环形铸坯热辗扩成形微观组织演变规律研究

建立了铸态42CrMo钢的微观组织演变模型和环件热辗扩三维刚塑性有限元模型.基于DEFORM-3D软件平台,对微观组织演变和宏观热力学行为进行了耦合模拟,研究了环形铸坯热辗扩成形过程中芯辊进给速度对动态再结晶晶粒尺寸和体积分数分布的影响规律.结果表明,在变形量比较大的环件外层和内层容易发生动态再结晶,晶粒尺寸细化明显;...     

基于铸坯的环件热辗扩过程微观组织演变研究

根据建立的环形铸坯热辗扩成形刚塑性有限元模型和铸态42CrMo钢动态再结晶数学模型,基于DEFORM-3D软件平台,综合考虑了变形、传热和微观组织演变等因素,对辗扩过程中的变形和动态再结晶行为进行宏微观耦合模拟.结果表明:在环形铸坯热辗扩成形过程中,动态再结晶程度较低,等效应变和晶粒尺寸呈现环带状分布,环件的内外层等效...     

不同铸态42CrMo坯料尺寸下环件径轴向辗轧成形组织演变规律

基于铸坯的环件径轴向辗轧成形新技术面临的瓶颈问题,对坯料铸态组织的演变规律与合理控制进行研究。对某环件双向轧制,铸坯的尺寸将决定变形程度和径、轴向的变形量分配,从而对最终环件的组织起着至关重要的作用。该文基于ABAQUS平台,建立了42CrMo钢铸坯环件径轴向热辗轧宏微观耦合有限元模型;采用一种基于轧比K和径轴向变形量分配比tanα的坯料尺寸设计方法,针对同一环件的轧制过程,设计具有不同K和tanα值的不同尺寸的多个环坯,模拟不同铸态42CrMo坯料尺寸下环件径轴向辗轧过程组织的演变规律。结果表明,随着K值增大,环件动态再结晶程度增加,平均晶粒尺寸减小且分布逐渐均匀;随着tanα值增大,环件上下端面区域动态再结晶程度增大,平均晶粒尺寸减小,沿环件轴向平均晶粒尺寸分布逐渐不均匀,而内外表面区域动态再结晶程度减小,平均晶粒尺寸增大,沿环件径向平均晶粒尺寸分布逐渐均匀。     

42CrMo铸坯环件热辗扩有限元模拟与分析

基于ABAQUS软件平台,建立了42CrMo大型环形铸坯热辗扩三维热力耦合有限元模型,模拟了铸坯热辗扩过程中应变场和温度场,研究了初始辗扩温度对辗扩力的影响规律.模拟结果表明在环形铸坯热辗扩过程中：①铸坯等效应变呈阶梯状上升,内外表面应变大于中间层应变;在稳定成形阶段,沿环件径向方向,由于导向辊与芯辊直径差异,导致环件最大平均等效应变可能出现在环件内表面也可能出现在环件外表面;②初始阶段,变形区与成形辊接触处温度降低较快,非变形区温度变化不是很明显;随着辗扩的进行,芯部温度逐渐上升,边缘温度低,温度分布不均匀;③随着铸坯初始辗扩温度升高,平均辗扩力明显下降,但随时间变化趋势保持一致.     

外台阶截面环形铸坯热辗扩成形工艺有限元模拟

基于DEFORM软件,建立了外台阶截面环件热辗扩有限元模型,模拟分析了材质为42CrMo钢的环形铸坯热辗扩过程.结果表明:在选定的热辗扩工艺参数下,辗扩后环件表面平整光滑,无凹坑缺陷,成圆规整;与成形辊接触的环件内外层金属的流动速度大于芯部的流动速度;与成形辊接触的环件内外层金属等效应变较芯部大;在辗扩过程中,环件芯部的温度下降始终较内外层平缓,内外层金属的温度呈锯齿状下降趋势.本文的研究对外台阶截面环形铸坯热辗扩成形工艺的设计、环件质量预测与控制具有指导意义.     

