Effects of Cr and Ti Addition on Mechanical Properties and Thermal Conductivity of Al-7Si-3Mg Die-Casting Alloys

The effects of Cr and Ti on the thermal conductivity and mechanical properties of Al-7Si-3Mg die-casting alloys were investigated in the present study. For alloy design, solidification behavior and thermal conductivity of all phases were examined by thermodynamic calculations. The results showed that the alloys with intermetallic compounds exhibited good mechanical properties, but thermal conductivity was relatively low. As a result of predicting solidification behavior and physical properties of each phase through computational thermodynamic simulations, the effects of the addition of Cr and Ti on the change in the fraction of the precipitated phase in Al-Si-Mg-Fe alloys were examined. Yield strength and ultimate tensile strength were found to be the highest in the case of the Al-7.2Si alloy with the addition of 4.39 wt%Mg and 0.45 wt%Cr (S-5 alloy). The mechanical properties of other alloys (S-1 to S-4) showed almost similar values. Thermal conductivity was found to be similar in the case of S-1 to S-3 alloys. The thermal conductivity was found to be directly proportional to the fraction of alpha-Al phase and the lowest in the case of S-4 alloy due to the presence of low fraction of the alpha-Al phase. The S-5 alloy was found to have the best mechanical properties as well as optimal thermal conductivity. To improve the mechanical property and thermal conductivity simultaneously, it is important to control the fraction of the second phase of the alloy. In contrast with Cr which formed intermetallic compounds with major alloying elements, Ti did not produce intermetallic compounds at a small addition. However, Ti affected the solidification behavior of other phases which changed the properties of the alloys. It was found that not only the type of alloying element but also the amount of addition are important for the improvement of thermal conductivity and mechanical properties of Al-Si-Mg alloys.