Joining high strength steels

Progress in arc welding and brazing high strength steels. Tom Shelley reports

Everyone agrees that high strength steels are key to weight reduction in a host of products, but that traditional dip arc and spot welding are neither efficient nor metallurgically effective ways of joining them. There are designers of low volume cars, who prefer to join high strength steel and other metal alloys by adhesive bonding sometimes with additional riveting. Welding high strength steel in such a way as to melt part of the steel, and subject the zone immediately adjacent to the weld to high temperature, tends to ruin the microstructures that have been carefully designed to increase strength. One solution that has been around for years is to use Metal Inert Gas (MIG) brazing, in which a filler rod is melted to join the steel parts. But, this does not produce sufficient heat to melt the steel or significantly change the microstructure in the heat-affected zone. Peter Brace, technical sales manager of Qalitas Technology, recently demonstrated this. He explained that his German made, Rehm electronically controlled welding unit incorporated an expert system database, so that the user only had to select material and thickness, and the control system did the rest. In addition, it incorporated Stepless Dynamic Inductance that means that it was in effect 95% efficient. This is opposed to the 35% efficiency typical of a process undertaken using a conventional welding transformer and digital feedback speed control for the advancement of the filler rod. He told us that the process was used to braze roofs onto Vauxhall cars back in the 1970s and 1980s, but problems were found because the steel available at that time had so many inclusions in it. The latest idea that has also been around for a few years is called Cold arc. While the process is still hot, the 'Cold' refers to the fact that the heat input to the parent steel is significantly less than in a conventional arc welding process. The arc is pulsed in such a way that each metal globule from the filler rod is driven across the gap to the weld by a separate pulse of power in such a way that the globule size is similar to that of the filler rod, which is quite thin, and tightly controlled. The control parameters are quite critical if it is to be successfully used to weld high strength steels. The company says it has undertaken a lot of development work with a major UK automotive component company. We understand that the trials have all been highly successful, but gather that the purchase of equipment is currently delayed pending an easing of the present state of the automotive business generally. The main purpose of the demonstration, which was held at Canterbury College under the auspices of The Welding and Joining Society (TWI) Kent Branch, was to interest users engaged in the aftermarket and repair trade. The primary concern of automotive designers, even more than reducing weight and saving fuel, is to ensure the safety of vehicle occupants. This goal has, by and large, been attained to an outstanding degree. But repairs to modern cars made of high strength materials using old fashioned gas and arc welding equipment are liable to render all the careful intentions of the designers invalid, and result in badly weakened, heat affected zones adjacent to welds. Pointers * High strength steels can be brazed using metal inert gas brazing without affecting the strength of parent metals in any way * High strength steels can also be joined using Cold Arc welding without loss of strength but welding parameters have to be very carefully controlled using modern solid state, power electronic equipment