Abstract: The present study focuses on the influence of a 5-pass overlapped friction stir lap welding of AA6061-T6 on the mitigation of defects and its simultaneous effect on microstructure and mechanical properties. Tool pin overlapping percentage of 0%, 25%, 50% and 75% was employed, from which it was observed that defects significantly reduced at higher overlapping. Fine equiaxed grains were observed across all the nugget zones in both top, and bottom regions were developed due to simultaneous overlapped passes; the grain size of which varied from 8.5 µm to 3.5 µm. The grains became finer, which increased of overlapping percentage. The microhardness of the processed zone was significantly reduced during overlapping that varied from a range of 43 HV0.1 to 49 HV0.1 as compared to base metal which was interpreted due to changes in grain size and secondary precipitate distribution during overlapping. A maximum ultimate tensile strength of 230 MPa was achieved at 50% overlapping, whereas minimum ultimate tensile strength of 160 MPa was achieved at 0% overlapping. Fractography of fracture surface showed a ductile failure at 50% that showed a number of dimples of varying shapes and sizes.

Abstract: In this study, the submerged arc welded butt joint of P-91 ferritic-martensitic alloy steel plates has been chosen to examine the influence of microstructural changes on welding residual stress characteristics and macro-hardness in the weldment with two different heat input 2497.5 J/mm & 2040J/mm respectively. The stress magnitudes at a different location on the welded specimen are measured by X-ray diffraction technique at the post-weld heat-treated condition. Compressive stresses are observed in the weld zone (WZ) and tensile stresses are detected at the boundary of the heat-affected zone (HAZ) near the weld zone. Higher compressive stresses are found in the weld zone of both the specimens where maximum hardness values are observed. Different grain matrix in the microstructure contributes significantly towards the quality of weldment. Residual stress, as well as the hardness of weldment, is found to be influenced by the martensite phase transformation and dissolution of precipitates. A modeling concept is presented for dimensional analysis of martensite structure. Energy dispersive X-ray indicates the distribution of carbides at the grain boundaries. Optical micrograph substantiates the presence of δ-ferrite in the WZ.