Modelling Effects of Copper on Microstructure and Properties of Al–Si–Cu Alloys

Abstract

The combined impacts of copper content, cold working and homogenization in aluminium–silicon alloys were examined using microstructure study, thermodynamic modelling, image analysis and hardness measurement. Two aluminium alloys of varying copper contents were cast in a permanent mould using induction furnace. Microstructural observation revealed the formation of varying amount of intermetallic phases due to the presence of copper. Image analysis confirms the relative changes in percent phase fractions owing to presence of copper as intermetallic phases increases. Al₂Cu phase was most favoured with increasing copper. Cold working was performed by reduction of 5%, 10% and 15% of the cast alloys by equal amounts of strain. Hardness measurements indicated increase in hardness values attributed to both higher amounts of copper and cold working. Al₂Cu phase was predicted contribute compared to Al₇Cu₄Ni and Alpha phases. Homogenization at 300°C for 4 hr would suffice to obtain a uniform structure. The reasons for changes in microstructure and mechanical properties for different copper content were established by thermodynamic phase fraction calculation. Higher amounts of copper leads to greater volume fractions of intermetallic phases. Using solidification calculations, it is found that copper in liquid encourages dendrite formation which was also evident from microstructure.

Addition of copper in aluminium alloys can significantly alter the secondary phase formation, with abrupt changes in liquidus and solidus temperatures and solidification regions. At the same time, hardness values are also adjusted based on the phases predicted. Copper has more affinity for the Al₂Cu phase compared to Al₇Cu₄Ni in the investigated alloys systems.