Effect of heat-input level on microstructure and mechanical properties of SDS-FSW joints in thick AA2219

Simultaneous double-sided friction stir welding (SDS-FSW) enables solid-state joining of thick aluminum alloys, yet joint integrity and mechanical performance remain highly sensitive to heat-input conditions. Here, thick AA2219‑T6 plates were welded under two representative heat-input levels, quantified by an empirical heat-input index (HI = 7 and 22). Multi-point thermocouples were employed to quantify thermal cycles using peak temperature and high-temperature exposure time (defined as the cumulative duration above a critical temperature threshold), and weld formation, second-phase particle redistribution, grain structure, and mechanical responses were assessed by macro-etching / optical microscopy, microhardness testing, tensile testing, and EBSD. At HI = 7, the joint showed stable formation and a compact stirred region with a higher strength–ductility balance. In contrast, HI = 22 produced pronounced flash and a void defect, together with a deeper hardness trough and degraded tensile properties. Thermal measurements showed that the higher-HI condition resulted in broader thermal cycles and longer high-temperature exposure, providing a quantitative basis for correlating heat-input level with softening and defect susceptibility. EBSD further reveals that HI = 22 yields a coarser recrystallized structure with a lower HAGB fraction (dAVE = 8.11 μm, HAGB = 61.9 %) than HI = 7 (dAVE = 5.28 μm, HAGB = 71.4 %), accompanied by more evident particle clustering / banding. These results show that the heat-input level strongly affects weld formation, microstructural characteristics, and mechanical performance in SDS-FSW of thick AA2219‑T6 plates, and provide experimental support for process optimization.

1. National Natural Science Foundation of China - 52475028
2. National Natural Science Foundation of China - 52375507
3. National Natural Science Foundation of China - 52325501