Interactive study on phase morphology and mechanical characteristics of diffusion annealed and age hardened Al7075-steel powder reinforced metal matrix composites

Abstract

This paper expounds the effects of diffusion annealing and age hardening heat treatments on the phase morphology and mechanical properties of Al7075-eutectoid steel powder (0.8 wt% carbon steel) reinforced composites. Eutectoid steel powder ranging from 55-70 μm is selected due to its ability to improve the bulk hardness of the composite. In the present study three different weight fractions of eutectoid steel powder composites are prepared by conventional stir casting process. The material is cast in the form of rods inside the permanent stainless steel mould from where some samples are cooled in still air to room temperature. From these samples some are diffusion annealed from isothermal holding of 6 h i.e., above recrystallization temperature (600 °C) followed by boiling water quench and few are subjected to age hardening for a fixed time duration of five hours. Age hardening is carried out at solutionising temperature 560 °C tailed by aging at two temperatures 100 °C and 180 °C independently. All the samples in each category are subjected to critical property characterization and the phase arrangement information are characterized by scanning electron microscopy. From the analysis of microstructure and destructive tests, it is clear that the as-cast (stir-cast) samples display dendritic segregations with wider grain boundary area, is responsible for the inferior mechanical properties. It is also found out that diffusion annealing and age hardening heat treatments eliminates micro-segregation and improve mechanical properties of Al7075-eutectoid steel powder reinforced composites. It is concluded that chemical inhomogeneity in dendrites can be eliminated by selecting the suitable heat treatment with proper heat treatment parameters.