Improvement in as-cast strength of high pressure die-cast Al-Si-Cu-Mg alloys by synergistic effect of Q-Al_5Cu_2Mg_8Si_6 and θ-Al_2Cu phases

摘要

A novel high-strength die-cast Al-Si-Cu-Mg alloy has been developed in the present work. Experimental results indicated that the combination of different amounts of 0-Al_2Cu and Q-Al_5Cu_2Mg_8Si_6 phases could provide a same yield strength but different elongations due to the differences in size and distribution of these two phases. By optimizing the synergistic strengthening of Q-Al_5Cu_2Mg_8Si_6 and θ-Al_2Cu phases, the balanced strength-ductility trade-off was achieved at 3.0 wt% Cu and 0.9 wt% Mg, offering a yield strength of 225 MPa and an elongation of 4.3% under as-cast condition. Except the normal Q and 9 large lamellar or blocks located in n-Al grain boundary, numerous nanoscale Q-Al_5Cu_2Mg_8Si_6 precipitates (mostly under 200 nm) were also observed inside the α-Al grains in the newly developed alloy. Thermodynamic calculation was performed using Pandat 2018 software to explain the formation and distribution of strengthening phases. It was found that Q-Al_5Cu_2Mg_8Si_6Si6 can be precipitated from (1) the melt directly via liquid-solid reaction and (2) α-Al phase via solid-solid reaction. The nanoscale Q-Al_5Cu_2Mg_8Si_6 particles are considered to be precipitated via the latter solid reactions.