tensile Strength Proof Stress Elongation (%)Alloy (MPa) (MPa)
Parent Weld Parent Weld Parent WeldGrade 2 460 510 325 380 26 18
Ti-6A1-4V 1000 1020 900 880 14 8
Ti-3A1-2.5V 705 745 670 625 15 12
Typica l tensile prop erties o f TIG w eldm ent s
v
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PLASMA ARC (PAW) WELDINGPlasma arc welding retains the high quality associated
w ith TIG we lding whilst ha ving significa nt pe net ration
and speed ad vantag es. Similar to TIG, heat is transferred
by an arc generate d betw een a tungsten electrode a nd
the w orkpiece ; but, in the PAW proce ss t he a rc is
constricted by a copper alloy orifice to form a highly
collimated arc column (see figure). In addition to a
surrounding shielding gas , a ‘plasm a ga s’ flow s through
the copper orif ice and a portion of this is ionised
p r o d u c i n g t h e c h a r a c t e r i s t i c p l a s m a j e t . I n t h e
conduction-limited mode a weld pool similar to that
produce d during TIG w elding is gene rated , w hilst in the
keyhole mo de , the p las ma jet fully penet rates t he joint.
Molten metal flows around the keyhole and solidifies
behind the plasma jet as the torch traverses along the
joint line. In many w ays t he keyhole plasm a a rc proce ss
is akin to a slower version of one of the power beam
processes (electron beam and laser welding). A thirdproces s variat ion exists, referred t o a s microplasm a a rc
w elding. This is simply a low current va ria nt (typica lly
0.1-15A) of the conduction-limited mode, suitable for
prod ucing sma ll co ntrolled w eld bea ds . Welding is
generally performed w ith co ntinuous d irect current w ith
the e lectrod e neg at ive (DCEN), but a pulsed current c an
be used to broaden the tolerance window of welding
parame ters which prod uce accepta ble we lds.
Plasma arc w elding offers significa nt prod uctivity ga ins
over both TIG and MIG, espec ially whe n ope rated in the
keyhole mode. Although welds are only typically madein the 1G o r 2G positions, single pa ss w elds can be m ad e
in ma te rial up to 18 mm thick. Furthermo re, the keyho le
PAW process ap pea rs to o ffer grea ter imm unity to we ld
metal porosity than any other fusion welding process.
Because introduction of filler into the arc can cause
insta bilities in the ga s pla sm a, keyho le PAW is norma lly
performed auto geno usly, thus a sm all am ount o f underfill
is typical. Completing a second pass, adding filler with
the same torch operated in the conduction-limited
mo de , o r alternatively using TIG w elding, ca n co rrect
this. Pipe circumferential welding (e.g. pipe joints) is
certainly pos sible, but req uires a co ntrolled slope -dow n
of the plasm a ga s flow rate and a rc pow er to a void a ny
porosity defects a t the sto p position.
FLUXED WELDING PROCESSESThe a pplica tion of fluxed we lding proce sses such a s
submerged arc, electroslag and flux cored arc welding
have been investigated and reportedly used in the formerSoviet U nion fo r we lding t hick sec tion t itanium a lloy. The
main difficulty is the selection of an appropriate flux;
oxides ca nnot be used these w ould co ntaminate the we ld
metal and, for similar reasons, fluxes should not be
hygroscopic (i.e. adsorb moisture). Most of the fluxes
have be en rare ea rth and /or alka line meta l-base d
h a l o g e n s a n d h a v e b e e n r e p o r t e d t o p r o d u c e
contam inant-free we lds. Some tests carried out in the
UK on commercially pure titanium showed mechanical
properties in conforma nce w ith ANSI stand ards. Work
performed at the Pato n Institute, Kiev has sho wn that
j o i n t s c a n b e p r o d u c e d i n t i t a n i u m a l l o y s w i t hperforma nce co mpa rable to those of TIG welds. In
practice, how ever, the q uality o f w elds ma de using fluxes
is suspect , since the oppo rtunities for conta mination a nd
slag inclusions a re significa nt. Due t o thes e intrinsic risks,
f l u x e d w e l d i n g p r o c e s s e s c a n n o t c u r r e n t l y b e
reco mm end ed for joining tita nium. Further rese arch into
these we lding methods is need ed but the improvements
to be gained, over conventional arc welding, are
considerable and could present ma jor cost savings for
thick sect ion titanium alloy fabrica tion.
PAW
Advantages
• Fas ter than TIG
• Single pass welds possible in material up to
18mm thick
• Greater immunity to weld metal porosity
than any other fusion process
Disadvantages
• Lmited posit ional capability
ˇKeyho le PAW is used exte nsively in the fa bricat ion of the Ti-6Al-4V VSEL light w eig ht field g un.
MIG welding has found application for a ppliqué armour plate such as for the General Dynamics M1 Abrams tank.
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LASER WELDINGLaser welding is f inding increasing application for
titanium, for example in the production of welded tube
and pipe. The proces s, w hich offers low distortion and
go od prod uctivity, is po te ntially mo re flexible t ha n TIG
or electron bea m for auto ma ted we lding. Applica tion is
not restricted by a requirement to evacuate the joint
region. Furthermo re, las er beam s ca n be d irected ,
enabling a large range of component configurations to
be joined using different welding positions. CO2 lasers
offer the greate st po we r range, with single pa ss w elds
possible in 20mm thick titanium using 25kW systems.
Nd-YAG las er weld ing offe rs supe rior flexibility due to
the p os sibility o f fibre opt ic beam de livery syste ms, but
penetration is restricted by a lower power capability.Laser welds can suffer from weld spatter, which may
pose a problem on the root surface, particularly if
postw eld a ccess is restricted.
POWER BEAM WELDING PROCESSES
La se r we ld 10m m (.4 ” ) thick Gr 2 joined to Ti-6Al-4V
ELECTRON BEAM WELDINGElect ron bea m (EB) welding has trad itiona lly been t he
preferred process for making critical joints in titanium
alloys. High quality welds can be produced with low
distortion and with high reliability. Productivity can also
be good, especially for thick sections which can be
welded readily in a single pass. Conventional electron
beam we lding is performed in a vacuum o f abo ut 10 -4
mba r, requiring a se aled cha mber and pumping system.
This a dd s to the ca pital cost of the eq uipment, especially
for large components. A further drawback for large
compo nents is the extended time it takes to a chieve a
vacuum in the chamber, decreasing product iv i ty .
How ever, electron beam guns have been d esigned w hich
can operate a t lower vacuum or near a tmosphericpressure. So called ‘reduced pressure’ electron beam
(RPEB) welding show s great promise for dec reasing cost s
and increasing productivity beyond that achievable using
conventional EB welding. Simple seals can be used to
isolate the joint region of a large component, which is
evacuated to a pressure of around 10 -1mba r (achievable
using inexpens ive m echa nical vacuum p umps ). An RPEB
ste el pipe J-laying syst em is currently unde r developm ent
at TWI. High q uality w elds ha ve a lso been produce d in
titanium a lloy pipe a nd plate .
RPEB w eld in 13m m t hick Ti-6Al-4V
Laser Welding
Advantages
• Aut om a te d p ro ce ss
• H ig h w e ld ing sp ee d
• Fibre o ptic bea m d elivery with Nd-YAG
Disadvantages
• Exp ens ive e q uip m ent
• Thickness limited w ith Nd-YAG• We ld Spa tt er
Electron Beam Welding
Advantages
• Aut om a te d p ro ce ss
• Single pass welding of thick sect ions
• No filler or gas shield required
• Significant industrial experience
Disadvantages
• Exp ens ive e q u ip me nt
• Component s ize limited by vacuum
cham ber (not a pplica ble fo r RPEB)
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INTRODUCTIONResistance welding of titanium is quite straightforward
and is aided by the metal’s high resistivity and low
therma l conductivity. The a sso ciated process es rely on
the hea t generat ed by the resista nce to electrical current
flowing through the workpiece to fuse the metal with
the joint. Shaped electrodes apply the current and
pressure req uired to m ake a localised w eld. As with othe r
joining proce sses , cleanliness of t he a butting joint fac es
is ess ential for a succ ess ful we ld. Experience ga ined with
sta inless s teels is relevant for resis tance welding
com mercially pure titanium grad es, since the resistivity
and therma l cond uctivity of the t w o m eta ls are similar.
Titanium alloys , how ever, have q uite d ifferent t herma l
and electrical characteristics and should not be welded
using parameters established for stainless steel.
SPOT WELDINGSpot welding is performed using copper a l loy
electrodes with a spherical face, a current of 5-10kA
(increasing with shee t thickness) and an e lectrod e force
of s everal kiloNew to ns. Inert ga s shielding is not req uired
for spo t w elding since the therma l cycle resulting from
the brief electrical pulse is extremely rapid, minimising
loc al oxida tion.
SEAM WELDINGContinuous or intermittent seam welds are made
using rotating disc electrodes, again with a spherical
contact profile, and the repeated application of brief
electrica l pulses. During w elding, the e lectrod e is rot at ed ,
traversing the cont ac t po sition a long the joint line. Inert
gas shielding or flood water cooling may be desirable
for seam welds since the repeated thermal cycles can
result in the accumulation of heat and subsequent
oxidation.
TITANIUM RESISTA- CLADTM PLATEThis p at ented process is p rincipally used to supply
r e q u i r e m e n t s f o r v e s s e l c l a d d i n g a n d f l u e g a s
desulphurisation (FGD) duct and flue linings. Electrical
resistance heating is used to bond titanium to a less
expensive ste el backing, using an intermed iat e s ta inless
steel mesh. The bond is a sea m 12.7m m (0.5 inch) wide,having typical shear and peel strengths of 303MPa and
15kg/m respectively. The se am s ca n be spa ced to mee t
the application requirements of stresses imposed in
service by gravity, vibration, thermal expansion and
pressure cycling.
Pre-bonded sheets are supplied for installation with
the titanium offset o n tw o s ides to permit o verlap and
fillet sea l welding o f a djoining s heet s. For retrofit
insta llat ions, the thicknesses of t itanium a nd the backing
ste el wo uld e ac h be 1. 6m m, (1/16 inch). For new build,
the titanium sheet can be recessed on all sides to allow butt w elding of the ste el backer, follow ed by sea l w elding
of a titanium batten strip. Here, the thickness of the
titanium is 1.6mm , but the st eel backer ma y be hea vier
up to 25 .4mm (1 inch), or as req uired by the o perating
cond itions of t he vesse l or structure.
RESISTANCE WELDING
Resista-Clad TM plate. Methods of application
A sp ot w eld in ASTM gra de 2 she et
Resistance Welding
Advantages
• Aut om a te d p ro ce ss
• Lo w dis to rt io n
• Spot welds do not require gas shielding
Disadvantages
• Po o r fa t ig ue st re ng th
• Limited to sheet materia l
Typical pa tte rn of st eel
w a l l a t t a c h m e n t a n d
ove rla p o f Ti Res ista -Clad
plates for retofit linings.
Typica l Ti at ta chm ent a nd
Resista-Clad plate
contruction for new or tota l
duct /vesse l wa ll cons truction.
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INTRODUCTIONThere a re app roxima te ly 20 variants o f friction we lding,
mos t of w hich could be a pplied to titanium and its a lloys.
In practice only a few of these are used industrially to
join titanium. An important feature of friction welding is
its ability to join titanium to other materials, which,
although often requiring an intermediate material, is
virtually impo ssible t o do by a ny proce ss involving fusion.
