Friction Stir Processing (FSP) can be utilized to modify the microstructure of the surface layer and eliminate solidification defects. In this study FSP was used to improve the weldment microstructure of cast A356 aluminum alloy. Gas tungsten arc welding process was used for fusion welding with ER4043 filler metal and negative polarity. FSP was applied along the longitudinal direction of the weld with rotational and linear speed of 1000 RPM and 100 mm / min, respectively. To determine the phases produced during the process, X-ray diffraction (XRD) and X-ray spectrometer (EDS) was used. The microstructure of samples was studied by conventional light microscopy and scanning electron microscopy (SEM), to evaluate the abrasion resistance, a wear test with a constant load of 10 N was applied. Microstructural evaluations showed that FSP destroyed the coarse dendritic microstructure and dissolved the secondary hard and brittle phase (Al 5 FeSi) existing at grain boundaries of weld zone. Metallographical examinations showed that eutectic silicon particles are in the weld zone compared to the base metal indicating the greater influence of the FSP process in breaking and dispersing of silicon in the base metal. The average hardness of the as-received base metal, weld metal, weld zone after FSP and T6 heat treatment was 55, 59, 78 and 80 Vickers respectively. Tensile test results indicate a dramatic increase in yield strength, tensile strength and ductility of the samples after FSP. The tensile strength of the stir zone (SZ) was also greatly increased to 178 MPa, almost 37% higher than that of the base metal (BM). Wear test results showed that the FSP led to an increase increases in wear resistance of samples. The wear resistance of the SZ was also much (700%) that of the BM. Keywords: Optimize, Repair weld, Friction stir processing, GTAW, A356
