theory of joining and cutting ctu in prague faculty of...
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CTU in Prague
Faculty of Mechanical Engineering
Ing. Petr Vondrouš, PhD., IWE
Theory of Joining and Cutting
1st semester 2015/2016
TJC - Background
Subject name: Theory of Joining and Cutting
Abreviation: TJC
Department of Manufacturing Technology
Total hours: 52,
Lecture 2 hrs/week
Practice 2 hrs/week
Finished: assessment + exam
Language: English
Teachers:
Ing. Vondrous: [email protected]
Ing. Kovanda, Ing. Kolarik, Ing. Rohan, Ing. Kramar
During practices others
+ engineers from companies during visits
TJC - Background
Contents:
Course covers technologies of welding, brazing and thermal cutting - SMAW, GMAW, GTAW, SAW, oxyacetylene cutting, plasma cutting..
Welding equipment and typical application of technologies in the industry is done.
Covered is also topic of material weldability and assessment of joint quality by destructive and non-destructive methods.
Assessment: 1. presence at the practices – max. 2 absences
2. submission and presentation of project report
Exam: Final evaluation of the subject is based on project and oral examination.
TJC – Expected knowledge
for students with finished Bc. degree
good level of language and technical knowledge is expected
Expected: good level of English
knowledge of materials, drawings, technology on the level of Bc. degree
do study at home if you lack some knowledge
Books available at CTU library and also at National Technical Library.
Internet – Focus on scientific and company information.
Eg.: www.scirus.com, scholar.google.com
Responsibilities of welding engineer
1. What does he do?
2. What is his responsibility?
3. What he must know?
4. Who is he subjected to do?
5. Who is he supervising?
6. Who does he cooperate with?
KNOWLEDGE – process, machines, materials,
drawings, norms
COOPERATION – welders, clients, boss
LEGAL RESPONSABILITY
TJC – Overall content 1
Order /
Day
Contents - lecture Practice
1 Organization, Contents, Welding industry,
technologies, mat., weld joints
Project introduction,
safety, welding catalogue
2 SMAW – arc, technology, use, limits, flux SMAW-labs
3 GMAW, GTAW – technology, welding torch,
use, SAW
GMAW, GTAW-labs
4 Laser welding, EBW, RSW Labs
5 Material weldability CET, preheat temperature
6 Excursion Exc. –Meiller-Kipper
7 Types of weld joints, joint preparation Load capacity of weld
8 Thermal cutting Oxy-fuel, laser cutting
9 Quality control, DT, NDT DT, NDT
10 Welding quality – welders qualification Welders qualification
11 Welding process qualification Process qualification
12 Excursion Excursion-Metrostav
13 Project discussion Assessment, project
closure
TJC –Contents of 1st lecture
1.Joining, welding process
2.Some specifics of welding industry
3.Technologies overview
4.Materials used
5.Weld joint design
Manufacturing process
Basic group of technologies
change of material`s shape
material removal
material addition
Where and why is welding important technology?
1. casting2. forming, forging
3. machining
4.assembly5. welding6. additive manufacturing
Joining processes
Aim: to join parts together to form assemblies which provide useful mechanical functions
E.g.: sewing with thread, zips, buttons, bricks and mortar, wood joints(dove-tailed joint)
Evolution of joining: sewing, weaving, dove-tail joint, metal rivets, srewed connections, welding
Permanent
Welding
Brazing
Adhesive
Rivets
Press Fit joints
Nails
Removable Fasteners
Bolts
Screws
Pins
Circlips
Keys
Welding processes
Welding – technology creating pernament connection between parts by creating atomic bonds between welded pieces accompanied by diffusion
Conditions of weld creation can be fullfiled by:
a)heating the pieces to melting point and fusing them – FUSION
b)applying pressure to the pieces in cold or hot state – PRESSURE
Advantages of welding:
Strong and tight joining; Cost effectiveness; Simplicity of welded
structures design; Welding processes may be mechanized and
automated.
Disadvantages of welding:
Internal stresses, microstructure change, distortions in the weld. Harmful
effects: light, ultra violate radiation, fumes, high temperature.
