Mechanical anisotropy and deep drawing behaviour of AZ31 and ZE10 magnesium alloy sheets

The influence of the initial microstructure on the deep drawability and the associated microstructural evolution in two different magnesium alloy sheets, AZ31 and ZE10, has been examined. Tensile testing at room temperature shows that the AZ31 sheet has high plastic strain ratios, r = 2–3, which are caused by strong basal-type texture. The ZE10 sheet shows lower r values, r ～ 1, as a result of its weak texture. Deep drawing experiments carried out over the temperature range 100–300 °C revealed that the ZE10 sheet can be successfully deep-drawn at lower temperatures than the AZ31 sheet. The ZE10 cups show earing despite the weak texture and low normal anisotropy, while earing of the AZ31 cups is negligible. In the ZE10 cups, deformation is accommodated mainly by 〈a〉 slips and by compression as well as secondary twinning. The occurrence of dynamic recrystallization is observed in successfully deep-drawn AZ31 cups.     

Effect of the width on AZ31 sheet rolling

A pyramid sheet has been used in experiments and three-dimensional finite element simulation to study the edge cracks and effects of width on AZ31 sheet under rolling.Results show that the edge cracks of Mg sheet comply with the Normalized Crockroft & Latham theory.It can be predicted by D=-0.124+0.09X -0.008X2,if D ≥ 0,the edge cracks occur.The sheet shearing deformation at the edge brings about cracks.The strain rate changes periodically with different X.With initial width increases,the recrystallized gra...     

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

The fatigue behavior of friction stir spot welded (FSSW) AZ31 magnesium alloy sheet joints was investigated by tension–compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.     

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy

In situ growth of Mg–Al hydrotalcite conversion film on AZ31 alloy has been developed by a two-step method. The characteristics of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electronic microscope (SEM) observation, electrochemical and immersion tests. The film formation process was proposed based on the open circuit potential (OCP) measurements and surface analysis. A precursor film with network cracks is first formed and then this film is transformed into a compact and uniform hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O) film after the post treatment. This dense Mg–Al hydrotalcite film can provide effective protection to the AZ31 alloy.     

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。     

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.     

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.     

Influence of rolling ways on microstructure and anisotropy of AZ31 alloy sheet

Two rolling ways,unidirectional rolling and cross rolling,were carried out on twin roll cast AZ31 alloy sheet to study the influence of strain path change on the evolution of the rolling microstructure and texture as well as the anisotropic properties of AZ31 alloy sheet with microscopy,X-ray diffraction technique and tensile tests.It is found that cross rolling gives rise to more uniform microstructure and stronger texture intensities compared with unidirectional rolling.The differences in the microstructure and texture intensities are reflected in the anisotropy characterized by the difference in the yield stress and the fracture elongation that were measured along directions in the rolling plane at angles of 0■,45■and 90■from the rolling direction.     

AZ31镁合金高应变速率轧制边裂及力学性能各向异性

在300~400 ℃对铸态AZ31镁合金进行平均应变速率为10~29 s^-1的高应变速率轧制,研究轧制后镁板边裂、组织及力学性能的各向异性。结果表明:随着平均应变速率的增加,轧制边裂得到改善,350 ℃和400 ℃下边裂长度变化相比300 ℃时更加平缓;晶粒尺寸在温升和应变速率综合作用下并不随平均应变速率的增加而减小,反而出现波动;在相对较低的应变速率下,由于组织中长条形晶粒的存在,导致板材的各向异性明显;随着平均应变速率的增加,长条形晶粒减少,再结晶完全,组织趋于均匀,轧板的各向异性得到改善;轧板拉伸断口中可观察到撕裂棱和韧窝,以韧性断裂方式为主。     

Microstructure and Mechanical Properties of AZ31 Mg Alloy Fabricated by Pre-compression and Frustum Shearing Extrusion

The AZ31 Mg alloys were processed by 6% pre-compression and frustum shearing extrusion at various temperatures, and the microstructure, texture and mechanical properties of the resulting alloys are systematically investigated. The results show that the grain size monotonically increases from 6.4 to 12.6 lm and the texture intensity increases from 6.7 to 9.6with the increase in the extrusion temperature. The combining effect of the pre-twinning and the frustum shearing deformation is found to contribute to the development of the weak basal texture in Mg alloys. The Mg alloy sheet produced at the extrusion temperature of 563 K exhibits excellent mechanical properties. The yield strength, ultimate tensile strength and elongation for the extruded alloys are 189.6 MPa, 288.4 MPa and 24.9%, respectively. Such improved mechanical properties are comparable or even superior to those of the alloys subjected to other deformation techniques, rendering the pre-compression and frustum shearing extrusion a promising way for further tailoring properties of Mg alloys.     