The a dvant ag es o f friction w elding include no need for
s h i e l d i n g g a s e s f o r m o s t p r o c e s s e s , v e r y r a p i d
comp letion rate s, and goo d me chanical properties. Most
we lds result in flas h forma tion, w hich typically must be
removed . The ma in process va riants are d escribed be low :
ROTARY FRICTION WELDINGThere are t wo ma in varia nts o f rot ary friction w elding;
the continuous drive and inertia processes. In the
continuous drive rotary friction welding process, a
com ponent in bar or tube form is rota ted under pressure
ag ainst a similar com pone nt, or one of larger dimensions,
FRICTION WELDING PROCESSES
Schemat ic diagrams of the tw o rota ry friction welding variants
Rot ary frict ion w eld in 13m m thick Ti-6Al-4V pipe
Prop erties of a rota ry frictio n w eld in Ti-6Al-4V
Tensile Strength Proof Stress Elongation
Region (MPa) (MPa) (%)
Base Metal 949 834 15
Weld Zone 994 854 11
Friction Welding - general comments
Advantages
• Ra p id o ne sho t pro c es se s• Fully a ut o ma t ed
• Can join dissimilar Ti alloys
• Potential to join Ti to some other materials
Disadvantages
• Exquipment may be expensive
• Inspect ion can be dif ficult
Rotary Friction Welding
Advantages
• Extensive industrial experience
• No shielding gas or f iller
Disadvantages
• Ro ta t iona l symmetry required
unde r an a pplied pressure. Frictiona l hea t de velops,causing the material close to the rubbing surfaces to
softe n and flow. After a certain displac eme nt distance
(called burn-off) has been reached, the rotation is
stopped rapidly, and the contact force increased to
provide a forging action to consolidate the joint. Any
interfacial conta mination is expelled w ith the flas h tha t
is extruded from t he joint.
Inertia friction welding has o ne com pone nt att ac hed
to a flywheel, and spun to a predetermined rotation
speed before being pushed aga inst the other component
unde r pressure. The bra king ac tion res ults in the
generation of frictional heating, and the formation of aweld. Inertia friction welding delivers energy at a
decreas ing ra te through the weld cyc le , whereas
continuous drive friction welding delivers energy at a
constant rate. Inertia welding is more commonly used
in the USA, and less so in Europe, where continuous
drive we lding is pred om inant.
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RADIAL FRICTION WELDINGOne d rawba ck of rota ry friction we lding is the nec essity
to rota te o ne of the co mpo nents. With sma ll pa rts this
is not normally a problem, but for example with long
lengths of pipe there are obvious potential difficulties.
One solution t o this is t o use rad ial friction w elding, in
which the pipes a re held s ta tionary, a nd a V sect ion ring
of narrowe r angle than the ed ge prepa ration in the pipe
is rotated between them using a continuous drivemechanism, and simultaneously compressed radially to
force t he ring into t he joint. The e q uipme nt req uired fo r
this proce ss is more com plex than tha t req uired for rot ary
friction w elding, a s it req uires a radial com pression unit,
and also a n internal mand rel to resist the high radial loa ds.
One a dvanta ge o f the internal ma ndrel is that the internal
flash is eliminated, although there is generally a small
reduction of internal diameter which may need to be
removed.
Schemat ic d iagram of rad ial friction w elding process LINEAR FRICTION WELDINGThis proce ss variant wa s d esigned t o e liminate the
need for rota tional symmetry in one o r both of the p arts
being joined, a nd a s a result the process c an succe ssfully
join pa rts of d iffering se ctio n. As its nam e implies, linear
friction welding uses a reciprocating linear motion to
provide the friction. The freq uency is t ypically 25 to
100Hz, w ith an a mplitude of + /- 2mm.
The proc ess w ill be used extensively in the ae ro-
engine industry, in particular for joining compressor
blades to disks, but has not been taken up by other
industrial secto rs for joining a ny me ta l. Ho we ver, a close
variant of the process , v ibrat ion welding, is used
ex tens ive ly in severa l indus t r ies for jo in ing
thermoplastics.
Stolt Comex’s radial friction welding pipe laying barge
FRICTION STUD WELDINGFriction stud w elding eq uipment is porta ble and ca n be
used in-situ in remote and ad verse environments. Like other
friction w elding processes the a dd ed ad vantages of friction
stud we lding are its rapidity a t co mpleting the joint and itsability to join to dissimilar metals. Current applications
include s tud a tta chment t o a rchitectural tita nium pa nels.
Radial Friction Welding
Advantages
• No sh ie ld ing gas required
• Neither component is rota ted duringwelding
• No bo re fla sh
Disadvantages
• Expens ive equipment
• Internal bore support required
Linear Friction Welding
Advantages
• Shie ld ing gas no t required
• Very good posit ional accuracy
• Rotat ional symmetry not required
Disadvantages
• Exp ens ive e q uip m ent
Pro pe rties o f rad ial friction w eld in Ti-6Al-4V-0.1Ru
Tensile Proof Elongation
Region Strength Stress (%)
(MPa) (MPa)
Base Metal 910 840 14
Ring (as-received) 885 795 11
Ring (as-welded) 1055 925 9
Cross-weld 900 820 9
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In this process, one component is moved against the
ot her in an o rbita l mo tion. This remo ves the req uirement
for symmetry in both of t he com ponents.
FRICTION STIR WELDING
This no vel proce ss ha s bee n w el l de ve lope d foraluminium al loys . Progress is being made for i ts
application to titanium alloys, although it will be some
time before it ca n be co nsidered a compe titive process.
A n u m b e r o f a d v a n t a g e s h a v e a l r e a d y b e e n
demonstrated for aluminium that may also apply to
tita nium. Friction stir we lding involves m oving a sm a ll
ro ta t ing too l be tween c lose but ted components .
Frictional heating ca uses the ma terial to so ften, a nd the
forwa rd mot ion of the t oo l forces ma terial from the front
of the to ol to the ba ck, w here it consolida tes to form a
solid sta te we ld. The p rocess com bines the flexibility o f
me chanised a rc welding with the excellent cha racte risticsof a friction we ld. P rogress has be en sw ift in developing
the tec hnology for titanium and development w elds ha ve
bee n co mp leted succ ess fully at TWI.
THIRD BODY FRICTION WELDINGThird bo dy friction w elding is a useful techniq ue for
joining dissimilar materials which cannot normally be
joined by co nventiona l friction w elding. In this proce ss,
one component is rotated and plunged into a hole in
the seco nd co mponent, into w hich a t hird mat erial has
ORBITAL FRICTION WELDING
Schematic diagrams of the above welding processes
Friction Stir Welding
Advantages
• No filler
• Lo w dist ort io n• Fully mechanised
• Increasing industrial usage for non-ferrous
alloys
Disadvantages
• Still under development for titanium
• G a s s hie ld re q uire d
been place d. This third m at erial can be a met al which
softens at a lower temperature than either of the twocom pone nts being joined, o r it ca n be a brazing alloy.
FRICTION TAPER PLUG WELDINGIn this proce ss a t ape red plug is rot at ed a nd plunged
into a pre-machined hole. It was developed for weld
repair of alloys that are d ifficult to fusion w eld or a re in
a ha za rdous environm ent. By placing overlap ping friction
taper plugs into the material, linear features (such as
crac ks) ca n a lso be rep aired. This is know n a s Friction
Tape r Stitc h we lding.
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DIFFUSION BONDING
CONVENTIONAL DIFFUSION BONDINGTitanium is the ea siest o f all co mm on e ngineering
ma terials to join by diffusion bond ing, d ue to its a bility
to dissolve its own oxide at bonding temperatures.
Conventional diffusion bonding is a slow process, andrequires careful control of temperature, and joint face
alignme nt. The proce ss a lso need s to be unde rtaken in
a vacuum. Under ideal conditions a bond of very high
q uality ca n be m ad e w ith no flash fo rma tion. Ho wever,
the p rocess is slow, a nd req uires considerable precision,
ma king it unattrac tive for field use , a lthough it has been
widely used in the aerospace industry, in particular in
co njunction w ith superplast ic forming. The proc ess ,
including superplastic forming, is also used in the
successful development of t i tanium compact heat
exchangers.
Rolls Laval’s diffusion bo nded comp act heat exchanger
ELECTRON BEAM DIFFUSION BONDINGThis proc ess is a va ria nt o f diffusion bo nding in which
only the interface region is heated, resulting in a
co nside rable e nergy saving. The hea ting source is an
electron beam which is swept over the area of the joint
at such a speed that fusion of the titanium alloy is
prevented. A force is applied across the joint. As the
heated area is very limited, higher forces can be used
without the risk of plastic collapse of the components
being welded, resulting in a significant reduction of
we lding time, t ypically by a n orde r of m ag nitude . The
proces s ha s bee n investigat ed for joining several titanium
aluminide a lloys t o the mse lves and ot her tita nium alloys,
and for joining titanium a lloys. Very go od results ha ve
been reported from these trials, but to d ate the process
has not been used commercially.
Diffusion Bonding
Advantages
• Limited microstructural changes
• Can join dissimilar Ti alloys
• No fille r required
• Can fabricate very complex shapes ,
especially using superplastic forming
Disadvantages
• Slow
• H ig h va c uum re q uire d
• Expens ive equipment
Rolls Royce ’s SP FDB front fa n blad e fo r the Trent 800
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FORGE WELDING PROCESSES
FLASH BUTT WELDINGFlash welding is a forge welding process in which
heat is generated by resistance when a large current is
pas sed across the surfaces t o be joined. During the initial
flashing stage points of contact resistance heat, meltand blow out of the joint as the faces are progressively
mo ved to gether at a predete rmined rate. When a critical
metal displacement has been reached the faces are
forged to gether rapidly to consolida te the weld.
The proces s has be en used for many yea rs for the
product ion o f aeroeng ine sta to r rings, a nd w ith suita ble
eq uipme nt is capa ble o f joining pipe and othe r extruded
sections of any configuration. Properties close to those
of the parent meta l are o btained from substantially de fect
free joints.
HOMOPOLAR WELDINGHom opo lar we lding is a new met hod c urrently under
development in the USA, where it has been developed
primarily for welding pipes. Kinetic energy stored in a
flywhe el is rapidly converted to a high d irect current low
voltage e lect rica l pulse using a hom op olar generat or, and
this high current pulse is pa ssed ac ross a closely butted
we ld joint, ca using a resistance we ld to be ma de . A high
axial load is also applied, causing softened material to
be expelled. Neither of the components has to be
rota ted , a nd no shielding ga s is required . Trials have be en
undertaken on titanium, apparently successfully, but
results ha ve yet to be published .
EXPLOSIVE BONDINGEx p l o s i v e b o n d i n g s h o u l d b e c o n s i d e r e d f o r
applications when a thin uniform lining of titanium is
req uired o n a ba se m eta l. The te chnique is regularly used
for the prod uction of high pressure tubeplate s for tube
and shell heat exchangers, reaction vessels, chlorine
generators, and for lined plant and ductwork subject to
nega tive pressure. In the proces s, thin tita nium shee t is
placed at a closely controlled distance on top of a backing
plate. Explosive spread uniformly o n to p o f the titaniumis detonated from a single point, the explosion driving
the titanium down across the air gap to impact on the
bac king m eta l. A jet o f surface o xide s is expresse d from
the a pex of the co llapse angle formed, and this removes
any residual contamination from the mating surfaces,
producing a meta llurgical bond o f wa ve-like form and
guarante ed shea r strength. The co ntinuity of the bond
can be confirmed ultrasonically. All low to medium
strength t itanium g rade s, (ASTM 1, 2, 7, 11, 12 , 16, 17,
26, 27), can be bonded typically down to 2mm (.08
inch) thick ont o a variety of ferrous o r non ferrous backing
plates , no minally 12.7m m (.5 inch) or thicker. Plate s ha vebeen produced up to 3.5 me tres (137 inch) diam eter or
15 sq . metres area (160 sq. ft.).
Cross-section of an explosively bonded steel to titanium joint.The e xplos ive b ond ing proces s
Flash Butt Welding
Advantages
• Very rap id weld t ime
• S ing le s ho t p ro c es s
Disadvantages
• Flash remova l required
• Inspect ion may be dif ficult
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BRAZING
CONVENTIONAL BRAZINGTitanium a lloys ha ve bee n braze d s ucce ssfully using
silver, aluminium and ti tanium alloy braze metals.
Although there a re ma ny variants , o nly vacuum braz ing
has significant application for ti tanium due to the
requirement to protect the bas e me tal from oxida tion.
