Research Progress on Rheological Behavior of AA7075 Aluminum Alloy During Hot Deformation

Introduction

AA7075 is a structural grade aluminum alloy containing Al-Zn-Mg-Cu in the range of 5.6-6.1wt% Zn, 2.1-2.9wt% Mg, 1.2-2.0wt% Cu and it exists in many temper conditions. Wang et al. (2012) reported that it possess specific strength greater than the 5xxx and 6xxx series and comparable to many steels. The alloy superior stress corrosion cracking resistance relative to the 2xxx series makes it suitable for most critical applications; and as such finds extensive use in the aerospace and ordinance industries. But, in spite of these attractive attributes of the AA7075 alloy, it is considered that its response to plastic deformation over a wide range of temperature and strain rates is poor, particularly at deformation temperature beyond 0.4Tm. Therefore, its application is limited to simple shapes. Thus, the high temperature deformation of the alloy is associated with several challenges such as dynamic softening which reduces the ductility of the alloy.

Ashtiani et al. (2012) in their work expressed hot temperature deformation to be plastic deformation processing conducted at temperature above 0.4Tm which is accompanied by both recrystallization and the development of new stress free grains such that the dislocation density is at equilibrium. In another instance, Humphrey and Hatherly (2004) referred to hot temperature deformation as high temperature thermo mechanical processing, and may be accompanied by grain refinement. Doherty et al. (1997) had previously considered hot working as thermomechanical processing in the temperature range 0.6T_m, and this was dictated by dislocation substructures of low density including sub grain migration which was further characterized by subgrain diameter, interior dislocation spacing and wall spacing related to angle of misorientation.

The challenges of thermo mechanical processing such as increased productivity, reducing scrap rate, production of almost perfect microstructures, and improving the quality of manufactured parts during processes like rolling, forging and specifically extrusion require enhanced scientific understanding of the flow characteristics in the candidate materials in relation to stress, strain, temperature, microstructural evolution and geometry of deformation during such processes.

Tilak (2005) employed Liquid aluminum refining system to improve the extrudability of aluminum alloy and reported that the flow behavior during thermo mechanical processes such as high temperature extrusion is influenced by multiplicity of variables involving both technical and metallurgical process parameters. This expression is supported by Humphreys and Hatherly (1996) in their literature on recrystallization and related annealing phenomena.

Altan (1983) identified the technical parameters to include deformation temperature, strain, strain rate, shear rate, friction, ram speed, tool geometry; while metallurgical parameters include chemical composition, degree of prior strain, initial grain size, metallurgical structure and phases. Yang et al. (2014) associated deformation direction as another significant technical parameter influencing the flow behavior of AA7075 during hot deformation. It must be stated that initial grain size, metallurgical structure and phases are further influenced by type of grain refiner, degree of dispersion, solute content, size and distribution, inter-particle spacing, and coherency of second phase particles. Sun et al. (2014) for instance, reported the presence of η phase (MgZn₂), ή phase (MgZn), T phase (Al₂Mg₂Zn₃) and S phase (Al₂CuMg) in AA7075 alloy. This is reinforced by Abolhasani et al. (2012) of the evidence of presence and distribution of Mg(Zn₂, AlCu) M (or η) hexagonal phase, S (Al₂CuMg) orthorhombic phase, Al₃₂(MgZn)₄₉ T phase and Fe rich phases such as Al₇Cu₂Fe and Al₃Fe second phase particles in the grain interior and along the grain boundaries of a AA7075 alloy.