轧制工艺对汽车用镁合金板材组织和性能的影响

对AZ31镁合金铸轧板材进行了不同初轧温度的多道次不同路径轧制试验。通过显微组织观察、室温拉伸试验研究了不同初轧温度和轧制路径对AZ31镁合金板材的组织和性能的影响。结果表明:在300~450℃,随着初轧温度的升高,AZ31镁合金板材试样平均晶粒尺寸逐渐增大,初轧温度达到450℃时,晶粒发生明显长大。相同初轧温度下,轧制方向交替变化轧制的AZ31镁合金板材试样比单向轧制试样晶粒更为细小。随着初轧温度的升高,试样的抗拉强度和屈服强度逐渐降低,伸长率先降低后升高。采用轧制方向交替变化轧制的AZ31镁合金板材具有更优的力学性能。     

应变速率与温度对AZ31B镁合金板材各向异性的影响

由于镁合金复杂的变形机制以及现有制备工艺不够成熟,导致其各向异性明显,力学性能不太稳定,对镁合金成形性能有较大影响,因此,对镁合金板材各向异性的深入研究具有非常重要的意义。对AZ31B镁合金轧制板材进行了单向拉伸试验,主要研究其在不同应变速率和不同温度条件下的各向异性。结果表明:室温下,AZ31B镁合金力学性能随着应变速率的变化呈现出不同程度的各向异性,且45°方向试样的力学性能优于其他两个方向的;随着温度的升高,其力学性能的变化趋势在三个方向上均一致,强度的各向异性行为逐渐减弱,当温度升高到400℃时,强度的各向异性现象基本消失。     

连续轧制AZ31镁合金板材退火组织织构和力学性能

通过光学显微镜、背散射电子衍射分析(EBSD)和室温拉伸试验研究了多道次连续轧制AZ31镁合金板材经200~400℃不同温度退火1 h后晶粒尺寸和微观织构的演化及其与力学性能的关系。结果表明:轧制板材经250℃×1 h退火后,静态再结晶几乎完成,晶粒细小均匀,平均晶粒尺寸约5.5μm,综合力学性能良好,抗拉强度和断后伸长率分别达到261 MPa和26.7%;当退火温度不高于350℃时,退火态板材基面织构较轧态低且差别较小。随退火温度升高,晶粒缓慢长大,晶界取向角分布由10°和30°双峰连续分布转变为30°单峰连续分布。此时,抗拉强度主要与晶粒尺寸有关。当退火温度达到400℃时,再结晶晶粒发生异常长大,基面织构急剧增强,晶界取向角呈离散分布,导致抗拉强度增加,而伸长率显著降低。     

镁合金AZ31轧制板材的单向拉伸行为

通过单向拉伸试验研究了AZ31镁合金轧制板在不同温度和应变速率下的力学性能。根据镁合金在50℃~400℃范围内的单向拉伸曲线分析结果,找出AZ31镁合金的抗拉强度、伸长率随变形温度、变形速度的变化规律。结果表明:AZ31镁合金轧制板的塑性随着应变速率的降低有明显提高;温度的升高可明显改善轧制板的塑性;当应变速率为1.5×10-2s-1、温度为400℃时,伸长率达到123.9%。     

7075铝合金高温力学性能及本构方程研究

在变形温度为440、460和480℃,应变速率为0. 001、0. 01和0. 1 s~(-1)的条件下,依次沿0°、45°和90°的轧制方向,对7075铝合金板材进行热拉伸试验,研究7075铝合金的高温力学性能。结果表明:7075铝合金的力学性能受变形温度、应变速率及轧制方向的影响,7075铝合金的抗拉强度随变形温度的升高而降低,随应变速率的增大而增加,且抗拉强度的增长率比较大;抗拉强度在轧制方向为0°时最高,45°时次之,90°时最低。通过对7075铝合金热拉伸获得的试验数据进行参数拟合,建立了在不同轧制方向上的Arrhenius型本构方程。     

轧制温度对大应变轧制AZ31合金板材显微组织和力学性能的影响

采用大应变轧制技术制备AZ31合金板材,研究了轧制温度对板材显微组织、宏观织构和力学性能旳影响。结果表明,轧制温度为200℃时,板材发生开裂,轧制温度升高至250~400℃时,大应变轧制可以成功进行;在250~400℃的轧制温度范围内,板材再结晶晶粒尺寸和基面织构强度随轧制温度的升高而增大,其力学性能则随轧制温度的升高而下降;轧制温度为250℃时,板材具有良好的综合力学性能,其抗拉强度、屈服强度和伸长率分别为325.7 MPa、213.2 MPa和29.8%。     

