Intermetallic Phase Layers in Cold Metal Transfer Aluminium-Steel Welds with an Al-Si-Mn Filler Alloy

In welding of aluminium (Al) alloys to steels, a major challenge is excessive growth of brittle intermetallic phases along the bonded Al-steel interfaces. The formation and growth of these phases are influenced by the heat input and the alloying elements present. This work focuses on the phases formed between a low alloyed steel and an Al-Si-Mn alloy that was used as the filler wire in a cold metal transfer joint. During lap shear testing of the joint, fracture ran through the melted Al, and the joint reached a strength of 174 +/- 21 MPa. Scanning and transmission electron microscopy showed that the formed similar to 2.5 mu m thick intermetallic phase layer consisted of polyhedral alpha(c)-Al-(Fe,Mn)-Si, elongated or rounded theta-Fe4Al13 and near equiaxed eta-Fe2Al5 grains. Electron backscatter diffraction was used to study the crystal orientations of the formed phases. Altogether this work aims to contribute to better understanding of the formation and growth of intermetallic phases in Al-steel welds where the Al alloy contains Si and Mn.