大型锻件锻合内部空洞性缺陷及优化工艺规程的研究

由大钢锭锻成的大型锻件,内部空洞性缺陷在周围基体压应变作用下,变形而闭合,在105℃以上焊合。坯料拔长过程的应变分布和砧宽比及压下量相关,合理选择工艺参数和操作程序,优化工艺规程,使钢锭内部有均匀足够的变形量,可以锻合全部空洞性缺陷,获得密实高质的大型锻件。     

大钢锭内部空洞锻合过程的数值模拟

本文采用实验和塑性有限元法模拟相结合的方法研究了大钢锭在锻造过程中内部空洞的锻合机制。结果表明,空洞的闭合度λ和空洞周围的最大压应变ε 1成线性关系。工具与坯料间的磨擦影响坯料内部的应力应变分布,但不改变λ和ε 1的线性关系。当用上下平砧拔长大钢锭时,使ε 1峰值到达坯料中心的最小砧宽比为0.51,最佳砧宽比为0.7。对于FM法拔长,最小砧宽比为0.4,最佳砧宽比为0.6。     

触底式镦粗工艺研究

利用Deform-3D对触底式镦粗和悬漏式镦粗两种工艺方案进行数值模拟,对比分析镦粗变形过程中应力、应变在轴向和径向上的分布情况,以及坯料心部的静水压力的分布规律。通过在坯料轴线方向设置追踪点分析坯料心部金属的流动及压实效果。分析结果表明:触底式镦粗时坯料内部的等效应变、静水压力状态更好,分布更均匀,压实效果更显著,更有助于锻合坯料心部的空洞缺陷,改善坯料内部质量。     

大型锻件锻造新中心压实法及工艺参数研究

针对现有大锻件中心压实(Japan-Tefeno-Shikano,JTS)法工艺操作复杂且对设备吨位要求过大的缺点,提出一种新的大锻件中心压实方法——NJTS(New-Japan-Tefeno-Shikano)法,设计可用于生产的工装,在锻坯上、下、左、右四个方向同时施压。利用数值模拟与物理模拟方法,以在心部预置空洞的坯料为研究对象,对新中心压实法锻造工艺进行研究。结果表明,新中心压实法能在整个横截面上形成大的静水压力,在心部有最大的变形。对相对压下量、砧宽比和侧压角等主要工艺参数进行优选,在砧宽比为1、侧压角为10°时,NJTS法的锻造效果最好,空洞的体积变化率、周围的等效应变和应力三轴度最佳;单向压实时相对压下量为18%～20%,双向压实时相对压下量为8%～9%,都可锻合内部空洞。试验表明:物理模拟与数值模拟结果基本一致。     

平砧拔长长方体钛合金锻件的数值模拟研究

在钛合金长方体锻件的锻造中,平砧拔长长方体锻件是非常重要的一个工序,也是整个锻造工艺过程中变形量最大的一个工序。利用Dform软件,研究钛合金长方体锻件的拔长过程,不同的砧宽及平砧压下量对锻件内部应变大小分布、质量及其内部孔洞锻合效果的影响。     

大型锻件锻造拔长新工艺

在分析现有镦粗、拔长变形工序中难变形区对大型锻件锻造过程变形分布和应力状态影响的基础上,提出一种新的拔长工艺——凹面砧拔长。借助数值模拟方法和物理模拟方法比较了普通平砧与凹面平砧、普通V砧与凹面V砧拔长过程中大型锻件内部的等效应变、静水应力和空洞缺陷的闭合情况。结果表明,对应力状态而言,在变形量相同的情况下,对于凹面平砧锻造(CFAF)工艺,静水应力提高近30%,对于凹面V砧锻造（CVAF）工艺,静水应力提高近50%。对变形而言,静水应力相同情况下,CFAF工艺减少压下量33.5%;CVAF减少压下量26.5%。对空洞缺陷而言,凹面砧拔长空洞变形后的椭圆度较普通拔长小10%以上。与其他特殊拔长方法比较,凹面砧拔长有操作方便,附具简单的优点。     

带背压的高硅铝合金等径角挤压数值模拟

采用DEFORM-3D分析软件对高硅铝合金进行了带背压的等径角挤压过程单道次三维有限元模拟,分析了坯料的变形、变形载荷以及等效应变。模拟分析结果表明,带背压时坯料头部无翘曲行为,变形载荷一开始就急剧增加,随着背压的增加,成形载荷近线性增加;坯料的等效应变也随着增加。并总结出,适当增加背压可以有效提高材料的力学性能。     

