Continuous dynamic recrystallization during hot torsion of an aluminum alloy.

Abstract

Most commercial aluminum alloys are characterized by dynamic recrystallization at very large deformations in a continuous manner. The present study deals with the characterization and modeling of the evolution of the microstructure of an aluminum wrought alloy at large plastic deformations. Hot torsion tests of the AA6082 aluminum alloy are carried out using the thermomechanical simulator Gleeble®3800 in a wide range of temperatures and strain rates. The use of water quenching immediately after deformation avoids any static restoration during cooling. Microstructural investigations are carried out by means of electron back scattered diffraction using a scanning electron microscope to determine the grain and subgrain structures, as well as the misorientation distributions. In-situ synchrotron radiation tests during hot torsion are used to confirm the continuous dynamic recrystallization (CDRX) by the evidence of the conversion of low angle boundaries (LAGBs) into high-angle boundaries (HAGBs) and the formation of new texture. Experimental investigations show that CDRX starts with the formation of LAGBs at low strains (center of the sample). By subsequent straining (close to the surface of the sample), the accumulation of dislocations at the LAGBs causes an increase in their misorientation until a critical value is reached and LAGBs transforms into HAGBs. The developed model consists of a microstructural model, equation rates and constitutive equations. The microstructure is described by three internal variables. Their rates are evaluated using the Kocks-Mecking model. The modelled and experimental flow stresses show softening due to the consumption of dislocations and the continuous formation of new HAGBs.

A Review of Microstructural Evolution and Modelling of Aluminium Alloys under Hot Forming Conditions

“…Grain bulging, dislocation rearrangement and boundary movement are the dominant mechanisms [28]. CDRX is the most commonly occurring mechanism observed in aluminium alloys under hot forming conditions, having been observed in alloys such as 7075 [21,[29][30][31], 6082 [32,33], 7050 [34] and 5052 [35]. During this process, dislocations are continuously generated and rearranged to form subgrains bounded by low angle grain boundaries (LAGB, with misorientation of 2-15 • ).…”

“…Unlike for DDRX, the essence of CDRX is the continuous formation and rotation of subgrains created by dislocations, directly changing the orientation of subgrains and forming new grains [32,33,41].…”

“…CDRX usually begins along GB due to their high stored energy, especially where microshear bands form under a large localised strain [13]. Unlike for DDRX, the essence of CDRX is the continuous formation and rotation of subgrains created by dislocations, directly changing the orientation of subgrains and forming new grains [32,33,41]. As illustrated in Figure 2a, CDRX can be expressed in terms of cell structure formation in images (1) and (2), subgrain rotation (image 3), and conversion of grain boundaries from LAGB to HAGB (image 4), resulting in the final recrystallised grain microstructure seen in image (5).…”

“…An example of this is given in Figure 2b, in which the numbers of the images correspond to the schematics in Figure 2a. CDRX is usually observed during tension, compression and torsion testing up to 0.7 T m , (where T m is the melting temperature) and has been seen under different strain rates from 0.0005 to 10 s −1 [32,33,35,[45][46][47]. CDRX has also been shown to exist, according to observations such as (i) a continuous increase in boundary misorientation with strain and (ii) recrystallised grains of smaller size than the original grains, in severe plastic deformation (SPD) processes, such as equal channel angular pressing (ECAP) [48,49], high pressure torsion (HPT) [50] and accumulative roll bonding (ARB) [14].…”

“…Several such oscillations in flow stress may occur before a final decrease towards a steady state. During CDRX or GDRX, the stress increases monotonically up to a peak value and then decreases into a steady state, as shown in curve (3) [29,31,33,42,44]. Higher temperatures and lower strain rates accelerate the recovery, giving rise to lower stresses both at the peak and in the steady state [98].…”