不锈钢焊接凝固裂纹应力应变场数值模拟结果分析

对不锈钢凝固裂纹的驱动力即熔池尾部应变场进行了模拟.首先,通过单元再生方案,消除了焊接构件中熔池变形对熔池尾部应力应变场的影响.其次,通过加大材料的线膨胀系数,考虑了凝固收缩对熔池尾部应力应变场的影响,最终得到了各种条件下的焊接应力 -应变场.此外,本文将机械应变随时间变化的关系曲线处理成机械应变随温度变化的关系曲线,从而获得了凝固裂纹驱动力,为凝固裂纹的预测奠定了基础.     

NUMERICAL SIMULATION OF STRESS-STRAIN DISTRIBUTIONS FOR WELD METAL SOLIDIFICATION CRACKING IN STAINLESS STEEL

1.ItisknownthatweldsolidificationcrackingresultsfromthecompetitionbetweentheInaterialresistancetOcrackingandthemechAncaldrivingforceinthecon-rseofsolidificationOfweldmetal['--sJ.Essentially,weldmetalsolidificationcrackoccurswhenthemechanicaldrivingforceexceedsthematerialresistance.ThedrivingforceisthemechanicaltensilestainthatisdevelOPedinthetrailOfaweldPOolduetOweldmetalsolidificationshrinkage,thennalcontractionoftheparentmetalandtheexternalrestraintoftheweldedstructure.HoWever,analyzing…     

不锈钢焊接凝固裂纹应力应变场数值模拟模型的建立

依据焊接过程的特点,本文把焊接构件分成熔池液相区,固液共存区和固相区三个区域,为了建立焊接凝固裂纹驱动力数值模拟模型,本文详细分析了这三个区域的力学行为,在实测了液态金属的凝固速率和固液态金属的加载卸载响应曲线的基础上着重研究了熔池的变形、固液共存区金属的流变性能和凝固收缩对熔池尾部应力,应变演变过程的影响。最后,采用单元死活的方法解决了熔池的变形问题,采用热、弹、塑性力学方法处理了固相区的应力,     

可调拘束实验测量的焊接凝固裂纹阻力曲线的有限元法修正

利用有限元法对横向可调拘束实验进行了数值模拟,得到了不同弯曲应变条件下熔池尾部的局部应变,结果表明：在进行可调拘束实验过程中,焊接熔池尾部确实存在应变集中,从而对实验测得的平均应变即材料凝固裂纹阻力进行了修正．此外,将横向可调拘束实验模拟得到的局部应变与采用高速摄影和可调拘束相结合测量得到的局部应变进行了对比,能够较好的吻合．同时,计算了不锈钢SUS310焊接凝固裂纹驱动力,并与修正后的阻力曲线进行比较,预测了凝固裂纹敏感性,预测结果与实验结果相符．说明利用横向可调拘束实验结合数值模拟方法,能够定量预测焊接凝固裂纹．     

Software package for simulation and prediction of welding solidification cracks

Abstract

In the present work, a software system is developed that can simulate and predict welding solidification cracks, based on previous work on welding solidification crack simulation in stainless steels. First, it assists users in discretising the welding workpiece, calculating welding heat input distribution, inputting material properties, and generating input data cards. Second, the system runs commercial finite element calculation software on the basis of input data cards. Third, the system performs data treatment to provide the user with simulated results in the form of contours, three-dimensional plots, and featured curves such as welding temperature cycles and strain - stress cycles. Next, the system carries out regression analysis on experimental data from transverse Varestraint testing to determine the resistance of the material to welding solidification cracking. Finally, the system calculates the curves of strain evolution with temperature at the trailing edge of the mushy region in the weld pool to obtain the driving force for welding solidification cracking. Consequently, the system can predict welding solidification cracking by presenting users with the driving force and resistance curves in one figure. The software package is developed mainly using Visual Basic and its graphical functions are achieved with the assistance of the Matlab software packages.     

