Microstructure And Mechanical Properties Of Die-Cast Al-Si-Cu-Zn Alloys

Because of their higher specific strength and corrosion resistance,good foundry and processing properties as well as excellent electric and thermal conductivity,die-cast aluminum alloys have found a wide application in aeronautical and automotive industries. And thus,the systematic research concerning the microstructure and property of die-cast aluminum alloys is of both important theoretical referring value and practical directing significance.In this investigation,the pure Al,Si,Cu and Zn are chosen as the raw material. Through melting,permanent-mold cast and die casting,the Al-Si-Cu-Zn alloys with different processing states have been fabricated,and some die-cast alloys are solid solution treated.The microstructures,tensile properties and low-cycle fatigue properties of the die-cast Al-Si-Cu-Zn alloys with different treatment states are studied,and compared with the permanent-mold cast Al-Si-Cu-Zn alloy.Microstructural observations show that the die-cast Al-Si-Cu-Zn alloy possesses the much finer microstructure than the permanent-mold cast Al-Si-Cu-Zn alloy.And the formation of the needle-like eutectic Si phase gets inhibited,while the globule-like eutectic Si phase can be obtained.However,the solid solution treatment will lead to the microstructural coarsening of the die-cast Al-Si-Cu-Zn alloy.The tensile experimental results show that compared with the permanent-mold cast Al-Si-Cu-Zn alloy,the ultimate tensile strength of the die-cast Al-Si-Cu-Zn alloy at room temperature increases by 27.3%,the yield strength by 18.1%,and the elongation to fracture at room temperature,150℃and 200℃increases by 72%,86%and 90%,respectively.It is noted that the solid solution treatment leads to a decrease in the tensile properties of the die-cast Al-Si-Cu-Zn alloy.The results of low-cyclic fatigue tests show that the Al-Si-Cu-Zn alloys with different processing states exhibit the cyclic strain hardening and cyclic stability,mainly depending on the imposed total strain amplitude.For the Al-Si-Cu-Zn alloys with different processing states,the relation between plastic and elastic strain amplitudes as well as reversals to failure can be described by Coffin-Manson and Basquin equations,respectively.In addition,a linear relation between tensile hysteresis energy and fatigue life can be also noted for the Al-Si-Cu-Zn alloys with different processing states.The fractographic results reveal that under tensile loading conditions,the die-cast Al-Si-Cu-Zn alloys with different treatment states mainly exhibit the ductile fracture feature,while under low-cycle fatigue loading conditions,the cracks of the Al-Si-Cu-Zn alloy with different processing states initiate transgranularly at the surface of fatigue samples and propagate in a transgranular mode.