The Effect of Si Content on the Microstructure and Mechanical Properties of Al–1.2Mg–xSi–1.2Cu–0.6Mn Cast Alloy

In this paper, the effect of Si content on the microstructure and mechanical properties of the Al–1.2Mg–xSi–1.2Cu–0.6Mn cast alloy was investigated. The study aimed to explore the optimal addition of Si element to improve the comprehensive properties of alloys. The microstructure, phase composition and fracture morphology of the alloy were determined through OM, SEM, EBSD and XRD analysis. The Si content in the alloy ranged from 0.48 to 2.4 wt.%. With the increase of Si content, the number of strengthening phases increases, which improves the comprehensive properties of the alloy. When the Si content is 0.8 wt.%, the eutectic Mg₂Si transforms from rod-shaped to larger block shaped, and the formation of coarse Mg₂Si phase limits the elongation of the alloy. When the Si content is 2.4 wt.%, fine Al₂Cu phases are present in this alloy and coexist with Al(Fe,Mn)Si phases, while an increase in the Si content appears as partially accompanied by an incipient crystalline Si phase around the Al(Fe,Mn)Si phase. The alloy exhibits a maximum tensile strength, yield strength and elongation of 198.2 MPa, 101.2 MPa and 4.96%, respectively, with a hardness of 80.94 Hv. It consists of five alloy phases, mainly α-Al, Mg₂Si, Al(Fe, Mn)Si eutectic phase, Q-AlCuMgSi eutectic phase and the beginning crystalline silicon phase formed due to the increased Si content. The Si content can improve alloy strength, but there is some damage to the toughness of the alloy. Higher silicon alloy content results in the formation of fine Al₂Cu eutectic. This result makes it possible to achieve a higher level of strength with a reduced loss of ductility in the alloy.