AZ31B镁合金及其焊接接头的疲劳断裂机理

对AZ31B镁合金进行疲劳实验,在2×106循环次数下,母材、对接接头、横向十字接头和侧面连接接头的疲劳强度分别为66.72,39.00,24.38和24.40MPa。采用光学显微镜对裂纹扩展特征进行分析,结果表明,AZ31B母材的疲劳裂纹宏观扩展路径平滑,但微观观察发现疲劳裂纹扩展方向曲弯,有些裂纹分成两岔;裂纹尖端扩展均为沿晶扩展。焊接接头裂纹均在焊趾部位起裂,对接接头和横线十字接头的裂纹沿着热影响区扩展;侧面连接接头的裂纹起裂位于焊脚部位。采用扫描电子显微镜对疲劳断裂机理进行分析。疲劳断口由准解理或解理台阶组成,均为脆性断裂,断口中存在二次裂纹,对接接头中存在疲劳条纹,其间距约为5μm。     

Failure Analysis of Warm Stamping of Magnesium Alloy Sheet Based on an Anisotropic Damage Model

Based on the frame work of continuum damage mechanics, a research work of anisotropic damage evolution in warm stamping process of magnesium alloy sheets has been carried out by means of a combined experimental-numerical method. The aim was to predict formability of warm stamping of AZ31 Mg alloy sheets by taking the thermal and damage effects into account. In the presented work, a temperature-dependent anisotropic yield function suitable for cold rolling sheet metals together with an anisotropic damage model was implemented into the a VUMAT subroutine for ABAQUS/EXPLICIT. The evolution of internal damage in the form of void growth and coalescence in AZ31 Mg alloy sheet was observed by means of scanning electron microscopy (SEM). Moreover, a coupled thermo-mechanical simulation of the stamping process was performed using the implemented code at different temperatures. The parameters employed in the simulation were determined by the standard tensile tests and algebraic manipulation. The overall anisotropic damage process from crack initiation to final propagation in local area of blank was simulated. Numerical results show that the prediction of the site of crack initiation and the orientation of crack propagation are consistent with the data observed in warm stamping experiments.     

Preliminary investigations on the Mg–Al–Zn/Al laminated composite fabricated by equal channel angular extrusion

In order to utilize the advantage of low density and overcome the disadvantages of poor corrosion resistance and low strength, the laminated composites of Mg alloys fabricated by different methods have attracted extensively attentions. In this paper, the equal channel angular extrusion (ECAE), which has been widely used to fabricate ultrafine grain bulk materials, was introduced to fabricate the Mg–3Al–1Zn/Al (AZ31/Al) laminated composites at different temperatures. After fabrication and annealing treatment at different temperatures, the microstructural evolution and phase constitution near the joining interface of AZ31/Al composites were evaluated using optical and scanning electron microscope as well as X-ray diffraction (XRD). The results indicated that the higher annealing temperature promoted the formations of thicker bonding layers and more reaction phases near the joining interface, which mainly included Mg₂Al₃ and Mg₁₇Al₁₂ phases. The AZ31/Al laminated composite fabricated at 300 °C revealed thicker diffusion layer than that fabricated at 200 °C after anneal under the same condition, and the appropriate technological parameters of fabricating AZ31/Al laminated composites were also discussed.     

A new method of determining forming limit diagram for sheet materials by gas blow forming

A new methodology is proposed to obtain forming limit diagrams (FLDs) of sheet materials using gas blow forming process at elevated temperatures. Tension–tension side of the forming FLD is achieved by using circular as well as elliptical dies of different aspect ratios. To achieve tension–compression side of FLD un-bonded bi-layer specimen with slots are utilized. The widths between the slots are varied to achieve different strain paths. A correlation is established between the hemispherical punch-based tests and GBF tests of samples with slots to achieve different strain paths. FLDs for automotive AZ31 magnesium sheet at 300 °C and 400 °C in two different orientations are determined. Increase in forming limits of AZ31 with increase in temperature is observed.     

AZ31镁合金的织构对其力学性能的影响

利用电子背散射衍射(EBSD)取向成像技术,分析AZ31镁合金热挤压棒材和轧制薄板的织构特点;对具有不同初始织构的镁合金棒材和薄板进行力学性能分析,并从织构角度分析棒材的拉压不对称性和薄板的力学各向异性。结果表明:挤压镁合金棒材具有主要以(0001)基面平行于挤压方向的基面纤维织构,存在严重的拉压不对称性,其原因在于压缩时的主要变形方式为{1012}1011孪生;热轧镁合金薄板具有主要以(0001)基面平行于轧面的强板织构,具有显著的力学性能各向异性,其原因在于拉伸时不同方向的基面滑移Schmid因子不同。     

AZ31镁合金板材室温拉深破裂行为

为了探索提高AZ31镁合金板材室温冲压性能的途径,通过断口形貌分析,对板材室温拉深变形过程中的破裂机理进行了研究.结果表明,室温下,对于普通轧制板材,在拉深比达到1.2后即在冲头肩部发生破裂;对于等径角轧制板材.其拉深比可达1.6以上;当拉深比达到1.8时,在杯形件凸缘发生破裂,断面光滑平整.为解理断裂.这主要与板材在拉深变形中的应力应变状态和其非基面织构有关.单向拉伸与断口分析表明其冲压性能和破裂行为的差异,主要是因为这两种板材织构不同所致.     

AZ31镁合金板材等径角轧制变形规律研究

对等径角轧制过程中AZ31镁合金板材的应力应变状态进行了分析,采用有限元对不同通道间隙下板材的应变状态进行了模拟,研究了不同通道间隙下镁合金板材晶粒取向的演变规律及其对晶粒取向的影响。结果表明,在等径角轧制过程中,板材在模具转角处受到剪应力和压应力的作用;随通道间隙的增加,板材的变形由剪切变形演变为剪切+弯曲变形,甚至弯曲变形;由于剪应力的作用,AZ31镁合金板材的晶粒取向由普通轧制所形成的基面取向转变为等径角轧制后的非基面取向,随着剪切变形量的减小,基面沿轧制方向的偏转角度也逐渐减小。     

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。     

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。