核电设备用钢20MoNiMn内部空洞愈合过程分析

通过热物理模拟试验获得20MoNiMn钢在不同变形条件下的真实应力一应变曲线,将数据导入到有限元软件DEFORM一3D中数值模拟20MoNiMn铜内部空洞愈合过程。结果表明,空洞闭合是由球形空洞到椭球形空洞,从椭球形空洞的两长轴端开始闭合,并随着压下量的增加逐渐向心部扩展达到完全闭合。闭合后的空洞裂隙需要在高温下的原子扩散和再结晶才能焊合。当压下量为6．35mm时,坯料中有85．4％的区域发生了完全动态再结晶,空洞裂隙周围原子充分扩散和再结晶,最终达到闭合后的空洞裂隙焊合。     

25Cr2Ni4MoV钢锻造过程孔洞缺陷愈合规律研究

实测了核电转子用25Cr2Ni4MoV钢的高温应力应变曲线和热物性参数,基于ABAQUS软件建立了锻造过程孔洞闭合的有限元模型,模拟了不同温度和变形量下的孔洞闭合行为.研究发现,典型的孔洞闭合过程分为3个阶段,即闭合速率减小-增加-再次减小.模拟结果表明,900—1200℃时孔洞几乎在同一压下率(约25%)下闭合,即孔洞闭合对变形温度不敏感.在模拟结果的基础上,进一步设计了孔洞闭合后焊合过程的物理模拟实验,研究了恒应变速率(0.01s~(-1))下压下率(25%-45%)和变形温度(900-1200℃)对闭合孔洞焊合过程的影响.焊合界面的拉伸实验结果表明,较高的温度和闭合后持续的塑性变形能极大地促进闭合界面的焊合,当变形温度大于1000℃,压下率为35%时,闭合界面结合强度可达基体强度,孔洞实现完全焊合;当温度降至900℃时,孔洞需45%的压下率才能完全焊合.最后,基于拉伸断口的形貌,对影响孔洞焊合的因素进行了讨论.     

45钢内部裂纹愈合过程孔洞闭合的数值模拟

在塑性变形条件下,内裂纹愈合过程一般经历裂纹的闭合及焊合两个阶段.闭合阶段可使裂纹内界面充分接近,为焊合阶段实现界面的金属键结合提供有利条件.本文以带有内部球形孔洞的45钢圆柱体试样为例,利用大型有限元分析软件ANSYS,数值模拟了压缩变形时试样内部球形孔洞的闭合过程.分析了孔洞闭合过程形态演化特点和孔洞附近的应力状态.     

大钢锭锻造时内部孔洞性缺陷压合的试验研究

对含有内部孔洞钢试件进行的热锻模拟实验结果表明,封闭孔洞在高温下锻造时,在一定的应力应变条件下,经过变形一压合一扩散焊合后可以消除,孔洞焊合后的部位为新的晶界所取代,应力和应变状态对孔洞闭合有明显的影响. 对不同砧宽比的平砧拔长过程内部的应变分布进行了量测和有限元法计算,结果表明拔长时坯料内部的变形分布很不均匀,大应变区只占小部分,且随砧宽比和压下量的不同而移动和变化.当砧宽比W/H<0.51时,最大应变区不在坯料中心.     

镦粗过程中摩擦对空洞演变行为的影响

采用有限元模拟方法,模拟计算了摩擦因子为0. 3,0. 5和0. 7时,锻件不同位置空洞的闭合情况。分析了不同摩擦因子下,锻件不同位置的空洞高度的变化规律,结果发现:锻件内部空洞比边缘的空洞更容易闭合,并且随着摩擦因子的增大,内部空洞闭合速度加快,边缘空洞闭合速度减慢。分析摩擦因子、静水应力、等效应变、空洞闭合速度的关系发现:在锻件的内部位置,随摩擦因子增大,静水应力绝对值增大、等效应变增大、空洞闭合速度增大;在锻件的边缘位置,随着摩擦因子增大,等效应变减小、空洞闭合速度减小。摩擦因子对锻件边缘位置空洞闭合情况和等效应变的影响大于对锻件内部位置空洞的影响。     

材料Norton指数对空洞闭合行为与闭合规律的影响

基于单轴压缩数值模拟,系统地研究了空洞的闭合行为,分析了材料Norton指数对空洞闭合的影响规律。研究发现,材料Norton指数的变化对等效应力、等效应变影响区的形貌的影响不大,但对等效应力的大小影响十分明显。Norton指数n越大的材料,空洞闭合所需变形量越小。     

基于修正的Gurson模型预报大锻件内部空洞型缺陷的演化

Gurson模型要求材料的静水应力或等效应变无穷大时才能实现材料内部的空洞闭合,而这一条件并不符合实际情况。文章对Gurson模型中的各参数进行了试验设计,通过开发子程序,对不同试验参数下的空洞体积变化过程进行了模拟仿真,据此获得了修正的Gurson模型参数,从而将其应用于大锻件锻造过程的空洞体积变化模拟中。结果表明,修正后的Gurson模型给出的空洞体积变化率,与直接设置空洞的有限元模拟结果很接近,从而大大改善了空洞闭合的预报结果。     

圆柱体锻件内多空洞闭合过程的模拟研究

为了提高锻件质量,减少锻件由于空洞缺陷导致的报废,采用数值模拟和物理模拟相结合的方法,研究了镦粗时圆柱体锻件内多个空洞闭合的过程.模拟结果表明,以双空洞为例,如空洞尺寸不同,大空洞附近变形大,小空洞对变形的影响是局部的;相同大小空洞在闭合过程中,圆柱体中部有封闭的应力、应变的等值线分布区域出现.研究结果推进了对这一问题地深入研究,指出数值模拟的有效性和其与物理模拟相结合的重要性.     

