STUDIES ON STRUCTURE AND PROPERTIES OF STEEL – ALUMINIUM ALLOY WELD JOINTS BY SPOT WELDING

Abstract

Al 5052- H32 (1.5 mm thick) alloy welded with Zn coated galvannealed steel (1.0 mm thick) by resistance spot welding with and without use of cover plate (CRC steel- 1.0 mm thick) with the objective of to achieve higher strength of joint at lower range of welding current. Effect of Zn coating on microstructure and mechanical behaviour of joint was explored and effect of nugget diameter, and thickness of reaction layer on tensile shear strength was discussed. Macrostructure of joint cross section and fractured surface were inspected using stereo microscopy, while microstructure and intermetallic layers were analysed using optical and field emission scanning electron microscopy. Shear tensile test was carried out at universal testing machine of 100 KN capacity and nugget diameter was measured from fractured surface. Fluxing action of Zn ensures thinner uniform reaction layers and good metallurgical bonding at low temperature. Reaction layer thickness is maximum at nugget centre and decrease with distance from centre for both joint.With increase of welding current, thickness of reaction layer slightly increases for joint without cover plate and drastically reduces for joint with cover plate. Nugget diameter and shear tensile strength increased for both joint with increasing current. However, joint with cover plate have higher growth rate of nugget diameter due to availability of some additional heat at interface conducted from cover plate. Excess increase in nugget diameter and reduction in thickness of intermetallic layer is the reason for dramatic improvement in shear strength for joint with cover plate. Maximum tensile shear load is 5675 N at 10.0 kA for joint with cover plate and 4049.3 N at 11.0 kA for joint without cover plate i.e. 40.14 % improvement in joint shear strength with the use of cover plate.