Abstract
This paper reports the effects of annealing and age hardening heat treatments on the microstructural morphology and mechanical properties of 7075 Al alloy. The material was cast in the form of round cylindrical rods inside green sand mould from where some samples were rapidly cooled by early knockout and others gradually cooled to room temperature. From the samples that were gradually cooled some were annealed while others were age hardened. Both the as-cast in each category and heat treated samples were subjected to some mechanical tests and the morphology of the resulting microstructures were characterised by optical microscopy. From the results obtained there is formation of microsegregations of MgZn2 during gradual solidification which was not present during rapid cooling. It was also found out that age hardening and annealing heat treatment operation eliminated these microsegregations and improve mechanical properties of 7075 Al alloy. It is concluded that microsegregation can be eliminated by rapid solidification and appropriate heat treatment process.
Highlights
Aluminium and its alloys are used in a variety of cast and wrought forms and conditions of heat treatment
From the outcome of this study, there is formation of microsegregations of MgZn during the gradual solidification of 7075 aluminium alloy due to solute redistribution of Mg and Zn but this was suppressed during rapid solidification
The microsegregations that were formed when it was gradually cooled are dissolved to form a homogeneous phase during the soaking period of heat treatment operations
Summary
Aluminium and its alloys are used in a variety of cast and wrought forms and conditions of heat treatment. The optimum properties of aluminium are achieved by alloying additions and heat treatments. One of the most commonly used aluminium alloy for structural applications is 7075 Al alloy due to its attractive comprehensive properties such as low density, high strength, ductility, toughness and resistance to fatigue[8,9,10,11]. It has been extensively utilized in aircraft structural parts and other highly stressed structural applications[12,13,14,15,16]
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