Applications of welding: Buildings and bridges structures; Automotive,
ship and aircraft constructions; Pipe lines; Tanks and vessels; machinery
Fusion welding processes
Pressure welding processes
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Importance: To Manufacturing
• Welding plays a key role in the
manufacturing challenge
• Welding is a key contributor to:
• Heavy Manufacturing
• Light Manufacturing
• Construction
• Transportation
• Electronic/Medical
• Maintenance & Repair
• Energy
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• Welding is viewed as “dark & dirty”
• The critical role of welding is not fully appreciated in all corporate
and government offices.
• High schools phasing out welding programs and discouraging
careers despite annual earnings potential of $30,000 - $60,000.
• 1/2 of US industries report difficulties locating qualified welders.
• The US infrastructure is aging.
• Shortage “barrier” resulting in fewer competitive welding
techniques, less productivity and outsourcing to foreign markets.
• Natural disaster prone areas have immediate need for welders to
quickly rebuild homes and cities.
Status: Welding Industry
– Not the case anymore.
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Status: Career Paths• Welder
Extensive and
growing job variety
• Engineers
• Researchers
• Business Owner
• Teachers
• Salesperson
• Machine Operator
• Underwater Welder
• Brazer
• Robotic Arc Welding
Technician/Operator
• Radiographic
Interpreter
• Inspector
• Maintenance and
Repair Tech.
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Status: Opportunities
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Status: Where Welders Work
• Military
• Aircraft and Aerospace
Industry
• Building Construction
• Automotive Industry
• Bridge/Highway
Construction
• Shipbuilding Industry
• Universities and Schools
• Safety Products
• Boiler Industry
• Medical Industry
• Mining Industry
• Robotics and Computer
Engineering
• Farming Equipment
Manufacturers
• Job Shops
• Specialty Gases
• Consumer Electronics
Civil engineering - Structures
Bridges
Buildings
Highways
Used technologies?Used materials?
Energy generation, pipelines
Used technologies?Used materials?
Transportation
Used technologies?Used materials?
Car
Used technologies?Used materials?
Is result of welding acceptable?
What is weldability?weldability is technological property that expresses suitability of material to create acceptable weld joint fulfilling all requirements by specific welding method and conditions
– Metallurgical Capacity• Parent metal will join with the weld metal without formation of deleterious constituents or alloys
– Mechanical Soundness• Joint will be free from discontinuities, gas porosity, shrinkage, slag, or cracks
– Serviceability• Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments,
fatigue, high pressures, etc.)
Welding technology
Welded material Structure design
All 3 have influence on the welding result.
Brief History of Welding
Late 19th Century
– Scientists/engineers apply advances in electricity to heat and/or join
metals (Le Chatelier, Joule, etc.)
Early 20th Century
– Prior to WWI welding was not trusted as a method to join two metals
due to crack issues
1930’s and 40’s
– Industrial welding gains acceptance and is used extensively in the
war effort to build tanks, aircraft, ships, etc.
Modern Welding
SSSR - Paton
USA -
France -
UK – TWI
Czechoslovakia
NOWADAYS SO MANY METHODS
Types of Welding
Fusion Welding Pressure Welding
Homogeneous Heterogeneous
Brazing SolderingGas Welding
Electroslag
High Energy Beam
Electric Arc
MIG
TIG
Shielded Metal Arc – “Stick”
Friction Welding
Other schemes, more detailed may exist as well-but basically divided by ENERGY SOURCE.
Fusion welding X Pressure welding
Fusion welding - Material is over the melting, fusion
temperature, upon cooling joint is created.
Base metal is melted, filler metal may be added
Heat is supplied by various means
– Oxyacetylene gas
– Electric Arc
– Plasma Arc
– Laser
Pressure welding – external force is needed, in molted or solid
state
– Diffusion welding
– Resistance welding
– Explosion
– Solid state welding
Joint Design
BUTT
JOINT
STRAP JOINT
LAP JOINT
FILLET JOINT
CORNER JOINT
Welding Positions
FLAT
HORIZONTAL
VERTICAL
OVERHEAD
INCREASING DIFFICULTY
Generalized Welding Symbol
FAR SIDE DETAILS
ARROW SIDE DETAILS
Field weld symbol
Weld all-around for
pipes, etc.
