Influence of rolling route on microstructure and mechanical properties of AZ31 magnesium alloy during asymmetric reduction rolling

Four different routes of asymmetric reduction rolling were conducted on AZ31 magnesium alloy to investigate their effect on the microstructure evolution and mechanical properties. Route A is the forward rolling; while during routes B and C the sheets are rotated 180° in rolling direction and normal direction, respectively; route D is the unidirectional rolling. The strain states of rolled sheets were analyzed by the finite element method, while the microstructure and texture were observed using optical microscopy, X-ray diffraction and electron back-scattered diffraction techniques, and the mechanical properties were measured by tensile test. The results show that route D produced the largest effective strain. Compared with other samples, sample D exhibited a homogeneous microstructure with fine grains as well as a weak and tilted texture, in corresponding, it performed excellent tensile properties, which suggested that route D was an effective way to enhance the strength and plasticity of AZ31 sheet.     

脉冲电流轧制对AZ31镁合金微观组织与力学性能的影响

对比研究脉冲电流轧制工艺与温轧工艺对AZ31镁合金板材的力学性能、织构、微观组织与沉淀相等方面的影响。结果表明：脉冲电流具有促进冷轧AZ31镁合金低温再结晶能力的作用。脉冲电流轧制后的镁合金板材组织由细小的等轴再结晶粒与析出相构成,没有发现孪晶组织,并且完全再结晶,原始晶粒均被细小的再结晶晶粒取代,再结晶晶粒内的位错密度低。而温轧镁合金组织则由稍拉长变形孪晶、粗大的再结晶晶粒和析出相构成,再结晶的晶粒内位错密度高。两种轧制方式下的镁合金析出相均为Mg17Al12。脉冲电流轧制后镁合金的织构具有典型基面织构的特征,而脉冲电流轧制镁合金的织构则出现横向偏转;脉冲电流轧制后镁合金的屈服强度与伸长率均比温轧镁合金的大,但抗拉强度正好相反。     

AZ31镁合金轧制板材退火后的组织与力学性能

采用单向轧制和交叉轧制工艺分别进行了AZ31镁合金板材轧制实验,分析了轧制板材经退火处理后的组织与力学性能。结果表明:采用单向轧制工艺,当板材最终变形量相等时,经退火处理后的板材大压下量比小压下量得到的微观组织更为细小;交叉轧制得到轧制板材可以缓解镁合金轧制板材在室温下的各向异性,其微观组织较单向轧制均匀,具有更好的冲压成形性能;交叉轧制和单向轧制两种工艺得到的板材微观组织细化效果相似。     

Effect of deformation mode,dynamic recrystallization and twinning on rolling texture evolution of AZ31 magnesium alloys

In order to obtain quantitative relationship between (0002) texture intensity and hot rolling conditions, conventional rolling experiments on AZ31 magnesium alloys were performed with 20%–40% reductions and temperatures within the range of 300–500 °C. Shear strain and equivalent strain distributions along the thickness of the rolled sheets were calculated experimentally using embedded pin in a rolling sheet. Rolling microstructures and textures in the sheet surface and center layers of the AZ31 alloys were measured by optical microscopy (OM), X-ray diffractometry (XRD) and electron back scatter diffraction (EBSD). Effects of the rolling strain, dynamic recrystallization (DRX) and twinning on the texture evolution of the AZ31 alloys were investigated quantitatively. It is found that the highest (0002) basal texture intensities are obtained at a starting rolling temperature of 400 °C under the same strain. Strain–temperature dependency of the (0002) texture intensity of the AZ31 alloy is derived.     

Improving single pass reduction during cold rolling by controlling initial texture of AZ31 magnesium alloy sheet

The AZ31 magnesium alloy sheets obtained by multi-pass hot rolling were applied to cold rolling and the maximum single pass cold rolling reduction prior to failure of AZ31 magnesium alloy was enhanced to 41%. Larger single pass rolling reduction led to weaker texture during the multi-pass hot rolling procedure. The sheet obtained showed weak basal texture, while the value was only 1/3–1/2 that of general as-rolled AZ31 Mg alloy sheets. It was beneficial for the enhancement of further cold rolling formability despite of the coarser grain size. The deformation mechanism for the formation of texture in AZ31 magnesium alloy sheet was also analyzed in detail.     

