Abstract

Cast Al-3Cu-Li alloys are limited by coarse grains and non-uniform Cu-rich phases, but their traverse-speed-dependent response to friction stir processing (FSP) has not been systematically clarified. The effect of FSP traverse speed on the microstructural evolution and mechanical properties of an Al-3Cu-Li alloy was systematically investigated. The alloy was processed at traverse speeds of 20 mm/min and 100 mm/min, and the resulting microstructures were characterized by XRD, SEM, EDS, EBSD and TEM. The results show that FSP does not significantly change the main phase constitution of the alloy, which remains dominated by the α-Al matrix with Al-Cu-related secondary phases. However, FSP markedly modifies the grain structure and precipitate distribution. Compared with the as-cast alloy, both FSP-treated samples exhibit refined equiaxed grains formed through dynamic recrystallization. The FSP-20 sample shows a finer and more homogeneous recrystallized structure, with a mean grain size of 4.60 ± 0.62 μm based on 982 measured grains, whereas the FSP-100 sample exhibits a coarser grain structure with a mean grain size of 15.20 ± 2.17 μm based on 109 measured grains. EBSD analysis further reveals that the FSP-20 sample possesses lower grain orientation spread and more dispersed local misorientation, indicating more sufficient dynamic recrystallization and lower residual deformation. TEM observations confirm the presence of plate-like T1 precipitates in both FSP samples, while the precipitates in the FSP-100 sample are relatively coarser. Mechanical testing showed that the microhardness, yield strength, ultimate tensile strength and elongation increased from 62.4 HV, 92.8 MPa, 178.5 MPa and 10.86% in the as-cast alloy to 85.5 HV, 170.0 MPa, 228.9 MPa and 20.41% in FSP-20, and to 78.2 HV, 166.8 MPa, 202.9 MPa and 17.63% in FSP-100, respectively. Fracture analysis indicates that FSP-20 is dominated by ductile dimpled fracture, whereas FSP-100 shows more obvious quasi-cleavage features associated with coarse precipitates.

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