Dissimilar resistance spot welding of 6061-T6 aluminum alloy/St-12 carbon steel using a high entropy alloy interlayer

Abstract

The microstructure and mechanical properties of dissimilar resistance spot weldments between the sheets of 6061-T6 aluminum alloy and St-12 low carbon steel using Al0·5FeCoCrNi high entropy alloy (HEA) interlayers (with the thicknesses of 0.4, 0.7 and 1 mm) were studied and compared to interlayer-free weldment. Although HEA contained Al and Fe, its microstructure showed no evidence of Al–Fe intermetallic compounds (IMCs). Barely visible interfaces formed at the carbon steel/HEA side of all weldments, suggesting metallurgical consistency between both materials. Conversely, complex IMCs formed at the interfaces of Al/HEA, as a result of strong chemical tendency of HEA constituent elements towards the aluminum alloy. The application of HEA interlayer was found to change the type of the formed IMCs from AlxFey in the interlayer-free weldment to complex IMCs. The latter prevented cracking in the Al-side of the weldments. Meanwhile, it increased the tendency to hot tearing in the Al-side of the weldments. The thickness of HEA interlayer affected the morphology of IMCs. For instance, a 1 mm-thick interlayer produced needle-shaped complex IMCs owing to initial dissolution of IMCs into the molten aluminum and subsequent eutectic solidification. The weldments fabricated by HEA interlayer exhibited improvement of tensile shear load compared to the interlayer-free joint. Distinct fracture mechanisms were involved in the weldments produced with and without HEA interlayer. The pre-existing cracks and hot tearing at the Al-side of the joint were deduced to be responsible for the fracture initiation in the fabricated weldments without and with HEA interlayer, respectively.