Friction Stir Welding of austenitic stainless steel: a study on microstructure and effect of parameters on tensile strength

Friction stir welding was performed on austenitic stainless steel plates on an indigenously retrofitted vertical milling machine. Machine was retrofitted with a robust work fixture capable of measuring welding force and a tool adapter. AISI-304 equivalent grade stainless steel was welded by FSW using tungsten carbide tools with tapered cylindrical (conical) pin. The experiments were performed using L-4 Orthogonal Array of Taguchi design. The tensile tests revealed failure of test specimens outside the weld region. The results of ANOVA showed the order of importance in which the parameters have affected the UTS in terms of percent contributions i.e. welding speed with (56.83% contribution), shoulder diameter (27.44% contribution) and tool rpm (15.73% contribution). The microstructure study was performed to investigate microstructure evolution in Stir Zone (SZ), Thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). The microstructure characterization in three distinct FSW zones showed: (i) Fine grain structure with a circular and non-uniformly distributed darkly etched band formation in SZ, (ii) Finer grains in TMAZ, and (iii) Refinement of grains in HAZ Micro-hardness distribution across the weldline was also obtained. The highest hardness was seen to occur in the TMAZ on advancing side. (C) 2015 Elsevier Ltd. All rights reserved.