大型核电低压转子锻件倒棱新工艺的模拟

低压转子是核电机组关键零部件.低压转子锻件方截面坯料倒棱工艺一般采用普通平砧或无曼内斯曼效应的锻造法(Free from Mannesmann effect,FM)法,锻坯的变形仅集中在表层,难以深入心部,这浪费了倒棱工艺的大压下量.提出一种新的改进工艺方案,采用V形砧倒棱工艺来改善锻件心部应力应变状态,并通过数值模拟和物理模拟相结合的方法对其应力应变的分布进行研究.模拟结果表明,采用V形砧倒棱工艺,锻件心部等效应变增加,静水压力增大,随V形砧角度的减小应力应变分布效果更好;本质为型砧侧壁和锻件的接触面积的增加,使得接触处形成难变形区,从而使锻件取得更好的心部应力应变条件.连砧倒棱在两砧相搭接处对应区域存在小变形区,可以通过错砧工艺进行消除.建议倒棱工艺采用V形砧,角度小于等于120°,压下率大于20%,连砧错砧.     

大锻件内部空洞热锻闭合的Z-C判据及其应用

大锻件内部空洞闭合是一个多尺度问题.基于细观塑性理论和体胞模型,推导出大锻件内部空洞的体积演化方程,通过分析空洞闭合的力学条件,建立大锻件高温成形过程中内部空洞闭合的Z-C判据.Z-C判据对人们熟知的定性结论能够从理论上给出定量解释,发现硬化指数、远场应力三轴度和等效应变对空洞闭合均有重要影响,并揭示出影响规律.数值模拟研究表明,运用Z-C判据和CAE技术不仅可以判断空洞是否闭合,而且可以模拟空洞的体积变化过程,揭示空洞演化规律.Z-C判据为大锻件锻造工艺的优化设计提供了一条新途径.     

关于不同规格球面砧对圆柱体镦粗的研究

使用有限元方法模拟了不同规格的球形砧的镦粗过程,同时对不同变形条件下工件的等效应变、动态再结晶体积分数、晶粒尺寸大小以及压力机载荷状况进行了比较。结果表明:在相同变形条件下,随着球面砧球面半径的增大,压力机载荷也不断增大,等效应变、动态再结晶体积分数分布情况与晶粒细化情况有所不同。     

Photoelastic investigations on initial plastic straining

Photoelasticity has been used for an investigation of the stress distribution in billets forged on flat dies. The flat models of the billet were made of birefringent material which were loaded to a state approaching the elastic limit.The authors' experiments were based on Nadai's hypothesis that in the first stages of plastic straining the stresses and strains are commensurate with the corresponding extremal values in the elastic range.The stresses were analysed for different values of feed ratio, i.e. relation of (actual) width of contact area between the die and the billet to its height. The experiments showed that if the feed ratio is less than 1·3, tensile stresses are present in the central zone of the billet under the die centre.There are no unfavourable tensile stresses in the central zone of the billet if the feed ratio is more than 1·3. However, for all values of feed ratio at points on the billet axis at a distance of 0·2 of the height of the billet from the die edge, tensile stresses are present.These conclusions lead to practical recommendations, concerning the forging of ingots and bars of complex brittle alloys at a maximum feed ratio.     

Using a <670> zone axis for convergent beam electron diffraction measurements of lattice strain in strained silicon

Convergent beam electron diffraction patterns of silicon from the gate channel region of a complementary metal‐oxide‐semiconductor transistor with recessed Si_.82Ge_.18 stressors were analysed using three zone axes: <230>, <340> and <670>. Values measured using these axes were compared with each other with regards to strain along the [] and the [001] directions. It was demonstrated that strain measurements made using all three axes showed reasonable agreement with each other: an increase in the [] compressive strain and a switch from compressive to tensile strain in the [001] with decreasing distance below the gate. It was also observed that the strain calculations using the <230> axis had the lowest uncertainty whereas the <670> axis allowed for measurements closest to the gate due to the improved lateral resolution at that tilt angle.     

Shear deformation of voids with contact modelled by internal pressure

The behaviour of voids in a ductile material subject to simple shear or to a shear-dominated stress state is analyzed numerically. Here the stress triaxiality is so low that instead of void volume growth to coalescence there is void closure leading to micro-cracks that rotate in the shear field. At some stage of the deformation, the void surfaces will come in contact so that sliding with or without friction will start to occur. To avoid problems with strong mesh distortion in the large strain field around the deforming void and with mesh resolution at the tip of the crack, an internal pressure is applied as an approximate representation of void surfaces pressed together in frictionless sliding, and also remeshing is applied. This micromechanical model for a strain hardening elastic–plastic material shows that a maximum overall shear stress is reached, at which localization of plastic flow occurs, leading to final failure in the material.     