Ho we ver, de velopm ent w ork has bee n performed in the
use o f sliver chlo rid e-lithium fluo ride fluxes a nd TIG
brazing has proven succes sful in so me ap plica tions. Silver
alloy braze s we re the first to be a pplied t o titanium and
commercially pure silver, silver alloys with copper and
manganese, and silver–copper alloys with zinc and tin
have all shown some success. Although joints tend to
have good duct i l i ty , s trength is poor a t elevated
temperatures and corrosion resis tance is poor in
chloride-containing environments. However, the silver
alloy braze met als have liq uidus te mpe ratures below the
bet a trans us o f a lloys s uch a s Ti-6Al-4V, t hus t he braz ingcycle will have little or no effect on the base metal
microstructure and properties. The use of aluminium-
silicon fillers also ena bles low tem perat ure braz ing to be
performed, w ith the ad ded benefit of de creased w eight.
It is crucial, how ever, to ma inta in as short a braze c ycle
as poss ib le to min imise the format ion o f b r i t t l e
intermetallics.
Titanium a lloy bra zing a lloys are by fa r the mo st
common for joining t i tanium, the most avai lab le
commercial alloys being titanium-copper-nickel alloys.
These offer high strength a nd g oo d corrosion resistance,
but t he m os t rea d ily ava ila ble alloy (Ti-15Cu-15 Ni)
requires braz ing a t te mpe ratures over 1000°C (1830° F).
A Ti-20Cu-20Ni a lloy a nd a mo rpho us Ti-Zr-Cu-Ni braz e
f o i l h a v e b e e n d e v e l o p e d f o r b r a z i n g a t l o w e r
tempera tures (850 and 950°C , (1560 - 1740°F)
respect ively). These ha ve ad vanta ges fo r applica tion with
Ti-6Al-4V. For t he highes t t em pe rature joint a pplica tions ,
pa llad ium bas ed a lloys ha ve been used a lthough brazing
must a lso be performed at high tempe ratures.
The braz ing proc ess offe rs the ca pa bility o f dissimila r
metal joining, using a silver alloy braze metal. Dissimilarti tanium alloy and ti tanium to ferrous, nickel and
Brazing
Advantages• Complex geometry ’s can be joined
• Dissimilar metal joints are possible
Disadvantages
• Slow, unless batch processing is possible
• Mus t be performed in a vacuum
• Galvanic corrosion may limit application
r e f r a c t o r y m e t a l j o i n t s a r e p o s s i b l e . C o m p l e x
configurat ions can be joined, limited only by the neces sity
to ma inta in closely abutting joint face s.
Titanium braze d w ith a silver braze met al.
TRANSIENT LIQUID PHASE BONDINGThis proces s has been d escribed as a dif fusion
bonding proce ss, but trans ient liquid pha se (TLP) bond ing
has m ore in commo n with braz ing tha n diffusion bo nding.
An interlayer, or melting point suppressant, is placed
between the joint faces prior to heating in a vacuum.
The interlaye r mat erial is cho sen t o rea ct w ith the bas e
m e t a l , f o r m i n g a e u t e c t i c l i q u i d a t t h e j o i n i n g
tem perat ure. The rea ction prog resses until the liq uid
meta l reso l id i f ies i so thermal ly , l eav ing a jo in t
microst ructurally similar to the bas e m eta l. P ure nickel
and copper, and copper-nickel alloy interlayers haveshown good performance for joining titanium alloys. A
further benefit of t he proce ss o ver conventiona l braz ing
is the reduce d w eight of the st ructures, since only a very
thin interlayer is required. However, a signif icant
perpendicular loa d m ust be a pplied to the com ponents
to maintain good surface contact during the bonding
process.
SOLDERINGTitanium is extreme ly difficult to so lde r beca use o f
the same properties that confer its superb corrosion
resista nce - the tena city and sta bility o f its surface oxide.Conventional soldering methods depend on aggressive
fluxes to allow the so lde r alloy to w et the surfac e o f the
bas e me ta l. None of the conventiona l fluxes is effective
for titanium and so the surface is typically precoated
with a m ore com pa tible me ta l, such as c opp er, by PVD
or sputte r coat ing. It is a lso p oss ible to ‘tin’ the s urface
of the titanium by extended immersion in a molten tin
bath at 600°C (1110°F); the titanium oxide is adsorbed
by the base met al, allow ing the tin to we t a no n-oxidised
surface. Some success has also been reported in the
use o f mo lten silver or tin halide s, w hich reac t w ith the
oxide surface to produce a tin or silver coating; and in
the use of conventional fluxes whilst disrupting the
surfac e o xide with a n ultraso nic s oldering iron.
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ADHESIVE BONDING
Adhes ive bo nding provide s a n a lternative to we lding,
particularly for joining sheet material and for joints
between titanium and non-metals such as polymer
co mpo sites. The use o f ad hesives is ofte n a viable
alternative or companion process (i.e. hybrid bonding)to res i s t ance spo t weld ing in jo in ts des igned to
experience predominantly shear stresses in service.
Fac to rs such as t he service e nvironment d om inat e the
selection of a dhes ive, but this subject is too co mplex to
discus s in det a il in this publica tion. The high streng th o f
modern structural adhesives is entirely appropriate to
the use of bonded titanium in structural applications,
although careful pre-treatment of the bond surfaces is
critical for achieving maximum properties. It is strongly
recommende d that t echnical consultation is sought for
ad vice o n all aspec ts o f the bond ing process.
Adhesively bo nded carbon fibre com posite/titanium w ishbone
of a Willia ms fo rmula one rac ing ca r.
MECHANICAL FASTENING
Mecha nica l joining proc esse s fo r titanium include all
t y p e s o f f a s t e n e r , m a n y o f w h i c h a r e r o u t i n e l y
manufactured in t i tanium and widely used in the
ae rospa ce industry. Non-titanium fa stene rs in ma terials
of low er corrosion resistance co mpa red to titanium ma y
be used w here no da nger of galvanic corros ion is present,
or where the fastening is totally isolated from the
corrosive e nvironment. In environment s w hich po se a risk
Adhesive Bonding
Advantages
• Rapid
• Titanium to polymer/compo site joints arepossible
Disadvantages
• Po o r p erfo rm a nc e in pe e l
• Application is limited for most corrosive
environment
of ga lvanic co rrosion, non titanium fast eners can be used
provide d they are insulat ed from the titanium using
suitable gaskets.
Other mechanical jointing methods such as lock
seaming (e.g. in automotive exhaust box manufacture)
and overlap joints, (used for architectural panels) are
limited to the mo re ductile grad es o f the me tal and its
alloys.
Selection of titanium bolts, fast eners and capt ive nuts
Close up of joint
Both lock seaming and welding are used in the manufacture
of titanium exhaust system s
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JOIN ING TITANIUM TO OTHER
M ATERIALS
Titanium is incom pa tible w ith m ost othe r meta ls a nd
w ill form brittle co mpo unds if fusion we lde d directly to
them. Indeed the only commercial alloys that can be
directly fusion welded to titanium are those based onzirconium, niobium and certain other refractory alloys.
More common structural materials, such as all ferrous
a nd a luminium a lloys , a re invariably unsuitab le fo r direct
fusion welding to t i tanium. Several novel joining
techniques have been adopted for making dissimilar
joints, but the range o f possibilities is to o va st to ad dress
here in any detail . Many of the welding processes
discussed in the preceding sect ions c an be applied to
dissimilar material joints between titanium and other
metals. Indeed, explosively bonded titanium clad steel
and Res i s t a-C lad TM p l a te a re pr ime examples o f
successful dissimilar bonding technologies. Explosive
bonding has also been used to form transition joints
betw een t ita nium a nd ferrous alloys, for exam ple titanium
pipe to s ta inless stee l flange joints.
The fo llow ing tab le is intend ed to highlight ge neric
processes that may be capable of fabricating joints
betw een titanium a nd othe r ma terials. These will normally
require particular practices to be adopted to achieve a
sa tisfac to ry joint. The suitability of the va rious p roces ses
will depend on the components to be joined and the
properties req uired and it is strongly reco mm ende d tha t
technical consultation be sought prior to finalising a
com pone nt d esign incorpo rating dissimilar joints.
EB Laser Friction1 Adhesives Explosive Resistance Brazing
bonding welding
Steel
Stainless Steel
Nickel alloy
Refractory
metals
Copper
Aluminium
Cobalt alloy
Ceramics
Polymer composites
Joining processes t hat m ay be capa ble of forming sound joints betw een titanium a nd o ther materials.
Notes 1 Does not indicate that all friction processes are appropriate for a given dissimilar joint.
SELECTION OF A WELDING
PROCESS The fo regoing sect ions have p rovide d a br ief
summary of the characteristics of the various joining
processe s that ca n be used to we ld titanium structures.
Most fabrica tion is performed by TIG we lding and this
is unlikely to change, however it is crucial to the
production o f low cost titanium co mponents that higher
produc t iv i ty , more cos t e f f ec t ive processes beco nside red w here po ssible. For exam ple, PAW oft en
enables significant productivity gains for a low capital
investme nt, while achieving similar o r greate r q uality t o
tha t a chievable using TIG w elding. More ‘exot ic ’
processes such as power beam and friction welding
should also be considered, since even if no in-house
capability exists, work can often be subcontracted to
experienced fa brica tors.
CAUTION:GALVANIC CORROSIO N Titanium is highly co rros ion resista nt, a nd c an a cce lerate t he co rros ion of d issimilar
meta ls w hen coupled to a less noble meta l. In add ition to accelerated co rrosion, w hen such a ga lvanic couple exists,
hydroge n can be t aken up by the titanium, lead ing in som e circumst anc es to hydride c racking and failure. Alloys w hich
occupy a similar position in the galvanic series as titanium may be safely coupled to titanium in environments which
w ould not o rdinarily lead t o co rrosion o f the uncoupled ba se me ta l. For example duplex and supe r-auste nitic sta inless
steel, a nd Ni-Cr-Mo alloys ca n often be sa fely coupled to titanium. Ho wever, it is recommend ed that specific tec hnical
ad vice is so ught fo r any given ope rating environm ent a nd d issimilar joint. Further de ta ils o n simple me chanical co uplings
ca n be found in TIG Dat a Shee t No 6.
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OPEN AIR WELDINGThe req uirement for ad ditiona l gas s hielding o f bot h
the back-face and cap regions during open-air welding
is the only significa nt fac to r w hich differentiate s titanium
from mo st s ta inless s tee l fabricat ions . There a re,
how ever, ma ny com mercial so lutions fo r the prote ction
o f t i t a n i u m w e l d m e n t s , m a k i n g b o t h l i n e a r a n d
circumferential welds quite straightforward. Greaterexperience is necess a ry for mo re difficult configurat ions,
but many commercial fabricators weld more or less
complex shapes on a regular basis.
Conventional back purging techniques, as used for
high quality stainless steel welding, are commonly
adopted for titanium. Straight runs employ a grooved
backing bar which is purged with a moderate gas flow.
For m ore c om plex configurat ions aluminium o r cop per
foil ca n be tape d to the underside fo rming the necess ary
channe l for the ga s purge. In this instance, ca re must be
taken to prevent the foil coming into contact with thehot titanium. Purging dams or bladders are used to
protec t the underside o f circumferential welds, o r difficult
to access regions. It is important that sufficient time is
allow ed during ba ck purging to reduce the air content in
the purged region to very low levels. No hard and fast
rule exists for purge time, since this d epend s largely on
the purged volume, its complexity and the flow rate of
Purging blad de rs (Hunt ingdo n Fusion Tec hniq ues )
argon. Use of an oxygen meter is advised in most
instances to ensure that oxygen content of the purgegas i s lower than approx imate ly 20ppm pr ior to
com menc ing we lding. The cos t of so lid stat e oxygen
meters capable of reading to these low levels has
dropped in recent years and the capital investment can
be quickly recouped by a reduced use of argon, rework
and scrapped comp onents. If possible w elding should
be performed with touching root faces , since a root ga p
makes purging of the underside significantly more
difficult. If a roo t ga p is unavoida ble the n, w here there is
acce ss to the underside, a root side ga s shield’ similar
to that used for protecting the weld cap (discussed
below), is the best solution. If access is limited, then
aluminium or coppe r foil can be t ap ed o ver the to p face
and remo ved ahea d o f the torch during the com pletion
of the root weld pass.
