/ by Aldo Avancini /

Aldo Avancini looks at gas metal arc welding, a process developed after the Second World War that over the years has grown in importance for welded products /

Excluding some noble metals (stainless steel and titanium in their various alloys), today most permanent joints between steel elements are made using gas metal arc welding, with a continuous electrode (wire) and a shielding gas, commonly called “wire welding”. This technology, with the aforementioned exclusion of noble alloys, has now supplanted the shielded metal arc welding technology, especially in processes where time is of the essence.

Keeping the welding parameters under control is a determining factor in maintaining the right quality and guaranteeing performance; in both cases, the welding guns, either manual or robotic, are fundamental in obtaining the desired result.

Today, both air-cooled and water-cooled welding guns are available; these are also called “torches” because, once the arc is triggered, it also illuminates the area being welded.

I would like to make some considerations regarding these tools, starting with identification of the torch. First of all, the declared nominal output applies to short beads or in any case for a non-excessive continuous use, while for longer sections the value will be lower. This is because excessive use overheats the gas, which makes it harder to protect both the fused metal and the heated material. What does this mean? While for short welds (tack-welding) the unit ø can be used, for longer welds there will be a decline of 10-15% of nominal output.

Similarly, it should be considered that passing through the wire conduit and the power cable, the gas cools, heating up immediately at the outlet; therefore the most thermally stressed part is the torch outlet, which must be accurately controlled as the continuous supply of metal splashes adds to the high temperature in an area that is stressed by strong electrical discharges. It must be emphasised that these metal splashes are deposited at the outlet of the nozzle, in particular around the edges.

Correct gas flow (approximately 18/20 l/minute) and other precautions, such as not creating too-tight curves in the cable, perhaps wrapping it around your body, help avoid wire feed problems. It should also be remembered that if a difficulty is encountered in the continuity of wire feed, immediately highlighted by a non-constant flow, the first thing to consider is the path of the wire.

Automatically increasing the pressure on the feed rollers should be avoided. This action has negative effects: it increases the mechanical stress along the entire wire, in the worst case scenario distorting the cross-section circular to almost elliptical in shape and thus creating further friction at the outlet and worsening the result.

Furthermore, in the case of copper-plated wire, a situation may arise whereby the copper plating detaches due to mechanical action on the wire itself, with tiny crumbs of coating getting stuck either in excessively tight curves or at the wire outlet, with negative consequences on the weld quality.

Taken from Games&Parks Industry January 2021, page 76

Ing. Aldo Avancini /  Proposta Srl / proposta_design@yahoo.it

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