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Manufacturing Processes > Joining processes > Metal Inert-Gas Welding(MIG)

 

Metal Inert-Gas Welding(MIG)

 

Process description

An electric arc is manually created between the workpiece and a consumable wire electrode at the joint line. The parent metal is melted and the weld created with the continuous feed of the wire which acts as the filler metal. The weld area is shielded with a stable stream of argon or CO2 to prevent oxidation and contamination.

Materials

Carbon, low alloy and stainless steels. Most non-ferrous metals (except zinc) are also weldable; aluminum, nickel, magnesium and titanium alloys and copper. Refractory alloys and cast iron can also be welded. Dissimilar metals are difficult to weld.

MIG

Process variations

  • Portable semi-automatic (manually operated) or fully automated d.c. systems and robot mounted.
  • Three types of metal transfer to the weld area: dip and pulsed transfer use low current for positional welding (vertical, overhead) and thin sheet; spray transfer uses high currents for thick sheet and high deposition rates, typically for horizontal welding.
  • Shielding gases: pure CO2 or argon/CO2; mix commonly used for carbon and low alloy steels, or a mix of argon/helium, also used for nickel alloys and copper. Pure argon is used for aluminum alloys. High chromium steels use an argon/O2 mix.
  • MIG spot welding: used on lap joints.
  • Flux Cored Arc Welding (FCAW): uses a wire containing a flux and gas generating compounds for self-shielding, although flux-cored wire is preferred with additional shielding gas for certain conditions. Limited to carbon steels and lower welding rates.

Economic considerations

  • Weld rates from 0.2 m/min for manual welding to 15 m/min for automated setups.
  • Production costs reduced by high weld deposition rates with continuous operation.
  • Well suited to traversing automated and robotic systems.
  • Choice of electrode wire (Ø0.5–Ø1.5 mm) and shielding gas important cost considerations.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low to moderate.
  • Equipment costs low to moderate, depending on degree of automation.
  • Direct labor costs moderate to high. Skill level required is less than TIG.
  • Finishing costs low generally. There is no slag produced at the weld area, however, some grinding back of the weld may be required.

Typical applications

  • General fabrication
  • Structural steelwork
  • Automobile bodywork

Design aspects

  • All levels of complexity possible.
  • Typical joint designs possible using MIG: butt, lap, fillet and edge. MIG excellent for vertical and overhead welding .
  • Design joints using minimum amount of weld, i.e. intermittent runs and simple or straight contours wherever possible.
  • Welds should be balanced around the fabrication’s neutral axis where possible.
  • Design parts to give access to the joint area, for vision, electrodes, filler rods, cleaning, etc. MIG good for welds inaccessible by other methods.
  • Sufficient edge distances should be designed for and avoid welds meeting at the end of runs.
  • Provision for the escape of gases and vapors in the design important.
  • Distortion can be reduced by designing symmetry in parts to be welded along weld lines.
  • The fabrication sequence should be examined with respect to the above.
  • Minimum sheet thickness =0.5mm (6mm for cast iron).
  • Maximum thickness, commonly:
    • Carbon, low alloy and stainless steels; cast iron, aluminum, magnesium, nickel, titanium alloys and copper =80mm
    • Refractory alloys =6 mm.
  • Multiple weld runs required on sheet thicknesses ≥5 mm.
  • Unequal thicknesses possible.

Quality issues

  • Clean, high quality welds with low distortion can be produced.
  • Access for weld inspection important, e.g. Non-Destructive Testing (NDT).
  • Joint edge and surface preparation important. Contaminates must be removed from the weld area to avoid porosity and inclusions.
  • Shielding gas chosen to suit parent metal, i.e. it must not react when welding.
  • Wire electrode must closely match the composition of the metals being welded.
  • Slag created when using a flux-cored wire may aid the control of the weld profile and commonly used for site work (windy conditions where the shielding gas may be gusted or positional welding) and large fillet welds.
  • A heat affected zone always present. Some stress relieving may be required for restoration of materials original physical properties.
  • Cracking may be experienced when welding high alloy steels.
  • Self-adjusting arc length reduces skill level required and increases weld uniformity.
  • Backing strips can be used for avoiding excess penetration, but at added cost and increased setup times.
  • Need for jigs and fixtures to keep joints rigid during welding and subsequent cooling to reduce distortion on large fabrications.
  • Welding variables should be preset and controlled during production.
  • Automation can limit the ability to weld mating parts with large size and shape variations, however, the use of dedicated tooling does reduce distortion, improve reproduction and produces fewer welding defects.
  • ‘Weldability’ of the material important and combines many of the basic properties that govern the ease with which a material can be welded and the quality of the finished weld, i.e. porosity and cracking. Material composition (alloying elements, grain structure and impurities) and physical properties (thermal conductivity, specific heat and thermal expansion) are some important attributes which determine weldability.
  • Surface finish of weld good.
  • Fabrication tolerances typically ±0.5 mm.

costing