Abstract: AlSi7Mg is an age hardening aluminium alloy that is growing in popularity in additive manufacturing as its good processability makes it well suited to the powder bed fusion process, allowing for its use in the aerospace and automotive industries. The alloy relies on heat treatment to reach its peak strength, with precipitates in the Mg₂Si precipitation sequence providing the majority of this strengthening. Powder bed fusion, however, produces components with unique microstructures that differ to those of traditional manufacturing methods such as casting, meaning heat treatments must be carefully tailored for additively manufactured parts. This study therefore investigated the effects of solution treatment temperatures on the final properties and microstructures of T6 treated, additively manufactured AlSi7Mg. Due to the use of a heated build plate, samples aged from the as built and stress relieved conditions exhibited over ageing behaviour, necessitating a solution treatment step to induce useful strength properties. Solution treatments below 500 °C were found to provide minimal strengthening upon ageing due to the presence of the π-Al₁₈Fe₂Mg₇Si₁₀ phase, which reduced the amount of magnesium in solid solution and so reduced the age hardening response of the alloy by restricting the growth of strengthening precipitates. Solution treatments over 500 °C, however, demonstrated excellent age hardening due to the replacement of the π-Al₁₈Fe₂Mg₇Si₁₀ phase with the β-Al₉Fe₂Si₂ phase, increasing the concentration of magnesium in solid solution and therefore improving its age hardening response. A T6 treatment then demonstrated an improvement in yield strength and ductility when compared to the as built state, but a decrease in tensile strength and impact toughness. The variability in the age hardening response of AlSi7Mg suggests that the careful selection of a solution treatment temperature is crucial to property optimisation.

Key words: Additive manufacturing, Aluminium alloy, Microstructure evolution, Ageing behavior