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.
