Themo-elasto-viscoplastic modelling of friction stir welding

Abstract

A coupled two-dimensional Eulerian thermo-elasto-viscoplastic model has been developed for modelling the friction stir welding process. First, a coupled thermo-viscoplastic analysis is performed to determine the temperature distribution in the full domain and the incompressible material flow around the spinning tool. Next, an elasto-viscoplastic analysis is performed outside the viscoplastic region to compute the residual stress. Both frictional heat and plastic deformation heat generation are considered in the model. Furthermore, this is the only known model computing residual stress accounting for plasticity caused by both thermal expansion and mechanical deformation due to material spinning. The computed residual stress is verified by comparing to experimentally measured data.     

弹粘塑性材料模型在H型钢热轧模拟中的应用

采用弹粘塑性材料模型，利用有限元分析软件ANSYS-DYNA对H型钢开坯6道次连轧进行了模拟。说明了建模和材料参数的选取方法#以及轧件的变形、应力和应变等计算结果，有效解决了模拟过程中易出现的堆钢问题。模拟结果与理论值较接近，为生产提供了理论依据。     

应用人工神经网络理论建立热粘塑性材料的本构关系

本文将人工神经网络（ＡＮＮ）用于建立热粘塑性材料的本构关系，意在探索出一种描述材料变形力学行为的新方法。文中给出了应用人工神经网络建立热粘塑性材料本构关系的ＢＰ模型和学习算法过程，并应用于４５号钢在高温和高速变形条件下的流动应力计算。计算结果与实测结果比较表明，二者吻合良好。因此，应用人工神经网络建立热粘塑性材料的本构关系具有重要的工程应用价值。     

考虑不连续屈服的钛合金高温变形本构模型

以发生不连续屈服的钛合金高温变形流动曲线特性为基础,讨论各阶段的变形机制。利用位错增殖动态理论和统一粘塑性理论,构建反映变形温度和应变速率影响且能描述不连续屈服、下屈服点后存在轻微应变硬化、动态再结晶等变形特性的高温粘塑性本构模型。将所建模型应用于新型亚稳β钛合金Ti2448发生明显不连续屈服的高温变形,并用改进的遗传算法确定模型中的相关材料参数。预测值与实验值误差在5%以内,表明这种基于内变量构建的本构模型不仅物理意义清晰,能够有效描述发生不连续屈服的钛合金高温变形,而且具有较强的外推能力,能为其它钛合金的本构模型构建提供参考。     

Finite element analysis for the semi-solid state forming of aluminium alloy considering induction heating

The major objective of this study is to establish an analytical technique in order to investigate the behavior of semi-solid material considering induction heating of the workpiece. The induction heating process is analyzed using the commercial finite element software, . The finite element program, , for the simulation of deformation in the semi-solid state is developed in the present study. The behavior of semi-solid material is described by a viscoplastic model for the solid phase and by Darcy’s law for the liquid flow. Simple compression and closed-die compression processes considering induction heating are analyzed. To validate the effectiveness of the proposed analytical technique, the results of simulation are compared with those of experiment.     

半固态AlSi7Mg合金的触变力学模型研究

采用G1eeble—1500热模拟实验机研究了半固态A1Si7Mg合金的触变力学行为.通过回归分析建立了半固态A1Si7Mg合金在不同温度下的粘塑性本构关系模型；利用自行开发的半固态金属触变成形装置进行了实验研究，并采用建立的本构关系应用DEFORM^TM商用软件进行了模拟计算与实验条件相同的变形过程．由模拟计算与实验结果相比较可知，建立的本构关系模型是合理的，模拟结果与实验结果基本吻合．     

轴对称热弹塑性应力有限元分析在焊接中的应用

本文运用热弹塑性理论,给出了轴对称条件下热弹塑性应力有限元分析关系式。所编制的轴对称热弹塑性应力分析程序考虑了温度对材料性能的影响以及进入塑性变形后卸载问题和应变硬化问题。利用该程序对补焊时的应力分布进行了实例分析。结果表明,该程序可以作为了解轴对称条件下热弹塑性应力的有力工具。     

接触单元在铸件铸造过程应力数值模拟中的应用

铸件-铸型之间的相互作用对铸件凝固过程中热应力的分布有很大的影响。本文使用ANSYS软件中的接触单元处理了铸件-铸型之间的相互作用,对应力框进行了热应力分析,并与将砂型处理为完全退让性和完全刚性的情况进行了比较。结果表明,采用接触单元的计算结果更为准确。同时研究了在砂型铸造中湿型砂厚度对应力结果的影响,表明接触单元模型可以进行一定的简化从而减少计算量。     

