Abnormal particle-coarsening phenomena in friction stir repair welding of a 2219-T6 Al alloy: formation mechanisms

Abstract

Without any post-weld heat treatment, abnormal Al₂Cu particle-coarsening phenomena have occurred in a friction stir repair weld of a 2219-T6 aluminium alloy. The size of the biggest particle in the dark bands of the ‘onion rings’ was about 150 mm. After a related investigation, three types of formation mechanisms were proposed to explain the abnormal phenomena: aggregation mechanism, diffusion mechanism I, and diffusion mechanism II. The aggregation mechanism was strongly related to the motion laws of plastic metal flow in the weld zone during friction stir repair welding. Diffusion mechanisms I and II were based on classical theories on the growth of precipitates in metallic material systems.