变形参数对AZ31镁合金板材织构的影响

利用X射线衍射(XRD)方法测量了不同轧制状态,即不同变形温度和变形量条件下AZ31镁合金板材织构的变化特征。结果表明,经过轧制之后的AZ31镁合金板材形成强烈的基面织构;在250℃~400℃范围内,变形温度的升高、变形量的增大都会促进镁合金板材棱柱面、锥面等非基面滑移系的启动,从而影响各织构组分的锋锐程度和板材各向异性的强弱。随着变形温度的升高,镁合金板材的各向异性减弱;变形量的增大,镁合金板材的各向异性增强。     

Tensile properties and microstructure of AZ31B magnesium alloy sheet processed by repeated unidirectional bending

The mechanical properties of the AZ31B magnesium alloy sheet processed by repeated unidirectional bending (RUB) with a basal pole tilted in the rolling direction were systematically investigated at different temperatures. Compared with the as-received sheet exhibiting approximately the same average grain size, the fracture elongation increased while the proof stress decreased at room temperature. The initial texture had a significant influence on the mechanical properties at room temperature, while it showe...     

同步和异步轧制AZ31镁合金板材的织构及力学性能

采用同步轧制(NR)和异步轧制(AR)工艺对AZ31镁合金挤压板材进行了轧制,研究了轧制过程中组织和织构的演化,以及总压下量和异步比对轧材组织、织构和力学性能的影响。结果表明,在压下量为3%～15%的范围内,同步轧制与异步轧制板材在晶粒尺寸以及均匀性上有相似的变化趋势。轧制过程中,在变形初期,随压下量的增加,孪晶数量不断增加,孪晶使同步轧制与异步轧制板材中晶粒取向都发生偏转,即C轴趋向于垂直于法向(ND),从而使初始挤压板材的丝织构强度减弱;而当压下量达到24%时,孪晶大量减少或消失。在压下量为3%～24%的范围内,同步轧制对板材力学性能的影响并不明显,峰值应变呈交替变化;异步轧制板材在压下量达到24%左右时,表现出了良好的塑性变形能力,抗拉强度达到309MPa,峰值应变达到0.163。     

Cold stamping formability of AZ31B magnesium alloy sheet undergoing repeated unidirectional bending process

The repeated unidirectional bending (RUB) process was carried out on an AZ31B magnesium alloy in order to investigate its effects on the cold stamping formability. The limiting drawing ratio (LDR) of the RUB processed magnesium alloy sheet with an inclination of basal pole in the rolling direction can reach 1.5 at room temperature. It was also confirmed that cell phone housings can be stamped successfully in crank press using the RUB processed AZ31B magnesium alloy sheet. The improvement of the stamping formability at room temperature can be attributed to the texture modifications, which led to a lower yield strength, a larger fracture elongation, a smaller Lankford value (r-value) and a larger strain hardening exponent (n-value).     

退火工艺对高温叠轧AZ31镁合金板材组织与性能的影响

将高温叠轧变形和退火再结晶相结合,尝试共同调控AZ31镁合金板材的组织与织构。在300℃下对高温叠轧AZ31镁合金板材进行不同时间的退火处理,并研究了退火对高温叠轧板材组织、晶粒取向和力学性能的影响。结果表明:随退火时间的增加,界面结合质量逐渐提高,当退火时间为30 min时,部分区域出现冶金现象;显微硬度随退火时间的增加而降低;延长退火时间,高温叠轧板材非基面取向晶粒比重显著增加,同时,高温叠轧历史累积应变量、后续退火两者共同作用促使AZ31镁合金板材基面织构显著弱化。     

The effect of texture and strain conditions on formability of cross-roll rolled AZ31 alloy

The effect of texture evolution of AZ31 Mg sheet on cross-roll rolling process has been investigated for the commercial AZ31 Mg sheet. The large ?₂₃ was operated for the cross-roll rolled sample throughout the whole thickness, leading to the homogeneity of the (0 0 0 2) basal texture. After recrystallization annealing at 673 K for 30 min, cross-roll rolled sample showed uniform texture intensity from surface layer to middle layer different from the normal-roll rolled sample. An excellent formability of the cross-rolled specimen was achieved due to the developed homogeneity of the texture and microstructure refinement for the cross-roll rolled specimen. The strain conditions of surface and center layers were discussed in terms of experimental evaluations and three-dimensional finite element method (FEM) for conventional rolling and cross-roll rolling.     