GCr15铸环热辗扩孔洞形状演变预测模型研究

环件铸辗复合成形是利用铸造环坯热辗扩成形环件的一种短流程成形新工艺。然而铸造环坯中存在的夹杂、缩松、缩孔等孔洞类缺陷能影响到热辗扩成形环件的最终质量。基于ABAQUS平台建立了GCr15铸环孔洞热辗扩三维热力耦合模型,通过热轧实验验证了模型的可靠性,并研究了GCr15铸态环件不同径向位置孔洞缺陷在热辗过程中的演变规律,提出了一种铸环热辗过程中孔洞形状演变的预测模型。模拟结果表明,孔洞各向尺寸变化与孔洞位置密切相关,孔洞距离内径越近,其各向尺寸变化越大;根据初始铸坯的几何参数、相关热辗扩工艺参数和初始孔洞尺寸可以预测该孔洞在热辗扩过程中的形状演变情况。     

基于铸坯的42CrMo环件辗扩成形工艺热力耦合分析

以42CrMo钢为研究对象,对铸态坯料的环形零件辗扩成形过程进行了热力耦合分析。通过铸态42CrMo材料的热压缩实验,建立了两段式流变应力模型;对环形铸坯热辗扩成形过程进行了数值模拟,得出了基于铸坯的环件热辗扩成形过程等效应变、温度场及载荷的分布情况与演变规律,分析了辗扩工艺参数对成形过程中应力、应变和温度变化的影响。结合工业性铸坯环件辗扩成形试验,对改善和提高辗扩成形零件的组织和性能提出了建议和措施。所得结果对于完善环形零件铸辗复合成形新工艺具有指导意义。     

进给速度控制方法对铸环坯辗扩微观组织的影响

径轴向进给速度是铸环坯热辗扩成形中重要的工艺参数,直接影响环件的"成形"和"成性"质量。本文以铸态25Mn环坯为研究对象,基于Simufact.forming软件平台,对比分析和研究了两种不同的速度控制方法对成形环件微观组织质量的影响规律。研究结果表明:通过调整进给速度来控制整个辗扩过程和通过调整环件直径增长速度来控制辗扩进给量都能达到"成形"的要求,但后者在改善铸态材料的微观组织、细化晶粒尺寸等"成性"方面比前者具有更加明显的优势。该研究为基于铸环坯的铸辗复合成形工艺中进给速度的控制方法提供了一定的理论指导。     

基于铸坯的25Mn钢环件热辗扩成形力学性能分析

环形铸坯辗扩成形是一个多场耦合、局部加载和连续成形过程,与传统辗扩工艺相比,缩短工艺流程,减少加热次数,节材、降能耗,具有重要的研究价值与推广应用前景。辗扩过程中,不仅要控制环形铸坯辗扩后的成形精度,还应通过高温塑性变形获得所需微观组织,提高综合力学性能,实现成形/成性一体化调控。根据环件轧制技术及基于铸坯的辗扩成形基础理论,通过对25Mn钢环形铸坯进行辗扩,分析辗扩成形环件不同部位微观组织的变化及力学性能,并借助扫描电镜观察,揭示其拉伸与冲击断口的断裂机理。结果表明:辗扩后环件的外形好,宽展小,椭圆度为0.4;组织内部平均晶粒尺寸小,环件各个区域的力学性能均较高,且中层的略低于内层和外层;拉伸与冲击断裂形式主要为准解理,并伴有韧窝断裂。可见,基于铸坯的25Mn钢环件热辗扩"成形"/"成性"质量较高,为其进一步研究以及实际生产提供了理论指导价值。     

鼓形环坯轧制的宏微观变化数值模拟研究

针对鼓形环坯建立了环件径轴向轧制三维有限元模型,通过Simufact软件对鼓形环坯轧制的宏微观变化进行了耦合模拟,模拟揭示了鼓形环坯在轧制过程中的温度、等效应变、晶粒、动态再结晶的分布和演化规律;深入研究了轧制成形过程中径向每圈压入量对环锻件微观组织大小的影响规律。结果表明:基于鼓形环坯获得的环锻件,其内外侧棱边处发生的动态再结晶体积分数最大,晶粒最为细小,其次是内外表面和上下表面,心部动态再结晶体积分数最小,晶粒尺寸最大;适当增大径向每圈压入量,能够扩大动态再结晶区域,获得晶粒尺寸细小的环锻件。     

