A Comparison of Interface Characteristics and Mechanical Properties of Aluminium-Steel Joints Made by Pulsed-MIG and Cold Metal Transfer (CMT) Processes

Selective replacement of steel by aluminium alloys can significantly contribute to light-weighting of automotive bodies. Some of the potential applications include closures (trunk lid, bonnet), roof to side members etc. Several joining techniques: thermal, mechanical, electromagnetic pulse and adhesive are being investigated to accomplish sound joints in this dissimilar material combination. However, possibility to join with techniques familiar to the automotive industry, such as arc welding, enables faster adaption. In thermal joining of aluminium to steel, formation of brittle intermetallic phases at the interface is the critical issue to be addressed, in order to achieve acceptable joint efficiency. In the present study thermal joining of aluminium alloy 6061-T6 to galvanised IF steel in lap configuration is investigated using two welding techniques Pulsed-MIG and Cold Metal Transfer (CMT) processes. Al-Si type filler is used. In order to minimise formation of intermetallic phase formation, low heat input parameters which can limit dilution of steel into aluminium are used. A joining process similar to brazing, wherein only aluminium alloy melts and wets the steel surface to form the joint is targeted. The two chosen processes are generally suitable for welding at low heat inputs. However, by the nature of process, the metal transfer characteristics are different which can affect the interface characteristics. Macrostructural analysis indicated that, for equivalent welding parameters, the bead profiles and wetting behaviour is different. So is the interface microstructure. Detailed investigation into the characteristics of the interface of the joints made using the two chosen processes was conducted and compared. Investigation included microstructural analysis using SEM, EBSD, FIB and TEM and mechanical properties by tensile and cupping tests. Nature of intermetallic phases vis-à-vis the process and welding parameters are presented. The mechanical properties of the joints are correlated with the interfacial features and optimum welding process parametric window is suggested.