review, high speed fusion weld bead defects

REVIEW

High speed fusion weld bead defects

T. C. Nguyen1, D. C. Weckman*2, D. A. Johnson2 and H. W. Kerr2

A comprehensive survey of high speed weld bead defects is presented with strong emphasis on the

formation of humping and undercutting in autogenous and non-autogenous fusion welding

processes. Blowhole and overlap weld defects are also discussed. Although experimental results

from previous studies are informative, they do not always reveal the physical mechanisms

responsible for the formation of these high speed weld bead defects. In addition, these

experimental results do not reveal the complex relationships between welding process parameters

and the onset of high speed weld bead defects. Various phenomenological models of humping and

undercutting have been proposed that were based on observations of events in different regions

within the weld pool or the final weld bead profile. The ability of these models to predict the onset of

humping or undercutting has not been satisfactorily demonstrated. Furthermore, the proposed

formation mechanisms of these high speed weld bead defects are still being questioned. Recent

welding techniques and processes have, however, been shown to be very effective in suppressing

humping and undercutting by slowing the backward flow of molten metal in the weld pool. This

backward flow of molten weld metal may be the principal physical phenomenon responsible for the

formation of humping and undercutting during high speed fusion welding.

Keywords: High speed weld bead defects, Humping

IntroductionTo remain competitive in today’s manufacturing envir-onment, companies must continuously improve theirproductivity without sacrificing the quality of theirproducts. Increases in productivity will reduce overallproduction costs thereby maintaining and strengtheningthe company’s competitiveness. Since welding is ubiqui-tous and an integral part of most manufacturingindustries such as the construction, shipbuilding, aero-space, automotive, petrochemical and electronic indus-tries, overall production costs can usually be reduced byevaluating the productivity of the welding processesused.

There are a number of general approaches that can beused to improve the productivity of a particular weldingoperation. The existing welding process can be opti-mised in order to maximise welding speeds. Furthergains in productivity, repeatability and weld quality maybe realised by automating the welding process. Finally,selecting and implementing newer welding processes thatare intrinsically capable of much greater welding speedsthan is possible with the existing welding processes mayhelp to realise significant gains in productivity. Inaddition to productivity increases, high welding speedscan also have other benefits such as improved melting

efficiency and lower distortion, because as welding speedis increased, most of the incident heat contributes toforming the weld and less is lost by conduction into thesurrounding weldment.1,2

All fusion welding processes are multivariate, fre-quently with synergistic, nonlinear interactions betweenthe numerous process variables. Therefore, increasedproductivity in a fusion welding process cannot berealised simply by increasing the welding speed withoutaffecting other welding parameters. Some insights intothese interactions are possible through consideration ofRosenthal’s3 analytical models for heat conduction infusion welds made in thick or thin plates using point orline heat sources respectively. For example, workingfrom Rosenthal’s models,3 Adams4 has shown that thecooling rate down the centre line of the weld and thewidth of the heat affected zone (HAZ) are proportionalto the heat input per unit distance of weld Hnet (J m21),as given by

Hnet~g Q

vw(1)

where g is the arc efficiency; Q (W) is the powergenerated by the heat source and vw (m s21) is thewelding speed.4 From equation (1), it should be possibleto increase welding speed and productivity whilemaintaining the same cooling rate or HAZ size providedthat Hnet is maintained constant, i.e. Q must beincreased at the same rate as vw.

The theoretical predictions embodied in equation (1)suggest that unbounded increases in productivity arepossible by simply increasing Q at the same rate as vw. In

1School of Engineering and Information Technology, Conestoga College,299 Doon Valley Dr., Kitchener, Ontario N2G 4M4, Canada2Department of Mechanical Engineering, University of Waterloo, Waterloo,Ontario N2L 3G1, Canada

*Corresponding author, email [email protected]

� 2006 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 22 June 2006; accepted 6 July 2006DOI 10.1179/174329306X128464 Science and Technology of Welding and Joining 2006 VOL 11 NO 6 618

practice, however, the welding speed will be limited bythe deterioration of the weld bead quality as welds beginto exhibit serious geometric defects such as undercuttingand humping.5,6 Located in the weld metal adjacent tothe weld toe, undercut is a sharp groove left unfilled bythe weld metal during solidification.6 An example of aweld with severe undercutting is shown in Fig. 1.Undercut creates a mechanical notch at the weldinterface that lowers the static, fatigue and fracturestrength of the welded assembly. In certain advancedmanufacturing applications such as tailor weldedblanks, undercut will also reduce the formability of theblank.7

Humping can be described as a periodic undulation ofthe weld bead. An example of humping in a bead onplate gas metal arc (GMA) weld made in mild steel isshown in Fig. 2a.8,9 Transverse sections of this weld at avalley (section a-a) and a hump (section b-b) are shownin Fig. 2b and c respectively. At both the valley and thehump, the depth of penetration is the same. However,there is a large accumulation of weld metal at the hump.The overall appearance of a humped weld suggests thatits formation is a periodic physical phenomenon in anotherwise steady welding process.

The undercut and humping weld defects producedat high welding speeds will compromise the mechan-ical integrity of the joint, thereby imposing an upperlimit to the welding speed and overall productionrates. To achieve further gains in productivity, theformation of these high speed weld defects must beeliminated or suppressed to higher welding speeds.This requires a thorough understanding of the physicalphenomena responsible for the formation of theseweld defects.

In the present article, a review of the current literaturerelated to high speed weld bead defects for differentfusion welding processes will be presented. The reviewwill concentrate mostly on humping and undercuttingweld defects; however, blowhole and overlap defects inwelds will also be briefly discussed. The present articlewill be divided into three sections. First, parametricstudies of the effects of various welding parameters onthe formation of these weld defects will be discussed.Second, the different phenomenological models thathave been proposed to explain the physical phenomenaresponsible for the development of these defects will beexamined. Last, welding techniques and processes thatare currently known to be capable of suppressing oreliminating these high speed weld defects therebyfacilitating higher welding speeds and increased produc-tivity will be reviewed.

Effects of weld process parameters onhigh speed welding defectsOver the years, as welding processes have been auto-mated and greater welding speeds were made possible,welders have become increasingly aware of the occur-rence of humping and undercutting at higher weldingspeeds. To eliminate these weld defects, a slower weldingspeed is often recommended. This option, however, iscontrary to the continuous pressures for ever increasingwelding speeds and productivity.

Because fusion welding processes have many parameters,frequently with multiple, nonlinear interactions, it shouldnot be surprising that welding speed is not the only weldprocess parameter that influences undercut and humpingdefect formation.5,8,9 Clearly, the first step towards devel-oping a comprehensive understanding of the undercut andhumping phenomena in any welding process is thedetermination through experiments of all welding para-meters that affect these defects. Performing weldingexperiments designed to map the range of these parametersthat will produce welds with the undercut and humpingdefects and identification of process parameter interactionswill then provide better understanding of the effects ofprocess parameters on the occurrence of these defects.

