TA15钛合金热挤压过程中金属变形行为及组织分析

运用MSC.Marc有限元软件对TA15钛合金热挤压变形过程进行有限元模拟,得到热挤压成形过程中应力场和应变场分布情况。对有限元模拟所得到的应力、应变数据进行后处理,利用应力偏量不变量J2进行变形分区,采用罗德系数对塑性区内材料的应变类型进行划分。结合有限元模拟结果以及变形分区和变形类型的划分,对热挤压实验后的试样进行组织取样,在不同温度条件下对试样进行微观组织观察,分析微观组织的演化规律,这对于分析金属挤压成形问题及其在实际中的应用具有重要意义。     

摩擦条件对铝合金挤压变形流动行为的影响

通过有限元模拟和实验,对不同摩擦挤压过程中铝合金变形流动行为的机理进行分析,并利用罗德系数和应力偏量不变量(J2)等特征量进行变形分区.结果表明:采用积极摩擦可使挤压时的"死区"缺陷完全消失,且塑性区内材料的应变类型由3种变为均一的拉伸类变化,显著地提高了挤压过程中坏料横断面上金属流速的均匀性,更利于金属的挤出成形.     

不同摩擦挤压过程中金属流动行为的变形分区研究

采用数值模拟和实验,深入分析了积极摩擦挤压过程中金属变形流动行为的力学机理,利用应力偏张量第二不变量J2、Lode系数μ等特征量对挤压过程进行了变形分区.结果表明:积极摩擦挤压时筒底不出现分流的现象,"死区"缺陷完全消失,处于塑性区内材料的应变类型由三种变为均一的拉伸类,且显著改善了成形过程中金属变形和流动的均匀性,利于金属的挤出成形及制品质量的提高.     

纯钼粉末多孔烧结材料ECAP的数值模拟及实验

采用DEFORM-3D软件对纯钼粉末多孔烧结材料等径角挤压过程进行单道次三维有限元模拟和实验研究,获得变形过程中试样的应力、应变、致密行为等相关场量变化规律.模拟结果表明:等径角挤压工艺对粉末材料具有强烈的致密效果,整个变形过程可分为3个阶段,即初始变形、过渡变形及稳定变形;试样纵横截面上,等效应变均存在不均匀分布现象,靠近模具内角和上表面处试样所获应变较大,相对密度也较高.试样不同部位所处应力状态及应变速率分布状态的不一致是导致其应变分布不均匀的根本原因.单道次挤压实验结果与模拟结果具有较好的一致性,证明了所建立有限元模型的可靠性.     

某雷达连接杆焊接温度场及残余应力分析

以长轴薄壁连接杆为研究对象,采用MARC有限元软件对连接杆的焊接过程进行数值模拟,分析不同焊接热输入对温度场、残余应力及变形的影响规律。结果表明,焊接热输入由5 600J·mm-1减小到2 800J·mm-1,残余应力及变形均显著增加,纵向残余应力增加17%,横向残余应力增加33.9%,变形量增加63.5%。焊接实验结果与模拟结果吻合,验证了模拟结果的准确性,对焊接变形的预测及控制具有重要意义。     

管料无芯轴凹进式旋转锻造变形规律研究

无芯轴凹进式旋转锻造是一种用于管料精密制造的渐进近净成形工艺,广泛应用于航空航天、汽车制造等领域。本文基于有限元软件TRANSVALOR FORGE 2011,建立了管料无芯轴凹进式旋转锻造成形三维有限元模型,研究了成形过程中的应力状态和残余应力状态。结果表明,工件不同区域材料应力状态不同,工件外壁材料受三向压应力,应力变化较为剧烈;工件中间层材料受环向和径向两向压应力,而轴向应力趋于零;工件内壁材料受轴向拉应力,环向和径向压应力。卸载后,材料内部仍然存在保持自相平衡的三向残余应力,最大值为57.2MPa,仅为材料屈服应力的63.6%。凹进式旋转锻造成形过程中所产生的变形,满足应力偏张量的第三不变量J3′<0,属于压缩类变形。     

