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Welding - Processes, Metallurgy and Defects. Summer 2010. Mikal Balmforth, P.E., CWEng, CWI MIT Research Associate MIT Welding Laboratory. Overview. Major welding processes Some welding metallurgy topics Common weld defects Other welding related issues. Arc Welding Processes.

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welding processes metallurgy and defects

Welding - Processes, Metallurgy and Defects

Summer 2010

Mikal Balmforth, P.E., CWEng, CWI

MIT Research Associate

MIT Welding Laboratory


Major welding processes

Some welding metallurgy topics

Common weld defects

Other welding related issues

arc welding processes
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)

Also TIG welding

Non-consumable tungsten electrode

Inert gas shielding

Clean, precision work

Filler added externally

arc welding processes4
Arc Welding Processes

Gas metal arc welding (GMAW)

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 processes5
Arc Welding Processes
  • Others
    • Submerged arc welding (SAW)
    • Plasma arc welding (PAW)
    • Underwater welding
resistance welding processes
Resistance Welding Processes

Spot welding

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
Solid State Welding Processes

Friction welding (FRW)

Inertia welding (IFW)

Friction stir welding (FSW)

solid state welding processes8
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
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 processes10
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
Allied Processes

Brazing and soldering (450°C, 840°F)

Oxyfuel gas welding

Thermal cutting

Oxygen, plasma arc, other


regions of a fusion weld
Regions of a Fusion Weld

Fusion zone (FZ) – weld metal

Fusion boundary (FB)

Heat affected zone (HAZ)

Unaffected base metal (BM)





fusion zone
Fusion Zone

Autogenous, homogeneous or heterogeneous

Molten to solid

heat affected zone
Heat Affected Zone

Grain growth, recovery and recrystallization, precipitates, stresses

welding metallurgy issues
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
Classification of Weld Defects


Delayed (hydrogen-induced)


fabrication defects

Incomplete fusion

Incomplete penetration


Fabrication Defects
fabrication defects cont
Fabrication Defects Cont.

Solidification cracking

Also called hot or centerline cracking

HAZ hot cracking

Inclusions – slag or tungsten

fabrication defects cont19
Fabrication Defects Cont.

Reheat cracking

Liquid metal embrittlement (LME)

fabrication defects cont20
Fabrication Defects Cont.

Lamellar tearing

Incorrect weld shape or size


delayed h induced defects
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
Service Defects

Fatigue failures

Cyclic loading conditions

Cracks propagate from weld defects

Mechanical overload

Ductile rupture can initiate at defects

service defects23
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
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
Other Welding Related Issues

Codes and Standards / Certification


AWS, ASME, API, etc.

Testing / Inspection

Metallography, SEM, mechanical testing


Visual, dye penetrant, ultrasonics, x-ray, magnetic particle, eddy current


Worker, workplace, fumes, fire prevention

Weld repair

Stresses and distortion