1 Top weld bead and transverse section of high speed

variable polarity plasma arc weld made in AA5182 alu-

minium alloy sheet exhibiting severe undercut

a humped weld bead; b transverse section at a-a; ctransverse section at b-b (taken from Nguyen et al.8,9)

2 Humping in bead on plate GMA weld made on cold

rolled AISI 1020 plain carbon steel

Nguyen et al. High speed fusion weld bead defects

Science and Technology of Welding and Joining 2006 VOL 11 NO 6 619

Humping and undercutting are found in both non-autogenous (filler metal added) welding processes suchas gas metal arc welding (GMAW) and submerged arcwelding (SAW)5,8–10 and autogenous (no filler metal)processes such as gas tungsten arc welding (GTAW),11,12

laser beam welding (LBW)13,14 and electron beamwelding (EBW).15–17 The formation of humps andundercuts when using these welding processes have beenattributed to the effects of various process parameterssuch as welding speed, power input, surface conditionof the work piece, base metal chemical composition,the electrode geometry, the type and composition ofshielding gas used, the travel angle of the heat sourceand the orientation of the work piece with respect togravity, i.e. whether welding is in the flat position, uphillor downhill.5,8,9,11 Often, systematic mapping of various

weld bead profiles observed versus the relevant weldprocess parameters has been performed.8–12,14–16 InFig. 3, for example, the influences of welding speed,welding current or power input on the formation ofhumping and undercutting in autogenous GTAW of aprecipitation hardenable steel12 (Fig. 3a), in autogenousCO2 LBW of thin 304 stainless steel sheets14 (Fig. 3b)and in non-autogenous GMAW of mild steel plate10

(Fig. 3c) are shown. Although the fusion weldingprocesses and their controlling parameters used togenerate the process maps shown in Fig. 3a–c arefundamentally different, there are common trends inbehaviour. In all cases, the welding speed limit is definedby the occurrence of undercuts at low powers andhumps at medium power input levels.11,12,16 At highpower input levels, the welding speed is restricted by acombination of humped welds and severe under-cuts5,11,14,15 Finally, there appears to be an inverserelationship between the welding speed limit and totalpower, i.e. the welding speed limit decreases as the totalpower increases.

The presence of severe undercutting at high powerinput levels is not always observed. Nishiguchi et al.10

and Tsukamoto et al.16 reported that humped weldswithout undercut were observed at high power inputlevels in GMAW and EBW respectively. In contrast,Bradstreet5 and Hiramoto et al.13 reported humpingwith undercutting in high power GMAW and EBWrespectively. Interestingly, Hiramoto et al.13 alsoobserved humping without undercutting in LBW.

A comparison between the GMAW experimentalprocedures used by Bradstreet,5 Nguyen et al.8,9 andNishiguchi et al.10 reveals that different modes of fillermetal transfer were used in their respective studies.Nishiguchi et al.10 employed short circuit and globulartransfer modes while Bradstreet5 and Nguyen et al.8,9

used spray transfer mode. Furthermore, Nishiguchiet al.10 employed the buried arc technique to minimisethe amount of weld spatter. Therefore, the lack ofundercut may be related to the different metal transfermodes of GMAW and the effects of burying the weldingarc. The observed difference indicates that the weldingtechnique and the metal transfer mode in GMAWinfluence the weld bead geometry. The interactionsbetween metal transfer mode and the occurrence ofhumping and undercutting further complicate weldingprocess maps such as that shown in Fig. 3c. Theseinteractions are not yet clearly understood.

The influences of a number of other process variableson the formation of undercutting and humping havealso been reported. Yamauchi and Taka18,19 and Savageet al.12 showed that GTAW speeds could be more thandoubled when He was used instead of Ar as shieldinggas. The beneficial effects of using lower molecularweight of He shielding gas was attributed to theinfluences of arc forces on depression of the weld poolunder the arc Savage et al.12 found that there was nomeasurable difference between total arc force whenusing either Ar or He and concluded that the observedeffects must be due to lower peak arc pressures and thecharacteristic increased width of He arcs. These conclu-sions were verified by arc pressure measurements madeby Yamauchi and Taka.18,19 They found that the peakarc pressure of He arcs was significantly lower than Ararcs under the same welding conditions.

a GTAW of plain carbon steel after Savage et al.;12

b CO2 laser welding of 304 stainless steel after Albrightand Chiang;14 c GMAW of mild steel after Nishiguchiet al.10

3 Mapping of various weld bead regions



The ambient pressure also has been shown to have aneffect on the humping phenomenon. Nishiguchi andMatsunawa20 made GMA welds on plain carbon steel ina hyperbaric chamber at Ar atmosphere pressures up tofive bars and found that the critical welding speed abovewhich high speed defects such as humping occurredincreased with increased hyperbaric pressures. Theynoted that the arc and oxide cleaning by the arc becamemore focused with increased ambient pressure and thatthe weld pool profile changed from the normal nailhead profile commonly observed with spray transferto a semicircular profile with increased penetration.Unfortunately, the increased costs and welding timesassociated with hyperbaric welding would be prohibitivein most production situations.

The use of reactive shielding gases such as CO2 andAr–CO2 or Ar–O2 mixes5,8,9,21 as well as trimixes such asAr–CO2–O2

21 and quad mixes such as Ar–CO2–O2–He8,9 in GMAW and He shielding gas in GTAW11 havebeen found to expand the range of operating conditionsin which good welds are produced to as much as 400%higher welding speeds relative to welds produced usingAr shielding gas.8,9 Bradstreet5 and Nguyen et al.8,9

noted that the contact angle between the molten metaland the fusion boundary of GMA welds in mild steelwas reduced and wetting between the weld metaland base metal improved when even a small percentage(2–5%) of oxygen was added to the Ar shielding gas.Because an interfacial surface tension force balanceexists at the point of contact between the liquid metaland the solid fusion boundary,5 this reduced contactangle is direct evidence that the liquid metal surfacetension has been reduced by the addition of O2, possiblythrough formation of SiO2 or FeO oxide films on thesurface of the molten steel. In fact, Subramaniam andWhite22 have made in situ measurements of the surfacetension of molten metal droplets in a GMAW plasmaand found that the surface tension of the molten metal inthe plasma is decreased from y1.56 to y1.1 N m21

when O2 or CO2 is added to the Ar shielding gas.

When using reactive shielding gases, the resultantlower values of liquid metal surface tension influence theinitial contact angle between the molten weld metal andthe surface of the work piece.5 Bradstreet5 argued thatthis change in the initial contact angle delays the onset ofhumping. This is further supported by an analoguestudy of bead geometry stability by Schiaffino andSonin,23 in which a fine stream of microcrystalline waxdroplets was projected onto the surface of a movingplate of Plexiglass. They also found that the contactangle between the molten wax and the surface of thePlexiglass work piece was responsible for the instabilityof the wax bead geometry. When this angle was ,p/2radians (90u), humping would not occur. On the otherhand, if the initial contact angle was .p/2 radians (90u),the wax bead was unstable and formed periodicundulations similar to the humping phenomenon inwelding. Schiaffino and Sonin23 suggested that the initialcontact angle is a function of the liquid’s properties andis also strongly dependent on the temperature differencebetween the initial temperature of the work piece andthe melting temperature of the deposited liquid.

In GTAW, the electrode geometry has also beenshown to suppress the onset of humping and under-cutting to higher welding speeds. Use of larger electrode

tip angles or smaller electrode diameters is reported tobe beneficial to achieving higher welding speeds duringGTAW.11,18 Yamauchi and Taka19 have also shownthat significantly higher welding speeds are possiblewhen hollow electrodes are used. Similarly, Nishiguchiet al.10 have found that use of a larger electrode wireduring GMAW will suppress the creation of high speedweld defects in GMA welds. All of these electrodegeometry effects are known to reduce arc forces and theamount of depression of the weld pool surface directlyunder the welding arc.18,19,24 Arc forces are created bymomentum transfer that takes place when the highvelocity stream of plasma in the welding arc impinges onthe weld pool surface. The plasma within the arc isdriven down from the electrode to the weld pool surfaceby the difference in Lorentz force ~FFL~~JJ|~BB, betweenthe electrode and the weld surface as well as the axialcomponent of the Lorentz force where ~JJ is the currentdensity and ~BB is the magnetic field vector. As theelectrode tip angle increases, for example, the arcchanges from a bell shape with a large difference in ~JJat the electrode relative to ~JJ at the weld surface to amore column like arc with less difference between ~JJalong the length of the arc and, therefore, less drivingforce for flow of the plasma. Therefore, any electrodegeometry such as a hollow electrode or larger electrodetip angle that produces a more column like arc will resultin lower plasma velocities, lower arc forces and, there-fore, less depression of the weld pool surface anddecreased propensity for humping and undercut-ting.18,19,24 While electric arcs and arc forces are notpresent in high energy density processes such as EBWand LBW, lowering the energy density of the heat sourceat the work piece surface13 or over focusing15 have beenshown to facilitate production of defect free welds athigher welding speeds, because there is less depression ofthe weld pool surface owing to vaporisation.

