Numerical simulation of plate rigid restraint cracking tests based on cohesive element model

Cohesive element is developed from the Dugdal-Barenblatt model in the field of fracture mechanics. The mechanical constitutive relation of cohesive element can be artificially assumed depending on the specific applications. It has been successfully applied in the study of crystal plasticity/ brittle fracture process and decohesion between delaminations. In this paper, tensile experiments of large steel plate with different length of pre-existing cracks are conducted. Based on commercial software ABAQUS, cohesive element is adopted to simulate the tensile tests, and appropriate parameter values are obtained by fitting displacement force curves. Using these parameters, a numerical method is presented by applying cohesive element to thermo-elastic-plastic finite element method ( TEP-FEM) to simulate plate rigid restraint cracking ( PRRC) tests. By changing constitutive relation of cohesive element, dimensions of the model and welding conditions, the influence of welding restraint intensity and welding conditions on the crack propagation are discussed, respectively. Three types of welding cold cracking are simulated. Significant influence of welding cold cracking on resistant stress in welding line is captured by this numerical method.     

Preventing welding hot cracking by welding with an intensive trailing cooler

Welding with an intensive trailing cooler has been investigated as a means to prevent welding hot cracking. Hot cracking initiated from the crater and extending backwards has been reproduced using a welding fixture with variable restraint. Welding with a trailing cooler can effectively prevent this kind of hot cracking and is feasible in welding production. The temperature field and the strain distribution have been analyzed using the non-linear finite element method. The results of experiment and numerical simulation expound the mechanism of the prevention of hot cracking.     

Hot cracking in pulsed laser processing of a nickel based superalloy built up by electrospark deposition

A cast precipitation hardened nickel based superalloy was built up by electrospark deposition and then subjected to pulsed laser welding. The resistance of electrospark deposition layer to liquation and solidification cracking in pulsed laser welding was significantly higher than that of the cast alloy. It is shown that the extreme cooling rates involved in the solidification of electrospark deposition droplets result in the prevention of formation of grain boundaries terminal solidification constituents, which are the sources of liquation cracking in the cast alloy.     

焊接凝固裂纹数值模拟的研究及其进展

焊接凝固裂纹的产生与否取决于两个方面的因素,材料凝固裂纹阻力和凝固裂纹驱动力,凝固裂纹阻力可以通过试验方法来获得,而凝固裂纹驱动力主要采用有限元计算方法得到.对焊接凝固裂纹的形成机理、凝固裂纹驱动力曲线和焊接凝固裂纹阻力曲线的试验测量方法的数值模拟,以及防止凝固裂纹方法的数值模拟过程都进行了总结及展望.     

CO2焊接逆变电源-电弧系统的仿真

由于短路过渡CO2焊接电弧负载的非线性与时变性的特点，CO2焊接电源-电弧系统的研究与设计有其固有的难点。作者采用在控制领域最具影响的MATLAB软件，在对CO2焊接电弧建模、逆变电源-电弧系统建模的基础上，仿真了波形控制CO2焊接逆变电源-电弧系统的动态过程，并且在模拟示波器上输出了CO2焊接电弧的几个典型参数的动态波形。通过计算机仿真，提出了CO2焊接过程平衡长度的概念，揭示了焊接过程平衡长度的物理过程，CO2焊接过程平衡长度的变化范围必须是大于零、最大值是几毫米(具体值与焊接参数有关)的数量程度。给出了波形控制CO2焊接逆变电源-电弧系统的控制结构，通过仿真可以看出短路过渡CO2焊接逆变电源控制参数对系统的影响规律，辅助优化设计系统。     

In situ strain investigation during laser welding using digital image correlation and finite-element-based numerical simulation

In situ strain evolution during laser welding has been measured by means of digital image correlation to assess the susceptibility of an advanced high strength automotive steel to solidification cracking. A novel method realised using auxiliary illumination and optical narrow bandpass filter allowed strain measurements as close as 1.5?mm from the fusion boundary with good spatial and temporal resolution. A finite-element thermomechanical model of the welding process supports the experimentally measured transverse strain. The validated finite-element numerical model can be used to assess the local strain and associated stress conditions which influences weldability and in particular, solidification cracking.     

裂解炉管焊接残余应力场中裂纹扩展行为的数值分析

针对裂解炉管对接焊接热影响区中常见的环向与斜裂纹在焊接残余应力的作用下的扩展行为进行数值预测.采用基于VNA算法的有限单元交互法(FEAM),结合有限元方法研究焊接残余拉应力对炉管裂纹扩展的影响,通过研究裂纹尖端区域的应力强度因子的变化来判断裂纹的准静态扩展倾向.结果表明,在多道焊焊接残余应力的影响下,在炉管焊缝热影响区内表面的环向裂纹有沿环向变长的倾向,而斜裂纹则同时有变长、变深的倾向.     