L1-L2
L1-L2
D = Weld Depth (usually equal to plate thickness)
L1 = Weld Length
L2 = Distance between centers for stitched welds
The Field Weld Symbol is a guide for installation. Shipyards
normally do not use it, except in modular construction.
Electrode
Material
D
D
Weld Geometry
Example Welding Symbol
5 5
5
5
One-sided welds are max 80% efficient
Two sided are 100% efficient
Geometry symbol for Y-groove5 150
Welded materials
Most widely used engineering materials:
Ferrous materials
Steels
Cast irons
Non ferrous
Al, Ti, Mg, Ni alloys
Where are these materials used?
What properties of the material and about welded structure we must know
before commencing the welding?
Material properties and weldability
What properties influence weldability?
Properties Examples
Mechanical properties Strength Young modulus Ductility
Physical properties Density Thermal expansion Heat conductivity
Chemical properties Reactivity Corrosion Flammability
Manufacturing properties Castability Weldability Formability
Cost, availability Prize Terms Place
Fusion welding
• These processes are associated with molten metal-
atmospheric reactions
– Oxides and nitrides are formed –oxide layer,
– Change of chemical composition
– Discontinuities such as porosity – emental cheese, leak proof ?
– Poor weld metal properties
• Need of shielding – during heating and cooling
– Vacuum
– Gas shielding – inert, active gasses
– Gas forming components
– Liquid layer – molten slag
Heat source energy density
• Heat source density influences speed of welding,
heat input per unit length, fase transformations and their
dynamics.
with increasse of energy density HAZ area, distortions
become smaller and welding speed increaces.
Oxyacetylene Welding
• Flame formed by burning a mix of acetylene (C2H2) and oxygen
• Fusion of metal is achieved by passing the inner cone of the flame over the metal
• Oxyacetylene can also be used for cutting metals
Inner Cone: 3300°C Combustion Envelope 2000°C
1200°CTORCH TIP
VIDEO - OAW
Arc welding processes
• Welding processes that employ an electric arc are the most
prevalent in industry
– Shielded Metal Arc Welding
– Gas Metal Arc Welding
– Flux Cored Arc Welding
– Submerged Arc Welding
– Gas Tungsten Arc Welding
SMAW
• Easy, equipment inexpensive, portable
• Filler metal and shielding of the weld puddle are provided by the
covered electrode
• Less sensitive to drafts, dirty parts, poor fit-up
• Can be used on carbon steels, low alloy steels, stainless steels, cast
irons, copper, nickel, aluminum
SMAW - MMA - drawbacks
• slow, low deposition rates, low productivity
• manual – operator dependent
• heat input is very high, small energy density
• Inadequate weld pool shielding for reactive metals – Ti, Zr, Ta
GMAW – MIG/MAG
There are four methods in which the wire is transferred to the molten pool:
•short circuiting
•globular
•pulsed spray
•spray transfer
AWS - Welding Handbook
GMAW – MIG/MAG – Metal transfer modes
According to setting of the welding parameters, there are 3 modes of molten
wire transfer to the molten pool:
•short circuiting – small welding parameters,
till 150 A, 20-200 Hz, typical for MMA
•globular – 180 A , Ar–2% O2 shielding
•spray transfer (for special invertor welders pulsing of current can be done -
pulsed spray transfer)
Above a critical current level, high
electromagnetic force (Lawrence force) -
small discrete metal drops, figure shows
spray transfer during GMAW of steel at 320A
and with Ar–2% O2 shielding.
Shielding in GMAW
Shielding gas can affect
– Weld bead shape
– Arc heat, stability, and starting
– Surface tension
– Drop size
– Puddle flow
– Spatter
Important gas characteristics:
ionization potenrial – arc start, stability, arc heat – heat input
heat transfer ratio – heat input
influence on surface tenstion – drop size, puddle flow, spatter
Ar Ar-He He CO2
GMAW characteristics
Advantages
•High deposition rates (higher than SMAW)
•Continuous electrode
•No slag creation
•Easily automated, versitality
•Deeper penetration (compared to SMAW)
Drawbacks
•Gun size
•Many process variables
Use in automotive:
•continuos, sealing welds - exhaust systems
•robotic welding of steel and Al
•arc spot welding can be variant to RSW
FCAW Flux cored arc welding
Uses tubular wire with flux inside.
