The S-Al₂CuMg precipitates and L1₂-Al₃(Sc,Zr) dispersoids co-existing in the SLM fabricated Al-Cu-Mg-Sc-Zr alloy

This study delves into the effect of heat treatment on the microstructure and mechanical properties of selective laser melted (SLM) Al-Cu-Mg-Sc-Zr alloys. Subjecting the manufactured Al-Cu-Mg-Sc-Zr alloy to direct aging at a low temperature of 200 °C and multi-stage aging protocols revealed insights into its microstructure and performance characteristics. Two main strengthening phases emerged post-aging: the S-Al₂CuMg phase, forming primarily at temperatures under 225 °C, and the L1₂-Al₃(Sc,Zr) phase, developing when the temperature surpasses 225 °C. After direct aging at 200 °C, two types of S-Al₂CuMg precipitates were observed, and the reconstruction of Cu atoms at the interface and the formation of (021)_S-II interface are mainly discussed. Introducing aging at 400 °C for 130 min before heat treatment at 200 °C, the coexistence of Al₃(Sc,Zr) and S-Al₂CuMg phases can be observed in the microstructure. Findings demonstrated that L1₂-Al₃(Sc,Zr) dispersoids precipitated at 440 °C for 130 min, fostered the nucleation of S-Al₂CuMg phases at 200 °C while restraining their growth. However, the L1₂-Al₃(Sc,Zr) dispersoids formed by heat treatment at 320 °C for 2.5 h hinder the formation of the S phase at 200 °C. Moreover, when subjected to a temperature of 225 °C for 2.5 h, these L1₂-Al₃(Sc,Zr) dispersoids were speculated to disintegrate due to their thermodynamic instability at 200 °C. Room temperature assessments of mechanical properties showed that direct aging at 200 °C for 4 h achieved a yield strength of 381 ± 6 MPa. In contrast, multi-stage aging—comprising a 440 °C phase for 130 min followed by aging at 200°C—resulted in a maximum yield strength of 406 ± 7 MPa after a 1-h isothermal hold at 200 °C.