热变形对高强汽车钢组织及性能的影响

在Gleeble-3500热模拟试验机上对高强汽车钢进行了热变形处理,研究了热变形温度、应变速率、变形量和初始冷却速度对其应力应变曲线和显微组织的影响。结果表明,在相同的热变形温度下,应变速率越大,其同样应变量下的应力值越高;在相同应变速率下,随着热变形温度的升高,峰值应力逐渐降低;热变形温度、应变速率、变形量和初始冷却速度对实验钢中碳化物的微观结构和尺寸都有明显影响。     

坡口角度对核电站压力容器J形接头应力和变形的影响

本文模拟核电站反应堆压力容器顶盖与CRDM贯穿件J形焊缝结构,采用自动钨极氩弧焊工艺,制作了3个不同J形焊缝坡口角度的模拟件,测量并比较了贯穿件的焊接变形、模拟件J形焊缝表面和管内壁的焊接残余应力,分析了相应的变化规律。结果显示,随着J形焊缝坡口角度的增大,CRDM管头、管尾的焊接变形均有所变大;焊缝表面的残余应力整体水平有所起伏,变化规律不明显;而CRDM管内壁残余应力分布趋势基本相同,但应力峰值有所不同。     

温成形工艺参数对热镦GCr15钢球坯晶粒尺寸和均匀性的影响

在Gleeble-3500热模拟试验机上研究了实际热镦生产中变形温度、变形量对热镦GCr15钢流变应力和晶粒尺寸的影响。结果表明,GCr15钢温成形的峰值应力随变形温度的升高而降低;变形温度相同时,应变量越大,平均晶粒尺寸越小;应变量一定时,变形温度越高,基体的再结晶程度增加,平均晶粒直径减小;当应变量为50%时,晶粒尺寸随温度的变化不明显;从晶粒尺寸的均匀性来说,小应变量变形时,应选择较高的变形温度,而当应变量较大时,则选择较低的变形温度。     

TC4钛合金叶片等温成形多塑性变形机制模拟分析

以TC4钛合金等温锻造为例,提出了变形过程中材料所处塑性状态的判断依据.通过正交试验,得到了基于本构方程的应变速率敏感指数的计算方法,同时计算出不同应变下应变速率敏感指数等值线,便于判断材料所处的塑性状态.通过数值模拟,验证了多塑性变形机制,进一步说明在变形过程中材料内部同时存在多种塑性变形机制,且随着变形的不断进行,各种塑性变形机制会发生相互转变.     

Superplastic deformation under complex loading conditions

The initial yield surface of a superplastic material was investigated by using a combined loading of axial force and torque. The thin-walled tubular specimen made of Zn-22 wt.%A1 alloy was used in the experimental part of this study. Tests were carried out at room temperature (293 K) and at elevated temperature (523 K). The experimental results show that the material tested did not deform superplastically at 293 K exhibiting a yield surface that can be described by the second invariant of stress deviator (by Mises’ criterion). On the other hand, the material was found to deform superplastically at the higher temperature of 523 K with a yield surface more complex compared to that observed at 293 K. This difference may be attributed to the difference in major deformation mechanism at two applied temperatures; that is, the slip within the grains at 293 K and the grain boundary sliding at 523 K.     

Research of Metal Flow Behavior during Extrusion with Active Friction

Using numerical simulation and experiment, the metal flow behavior mechanical mechanisms in the extrusion process with active friction were investigated. The characteristic variables, second invariant of the stress deviator J ₂ and the Lode’s coefficient μ were employed to partition the deformation region. It is shown that no metal flow interface occurred at the container bottom in the extrusion with active friction and the dead zone disappeared completely. The strain types of the material in the plastic deformation area decreased from three types into a single type of tension, and the homogeneity of metal deformation as well as metal flow was greatly improved. It was also indicted that the active friction was beneficial to the extrusion process and the promotion of product quality. After contrasting the result of experiment and the simulation, the displacement and the load were well correlated on both values and trends.     