Advantages
•High deposition rates (higher than SMAW)
•Continuous electrode
•No slag creation
•Easily automated
Drawbacks
•Gun size
•Many process variables
FCAW characteristics
Advantages
•Less cleaning, good self shielding protection
•High deposition rates (higher than SMAW)
•Continuous electrode
•Easily automated
•Deeper penetration
Drawbacks
•Slag creation
•Fumes
•Price
•spatter
Very reduced use in automotive:
•only small series production
•stainless welding, mufflers
SAW submerged arc welding
- use of granular flux which is fed into the joint from a flux hopper
- arc is struck between the wire and the workpiece beneath the flux
- arc and weld pool are shielded by the resulting envelope of molten flux
SAW submerged arc welding
Advantages
•continuous wire electrode – high diameter (max 5-6
mm)
•high current (1000 A), high deposition rates – for
thick materials
•no arc flash or glare
•Minimal smoke and fumes
•flux and wire added separately - extra dimension of
control
•easily automated
•joints can be prepared with narrow grooves
•used to weld carbon steels, low alloy steels,
stainless steels, chromium-molybdenum steels,
nickel base alloys
•twin wire, multiple torch …
Drawbacks - slag creation, much heat input, welding
process not visible
GTAW – TIG
- gas tungsten arc welding - arc is established between the tip of a tungsten
electrode and the workpiece to melt the base material (possibly also filler)
-tungsten electrode is considered to be non-consumable
-inert shielding gas protects the molten weld pool
GTAW – change of polarity
- non consumable electrode and invertor welder makes possible change of
polarity, stable arc, pulsating
Superior quality welds, generally free from spatter, porosity, or other defects
-Precise control of arc and fusion characteristics
-Weld almost all metals
-Used with or without filler wire
-Easily automated
-Used in all positions, intricate geometries weldable
GTAW - use
for thin sheets – very precise
sealing joints
precision welding
atomic energy, aircraft – turbine blades
repair of dies, forming tools
no spatter,
small distortion, high precision
especially for highly alloyed materials - SS, HSS, AHSS
especially for light alloys - Al, Mg
heat exchangers, exhausts, pressure wessels
PAW – Plasma arc welding
energy level of the arc plasma is constricted by flow of gas to increase energy
density in controlled manner
use of special gas nozzle around a tungsten electrode operating on DCEN
two variants - transferred arc process
- non-transferred arc process
PAW
-plasma gas – clean argon
-shielding gas – argon, helium, mixtures
Narrower welds can be made with PAW than with GTAW because of the
constricted arc. PAW can also be used on Titanium and other reactive metals.
High energy density – keyholing effect
Advantages
Greater Energy Concentration
Improved arc stability over other processes
Higher heat content, Higher travel speeds
Greater penetration capabilities
Finer sections can be welded with low current PAW than GTAW
Disadvantages
Operator skill required is slightly greater than for GTAW
Equipment more expensive
Orifice cleanliness, replacement
PAW - use
Gives better control than GTAW.
for thin sheets – very precise
sealing joints
precision welding
atomic energy, aircraft – turbine blades
repair of dies, forming tools
no spatter,
small distortion, high precision
especially for very thin materials – heat exchangers (Al)
fast – 5,5 m/min
high alloy materials, SS,
tailored welded blanks
very deep penetration
High energy beam welding
High Energy Density Beams – increase of energy density
possibility of keyhole welding
Plasma Arc welding
Electron beam welding
Laser beam welding
These processes focus the energy onto
a small area
EBW – electron beam welding
Deepest single pass weld penetration – max 300 mm
Fast travel speeds
Low heat input welds produce low distortion
Question:
Why EBW can produce the
deepest welds? What is the
biggest drawback of this
method?
LBW – Laser Beam Welding
principles of LBW will be explained in detail in specialized presentation
Heat
treatment Welding Cutting
Focal spot positionQuestion:
How can focal spot
position influence
the process?
