Welding Processes, Metallurgy
and Defects
Summer 2010
Mikal Balmforth, P.E., CWEng, CWI
MIT Research Associate
MIT Welding Laboratory
Overview
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Major welding processes
Some welding metallurgy topics
Common weld defects
Other welding related issues
Arc Welding Processes
• Shielded metal arc welding (SMAW)
– Also stick or MMA welding
– Simple, common process
– Consumable, flux coated
electrode
• Gas tungsten arc welding (GTAW)
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Also TIG welding
Non-consumable tungsten
Inert gas shielding
Clean, precision work
Filler added externally
electrode
Arc Welding Processes
• Gas metal arc welding (GMAW)
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Also MIG or MAG welding
Consumable wire electrode
Gas shielding
Often automated
• Flux cored arc welding (FCAW)
– Similar to GMAW
– Hollow electrode wire filled
with flux
– Often no shielding gas
Arc Welding Processes
• Others
– Submerged arc welding (SAW)
– Plasma arc welding (PAW)
– Underwater welding
Resistance Welding Processes
• Spot welding
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Sheet metal
Two copper electrodes
Heat from resistance at interface
Autos have thousands of spot welds
• Seam welding
– Wheel shaped electrodes
• Projection welding
– Dimples or projections in sheet or components
concentrates heat
• Upset welding
– Components forged at elevated temperature
Solid State Welding Processes
• Friction welding (FRW)
• Inertia welding (IFW)
• Friction stir welding (FSW)
Solid State Welding Processes
• Diffusion welding (DFW)
– Often used in aircraft and aerospace
• Ultrasonic welding (USW)
– Used for plastics and electronic components
• Explosion welding (EXW)
– Often dissimilar metals or cladding
High Energy Density Welding
Processes
• Narrow, deep welds are produced
– Lower heat input and narrower HAZ
• High speeds are possible
• Laser beam welding (LBW)
– Solid state or gas lasers
– Usually done in air
High Energy Density Welding
Processes
• Electron beam welding (EBW)
– Concentrated stream of
high-velocity electrons
– Usually done in vacuum
– Can be high power
Allied Processes
• Brazing and soldering (450°C, 840°F)
• Oxyfuel gas welding
• Thermal cutting
– Oxygen, plasma arc, other
– Gouging
Regions of a Fusion Weld
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Fusion zone (FZ) – weld metal
Fusion boundary (FB)
Heat affected zone (HAZ)
Unaffected base metal (BM)
FB
FZ
HAZ
BM
Fusion Zone
• Autogenous, homogeneous or
heterogeneous
• Molten to solid
Heat Affected Zone
• Grain growth, recovery and
recrystallization, precipitates, stresses
Welding Metallurgy Issues
• How does the heat from welding affect the
material and its properties?
• Control microstructures and prevent
defects
• What filler metals, processing conditions,
and welding techniques to use
– Chemistry
– Heat input
– Welds often require pre- and/or post-heating
Classification of Weld Defects
• Fabrication
• Delayed (hydrogen-induced)
• Service
Fabrication Defects
• Porosity
• Incomplete fusion
• Incomplete penetration
• Undercut
Fabrication Defects Cont.
• Solidification cracking
– Also called hot or
centerline cracking
• HAZ hot cracking
• Inclusions – slag
or tungsten
Fabrication Defects Cont.
• Reheat cracking
• Liquid metal
embrittlement (LME)
Fabrication Defects Cont.
• Lamellar tearing
• Incorrect weld
shape or size
• Spatter
Delayed (H-induced) Defects
• Usually initiates in course grained HAZ,
but can occur in weld metal
• Four requirements:
– Susceptible microstructure
– Source of hydrogen
– Low temperature
– Tensile stress
Service Defects
• Fatigue failures
– Cyclic loading conditions
– Cracks propagate from weld defects
• Mechanical overload
– Ductile rupture can initiate at defects
Service Defects
• Corrosion
– Intergranular corrosion
(IGC)
• Localized grain boundary
attack
• Sensitization in austenitic
stainless steels
– Stress corrosion cracking
(SCC)
• Tensile stress combined
with corrosive media
• Transgranular or
intergranular crack
propagation
Significance of Weld Defects
• Failure initiation sites
• Reduced tensile and other mechanical
properties
• Reduced fatigue life
• Toughness / Fracture mechanics
– Allowable defect size
Other Welding Related Issues
• Codes and Standards / Certification
– WPS, PQR, WPQR
– AWS, ASME, API, etc.
• Testing / Inspection
– Metallography, SEM, mechanical testing
– NDE
• Visual, dye penetrant, ultrasonics, x-ray, magnetic
particle, eddy current
• Safety
– Worker, workplace, fumes, fire prevention
• Weld repair
• Stresses and distortion