Microstructure and mechanical properties of a basal textured AZ31 magnesium alloy cryorolled at liquid-nitrogen temperature

A strongly basal textured AZ31 magnesium alloy were cryorolled at liquid-nitrogen temperature at various strains. The microstructure and texture of the rolled sheets have been investigated using electron backscatter diffraction (EBSD) and X-ray diffraction. The microstructural and textural evolutions of the AZ31 magnesium alloy during cryorolling have been discussed. A lot of twins were observed in the rolled sheets. The influence of strain on the twin types and variant selection during cryorolling for the magnesium alloy has been discussed quantitatively based on the orientation data collected using EBSD. The influence of the twins on the microstructural and textural evolutions for the AZ31 magnesium alloy during cryorolling has also been discussed. The mechanical properties of the cryorolled sheets were tested by uniaxial tensile tests at the ambient temperature with a strain rate 10^-3s^-1 in the tensile direction respectively along the rolling and transverse directions of the rolled sheets. The relationships between the mechanical properties and microstructure of the cryorolled sheets have been discussed in the present work. The active twinning during rolling at that cryogenic temperature has been found to play an important role in influencing the microstructure, texture, as well as the mechanical properties of the AZ31 magnesium alloy.     

The Microstructure,Texture and Mechanical Properties of the Rolled Al/Mg/Al Clad Sheets

Two types of three-layered Al/Mg/Al clad sheets were fabricated by hot rolling. The first (sheet A) underwent a single pass with a small rolling reduction of 33% and the second (sheet B) underwent four passes with a large rolling reduction of 71%, and both were subsequently annealed at 200 °C for 1 h. Microstructural examination and tensile tests on the fabricated sheets revealed that 17.8-μm-thick intermetallic compound layers (IMCLs) appeared at AZ31/5052 interfaces in sheet B while none were observed in sheet A. The AZ31 layers in sheets A and B exhibited basal textures with intensities of 15.1 and 9.8, respectively, and only sheet A exhibited tensile twins (TTs) in the AZ31 layer. Recrystallization resulting in grains was preferred near the AZ31/5052 interface and the intersections between TTs. Owing to its larger rolling reduction, more extensive recrystallization was observed in the sheet B component layers than in sheet A. Sheet B exhibited better mechanical properties with a much higher ultimate tensile strength (UTS) than sheet A (230 versus 102 MPa) and a slightly larger elongation (19 versus 17%).This indicates that texture intensities and the extent of recrystallization of component layers have a significant effect upon the mechanical properties of clad sheets.     

Microstructure and temperature monitoring during the hot rolling of AZ31

This study details the microstructural evolution during hot rolling of AZ31 alloy sheet using a pilot-scale rolling mill. The aim is to understand the deformation mechanisms leading to grain refinement under industrial processing conditions and to design and optimize the hot rolling schedule for AZ31 in order to produce sheet with a fine and homogeneous microstructure. The study examined three different hot rolling temperatures, 350, 400, and 450°C, and two rolling speeds, 20 and 50 rpm. A total thickness reduction of 67% was obtained using multiple passes, with reductions of either 15% or 30% per pass. It was found that the microstructure of the AZ31 alloy was sensitive to the rolling temperature, the reduction (i.e., strain) per pass and the rolling speed (i.e., strain rate). The results show that the large cast grain structure is broken down by segmentation of the cast grain through localized deformation in twin bands, where dynamic recrystallization occurs in these bands as well as at the grain boundaries (necklacing).     

AZ31镁合金挤压棒退火组织和织构的演变

试验研究了退火温度对AZ31镁合金挤压棒组织和织构的影响.结果表明:铸态镁合金挤压后,初始强点织构向(80°,90°,0°)面聚集,主要织构组分强度提高.对热挤压后的AZ31镁合金进行退火,可以细化晶粒,使组织均匀,300℃退火时平均晶粒尺寸5μm为最小;随着退火温度的升高,形变织构(80°,90°,0°)逐渐减弱,再结晶织构(0°,90°,0°)和(90°,55°,0°)逐渐增强,300℃退火之后二者均被弱化,400℃退火之后取向分布漫散度增大.     