Regardless of the type of heat source, the travel anglebetween the heat source and welding direction has beenfound to affect the onset of undercut and humping. Useof a positive push angle of the heat source or forehandwelding has been shown to suppress the formation ofthe undercut and humping defects to higher weldingspeeds.5,11,25 This is thought to occur because the heatsource is impinging more directly on unmelted basemetal and is less able to depress the surface of the weldpool, whereas, use of a trailing heat source angle resultsin greater depression of the weld pool surface and activepushing of the molten metal towards the tail of the weldpool, thereby promoting humping at lower weldingspeeds and powers.5,8,9

Nguyen et al.8,9 have shown that the orientation of thework piece with respect to gravity also affects thepropensity for humping during GMAW. They foundthat welding could be performed at higher speedswithout humping when welding downhill, becausegravitational forces weaken the backward flow ofmolten metal in the weld pool and the molten metaltends to flow down from the tail of the weld back intothe weld pool. Alternatively, welding in the uphilldirection promoted humping at lower speeds, becausegravitational forces further strengthen the backwardflow of molten metal and push the molten metaltowards the tail of the weld pool thereby promotinghumping.



Besides undercutting and humping, two other types ofhigh speed weld defects have been reported: throughthickness holes or blowholes in full penetration LBWand EBW welds in thin sheets14,25–28 and overlap defectsin high speed welding of stainless steels.15,25,26,29–32 Anexample of through thickness hole or blowhole defect insemiautomated GTAW of 16 gauge mild steel sheets isshown in Fig. 4.33 The weld bead of welds exhibitingthrough thickness hole or blowhole defects is broken upby regularly or irregularly spaced holes.14,33 Accordingto Albright and Chiang,14 the occurrences of throughthickness holes are generally an indication of excessivelyhigh levels of heat input per unit distance (see Fig. 3b)which results in full penetration welds with excessiveweld widths relative to the sheet thickness. These widewelds are subject to blow through defects owing to thecombined effects of arc forces and gravity and are alsoaffected by capillary instabilities similar to the Rayleighinstability model used by Bradstreet5 to explain hump-ing in GMAW.

If the welding speed is further increased, the overlapdefect may begin to form.6,15,17 Figure 5 is a schematicof the top view and the transverse section of an overlapdefect in a weld produced using high power input andwelding speed. The overlap defect is formed when themolten metal overflows the weld pool and solidifies onthe top surface of the work piece without fusing to thebase metal. This creates an incipient crack between theover flowed weld metal and the base metal. The weldbead can also exhibit a deep groove along the centre ofthe bead. Under certain extreme welding conditions,overlaps have been observed to form two parallelhumped beads along the adjacent fusion boundaries ofthe weld.29–32 These overlap defects have been reportedmost frequently to occur in austenitic stainless steelwelds when using the EBW15,26 or the GTAW25,29–32

processes, but were also observed by Bradstreet5 duringGMAW of mild steel.

There are two proposed explanations for the forma-tion of the overlap defect. Tomie et al.17 speculated thatat higher EBW speeds, molten weld metal from the frontof the weld pool could not easily flow back to the tail ofthe weld pool owing to an enlarged beam cavity andsmaller weld pool width. The inclined front wall of theweld pool at high welding speed also caused molten weldmetal to flow out of the weld pool as it went around thebeam cavity to form the overlap on the top surface ofthe work piece.17,26 On the other hand, it has beensuggested that for low energy density welding processes

such as GTAW, the overlaps form as the molten weldmetal is split into two streams around the welding arcwhile being displaced from the front to the rear of theweld pool.29–32 According to Brooks and Lippold,34

molten austenitic stainless steel is highly viscous andsluggish compared to other grades of stainless steel. Thehigh viscosity reduces the flow and the wettability ofmolten weld metal thereby preventing merging of thetwo streams behind the welding arc prior to solidifica-tion of the liquid metal. At high welding speeds, thesetwo streams of molten metal may solidify individuallyon the cooler edges of the weld pool, thereby causing theoverlap defect.

While empirical plots such as those shown in Fig. 3are very informative, they are somewhat limited becausethey do not show the effects of all influential processparameters. In each plot, the effects of variouscombinations of welding speed and power input areevident while other process parameters were keptconstant. A minor change in one of the other processvariables such as selecting a different shielding gascomposition can significantly displace the boundaries ofthe weld profile regions.8–11 Therefore, a new plot isneeded to properly illustrate the effect of a minor changein one variable. This is time consuming and costly asmany experimental welds are required to generate theseprocess maps.

Table 1 contains a summary of the welding processes,materials, weld types and process parameters that havebeen examined by different researchers. In general, moststudies have used bead on plate welding to explore theinfluence of welding speed and welding power on theundercut and humping phenomena. However, it is clearfrom the experimental results discussed above and inTable 1 that there are a number of other welding processparameters that affect the undercut and humpingphenomena and that there are frequently nonlinearinteractions between all of these parameters. In general,

4 Severe blowhole defects in semiautomated GTA weld

made in 16 gauge mild steel sheets in corner joint

configuration (taken from Biglou33)

5 Schematic drawing illustrating cross-section and top

view of overlap defect in autogenous weld produced

at high welding power and welding speed



Tab

le1

Weld

ing

pro

cess

para

mete

rsth

at

have

been

exam

ined

instu

die

so

fu

nd

erc

utt

ing

an

dh

um

pin

gas

well

as

vari

ou

sp

rop

osed

mo

dels

used

toexp

lain

form

ati

on

of

these

hig

hsp

eed

weld

defe

cts

Year

1968

1971

1975

1975

1978

1978,

1979

1979

1982

1982,

1983

1987

1988

1989

1990

1992

2002

1998–

2003

2005

Au

tho

rsB

rad

str

eet

Pato

net

al.

Nis

hig

uch

iet

al.

Yam

am

oto

et

al.

Ara

taet

al.

Yam

au

ch

ian

dT

aka

Savag

eet

al.

Sh

imad

aet

al.

Tsu

kam

ota

et

al.

Hir

am

oto

et

al.

Alb

rig

ht

an

dC

hia

ng

To

mie

et

al.

Beck

et

al.

Gra

tzke

et

al.

Xie

Men

dez

et

al.

Ng

uyen

et

al.

Refe

ren

ce

No

.[5

][4

3]

[10,

20]

[11]

[48]

[18,

19]

[12]

[25]

[15,

16]

[13]

[14]

[17]

[47]

[40]

[54]

[29–3

2]

[8,

9]

Weld

ing

Pro

cess

GTA

W$

$$

$$

GM

AW

$$

$

SA

W$

LB

W$

$$

$$

EB

W$

$$

Mate

rial

Ste

els

$$

$$

$$

$$

$$

$

Sta

inle

ss

ste

els

$$

$$

$$

Alu

min

ium

allo

ys

$$

$

Cop

per

allo

ys

$$

Weld

typ

eB

ead

on

pla

te$

$$

$$

$$

$$

$$

$$

$

Part

ial/fu

llP

en.

weld

s$

$$

Keyhole

/cond

uction

$$

$

Weld

ing

pro

cess

para

mete

rs

Weld

ing

sp

eed

$$

$$

$$

$$

$$

$$

$$

$$

Pow

er/

curr

ent/

voltag

e$

$$

$$

$$

$$

$$

$$

$$

$

Pla

tesurf

ace

cond

itio

n$

Ar

shie

ldin

gg

as

$$

$$

$$

$

He

shie

ldin

gg

as

$$

$$

Ar–

O2

shie

ldin

gg

as

$$

CO

2shie

ldin

gg

as

$$

Flu

x$

Am

bie

nt

pre

ssure

$$

$

Torc

horienta

tion

$$

$$

Work

pie

ce

orienta

tion

$

Ele

ctr

od

etip

geom

etr

y$

$

Ele

ctr

od

ed

iam

ete

r$

$$

Ele

ctr

od

eg

ap

$$

Tand

em

heat

sourc

es

$$

$$

Pow

er

ratio

$$

Dualheat

sourc

es

gap

$$

$

Pro

posed

mod

els

Rayle

igh

insta

bili

ty$

$$

$

Pre

ssure

bala

nce

$

Sup

erc

riticalflow

$$

$

Hig

hvelo

city

flow

$$

Curv

ed

wall

jet

$



these experimental results do not directly reveal thephysical mechanisms responsible for the formation ofthese defects, e.g. they do not explain what causes thetransition from good welds to undercutting or humpingwith increased welding speed.

