TWI Industrial Member Report Summary 697/2000
D S Howse
Background
In the design of lightweight body in white steel structures for the automotive industry, one of the main objectives, in terms of joining processes, is to produce stiffer joint connections than conventional resistance spot welding. The advantage for design would be that weight savings could then be derived from a reduction in material gauge using higher strength steel without compromising component strength or stiffness. Hence, it is almost inevitable that a continuous process as opposed to a point joining process will find greater application in lightweight vehicles.
Although amongst the manufacturers of vehicles, Nd:YAG laser welding, weld bonding and riveting are likely to be the dominant joining processes for the future, suppliers to the automotive industry will still rely heavily upon arc welding as a joining technique to produce sub-component parts. In addition, although all steels used in the ultra light steel autobody design are considered to be weldable, as higher strength, thinner steels are used, the corrosion performance becomes more critical. The margin of safety is less than for thicker materials and all the steels used in a lightweight vehicle design are likely to be zinc coated.
Zinc coated steels can be arc welded but they are prone to porosity formation. For MAG fusion welding, work has been carried out to show that an increase in welding current and welding speed increases the tendency to form porosity. However, in manufacturing thinner gauge steels in a production environment, welding at high speed is important in terms of both increasing throughput and reducing heat input and, therefore, distortion potential of the welding process. Another factor that has been studied is joint preparation. It has been concluded that the joint most susceptible to porosity formation is a lap joint with no gap as there are two coated surfaces being melted and the only way for the vaporised zinc to escape is through the molten pool. Increased welding speed on a lap joint is precisely the condition that most manufacturers within the automotive industry require and as such, the tendency will be for porosity formation.
The work carried out here investigated possible solutions for manufacturing components in high strength, thin, coated steels using MAG welding techniques giving optimum productivity, reduced distortion and a low defect rate.
Objective
- To produce guidelines for making MAG welds in galvannealed zinc coated steels with low porosity at optimum productivity and reduced distortion.