Mechanical and microstructural characterization of optimized friction stir welded twin roll cast AZ31B sheets

Abstract

In order to select the optimum parameters for friction stir welding of twin-roll cast (TRC) AZ31B Mg-Al-Cu alloy, mechanical and microstructural characterizations are performed on friction stir welded 3 mm thick sheets under different processing parameters of rotational spindle speed and feed rate. Used was a tool made from high resistance Sverker 21 steel alloy with a 19 mm diameter cylindrical shoulder and a 6.4 mm diameter pin that extrudes 2.7 mm from the bottom of the shoulder. A large number of welded joints were prepared according to a test matrix of various combinations of rotational speed and feed rate. Tensile tests at different temperatures and strain rates were performed on specimens cut with their major axis perpendicular to the welded joints. Static strength tests of these specimens revealed that joints welded at a speed of 1200 rpm and a feed of 150 mm/min resulted in strengths as high as 94% of that of the as received TRC AZ31B metal. For specimens welded at spindle speeds larger than 1400 rpm, microstructural observations revealed the presence of micro cracks and voids located in the zone between the nugget and the thermally affected zone. In order to monitor the effect of lengthy testing times and the observed grain growth associated with slow strain tests, annealing tests under different temperatures and time durations were performed while monitoring grain diameter. Very limited grain growth have been obsereved after annealing and thus the mechanical performnace of the joints was solely due to the effect of the friction stir welding.