Strengthening Mechanism Of Wrought Al-12.7Si-0.7Mg Alloy

Al-Si alloys are commonly used in automotive and aerospace industry due to their low thermal expansion coefficient, high wear resistance and thermal conductivity as well as good castability. However, application of Al-Si alloys as structural materials has not been seen in literature. The reason is generally believed to be drawn from the fact that, in ingot metallurgy, in the cast Al-Si alloys with higher Si content, the brittle primary and/or eutectic Si phase usually make further deformation difficult.In this paper, DC casting without chemical modification was employed to generate an Al-12.7Si-0.7Mg billet with fine structure. Hot extrusion was performed following pre-heat-treatment to produce profiles which were subsequently subjected to solution and aging treatment. The effect of heat treatment process parameters on the microstructure and mechanical properties of the alloy was discussed. The precipitation behavior during aging was examined and analysed, and the strengthening mechanism of the alloy was summarized.The as-cast microstructure of DC cast Al-12.7Si-0.7Mg alloy consists of a-Al dendrites, Al-Si eutectics, and intermetallic compound of Mg2Si and Al8FeMg3Si6. The microstructure of the extruded alloy contains uniformly distributed silicon particles in the Al matrix of fine equiaxed grains. The averaged grain size of the Al matrix on the longitudinal section of the extruded alloy is 6.1?m. The averaged size of Si particles is 3.1?m.Traditional metallurgical process could be used to AI-12.7Si-0.7Mg alloy with mechanical properties comparable or even superior to that of the 6000 series alloys. The mechanical properties of extruded alloy in T1 and T6 condition are much higher than that of 6063 alloy in T4 and T6 condition, respectively.The ellipsoidal and needle-shaped precipitates coexist in the alloy at given aging condition. The ellipsoidal precipitate is consistent with needle-shaped metastable phases reported Al-Mg-Si alloys. The needle-shaped metastable phase along<100> direction of the matrix is not found by previous researchers. The ellipsoidal precipitates in the alloy aged at 160? are G.P.zone. Ellipsoidal precipitates in the alloy aged at 180? are mainly ?", of which there are few B'phases. Grain boundary precipitates in the alloy aged at 180? are mainly Type-Aand ?' phases. No ignificant change was found precipitate of the alloy aged at 180? accompany with increasing the aging time. The ellipsoidal precipitates in the alloy aged at 200? are mainly P", of which there are few B'and Type-A phases. The needle-shaped precipitate in the alloy aged at 160?,180? and 200? have same characteristics, which is coherent with the matrix. The dark spot precipitates viewing along [001]Al is not cross-sectional image of needle-shaped precipitates along<001>Al.The strengthening mechanisms of Al-12.7Si-0.7Mg alloy can be attributed to be the combining effect of particle, grain boundary, solid-solution and precipitation strengthening. The fracture surface characteristic of the tensile test specimens is ductile fracture. An important aspect of damage evolution in Al-12.7Si-0.7Mg alloys is fracture/cracking of Si particles.