| Automobile industry is moving in lightweight, high-speed, comfort, energy saving, low-emission direction, and an increasing number of cars using high-performance aluminum to replace steel, iron and other traditional materials. The use of aluminum engine can significantly improve the performance of the engine and cars, but also to reduce the weight of the engine. The subjects adopted a cast Al-Si alloy micro-alloying, the impact of trace elements Ni, Cr, and Zn on the alloy mechanical properties and microstructure were analyzed, and further enhanced the comprehensive performance to Al-7.5Si-3.5Cu-0.35Mg as the base of the Al-Si cast alloy.The matrix alloy was added trace elements Ni, Cr, and Zn to examine the effects of these trace elements to the mechanical properties and microstructure of the cast alloy. Experimental results show that:(1) When adding Ni in the range of 0.01%～0.03% (wt%), with the Ni content increased, the tensile strength has significantly increased while the elongation has no significant change. Added 0.03%Ni, generates a light sheet or mesh complexθ-phase, which containing trace amounts of Ni, can reduce the needle-like or flake-like Fe-containing split-phase, to improve the strength of alloys. (2) When Cr of 0.03%and 0.045%, the tensile strength and elongation are both high, and adding Cr generated the short rod-like or small flakes off-whiteπ-phase can partly replace the needle-like or flake-like Fe-containing phase, reducing the fragmentation of the role of alloy. (3) When Zn of 0.15%, the tensile strength reaches the maximum, while the elongation always decreases with the Zn content increases, Zn does not exist in a form of phase but as solid solution in the form of relatively uniform distribution in the matrix.By the orthogonal test, using the comprehensive evaluation of the weighted value of y * as the evaluation of the comprehensive performance of alloys, analysis showed that Cr affected the most, followed by Ni and Zn, and the optimal ratio is Ni0.02Cr0.03Zn0.2. Under this ratio the tensile strengthσb reach 233.51MPa, and the elongationδis 2.46%. When its heat treatment cycle is 505℃x6 h+170℃x8h, the tensile strength is up to 374.8MPa of the optimized alloy. Through the establishment of the fitting curves of 3 alloy elements Ni, Cr, and Zn affect the mechanical properties respectively, obtained the optimal range of their contents were:Ni 0.018%～0.022%, Cr 0.028%～0.032%, and Zn 0.18%～0.22%. The SEM organization and EDS analysis of the optimized alloy showed that the newly generated contain trace amounts of Ni, Cr, and Zn short rod-like or granular gray complexωphase, can neutralize the harmful effects of Fe, have a certain pinning and strengthening effects to the matrix alloy, but also induce the Al2Cu phase nucleation in its surroundings, thus enhance the strength of the Al-Si cast alloy.In the conditions of heat treatment, the new phase generated by adding three trace elements have tended to be round, evenly and distribution dispersed. The Al2Cu phase will be integrated into theα-solid solution in the solution treatment, and will be precipitated out in the aging process, thus strengthened the alloy. Those phases which formed by adding Cr are mostly dissolved into theα(Al), and the precipitation of Cr-richπ'-phase is the dispersed particle phase, playing a certain role in the dispersion strengthening. Zn are mainly in the form of solid solutions exist in the alloy which can be to promote dispersion of the distribution Al2Cu phase.By the contrast of electromagnetic levitation melting and normal melting, we can arrive that alloys melted by electromagnetic levitation melting grain size reduced significantly, the primary a-phase grain size is smaller, eutectic Si phase becomes small and dense, and distributed more evenly, organization spacing reduced greatly, and sharp edges are passivated mostly, also, porosities, inclusions and other defects have reduced. The performance of electromagnetic levitation melting alloys is superior than that the ordinary melting alloys.
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