Protection of the weld cap is routinely achieved by
the use o f a t r a i l ing sh ie ld , however in cer ta in
circumsta nces, such as a TIG roo t pa ss in a dee p groove,
the use of a n appropriate gas lens on the welding torch
can a chieve sa tisfacto ry results. Whilst no hard a nd fa st
rule ca n be s ta ted , the ceram ic no zz le is suita ble fo r TIG
welding currents up to about 35 amps and the annularga s lens for currents up to abo ut 90 am ps. It is stressed
that this depend s on a favourable joint geo met ry, a llow ing
the torch shielding gas to flood the joint and provide
ga s protec tion aw ay from t he to rch. Welding at higher
currents or anything other than slow traverse speeds,
should be carried out with a trailing shield attached to
the t orch. The a rgon supply to t his shield is via a s epa rate
supply rat her than by d iverting a propo rtion o f the to rch
argon. The bod y of the shield ca n be ma de from copper
or aluminium if l ightness is important and should
incorporate a stainless steel woven mesh gauze for
diffusing the gas stream. The d esign of a successful
t r a i l i n g s h i e l d r e q u i r e s e x p e r i e n c e , b u t p r o v e n
commercial products are available for circumferential,
fillet and straight w elding. Their length and w idth de pend s
on the we lding process: MIG and auto ma tic TIG we lding
req uire long er tra iling shields tha n for m a nual TIG since
traverse speed s a re greate r. Hea t resistant glass ma y be
employed instead of metal for shields where better
visibility is req uired .
Trailing shields (Hunt ingd on Fusio n Tec hniq ues
Desp ite t he high rea ctivity of t ita nium, shielding ga sesare no t no rmally req uired fo r friction w elding. Ma terial
conta minated by exposure to a ir is pushed into the flash,
and can be remo ved. There wi ll be som e surface
discolorat ion c lose to the weld, but the depth of
cont am inat ion is very sm all. If the ap plica tion is critical,
it m ay be ad visable to remove this mat erial after w elding.
An exception to the lack of requirement for shielding
gases occurs when processes are used which develop
little or no fla sh. Friction stir we lding of t itanium req uires
a high q uality ga s shield.
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The need for care a nd planning at t he ma terials
p r e p a r a t i o n s t a g e c a n n o t b e o v e r e m p h a s i s e d .
Frequent ly, w here problems ha ve been repo rted w ith
titanium fabrications, all or part of the cause can be
traced to t his sta ge. The correct preparat ion o f the we ld
joints is essential for arc welding, diffusion bonding,resistance welding and brazing, although friction welds
are typically more forgiving.
GEOMETRY OF WELD PREPARATIONS:For TIG we lding, a sq uare-cut ed ge ca n be used for
all butt or corner welds in thin gauge sheet and tube
where the thickness do es not exceed 1mm. Rough edge s
with burrs are difficult to set up and can result in high
levels o f we ld poro sity. Thicker shee t a nd t ube sho uld
be p rovide d with a V-prepa ration w ith the 90º included
angle V terminating in a 0. 6mm root fa ce. By t his me ans
it is possible to achieve consistent penetration duringt h e f i r s t w e l d r u n . Ex p e r i e n c e h a s s h o w n t h a t
co mm ercially pure titanium gives low er weld pene tration
tha n Ti-6Al-4V a lloy. Thus, a lthough t he d ifferenc e is no t
as extreme as is commonly found between different
cas ts o f sta inless steels, joint de signs sho uld be q ualified
for the titanium grad e to be w elded . For plat e w elding at
more than 6mm thickness, a simple open V can give
rise to unacceptable distortion due to thermal stresses.
In this case a machined J-preparation is used in which
the a ngle of t he sides is as steep a s pos sible co nsistent
with achieving complete side-wall fusion. As a guide,
the total included angle should be not greater than 65º
and not less tha n 45º. A do uble V-prepa ration is an
acceptable alternative to the J-preparation when there
is acce ss to bot h sides o f the weld.
Prepa rations tha t a re suita ble for TIG we lding are a lso
generally a ppropriate for MIG and plasm a when o perated
in the co nduct ion-limited mo de . Keyhole plasma we lding
requires only a simple square butt penetration for
thicknesse s up to ap proxima tely 18mm (.7”). Thicker
ma terial can be prepared as for TIG we lding, but w ith a
root fac e up to 15m m, and filled by PAW or TIG. Elect ron
bea m, laser and mo st friction we lds req uire simple butt
geometry.
CUTTINGA n y t h i c k n e s s o f t i t a n i u m c a n b e c u t w i t h
conventional flam e cutt ing eq uipment. How ever, it must
be remembered that contamination of the metal with
oxygen w ill result in harde ning o f the m eta l adjacent of
the cut edges. Thus a size to lerance of + 6mm, (.25”)
should be allowed for subsequent cleaning up. Plasma
arc cutting or the use of lasers are possible alternative
techniques to the oxyacet ylene proce ss. As-cut surface s
should not be w elded be fore the joint faces a re finished
using a machining technique capable of giving a non-cont am inate d goo d q uality surface. As-guillotined joint
20
faces sho uld not be w elded . Experience has shown t hat
this cutting tec hniq ue which produces a smea red ed ge
leads to excessive weld metal porosity.
MACHININGThe follow ing technique s a re suita ble for the prepa ration
of titanium joints:
(a) Turning, milling a nd planing: The surfac e ob ta ined by
conventional machining processes such a lathe-
turning, milling and planing are suitable for welding
w ith no ad ditional clea ning other than de greasing to
remove cutt ing lubrica nts. Ca re is need ed to ensure
that the me tal is not overhea ted during the m achining
operation and that other (non machined) surfaces
to be w elded are not oxidised.
(b) Grinding: This techn ique is w idely used for prepa ring
the ed ges o f med ium and thick ma terial for we lding.The aim should be to prod uce the smo othest, most
regular profile po ssible with the s crat ch lines running
along the line of the weld and never across it. If
overheat ing o f the ma terial occ urs it w ill be evide nt
from discoloration. Whenever practicable, grinding
should be followed by draw fil ing, or any other
technique which improves the smoothness and
profile of the weld and ensures that any grinding
particles are removed.
(c) Linishing: Belt or disc finishers are suitable for edge
preparation o f med ium gauge compo nents. A 100
grit grade o f paper can be used for most purposes.
Linishing is a relat ively slow ope ration w hich p roduces
fine d ust a nd is expensive on consuma bles. How ever,
it is very flexible and can give excellent results.
(d) Draw filing: Preparations made by grinding can be
improved by draw filing. A fine toothed flat file is
drawn repea tedly a long the meta l sur f ace , an
op erat ion w hich remo ves minor irregularities. Filing
requires skill and the use of a clean file or it can
worsen rather than improve the surface
(e) Scratch brushing: Surfaces can be scratch brushed
to remove a ny residua l conta mination. With st ainless
ste el brushes th ere is a slight risk of iron pick up a nd
t i t a n i u m b r u s h e s s h o u l d b e u s e d f o r c r i t i c a lapplications.
It is usual to ma chine the abut ting surfac e o f a friction
weld before w elding. It is not necessary to have great
accuracy in the finish, although a good square set up is
usually needed. Since there is always a loss of length
due to burn-off into the flash, there is little point in
ma chining o verall lengths t o a ny precision. This should
be done after welding, when the flash must also be
removed . Much grea ter precision is required for diffusion
bonding and, to a lesser extent, brazing since good fit-
up of the joint faces is essential to the success of thesejoining me thod s.
PREPARATION OF THE JOINT FOR WELDING
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Mac hined joint face s a nd m at erial likely to be fused
(i.e. nearby ma terial on the joint underside and top face),
should be cleaned and degreased prior to welding to
remove a ny cutting fluids or grease .
PICKLINGAcid p ickling ca n be used to remove oxygen
contaminated metal from the surface of titanium. It is
also useful for removing any surface iron conta mination
tha t m ay be prese nt from ma chining. Pickling so lutions
are typically aqueous solutions of hydrofluoric (48%
co nce ntrat ion) a nd nitric ac id (70%conc ent ration). The
acid ratio s hould always be maintained betw een 1:5 a nd
1:9 (5%HF/35%HNO3 has bee n found to be an effective
solut ion) . P ickl ing should be carr ied out a t room
tem perature, for 1-5 minutes d epe nding on the act ivity
of the bath. If the surface of the metal is dirty or oily,
de greasing or aq uablasting must preced e pickling or theac id dissolution w ill be non-uniform prod ucing a pitted
effect.
PREWELD CLEANINGThe surface o f the w eld preparat ion a nd a djoining
met al is critical to the q uality of t he joint and should be
scrupulously clea n prior to w elding. The surfa ce s ho uld
be inspected to see whether a f inal hand f inishing
operat ion is necessary , e .g . to smooth out rough
ma chining ma rks a nd rem ove s livers of m eta l. The
smoothness of abutting edges is particularly important
for reduced porosity in arc welds and diffusion bonds.Vapo ur and liq uid deg reasing metho ds are a pplica ble fo r
titanium alloys.
(a) Vap our: Imm ersion ta nks ba sed on trichlorethylene
vapour a re e f fec t ive in removing grease , o i l ,
fingermarks and general dirt from the surface of
titanium co mponents. It should be ensured that the
tanks are not located too near to the welding area
nor that components are transferred immediately
from the tank to the welding booth because of the
risk of pho sge ne fo rma tion. Trichlorethylene sho uldregularly be checked for HCl acidity.
(b) Liquid: Small components can be degreased by
immersion in, for example, acetone or isopropyl
alcoho l. Larger item s ca n be cleaned by wiping w ith
lint free cloths o r tissues so aked in the so lvent. Under
no circumstances should m ethanol be employed a s
a de greasing agent.
Once com ponents have been degreased, t he surfaces
must be handled only with clean gloves and preferably
not a t a ll: ba re hands, even ostensibly clean ones, depo sit
a surprising amount of grease and salt.
CLAMPING AND FIXTURESClamps and f ixtures for arc welding should be
designed to minimise distortion and, where necessary,
incorporate the purging system required to protect the
underside of the w eldme nt.
For co nventiona l rota ry friction w elds, the rota ting
part is norma lly held in a t hree jaw chuck, a lthough special
too ling ma y be req uired fo r the non-rotat ing pa rt if it is
not axially symme trical. For linear frict ion we lding, spe cial
tooling specific to the component is always required.
The to oling for the reciproca ting comp onent must be
de signed w ith ca re in order to m inimise the w eight and
hence inertia of the system, which will have to change
direction typically 100 times every seco nd.
ARC WELDING TECHN IQUE
POWER SOURCES AND TORCHESTitanium and its a lloys c an be we lded with mo st
conventiona l we lding pow er sources and torches. For
TIG welding a po we r source eq uipped with a non-cont ac t
arc strike facili ty is essential to prevent tungsten
conta mination of the w eld, which occurs if a touch do wn
technique is employed . The po we r source must also be
capable of breaking the arc on completion of a weld
run, without stopping the inert gas flow, or weld metal
conta mination by air may o ccur at the w eld st op p osition.
TUNGSTEN ELECTRODEThe choice o f electrod e c ompo sition a nd d iam eter
is no different t han fo r TIG w elding sta inless stee ls a nd
is influence d by t he req uirement s fo r elect rode long evity,
ease of arc initiation and stability. A simple 60º conegives sa tisfacto ry results for mo st ma nual TIG w elding.
With angles less t han 4 0º the re is a g reate r risk of
tungsten loss while above 80º arc initiation is difficult
and the arc has a t endency to w ande r. Should the
electrode touch the weld bo th must be ca refully examined
before restarting. Any tungsten in the weld, no matter
how sma ll must be excavated.
SELECTION OF WELDING PARAM ETERSTIG, PAW and MIG w elds c an be ma de using a va riet y
of current/spee d com binat ions, the d ifferences being the
result of operator preference. However, i t is worth
remembering that the aim should be to a chieve a go od
bead shape with minimum heat input. In that way,
disto rtion a nd a rgon shielding problems w ill be minimised.
TIG and plasma we lding are best ac hieved with direct
current electrode nega tive (DCEN) polarity and , fo r MIG
welding , pu lsed opera t ion i s genera l ly pre fer red .
Suggested welding parameters are given in the table,
although these should be used as a guideline only.
