Simultaneous Improvement of Thermal Conductivity and Strength for Commercial A356 Alloy Using Strontium Modification Process

Abstract

The Aluminum–Silicon (Al–Si) die-casting alloys, such as the commercial A356 alloy, are expected to be used in heat-sink and the device with high thermal conductivity due to their high production efficiency of casting process. These fields also required them to possess sound mechanical properties. To meet these demands, the Strontium (Sr) was often utilized to modify the silicon phase. According to our current work, the secondary dendrite arm spacing of the α-Al grains prominently was decreased when 0.05%–0.25% strontium was added. The decrease of the secondary dendrite arm spacing (SDAS) lead to the enhancement of the mechanical strength as well as the improvement of the thermal and electrical conductivity. When the Sr dosage was 0.15% in the commercial A356 alloy, the strength and the thermal conductivity of the A356 alloy simultaneously reached the maximum value. The improvement of the electrical and thermal conductivity might by contributed by the formation of a good conductor, Al2Si2Sr phase, on the Si surface. Further investigations suggested that the improvement of thermal conductivity was mainly due to the modification effect of Sr on the eutectic Si phase, which enlarged the specific area between the α-Al/eutectic Si interface. The WDS analysis indicated that the solubility of Si decreased in aluminum matrix by increasing in the Sr dosage. However, the excessive dosage of strontium would result in the coarsening of the modified silicon phase, deteriorating the strength and the thermal and electrical conductivity of the modified A356 alloys. In this work, the commercial A356 alloy was modified by adding small amounts of strontium (Sr). This kind of aluminum alloy with sound fluidity could be formed by using casting process, and it was potential to be applied in the structural parts. After the Sr modification, the plate-like silicon (Si) phase was transformed to fibrous shape in the raw commercial A356. Meanwhile, it was found that the content of silicon in the Al solid solution was reduced.The XRD and SEM analysis suggested that the Al2Si2Sr phase was formed on the modified silicon phase, which was favorable to the conduction of free electron.