Correlation between Microstructural Stability and Tensile Properties of FSWed Al-Mg-Mn Cast Plate during Subsequent Thermal Exposure

Abstract

Al-Mg-Mn cast plate was friction stir welded at different rotational speeds for studying the thermal stability of FSWed specimen. As-FSWed specimens possessed a typical fine grain structure, 5.5 μm at 450 rpm, 9.5 μm at 650 rpm and 12.8 μm at 850 rpm could be acquired. But for the specimens as raised up the rotational speed to 850–1650 rpm exerts little effect to the coarsening of grain size. On the other hand, a subsequent thermal exposure from 350 to 500°C for FSWed specimens will cause significant growth of fine dynamic recrystallization grains, especially as the rotational speed decreased to 450 rpm for the fixed traverse speed, 0.55 mm/s. Tensile properties revealed that extreme grain growth after thermal exposure at 500°C deteriorated the workability of the weld zone. It should be noted that a stored energy release by 300°C pre-annealing heat treatment was beneficial for avoiding extra grain coarsening. In this study, the optimum rotational speed for improving tensile properties pertaining to microstructural thermal stability could be recognized as 850 rpm.