Study On Microstructers And Properties Of In Situ Mg₂Si/Al Composites

In situ Mg2Si/Al is a kind of aluminum matrix composite with the characteristics of low density, high specific modulus, low expansion, abundant resources, simple preparation process, etc. having good applications prospects in the preparation automotive engine cylinder liner, brake discs and other light weight important component. However, the primary Mg2Si phases in normal in situ Mg2Si/Al composites are usually very coarse, thus, resulting in poor mechanical properties. Accordingly, the poor mechanical properties restrict its applications in many field. So it is a hot topic to modify the morphology, size and distribution of the reinforcement phase. In this paper, the effects of processing parameters on the microstructure and mechanical properties of the Mg2Si/Al composite has been studied. The primary Mg2Si particles have been modified by Al-P master alloy and extra addition of Si element. The results are shown as followings:Holding time and pouring temperature have large influence on the morphology of primary Mg2Si and mechanical properties. The processing parameters that the optimal modification effect of the primary Mg2Si particles can be obtained are the pouring temperature of765℃and holding time of15minutes at this temperature.The extra Si addition can obviously affect the microstructure and mechanical properties of the Mg2Si/Al composite. The results indicated that the size of the primary Mg2Si particles first increased and then decreased with increasing extra Si content. When the content of extra Si was5%, an average size of41μm was obtained. However, when the content of extra Si was increased to10%, an average size of24um was obtained. In addition, the amount of Al-Mg2Si binary eutectic structure gradually reduced as the extra Si content increases, and even disappeared when the content was increased to10%. Furthermore, as the extra Si content increased, the irregular a-A phase became spherical one gradually, and spherical α-Al phase could be formed when the content Si was increased to8%.The tensile strength of the in situ Mg2Si/Al composite increased from172MPa to200MPa, while the elongation was increased from1.4%to3.1%as the content of extra Si increased from0%to10%.The addition of Al-P master alloy can significantly improve the microstructure and mechanical properties of in situ Mg2Si/Al composite. When the addition of the Al-P master alloy was3%, the size of the primary Mg2Si particles reduced from30μm to17μm, and the holes in Mg2Si dendrites also disappeared. The fibrous-like eutectic Mg2Si phases become into rod-like or dot-like ones. When the content of Al-P master alloy was3%, the morghologies of the primary Mg2Si particles and eutectic Mg2Si basically not change. So the optimum amount of Al-P master alloy was2%. The tensile strength of the in situ Mg2Si/Al composite was improved from200MPa to225MPa, while the elongation was increased from3%to3.7%when the3%Al-P master alloy was added.