Abstract
Aluminum alloys are widely applied in the power industry due to their excellent conductivity, low cost, good formability, corrosion resistance and low density. In the case of electrical products, if heat dissipation characteristics are not secured, efficiency and lifespan rapidly drop, so heat dissipation performance is required for high output. However, when other elements are added to pure aluminum, the conductivity is lowered due to solute atoms, grain boundaries, dislocations and precipitates. Therefore, it is difficult to satisfy both high strength and high conductivity, and research on improving strength while minimizing the decrease in conductivity is required.
In Al alloys, Bi, Sb, Sc and Sr elements have been reported to be good modifiers. It is well know that modification treatment with addition of small amounts of Sr results in refining of the eutectic phases, specifically grain refinement of the eutectic Al phase. Additions in the Sr modify the eutectic Si morphology from coarse plate-like into fine fibrous, thereby improving the mechanical properties and castability.
The alloys used in this study had the nominal composition of Al-0.8wt%Zn-0.4wt%Mg-0.2wt%Mn-xSr alloys (x=0, 0.5 and 1.0 wt%). For alloy melting, a graphite crucible was filled with alloying elements and maintained at 820 °C for 20 minutes in a high-frequency induction furnace. It was gravity cast in a steel mold (diameter = 75 mm, height = 250 mm) preheated to 200 °C to remove residual moisture and prevent rapid cooling of the molten metal. The prepared cast billet was processed by lathe processing to form an extruded billet with a diameter of 70 mm and a height of 90 mm suitable for extrusion. The machined billets were hot extruded into a plate shape of 50 mm width and 4 mm thickness without any defects.
The microstructures of the alloys were examined using a field emission scanning electron microscopy (FE-SEM) and electron backscatter diffraction (EBSD) system. The phase composition of the alloys was examined using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (EDS). The electric conductivities of the alloys were measured using the eddy current method at room temperature. The mechanical properties of the as-extruded Al alloy specimens were measured by universal testing machine with ASTM E8M standard. Tensile tests were carried out at an initial strain rate of 1.0×10−3 s−1.