Proposed mechanisms for formation ofhigh speed weld defectsUnderstanding the mechanisms and driving forcesresponsible for undercutting and humping is essentialif techniques for suppression or elimination of thesedefects are to be developed. While several attempts havebeen made to view the formation of undercuts andhumping using high speed film5,15,16 and video ima-ging,8,9 these images are of the surface of a rapidlymoving opaque fluid directly under an intense plasmaarc light source. It is difficult to glean from such imagesthe physical phenomena that are taking place during theformation of humping and undercutting. Nevertheless,various mechanisms for the formation of defects havebeen proposed based on such observations obtainedfrom imaging of the weld pool, other experimentaltechniques and results and the final weld bead profiles.

Rayleigh jet instability modelBradstreet5 was the first to attempt to explain thehumping and undercutting phenomena. Based onobservations of the final weld bead profile, he suggestedthat the humping phenomenon was analogous to theRayleigh jet instability model of a cylindrical inviscidfluid jet freely suspended in space.35,36 The Rayleighinstability model is based on the conservation ofmechanical energy and as shown in Fig. 6, it predictsthat the lateral surface of the cylindrical jet will beunstable once its length l, exceeds its circumference 2pR,where R is the radius of the cylindrical jet. Theinstability grows most rapidly when l is equal to2.9pR35,36 causing a periodic swelling and necking ofthe jet and eventual break-up of the jet into a stream ofindividual droplets. It should be noted, however, that inthis highly idealised model, many important effects suchas the imbalance of the forces between the top freesurface and the bottom surface in contact with the solidfusion boundary, viscosity and viscous drag forcesacting on the liquid at the fusion boundary as well asthe effects of solidification on the time varying domainof the liquid jet are all ignored.

In the context of a weld, the similarity between theRayleigh jet instability model and humping is based onobservations of the periodic bead geometry of themolten weld metal behind the welding arc in the tail ofthe weld pool. At high welding speeds and currents, thetop surface of the weld pool is believed to transformfrom a flat to a convex shape behind the arc where

surface tension forces draw the molten metal into acylindrical shaped jet. The pressure inside this jet isincreased owing to the need to balance surface tensionforces.37 Once the lateral surface instability begins, themolten metal flowing through the jet from the centre ofthe weld pool towards the tail of the weld pool willbegin to form the periodic necking and swelling that ischaracteristic of the humped bead geometry. Bradstreet5

argued that the combination of the small radius of theneck and the relatively cold underlying work pieceresults in premature solidification of the molten metal inthe neck, thereby cutting off the fluid flow to theswelling. As welding continues, a new swelling or humpwill form again once the jet reaches the critical length l.5

This proposed Rayleigh instability model of humpingwas also adopted by Tsukamoto et al.15 to explainhumping in EBW of steel plate and by Simon et al.38 inCO2 LBW of metals.

There is ongoing debate about the validity of using theRayleigh instability model to explain the driving forcesresponsible for humping. As originally proposed, thesurface tension induced pressure in the swelling or humpis smaller than that in the neck because of the differencebetween their respective radii of curvature. Bradstreet5

argued that this pressure difference was the driving forcefor the build-up of molten metal at the tail of the weldpool that ultimately forms the hump. On the other hand,Ishizaki37 has theorised that the high internal pressure ofthe small molten droplets being transferred across thearc to the weld pool is the driving force responsible forthe humping in GMAW. He argued that the highinternal pressure in these droplets causes the moltenmetal to flow backward from the arc gouged regionthrough the jet of molten metal and eventually to pile upforming a hump at the tail of the weld pool. Accordingto Ishizaki,37 the backward flow of molten metal willstop and another hump will form when the pressureinside the jet equals the pressure within these moltendroplets. However, the relationship between surfacetension generated pressure within the droplets andpressures within the weld pool is unclear because, unlikemass, momentum or energy, pressure in a fluid is not aconserved quantity.

Hugel et al.39 have argued that humping in laser weldsis caused by the surface tension at the keyhole rim andthe shear force due to vapour flow inside the keyhole ofthe laser weld. These provide the driving force for thepile up of molten metal at the tail of the weld pool.Using a numerical model, they predicted a region of highmetallostatic pressure at the tail of the weld pool. Theseresults contradict previous suggestions by Bradstreet5

and Ishizaki,37 because the high local pressure at the tailof the weld pool should prevent further accumulation ofmolten metal.

Although the Rayleigh jet instability model mayexplain the periodic occurrence of the swellings in ahumped weld, it cannot be used directly to predict thecombination of welding process parameters that willresult in a humped weld. Furthermore, the Rayleigh jetinstability model assumes that the molten weld metalmust have a cylindrical shape and be freely suspended inspace.35,36 With severe undercut, the molten weld metaldoes assume a shape that closely approximates anunsupported cylinder. For these welds, Bradstreet5

obtained good correlation between the observed average

6 Schematic diagram showing cylindrical inviscid fluid

jet freely suspended in space with unperturbed radius

R and length l, described in the Rayleigh jet instability

model



hump spacing and the critical instability length predictedby the Rayleigh instability model. On the other hand,without severe undercut, the measured and calculatedresults were significantly different, although of approxi-mately the same order of magnitude.5

To overcome the deficiency of the Rayleigh instabilitybased model for humping noted by Bradstreet,5 Gratzkeet al.40 introduced a correcting function B(Qo) andestablished a new instability criterion, i.e. humping willoccur when

lw2p|R|B Qoð Þ (2)

where l and R are as depicted in Fig. 6 and B(Qo) is acorrecting function which is dependent on the angle Qo,as shown in Fig. 7. When the angle Qo50, the cylindricaljet only touches tangentially to a flat surface. Thiscorresponds most closely to the free standing jet in theRayleigh instability model. The main purpose of thecorrecting function B(Qo), is to account for the wettedperimeter that exists between the solid at the fusionboundary and the molten metal jet. Gratzke et al.40

suggested that humping would occur when B(Qo)51.5(i.e. Qo<60u) for GMAW and when B(Qo)52 (i.e.Qo<80u) for LBW. However, these values were notvalidated experimentally.

By rearranging equation (2), Gratzke et al.40 were ableto define a critical ratio between the width and the lengthof the weld pool below which humping was predicted tooccur. This model appears to be supported by Baggeret al.41 observed reduction in the area and the width ofthe weld pool upon formation of a hump. Thesearguments suggest that maximising the width to lengthratio of the weld pool during welding will suppress theformation of undercuts and humping. If this is true, thena side by side arrangement of two heat sources would beexpected to increase the width to length ratio of the weldpool thereby allowing higher welding speeds. Asdiscussed in the latter portion of the present review,the inline arrangement of the heat sources of a weldingprocess with two heat sources (i.e. one heat source is infront of the other) has been shown to be effective inachieving higher welding speeds.15,18,19 More experi-mental work is needed to examine the validity of thismodified Rayleigh jet instability model and its ability topredict the observed effects of different welding processparameters on the humping phenomenon.

While studying the sweep deposition of molten waxon a flat surface, Gao and Sonin42 observed that theresultant wax bead had periodic ripples at higherdepositing frequencies of the molten wax droplets. Theobserved undulation of the wax beads was very similarto the humping phenomenon in a weld. Gao and Sonin42

suggested that the formation of these undulations issimilar to the Rayleigh jet instability. Schiaffino andSonin23 later demonstrated that the undulation of thesewax beads would only occur if the initial contact anglebetween the liquid wax and the flat surface is .p/2 (90u).Using inviscid flow theory, Schiaffino and Sonin23

showed some correlations between the wavelength ofthe undulations and the wavelength associated with themaximum growth rate of the instability.

