异步轧制AZ31镁合金板材的晶粒细化及性能

采用上下轧辊速比1.125的异径异步轧制方法对AZ31镁合金板材进行轧制。采用光学显微镜、X射线衍射仪和电子拉伸机等设备分析轧制前后AZ31镁合金板材的微观组织和力学性能。结果表明:AZ31镁合金热挤压板坯在加热到350℃后,经一道次38%压下率的异步轧制,可获得平均晶粒尺寸为2.8μm的等轴晶粒,板材轧制方向的伸长率和抗拉强度显著增加;轧制过程中形成了非基面晶粒取向;伸长率的增大与晶粒细化和非基面织构的形成有关,抗拉强度的增大归因于晶粒的显著细化效应。     

异速比对异步轧制AZ31镁合金板材组织和织构的影响

采用不同异速比对AZ31镁合金板材进行异步轧制,并将轧后样品进行显微组织和X射线衍射分析,研究异速比对镁合金板材组织和织构转变的影响.结果表明:异速比的变化对晶粒形貌影响较大但晶粒细化效果不明显;当异速比为2.800时,板材内出现了长条晶粒;快速辊侧{0002}基面织构强度高于慢速辊侧,且板材两侧表面{0002}晶面的偏转方向相反;异速比对基面织构的强度影响显著,随着异速比的增大,基面织构的强度先增加后下降.这种特殊的织构变化与异步轧制过程中沿厚度方向引入的剪切变形有关.     

高温轧制对AZ31B镁合金组织和性能的影响

研究了高温轧制工艺对AZ31B镁合金微观组织、织构以及性能的影响规律.在轧制状态下,随着轧制温度从450℃升高至525℃,合金组织内部动态再结晶逐渐增多,孪晶数量不断减少,同时组织的均匀性也得到了改善,基面织构强度也呈下降的趋势.经350℃保温60min退火之后,合金板材内部发生了完全再结晶,孪晶组织消失,显微组织更均...     

非对称轧制对AZ31B镁合金组织及性能的影响

开展了非对称轧制对AZ31B镁合金晶粒细化影响的研究,分析了不同温度及不同压下率时宏观形貌和晶粒尺寸变化,并与对称轧制作了对比。结果表明,非对称轧制的整体晶粒尺寸比对称轧制更为细化;非对称轧制在温度为350 ℃、压下率为60%时晶粒最为细小均匀,上表面、中心层和下表面的平均晶粒尺寸分别为2.35、2.84和2.22 μm。在初轧温度为300～350 ℃范围内,组织产生充分动态再结晶;随着轧制温度继续升高,晶界产生充分迁移和扩散,晶粒随之长大,导致镁合金的综合性能变差。非对称轧制板材的抗拉强度和断后伸长率都优于对称轧制板材,在400 ℃轧制时,压下率为30%时获得较为优异的综合力学性能,抗拉强度为365.36 MPa,断后伸长率为34.9%。     

高成形镁合金板材最新研究进展

镁合金作为当前应用广泛的轻量化金属结构材料,具有高比强度和比刚度、优良的阻尼性能以及可回收等优点。同时,中国拥有丰富的镁资源,其应用与推广可起到缓解中国铁、铝矿等传统金属材料的短缺问题和降低污染的作用。变形镁合金在航空航天、交通运输和生物医用支架等领域受到广泛青睐。但是,大部分变形镁合金具有密排六方（hcp）晶体结构,室温下能够开动的独立滑移系较少,因而在塑性变形时易形成强基面织构导致其室温塑性成形能力差。如何提高镁合金板材的室温成形性能是扩大镁合金应用当前亟待解决的主要问题之一。综述了近年来国内外研究学者在改善镁合金板材室温成形性的工作及研究进展,主要集中在添加合金元素和塑性预变形调控来消融强织构与低成形壁垒,阐述了添加稀土元素、微合金化、新型轧制及挤压加工、预变形塑性加工等手段对镁合金板材微观组织结构、晶体取向及成形性能的调控规律,为制备高成形性镁合金板材制备提供参考。     

热处理对含Y元素AZ31镁合金板材组织和性能的影响

对含Y元素AZ31镁合金板材进行退火处理后的组织和性能进行了研究.结果表明:随着退火温度的升高,镁合金晶粒尺寸逐渐增大,力学性能略有提高然后降低;退火时间对镁合金晶粒尺寸影响不大;在300℃下退火1 h后板材性能达到最佳,抗拉强度为255 MPa,屈服强度为170 MPa,延伸率为24%;经过热处理后镁合金断裂方式为准解理断裂和韧性断裂的复合形式.     

