Paper Title
Effects of Tool Design and Process Parameters on Microstructure Evolution and Mechanical Properties of Friction Stir Welded Dissimilar Joints

Abstract
The present research was aimed for a review on effects of friction stir welding tool design and welding parameters on microstructure evolution and mechanical properties of dissimilar joints. Effects of welding parameters, including rotational speed, linear speed and pin offset on temperature distribution, microstructure evolution and mechanical properties of joints were investigated and optimized by response surface method. In order to study the effects of the tool, two types of tools were designed, one of which had a rotating shoulder known as conventional FSW (CFSW) and the other had a fixed shoulder known as stationary shoulder FSW (SSFSW). The XRD analysis showed the presence of intermetallic compounds in both CFSW and SSFSW processes. High Vickers microhardness values were measured at several joint interfaces, which corresponded with the intermetallic compounds. These peaks observed in all joints that welded by CFSW, whereas only one of SSFSW joints had a peak. In SSFSW, due to the fixed shoulder, the heat input was greatly reduced, which consequently the grains size and width of the HAZ region and the amount of intermetallic compounds were decreased. By increasing the rotational speed and shoulder diameter and reducing the linear speed, the generated heat flux, was increased. By increasing heat input, tensile strength of joints was increased to a maximum value and then decreased. This was due to increasing frictional heat and excellent mixture of materials that achieved with proper stirring. Further increasing of heat input, caused coarse grain structure in HAZ and increasing intermetallic compounds and consequently, tensile strength was reduced. For CFSW of AA1100-H18 to C10100, highest UTS of 144 MPa was obtained with tool rotation speed of 1650 rpm, tool linear speed of 50 mm/min and tool pin offset of 0.54 mm. For SSFSW of same materials, highest UTS of 149 MPa was achieved and mentioned parameters were 2400 rpm, 8 mm/min and 0.71 mm, respectively. Decrease of the heat input in SSFSW, leads to increase of strength as a result of; decrease of amount of intermetallic compounds, and reducing the grain size and width of the HAZ region. Keywords - Friction Stir Welding, Stationary Shoulder, Mechanical Properties, Microstructure, Optimization, Response Surface Methodology, Dissimilar joining.