Ultra-Fine Grain Development in an AZ31 Magnesium Alloy during Multi-Directional Forging under Decreasing Temperature Conditions 1647 400 1.0 AZ31 1.49 e=3×103s1 2.22 T=473K ①④…↑ ② △e=0.8 453 423 (a ③ 473(0.5Tm) 493 100 523 700 623 ① 600 4 兰 ② Cumulative strain,∑e 克500 ③ ④ Fig.2 True stress-true strain curves of AZ31 alloy during MFD with ⑤ ⑥ decreasing temperature from 623 to 423k at a strain rate of 3 x 10-3s-1 A broken curve at 473K for as-annealed sample is also shown for 400 comparison. W Q p Q p 300 temperature;the initial grain size of 22.3 um decreases to (b) about 6.7 um after 1st-pass compression at 623K(Fig.3(b)). Cumulative strain,Σe 3.8 um after 2nd-pass compression at 523K(Fig.3(c))and Fig.1 Schematic illustration of the thermo-mechanical processing method about 0.8 um after 4th-pass compression at 473K(Fig.3(d)). used in multi-directional forging (MDF)with continuous decreasing The flow curves in MDF at 623 K shows significant strain temperature in each pass.(a)The loading direction is changed in 90 with softening after a stress peak,followed by steady state flow pass to pass (x→y→z.A pass strain△E=0.&.(b)The temperature (Fig.2),where equiaxed fine-grains are homogeneously and decreases from 623 to 423 K.WQ indicates water quenching. almost fully developed in the whole area,as can be seen in Fig.3(b).These results are almost similar to the previous data obtained by single-pass compression at 673 K.+)Yang et Mg alloy AZ31 during MDF are shown in Fig.2.The o-s al.4.5 have found that initial coarse grains are fragmented by relationship of single-pass compression for an as-annealed the boundaries of kink bands developed at early stages of sample tested at 473K is also shown by a broken line for deformation.The regions fragmented by kink bands are comparison.The latter exhibits a sharp and higher stress peak bounded by high angle boundaries in high strain,finally of above 300MPa at a low strain of 0.2,immediately resulting in evolution of a new grain structure.They followed by brittle fracture.On the other hand,the flow concluded that dynamic formation of new grains in Mg curves during early MDF show work softening following a alloy can result from a series of strain-induced continuous peak stress,then followed by steady state flow at high strains reactions,that is to say continuous dynamic recrystallization in high temperature region.The amount of work softening (cDRX).Kink bands are commonly generated perpendicular after a peak stress decreases with dropping processing against the basal plane of HCP lattice when the latter lies to temperature,and at temperatures lower than 473K steady parallel to the compression axis.During hot compression,the state flow appears with no flow softening.It is interesting to basal planes in the initial grains rotate gradually and note that the flow curves in single-pass and MDF tests at approach near perpendicular to the compression direction. 473 K are clearly different from each other,i.e.the flow stress After that,the generation of kink bands as well as full of around 1100 MPa at∑△e兰3.2 in MDF is about one-third development of new grains hardly takes place even in high of that in single-pass compression.Furthermore,the MDF strain.5)During MDF,in contrast,kink bands are developed processing was successful to deforme the Mg alloy to EAg in various directions and therefore crossed mutually due to 4.8 at 423K which is below 0.5Tm (Tm is the melting point). change in the compression axis from pass to pass.This process should lead finally to homogeneous and full develop- 3.2 Optical microstructures ment of fine grains over the whole area.8) Figure 3 shows the microstructures of (a)as-annealed and The lower flow stresses appearing during MDF of 473K (b)to(d)developed during subsequent MDF with decreasing (Fig.2)can be resulted from such a drastic grain refinement temperature from 623 to 423 K.It can be clearly seen in taking place by repeated MDF with dropping temperature. Fig.3 that the grain size decreases by straining with reducing During MDF at temperatures from 623 to 473 K,the averageMg alloy AZ31 during MDF are shown in Fig. 2. The
