Effects of Ni and Cu Addition on Tensile Properties and Thermal Conductivity of High Pressure Die-cast Al-6Si Alloys

In this study, the effects of nickel and copper on the mechanical and thermal properties of Al-Si alloys were investigated, for different alloy compositions. Thermodynamic calculations were carried out to predict the alloys' solidification behavior (solidification temperature, solid fraction and physical properties (density, thermal conductivity, elastic modulus, bulk modulus)). The aluminum alloys were produced by high pressure die-casting process with a 125 ton die-casting machine. Microstructures in the Al-Si-Ni alloys were characterized by optical microscopy and scanning electron microscopy. The mechanical properties were evaluated by tensile tests. Thermal conductivity was measured using a laser flash method. The addition of Ni and Cu improved the mechanical properties of the alloys by different strengthening mechanisms. Solution strengthening was the more effective method of improving mechanical properties, compared with strengthening by intermetallic compounds. The addition of an appropriate concentration of soluble alloying elements consistently improved alloy strength. However, addition of alloying elements decreased the thermal conductivity of the primary alpha aluminum phase. EDS analysis confirmed that soluble alloying elements like Cu not only can form intermetallic compounds, but can also change other kinds of compounds. Accordingly, careful consideration should be given to the chemical composition of soluble/insoluble alloying elements when attempting to improve mechanical properties and thermal conductivity at the same time.