大型钢锭内部孔洞性缺陷锻合过程的数值模拟和实验研究

本文是作者一篇论文的详细摘要。作者对锻件内部孔洞消除机制作了实验研究,对锻造过程孔洞压合和应变分布用塑性有限元法进行数值模拟,并对锻造坯料内部应变分布作了试验量测。研究结果表明:大锻件内部孔洞的锻合过程由变形、压合、焊合三个阶段组成,孔洞只有在高温下才能焊合;孔洞闭合度λ与最大压应变ε_1呈线性关系,孔洞的几何因素和工具接触表面摩擦不改变这种规律,但可使应变向坯料心部集中,有利于闭合心部孔洞;用平砧拔长时,砧宽比应取0. 51以上,而用FM 法时,砧宽比应大于0. 4。作者建议制定大钢锭锻造工艺时,要注意:1) 尽量使最大压应变ε_1的方向同内部孔洞的短轴相一致;2) 拔长时应使用合理的砧宽比;3) 压合内部孔洞要在较高温度下进行。     

大锻件内空洞形状对闭合影响的模拟研究

利用有限元分析法研究了大锻件内部不同形状空洞在镦粗过程中的闭合情况,以及空洞周围的应力情况.结果表明,椭球形空洞的横轴与竖轴数值之比越大,空洞的闭合情况越理想;基于对空洞闭合过程中应力、应变的追踪分析,提出了一种判断空洞闭合的量化分析方法.     

Internal void closure during the forging of large cast ingots using a simulation approach

Large cast ingots often contain defects or undesirable microstructural features, such as voids and zones related to casting. Some of these features can remain after hot open die forging, which is an important process for converting large cast ingots into wrought components. During the initial cogging and deformation steps prior to the detailed open-die-forging operations, any internal voids should be eliminated. The present work focuses on the closure of internal voids during open die forging so as to produce a sound component. Hot compression tests were conducted to obtain the flow strength of the cast microstructure at different temperatures and strain rates. The measured flow strength data together with other appropriate material properties were used to simulate the forging steps for a large cast ingot. The numerical simulations for the forging deformation and for the internal void behavior were performed using DEFORM-3D™. Actual defects were measured in commercial ingots with an X-ray scanner. The simulation results for the void deformation behavior are compared with voids measured before and after forging. Through the comparison of experimental results and numerical simulation, a criterion for void closure is proposed. The criterion is that a local effective strain value of 0.6 or greater must be achieved for void closure during forging. Such a criterion can be used in conjunction with simulations to insure that a sound component is produced during the hot open die forging of large cast ingots.     

镁合金有限元与三维加工图相结合的材料驱动的可加工性模拟

基于PRASAD提出的传统的二维加工图理论,建立考虑应变的三维加工图,描述功率耗散系数和流变失稳区域随应变速率、温度和应变的变化。三维加工图说明了材料的内禀可加工性,而有限元分析方法可得到材料在特定工艺条件下应力、应变、应变速率及金属流动情况,说明了由模具形状和工艺条件决定的应力状态可加工性。基于此,提出一个新的由材料驱动的热变形可加工分析方法,联合考虑有限元和三维加工图,可以说明整个热加工过程的材料可加工性（包括应力状态可加工性和内禀的可加工性）。通过此方法,研究难变形金属镁合金的热锻过程,包括复杂热锻直齿锥齿轮的三维热力耦合有限元和三维加工图的集成模式。基于得到的研究结果,成功进行了热锻试验。试验表明新的方法用于确定最佳工艺参数是合理的。     

模锻成形过程中金属变形流动的测试方法

利用此方法对叶轮锻件成形过程中两个典型截面上4条流线及其轴向应变进行了测量分析,并定量给出该复杂锻件成形过程中变形及应变分布规律。因而该方法是测试模锻成形过程中金属塑性变形流动较为有效的实验手段。     

Analysis of void closure in open-die forging

Void closure studies have been conducted numerically and experimentally for open-die forging processes. The plane-strain FEM analysis was compared with bite forging experiments in order to determine how well the plane-strain approximation predicted the material flow in open-die forging. In addition physical modeling with plasticine was used to compare the measured and calculated deformation of the internal defect. The FEM analysis was in good agreement with the experimental results. Correlations for the computed effective strain and hydrostatic stress to the void closure were then calculated. Simulations of a solid cylinder side pressed with flat dies, V-shaped dies, and FML dies were done to determine the effectiveness of these dies at consolidating internal porosity based on the calculated strain and hydrostatic stress at the center of the billet. The V-shaped dies were found to be the most effective among those investigated. However, the press load for the V-shaped dies was also the highest.     

