Effects of Strain Rate during Multi-Directional Forging on Grain Refinement and Mechanical Properties of AZ80Mg Alloy

Abstract

Samples of AZ80Mg alloy were multi-directionally forged (MDFed) at various strain rates in the rage from 3.0 × 10 −3 s −1 to 3.0 × 10under decreasing temperature conditions. The MDFing pass temperatures employed were 653 K, 605 K and 553 K. These passes, each of 0.5 strain, were applied up to a cumulative strain of ΣΔε = 1.5 at maximum. The average grain size decreased with increasing cumulative strain and depended only weakly on the strain rate. Microstructures with average grain sizes of 0.84 μm, 0.88 μm and 1.2 μm were attained when MDFed at strain rates of 3.0 × 10 −1 s −1 , 3.0 × 10 −2 s −1 and 3.0 × 10 −3 s −1. The microstructure evolved at 3.0 × 10 −1 s −1 was less homogeneous. This difference is interpreted as being due to the higher stored energy causing static recrystallization in the latter MDFing condition. As a result, the best balance of the mechanical properties was attained by MDFing at 3.0 × 10 −2 s −1. In this case, ultimate tensile strength (UTS) of 445 MPa was achieved together with a fracture strain of 22%. By comparison, MDFing at 3.0 × 10 −1 s −1 led to lower UTS and ductility of 410 MPa and 15% while the yield stresses were comparable.

Multi-Directional Forging and Warm Extrusion of AZ80Mg Alloys

“…During MDFing, the forging axis was changed by 90 degrees from pass to pass. 5,6) Pass strains of ¦¾ = 0.8 were employed. The forging temperature, 623 K, 593 K, 543 K, 513 K, 483 K, 453 K, 433 K at each forging pass, was chosen referring the reports by Miura et al 6,10) The processing conditions are briefly illustrated by solid lines in Fig.…”

“…2(d)), respectively Such warm temperature forging as low as 433 K without any cracking was enabled by the effect of GBS as well as DRX. 5,6,10) GBS takes place even at room temperature 11) and it becomes more substantial with decreasing grain size and with increasing temperature. 5,12) The changes in the grain size and the hardness with increasing cumulative strain were summarized in Fig.…”

“…Nevertheless, it fatally derives softening and, therefore, decrease in strength at room temperature. 5,6) The softening is caused by the drop of dislocation density due to DRX and dynamic recovery. 1,3,5) From the point of view of strengthening of Mg alloys, therefore, plastic forming at lower temperature is desirable.…”

“…It is known that, under certain conditions of low strain rate and elevated temperature, grain-boundary sliding (GBS) acts as one of the main mechanisms for the plastic deformation of ultrafine-grained (UFGed) Mg alloys. 5,6) When GBS dominantly controls the plastic deformation, superplasticity can occur. This phenomenon is known to drastically facilitate large plastic deformation.…”

Grain Growth Model of Ultra-Fine Grain AZ80 Magnesium Alloy Multi-Directionally Forged During the Isothermal Heating Process

“…When grain size is refined to the ultra-fine grain state of submicron, magnesium alloy has superplasticity even at room temperature [6 − 7]. In the wrought magnesium alloy field, equal channel angular pressing (ECAP) [8,9], multi-directional forging (MDF) [10,11], accumulative roll-bonding (ARB) [12,13] and other severe plastic deformation modes are mainly used to make magnesium alloy of fine grains microstructure at the current stage.…”