Pressure welding
Resistance welding – spot
The resistance of metal to the localized flow of current produces heat
Process variables
Current
Time
Force
Spot and seam welding
used extensively for sheet metal joining
automotive, appliances …
High speed, < 0.1 seconds in automotive spot welds
Excellent for sheet metal < 5 mm
No filler metal
contact resistance between sheet metals is much higher than the bulk
resistance of the copper electrodes or of the sheet metal itself and the
highest resistive heating occurs between the two pieces of sheet metal
Resistance welding – seam
the welding electrodes are motor-driven wheels
power source is controled to produce ovelapped, water-tight
weld-gas tank
Roll spot weld Overlapping seam weld Continuous seam weld
Resistance welding
BIW has upto 5000 spot welds
process limitations:
voluminous equipment – non portable
only lap joints
high increase of current demand with thickness
dificcult NDT testing – nugget non visible
nugget acts like stress concentrator- low tensile
and fatique strength
Adaptive quality control for RSW
With increase of speed of electronics
devices, online NDT testing of weld
nuggets have become available.From Bosch Rexroth
Stud welding – arch stud welding
-stud is initially in contact with the plate
-use of ceramic sleeve (ferrule provides shielding and support)
-solenoid lifts the stud off the plate and arc is established an arc
-after an appropriate amount of arcing – heat input, the stud is driven into the
workpiece
Stud welding - capacitator discharge
-variation of the stud welding
-energy stored in a capacitor is discharged,
initiating an arc and melting
-stud is then forced into the workpiece
Stud welding - use
-very versitile
-fast
-economical
-automation industry - fixation of screws
-fastening, assembly of metal sheets (steel, Al, Mg), plastics …
-construction industry
-electronics, home eppliances
Pressure welding – solid state welding
Processes that produce a weld through the application of pressure at a
temperature below the melting temperature of the base material; no filler metal
is used.
Friction welding, friction stir welding
Diffusion welding
Ultrasonic welding
Explosion welding
By joining materials in the solid state, many of the difficulties of the fusion
processes are avoided.
dissimilar metals joining-thermal expansion mismatch between the two is
decreased in solid state processes because of the lower temperature
Friction welding - FRW
The workpieces are brought into contact and rotated very rapidly to produce
heat. Usually one piece is rotated against a stationary piece to produce the
heat at the junction.
Used for round bars, tubes, joining of bars to sheet …
fast, easy automation
easily joins Cu, Al, Cu+Al, Cu+steel, plastics
Drawbacks: clamping forces, rotational symmetry, forgeable
material is neccessary, S in steel is not good
Ultrasonic welding - USW
The parts to be joined are held together under pressure and are subjected to
ultrasonic vibrations at right angles to the contact area.
Ultrasonic welding processes occur when vertical oscillations at frequencies of 10
to 50 kHz are transmitted through polymers and dissipated in a bond line. Heat is
generated through a combination of friction and hysteresis.
Ultrasonic welding - USW
• Cost-effective process
• Large batch-sizes are possible – cycle time about 300 ms
• Very short cycle time
• clean
• precise
• Typical for thermoplastics, Al sheet, Cu wires
• Electrical connections, electronics, circuits – wires, chips, memories
• multiple ply joints – batteries
• hermetic sealing of containers at low temperature - lighter
Weldability of a Metal
• Metallurgical Capacity– Parent metal will join with the weld metal without
formation of deleterious constituents or alloys
• Mechanical Soundness– Joint will be free from discontinuities, gas porosity,
shrinkage, slag, or cracks
• Serviceability– Weld is able to perform under varying conditions or
service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.)
Weld Metal Protection
During welding, the molten and heated metal is susceptible to
oxidation – need to protect weld from the atmosphere
•How?
– Weld Fluxes
– Inert Gases
– Vacuum
SMAW, SAW - Fluxes can be SiO2, TiO2, FeO, MgO, Al2O3
– Produces a gaseous shield to prevent contamination
– Act as scavengers to reduce oxides
– Add alloying elements to the weld
– Influence shape of weld bead during solidification
GMAW, GTAW – gases as Argon, helium, nitrogen, and carbon dioxide form a protective envelope around the weld area
Data source
This presentation used data from:
-Ladislav Kolařík, Ústav strojírenské technologie,
CTU in Prague
-AWS, American Welding Society
-other sources – list upon request