Abstract
There is a strong drive to near net shape process 7000 series aluminium alloys, which are currently machined from the wrought state with much waste. Semi-solid processing is one potential near net shaping route. It relies on the thixotropic behaviour of alloys with non-dendritic, spheroidal microstructures in the semi-solid state. If such alloys are sheared they thin and flow to fill the die. One route to spheroidal semi-solid microstructure is by reheating worked material into the semi-solid state. During the reheating, recrystallisation occurs and as liquid forms it penetrates the recrystallised boundaries to form spheroids. Here we examine the formation of spheroids in as-supplied 7075 aluminium alloy. It is generally known that 7075 is very resistant to recrystallisation in the solid state due to the presence of dispersoid particles pinning grain boundaries. In this work, we have reheated 7075 in the extruded and T6 condition into the semi-solid state. As the temperature rises into the semi-solid regime there is a sudden increase in the appearance of spheroidal grains. The occurrence of recrystallised grains is closely associated with the location of the first liquid to form above the solidus. Fully spheroidal microstructures are obtained on reheating as-supplied material to temperatures in the region of 580 °C, although the fractions of liquid at that temperature are still low at around 5%. Conventionally there is extra cost involved in obtaining spheroidal microstructure feedstock for semi-solid processing because additional steps are introduced into the process route. The advantage here is that as-supplied material can be directly thixoformed once appropriate liquid fractions are attained.
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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