The proposed instability theory by Schiaffino andSonin23 may not entirely explain the humping phenom-enon in welding. As previously mentioned, by usingreactive shielding gases, a low initial contact anglebetween the molten weld metal and the work piece canbe achieved. Despite the low initial contact angle,humping will eventually occur at higher welding speeds.In fact, as observed by Bradstreet,5 the molten metal of ahumped weld produced with reactive shielding gas had asmall contact angle with the unmelted based metal. Thisis indicative of good wetting between the molten weldmetal and the unmelted base material. This wouldsuggest that the instability of the weld bead andhumping is not based solely on the contact anglebetween the liquid metal and the work piece.

Albright and Chiang14 used the Rayleigh jet instabil-ity model to predict the onset of blowhole defects in highspeed LBW of thin 304 stainless steel sheets. They usedthis model to predict the break-up length of a cylindricalfluid jet into individual droplets lc, as shown in Fig. 6.The driving force for this is minimisation of the overallsurface energy. The break-up length can be calculated asfollows

lc~12v

c=(8rR3)½ �1=2(3)

where lc (m) is the break-up length of the liquid jet; c(N m21) is the surface tension of the fluid; r (kg m23) isthe liquid density; R (m) is the nozzle or initial jet radiusand v (m s21) is the liquid flow speed, which was takenas the welding speed.14 If the weld pool was shorter thanthe break-up length lc, then the weld pool was stable andblowholes would not form. On the other hand, the weldpool would be unstable when its length was longer thanthe break-up length. Within an unstable weld pool, themolten metal is predicted to swell into spherical ballsseparated by through thickness holes or blowholes.Albright and Chiang’s results14 showed that the break-up length lc, predicted by the Rayleigh instability modelcould be used to predict the occurrence of blowholes inwelds.

Arc pressure modelAlthough it is difficult to make direct observations offluid flow in an arc weld pool, it is generally believed thatthe undercut and humping phenomena are initiated byevents taking place in the weld pool directly underneaththe welding arc. For example, Yamauchi and Taka18,19

and Paton et al.43 considered the static balance betweenthe arc pressure and the metallostatic pressure of the

7 Subtended wetted angle 2Qo, as defined by Gratzke

et al.40



molten metal at the tail of the weld pool to beresponsible for the final weld bead profile and humping.A schematic diagram of the static pressure balance isshown in Fig. 8. According to their pressure balancemodel of humping, when the metallostatic pressure ofthe molten metal exceeds the arc pressure, a thick layerof molten metal will exist beneath the arc. This willresult in good weld bead geometry. Conversely, whenthe arc pressure is much greater than the metallostaticpressure, the pressure from the arc depresses the weldpool surface and creates a crater with a very thin layer ofmolten metal. Most of the remaining molten metal isdisplaced toward the tail of the weld pool. In this lattersituation, the arc gouged crater is not filled at the lateralweld edges by the displaced molten metal prior tosolidification and therefore undercutting occurs. Basedon the pressure balance model, therefore, higher weldingspeeds can be achieved by selecting process parametersthat reduce the overall arc pressure.

The impinging of the high velocity plasma jet on theweld pool surface generates the arc pressure that acts onthe free surface to create a depression in the weld poolsurface.18,19,24,44 In the GMAW process, the momentumtransfer from the stream of molten filler metal dropletsalso contributes to the depression of the weld poolsurface. Higher welding speeds and currents have beenshown to reduce the metallostatic pressure of the moltenmetal within the weld pool43 and to increase the arcpressure.12,18,19,24,45,46 The arc pressure is also stronglydependent on the physical properties of the shieldinggas.12,19,24 For example, higher welding speeds havebeen obtained by switching from Ar to the lowermolecular weight He shielding gas when GTAW ofstainless steel11,24,45 or by welding in a low ambientpressure environment.25

The pressure balance model attributes the formationof undercut at high welding speeds to the inability ofmolten weld metal to laterally fill the crater gouged outby the welding arc before the completion of solidifica-tion. However, this is a steady state model that does notexplain the swellings and the periodic behaviour of thehumping phenomenon because it does not include theinteractions between fluid flow, surface tension andsolidification of the molten weld metal during humping.Finally, because the model is qualitative, it cannot beused to predict the actual welding conditions that willinitiate the formation of undercuts and humping.

Supercritical flow modelIn an attempt to include the influence of the molten weldmetal flow in GTA weld pools on the undercutting andhumping phenomena, Yamamoto and Shimada11 haveproposed a supercritical flow model for humping. Underthe influence of high arc currents and pressures, theynoted that GTA weld pool surfaces are significantlydepressed and that only a very thin layer of molten metalexists within the arc gouged region of the weld pool asshown in Fig. 9. Because most of the melted base metalmust flow from the front of the weld pool to the tail ofthe weld pool through this thin film or wall jet, the flowvelocities through this wall jet are very high. Yamamotoand Shimada11 argued that humping occurs when themolten metal velocity in the wall jet exceeds a criticalvalue and a hydraulic jump forms in the weld pool withthe swellings similar to those observed in humped welds.The supercritical flow model can also be applied to non-autogenous welding processes such as the GMAWprocess.

Although the supercritical flow model appears to beable to predict the fluid flow conditions that must existwithin the weld pool for humping to occur, it does notexplain the observed periodic nature of humps nor doesit provide the link between the principal welding processparameters and the fluid flow conditions that will existwithin the weld pool when humping occurs. Althoughthe molten weld metal velocity field can be obtainedfrom numerical simulations such as that of Beck et al.,47

the results cannot be readily verified, because the moltenweld metal velocities are difficult to measure. Therefore,the supercritical flow model cannot as yet be useddirectly to predict the conditions that will causehumping on the basis of the principal welding processparameters.

Mendez et al.29–32 have attempted to explain theperiodic nature of humping by modifying the super-critical flow model by Yamamoto and Shimada.11 In thismodified model, the weld pool is divided into two mainregions; an arc gouged region at the front of the weldpool directly under the arc and a trailing region at theweld pool tail as shown in Fig. 9. At high welding

8 Schematic diagram illustrating balance between arc

pressure and metallostatic pressure of molten weld

metal during arc welding9 Arc gouging region, trailing region, rim of molten weld

metal and transition line in high speed GTA weld

as defined by Yamamoto and Shimada11 and Mendez

et al.29–32 for their supercritical flow models of

humping



currents, arc forces depress the weld pool surface andcreate a very thin film of molten metal underneath thearc. On the upper portion of the weld pool wall,however, a slightly thicker wall jet or layer of moltenmetal has been observed to exist around upper rim of theweld pool fusion boundary. It is thought that this layerof molten metal around the rim of the weld pool isthicker because of the contact angle and capillary forcesacting on the liquid surface where it contacts the solidbase metal and because it is further away from the arcwhere arc forces that depress the molten metal surfaceare weakest. During welding, molten metal flows fromthe front to the tail of the weld pool through acombination of the rim and the thin film wall jets. Thetrailing region at the tail of the weld pool contains thebulk of molten weld metal. As shown in Fig. 9, thesetwo regions meet at a distinct transition line.

According to Mendez et al.,29–32 the location of thetransition line between the arc gouged and the trailingregions determines the onset of the humping weld defect.The location of the transition line can be estimated fromthe force created by the arc pressure and the combinedforce caused by the metallostatic pressure of the moltenweld metal and the capillary effect. The arc forcedisplaces the transition line toward the back of the weldpool and away from the welding arc, while themetallostatic pressure of molten weld metal andcapillary force resist this rearward displacement.

