Homogenisation of aluminium composite reinforced with Mg2Si

Abstract Al–Mg 2 Si composites are lightweight materials with potential for aerospace and automotive applications. In situ formation of the Mg 2 Si reinforcing phase in the Al–Si–Mg system enables the production of metal-matrix composites with favourable properties; however, coarse primary and eutectic Mg 2 Si particles in conventionally cast materials can limit their mechanical performance. The aim of this study was to investigate the effect of homogenisation heat treatment on the microstructure, phase constitution and hardness of an in situ Al–19Mg–11Si composite. An in situ Al–19Mg–11Si composite with a Mg/Si mass ratio of 1.69 was produced by casting. Samples 30 mm in length were homogenised in an electric furnace, while one sample was retained in the as-cast condition. The heat treatment consisted of homogenisation at 430 °C for 4 h, followed by heating to 520 °C and holding for either 12 or 48 h. Differential scanning calorimetry (DSC), light microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to characterise the thermal behaviour and microstructural evolution. The hardness of the composite matrix and primary Mg 2 Si particles was also evaluated. The results showed that homogenisation significantly modified the morphology of the Mg 2 Si phases. After 12 h at 520 °C, the initially polygonal primary Mg 2 Si particles began to round off, while after 48 h they exhibited substantially more rounded morphologies with approximately 11% reduction in the fraction of primary Mg 2 Si. The eutectic (α–Al + Mg 2 Si) was also progressively refined and spheroidised with increasing homogenisation time. EDS analyses confirmed the presence of primary Mg 2 Si, eutectic Al 13 Fe 4 , Mg-saturated α–Al and eutectic (α–Al + Mg 2 Si), while β–Al 3 Mg 2 was additionally detected in the as-cast condition. Although homogenisation did not significantly increase the overall hardness of the composite, the hardness of the primary Mg 2 Si reinforcing particles increased considerably. These results demonstrate that homogenisation at 520 °C effectively modifies the Mg 2 Si morphology and microstructural state of the investigated composite.