Abstract
7000 series aluminium alloys are currently machined from the wrought state with much waste. There is therefore a motivation to identify effective near net shaping routes. Semi-solid processing is one such potential route. It relies on the thixotropic behaviour of alloys with non-dendritic spheroidal microstructures. In the semisolid state the material thins when sheared and will flow to fill the die. When worked material is reheated, it recrystallises and moving into the semi-solid state, the required spheroidal microstructure develops. Here we examine the early stages of spheroid formation in 7075 aluminium alloy reheated from the as-supplied T6 condition. This alloy is very resistant to recrystallisation in the solid state due to the presence of dispersoid particles pinning grain boundaries. There is a sudden increase in the appearance of spheroidal grains. There is a close association with the position of the first liquid to form. On reheating as-supplied material to around 580°C (fraction of liquid ∼5%), a fully spheroidal microstructure can be obtained.
Highlights
The strongest wrought aluminium alloys are those in the 7XXX series
Fe and Si are present in the alloy as impurities and give rise to constituent particles 1-30μm in size, which are detrimental to mechanical properties [3] and are resistant to dissolution. 7075 is usually supplied in the worked and heat treated state
Recrystallisation in the solid state is defined as the formation and migration of large angle grain boundaries where the driving force is the decrease in the free energy from the decrease in dislocations
Summary
The strongest wrought aluminium alloys are those in the 7XXX series. These are based on the Al-Zn-MgCu system. The dispersoid particles are soluble only in liquid and include Al12Mg2Cr and Al18Mg3Cr2 (E-phase) [4]. These are 0.5-2μm in size and play an essential role in pinning grain and sub-grain boundaries. Provided the melting sequence matches the solidification sequence, according to Backerud et al [2], the first liquid to form during reheating would be based on the sub-micron sized MgZn2 at around 470 ̊C. This would be followed by a reaction between Al and Mg2Si at around 550 ̊C/560 ̊C. Understanding the development of the spheroidal microstructure will enable process optimisation
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