Effect of Cr, Ti, V, and Zr Micro-additions on Microstructure and Mechanical Properties of the Al-Si-Cu-Mg Cast Alloy

Uniaxial static and cyclic tests were used to assess the role of Cr, Ti, V, and Zr additions on properties of the Al-7Si-1Cu-0.5Mg (wt pct) alloy in as-cast and T6 heat-treated conditions. The microstructure of the as-cast alloy consisted of α-Al, eutectic Si, and Cu-, Mg-, and Fe-rich phases Al_2.1Cu, Al_8.5Si_2.4Cu, Al_5.2CuMg₄Si_5.1, and Al₁₄Si_7.1FeMg_3.3. In addition, the micro-sized Cr/Zr/Ti/V-rich phases Al_10.7SiTi_3.6, Al_6.7Si_1.2TiZr_1.8, Al_21.4Si_3.4Ti_4.7VZr_1.8, Al_18.5Si_7.3Cr_2.6V, Al_7.9Si_8.5Cr_6.8V_4.1Ti, Al_6.3Si_23.2FeCr_9.2V_1.6Ti_1.3, Al_92.2Si_16.7Fe_7.6Cr_8.3V_1.8, and Al_8.2Si_30.1Fe_1.6Cr_18.8V_3.3Ti_2.9Zr were present. During solution treatment, Cu-rich phases were completely dissolved, while the eutectic silicon, Fe-, and Cr/Zr/Ti/V-rich intermetallics experienced only partial dissolution. Micro-additions of Cr, Zr, Ti, and V positively affected the alloy strength. The modified alloy in the T6 temper during uniaxial tensile tests exhibited yield strength of 289 MPa and ultimate tensile strength of 342 MPa, being significantly higher than that for the Al-Si-Cu-Mg base. Besides, the cyclic yield stress of the modified alloy in the T6 state increased by 23 pct over that of the base alloy. The fatigue life of the modified alloy was substantially longer than that of the base alloy tested using the same parameters. The role of Cr, Ti, V, and Zr containing phases in controlling the alloy fracture during static and cyclic loading is discussed.