Study of homogenization heat treatment of AL-MG-SI billets produced by direct chill casting

Abstract

The homogenization heat treatment of as-cast billets is a significant process in aluminium extrusions. This process is characterized by a large evolution in the microstructure when going through the different stages of the morphological process. This change involves several key steps: the disruption of eutectic morphology by processing inter dendritic phases into round shapes, reduction of in grain segregation though diffusion, re-melting of low melting phases to achieve enhanced extrusion surface finish, and precipitation of nano sized dispersions (for alloys containing Cr, Zr, Mn, and Sc) to arrest grain boundary movement and avoid recrystallization. Post homogenization cooling may also cause reprecipitation of some phase that can change the flow stress required for the next extrusion. However, all these precipitates have an adverse effect on the mechanical characteristics of the alloy and inhibit age hardening ability requiring their dissolution during solution heat treatment. The processes of development of microstructure take place during homogenization and subsequent cooling. Therefore, the main aim of the present study is to systematically investigate the microstructural change during homogenization heat treatment and the chemical composition of the samples before and after the treatment. Investigation carried out on as-cast 6xxx aluminium billets shows that both ßAl5FeSi intermetallic phase and coarse Mg2Si particles are detrimental to the subsequent operations like extrusion. For proper surface quality of the extruded products, the billets are heat treated thorough homogenization before the extrusion is carried out. Typically, during the heat treatment, there are unwanted intermetallic particles, ß-AlFeSi platelets, are converted to rounded α-Al(FeMn)Si intermetallic particles. The reactions, which take place during homogenization process, were investigated up to the present time by experiments, providing some information about the process and alloys properties. In the present study the transformations of 6060 and 6063 have also been studied using Light Optical Microscope, hardness tester and Optical Emission Spectroscopy.

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