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SHIELDING GASESFor most purpose s, the co mme rcial grad e of argon
ma y be used for welding tita nium, a lthough prod uctivity
can be enhanced through the use of argon-helium
mixtures or pure he lium. The use of he lium-co nta ining
gases has particular advantages for MIG welding since
spatter can be reduced considerably. Commercially
available cylinders of welding grade argon and helium
are o f sufficient purity for a ll welding o perat ions, how ever
ca re should be ta ken to ensure that non-permea ble hose s
are used for all attachme nts to ensure tha t m oisture is
not incorporat ed into the shielding gas . If cylinders a re
used it is inevita ble t hat they w ill conta in a sma ll amo unt
of m oisture. This level is extreme ly low w hen the ga s
cylind er is full, but as t he pres sure in the c ylinde r drops ,
so t he mo isture co ntent rises . There is some justifica tion
for using gas from a cylinder for welding titanium only
until the pressure has fa llen to ~ 25ba r, a fter which it
should be used to supply gas for w elding less s ensitive
met als. Bulk supplies o f argon ha ve much low er moisture
contents. Where an on-site gas tank is used to supply
several welding stations gas purifiers, moisture and
oxygen met ers can be co nnected to the ma in feed line
to provide overall q uality a ssurance.
Inad eq uate shielding of the w eld cap can o ccur when
argon flow from the to rch is either too low so tha t all the
air is not d isplaced , or too high so tha t turbulence o ccurs.
Some experimentation on off-cuts of material may be
needed to estab l ish the most sui tab le condi t ions .
Keeping a record o f values used on p revious w ork
eventually helps to reduce the time spe nt in setting up.
It is ad vised that a gas lens be used to m aintain a lamellar
ga s flow. This a pplies e q ually to the ga s flow rate fo r
trailing shields, although the minimum flow rate willdepend on the size of the shield.
22
Dia meter Flow
in. mm in. mm cfh l/min ipm mm/s ipm mm/s
0.008 0.20 Melt-in 0.030 0.76 0.5 2.3 Ar 5 - 5 2.1 - -
0.015 0.38 Melt-in 0.030 0.76 0.5 2.3 Ar 6 - 5 2.1 - -
0.125 3.18 Keyho le 0.136 3.45 9 42 Ar 150 24 15 6.3 40 16.90.188 4.78 Keyho le 0.136 3.45 10-12 47-57 Ar 175 30 15 6.3 42 17.8
0.250 6.35 Keyho le 0.136 3.45 16 76 Ar 160 30 12 5.1 45 19
0.313 7.95 Keyho le 0.136 3.45 15 71 Ar 172 30 12 5.1 48 20.3
0.390 9.92 Keyhole 0.136 3.45 32 151 He+ 25Ar 225 38 10 4.2 - -
0.500 12.7 Keyhole 0.136 3.45 27 127 He+ 50Ar 270 36 10 4.2 - -
Notes: * Direct current electrode negative** 0.062inch (1.6mm) diameter wire
Suggest ed Welding pa rame ters fo r PAW w elding titanium
ThicknessWelding
techniq ue Nozzle OrificeOrifice and
shielding gas
Welding
current, A*
Arc
vo lt a ge , V Tra ve l s pe ed Fille r w ire fe ed **
Suggested w elding param eters for automa tic TIG and MIG welding titanium (1/16” = 1.6mm)
TIG (GTA) w ithout filler TIG (GTA) w ith filler MIG (GMA)
Ga uge, in 0.030 0.060 0.090 0.060 0.090 0.125 0.125 0.250 0.500 0.625
Electrode diam ete r, in 1/16
1/16
1/16
-3/32
1/16
1/16
-3/32
3/32
-1/8
1/16
1/16
1/16
1/16
Filler w ire d ia meter, in - - - 1/16
1/16
1/16
- - - -Wire feed ra te, ipm - - - 22 22 20 200-225 300-320 375-400 400-425
Voltage, V 10 10 12 10 12 12 20 30 40 45
Amps, A 25-30 90-100 190-200 120-130 200-210 220-230 250-260 300-320 340-360 350-370
No zzle ID, in ¾ ¾ ¾ ¾ ¾ ¾ ¾ -1 ¾ -1 ¾ -1 ¾ -1
To rch shield , cfh 15Ar 15Ar 20Ar 15Ar 20Ar 20Ar 50Ar+ 50Ar+ 50Ar+ 50Ar+15He 15He 15He 15He
Tra iling shield , cfh 20Ar 30Ar 50Ar 40Ar 50Ar 50Ar 50Ar 50Ar 60Ar 60Ar
Ba cking ga s, cfh 4Ar 4Ar 5Ar 5Ar 6Ar 6Ar 30Ar 50Ar 60Ar 60Ar
Tra vel speed, ipm 10 10 10 12 12 10 15 15 15 15Pow er supply DCEN DCEN DCEN DCEN DCEN DCEN DCEP DCEP DCEP DCEP
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Bac king ga s flow rates d epend largely on the volume
being filled. Flow rates for backing bars will normally be
lowe r than tho se fo r the to rch. Simila rly, ba cking ga s flow
rates for a d amm ed pipe a re limited by the ma gnitude
of the pos itive p ressure m aintained inside the pipe. The
pressure must not be too great or the weld root ma y be
‘pushe d’ in, giving a conc ave p rofile. Sufficient t ime must
be allowed for the argon to sweep all air out of thebac king volume, and this will vary a cco rding to the exac t
volume a nd flow ra te s used . Typically, a g reat er flow rate
is used whe n purging a da mm ed a rea. Where a bac king
bar is used , localised oxida tion can result from either an
inadequate purge time or excessive argon flow rate. A
similar effect can be caused by a badly fitting jig or by
impure a rgon.
Strong a ir currents ca n reduce t he efficiency o f even
we ll de signed argon shields a nd sho uld thus be a voided .
Screens ma y be used indoo rs to minimise the effect of
draughts while for on-site work, a polythene sheet tent
or other draught proof enclosure may be necessary.
SELECTION OF FILLER WIREFiller wires are p roduced for a wide range o f titanium
a lloys , a nd tho se for gra de s 2 (CP) and 5 (Ti-6Al-4V) are
read ily available t o AWS spe cifica tions in st raight lengths
and spo ols. The expedient o f cutting strips from s heet
to provide filler ma terial is one w hich may pro ve far from
sa tisfacto ry. Wire for welding is ma de to a s pecifica tion
which includes com position, dimensions, surface q uality
a nd c lea nlines s. Edge s littings are unlikely to c onfo rm in
all these a spects a nd their use without great care ma y
prove troublesome.
Unde r normal circumsta nces, the gra de of filler wire
will be identical with tha t o f the pa rent ma terial. Thus,
when two grade 2 components are to be welded, a grade
2 filler wire should be used. Where some atmospheric
contaminat ion can be ant ic ipated, for example on
positional welds in pipework, or where specifications
impose low hardness differences between weld bead
and pa rent meta l, a s ofter grad e of w ire such as grade 1can be e mployed. How ever, on no acco unt should the
use of a softer filler be used as a substitute for good
shielding pract ice. Welds bet we en d ifferent grade s o f
com mercially pure titanium ca n be ma de using filler of
either comp os ition. The cho ice will depe nd o n w hich is
the most important property of the weld, strength or
ductility.
For we lds in Ti-6A1-4V, TIG w eld ing w ith a ma tc hing
filler met al ca n lea d t o a reduction in ductility in the w eld-
because of metallurgical changes within the structure.
This ca n be o vercome to som e extent by the use o f Ti-
6Al-4V ELI, extra low interstitial grade wire. Joints
betw een low and higher alloy titanium g rade s (e.g. Ti-
6Al-4V to CP) sho uld be conside red c arefully, pa rticula rly
w h e r e p o s t w e l d h e a t t r e a t m e n t i s e m p l o y e d , a s
hydrogen embrittlement can be more likely.
When t he a rc is extinguished the tip of t he filler w ire
should remain, with the weld, in the argon stream from
the t orch until bot h a re sufficiently co ol not to oxidise. If
f i l ler wire does accidentally become oxidised, the
contam inated end m ust be removed before w elding is
recommenced.
23
Titanium we lding electrod e co mpo sitionsAWS WireCla ssifica tio n ASTM G ra de Co mpo sitio n, w t. %
C O H N Al V Fe Other TiERTi-1* 1 0.03 0.10 0.008 0.012 - - 0.10 - Rem.ERTi-2 1 0.05 0.10 0.008 0.020 - - 0.20 - RemERTi-3 2 0.05 0.10-0.15 0.008 0.020 - - 0.20 - RemERTi-4 2,3,4 0.05 0.15-0.25 0.008 0.020 - - 0.30 - RemERTi-0.2Pd 7,16,17,11 0.05 0.15 0.008 0.020 - - 0.20 Pd 0.12-0.25 RemERTi-3Al-2.5V 9 0.05 0.12 0.008 0.020 2.5-3.5 2.0-3.0 0.25 - RemERTi-3Al-2.5V-ELI* 9 0.04 0.10 0.005 0.012 2.5-3.5 2.0-3.0 0.15 - RemERTi-6Al-4V 5 0.05 0.15 0.008 0.020 5.5-6.75 3.5-4.5 0.25 - RemERTi-6 Al-4V-ELI* 5,23 0.04 0.10 0.005 0.012 5.5-6.75 3.5-4.5 0.15 - RemERTi-12 12 0.03 0.25 0.008 0.020 - - 0.30 Mo 0.2-0.4
Ni 0.6-0.9 RemNotes: * This clas sifica tion of filler met al restricts t he allowa ble interstitial content to a low level so tha t the high toughne ss req uired fo r cryo -genic applications a nd other special uses ca n be obta ined in the deposited w eld metal.
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TACK WELDINGTac k w elds a re used to fix parts into t he co rrect
relative position before welding. Since the tack is
eventually incorporat ed into the w eld, it must be shielde d
to t he sa me high sta nda rd as t he w eld itself. Tac ks ma y
be used in conjunction with a root gap i.e. where the
edges of the weld are deliberately set slightly apart to
as sist in achieving uniform penet ration. A tapering roo tgap, wider at the finish end, can be set to counteract
the scissor effect ca used by w eld co ntraction.
MULTIPASS WELDINGThe initial pa ss o f a m ultipa ss w eld w ill genera lly be
autogenous with only minor filler additions to correct
for small variations. It is advisable to X-ray the weld at
this stage if work is being carried out to radiographic
stand ards since porosity and lack of fusion d efects a re
more often associated with this first pass than with
subsequent runs.
Bright silvery coloured welds which have been
correctly shielded do not require any attention before
laying subsequent pa sses onto them.
Heat build up from previous weld runs can lead to
surface co ntamination on subsequent pa sses. In extreme
cases, the only solution may be to leave the work to
cool before further welding is carried out. Another
ap proac h is to m ake any long w elds in shorter sections.
In addition to helping with cooling, sequence welding
can also be effective in reducing distortion. Interpass
tempe ratures up to 500oC, depe nding on circumstances ,
ca n be us ed for co mm ercial purity t itanium a nd Ti-6Al-
4V. This ensures tha t he a t build up o f the w ork piec e
does not reduce the effectiveness of the shielding
arrangements, which are typically based on single pass
welds.
RESISTANCE WELDING TECHNIQUE
24
Equipment and technique are very similar to those
required for austenitic stainless steels. As with fusion
welding techniques, the quality of the joints depends
largely on the cleanliness of the joint surfaces, which
should be free of grease oil and other contaminants.
Similarly, an oxidised surface, even one which is only
lightly discoloured, should be ground o r scratch brushedwith a titanium o r stainless stee l brush, prior to we lding.
Pickling achieves the lowest contact resistance, but
mechanical cleaning methods are more than adequate
for the production of sound joints. Gas shielding is not
typically necessary, since contamination is minor as a
resul t of the very rapid thermal cycle . However ,
met allographic and m echa nical testing should a lwa ys be
used to determine if shielding is required for a given
com bination of pa ramete rs, materials, req uirements and
machine.
The face of resistance w elding electrode s should havea domed profile, rather than the truncated cone profile
favoured for som e ot her mate rials, to prevent excessive
indentation of the titanium.