Thermo-mechanical coupled analysis of hot ring rolling process

A 3D rigid-plastic and coupled thermo-mechanical FE model for hot ring rolling（HRR） was developed based on DEFORM 3D software, then coupled heat transferring, material flow and temperature distribution of the ring in HRR were simulated and the effects of process parameters on them were analyzed. The results show that the deformation nonuniformity of ring blank increases with the increase of the rotational speed of driver roll and friction factor or the decrease of the feed rate of idle roll and initial temperature of ring blank. The temperature nonuniformity of ring blank decreases with the increase of the feed rate of idle roll or the decrease of initial temperature of ring blank and friction factor. There is an optimum rotational speed of driver roll under which the temperature distribution of ring blank is the most uniform. The results obtained can provide a guide for forming parameters optimization and quality control.     

铸态Ti-46Al-6(Cr, Nb, Si, B)合金的高温流变行为及其组织演变

以3次真空自耗熔炼的Ti-46Al-6(Cr,Nb,Si,B)(at%)(以下简称G4合金)合金为对象,采用恒温等应变速率热模拟压缩试验研究G4合金在1050～1250℃及0.001～1s-1应变速率下的高温流变行为和组织演变。结果表明,在高温变形过程中,G4合金呈现先硬化后软化的流变行为特征,组织由粗大的铸态γ+γ/α2近片层组织演变为细小的近等轴γ+α2组织;造成G4合金流变软化和组织演变的主要原因是动态再结晶(DRX)。变形温度和应变速率是影响G4合金高温流变和组织演变的2个主要因素。铸态G4合金在高温下的变形机制以γ/α2层片晶团的扭折、弯曲、球化和DRX以及γ晶粒的拉长、破碎和DRX为主,孪生变形也起到了一定的辅助作用。其最佳高温塑性变形温度为1150℃,应变速率应不大于0.1s-1。     

Simulation and experimental study of dynamic recrystallization process during friction stir vibration welding of magnesium alloys

A two-dimensional cellular automaton (CA) model was utilized to analyze the effect of mechanical vibration on microstructure evolution of AZ91 alloy during friction stir welding (FSW). The simulated results, namely grain topology, grain size distribution, average grain size, and also the dynamic recrystallization (DRX) fraction were compared with measured data. The adequate comparability between FEM and experimental data shows that the CA method can be applied to the analysis of the microstructure progression during the friction stir welding of AZ91 alloy. It is concluded that the dislocation density during the friction stir vibration welding (FSVW) is higher than that in the FSW process and the process of nucleation and grain growth is faster for samples during FSVW compared to FSW. The grain size modification and DRX phenomenon with various vibration frequencies were also simulated in detail during FSVW. It is found that vibration makes nucleation start earlier and decreases the proportion of the incubation period and the percentage of recrystallization as vibration frequency improves.     

用热加工图研究铸态和挤压态ZE41A镁合金的热加工性能

采用热加工图研究铸态和挤压态ZE41A镁合金的热变形行为和组织演变。在温度250-450℃,应变速率0.001-1.0s-1的条件下,对铸态和挤压态合金进行抗压测试,建立热加工图。通过显微组织观察确定动态再结晶和不稳定区域。挤压态ZE41A镁合金比铸态合金具有较高的流变应力,较高的能量损耗率和较小的不稳定区域。由于晶粒的细化、材料孔隙度的降低、硬化和强化,挤压态镁合金具有良好的热加工性能。     

Effect of welding speed on microstructure and mechanical properties of Al−Mg−Mn−Zr−Tialloy sheet during friction stir welding

Effects of welding speed on the microstructure evolution in the stir zone (SZ) and mechanical properties of the friction stir welding (FSW) joints were studied by OM, XRD, SEM, TEM, EBSD and tensile testing. Compared with the base metal (BM), an obviously fine dynamic recrystallization (DRX) microstructure occurs in the SZ and the DRX grain size decreases from 5.6 to 4.4 μm with the increasing of welding speed. Fine DRX microstructure is mainly achieved by continuous dynamic recrystallization (CDRX) mechanism, strain induced boundary migration (SIBM) mechanism and particle stimulated nucleation (PSN) mechanism. Meanwhile, the geometric coalescence and the Burke−Turnbull mechanism are the main DRX grain growth mechanisms. Among all the welding speeds, the joint welded at rotation speed of 1500 r/min and welding speed of 75 mm/min has the greatest tensile properties, i.e. ultimate tensile strength (UTS) of (509±2) MPa, yield strength (YS) of (282±4) MPa, elongation (El) of (23±1)%, and the joint efficiency of 73%.     