AZ31镁合金板在不同温度下轧制中的数值模拟

运用DEFORM-3D软件,模拟了在250~400℃温区内不同温度下轧制变形量为30%的AZ31镁合金板材的轧制过程,分析了温度对镁合金板材轧制过程中轧制力、等效应力以及温度场分布的影响。研究结果表明:AZ31镁合金板材在轧制过程中存在着明显的温度效应,并且随着轧制温度的升高,温度效应减弱;随着轧制温度的升高,AZ31镁合金板材在轧制过程中的塑性变形抗力、轧制力与等效应力均显著降低;若单从温度角度考虑,在其他条件不变的前提下最佳轧制温度在350~400℃的温区内。     

AZ31B镁合金铸轧板的织构和冲压性能的分析

在室温条件下,AZ31B镁合金的塑性加工难度较大,而影响其加工难度的主要因素就是AZ31B镁合金的织构以及冲压性能。基于此,笔者自制AZ31B镁合金铸轧板,分别制备了普通AZ31B镁合金以及复合能场AZ31B镁合金铸轧板,对两者的织构与冲压性能进行了分析,分析的结果显示,普通AZ31B镁合金铸轧板的织构强度更大,冲压性能相对较差。     

弯轧过程加热温度对AZ31镁合金板材的影响

为研究加热温度对镁合金板材弯轧工艺的影响,对原始板材采用不同的加热温度,并进行多道次的"反复弯轧",对弯轧后的板材进行拉伸试验,分析弯轧后板材的显微组织。结果表明:经过弯轧后板材试样的室温延伸率得到了改善,且得出有利于AZ31镁合金室温成形的加热温度。     

挤压-室温轧制对尺寸不对称喷射沉积含Gd镁合金组织及织构影响

采用喷射沉积技术制备Mg-8Zn-2Ca-2Gd-1Zr-1Nd合金沉积坯,对其进行挤压预变形后再进行室温轧制变形(ε=5%,10%,15%)。利用蔡司金相显微镜(OM),扫描电子显微镜(SEM)和X射线衍射仪(XRD)分析挤压—轧制变形对镁合金显微组织及微观织构演变的影响。结果表明:尺寸不对称喷射沉积镁合金挤压坯在室温轧制变形时,随压下率(ε=5%,10%,15%)增加,硬度值升高且存在硬度值分布不均匀现象,团聚第二相粒子逐渐呈弥散分布,其中尺寸小于1μm的第二相粒子弥散分布于基体上,而部分富稀土元素第二相碎化后偏聚在基体晶粒晶界处,大量弥散分布于基体和晶界处的第二相粒子阻止位错滑移是镁合金轧制后硬度提高的重要原因。尺寸不对称喷射沉积镁合金在挤压-轧制变形过程中,当ε=10%时,(0002)基面织构强度降低、非基面织构形成且极密度增强,实现了形变织构随机化。晶粒细化、挤压坯初始织构遗传性、尺寸不对称变形三者综合作用是实现形变织构随机化的主要原因。     

变形Mg-1.5Zn-xCe合金织构及室温成形性能

实验研究了不同Ce元素含量对变形Mg-1.5Zn合金的织构及室温成形性能的影响.结果表明:在相同的热轧和退火工艺处理后,添加不同含量的Ce元素均可以有效弱化镁合金织构强度.Mg-1.5Zn合金中添加质量分数为0.2%的Ce元素后表现出了优异的室温成形性能,织构强度最大值仅为2.20,织构沿着横向分裂,并且基面法向即c轴沿着横向发生约为±35°偏转,室温下轧向方向延伸率达到23.2%,埃里克森杯突值为5.46,平面各项异性系数Δr=0.01;然而,Mg-1.5Zn合金中添加质量分数分别为0.5%和0.9%的Ce元素后,织构强度增加,埃里克森杯突值减小,由于在合金中生成了粗大的第二相粒子,使得Mg-1.5Zn-xCe合金的室温成形性能变低.      

