Evolution of segregation, microstructure and mechanical properties of a semisolid die casting Al–6Si–3Cu–0.4Mg alloy

Study on segregation, microstructure and mechanical properties of Al–6Si–3Cu–0.4Mg alloy was achieved by controlling the filling length in semisolid die casting. Results show that the microstructure is comprised of globular α–Al, eutectic Si, θ–Al₂Cu, Q–Al₅Cu₂Mg₈Si₆ and Fe–rich phase. After solution–ageing treatment, abundant θ–Al₂Cu and Q–Al₅Cu₂Mg₈Si₆ phases dissolve into the matrix. The liquid difference between the edge and middle area is 21.3% for 1/3 filling length, which is much higher than that of 15.0% for full filling length. As the filling length decreases, the increment of segregation degree at the bottom of the sample is detected. This segregation behavior leads to the strength and ductility both decreases from 425.0 MPa and 6.5% for 1/3 filling length to 405.3 MPa and 4.4% for full filling length, respectively. Besides, along with the formation of abundant fine θ–Al₂Cu phase and higher liquid fraction at the edge area, the ultimate tensile strength and elongation reaches 443.5 MPa and 5.2%. While in center area, the occurrence of coarsen θ–Al₂Cu phases leads to the tensile properties dramatically reducing to 378.7 MPa and 1.9%.