一点的正应力三维图形

过一点的不同平面上的正应力随着平面法向的变化而改变。本文首次给出了一点的应力全量及其应力偏量中的正应力变化的三维图形 ,分析了该图形和应变类型的对应关系     

铝合金圆环压缩过程中的分流行为

利用有限元方法对不同圆环压缩过程中的分流行为进行模拟研究,揭示铝合金圆环压缩过程中分流行为的产生机理,并利用罗德系数、J_2应力偏量不变量等特征量对成形机理进行分析.结果表明:外侧施压时,随着高径比增大,难变形区明显扩大,向内侧流动的金属量逐渐减少,分流面位置明显左移;内侧施压时,随着摩擦因子增大,塑性区内应变类型由均一压缩类变为3种共存,此时向内侧流动的金属量增大,其流动分界面相对右移.     

用嵌入螺柱法测量金属体内塑性应变分布

为解决传统塑性应变测试方法无法有效地测量体积成形中金属体内应变分布的问题,提出了一种测量金属塑性应变的新方法.其原理是将工件加工成内部嵌入螺柱的组合件,对组合件进行塑性变形,测量变形后组合件内部的螺纹界面的变形,进而计算相应位置上的应变.该方法可以定量给出变形体内部的应变分布,与传统的坐标网格法相比,具有不需要剖分工件、不改变应力状态的优点.该方法可应用于镦粗、挤压、模锻等塑性变形工艺中金属变形流动行为的研究.     

Thermomechanical behavior of an Fe-Based shape memory alloy :Transformation conditions and hystereses

The transformation conditions of an Fe-based polycrystalline shape memory alloy are determined experimentally under the multiaxial stress state. The martensite start condition is represented as an oval conical surface in the axial stress-shear stress-temperature space, meaning the transformation condition is not the von Mises type condition in plasticity and clearly depends on the third invariant of the stress tensor. The austenite start and finish conditions are also determined as the planes in the multiaxial stress-temperature space. The uniaxial stress-strain-temperature hysteretic behavior determined under the uniaxial thermomechanical loads is simulated by introducing the internal state variables which represent the status of the microscopic internal structures.     

关于对理想刚塑性材料本构方程的见解

本文对Ｌｅｖｙ－Ｍｉｓｅｓ增量理论的若干论述进行了综合分析，指出了其中的不足，提出理想刚塑性材料的应力—应变关系为应力偏张量各分量之比决定应变增量各分量之比，应变增量的大小由瞬时位移条件决定的新论述。最后用Ｌｏｄｅ实验说明了新论述的正确性。     

Investigation of the tensor character of mesoscopic internal stresses in tensile-deformed nickel single crystals by X-ray diffraction

X-ray diffraction profiles of tensile-deformed nickel single crystals with two different orientations of the loading axis were investigated. To characterize the internal elastic strain state on a mesoscopic scale, the profile shapes of different Bragg reflections were analysed. The asymmetric profile broadening is interpreted on the basis of a two-component model for internal stresses in a dislocation cell structure. A least-squares method was applied to determine the stress tensor for the cell interior from the asymmetry parameters of at least six independent reflections. Furthermore, this method was advanced to evaluate the distribution of the plastic deformation across the slip systems. Taking into account an inclusion approximation, by this way the calculated internal stresses were correlated with the plastic strain in the cells. The orientation of the principal axes of the stress tensor and the heterogeneity of slip in the slip systems found agree with model assumptions for the tensile deformation.     

Determination of the profile of the complete residual stress tensor in a VPPA weld using the multi-axial contour method

The multi-axial contour method is a recent development of the contour method of stress measurement. It permits measurement of the 3D residual stress distribution in a body, based on the assumption that the residual stresses are due to an inelastic misfit strain (eigenstrain) that does not change when a sample containing residual stresses is sectioned. The eigenstrain is derived from measured displacements due to residual stress relaxation when the specimen is sectioned. By carrying out multiple cuts, the full residual stress tensor in a continuously processed body can be determined, where the specimen has an initial length-to-width aspect ratio of 2:1. In the present study, first a finite element simulation of the technique was carried out to verify the method numerically. The method was then used to determine the residual stresses in a VPPA-welded sample, and the results were validated by neutron diffraction measurements.