Enhancement of the mechanical properties of semi-stationary bobbin tool friction stir welded joints in AA2219 through post-weld heat treatment

This research investigates the enhancement of AA2219 joints formed by semi-stationary bobbin tool friction stir welding (SSBT-FSW) through post-weld heat treatments. Although solid-state processes such as SSBT-FSW are able to produce superior welds without common defects seen in conventional fusion welding of aluminum alloys of the 2XXX series, the thermal cycle in SSBT-FSW can deteriorate mechanical properties by causing precipitate dissolution and over-aging. The research proposes using a heat treatment sequence to induce solubilization and re-recipitation of precipitates in a more favorable distribution, re-activating the precipitation-hardening. Since abnormal grain growth is very difficult to avoid during heat treatments of welded samples, the analysis focuses on the evolution of the microstructure during solution heat treatment and on the influence of grain size, texture, and precipitate distribution on grain growth. The origin and evolution of the grain growth during solution heat treatment have been observed. Hardness measurements and tensile tests show that post-weld heat treatments can effectively restore the mechanical properties of the joints to the levels of those of the base material. Understanding the relationships between microstructure, abnormal grain growth and mechanical behavior of the joints aids the optimization of the SSBT-FSW process for broader industrial application, especially in the aerospace industry, where high-performance aluminum welded structures are critical.