Property Changes of Pure Aluminum Castings with Inserted Aluminum Nitride Substrates Caused by Thermal Cycling

In this study, changes in the properties of pure aluminum (trace elements: Mg, Si, Ti, and Fe) castings with inserted aluminum nitride (AlN) substrates caused by thermal cycling similar to that of in-use power modules were examined. The hardness of the late-solidified part of the casting with AlN inserts after thermal cycling (500 cycles) was considerably higher than that before cycling; notably, this increase in hardness was not observed in castings without AlN inserts. In contrast, the electrical conductivity, density, and crystal grain size of the castings remained almost constant irrespective of thermal cycling and the presence of AlN inserts. The microstructures of these castings revealed no notable precipitate phase. Electron backscattering diffraction analysis indicated that the transgranular crystal orientations of the castings with AlN inserts before and after thermal cycling were uniform and nonuniform, respectively, suggesting that thermal cycling increased the number of dislocations. Therefore, work hardening due to repetitive plastic deformation produced by the difference between the thermal expansions of pure Al and AlN during thermal cycling is key to increasing the hardness of castings.