–" relationship of single-pass compression for an as-annealed sample tested at 473 K is also shown by a broken line for comparison. The latter exhibits a sharp and higher stress peak of above 300 MPa at a low strain of 0.2, immediately followed by brittle fracture. On the other hand, the flow curves during early MDF show work softening following a peak stress, then followed by steady state flow at high strains in high temperature region. The amount of work softening after a peak stress decreases with dropping processing temperature, and at temperatures lower than 473 K steady state flow appears with no flow softening. It is interesting to note that the flow curves in single-pass and MDF tests at 473 K are clearly different from each other, i.e. the flow stress of around 110 MPa at
" ¼ 3:2 in MDF is about one-third of that in single-pass compression. Furthermore, the MDF processing was successful to deforme the Mg alloy to
" ¼ 4:8 at 423 K which is below 0:5Tm (Tm is the melting point). 3.2 Optical microstructures Figure 3 shows the microstructures of (a) as-annealed and (b) to (d) developed during subsequent MDF with decreasing temperature from 623 to 423 K. It can be clearly seen in Fig. 3 that the grain size decreases by straining with reducing temperature; the initial grain size of 22.3 mm decreases to about 6.7 mm after 1st-pass compression at 623 K (Fig. 3(b)), 3.8 mm after 2nd-pass compression at 523 K (Fig. 3(c)) and about 0.8 mm after 4th-pass compression at 473 K (Fig. 3(d)). The flow curves in MDF at 623 K shows significant strain softening after a stress peak, followed by steady state flow (Fig. 2), where equiaxed fine-grains are homogeneously and almost fully developed in the whole area, as can be seen in Fig. 3(b). These results are almost similar to the previous data obtained by single-pass compression at 673 K.4) Yang et al.4,5) have found that initial coarse grains are fragmented by the boundaries of kink bands developed at early stages of deformation. The regions fragmented by kink bands are bounded by high angle boundaries in high strain, finally resulting in evolution of a new grain structure. They concluded that dynamic formation of new grains in Mg alloy can result from a series of strain-induced continuous reactions, that is to say continuous dynamic recrystallization (cDRX). Kink bands are commonly generated perpendicular against the basal plane of HCP lattice when the latter lies to parallel to the compression axis. During hot compression, the basal planes in the initial grains rotate gradually and approach near perpendicular to the compression direction. After that, the generation of kink bands as well as full development of new grains hardly takes place even in high strain.5) During MDF, in contrast, kink bands are developed in various directions and therefore crossed mutually due to change in the compression axis from pass to pass. This process should lead finally to homogeneous and full develop￾ment of fine grains over the whole area.8) The lower flow stresses appearing during MDF of 473 K (Fig. 2) can be resulted from such a drastic grain refinement taking place by repeated MDF with dropping temperature. During MDF at temperatures from 623 to 473 K, the average (a) σ σ 1.0 2.22 1.49 X Y Z ∆ε=0.8 0 1 2 3 4 5 300 400 500 600 700 Cumulative strain, Σ ε Temperature,/K W Q W Q W Q W Q W Q W Q (b) Fig. 1 Schematic illustration of the thermo-mechanical processing method used in multi-directional forging (MDF) with continuous decreasing temperature in each pass. (a) The loading direction is changed in 90 with pass to pass (x ! y ! z). A pass strain
" ¼ 0:8, (b) The temperature decreases from 623 to 423 K. WQ indicates water quenching. 0 1 2 3 4 5 0 100 200 300 400 Cumulative strain, True stress, 623 523 493 473 ε = 3 10-3 s -1 453 423 T=473K (0.5Tm) AZ31 σ /MPa Σ ε Fig. 2 True stress-true strain curves of AZ31 alloy during MFD with decreasing temperature from 623 to 423 k at a strain rate of 3
103 s1. A broken curve at 473 K for as-annealed sample is also shown for comparison. Ultra-Fine Grain Development in an AZ31 Magnesium Alloy during Multi-Directional Forging under Decreasing Temperature Conditions 1647