Investigations on joining of dissimilar materials by resistance spot welding

Abstract

Emerging trends in manufacturing such as light weighting, increased performance and functionality increases the use of multi-material, hybrid structures and thus the need for joining of dissimilar materials. The properties of the different materials are jointly utilized to achieve product performance. The joining processes can, on the other hand be challenging due to different properties. This thesis focused on state of the art research in joining dissimilar materials by resistance welding. 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. Two different materials, mild steel HR1 and aluminium alloy 6061 or stainless steel (SS 316L grade) and aluminium alloy 6061 were used to be welded.Welding current in the range of 4 kA to 6 kA and welding cycle time of 15 to 21 milliseconds 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. 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 time and this affected the mechanical properties. Optical microscopy and SEM analysis clearly showed different zones like base metal, heat affected zone and fusion zone. Micro hardness of the weld nugget was maximum because of the formation of brittle intermetallic layer. An increase in both welding current and cycle, increases tensile strength of the weld coupon up to certain limit but after that it decrease in case of joining Al alloy with mild steel but in case of Al alloy and stainless steel it increase with increase of welding current and time.