基于ANSYS的高强钢厚板对接焊缝热裂纹成因分析

利用ANSYS的APDL参数化设计语言,以节点方式建立模型,对实际生产中JFE-HITEN 780S高强钢CO2气体保护焊的三维焊接温度场和应力场进行了数值模拟。模型实现了对实际生产中JFE-HITEN 780S高强钢36 mm厚板的13层、29道实体焊接工艺过程进行的仿真,在仿真和理论基础上对焊接热裂纹的成因和防止措施进行了分析。     

基于TIG堆焊技术的低碳钢零件精密快速成形

建立了基于TIG堆焊技术的熔焊快速成形系统,包括焊接电源、TIG焊枪、送丝机构、数控机床、工控机和电压电流采集卡等.TIG焊机进行堆焊成形,数控机床控制焊机的行走并进行后续的切削加工,工控机采集焊接过程中的成形数据,进行反馈控制.针对低碳钢TIG堆焊成形过程中热量积累导致工件边缘塌陷的问题建立了电压反馈模糊控制系统.结果表明,在电压反馈模糊控制系统的辅助下进行堆焊可获得成形良好的墙体工件.进行了圆筒形状低碳钢零件的熔焊快速成形,在TIG堆焊的基础上进行了数控切削加工,获得了高精度的成形工件.     

A simple Eulerian thermomechanical modeling of friction stir welding

A simple three-dimensional thermomechanical model for friction stir welding (FSW) is presented. It is developed from the model proposed by Heurtier et al. (2006) based on a combination of fluid mechanics numerical and analytical velocity fields. Those velocity fields are introduced in a steady state thermal calculation to compute the temperature field during welding. They allow partial sliding between the shoulder and the workpiece, the amount of which is provided as an additional result of the model. The thermal calculation accounts for conduction and convection effects by means of the particular derivative. The complete thermomechanical history of the material during the process can then be accessed by temperature and strain rate contours.The numerical results are compared with a set of experimental test cases carried out on an instrumented laboratory device. The choices for modeling assumptions, especially tribological aspects, are discussed according to agreements or deviations observed between experimental and numerical results. The amount of sliding appears to be significantly influenced by the welding conditions (welding and tool rotational velocities), and physical interpretations are proposed for its evolution.     

新型耐磨钢NM400斜Y形坡口焊接抗裂性分析

采用斜Y形坡口焊接抗裂性试验对NM400钢的焊接抗裂性进行了研究,通过测定其表面裂纹率、断面裂纹率及根部裂纹率,评定NM400的抗裂性能,并分析焊前预热对材料抗裂性的影响.同时,应用热弹塑性有限元法对斜Y形坡口试样进行了焊接过程的有限元数值模拟计算,得出横向焊接残余应力的分布情况及峰值位置.结果表明,常温下NM400的根部裂纹率较高,预热150℃时各裂纹率均降至0.因此,焊前预热能够提高NM400的焊接抗裂性.另斜Y形坡口根部产生较大的应力集中,局部横向残余应力超过材料的强度,是致使其产生裂纹的力学因素.     

Prediction of solidification cracking in laser welds of type 310 stainless steels

The occurrence of solidification cracks in laser welds of type 310 stainless steels was predicted by numerical analyses of the solidification brittle range (ductility curve for cracking) and thermal strain in the weld metal. The solidification brittle range in laser welding was estimated from that in arc welding based on the numerical analyses of supercooling (for calculating dendrite tip temperature) and segregation (for calculating completely solidified temperature) during rapid solidification. The calculated solidification brittle range was reduced with an increase in the welding speed because of the enhanced supercooling and the inhibited solidification segregation. The thermal strain analysis by FEM suggested that solidification cracks would occur in SUS310S welds at laser travelling velocity of 60 mm/s applying the initial strain of 1.5%, while no solidification cracks in SUS310EHP welds at any laser travelling velocities applying the higher initial strain of 2.2%. The cantilever type cracking test in laser welding revealed that the predicted results of occurrence of solidification cracks were consistent with experimental ones.     

镍基合金焊接裂纹研究现状

近年来,镍基合金焊接件在航空航天、核电、火电和石油化工等工程领域的应用需求快速增长。本文介绍了镍基合金的分类以及镍基合金焊接方法的研究,由于成本以及技术等的限制,镍基合金的焊接主要采用熔化焊焊接方法。重点综述了镍基合金焊接裂纹的产生机理以及各元素对裂纹的影响。镍基合金熔化焊焊接过程中易产生4种焊接裂纹:结晶裂纹、液化裂纹、失塑裂纹和应变时效裂纹。总体上,结晶裂纹和液化裂纹产生机理已较为明确,焊接过程中低熔点液态薄膜的出现是结晶裂纹和液化裂纹产生的主要因素。失塑裂纹目前仍没有对其明确的定义,镍基合金失塑裂纹产生机理也存在着较大的分歧。镍基合金应变时效裂纹是沉淀强化镍基合金所特有的,裂纹产生与沉淀相的沉淀速率密切相关。杂质元素和添加元素对镍基合金焊接裂纹敏感性有着重要影响,元素的影响虽然已经进行了大量的研究,但元素单独或者协同对裂纹敏感性的具体影响仍需进一步的研究。     

