Microstructural and mechanical evolution of Al/steel interface with Fe2Al5 growth in resistance spot welding of aluminum to steel

In present study, the interfacial microstructural evolution with intermetallic compound (IMC) growth and its influence on mechanical properties was investigated in resistance spot welding of aluminum to steel. Fe2Al5 was identified as the dominant phase in the reaction layer formed at the joint interface. It was observed that as Fe2Al5 grew, it evolved from randomly oriented fine grains to coarse grains with preferred orientation along [001] direction, perpendicular to the Al/steel interface, while the volume fraction of iron phase between the Fe2Al5 grains decreased. Via numerical simulation, it was found that the majority of Fe2Al5 growth occurred when interfacial temperatures was over 900 degrees C. Fe2Al5 growth tended to reduce the joint strength and lead to interfacial fracture. Cracks within Fe2Al5 mainly consisted of primary cracks propagating along the interface and secondary cracks approaching the steel side, while the primary cracks became the dominant ones as the IMC thickness increased. The estimated in-situ fracture toughness of Al/steel interface decreased from 1.07 MPam(1/2) to 0.35 MPam(1/2) as the Fe2Al5 layer thickness increased from 3 mu m to 6.5 mu m, where a dramatic drop to 0.56 MPam(1/2) happened when the thickness reached 4 mu m. The IMC growth-induced degradation of mechanical performance was attributed to the increasing texture and grain size of the Fe2Al5 phase and diminishing iron phase between the Fe2Al5 grains.