Study of Forming Limit for Rotational Incremental Sheet Forming of Magnesium Alloy Sheet

As a lightweight material, magnesium is being increasingly used for automotive parts. However, due to a hexagonal-closed-packed (hcp) crystal structure, in which only the basal plane can move, magnesium alloy sheets exhibit a low ductility and formability at room temperature. Press forming of magnesium alloy sheets is conventionally performed at elevated temperatures of 200 °C to 250 °C and thus is known as energy consumed forming. Therefore, in view of an energy saving forming technology, we study magnesium alloy sheet forming by a rotational incremental sheet forming (RISF) at room temperature, where the rotational tool generates local heat of specimen enough to accelerate plastic deformation. The flow curves of the magnesium alloy sheet are obtained and calculated at elevated temperatures, while the yield loci of the magnesium alloy sheet are measured at room temperature. Using RISF, a square cup of 80-mm width, 80-mm length, and 25-mm height is then formed from a magnesium alloy sheet at room temperature. In addition, the strain distribution is obtained and compared with the forming limit curve (FLC) by considering the effect of the tool radius and is found to effectively predict the forming limit of a magnesium alloy sheet in RISF.     

Warm Forming of Mg Sheets: From Incremental to Electromagnetic Forming

Magnesium alloys are generating interest in the automotive and aeronautic industries due to their low density and potential to reduce gross vehicular weight. However, the formability of these alloys is poor and they are very difficult to be formed at room temperature due to their strong basal texture in rolled form. In this paper, the potential of magnesium alloy sheets to be processed at warm conditions is studied for four different forming technologies: incremental forming (IF), deep drawing (DD), hydroforming (HF), and electromagnetic forming (EMF). Forming mechanisms and process window are experimentally characterized by monitoring different process parameters. Special focus is made on the influence of the forming temperature and the strain rate. Thus, experiments at temperatures from room to 523 K (250 °C) and a wide range of strain rates, between 10^?3 up to 10³ s^?1 according to each process nature and scope, are conducted. It is observed that, even the inherent forming rate range of each process vary considerably, increasing forming temperature increases formability for all of these forming processes. In the other hand, an opposing effect of the strain rate is observed between the quasi-static processes (IF, DD, and HF) and the high speed process (EMF). Thus, a detrimental effect on formability is observed when increasing strain rate for quasi-static processes, while a mild increase is observed for EMF.     

Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close‐packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi‐static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson‐Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.     

Formability behavior of brass alloy sheet: the role of twins in microstructure

Quantitative understanding of the process and formability parameters involved in grain size and the formation of annealing twins after plastic straining is important in the control of the manufacturing process. There is a synergistic effect of strain and temperature on the density of annealing twins. Formability of brass alloy sheets was studied after annealing of 65% cold worked (CW) samples at different temperatures (300–600°C). Tensile, deep drawing and Erichsen tests were carried out at room temperature to evaluate formability of alloy. Effect of annealing temperature on density, distribution and size of twins is investigated. It was shown that annealing of brass alloy resulting in formation of annealing twins which at higher annealing temperature were reduced by increasing grain size. Best deep drawability would be achieved by annealing at moderate temperature 400–450°C which microstructure consists of fine grain and twin bands. Work hardening exponent of samples was calculated based on the tensile test data and correlated with stretch ability of annealed brass sheets. It was found that the sheets annealed at 600°C possess best ductility and high average n-value.     

Texture,Structure, Mechanical Properties,and Deformability of MA2-1pch Magnesium Alloy Rolled Sheets Preliminarily Subjected to Direct Extrusion or Equal-Channel Angular Pressing and Annealing

The textural and structural evolution in MA2-1pch magnesium alloy sheets fabricated from initial workpieces after (1) hot direct extrusion or (2) equal-channel angular pressing performed by route B_c in four passes at 245°C and subsequent recrystallization annealing is investigated during warm rolling and subsequent uniaxial tension. The same sharp basal texture, but different structures with different average grain sizes and fractions of twinned grains form in sheets (made of two different workpieces) after warm rolling. Subsequent uniaxial tension arranges basal planes in the sheets made from workpiece 2 along prismatic directions. Modeling of texture within the framework of a thermoactivation model shows that the texture changes due to the activation of prismatic slip. Structural evolution during uniaxial tension in the sheets made of workpiece 2 is accompanied by more intense twinning than that in the sheets made of workpiece 1. These textural and structural changes are responsible for the enhanced mechanical properties and the deformability parameters of the sheets fabricated from workpiece 2.