What is welding?

The base metal melts at the joint, and once it solidifies the parts form one piece. TIG uses a non-consumable tungsten electrode under an inert gas, with filler added separately or no filler at all. It is a controlled, clean process, suited to thin material, stainless steel, aluminium and cosmetic welds. In MIG and MAG the wire itself is the consumable electrode, fed from a spool through the torch. MIG uses an inert gas, for example on aluminium; MAG an active gas mixture, on steel. Both deposit more weld metal per hour than TIG.

What decides the choice of welding process?

Which process fits depends on material, thickness, joint type, welding position, access, required appearance and the amount of weld metal. Heat input makes the part shrink and distort and leaves residual stress; welding sequence, tacking, a welding fixture and pre-setting limit this but do not remove it. On stainless steel, heat tint lowers corrosion resistance until it is removed. Some steels and thicknesses need preheating to avoid cracking.

What does the standard say about welding?

Welds appear on the drawing as symbols to ISO 2553, with joint type, size and optionally the process number. For a fillet weld, the a dimension is the throat thickness and the z dimension the leg length; the letter before the value says which is meant. Process numbers come from ISO 4063, for example 141 for TIG, 131 for MIG and 135 for MAG with solid wire. ISO 5817 sets quality levels B, C and D for imperfections in fusion welds in steel, nickel and titanium, B being the most stringent; aluminium has ISO 10042. Load-bearing steel and aluminium structures fall under EN 1090; the requirements for welding coordination, qualified welders and qualified procedures depend on the execution class.

How does welding work in practice?

A welding line in the estimate is more than weld length: joint preparation such as bevelling, tacking in a fixture, the welding itself, straightening and often machining afterwards. Fits, bores and mounting faces with a tight tolerance are usually machined after welding, in a setup of their own. Stainless work may be followed by pickling and passivation. A structure under EN 1090 also carries documentation and qualifications that take time. How much each item counts varies by structure and by shop.

What is welding often confused with?

TIG is not the same as MIG: TIG uses an electrode that does not melt and is slower, while in MIG and MAG the wire melts off and weld metal goes down faster. And welding is not brazing or soldering, where only the filler melts and the base metal does not.