Investigation into physical and mechanical properties and failure mode of resistance spot welded dissimilar metal joints

Abstract

Resistance spot welding is a widely used technique for fabricating sheet metal assemblies because of its high speed, efficiency, reduced cost and suitability for automation. The objective of this thesis was to study the effect of process parameters particularly on the structure and properties of resistance spot welded dissimilar sheet metal joint. Process parameters were optimized and a discussion was made on failure mode of this welding technique. Two different materials, austenitic stainless steel and low carbon steel were used to be welded. Thickness of these two sheets were around 1.5 mm. Welding current in the range of 3 kA to 9 kA and welding cycle of 30 to 90 was used as process variables. The welded coupons were examined by different characterization techniques such as macroscopic, optical microscopic and scanning electron microscopy, EDX analysis, tensile testing and micro hardness measurements. Failure mode was also identified by analysis of the fractured samples. In this study, asymmetrical shape weld nugget was formed to join two different materials. Weld nugget size increased with an increase in welding current and welding cycle and this affected the mechanical properties. Optical microscopy and SEM analysis clearly showed different zones like base metal, heat affected zone and fusion zone. The heat affected zone consist martensite, grain boundary ferrite and widmanstatten ferrite in low carbon steel side but in case of austenitic stainless steel no phase change occurs in the heat affected zone. The fusion zone had a cast structure with coarse columnar grain and dendrite with excess delta ferrite in austenitic matrix. Micro hardness of the weld nugget was maximum because of the formation of martensite. An increase in both welding current and cycle, increases tensile strength of the weld coupon. Three types of failure mode were found and these are base metal tearing, pullout failure mode and interfacial failure mode.