基于MATLAB平台有限单元法的铸造热应力数值模拟

采用有限单元法(FEM)对铸件凝固过程热应力场进行了三维数值模拟研究,开发了基于MATLAB平台的FEM铸造热应力场模拟程序,采用典型应力框试件对所开发的算法和程序进行了算例验证,并与采用商品化软件ANSYS进行模拟的结果进行了对比,二者吻合良好.结果表明,所采用的算法及开发的程序能较好地预测铸件变形及热应力的集中位置.     

基于有限单元法的铸件热应力场数值模拟

重点论述了基于有限单元法(FEM)对铸件热应力场进行数值模拟的理论基础和实际方法,开发了基于有限单元法的铸造热应力场模拟程序.采用典型试件进行了算例验证,并与ANSYS的模拟结果及实际铸件浇注结果进行了对比,结果吻合.研究表明,开发的基于有限单元法的铸件热应力场数值模拟程序,模拟结果精确,运行可靠.     

W/ODS铁素体钢功能梯度材料热应力分析

目的 研究W/ODS铁素体钢功能梯度材料（W/ODS FGM）服役条件下的热应力,期望获得较合理的W/ODS FGM材料设计,以达到热应力优化的效果。方法 采用有限元分析方法,结合偏滤器的服役条件,通过改变W/ODS FGM材料梯度层成分分布指数p、梯度层厚度HFGM以及金属W涂层厚度HW,探索各参量的变化对热应力大小及分布的影响。结果 梯度层成分分布指数p值增大,梯度层的应力值会随之增大,而W层的热应力先减小后增大。当p=0.5时,最大热应力出现在梯度层的中段;当p=1、2时,最大应力由FGM层中段转移至FGM/W层的交界处。梯度层厚度HFGM增大,涂层的热应力会大幅提高。梯度层厚度较厚或较薄都会导致热应力在FGM/W交界处集中。W涂层厚度HW增大,会导致W/FGM界面的热应力增大,增添了涂层自身的不稳定性。结论 梯度层成分分布指数和厚度的增大均会引起涂层热应力的增大,并导致最大热应力区的转移。W涂层的增厚会使结构的热应力增大,且最大应力值位于W/FGM界面,不利于涂层寿命的提高。HW=HODS=1 mm、HFGM=8 mm、p=0.5和HW=HODS=1 mm、HFGM=4 mm、p=1的最大热应力区位于梯度层中段,且后者的最大应力值小于前者,故HW=HODS=1 mm、HFGM=4 mm、p=1的结构较优。     

Experimental investigation of spark generation in electrochemical discharge machining of non-conducting materials

Electrochemical discharge machining (ECDM), also known as spark assisted chemical engraving (SACE), is an effective micro-machining process for non-conducting materials. Process modeling of ECDM, including spark generation and material removal, is not fully established however. Empirical estimation for discharge energy results in large prediction error of material removal and is hard to experimentally validate. In this paper, an experiment-based stochastic model for spark energy estimation is presented. Tapered tool electrodes were fabricated by electrochemical machining (ECM) to improve the consistency of spark generation. Energy of sparks was experimentally determined and fit into a two-component mixture log-normal distribution to reveal electrochemical characteristics of tool electrodes. A finite element based model was established to correlate spark energy and the geometry of removed material. Material removal was treated as heat transfer problem because electrical energy released by spark generation transfers into thermal energy on the workpiece, resulting in material removal due to thermal melting and chemical etching. Predictions of material removal by the model demonstrated good consistency with experimental results.     