Visioplasticity analysis of 2024 aluminum alloy extrusions

Visioplasticity studies were conducted on axisymmetric extrusions of 2024 aluminum alloy extruded in the temperature range of 300–537°C. Two types of flow pattern occurred: (a) the usual type consisting of a single maximum in the transverse grid lines located at the extrusion axis; and (b) an uncommon wavy-type with two maxima located symmetrically about the axis. The occurrence of the latter pattern was accentuated as the temperature was lowered or the extrusion speed increased. Good agreement was obtained between computed and experimental positions of the transverse grid lines for both types of flow pattern using flow function analysis. The width of the deformation zone in the double maximum pattern was reduced compared with the single maximum pattern; it moved downstream at the die entrance and upstream at the exit. Derived values of the shear strain and principal strain suggest that the double maximum pattern observed here is determined more by material behavior than by lubrication conditions.     

温度和背压方式对等通道转角挤压过程的影响

建立了用于分析等通道转角挤压过程的热力耦合有限元模型。通过对纯钛等通道转角挤压过程的数值模拟,获得了模具及试件内部的应力、应变和温度分布。研究结果表明,等通道挤压过程中试件温度分布不均匀,在模具转角剪切部位温度最高且存在明显的温度梯度。在较高的温度条件下进行挤压,有利于降低成形压力和获得较大的变形量;接触摩擦的存在导致模具受力状况恶化及试件变形的不均匀;而带背压的挤压方式可以在有效地增加试件变形量和变形均匀性的同时降低材料产生破坏的可能性。     

Void fraction of supersonic steam jet in subcooled water

For most operations in process, petroleum and power industries gas-liquid two phase flows occurs, so an accurate estimation of void fraction is vital because it affects the calculations of heat and mass transfer as well as hydrodynamics. Any inaccuracy in estimation may lead to drastic incidents along with heavy monetary loss. An effort has been made here to estimate the approximate void fraction of supersonic steam jet into the sub-cooled water. Electrical Resistance Tomography (ERT) has been used for the purpose along with the Electrical Impedance Tomography and Diffuse Optical Tomography Reconstruction Software (EIDORS) to generate the conductivity scans obtained by ERT setup. Before the experimentation, for further assurance on our void fraction estimations, the measurement system has been calibrated by securing scans of heated Teflon rod of diameter 6 mm, which is approximately having the same cross-sectional surface area as the steam jet have and reported in previous studies at the same hydrodynamic conditions. Images of supersonic steam jet in subcooled water have been processed by the help of EIDORS and image processing technique. The over estimation in void fraction of Teflon rod is ranging from 46.17 to 83.44% and when it is subtracted from the total void fraction of supersonic steam jet (46.51–83.79%) at steam inlet pressure of 1.5–3.0 bar and surrounding water temperature 30–60 °C respectively, the actual void fraction of Teflon rod comes out to be ranging from 0.34% to 0.35% of the total cross-sectional area of vessel. When these results are compared with the previous studies, a close agreement has been observed between these two sets of results.     

SiCP/Al复合材料延性去除加工的有限元分析

以断裂力学的有关理论为依据,运用ABAQUS有限元分析软件研究在固定的磨削速度下,单个磨粒磨削单颗粒SiCp/Al复合材料的延性去除问题.结果表明:SiC颗粒所受到的拉伸应力和压缩应力都随磨削深度的增加而增大;SiC颗粒发生延性去除的极限切深为0.6μm,SiC颗粒发生延性去除时其最大压缩应力大于26GPa,同时最大拉...     

喷射成形沉积坯热应力及残余应力分析

利用有限元技术计算了沉积坯在生长形成过程及形成后的温度场,利用该温度场计算了沉积坯的瞬时热应力场和残余应力场,计算结果表明沉积坯在沉积的初始阶段冷却速度较大,沉积坯底部的残余应力和瞬时热应力最大。     

镦粗过程中孔洞应变分布的光塑性研究

采用内部带有人工生成孔洞的聚碳酸酯圆柱体作为模拟试件，对镦粗过程进行了光塑性模拟研究，直观地描绘了孔的闭合形式与过程。同时得出，孔洞的闭合需要足够大的等效应变和剪应变。采用Ｅｕｌｅｒ坐标进行应变的计算分析，绘出了孔洞周围应变分布曲线。结果表明，孔洞周围存在强烈的应变集中，孔洞的存在对全场应变分布有局部影响，其范围为原始直径的３～５倍，区域内应变分布不均匀。