KNOW YOUR MATERIALS
While this feature focuses on welding mild steel, which remains a major material used for car bodies, the need to reduce weight and improve crash resistance has seen other materials being used, including aluminium. In less exotic mass-produced vehicles, various types and grades of both High Strength Steel (HSS) and Ultra High Strength Steel (UHSS) are employed in critical locations, such as door pillars and even bumper mounts. In general, the greater the steel’s strength, the harder it is to weld, and introducing heat could change its composition, or even cause it to crack. This will affect the crashresistance properties of a car in the future.
Should you be considering buying and repairing a fairly modern crash-damaged vehicle, it may be worth consulting the manufacturer about the material and construction methods used in the damaged area, because this will determine whether, or not, an economical DIY Mig-welded repair is feasible.
Some car-makers provide information on whether, or not, Mig-welding techniques are suitable to repair their grades of HSS and UHSS, such as these examples from General Motors: – www.genuinegmparts.com/pdf/techinfo/steelrepairability-matrix.pdf – and Ford – https://rts.i-car.com/ images/pdf/oem-info/ford/16733.pdf
Should vehicle-specific data be unobtainable, Thatcham Research specialises in model-specific crash repair methods, but its data tends to be available only to professional bodyshops, a local branch of which might be willing to share this information with you. Attempting to Mig-weld two steel sections with incompatible tensile strengths may be almost impossible and the most common advice is to drill-out the original spot-welds and replace the complete panel. However, even this may require more specialist hardware, such as a spot-welder or even a pulse MIG welder, both of which tend to be beyond the reach of a DIY mechanic.
Consider also that manufacturers have found ingenious ways of bonding different materials, such as joining together aluminium and steel using structural adhesives. Therefore, be wary that Mig-welding an adjoining panel might introduce heat that causes further damage.