Laser joining of aluminium to hot-dip galvannealed steel for the use of car bodies

Abstract

Laser welding-brazing (LWB) process with filler wire was developed to join 6000 series aluminium alloy (AA6451) and hot-dip galvannealed (GA) mild steel. LWB is useful to minimize brittle Fe-Al intermetallic compounds (IMCs) that are generated at the interface of the joint and severely degrade the joint strength. The laser melts only filler wire and part of the aluminium and the molten wire braze onto the solid state surface of the steel. Hot-dip galvanized (GI) mild steel also tested to compare the effects of zinc coating layers of each steels. For the brazing trials, these materials were cleaned and fitted up in one of two joint configurations: overlap or flare bevel groove joint. And four kinds of filler wires, AlSi12, AlMg5, ZnAl15 and flux cored wire (FCW) were tested for each joint configuration. Having identified the most suitable LWB parameters that best met requirements for braze quality and property characterisations, the selected braze joints have undergone a detailed qualitative and quantitative analysis. At the high laser moving speed of 3m/min, braze joints of aluminium on GI steel shows better joint strength than those of aluminium on GA steel. The failures of tensile test samples with the GI steel occurred across the heat affected zone (HAZ) of the aluminium, whereas those with the GA steel occurred at the joint interface. This result could be explained by the presence of the pure zinc layer (melting temperature of 419℃) of GI steel which melts before AlSi12 filler wire (melting temperature of 582℃) and help to spread the molten filler wire by forming liquid layer on the steel surface. On the other hands, since Fe-Zn alloy layer of GA steel melts at a high temperature close to 1200℃, thus this layer requires higher heat energy to remove it before wetting. The higher heat energy can be assumed that promotes the growth of IMCs at the interface of the joint which reduce the joint strength. When the laser moving speed was slower down to 1m/min, the joint strength of aluminium and GA steel was increased and the failure of the tensile samples occurred in a combination of the braze throat and the interface of the joint. This is likely to be due to the larger braze widths with thin IMC layers by granting more time to feed the filler wire and remove the zinc coating of the steel parent material with lower heat energy conditions.