Design aspects and Quality issues of Metal Inert Gas Welding

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 (see Appendix B – Weld Joint Configurations).
• 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 thickness¼0.5mm (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 thicknesses5 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 typically0.5 mm.

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