AZ31镁合金挤出板降温热轧的组织和织构的演变

研究AZ31镁合金挤出板坯在降温热轧过程中的组织和织构的演变规律.结果表明:退火前滑移和孪生是主要的变形机制和取向硬化机制;退火后长条晶的滑移和细小等轴晶晶界扩散迁移的共同作用成为主要的变形机制;随着压下量的增大,析出物开始破碎和分散,压下量在70%～80%之间时,基面织构组分的取向密度存在突变最大值,形成硬取向较强的{0001}基面织构,软化退火能大幅减弱硬取向;通过一道67%大压下量和一次软化退火可顺利地将AZ31镁合金轧制成厚0.5 mm的薄板.     

Effect of Change in Strain Path During Cold Rolling on the Evolution of Microstructure and Texture in Al and Al-2.5%Mg

The effect of change in strain path during cold rolling on the evolution of microstructure and texture is investigated. For this purpose, high purity aluminum and Al-2.5%Mg alloy are deformed (~90% reduction in thickness) by unidirectional and cross cold rolling. Irrespective of the alloy system, copper-type texture is observed in unidirectional processed materials, while strong brass ({011}〈112〉) texture is developed during cross rolling. Unidirectionally rolled aluminum showed higher HAGB fraction, but similar HAGB spacing as compared to the cross-rolled aluminum after 90% reduction in thickness. At the same time, the internal misorientation in the cross-rolled 2N-Al is higher than in the unidirectionally rolled material. In contrast, Al-2.5% Mg alloy processed differently in both ways shows similar HAGB fraction, spacing, and internal misorientation distribution. These observations indicate that microstructure evolution due to strain path change is more strongly affected by dynamic recovery as compared to texture evolution.     

Cold stamping for AZ31B magnesium alloy sheet of cell phone house

Electric product house of magnesium alloy sheet is usually obtained by warm stamping owing to its poor plasticity and formability at room temperature. The formability of AZ31B magnesium alloy sheet can be improved by repeated unidirectional bending (RUB) process through control of (0002) basal texture. Compared with as-received sheet, the Erichsen value (IE) of the sheet underwent RUB process increases to 5.90 from 3.53 at room temperature. It is also confirmed that cell phone houses could be stamped successfully in crank press with AZ31B magnesium alloy sheets underwent RUB process. It provides an alternative to the electronics industry in the application of magnesium alloys.     

AZ31B镁合金轧制板材室温动态拉伸的SEM观察

利用动态拉伸台和扫描电镜(SEM)对AZ31B镁合金轧制板材进行了动态拉伸的微观形貌观察。试验证明,材料宏观织构和显微组织都会对镁合金轧制板材的室温拉伸性能产生影响;在轧制镁合金拉伸变形时滑移和孪生同时存在;裂纹的产生和扩展具有明显的方向性。轧制组织中大量的变形孪晶与应力方向的位向关系,是轧向和横向上裂纹产生及扩展机制产生较大差异的主要原因。     

AZ31镁合金板材纵波轧制形变规律及边部损伤分析

纵波轧制+平辊轧制(LFR)是一种减轻镁合金轧制边裂的新型轧制工艺,通过一道次纵波轧制+二道次平轧,可有效减少镁合金板材边裂。为了进一步明晰LFR变形规律,本文通过对比AZ31镁合金板材纵波轧制+平轧(LFR)及平轧+平轧(FFR)热-力耦合有限元虚拟轧制对比和物理实验,分析了纵波轧制变形区金属变形规律及其对板材边部损伤的影响。结果表明：纵波轧制形成了异形搓轧区,板材各部位受到较大的三向剪切作用;急速金属流动产生的塑性变形热避免了板材边部温降,有利于提升塑性;剪切及温度影响促使LFR板材形成混晶组织,降低了波谷部位损伤,进而有效抑制了镁合金板材边裂的产生及发展。