Figure 10 shows generalised relationships between thenormalised arc pressure, the normalised metal pressureand the normalised heat input plotted against thenormalised length of the arc gouged region respectively.In this case, the arc pressure was assumed to be aGaussian distribution and was normalised with respectto the predicted peak pressure directly under theelectrode and the assumed Gaussian distribution heatinput has been normalised with respect to the peak heatflux from the arc again, directly under the electrode. Thenormalised length scale l, is the size of the gougingregion of the weld pool L, normalised with respect to thelength scale of the pressure distribution LP, i.e. l5L/LP.These relationships were obtained by using a magnitudescaling technique.29–32 In this plot, the metal pressureline represents the combined force of the metallostaticpressure of the molten weld metal and the capillaryeffect. As labelled in Fig. 10, there are two intersections

between the arc and the metal pressure curves. Atintersection #1, the relationship between the arcpressure and the metal pressure is unstable. Anydisturbance will either eliminate the arc gouged regionowing to the increased metal pressure and the reducedarc pressure or develop a gouged region owing to higherarc pressures and reductions in metal pressure. For thefirst scenario, humping will not form. If the latterscenario occurs, the arc pressure and the metal pressurecurves will cross at intersection #2. At this intersection,the relationship between the arc pressure and the metalpressure is stable. Any reduction in the length of thegouged region will increase the arc pressure that pushesthe transition point backward. As a result, the arcgouged region will regain its original length. On theother hand, an elongation of the arc gouged region willincrease the metal pressure that pushes the transitionline forward and eliminates any length increase.

Under normal welding conditions, the arc gougedregion does not extend very far from the arc. At higherwelding speeds or currents, however, the higher arcpressure moves the arc gouged region further backtowards the tail of the weld pool, therefore pushing thetrailing region further back and away from the weldingarc. As shown in Fig. 10, the amount of normalised heatinput received at intersection #2 between the arcpressure and the metal pressure curves is less than unity.Therefore, the transition line between the arc gougedregion and the trailing regions is normally located at thetail portion of the heat input distribution. With less localheat input from the arc, the thin film solidifies, therebypreventing the molten metal in the thin film fromflowing to the back of the weld pool. As weldingcontinues and molten metal continues to flow towardsthe tail of the weld through the thin film and rim, a newhump begins to form at the point where the thin filmsolidified and the cycle is repeated.

In the modified supercritical flow model by Mendezet al.29–32 the effects of fluid motion, solidification ofmolten metal in the thin film and the balance betweenarc pressure and metallostatic pressure effects on theundercut and humping phenomena have been consid-ered. However, in its present form, the model cannotpredict the onset of humping based on the independentwelding parameters used and does not directly suggestany solutions to elimination or suppression of undercutand humping. As well, there is heavy emphasis placedon the solidification of the thin film directly underneaththe welding arc while the significant role of the rim intransporting liquid metal to the trailing region isignored.

Curved wall jet modelBased on video imaging of GMA welds made on mildsteel plates and corroborating experiments, Nguyenet al.8,9 have recently proposed a curved wall jet modelof humping in non-autogenous welding processes suchas GMAW. Figure 11 shows schematic diagrams oflongitudinal sections of welds produced using the samewelding parameters, but at low and high welding speedsrespectively. On these diagrams, d is the longitudinaldistance along the weld centre line from the leading edgeof the weld pool to the location where the filler metaldroplet impinges the top surface of the weld pool. Asillustrated in Fig. 11a, during low speed welding, themolten weld metal is contained within a large weld pool

10 Relationship between arc pressure and metal pres-

sure in GTA weld as presented by Mendez et al.29–32



underneath the welding arc. The depression of the freesurface of the weld pool due to the arc force and the fillerdroplet momentum is possible, but limited by theforward recirculation of the molten weld metal. Thisrelatively large pool of molten metal absorbs anddissipates the momentum of the incoming filler metaldroplets in the GMAW process and resists the effects ofthe arc forces. Consequently, the backward momentumof molten metal within this weld pool is low and there islittle chance of forming a humped weld.

As illustrated in Fig. 11b, at high welding speeds, theweld pool becomes elongated, shallow and narrow.Also, the electrode, the welding arc and the metaldroplet stream move forward and closer to the leadingedge of the weld pool, i.e. d decreases. Nguyen et al.8,9

observed an inverse relationship between d and thewelding speed. Owing to the reduction in penetrationand volume or mass of molten metal in the weld pool,the combined actions of the arc force and the dropletmomentum create a depression or gouged region at thefront of the weld pool that contains a thin layer of liquidmetal underneath the welding arc. Without the presenceof a thick liquid layer underneath the welding arc, themomentum of the incoming filler metal droplets are not

absorbed or dissipated. Rather, the droplets hit thesloping leading edge of the weld pool (see Fig. 11b) andthis molten filler metal is then redirected towards the tailof the weld pool at high velocity through a semicircularcurved wall jet frequently similar in shape to that shownin Fig. 7, dragging with it any liquid metal in the frontof the weld pool from the melting base metal. This highvelocity, rearward directed flow of molten weld metal inthe weld pool is consistent with numerical simulationsby Beck et al.47 and observations in tandem EBW byArata and Nabegata.48 Backfilling of the front portionof the weld pool as described by Bradstreet5 does notoccur because the recirculated molten weld metal fails tokeep up with the forward moving welding arc and ispushed or held back by the high velocity and momentumof the backward directed fluid flow within the wall jet.At the tail of the weld pool, the molten weld metal accu-mulates to form a swelling that is drawn into a sphericalbead shape by surface tension as molten metal is fed intothe swelling from the front of the weld pool through thewall jet. Therefore, the momentum of the backward flowof molten weld metal is responsible for not only theinitial formation, but also the growth of the swelling.

Although the swelling increases in size as moltenmetal flows into it, the new swelling is stationary withrespect to the base plate. As the welding arc continues tomove to the left along the weld joint, the wall jetbecomes increasingly elongated and the thermal mass ofmolten metal inside the wall jet becomes distributed overa longer distance until continued solidification of theweld and the molten metal in the elongated wall jetchokes off the flow of molten metal to the swelling.Solidification of the wall jet forms the valley typicallyobserved between swellings in a humped GMA weldbead such as that shown in Fig. 2. Initiation and growthof a new swelling closer to the arc and further along theweld bead occurs very soon after fluid flow in the wall jetis choked off. This sequential formation of a swelling orhump at the tail of the weld pool and solidification of thewall jet is a periodic phenomenon that results in theformation of humping in GMA welds. Nguyen8 hasshown that the humping frequency of this periodicphenomenon increases with increasing welding speed ordecreasing welding power.

The studies and proposed phenomenological modelsfor undercut and humping in various welding processesare summarised in Table 1. The arc pressure model,43

the supercritical flow model11 and the modified super-critical flow model29–32 were developed from observa-tions of the autogenous GTAW process in which thewelding arc plays a critical role in the formation of highspeed weld defects. However, these models may not beas directly applicable to the autogenous, high energydensity processes such as LBW and EBW, especiallywhen keyhole mode welding is used. In both of the latterwelding processes, the material laser beam interaction isfundamentally different from that found in arc weldingprocesses. The Rayleigh jet instability model byBradstreet,5 the pressure balance model by Patonet al.43 and more recently the curved wall jet model byNguyen et al.8,9 have been used in attempts to explainthe undercutting and humping phenomena in non-autogenous welding processes such as GMAW andSAW. In these processes, the momentum from the fillermetal droplets also plays a role in the heat and mass

11 Longitudinal sections of GMA weld pool and filler

metal droplet impingement locations at a low welding

speeds and b high welding speeds with wall jet and

humping (after Nguyen et al.8,9)



transfer responsible for these defects. It is possible thatthese proposed models are quite process specific and thatdifferent physical phenomena are responsible for hump-ing and undercutting in the various autogenous andnon-autogenous fusion welding processes.

The proposed phenomenological models of under-cutting and humping suggest that fluid flow and thehigh velocity and momentum of the backward directedflow of molten metal in the weld pool, arc pressure,metallostatic pressure, capillary forces and lateralinstability of a cylindrical or curved wall jet of moltenweld metal and the periodic solidification of this wall jetare responsible for the formation of humping andundercutting. However, the exact mechanism is unclearand is still being debated. Although some of theproposed models can provide plausible explanations ofthe periodic behaviour of the humping phenomenon,their ability to predict the onset of humping or under-cutting using the independent welding parameters havenot been experimentally validated. More importantly,from a technological perspective, these models do notalways directly suggest possible techniques that mightbe used to achieve higher welding speeds without theformation of undercut or humping weld defects.