Guideline spot welding parameters are given in the
Ta ble fo r Ti-6Al-4V, a lthough t he re q uired pa ram et ers for
a given job d epends o n ma ny factors and the values in
the table should only be regarded as a starting point
when esta blishing proced ures. For sea m we lding, an
ap preciably grea ter welding loa d sho uld be a pplied t han
is necessary for spot welding (3 times the spot welding
loa d is typically a goo d sta rting po int for w elding trials).
Current and on/off cycle ratios sho uld be d ete rmined by
trial and error. Ca re must be ta ken w hen evaluating the
welds to ensure that goo d o verlap is achieved betw een
succes sive w eld nugget s. Weld pene tration is norma lly
high but t he grain coa rsened H AZ can ea sily be m ista ken
for the nugget z one.
Paramete rs for spot welding (1 inch = 25.4mm; 1lb = 0.454 kg)
Sheet thickness (inches) 0.035 0.062 0.070 0.093
Jo int overla p (inches) ½ 5/8
5/8
¾
Sq ueeze time (ms) 60 60 60 60Weld time, cycles 7 10 12 16
Hold time (ms) 60 60 60 60
Electrode force (lbs) 600 1500 1700 2400
Weld current (A) 5500 10600 11500 12500
Cross tensile strength (lbs) 600 1000 1850 2100
Tensile shea r strength (lbs) 1720 5000 6350 8400
Ra tio C-T/T-S 0.35 0.20 0.29 0.25
Weld d ia meter (inches) 0.255 0.359 0.391 0.431
Nugget d iameter (inches) - 0.331 - -
Weld penetra tion (%) - 87.3 - -
Electrode indenta tion (%) - 3.1 - -
Sheet separa tion (inches) 0.0047 0.0087 0.0079 0.0091Notes : Electrode type : 3“ (75mm) spherical radius, 5/8” (15.9mm ) dia, class 2 cop per
Guide line pa ramet ers fo r spo t w elding Ti-6Al-4V.
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LIKELY DEFECTSTitan ium, like a ll me ta ls, is sus ce pt ible to ce rta in
we lding defects . How ever, the range o f possible defec tsis m uch less extensive t han, sa y, fo r ferrous fabrica tions.Solidificat ion crac king, a c om mo n de fect in stainless s tee l
and a luminium we ldments , is not fo und in Ti-6Al-4V. orCP. Likew ise, liq uat ion and rehea t cra cking a re notencountered in titanium fabricat ions. Contam ination d ueto inadeq uate ga s shielding is one o f the more comm ondefects responsible for rework or scrap and applies toall welding processes with the exception of frictionw elding. Tiny po res, irreleva nt to ma ny app lications , ca nbe formed in titanium weld metal but careful surfacepreparat ion w ill substantially reduce their presenc e.
Most of the defects commonly encountered intitanium TIG weldme nts ca n be traced to a de viat ionfrom ideal welding parameters. Molten titanium metalis fluid and its combination of low density and high
surface te nsion ena bles go od control of the weld surfaceprofile a nd pe net ration. Thus, tita nium is mo re forgivingin this respe ct tha n many othe r met als, but defects suchas lack of fusion, incomplete penetration and underfillare still possible. Porosity can also be encountered intitanium weld metal, typically at the fusion boundary.Pores a re spherica l and be tw een 50 -300µm in diame ter.MIG welds are susceptible to similar defects, but arealso prone to spa tte r. For critica l applica tions, it isimportant that the parent material be protected usingm e t a l f o i l o r h e a t r e s i s t a n t f a b r i c . Hy d r o g e nconta mination in the we ld o r pa rent mate rial can lea d tohydride cracking (typically in positions of maximumresidual stress), but this is typically encountered only
when Ar-H shielding ga ses, used com mo nly for sta inlesssteels, are used for titanium fabrication.
Plasm a a rc welds a re susceptible to the sam e rangeof defe cts a s TIG welds. Incomplete penet ration whenope rated in the keyhole mod e typically results in grosstunnel porosity. Autoge nous keyhole plasm a we lds inthick ma teria l typica lly exhibit a m inor amo unt o f unde rfill,but this can be read ily ad dresse d by a pplying a PAW orTIG final pa ss. One of t he m ajor benefits of keyholeplasm a welds is their seeming immunity to weld m eta lporos ity. Elect ron bea m a nd lase r welds a re suscept ibleto poros ity, voids, und erfill, incomplete penet ration andmissed seams. Again, the likelihood of these defects is
no grea ter than fo r most o ther meta ls. A lac k of bo ndingis the m ost co mmo n defect in diffusion bond s, brazedjoints, ad hesive bonds and resistance w elds.
The mo st likely defec t in a friction w eld, a nd t he mo stdifficult to detect non-destructively, is the so called“ kissing bond” , which is a region where intimate co ntac tis made between the two parts of the weld, but whereeither the joint is w ea k or no m eta llurgical bond exists.If insufficient flas h is ge nera te d d uring w elding, the jointcan be seriously embrittled, but this can typically beas sess ed visually. Po rosity is no t e ncountered in friction
welds.
25
RADIOGRAPHY.Rad iograp hy is one o f the mo re useful we ld inspection
techniq ues for titanium a nd its a pplication d oes not d iffersubstantially from the radiography of o ther meta ls, e itherin execution o r interpretat ion. Allow ance must be m ad e
for the low er abso rption o f X-rays than is found withiron or copper. One minor difficulty is that a titaniumimage qual i ty indicator ( IQI) is not avai lab le : thealuminium IQI is probably the best choice rather thaniron or copper.
Radiography will reveal:-• tungsten inclusions as sha rp white spots• porosity which shows up a s da rk spots that usually appearcircular• lack of root o r sidew all fusion indicat ed a s a da rk line orarea, often w ith asso ciated porosity• cracking, which is evident a s a da rk line, so metimes a ngular
and sha rp
DESTRUCTIVE TESTSThe principles used in approva l a nd q ualification
test ing of ot her meta ls ap ply eq ually to titanium but someprovision is necessary for assessing contamination.Colour should certainly be noted, but is an inadequateindicat or on its ow n. Transverse t ens ile te sts no rma llywill not show contamination, since the weld is usuallystronger than the p arent me tal . For plat es tha t a resufficiently thick, results of side bend tests will give aguide. For thinner plate or shee t, the long itudinal bendtest is preferable to the transverse, since this gives adirect co mparison w ith base m eta l performance. So me
care is needed, because the weld zone will usually beless ductile even in the absence of contamination,particularly with some alloys. Comparison should bemad e betw een the we ld and HAZ (rather than p arent),so as to account for hardening that occurs during thew eld the rmal cycle. Finally, the oxygen a nd nitroge nconte nt of the welds ma y be analysed to provide a directmea sure of a ny contamination.
Typical bend radii for a s-we lde d titanium
ASTMGrades
1, (11, 17, 27)
2, (16, 7 26)
3, 4
5, (24)
23, (29)
12
-
Alloy type
CP (+ Pd/Ru)
CP (+ Pd/Ru)
CP
Ti-6A1-4V (+ Pd)
Ti-6A1-4V ELI (+ Ru)
Ti-0.7Ni-0.3Mo
Ti-6A1-6V-6Sn
Minimum
bend radius
2t
3t
4t
10-12t
8-10t
5t
16-18t
EVALUATION OF WELD QUALITY
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ALTERNATIVES TO THE COLOUR CRITERIAHardness te sting and edd y current inspection ca n be
used to provide supporting evidence for contamination
i n c o m m e r c i a l l y p u r e g r a d e s o f t i t a n i u m , s i n c e
contaminated welds will exhibit greater hardness and
resistivity. Porta ble ha rdness te sting proced ures t hat can
be a pplied in-situ are c urrent ly being d evelope d a t TWIfor grade 2 a nd 5 titanium. The succe ssful app lica tion of
these techniques will allow rejects to be minimised a nd
the highest quality to be achieved.
Cont am ination of TIG electrode s from a ir entrainment (from
left to right 0%, 0 .5%, 1%, 1. 5%a ir in the Ar to rch s hielding ga s)
.
DYE PENETRANT INSPECTIONUnder normal circumstances, weld cracking is very
rare with tita nium. Ho we ver, problems c an so met imes
arise where several weld seams intersect or where
contam ination has occurred. In these ca ses, the d efects
can be detected by dye penetrant inspect ion, the
technique also being suitable for locating porosity in
partly mac hined w elds. It should be no ted , how ever, thatthe d ye penetrant m ust be completely removed prior to
at tem pting weld repa ir.
NOTES FOR FRICTION WELDSFriction w elded com pone nts a re by d efinition very
difficult to inspec t. As the process is o nly eco nom ic for
mass produced items, individual inspection of each
component can seldom be justified. Experience has
shown that, as the process is fully mechanised, and
therefore repeatable, reliance on statistical processcontrol is normally satisfactory. In this approach, the
tolerance of the process to key w elding pa rameters is
first determined, and much tighter tolerances are then
imposed on the production process. Providing the
process is kept within these tolerance levels, the
probability of getting a poor weld should be very small
indeed . If a pa rameter is recorded to be just outside the
tolerance range, t he we ld sho uld st ill be ac cept able, but
this issues a wa rning tha t so me intervention is required
to return the pa rameter to its intended set ting, and to
investigate the cause fo r the change.
27
TIG welds in comm ercially pure titanium shee t ma de w ith successively great er air cont am ination of the shielding ga s.
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REPAIR OF DEFECTS
LOCALISED MINOR REPAIRSDefects in titanium welds such as isolated tungsten
inclusions a nd poro sity are q uite ea sy to repair. The
affected area is removed by drilling or grinding and
cleaned prior to filling the hole or depression with theappropriate filler material, taking care that any metal
added is properly fused into the existing weld metal.
SEAM REPLACEMENTWhere a line of pores is found by radiography, the
weld ca n be re-melted up to a maximum o f, say, 3 times
subject t o a satisfactory contam ination check after each
sta ge . This re-m elt w ill req uire a higher current tha n tha t
used o n the o riginal we ld but ca n pote ntially remove a ll
or most of the porosity. Should this fail, however, or if
the defect is of a more serious nature, the entire weld
bead must be removed by machining or grinding and
then rewe lde d. These t ypes o f ma jor we ld repa irs are
usually slow and cos tly and conside ration should be g iven
to p atching or even complete replacem ent of the items.
REPAIR OF LARGE AREASAny substant ial area s ca n be repaired by cutting out
and replacing with new material or by welding a patch
over the e ntire a rea. G enera lly, replacem ent is preferable
to patching except for the repair of thick sheet with
access to only one side, a nd for the repair of titanium/
stee l explosively clad p lat e. Electron be am we lding has
proven particularly successful for replacement repair
welding, allow ing sections to be w elded into co mponents
w ith minima l dist ortion a nd high ac curac y. For exam ple,
fla p t racks fo r the Torna do fighter/bo mbe r aircraft ha ve
been repa ired using this technique.
TIG w eld repair and H VOF surfac ing wa s used to repa ir slat
tra cks on t he Trist ar a ircraft
REPAIR OF DETAILSDetails on components that are damaged either in
service or during fabrication are routinely repaired by
the build up o f we ld be ad s using, for exam ple, TIG
we lding. Fine d eta ils c an be repa ired using microplas ma
we lding, allow ing the precise pos itioning o f sma ll weld
beads prior to machining to the required geometry.
FURTHER NOTES ON POROSITY Weld me ta l po rosity occurs com mo nly in mo st
materials, including for example nickel alloys and
sta inless ste el. Tita nium fusion w eldment s ca n also
exhibit pores, but under most circumstances they do
not have a ny pa rticular d etriment al effects. For examp le,
pores a re typica lly isolate d a nd less than 0.3m m (.012”)
diameter, and have no discernible consequence for
tens ile prop erties or to ughnes s. Similarly, if the w eld ca p
and root profile a re left intact (i.e. not ground flush w ith
the p arent m eta l), fatigue life is typically de termined by
the severity of the stress concentration a t the w eld to e
and will not be influenced by the presenc e o r otherwise
of small pores.
