Development of heat treatments for automotive components die-cast with secondary aluminium alloy at semi-solid state

A great contribution to the use of aluminium alloys comes from improvements in casting process. High-pressure die-casting allow to increase production and permit to realise complex-shaped castings with thin wall thickness. This is possible because the molten metal is injected at high velocity inside the die cavity and then it rapidly solidifies. However, these features reduce the quality of the component due to a substantial amount of air/gas entrapped during the filling phase. This denies the opportunity for further property enhancement by heat treatment at high temperature. Due to difficulty to produce high integrity die-castings, generally inexpensive secondary alloys are used. In order to eliminate or at least reduce the amount of entrapped porosity, semi-solid metal forming processes have been developed. In this work, the microstructural and mechanical evolutions of an AlSi7Cu3Mg secondary alloy die-cast at semi-solid state by SEED process are investigated over different heat treatment conditions. The spheroidization of eutectic Si, the progressive dissolution of Cu-rich phases and the hardness variations of the alloy were observed and quantified. The results indicate that the mechanical properties increase with the increase of solution temperature. The effects of artificial ageing temperature and time were then studied. By increasing the ageing temperature, the maximum hardness value decreases but it is reached at shorter holding time.