焊接残余应力数值模拟研究技术的现状与发展

在焊接过程中产生的动态应力应变及随后形成的残余应力，是导致焊接裂纹和接头性能下降的重要因素。因此，焊接残余应力一直是人们关注的热点问题。结合焊接残余应力的研究方法与手段。介绍近年来固内外关于焊接残余应力的研究现状与发展。     

Analysis of crystallisation and growth behaviour of NbC on solidification process of SUS347H weld metal

Abstract

In order to investigate the solidification cracking susceptibilities of SUS347H weld metal containing high niobium and carbon contents, the crystallisation behaviours of niobium carbide as well as those of and γ phases during welding solidification were numerically analysed to compare with the results from in situ observations of the behaviour of the phase selection for γ, and niobium carbide phases for Fe–18Cr–0·2C–1Nb–(5–12)Ni weld metals. The in situ observation was carried out during tungsten inert gas welding using synchrotron radiation. On the other hand, to evaluate the segregation in liquid phase, which has significant influences on the solidification cracking susceptibility, a numerical model to calculate the segregation in liquid phase considering the crystallisation of γ, and niobium carbide phases was developed. The developed numerical model was verified by the comparison with experimental results from the in situ observation. It was suggested that the solidification cracking susceptibilities of Fe–18Cr–0·2C–1Nb–(5–12)Ni weld metals estimated from the results of the developed numerical model coincided with the experimental results evaluated by transverse Varestraint test.     

不锈钢0Cr18Ni9熔滴沉积参数优化

在熔融沉积成型(FDM)过程中,为保证熔滴与基板之间有较好融合,本文首先对送丝速度、焊炬不同姿态以及焊炬与基板距离等参数下的熔滴沉积质量以及与基板融合变形过程进行了分析,采用VOF方法对熔滴的自由表面进行跟踪和修正,通过数值计算分别模拟了熔滴以一定速度撞击基板后铺展、融合的过程。通过模拟与试验验证得到可视化结果,表明不锈钢0Cr18Ni9熔滴沉积融合变形过程受多种因素影响,与实际相吻合。     

电磁搅拌技术在焊接中的应用

作为控制金属凝固过程的有效手段,电磁搅拌技术在焊接过程中的应用日益广泛.综述了该技术在焊接过程中的具体应用,包括控制飞溅、影响焊缝成形、抑制焊缝中气孔的数量、防止焊缝结晶裂纹的产生及改善焊缝组织和力学性能,介绍了外加磁场数值计算的研究状况,指出了电磁搅拌技术的研究方向、应用前景和实用价值.     

不锈钢厚板焊接凝固裂纹驱动力的三维数值模拟

利用双椭球形热源模型研究了SUS310不锈钢的三维焊接温度场,并与实验测量进行了比较．在计算温度场的基础上,重点研究了材料在凝固裂纹敏感温度区间内,焊缝金属应变场和位移场的动态场演变过程,得到了10mm厚板的凝固裂纹驱动力曲线,并利用动态单元再生方法,消除了焊接凝固过程对应变场的影响．模拟计算得到的驱动力曲线与前人实验测量的凝固裂纹阻力曲线进行了比较,初步预测了SUS310不锈钢凝固裂纹敏感性．     

Nickel based superalloy welding practices for industrial gas turbine applications

Abstract

The continued drive for increased efficiency, performance and reduced costs for industrial gas turbine engines demands extended use of high strength-high temperature capability materials, such as nickel based superalloys. To satisfy the requirements of the component design and manufacturing engineers, these materials must be capable of being welded in a satisfactory manner. The present paper describes the characteristic defects found as a result of welding the more difficult, highly alloyed materials and reviews a number of welding processes used in the manufacture and repair of nickel alloy components. These include gas tungsten arc (GTA) and electron beam (EB) welding, laser powder deposition and friction welding. Many of the more dilute nickel based alloys are readily weldable using conventional GTA processes; however, high strength, precipitation hardened materials are prone to heat affected zone and strain age cracking defect formation. A number of factors are found to affect the propensity for defects: composition (aluminium and titanium content), grain size, pre- and post-weld heat treatment, as well as the welding process itself (control of heat input and traverse speed). Process parameter identification is still largely empirical and a fuller understanding of the joining processes is dependent upon the development and application of more sophisticated numerical modelling techniques.     

Development of a welding system for 3D steel rapid prototyping process

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