Stable and Unstable Flow in Materials Processed by Equal-Channel Angular Pressing with an Emphasis on Magnesium Alloys

Abstract

Magnesium alloys such as ZK60 exhibit strain softening when processed by equal-channel angular pressing (ECAP). Finite element modeling (FEM) was used to examine the flow process during ECAP with an emphasis on the importance of the strain-rate sensitivity m. The simulations show there is unstable flow and shear localization for values of m of 0 and 0.01, but the flow is stable for values of 0.05 and 0.1. The flow softening reduces the cross-sectional area of the billet and leads to an enhanced accumulation of damage at the upper surface. The simulations show that the presence of a back pressure increases the ability of the billet to fill the exit channel but does not remove the development of plastic instabilities such as shear concentrations. It is shown that an imposed back pressure reduces the level of the maximum principal stresses in the area in which deformation takes place, and this reduces the tendency for cracking of the billet during the pressing operation.

FEA of Plastic Flow in AZ63 Alloy during ECAP Process

“…However, as partial results suggest, even this eect cannot completely eliminate the emergence of cracks or other forms of failure, although they are greatly suppressed [15].…”

“…Together with other factors, the plastic ow of the material aects both the magnitude of the imposed strain and its homogeneity and, last but not least, the disposition of the processed material to the development of cracks. The plastic ow has been researched by several authors who checked various methods of evaluation from theoretical determination [18] through experimental tests [19] up to utilization of numerical modeling which probably includes the largest number of published works especially as regards application of the nite element method (FEM) [15,20]. Most of the published works consider absence of friction as an initial condition in the process and are most frequently executed for the angle φ = 90…”

Role of starting texture and deformation modes on low-temperature shear formability and shear localization of Mg–3Al–1Zn alloy

“…It is widely reported that crack-or localization-free ECAP (or other thermo-mechanical processing) of Mg alloys is possible at temperatures >200°C [9][10][11][12][13]20,21]. At these temperatures, a much higher strain rate sensitivity [2,22], significant dynamic recrystallization (DRX) activity [23] and non-basal slip activity are evident [2]. However, grain refinement is limited to the micron level, since 200°C is still relatively high for Mg alloys ($0.5T m ).…”

“…However, macro-defect-free thermo-mechanical processing of Mg-3Al-1Zn (AZ31), the most common wrought Mg alloy, is a challenge at temperatures below 200°C, due to deformation localization in combination with the formation of shear bands [20]. To the best of the present authors' knowledge, only the effects of macroscopic parameters on formability, such as the strain hardening rate and the strain rate sensitivity, have been investigated previously [20,22]. The microscopic reasons for shear band formation and localized DRX during the ECAP of Mg alloys remain unclear.…”

Deformation Heterogeneity on the Cross-Sectional Planes of a Magnesium Alloy Processed by High-Pressure Torsion

“…In practice, however, it is now recognized that there are often significant difficulties in the processing of magnesium-based alloys through ECAP at low temperatures due to the availability of only limited slip systems, the nature of the grain refinement process, and the development of unstable flow and segmentation. [10][11][12][13][14] By contrast, it is relatively easy to use HPT to process magnesium [15,16] and its alloys [15] at room temperature without the introduction of any cracking into the samples.…”