Mechanical and metallurgical properties of dissimilar AA6061-T6 and AA7277-T6 joint made by FSW technique

Aluminum 6xxx and 7xxx series are among the most commonly used alloys in industrial applications whereas their joining and welding using conventional fusion-based weld processes are very difficult. In this paper, friction stir welding process of dissimilar AA6061-T6 and AA-7277-T6 aluminum plates was investigated. The effects of tool travers and rotational speeds and the type of material located in the advanced side (AS) or retreating side (RS) of weldment were studied experimentally on the mechanical and metallurgical characteristics of the joint. Macro- and microstructure pictures of weld region were used to evaluate the quality of the weld and defect detections. Tensile strength tests and micro-hardness measurements of different areas in the weld section showed that the most strength and hardness values are achieved when the AA-7277-T6 was located in the AS of the weld. Accordingly, the highest tensile strength was about 78 % of the ultimate strengths of AA6061-T6 base metal which was obtained at the rotational and linear speeds of 825 rpm and 60 mm/min, respectively. Most of the tensile fractures were initiated from the heat-affected zone (HAZ) of AA6061-T6 alloy. The minimum hardness values were obtained from HAZ for both alloys, and the average hardness of joint was reduced by increasing the rotational speed. Also it was observed from the microstructure analyses that the size of grains in the HAZ area was greater than that of the nugget zone. Using artificial neural network (ANN) approach, the hardness and ultimate tensile strength of tested AA6061-T6 and AA-7277-T6 aluminum joint was predicted for different friction stir welding (FSW) process variables including the pin speeds and material position on AS and RS. Also, using a multi-objective optimization technique, a Pareto front (or Pareto optimal set) was obtained for both hardness and tensile strength results.