Optimization of CMT welding process parameters of dissimilar hot rolled E250 and polymer sandwich steel lap joints

Abstract

Lap joints of 1.5 mm thin sheets of dissimilar hot rolled E250 with polymer sandwich steel (MPM) were produced by cold metal transfer (CMT) process, closer to the real application. The polymer layer polystyrene-butadiene-styrene sandwiched between two DC06 sheets of MPM was found to be intact at significantly lower heat inputs (0.16 − 0.24 kJ/mm), which otherwise had been quite challenging during TIG welding leading to huge rejections. After several iterative trials for acceptable weld quality, experiments were conducted as per L9 orthogonal array, Taguchi technique with welding speed (WS), wire feed rate (WFR) and welding torch orientation (TO) as the process parameters. The joints were investigated through optical macrograph, micrographs, hardness, tensile lap shear tests and fractography on the fractured specimen. Optimum parameters were determined for maximizing shear strength. The sample with optimized parameters exhibited 6% improvement in shear strength achieving 152.09 MPa and polymer layer retention. ANOVA analysis suggested WS to be most significant parameter with 69.85% contribution affecting shear strength. Coefficient of determination (R 2) for the shear strength was 87.26% derived from the linear regression model. Significantly, lower error 0.96% computed from the confirmatory test concluded very effective optimization. Highlights Thin sheets of polymer sandwich steel (MPM) sheets were joined successfully with hot rolled E250 sheet without thermal disengagement of the polymer layer in MPM by CMT welding. Under optimal parameters MPM/E250 joints exhibited excellent mechanical properties. Fractography analysis revealed ductile failure mode. The error between experimental and predicted results by Taguchi was significantly lower and very well within the acceptable range.