轧制温度对AZ91镁合金组织及性能的影响

研究了不同变形温度对AZ91铸态镁合金轧制成形能力、微观组织及力学性能的影响,对热轧过程中第二相的分布及边裂行为进行了分析。结果表明,变形温度对AZ91铸态镁合金轧制成形性能影响显著。350℃和450℃轧制时,试样边部及表面出现明显横向裂纹,变形温度为400℃时,可获得较前两者更均匀细小的轧制组织,轧制成形性能明显提高。轧制试样的室温拉伸性能指标呈现先增加后减小的趋势,在400℃时达到最大值,抗拉强度和伸长率分别达到289 MPa和10.4%。     

温度对AZ31镁合金轧制板材冲压性能的影响

通过在不同温度下单向拉伸实验,分别沿轧向、45°方向和横向对AZ31镁合金轧制板材的冲压性能进行了研究.结果表明:随着变形温度的升高,板材抗拉强度和屈服强度下降,断裂伸长率提高,应变硬化指数和塑性应变比降低.拉伸性能得到改善;温度高于200℃时,板材的冲压性能得到改善,其屈强比为0.876,应变硬化指数为0.158,塑性应变比为1.307.     

AZ31B镁合金高温拉伸变形行为的研究

通过高温拉伸试验,研究了AZ31B镁合金板材在250～450℃以及应变速率0.001 s-1、0.01 s-1条件下的高温变形行为,获得了材料的厚向异性系数、伸长率等成形性能参数及有关组织特征.结果表明,不同变形条件下AZ31B合金的真应力-真应变曲线均出现峰值,峰值应力随变形温度的升高和应变速率的降低而减小;硬化速率随变形温度的升高而降低,在温度高于250℃时变化不大.当变形温度为250 ℃,应变速率为0.001 s-1时,合金的厚向异性系数达到最大.随变形温度的升高,AZ31B镁合金的塑性显著提高.合金的动态再结晶温度为250℃,随着应变速率增大,合金发生动态再结晶的速度加快.     

轧制温度对汽车用ZK61镁合金组织和力学性能的影响

采用OM、SEM、室温拉伸性能测试研究了不同轧制温度对ZK61镁合金板材组织与力学性能的影响。结果表明:在轧制温度300～420℃范围内,380℃轧制的ZK61镁合金试样内基本无孪晶组织,均匀性最好,晶粒平均尺寸9.6μm,晶粒最为细小;在420℃轧制的试样晶粒长大,组织均匀性下降。随着轧制温度的升高,抗拉强度逐渐降低,伸长率先升高后降低。在300℃轧制的试样抗拉强度最大,达到349 MPa。380℃轧制试样塑性最佳,伸长率达到24.2%。     

Influence of different rolling routes on mechanical anisotropy and formability of commercially pure titanium sheet

Influence of three different rolling routes on mechanical anisotropy and formability of commercially pure titanium sheet was investigated. Route A and Route B are unidirectional rolling (UR) where the rolling direction is along initial rolling direction (RD) and transverse direction (TD), respectively. Route C is cross rolling (CR) where the rolling direction is changed by 90° after each rolling pass. The microstructure and texture, tensile mechanical properties including strength and elongation, and also the anisotropy of the UR and CR sheets were investigated at room temperature. The XRD results indicate that the texture intensity of rolled samples gradually weakens from Route A to Route C. Compared with Route A and Route B rolled samples, the Route C rolled samples show a smaller planar anisotropy. The deep drawing tests reveal that cross rolling can avoid the occurrence of earing. Erichsen tests indicate that rolling routes have an effect on stretch formability of pure titanium sheet.     

Effect of temperature,strain rate and fibre orientation on the plastic flow behaviour and formability of AZ31 magnesium alloy

The sheet formability of AZ31 magnesium alloy has been widely investigated by means of uniaxial tensile and hemispherical punch tests, performed at different temperatures and strain rates, using samples with different fibre orientations. The results of the uniaxial tensile tests were analysed in terms of flow curves, ductility and microstructural evolution. They show that the flow stress decreases and ductility increases as temperature rises and strain rate reduces; the ductility is almost independent of the fibre orientation that, however, slightly affects the flow stress values. The formability, described by the forming limit curves (FLCs), improves with increasing temperature and decreasing strain rate. Moreover, formability along the rolling direction (RD) is higher than that along the transversal one (TD), even if the FLCs obtained along the TD have a larger extension in the drawing side than the ones along the RD. Such behaviours were related to the constitutive parameters and microstructure developed during deformation.     

Microstructure evolution leading to high strains during high temperature deformation of a Ti–Al intermetallic

The high strains achieved during high temperature deformation of a Ti–Al rolled sheet have been evaluated by following the microstructural evolution of tensile samples tested along the transverse and rolling directions. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations have confirmed that in both types of samples extensive twinning activity occurs during deformation at temperatures of 700 and 800°C but not at 900°C. Microstructural refinement occurs by subdivision of the grains either by the twin interfaces or by subgrain formation followed by recovery/recrystallization processes. The lower strains achieved in the samples deformed along the transverse direction are a result of a more inhomogeneous microstructure due to the different deformation mechanisms involved, that include activation of superdislocations at high strains.     

