Effect of heat treatments on microstructure and mechanical properties of sand cast Al-2Li-2Cu-0.5Mg-0.2Sc-0.2Zr alloy
(National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China)
Abstract: This study focused on modifying heat treatment schemes to enhance the mechanical properties of sand cast Al-2Li-2Cu-0.5Mg-0.2Sc-0.2Zr alloy. Different three-stage solution treatment schemes ((460 °C, 32 h) + (520 °C, 24 h) + (530/540/550 °C, 4/12/24/32 h)) and aging temperatures (125, 175, 225 °C) were designed for comparison. The microstructure evolutions were analyzed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the three-stage solution treatment of (460 °C, 32 h) + (520 °C, 24 h) + (530 °C, 12 h) could dissolve most of the secondary phases. The TEM results illustrated that fine δ''''(Al3Li) particles were homogeneously distributed in the matrix after aging at 175 °C for 8 h, accompanied by a small amount of lath-shaped S′(Al2CuMg) and plate-like T1(Al2CuLi) phases. The best comprehensive properties of yield strength of 376 MPa, ultimate tensile strength of 458 MPa and elongation of 4.1% were obtained by the optimal heat treatment scheme ((460 °C, 32 h) + (520 °C, 24 h) + (530 °C, 12 h) + (175 °C, 8 h)).
Key words: Al-Li-Cu alloys; sand-casting; heat treatment; precipitation; microstructure; mechanical properties