Effect of cold metal transfer process parameters on microstructural evolution and mechanical properties of AISI 316L tailor welded blanks

Abstract

Annealed austenitic stainless steel AISI 316L sheets of 1.6-mm and 2-mm thickness respectively was successfully joined by cold metal transfer (CMT) process at two different heat inputs of 0.125 to 0.236 kJ/mm. The process parameters such as welding current and welding speed were optimized to get defect free joints with full penetration depth. The geometry of the weld bead and microstructure of AISI 316L tailor welded blanks using ER308L filler wire are evaluated. Weld metal microstructure inspection by optical microscopy (OM) revealed that higher heat input used to melt the material led to equiaxed grains in the center of weld metal (WM). Due to the lower heat input, the microstructure at the heat-affected zone (HAZ) was similar to base metal (BM). Tailor welded blank’s (TWBs) were formed by designing L9 orthogonal array for 1.6 mm, 2 mm, and dissimilar thickness joint (1.6 mm and 2 mm) wherein the joints exhibited higher tensile strength and hardness as compared to base metal. The increase in tensile strength is corroborated to the increase in δ-ferrite and content of the alloying elements and the grain size in the WM. The microstructure refinement and the δ-ferrite presence contributed to the increase in hardness at the WM compared to HAZ and BM. Bend test revealed quality and soundness of the TWBs with no cracks or openings. The X-ray diffraction plots exhibited an increase in ferrite in WM and the ferrite number has been quantified at an average of 6.36 FN for TWBs and less than 0.5 FN for BM using Fischer Feritscope, which results in better mechanical properties. Fractography examinations indicated that samples failed as a result of the presence of dimples and microvoids resulting in ductile mode fracture at the welded joints.