Joining immiscible steel and magnesium alloy by Friction Melt Bonding process

In the automotive industry, the main driving force for joining Mg alloys to steel is to obtain a lighter structure that reduces fuel consumption, therefore decreasing greenhouse gas emissions. Joining steel with lighter materials, i.e., aluminum alloys, has been one of the strategies used by automotive manufacturers to deal with environmental regulations. Nevertheless, magnesium alloys are approximately 30 % lighter than their aluminum counterparts, thus making them potential candidates for replacing the latter in automotive applications. In this context, the present work focuses on characterizing and understanding the Mg-to-steel joint as produced by a UCLouvain patented welding process, Friction Melt Bonding (FMB). The materials used in this study were AZ31 magnesium alloy and galvanized S235 steel. Through the use of several microstructural characterization techniques and fractography analysis, preliminary results suggest that the main joining mechanism is related to brazing/soldering provided by the Zn coating being rapidly squeezed out of the weld center, in a similar way to what occurs in resistance spot welding (RSW). In addition, increasing the tool angular speed (rpm) was found to be the most effective way to increase the heat input, which, in turn, translated into lower values of hardness due to the softening produced by grain growth, detwinning, and second-phase dissolution. However, an inverse relationship was found for lap shear strength (LSS), where samples welded with higher energy (i.e., with higher rpm) exhibited greater strength, which based on the joining mechanism might be attributed to overall better conditions for interdiffusion to take place.