Techniques and processes for higherwelding speedsThe lack of a comprehensive phenomenological modelfor the formation of humps and undercuts at highertravel speeds does not prevent the unrelenting searchfor welding techniques or processes to achieve higherproductivity. Over the years, practitioners have discov-ered several welding techniques and processes that canbe used to increase the overall productivity withoutsacrificing weld quality. Although it is frequentlydifficult at the present time to properly explain whythese techniques work, a discussion of these weldingtechniques and processes is valuable. These modifiedwelding techniques and processes may offer usefulinsights into the formation and the suppression of highspeed weld defects such as undercut and humping.

Experimental results from past investigators havedemonstrated the significant effect of shielding gas onthe suppression of high speed welding defects.5,8–12

Savage et al.12 reported that higher GTAW speeds wereachieved when He shielding gas was used instead of Arduring bead on plate welding of stainless steel. Althoughthe measured arc force was not altered,12 the arcpressure was uniformly distributed over a larger areadue to the lower density and higher viscosity of Hecompared with Ar.45 As a result, the overall arc pressurewas reduced allowing higher welding speeds to be usedwithout the formation of defects. Yamauchi and Taka19

have also shown that measured arc pressures in GTAWarcs are significantly lower when He is used instead ofAr with conical shaped electrodes; however, there waslittle difference in arc pressures of the Ar or He arcswhen hollow electrodes were used.

In GMAW with CO2 shielding gas, the buried arctechnique has been reported as an effective means ofachieving higher welding speeds and filler metal deposi-tion rates.5,10 With this technique, the arc is actuallylocated beneath the original surface of the work pieceduring welding. This reduces the weld spatter and

increases the metal deposition rate. In addition, theoverall arc pressure may be reduced since the cathode islocated within the weld pool crater.20 This is consistentwith the arc pressure and supercritical models wherein areduction in arc pressure is predicted to decrease the arcgouging directly under the arc, thereby suppressing theformation of humps and undercuts to higher weldingspeeds and allowing improvements in productivity.43

During GMAW of mild steel, Bradstreet5 observedthe beneficial effects of using a small amount of O2 in theAr shielding gas. He attributed this to improved wettingat the solid/liquid interface within the weld pool whenAr with 2–5%O2 shielding gas was used. These resultsare consistent with the current practice of using a smallpercentage of O2 in pure Ar shielding gas to improve thewetting of molten metal in the GMA weld pool.49 Theseresults are also consistent with the Rayleigh jetinstability model, where the improved wetting at thesolid/liquid interface would increase the equivalentradius and therefore the overall circumference of thecylindrical jet. Moreover, according to Schiaffino andSonin,23 a low contact angle between the molten weldmetal and the unmelted work piece will suppress thehump formation. As a consequence, higher weldingspeeds can be achieved without defect formation whenreactive shielding gas is used.

Figure 12 shows the limiting welding speeds abovewhich humping was observed by Nguyen et al.8,9 versuswelding power for GMA welds produced in mild steelplate when using Argon and two reactive shieldinggases, MMGTM (Ar–8CO2) and TIMETM (Ar–8CO2–26.5He–0.5O2) shielding gases. When using MMGTM

and TIMETM shielding gases, the limiting welding speeddecreased as the welding power increased. This trend isvery similar to those reported in previous studies of thehumping phenomenon.12–17,20 However, at the lowestpowers, use of reactive MMGTM and TIMETM shieldinggases suppressed humping and allowed up to 400%higher welding speeds than are possible with pure Argon

12 Limiting welding speeds above which humping was

observed versus welding power when making GMA

welds in plain carbon steel using Ar and reactive

shielding gases MMGTM and TIMETM (taken from

Nguyen et al.8,9)



shielding gas. This significant improvement in produc-tivity when using the reactive shielding gases was explainbased on video observations of the metal dropletdistribution patterns when using the different shieldinggases. As shown in Fig. 13, with reactive shielding gases,the arc gap is shorter and the stream of molten fillermetal droplets from the electrode was less focused andwas spread out over a larger area of the weld pool. Thiscaused an increase in weld width relative to welds madewith Ar shielding gas. They suggested that the smallerarc gap and therefore lower arc forces, the larger fillermetal droplet impingement area, the less focused andmore lateral flow of molten filler metal droplets and theextra opposing viscous drag force from the large fusionboundary all reduced the depression of the weld poolsurface and the overall momentum of the backward flowof molten weld metal in the weld pool thereby allowingsignificantly higher welding speeds without humping.8,9

The tandem arrangement of GTAW torches,18,19

GMAW torches,50,51,52 laser beams53,54 and electronbeams15,48 have also been shown to be very effectivein delaying the onset of humping and undercuts to

higher welding speeds. In recent developments, twoGMAW electrodes, arranged in line with the weldingdirection, were reported to provide between two andthree times higher welding speeds and filler metaldeposition rates without the occurrence of defects suchas undercut and humping.50,52 This new GMAWvariation is often referred to as tandem wire GMAW.In tandem wire GMAW, the leading power supply andwire feed rate are adjusted to achieve the desired weldpenetration and filler metal deposition rate. A short arclength, long electrode stickout and high welding currentare commonly used.51 The trailing power supply andelectrode feed rate are adjusted to achieve the desiredweld bead profile and to prolong the degasificationtime.51,52 The process parameters of the trailing elec-trode are set to produce less severe welding conditions.In fact, there appears to be an optimal ratio between thepower levels of the two GTAW electrodes,18 the twoGMAW electrodes50–52 or between the two beams in thedual beam EBW48 and dual beam LBW27,28 weldingprocesses.

Although the addition of the second heat source into acommon weld pool allows significantly higher weldingspeeds to be used before undercutting or humpingoccurs, the exact mechanism responsible for suppressingthese weld defects and the actual roles of the secondelectrode or beam are not thoroughly understood.Yamauchi and Taka18,19 and Ueyama et al.52 suggestedthat the trailing arc stopped or significantly reduced thebackward flow of molten metal caused by the arc forcesfrom the leading arc. This is consistent with Nguyenet al.8,9 curved wall jet model of humping. Dilthey et al.53

suggested that the main functions of the second beam inthe tandem dual beam LBW process are to reheat theweld metal at the tail of the weld pool and to allow timefor surface tension and gravitational forces to smoothout the top surface of the weld pool prior to solidifica-tion, thereby avoiding the formation of geometricdefects such as undercuts or humps. On the other hand,Hugel et al.39 suggested that the second beam in tandemdual beam LBW enlarges and stabilises the keyhole. Thelarger keyhole is then responsible for an increase in theweld pool cross-section and a reduction in the fluidvelocity and pressure at the tail of the molten weld pool.Both of these changes help to suppress the formation ofdefects at high welding speeds. Using real time highspeed X-ray imaging, Iwase et al.55 have shown that thedistance between the two beams influences the top weldwidth and the stability of the keyhole during partialpenetration, bead on plate, dual beam laser welding of5xxx series aluminium alloy plate. Deutsch et al.27 andPunkari et al.28 have also observed the beneficial effectsof the second tandem beam on keyhole stability andweld bead quality during dual beam Nd:YAG laserwelding of AA5182 and AA5754 aluminium alloy sheet.They found that good quality weld beads were producedin these alloys provided that the lead/lag beam powerratio was >1. This is similar to the lead/lag power ratiofrequently used in tandem GMAW.51,52

Using a tandem dual beam CO2 LBW process, Xie54

has shown that undercut and humping defects in plaincarbon steel and aluminium LBW welds can be signi-ficantly reduced or eliminated. He argued that the effectsof the second beam on the welding process depended onthe relative distance between the two laser beams.

