Remote laser welding of Zn coated IF steel and 1050 aluminium alloy: processing, microstructure and mechanical properties

Abstract

The integrity of steel-aluminium dissimilar alloy joints is dependent on the intermetallic phases (IMCs) and the extent of the bonding area. The excessive growth of brittle AlxFex IMCs within the weld pool and interfaces is disadvantageous due to the initiation and propagation of hot and cold cracking during the solidification. The purpose of this work was to assess the development of Remote Laser Welded (RLW) joints of Zn coated interstitial free (IF) steel to 1050 aluminium alloy, which can be used in cooling circuits and electrical connectors in automotive applications. Welding experiments with variable RLW parameters (power, welding speed and focal offset) were performed to study the formation of IMCs and impact of joint integrity. Results showed that while in conduction mode (at power densities of 0.13–0.18 MW/cm2) three IMCs were identified through SEM/EDX and EBSD: η – Al5Fe2, κ – AlFe3 and θ – Al13Fe4 which possessed nano-hardness indentation values of approximately 12, 5 and 5 GPa, respectively; they formed in a non-continuous interfacial layer, the weld pool composition remained homogenous, and cracking was minimal. On the contrary, in keyhole mode (at power densities of 0.16–0.40 MW/cm2) welded samples produced a continuous and thick IMC layer, continuous and/or excessive cracking and an inhomogeneous weld pool composition due to the excessive mixing of steel and aluminium, of up to 10 wt.% of Al in the weld pool. The nominal lap shear strength for the sample produced in conduction mode was of 77%, with respect to the weakest joint material (Al). This work found a close link between the welding mode and weld pool chemistry which significantly determined the IMCs distribution and thickness, extent of cracking within the weld pool and mechanical properties.