ABNORMAL GRAIN GROWTH DURING HIGH TEMPERATURE EXPOSURE IN FRICTION STIR PROCESSED 7050 AND 2519 ALUMINUM ALLOYS

摘要

Friction stir welding (FSW) and friction stir processing (FSP) produce a fine grained 'nugget' region. The microstructural refinement and uniformity depends on processing parameters and tool design. During post-processing at high temperatures, friction stir processed 7050 and 2519 aluminum alloys show abnormal grain growth not reported previously. The locations for onset of abnormal grain growth are different for these two alloys. The nucleation and growth characteristics of abnormal grains have been investigated using differential scanning calorimetry, optical microscopy, and transmission electron microscopy. Precipitate distribution, precipitate stability, and microstructural non-uniformity in abnormal grain growth are discussed. The observation of abnormal grain growth has important implications for post-FSW heat treatment and superplastic properties of FSP material.