Metallurgical investigation of direct drive friction welded joint for austenitic stainless steel

The present study investigates the metallurgical behavior of direct-drive friction-welded (DDFW) joints for high Cr-Ni-Mo steel (AISI 316). The selected welding conditions are rotation speed of 3000 rpm, friction time of 10 s, friction pressure of 130 MPa, forge time of 5 s, and forge pressure of 260 MPa. The results were conducted by macroscopic observation to detect flash and macro defects such as cracks or cavities, microscopic observation using to investigate the grain size before and after the welding process to detect the regions around the welding joint, microhardness observation along the axial direction to determine the dimension of welding regions, and X-ray diffraction (XRD) to show the thermal influence alone and evaluation of the thermo-mechanical properties of the as-welded case. The results for the macroscopic view showed that the narrow welding center did not exceed 600 & mu;m and there were no macro cracks or defects, indicating a uniform structure. The microstructure exhibited a significant welded joint interface with microcavities and grain refinement of 10 & mu;m, about three times smaller than the grain size of AISI 316, due to the creation of a highly plastically deformed zone (HPDZ), while the thermo-mechanically affected zone (TMAZ) formed in the neighboring area with a grain growth of about 100 & mu;m, relatively enlarged three times compared to AISI 316 and ten times related to HPDZ. Thus, the high level of microhardness was investigated at the welding center and low values in the neighboring area were caused by the formation of the HPDZ and TMAZ, respectively. XRD also illustrated the concentration of gamma iron at the 111 level due to the grain refinement resulting from high plastic deformation.