复合SnPb焊点的形态与可靠性预测

建立了复合焊点形态的能量控制方程,采用Ｓｕｒｆａｃｅ Ｅｖｏｌｖｅｒ软件模拟了复合ＳｎＰｂ焊点的形态,利用复合ＳｎＰｂ焊点形态的计算结果。采用统一型粘塑性Ａｎａｎｄ本构方程描述复合焊点Ｐｂ９０Ｓｎ１８和Ｓｎ６０Ｐｂ４０的粘塑性力学行为。采用非线性有限元方法分析复合ＳｎＰｂ焊点在热循环条件下的应力应变过程,基于Ｃｏｆｆｉｎ－Ｍａｎｓｏｎ经验方程预测焊点的热循环寿命,考察焊点形态对焊点可靠性的影响,研     

热挤压钢管刚—粘塑性有限元分析

本文在刚-粘塑性材料变分原理的基础上,导出了相应的有限元列式。用实验确定了粘塑性材料的流动应力方程,编制了适用于轴对称钢管热挤压问题的有限元计算程序RVP-11,并分析了各种工艺参数(温度、挤压速度、挤压比、摩擦等)对挤压过程的影响。     

金属流变成形机理探讨与实例分析

结合工程实际，对金属流变成形机理进行了初步探讨，从一维流变学模型理论出发，求出具有弹粘塑性性质的金属材料的本构方程，进而分析限三维复杂应力状态下，各向同性不可压粘塑性材料在轴对称流变成形过程中的应力场，速度场和流量计算公式，由此发现线性粘塑性介质只是非线性粘性介质的一种极限情况，文章详细讨论了该零件的试验条件和成形方法，最后进行了试验结果分析。     

垂直裂纹对EB-PVD热障涂层热循环失效模式的影响

对电子束物理气相沉积（EB-PVD）双层结构热障涂层在热循环过程中形成的陶瓷层垂直裂纹对涂层失效模式的影响进行了研究.结果表明,陶瓷层表面垂直裂纹出现在热循环初期,但并未造成涂层的早期剥落;随着热循环次数增加,垂直裂纹网格变密.有限元计算表明,垂直裂纹的形成在试样中部产生了类似于试样边缘的剪应力集中效应.经过长时间热循环后,当陶瓷层等轴晶区的强度或者热氧化生长层（TGO）的强度小于边缘效应产生的剪应力时,涂层在试样的中部以垂直裂纹网格形状发生剥落失效.     

Temperature Variation in NiTi Shape Memory Alloy During Cyclic Phase Transition

Superelastic NiTi shape memory alloy (SMA) has high recoverable strain and outstanding damping capacity, and has been used as a damping material for many applications. When subjected to displacement-controlled cyclic deformation, the material exhibits distinctive temperature and stress oscillations due to the release of latent heat and hysteresis heat and the heat transfer with the ambient. In this paper, we establish a model to predict the temperature variation of NiTi SMA wire specimen under the cyclic phase transition by lumped heat transfer analysis. Closed-form solution on the evolution of the temperature is obtained. It is shown that, for all the test frequencies, steady-state cyclic thermal response of the specimen can be reached after a certain number of loading cycles in a transient stage, exhibiting a kind of ??thermal shake down.?? In the steady state, the temperature profile oscillates around a mean temperature plateau. We show that the temperature oscillation is mainly due to the release/absorption of latent heat during cyclic phase transition, while the mean temperature rise of the specimen is caused by the accumulation of the hysteretic heat of the phase transition. The model predictions agree well with the experimental results.     

Spark Plasma Sintering of Ultrafine YSZ Reinforced Cu Matrix Functionally Graded Composite

Copper matrix composites have received more attentions as possible candidate for thermal and electrical conductive materials to be used in electrical contact applications. In this study, five-layered Cu/YSZ(yttria-stabilized zirconia) functionally graded material(FGM) and copper matrix composite specimens containing 3 and 5 vol% YSZ particles plus pure Cu specimen were synthesized using powder metallurgy(PM) route and spark plasma sintering(SPS)consolidation process. The microstructural and some physical, mechanical features of all specimens were characterized.Microscopic examinations showed that ultrafine YSZ particles were distributed in the copper matrix almost homogeneously. An appropriate interface was observed at each layer of FGM. The density measurement indicated that the graded structure of the composite could be well densified after the SPS process. The microhardness values of various layers of Cu/YSZ FGM specimen were gradually altered from 56.3(pure copper side) to 75.2 HV(Cu-5 vol% YSZ side). The increase of YSZ content resulted in a decrease in electrical conductivity. Additionally, thermal conductivity of Cu/YSZ FGM specimen [308.0 W/(m K)] was determined to be higher than that of the Cu-5 vol% YSZ composite specimen [260.7 W/(m K)]. Accordingly, it can be concluded that the Cu/YSZ FGM can be a good candidate for the electrical applications, like sliding electrical contacts, where different material characteristics in the same component are required.