For so me ap plica tions, ho w ever, a ll geom etry-specific
stress-raisers, such a s w eld t oes, are removed in order
to m aximise the fat igue performance o f the joints. Under
these circumstances fatigue strength can be lowered
significantly by the presence of weld metal pores,
espe cially tho se ne ar the surfac e. This is true of m ost
ma terials, but the de grad at ion in fatigue life is typically
greate r for titanium, than, sa y, s tee l. Und uly restrictive
maximum specified pore sizes in welding codes maynot ha ve any profound effect on fa tigue performa nce.
Indeed, the removal of such defects and subsequent
weld repa i r may be more de t r imenta l to f a t igue
performance than the original defect. It is stressed,
however, that if the weld cap and root are left intact,
then weld metal porosity becomes irrelevant, and
allow able stresses ca n be calculat ed s olely on the bas is
of the severity of the profile of the weld toe.
Porosity in titanium fusion welds can be formed for
a va riety o f rea sons , but the m ost profound influence is
the condition of the joint surfaces. In principle, final
machining of the joint surfaces wi thout aqueous
lubrica nts a nd w elding in the sa me 24h period is advised,
although a cid pickling ma y be used successfully o n ‘o ld’
joint surfaces, provided welding is performed shortly
a fter the p ickling treat me nt. The joint surfac es sh ould
always be ca refully d egreased.
The prese nce o f a hyd rate d s ca le a nd/or surface
cont am inants ca n be ide ntified during ta ck w elding. If a
discoloured ring has formed within the protection of
the ga s shielded area a nd surrounds silver weld me tal
then, if porosity is to be minimised, the joint surfaces
should be re-prepared . Not a ll we lding proce sses sho w
the sam e s usceptibility to we ld met al po rosity. AlthoughTIG, MIG, EB a nd lase r welds o ften exhibit we ld met al
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pores, keyhole plasma welds are typically pore-free,
show ing tha t this lat ter process ha s significa ntly greate r
to lerance to the c ond ition of the joint surfac es. Weld
meta l porosity can o f course be com pletely avoided by
using a solid state welding process.
DISTORTION Beca use w elding involves highly localised hea ting of
joint edges to fuse the material, non-uniform stresses
are set up in the component. Initially, compressive
stresses a re creat ed in the surrounding co ld parent meta l
when the weld pool is formed due to the thermal
expansion of the hot m eta l (heat a ffected z one) adjacent
to the weld pool. However, tensile stresses arise on
coo ling when the c ontraction o f the we ld m etal and the
immed iate heat affected zone is resisted by the bulk of
the cold parent metal. If the stresses generated from
therma l expa nsion/cont raction exceed ed the yield
s t r e n g t h o f t h e p a r e n t m e t a l , l o c a l i s e d p l a s t i cdeformation of the metal occurs. Plastic deformation
causes a permanent distortion in the structure.
The ma in facto rs affecting the type and de gree of
distortion, are parent material properties, amount of
restraint, joint de sign, part fit-up and w elding proce dure.
PARENT MATERIAL PROPERTIESParent material properties which influence distortion
are coefficient of thermal expansion (greater values
increase distortion) and specific heat per unit volume
(lower values increase distortion). As distortion is
de termined by expans ion and cont rac t ion o f thematerial, the coefficient of thermal expansion of the
material plays a significant role in determining the
stresses generated during welding and, hence, the
degree of distortion. Simple calculations and practical
experience shows that the level of distortion expected
in a titanium co mponent lies betw een thos e o bserved
for stee l and s ta inless stee l (i.e. distortion w ill be greate r
that for steel, but lower that observed in many austenitic
sta inless stee ls).
RESTRAINT
I f a component is welded wi thout any externalrestraint, it distorts to relieve the welding stresses. So
met hods of restraint such as ‘strong ba cks’ in butt welds
ca n prevent mo vement a nd reduce disto rtion. It should
be no ted , how ever, tha t restraint produces higher levels
of residual stress in the m at erial.
JOINT DESIGNBoth butt and fillet joints are prone to distortion. It
ca n be m inimised in butt joints by ad opt ing a joint type
which balances the thermal stresses through the plate
thickness. For exam ple, a do uble-sided in preference to
a single-sided weld. Double-sided fillet welds shouldeliminate angular distortion of the upsta nding mem ber,
STRESS RELIEF As for welds in any other metal , postweld heat-
treatme nts are performed to reduce the residual stresses
encountered in the weld zone and improve fatigue
performance. Residual stresses in ferrous fabrications
can e q ual the yield stress o f the a lloy, but residual stresse s
in tita nium are t ypica lly low er. For exam ple a m a ximum
residual stress of approximately 85%of yield can beenc ounte red in Ti-6Al-4V in highly rest rained me ta l, such
as t ypical for repair welds. Postwe ld hea t treatm ents of
different durations are required for stress relief of the
various titanium alloy grad es. Hea t-trea tme nt sched ules
for weldable higher s trength a l loys are commonly
combined so that postweld heat-treatment relieves
residual stresses a nd a ges t he pa rent ma terial.
Postweld heat-treatment may be performed in a
vacuum o r argon atmo sphere to prevent the fo rma tion
of co nta minated laye rs. Ads orbed o xygen forms a brittle
surface, or ‘a lpha case’ , and is best avoided. Heat
treatment in air is possible provided that the oxidised
surface is removed by pickling, grinding or blasting and
descaling.
Welde d fa brica tions in com me rcia lly pure titanium,
including pipe and fittings, w ill not norma lly req uire stress
relief. Alloy fabrications, however, typically do require
stress relief. One to t wo hours at 60 0°C (1110°F) max is
usua lly ad eq uat e fo r bot h CP a nd Ti-6Al-4V, t o reduc e
residual stress to manageable levels whilst avoiding
excess ive therma l oxida tion. Indeed , higher temp eratures
should be avoided, since microstructural ageing can
reduce toughness and ductility. Suppliers should be
consulted for a suita ble hea t treatm ent cycle for welded
alloys req uiring post we ld so lution trea tme nt and ag eing.
29
especially it the two welds are deposited at the same
time.
PART FIT-UPFit-up should be uniform to produce predictable a nd
consistent shrinkag e. Excess ive joint ga p ca n a lso increase
the de gree of distortion by increasing the a mount o f weldme ta l neede d to fill the joint. The joints s hould be
ad eq uately tacked to prevent relative mo vement betwe en
the pa rts during w elding.
WELDING PROCEDUREThis influence s the d egree of d istortion mainly through
its effect on the heat input. As welding procedure is
usually selected for reaso ns of q uality a nd p roductivity,
the w elder has limited sco pe fo r red ucing d isto rtion. As
a g eneral rule, w eld volume sho uld be kept t o a minimum.
Also the w elding seq uence a nd tec hniq ue should a im to
balance the therma lly induced stresse s around the neutralaxis of the co mponent.
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Design Cost Control: The prac tical points o f succes sful de sign cos t co ntrol are principally tho se o f value eng ineering
using a light, strong, corrosion-resista nt m at erial.
Surface Treatment: The practical points of succes sful surfac e treat ment s are the t hose of know ing the problem to
be so lved a nd de ciding the appropriate treatment.
Do: Check ava ilable stand ard prod ucts and s pecificat ions
to obtain best availability and lowest cost.
Use de sign strate gies based o n using minimum ma terialthickness.
Exploit corrosion resistant characteristics to the full.
Consider the use o f liners and cladd ers in preference t o
solid design where heavy sections are unavoidable.
Consult suppliers and fa brica tors at t he ea rliest st ag e of
design.
Do not: Simp ly substitute titan ium into existing designs.
Budget for titanium project costs by weight, especiallynot by the weight of steel or other alloys.
Specify little-used alloys or fo rms.
Machining: The pra ctical points o f succe ssful machining a re principally tho se of o bserving the different mec hanical
and surfac e cha racteristics o f titanium. Fire safet y proced ures m ust be app lied for handling a nd co ntrol of t ita nium
fines and turnings.
Do: Use rigid set ups, correct speed s, feed s and too ling.
Use floo d lubrication.Use roller stea dies a nd running ce ntres.
Regularly remove turnings from machines.
Employ sp ecial closea ble cont ainers for titanium turnings.
Do not: Allow titanium t o rub on blunt to oling o r smea r
on the o ther metals.Mix co mbus tible rubbish with tita nium fines o r turnings.
Allow ope n flam es o r we lding nea r tita nium fines.
Fabrication: The pract ica l points o f success ful fabrica tion are principally thos e o f goo d ho usekeep ing a nd clean
practice in the workshop.
Do: Use the correct weld preparation and remove all
burrs.
Remove all grease, oil, paint and dirt before welding or
heat treatment.
Clea n we ld a reas w ith ace tone o n a lint-free cloth or use
sta inless ste el or tita nium w ire brushes.Dry tita nium surfaces before w elding.
Use clean d ry tita nium filler wire o f the c orrect grad e.
Ensure that the top and back face of the w eld a nd w eld
areas a re adeq uately shielded with argon gas.
Do not: Hea t treat t ita nium in a reducing atmo sphere, it
will absorb hydrogen and become embrittled.
Use methyl alcohol (methanol) as a cleaning fluid, dry
methanol can cause stress cracking.
Use sulpho -chlorinat ed or s ulphurised cleaning fluids.
Apply cleaning fluid with tissue paper, wool or rags.Wire brush with mild steel brushes.
Use hydorgen containing shielding gases.
Do not: At t e m p t t o a p p ly c o a t in g s t o s o i le d o r
contam inated s urfaces.
Exceed recommended inspection and maintenance
intervals.
Re-use components showing excessive wear or surfacedamage.
Do: Confirm that any side effects are a ccounted for in
the d esign and application.
Ensure that surface preparation is appropriate to the
coa ting selected .
Provide the sp ecified grad e o f lubrica tion.
Installation: The prac tical points o f succes sful installat ion a re principally tho se o f obs erving the d ifferent me cha nica l
properties, corrosion resistance and surface characteristics of titanium.
Do: Allow for the lower modulus of titanium in struts
and support spans.
Provide surface treatment for titanium parts in sliding
conta ct, o r on bearing surfaces.
Coa t external surfac es o f exposed titanium st ructures in
areas where dynamically induced sparking is a defined
hazard.
Do not: C o n n e c t t i t a n i u m w i t h o u t i s o l a t i o n t o
immediately adjoining less corrosion resistant metals,
(to red uce t he likelihoo d of ga lvanic corrosion).
PRACTICAL POINTS..... THE DO’S AND DON ’TS OF TITANIUM
CO MPREHENSIVE GU IDES ON MACH INING AND FABRICATION ARE AVAILABLE FROM TIG
30
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STANDARDS AND
SPECIFICATIONS ASME provides the only international non-aerospace
sta nda rd for we ldm ent q ualifica tion in titanium. For
pressure vessel construction, the ASME Boiler and
Pressure Vesse l Cod e de ta ils proced ure and performa nce
tests w hich must be met for coded grades 1, 2, 3, 7, 9,and 12. Tensile a nd bend tests on trial welds m ad e under
conditions intended fo r prod uction a re the acce ptance
criteria. Impact or notch tensile tests may also be
required , pa rticularly for low tem perature a pplica tions.
Once good procedures are established, as evidenced
by tensile a nd bend tes ts, the y should be strictly follow ed
in subsequent production welding. Although weld colour
should ce rtainly be included in a ny w elding q ualifica tion
testing, ASME Code suggests that , if titanium we ld m eta l
hardness is more than 40 BHN (50VPN) greater than
base meta l hardness , excess ive contamina t ion i s
possible. A substantially greater hardness differential
necessitates removal of the affected weld-metal area.The Co de further spe cifies tha t a ll tita nium w elds be
examined by liq uid penet rant. In a dd ition, full rad iograp hy
of many titanium joints is required by the Code.
A Europe an s ta nda rd is currently being discussed for
we lde r approval for tita nium fabricat ion, but a s yet (May
’99) the final content of this document has not been
ratified.
HEALTH AND SAFETY Welding fume g eneration from the co mm on t ita nium
a lloys during TIG w elding is minima l a nd t here a re no
extract ion req uirements beyond those necess ary for TIG
w elding ot her struct ural ma te rials. Titanium w ill not
com bust d uring w elding. Instances in w hich titanium has
caught fire are associated with finely divided material
and ignition from co mbustible fluids or ma terials.