Mesoscale simulation of discontinuous dynamic recrystallization using the cellular automaton method

A dynamic recrystallization (DRX) cellular automaton (CA) model that can mark the microstructure with DRX circle was developed. The effects of initial grain size on the stress-strain curve, mean grain size and DRX fraction were mainly investigated, and the simulated results were compared with those obtained from previous researches. The results show that the shape of the stress-strain curve is sensitive, while the stress and mean grain size at the steady state are insensitive to the initial grain size. The transition from a multiple-peak stress-strain curve to a single-peak one can be explained by variations in DRX circle fraction, and the initial grain size to make this transition is between 70 and 80 μm.     

Formation of shearing bands in the hot-rolling process of AZ31 alloy

Three types of AZ31 alloy samples, numbered with A, B, C, with various texture and microstructure condition were hot-rolled in single pass to investigate the different mechanism of shearing bands formation. Shearing bands came into being via twinning related grain fragmentation and DRX in Sample A while via rotational recrystallization in Sample B. Twinning played the most important role in shearing bands formation in Sample C. DRX and twinning are the two major elements in the formation of shearing band in magnesium alloys. Contrastive study indicated that sharper texture would increase the influence of twining while small size would promote the recrystallization in shearing bands formation.     

Dynamic recrystallization kinetics of as-cast AZ91D alloy

The flow behavior and dynamic recrystallization (DRX) behavior of an as-cast AZ91D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220–380 °C and strain rate range of 0.001–1 s^?1. The effect of temperature and strain rate on the DRX behavior was discussed. The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates. To evaluate the evolution of dynamic recrystallization, the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves. It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves. A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91D alloy.     

Effects of deformation parameters on microstructure and mechanical properties of magnesium alloy AZ31B

Plastic deformation and dynamic recrystallization (DRX) behaviors of magnesium alloy AZ31B during thermal compression and extrusion processes were studied.In addition, effects of deformation temperature and rates on the microstructure and mechanical properties were investigated.The results show that the DRX grains nucleate initially at the primary grain boundaries and the twin boundaries, and the twinning plays an important role in the grain refinement.The DRX grain size depends on the deformation temperature and strain rate The average grain size is only 1 μm when the strain rate is 5 s-1 and temperature is 250 ℃.It is also found that the DRX grain can grow up quickly at the elevated temperature.The microstructure of extruded rods was consisted of tiny equal-axis DRX grains and some elongated grains.The rods extruded slowly have tiny grains and exhibit good mechanical properties.     

不同状态条件下AZ80镁合金微观组织演化

利用电子背散射衍射(EBSD)技术观察研究了不同状态条件下AZ80镁合金的微观组织,分析了不同状态条件下AZ80镁合金的微观组织演化。结果表明:按照铸态、均质化热处理态和塑性变形态顺序,试样平均晶粒尺寸逐渐减小,平均晶粒形状纵横比呈先增大后减小的变化趋势,网状β-Mg₁₇Al₁₂相逐渐消失,材料塑性和强度得到提高;晶界的协调作用主要受晶界迁移与几何必须位错(GND)密度两方面的影响,伴随晶界的迁移,小角度晶界逐渐增多并阻碍再结晶的进行,使再结晶区呈先增大后减小的变化趋势,而亚结构区则逐渐减小,亚结构区的消失为孪晶的形成提供了能量,促进了孪晶的形成;几何必须位错密度呈先减小后增大的变化趋势,几何必须位错密度的减小促进了孪晶的生长和晶粒间的旋转移动,而几何必须位错密度的增大则使孪晶的生长和晶粒间的旋转移动受到阻碍。塑性变形态镁合金出现了典型的基面织构。