Prediction of void closure in steel slabs by finite element analysis

Closure of a spherical voids in a steel slab under plane-strain deformation was investigated using the rigidplastic finite-element method. Variations in the major and minor axes of a void from finite element analysis of a void model were related to the minimum principal strain and hydrostatic stress from finite element analysis of a non-void model. The correlation curves were obtained and a method using these curves to predict the void closure progress was proposed. The method was successfully applied to deformation processes such as simple compression, forging and rolling. Since hydrostatic stress also influenced void closure, the effective strain by itself was not sufficiently capable of predicting void closure. However, the effective strain was used to predict void closure for a specific process because it reached about 0.7 in compression or forging and about 0.78 during rolling as the void completely closed.     

Deformation characteristic of the precision forging of a blade with a damper platform using 3D FEM analysis

A blade with a damper platform is one of the most important types of blades being developed in aeronautical engines. However, this type of blade is complicated in shape, consisting of four feature parts with different geometrical shapes. Besides obvious flow of material along the transverse direction, there is material flow along the longitudinal body during its forging process. In addition, the material used is difficult to deform. All these have a significant influence on the precision forging process of the blade. Thus, it is necessary to choose some feature sections along the transverse and longitudinal direction of blade and systematically understand the deformation characteristics of blade forging process. In this paper, by means of 3D rigid–viscoplastic FEM simulation of the precision forging process of the blade, the deformed meshes, distributions of some field variables, including velocity, effective strain, effective strain rate and effective stress are presented for the four chosen feature cross-sections and the velocity fields are obtained for a selected typical longitudinal feature section, and further the deformation characteristics of forging this blade have been revealed. This research may serve as a guide to the optimization design of the relevant processes and dies.     

非对称复杂形状连杆模锻的三维有限元模拟

作为重要的传力件，连杆在工业中具有广泛的应用。以一个非对称复杂形状连杆为对象，在对其进行工艺分析的基础上，采用大型体积成形有限元数值模拟软件DEFORM3D对其模锻工艺进行了数值模拟，得到了变形过程不同时刻连杆的变形情况、等效应变和等效应力的分布以及连轩模锻成形时的压力行程曲线，最后给出了结论。     

A criterion for void closure in large ingots during hot forging

A quantitative investigation of the mesomechanism of void closure in large ingots during hot forging is undertaken in the present study. The constitutive relation of the void-free matrix is assumed to obey the Norton power law. A cell model which includes matrix and void is employed and a Ritz procedure is developed to study the volumetric strain-rate of the void. On the basis of a large number of numerical computations, a criterion for void closure in large ingots during hot forging is proposed. In addition, the significant effects of the Norton exponent, the remote stress triaxiality and the remote effective strain on void closure are revealed: (1) the volumetric strain-rate of the void increases monotonically as the stress triaxiality level and the Norton exponent of the material increase, (2) the remote effective strain required for void closure decreases as the stress triaxiality level and the Norton exponent increase and (3) the void becomes unstable and the collapse rate decreases at the final stages of void closure. With the criterion for void closure, process design and optimization in terms of elimination of voids in large ingots will be convenient since the criterion can be easily applied in CAE analysis.     

A study of pore closure and welding in hot rolling process

The design of processing conditions to eliminate porosity in steel during hot rolling has become more critical with the advent of continuously cast feed stock. To predict appropriate process parameters, experiments were performed in which holes were drilled in steel slabs, at different depth below the surface perpendicular to the rolling direction. After hot rolling, the state of the deformed holes was examined by optical microscopy. The fracture surfaces of tensile specimens notched in the plane of the closed holes were examined under a scanning electron microscope to investigate the bonding ( welding ) between the surfaces of the closed holes because this bonding is the most important factor indetermining the transverse mechanical properties of the rolled product. The deformation of such holes in the roll gap was modelled as an clastic / plastic plane strain problem using the FE software ABAQUS to investigate the strain and stress around holes and to analyze the conditions required to promote pore closure and welding between the closed surfaces.

Experiment results showed that the rate of pore closure was affected by the parameters of rolling process and the position of holes relative to the rolling contact surfaces. FE simulation of pore closure showed a good agreement with experimental results and showed that a certain level of hydrostatic pressure resulted in the closure of pore and that the holding period of the pressure in the compressive state determined the degree of welding of surface of pores; shear is favourable to this welding.     

AZ80镁合金多向锻造变形过程中晶粒取向的演变

用XRD图定性分析了多向锻造变形下AZ80镁合金择优取向的变化.除第4道次外,其余前9道次锻压面均出现明显的基面择优取向,10道次后材料开始呈现随机取向分布.变形温度和变形方式对材料晶粒取向影响较大,适当降低变形温度和采用单向变形方式均能促使锻压面形成强烈的基面择优取向,而道次压下量在本实验中对晶粒取向影响不大.