GLOSSARY TIG: tungsten inert gas or gas tungsten arc welding
(G TAW)
MIG: met al inert ga s o r gas met al arc w elding (GMAW)
PAW: plasm a arc w elding
DCEN: Direct current e lectrod e neg at ive
DCEP: Direct current electrode pos itive
EB: electron bea m
RPEB: reduced pressure electron beam
Nd-YAG (las er) Neo dym ium-yttrium-aga te -garnet
ASME American Society of Mechanical Engineers
Welding positions fo r plat e a nd c ircumferent ial welds.
31
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FOR FURTHER INFORMATION The list o f TIG Mem bers over includes fabrica tors w ho w ill be pleased to as sist w ith further informa tion a nd a dvice.
In ad dition, there a re ma ny industrial membe rs of TWI that have conside rable e xperience in fabricating titanium.
Informa tion on these com pa nies c an be o bta ined from TWI. Those loo king for US fa brica tors sho uld visit the
following web sites: http://www.welding-services.com. The me mbe rship o f TIG a nd the internat iona l Titanium
Asso ciation includes m any international specialist fabrica to rs and t hese c an be fo und on http://www.titanium.net.
Titanium International LtdKeys Ho useGra nby Avenue
Garretts GreenBirmingham B33 0SP, U KTel: + 44(0)121 789 8030Fax: + 44(0)121 784 7694
Huntingdon Fusion Techniques LtdStukeley MeadowsHutingdonCambridgeshire PE18 6EJ, UKTel:+ 44(0)1480 412432Fax: : + 44(0)1480 412841
FOR BRAZING CONSUM ABLES
WESGOGTE Prod ucts Corpo ration477 Harbor BoulevardBelmont, CA 94002, USATel: + 01 (415) 592 9440
32
FOR FURTHER READING There is a va st bo dy o f literat ure c overing the joining o f titanium a nd its a lloys, far mo re than ca n be highlighted in
this brochure. Req uest for informa tion o n spe cific w elding issues should be directe d to TWI.
L S Smith and M F Gi t tos : “Highproduct ivity p ipe w eld ing o f Ti-6Al-4Va lloys” . TWI mem bers repo rt 660,November 1998.
M F Gitto s: “ Welding of t itanium”.Titanium World, Sept em ber 1 996.
P L Threa dgill: “ The p ot ent ial for so lidsta te welding of t i t an ium pipe inof f shore industr ies” . P resented a t‘Right use of titanium III’, StavangerNorwa y, 4-5 November 1997.
D Howse, R Wiktorowicz and M F
Gi t tos : “Using shielding gases forimproved productivity arc welding oftita nium”. TWI Bulletin,
“AWS Welding H an db oo k, Ma te ria lsand Applica tions Part 2” , 8th e dition ,Ame rica n Welding So ciet y, Miam i.
M J H u ss i on : “ P r ac t i c a l u se o f aco lla psible purge cha mbe r for titaniumw elding” . Welding Journa l 66, July1997.
Ti tanium: Des ign and Fa br ica t ionHandbook for Industrial Applications,TIMET, Tita nium M et a ls Co rpo rat ion1997
B.Ha nson, “ The Selection and Us e of
Titanium - Des igners Guide ” , Instituteof Materials, 1995
TIMET UK LTDP.O. Bo x 704,Witton,
Birmingham B6 7UR UKTel : + 44(0)121-356-1155Fax: + 44(0)121-356-5413
RMI Titanium CompanyRiverside Esta teFa ze ley Ta mw orth
Sta ffordshire S78 3RWTel : + 44(0)1827 262266Fax: + 44(0)1827 262267
FOR SHIELDING AND WELDING EQU IPM ENT
FOR WELDING CONSUM ABLES
Vacuum Brazing Consultants LtdThe Ho b Hill, Chap el Roa dSteetonKeighley BD20 6NUTel: + 44(0)1535 653598Fax:+ 44( 0)1535 656707
FOR HELP FROM TWI
TWI,Grant a Park, Grea t Abingto n, Cam bridge , CB1 6ALTel: + 44(0)1223 891 162Fax: + 44(0)1223 892588Ema il: tw i@ tw i.co.uk
L S Smith a nd M F Gitto s: “A review ofwe l d m eta l p or os i t y an d h ydr i decracking in titanium and its alloys”.TWI Memb ers Report 658 , Nove mbe r1998.
M B D Ellis a nd M F Gitto s: “ Tungst eninert gas welding of titanium and itsalloys” . Welding & Met a l Fa brica tion,Janua ry 1995.
M H Scott: “Arc welding titanium”.Welding Inst itute Resea rch B ulletin,20, June 1979.
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M EM BER COM PANIES OF TIG
33
Time t UK Limite dPO Box 704WittonBirmingham B6 7URTel: 012 1 356 1155Fax: 0121 356 54 13
Manufacturers and stockists of titaniummill products
Doncasters Plc28-30 Derby RoadMelbourneDerby DE7 1FETel: 0 133 2 86 490 0Fax: 01332 864888Manufacture titanium forgings andcastings - specialist fabricator includingsuper-plastic forming/diffusion bonding
Wyma n Go rdon LtdHouston RoadLivingston
West Lo thian EH54 5 BZTel: 015 06 446200Fax: 01506 446330Manufacturer of large titanium forgingsincluding large diameter extruded tube
Bunting Tita nium Ltd34 Middlemore Industrial EstateSme thw ick Wa rleyWest Midland s B66 2 EETel: 012 1 558 5814Fax: 0121 558 80 72Titanium fabricator specialising in pipespools - manufacturer of a range of titanium valves
Aero spa ce Forgings LtdChurchbridgeOldburyWa rleyWest Midland s B6 9 2AUTel: 012 1 552 2921Fax: 0121 544 57 31Manufacturer of closed die and handforged titanium forgings
TWIAbington HallAbingtonCambridge CB1 6ALTel: 0 122 3 89 116 2Fax: 01223 892588Research, development and consultancy
on joining techniques for materialsincluding titanium
Meta l Improvement Co IncNavigation H ouseHam bridge LaneNewburyBe rks RG14 5 TUTel: 0 163 5 3 107 1Fax: 01635 31474Surface treatment of titaniumcomponents to improve mechanicalproperties and to prolong service life
RMI Titan ium Com pa nyRiverside Esta teFa ze ley Ta mw ort hStaffordshire B78 3RWTel: 01 827 262 266Fax: 0 1827 26 2267
Manufacturer and stockist of a full rangeof titanium mill products including pipingand OCTG tubulars
Ore me t Titan iumKeys HouseGra nby AvenueGarretts GreenBirmingham B33 0SPTel: 012 1 789 8030Fax: 0 121 784 8054Manufacturer and stockist of titaniummill products and castings
Aurora Forgings LtdPa rkga te St ee l WorksPO Box 16 RotherhamSo uth Yorkshire S62 6EBTel: 01 14 2 61 5 000Fax: 0 114 261 5025Open and closed die forgings, extrusionsand rolled rings
Euro-Tita n Ha nde ls AG/Ha nse a tischeWaren Ha nde lsgesellscha ft mbH & Co Kgc/o Interne t Agencies18 Cofton Church LaneBirmingham B45 8PTTel: 01 21 4 47 7 492Fax: 0 121 447 7493Stockist of titanium ingot, bar, plate,sheet, profile, tube and wire products
Rolls LavalPO Box 100Wolverha mp to n WV4 6JYTel: 01 902 353 353Fax: 0 1902 40 3334Manufacturers of compact heatexchangers
Super Alloys International Ltd5 Ga ramond e DriveClarendon Industrial EstateWymbushMilton Keynes MK8 8DFTel: 01 908 260 707Fax: 0 1908 26 0494Stockist of titanium wrought products
Scomark Engineering LtdHartshorne RoadWoo dville Sw ad lincot eDerbyshire DE11 7JFTel: 01 283 218 222Fax: 0 1283 22 6468Fabricator of high performance materialsincluding titanium
DERAGriffith Building A7Structural Materials CentreDERA Fa rnbo roughHampshire GU14 0LXTel: 01 252 392 540
Fax: 0 1252 39 4135Research and development on materialsincluding titanium
Tec va c Limite dBuckingway Business ParkSwaveseyCambridge CB4 5UGTel: 0 195 4 23 370 0Fax: 01954 233733
Surface treatment of titaniumparticularly nitriding
Titan ium Mill Prod ucts LtdLöwe House1 Ranmoo r CrescentSheffield S10 3GUTel: 0 114 2 30 8 85 5Fax: 01142 302 83 2Stockists of titanium wrought products
Alba ASLilleakerveien 23N-0283 OsloNorwayTel: 4 7 2 2 50 00 20
Fax: 4 7 22 50 01 11Titanium castings
Deutsc he Titan G mbHAltendorfer Strasse 10445143 EssenGermanyTel: 00 4 9 020 1 188 2 593Fax: 49 0201 18 8 3520Manufacturer of a wide range of titaniummill products
Rolls Royce PlcP.O. Box 200 0Derby DE21 7XXTel: 0 133 2 66 146 1Fax: 01332 622948Fabrication and design of components formarine power systems
Aerom et Inte rnationa l PlcWat chmea dWelwyn G arde n CityHert ford shire AL7 1LTTel: 0 179 5 41 500 0Fax: 01795 415050Fabricator specialising in super plasticforming and diffusion bonding
Azko NobelPermascand ABBox 42S-840 10 Ljungaverk
SwedenTel: 069 1 3 550 0Fax: 0691 33040Titanium fabricators
Euro -Tita n Ha nd els AGKatternberger Strasse 155-159Solingen 42655GermanyTel: 0 2 12 248 16-0Fax: 02 12 248 1 6-16Stockist of titanium mill products
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TITANIUM
TITANIUM is the fourth m os t a bunda nt st ructural meta l in the e a rth’s crust, a nd t he ninth industrial meta l. No
othe r engineering met al has risen so sw iftly to p re-eminence in such a w ide range o f critical and d ema nding
applications.
TITANIUM AND ITS ALLOYS OFFER:Availa bility in a ll forms
Comparable cost to other high performance engineering materials
Weight sa ving - as strong a s st eel but half the w eight
Outstanding corrosion resistance in a wide range of aggressive media
Resistance to erosion a nd ca vitation
Fire a nd s hoc k resista nce
Fa vourable cryog enic prop erties
Biocom pat ibility and non t oxicity
TITANIUM AND ITS ALLOYS DELIVER:High performance co mponents a nd systems
• Aerospace engine and airframe parts
• Automot ive compo nents including valves, springs, connecting rods
• Orthopaed ic implants surgical instruments, medical centrifuges
• Lightweight vehicle and body armour
• Offshore oil and g as e q uipment, stress joints, risers, flow lines, valves
• Seawa ter pipework systems for ballast, coo ling and f ire protection
• 100 million metres of stea m conde nser tubing in power plant wo rldw ide
• High pressure compact heat exchangers
• Process plant eq uipment, vessels, heat exchangers, pumps, mixers
• High strength corrosion resistant fasteners
• Strong co rrosion resistant a lloys for high pressure high temperature processe s• Naval ball valves up to 600mm diameter
• Equipment for safe handling of food, beverage a nd pharmaceuticals
• Erosion resistant com poments for high velocity wa ter duties
• Computer hard drive substra tes
• Flue gas d esulphurisation plant d uct and flue linings
• Lightweight steam turbine blading
• Safe long term storage of nuclear waste
• Corrosion resistant wet air oxidation process plant
Sports, leisure and fashion goods
• Rac ing and mounta in bicyc les
• Go lf clubs
• Ya c ht fit ting s
• Watches and personal jewellery
• Ultra lightweight spectac le frames
Attract ive a nd d urable a rchitect ural finishes and ornam ents
• Curta in walling and roof ing
• Elec tro pa in ted p ic tures
• Sculp tures , plaques and monuments
• Corrosion resistant cladd ing for marine structures
In the ma jority of the se a nd o the r applications TITANIUM has rep la ced hea vier, less se rvicea ble and les s
co st e ffect ive m at erials. D esigning with TITANIUM and ta king all fac to rs including selection o f the a pprop ria te
surface trea tme nt into a ccount has resulted in reliable, econo mic and mo re durable systems a nd com ponent s