Microstructure and mechanical properties of friction stir welded aluminum alloy Al 6061-T651

Since the beginning of the 21st century and rapid developments in various industries, the demand for strong, light weight, and robust materials that can be used in extreme environments has increased. The use of such materials has been significant in the transportation sector, where reduction in fuel consumption and emission of harmful gases are required. Aluminum is one of the most suitable metals that can be alloyed with different elements to obtain desired properties. However, joining of aluminum using conventional welding processes for special applications in aerospace, shipbuilding, and automobile industries has been a challenge. Friction stir welding (FSW) is a widely used method to weld aluminum since its invention because it overcomes most of the challenges related to aluminum welding. The objective of this research was to analyze the role of preheating and post-cooling applied before and after FSW on the microstructure and some mechanical properties of Aluminum alloy Al6061-T651. Various characterization techniques such as optical microscopy (OM), scanning electron microscopy (SEM), and electron backscattered diffraction have been used to examine the weldment of Al 6061- T651 plates. Analysis of different weld zones formed after the process such as nugget zone (NZ), thermomechanically affected zone (TMAZ), heat affected zone (HAZ) along with the parent metal or base metal (PM or BM) showed equiaxed and recrystallized grains in NZ, deformed and partially recrystallized grains in TMAZ and overaged grains in HAZ. 10% decrease and a 3% increase in the average hardness of NZ was seen in preheated and post-cooled samples, respectively. Hardness was lowest in the HAZ, a bit higher in TMAZ and NZ and highest in PM. Moreover, it was concluded that hardness is not dependent on the grain size to a large extent. SEM of the fractured samples revealed that the mode of failure is ductile and took place in the HAZ. The tensile test results showed only less than 5% and 1% change in tensile strength and percent of elongation of the welded samples, respectively. Overall, it was found that the peak temperature and the duration of the heat cycle have a higher impact on the microstructure and mechanical properties compared to the plastic deformation