Mechanical properties and fracture modes of thin butt-joined aluminum-steel blanks for automotive applications

The influence of different filler alloys and joining parameters on the mechanical properties and on the fracture modes of aluminum-steel blanks for automotive applications was experimentally investigated. Sheets of 1.15-mm-thick aluminum alloy EN AW-6014 T4 were butt-joined with sheets of 0.80-mm-thick zinc-coated steel DC04 using the single-sided Cold Metal Transfer (CMT) process. The quasi-static strength and the fracture modes of the joints were determined using uniaxial tensile and three-point bending tests. The width of the heat-affected zone (HAZ) was estimated based on hardness profiles and maps. The microstructure inside the HAZ was studied using optical micrographs, and selected fracture surfaces were investigated by means of scanning electron microscopy (SEM). The influence of the filler alloy composition was more significant under tensile load than under bend load. Under tensile load three different fracture modes depending on the actual location of fracture were identified. The most brittle behavior of the joint was observed, if fracture occurred directly at the weld seam. Both, thickening of the intermetallic (IM) layer between the weld seam and the steel sheet as well as the porosity forming during the joining process, facilitated fracture at the weld seam. Most favourable combinations of strength and ductility under both tensile and bend loads were obtained when using comparatively silicon-rich filler alloys Al-3Si-1Mn or Al-1Si-Mg-Mn at the welding speed of 0.4 m/min. Increasing the welding speed to 0.7 m/min may still result in acceptable strength and ductility of the joints; however, further increase to 1.0 m/min reduces these properties considerably.