13 Differences in arc gaps and overall velocity V and

velocity components Vv and Vh of filler metal droplets

during GMAW when using Ar (a) and reactive (b)

MMGTM and TIMETM shielding gases (taken from

Nguyen et al.8,9)



Similar to Dilthey et al.,53 he suggested that when thebeams were far apart, the second beam reheats andsmoothes the weld bead surface, effectively eliminatingany undercut or humping defects that had been createdby the leading beam. Alternatively, when the beams areclose together, the second beam acts to stabilise thekeyhole and enlarge the weld pool thereby suppressingthe formation of the undercut and humping defects untilhigher welding speeds are used. This is similar to thearguments of Hugel et al.39 When the distance betweenthe beams is intermediate to these two cases, Xie54

argued that the second beam creates a second keyhole inthe weld pool behind the keyhole created by the leadingbeam. This is similar to the results of Arata et al.48

which showed that the speed at which humping occurredin EBW welding could be increased up to 50% by using atandem dual beam EBW system. They attributed thesuppression of undercutting and humping to the changein momentum of the backward flow of the molten weldmetal as it was forced to flow around the second beamcavity. The second keyhole diverts the flow of super-heated liquid metal towards the lateral solid/liquidinterfaces and remelts any previously solidified metal.As a result, the flow channel is broadened, therebyreducing the pressure and velocity of the fluid stream.With reductions of both the pressure and velocity, themolten weld metal will not accumulate to form a humpat the tail of the weld pool. The laterally diverted flowalso prevents the formation of undercut.

The techniques described above and the relatedexplanations for why they are able to effectivelysuppress the formation of the undercut and humpingweld defects suggest that the backward momentum ofthe molten weld metal is a very influential factor. Theability to slow down the molten metal as it is beingdisplaced at high velocities toward the tail of the weldpool appears to be the key to suppressing or eveneliminating these high speed welding defects. This issupported by the work of Beck et al.47 and was utilisedby Kern et al.56 in CO2 LBW of construction steel. Inthe numerical simulations by Beck et al.,47 a highvelocity jet of molten metal directed toward the tail ofthe weld pool was predicted. This flow caused a steadyaccumulation of the molten metal and subsequentformation of a hump. According to Beck et al.,47 anytechnique that will reduce the flow velocities of therearward directed jet would lead to the suppression ofhumping. For example, by applying a magnetic fieldtransverse to the welding direction, Kern et al.56 wereable to suppress the formation of humping in CO2 laserwelding of fine grained construction steel under someconditions. They hypothesised that the transverselyimposed magnetic field altered the fluid flow profilewithin the weld pool and that this was responsible forthe elimination of humping in their laser welds. At thesame time, the high width to length ratio, alsorecommended by Gratzke et al.,40 widens the weld pooland therefore slows the velocity of the backward flow ofmolten weld metal. Ueyama et al.52 showed that therewas always an optimum combination of GMAW trailingtorch push angle, distance between the two GMAWelectrodes and ratio of leading/trailing arc currents thatwould facilitate the highest welding speed withouthumping. Finally, Nguyen et al.8,9 showed that thewelding position and orientation of the work piece

relative to gravity affects the backward flow of moltenweld metal and the critical welding speed at whichhumping occurs during GMAW of mild steel. With 10udownhill welding position, the gravitational forceweakened the backward flow of molten weld metal andallowed significantly higher welding speeds without theformation of humping. Conversely, 10u uphill weldingposition strengthened the backward flow of moltenmetal within the weld pool and caused humping to occurat much lower welding speeds. Therefore, higher weldingspeeds and improved productivity is possible by orient-ing the weldment such that welding is done in thedownhill position. Based on these arguments and obser-vations, future experimental work and models should bedesigned to carefully examine the role of molten metalfluid flow behaviour and its influences on the formationof undercut and humping weld defects.

The weld joint geometry has also been used toadvantage to confine and restrict the molten weld metalflow in the weld pool, thereby suppressing the formationof undercut and humping defects.16,57 Tsukamoto et al.16

argued that a deep and narrow square groove preventsthe molten metal from behaving as a cylindrical jet,which, according to the Rayleigh jet instability model, isa prerequisite for the formation of defects. However, it isnot unreasonable to argue that the additional wettedcontact with the fusion boundary provided by thegroove increases viscous drag thereby decreasing thevelocity of the molten metal and providing stability tothe cylindrical jet.35 Furthermore, according toShannon,57 certain weld joint geometries can result inwider weld pools. This will reduce the velocity of themolten weld metal towards the tail of the weld andtherefore suppress the humping phenomenon.

In tandem electrode GTAW, tandem wire GMAWand dual beam EBW, the trailing electrode or beam isnormally pointed in the direction of travel. Thisimproves the final weld bead appearance.18,19,48,50,52

Similarly, in multi-electrode SAW, forward deflection ofthe trailing arc has been shown to flatten the weld beadand eliminate weld defects such as undercut andhumping at high welding speeds.58 The forward deflec-tion of the welding arc was also reported to be beneficialin obtaining higher welding speeds in single wireGMAW5 whereas a lagging torch angle was found topromote undercutting and humping at much slowerwelding speeds. These observations suggest that theforward pointing of the trailing electrode or thetrailing beam in tandem processes will influencethe overall arc pressure or the backward momentum ofthe fluid flow within the weld pool and have a beneficialimpact on the formation of hump and undercut athigh welding speeds. This concept merits furtherinvestigation.

Several welding techniques and processes have beenidentified as being capable of suppressing the formationof undercutting and humping thereby permitting higherwelding speeds and increased productivity. Althoughpreviously proposed phenomenological models of thesedefects are unable to comprehensively explain theapparent success of these techniques and processes, thebenefits appear to be derived from their ability to slowdown the backward flow of molten weld metal. Thesewelding techniques and processes have revealed usefulinsights into the formation of high speed weld defects.



Further work is needed to examine the influence of thebackward flow of molten weld metal on the formation ofweld defects and to derive models that can predict theoccurrence of undercutting and humping on the basis ofthe independent welding process parameters.

Concluding remarksThe formation of high speed weld defects such asundercutting and humping is a very complex phenom-enon and dependent on many process parameters.However, in previous investigations, the influences ofvarious welding parameters and their interactions on theformation of the weld defects were not always fullydocumented. As a consequence, it is difficult from theexperimental results presented in these individual studiesto derive techniques, other than reducing the weldingspeed, that can be used to eliminate or to suppress theseweld defects.

Over the years, various conceptual models have beenproposed to explain the humping phenomenon. Thesemodels suggest that fluid flow and the high velocity andmomentum of the backward directed flow of moltenmetal in the weld pool, arc pressure, metallostaticpressure, capillary forces and lateral instability of acylindrical or curved wall jet of molten weld metal andthe periodic solidification of this curved wall jet areresponsible for the formation of humping and under-cutting. Some models suggest that at high weldingspeeds and welding currents, a very thin film of moltenweld metal exists underneath the arc and that the moltenweld metal is displaced at very high velocities toward thetail of the weld pool through this thin film or curved walljet. The backward momentum of the molten metalappears to be a critical factor, because several weldingtechniques and processes have been demonstrated to beeffective in suppressing weld defects by slowing downthe backward flow of molten weld metal. Nevertheless,the exact formation mechanism of high speed welddefects for each different welding process is unclear andis still being debated. Although some of the proposedmodels can provide plausible explanations to theperiodic behaviour of the humping phenomenon, theirability to predict the onset of humping or undercuttinghas not been proven experimentally. From a technolo-gical perspective, most of these models do not directlyprovide nor comprehensively explain possible techniquesor processes that might be used to achieve higherwelding speeds without the formation of defects such asundercut and humping.

Acknowledgements

The present work was supported by Natural Sciences andEngineering Research Council of Canada (NSERC),Ontario Research and Development Challenge Fund(ORDCF) and its partners Alcan International, Babcock&Wilcox, Canadian LiquidAir Ltd, Centerline (Windsor)Ltd, John Deere, Magna International Inc., Ventra.

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