A study of microstructural stability of friction stir welded joints of Al-Mg alloys during subsequent thermal exposure

Abstract

Al-Mg wrought alloys such as Al-2.6Mg, Al-4.6Mg and Al-4.6Mg- 1Si were friction stir welded and a subsequent thermal exposure was carried out in this study. The effects of elements addition and exposure temperature on the microstructural stability of welded zone were investigated. A grain refinement was found in the weld zone of each alloy and the average grain size of friction stir welded Al-2.6Mg, Al-4.6Mg and Al-4.6Mg- 1Si was 10.1µm, 7.4µm and 4.7µm, respectively. Thermal exposure caused drastic coarsening of grains in the stirred zone and abnormal grain growth initiated at some preferred sites such as upper surface and bottom. The volume fraction of coarsening grains increased with increasing exposure temperature; however an unexpected decrease in grain size was found at 550°C in compare to 500°C. Besides, the addition of Mg and Si deteriorated the thermal stability of grain structure in stirred zone.