Failure analysis on copper-steel butt joints produced by friction stir welding

Commercial sheets of 1.6 mm-thin oxygen-free high conductivity (OFHC) copper were butt-joined with commercial sheets of 1.3 mm-thin DC04 deep drawing steel by force-controlled friction stir welding (FSW). The microstructure and the mechanical properties of the welded copper-steel joints were investigated using light optical and digital microscopy, hardness mapping and tensile testing. Strong welds with coarse copper grains but without excessive mixing of steel with copper were achieved even at the relatively high welding speed of 0.2 m·min −1 by offsetting the tool toward the copper sheet and maintaining the constant lateral penetration of the pin into the steel sheet. However, the exact alignment of the sheets relative to the tool was crucial. Lateral misalignment as indicated by the decreasing spindle torque caused bonding defects and overheating of the weld metal, which impaired the tensile properties of the copper-steel joints. Hence, the spindle torque monitored during welding was identified as potential indicator for the weld quality and thus for the mechanical properties of the joints.