Effect of pre-existing shear bands on the tensile mechanical properties of a bulk metallic glass

Bulk Zr_64.13Cu_15.75Ni_10.12Al₁₀ metallic glass has been rolled at room temperature in two different directions, and the dependences of microstructure and tensile mechanical property on the degree of deformation and rolling directions have been investigated. No deformation-induced crystallization occurs except for shear bands. Shear band formation in conjugated directions is achieved in the specimen rolled in two directions, while rolling in one direction induces shear band formation only in a single direction. Pre-existing properly spaced soft inhomogeneities can stabilize shear bands and lead to tensile plastic strain, and the efficient intersection of shear bands in conjugated directions results in work-hardening behavior, which is further confirmed by in situ tensile scanning electron microscopic observation. Based on the experimental results obtained in two different specimen geometries and finite element analysis, it is deduced that a normal-stress-modified maximum shear stress criterion rather than a shear plane criterion can describe the conditions for the formation of shear bands in uniaxial tension.     

Effect of pre-rolling path on mechanical properties of rolled ZK60 alloys

Pre-cold rolling with low reductions (<3%) was used to improve the mechanical properties of rolled ZK60 plates. The effects of rolling path on mechanical properties were investigated in detail. Both pre-cold rolling along the transverse direction (TD) and pre-cold rolling along the normal direction (ND) can increase the yield strength. However, pre-cold rolling along the TD is more effective than pre-cold rolling along the ND in improving the comprehensive mechanical properties. After pre-cold rolling to 3% reduction, the sample rolled along the TD and the sample rolled along the ND have similar tensile yield strength (～270 MPa). However, the former has a higher compressive yield strength, lower yield asymmetry and larger toughness than the latter. Moreover, pre-cold rolling can also enhance precipitation hardening effect. However, aging treatment cannot further improve the yield strength of pre-cold rolled samples. Finally, the related mechanism is discussed.     

热轧大型H型钢残余应力相关研究

在全轧程热力耦合计算结果的基础上,对大型H型钢冷却后的残余应力场进行仿真分析。得到了沿着长度方向大型H型钢腿腰连接部位及翼缘整体表现为拉应力、腹板为压应力的残余应力场计算结果。H型钢冷却过程中,腹板的残余压应力有可能导致腹板冷却波浪的产生。对使用过程中切割翼缘时残余应力场的转变过程进行仿真分析,得到当切口到达H型钢腿腰连接部位时,切口处拉应力突然增大的结果。切口处应力场的突然变化与腹板开裂直接相关,H型钢腹板的开裂属于脆性断裂。对控制大型H型钢残余应力的方法进行仿真研究,得到了通过翼缘外侧强制冷却改善残余应力分布的结果。     

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.     

Study on Thermomechanical Treatment, Mechanical Properties and Fatigue of Nitinol Superelastic Thin Sheet

A wide range of different thermomechanical treatments was performed on commercially available superelastic Nitinol thin sheet. The ingot composition in the range of standard superelastic material with about 50.8?at.% Ni, balance Ti, was used to manufacture a series of samples with different thermomechanical conditions. Production parameters such as cold work, heat treatment temperature, and heat treatment time were varied. All finished samples were of the same final thickness of 0.3?mm and received the same industrial surface finishing process to obtain a smooth, defect, and oxide-free, shiny surface. Before carrying out the laser cutting, the material was characterized by tensile testing, DSC, and bend-and-free recovery test. Miniature dogbone specimens were cut from the as-manufactured sheets in both directions, longitudinal as well as transverse to the rolling direction. These samples were surface finished using standard deburring and electropolishing processes. For some specific parameter combinations, there were also samples taken at 45?? to the rolling direction. All qualified samples were then exposed to fatigue testing in a bending mode until fracture or run-out. The results showed there is a significant effect on the fatigue performance of the samples from both the applied thermomechanical treatment as well as the sheet anisotropy. It is also obvious that the achieved strain data is on average lower than the data obtained in comparable studies on tube or wire, which can be attributed to the different test setup (bending mode in air at 37???C) as compared to most other studies as well as the larger surface.     

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.     

应变方式对Bi-2223/Ag超导带材中微裂纹的形成及临界电流的影响

对比研究了拉伸、弯曲和压缩3种应变方式对Bi-2223/Ag高温超导带材中微裂纹的形成以及临界电流的影响。实验结果表明:带材的临界应变值在不同的应变方式作用下表现出明显的差异。同时,超导芯中裂纹的扩展途径也有所不同,弯曲时裂纹从带材上表面向内扩展,而拉伸时裂纹的产生则具有随机性。重点讨论了带材内部微裂纹的产生机理,实验发现,带材在拉伸过程中产生轧向裂纹和横向裂纹,而在压缩过程中以轧向裂纹为主,横向裂纹较少。正是这两种裂纹的交互作用导致带材临界电流的降低。