不锈钢焊接凝固裂纹温度场的数值模拟

进行不钢焊接凝固裂纹温度场分析与数据模拟。首先将电弧民辐射状对称并呈高斯分布的二维热流作用于工件表面,解决了电弧产热问题;其次,通过液淬法测试金属的固相分数随温度变化率,得到了凝固热放率;最皇采用增大热传导系数的方法,考虑了熔池内流体流动对整个温度场的影响。在此基础上,建立了二维焊接凝固裂纹温度场计算模型。此外,采用有限元方法计算了二维焊接凝固纹温度场,并研究了焊接规范参数及材料的热物理性能对 影     

电子束扫描横焊薄铌板的熔池动力学行为

在电子束焊接过程中,金属液体蒸发的反冲压力、表面张力、重力等驱动力共同作用于熔池,对焊缝成形有显著影响. 在扫描横焊的情况下,电子束作用范围的扫描摆动和重力方向的旋转使熔池的动力学行为变得更复杂. 采用试验和数值计算方法对电子束扫描横焊薄铌板的熔池形态和凝固后熔合区形貌进行研究,数值模拟得到的熔池形态和熔合区形状与试验结果吻合. 熔池流场分析结果表明,半熔透熔池的驱动力主要为液态金属蒸发引发的反冲压力;全熔透熔池的上表面Marangoni流动占主导,表面张力与反冲压力共同作为熔池流动的驱动力;重力与焊接扫描共同作用使得熔池两侧的质量分布和流场分布不对称,造成了焊缝两侧熔合线的不对称.     

焊接弧坑热裂纹的力学机制分析

应用断裂力学物理量和有限元方法,分析并揭示焊缝弧坑热裂纹开裂力学机理. 通过应力的无量纲化和正交变换,将热—弹塑性有限元分析获得的节点应力等效为应力强度因子计算模型的外载荷,建立了基于应力强度因子参量的焊缝弧坑热裂纹数值分析模型. 计算并比较焊接热应力作用下焊缝含裂纹单元的张开、剪切和撕裂三类应力强度因子,揭示弧坑裂纹断裂机制并断口试验验证. 结果表明,弧坑热裂纹以拉裂模式主导,在热裂纹敏感区间1100~1000 ℃扩展速度最快,且呈“之”字形开裂取向.     

Experimental and numerical analysis of solidification cracking behaviour in fibre laser welding of 6013 aluminium alloy

Abstract

Experimental observation and numerical modelling were employed to investigate the solidification cracking behaviour during fibre laser welding of 6013 aluminium alloy. The solidification cracking initiation location and propagation path were studied using a high speed camera system and via metallurgical analysis. A three-dimensional thermomechanical finite element model of fibre laser welding of aluminium alloys was developed, which considered cylindrical volumetric heat source, temperature dependent material properties, solidification shrinkage and stress relaxation in the weld molten pool. The transient evolution and distribution of mechanical strain in the brittle temperature range (BTR) were analysed in detail to find the factors which drove the crack initiation and propagation. The results showed that the solidification cracking initiated near the fusion line and then propagated along the centreline of the weld, which was the result of the strain distribution characteristic in BTR.     

关于焊接应力变形两个问题的进一步探讨

讨论了焊缝是否存在压缩塑性变形问题和拉达伊D应力应变原理图存在的问题。指出刚刚经历熔化—凝固过程的焊缝不存在压缩塑性变形 ,一直承受拉伸塑伸变形。拉伸塑性变形区的范围要比拉达伊D的原理图上给出的大得多 ,对拉达伊D的原理图进行全面修改 ,提出新的焊接热应力应变原理图。在新原理图中 ,三组曲线分别是升温—降温、压缩—拉伸和拉伸弹性—塑性变形的分界线。将等温线所处的温度明确为熔点 ,它包围的区域为熔池。取消弹性卸载区的提法 ,改为弹性变形区。整个焊接板由弹性变形区、压缩塑性变形区、拉伸塑性变形区和熔池组成。     

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.     

焊缝凝固裂纹的实时记录

研究了在实际焊接过程中的电弧光下如何拍摄记录焊缝凝固裂纹的启裂和扩展的过程。通过理论分析与实验，提出了怎样滤除弧光干扰，提高成象清晰度的方法，解决了对不同表面条件母材拍摄焊缝裂纹的技术，并成功地在以秒时标单位基准下，实时记录了３０ＣｒＭｎＳｉ和ＬＹ１６焊缝凝固裂纹的产生过程。此技术可为焊接中接头应力应变的各类数理模型提供实际动态验证手段，还可为实时调整焊接规范参数，控制焊接凝固裂纹扩展提供可靠的反馈信息。     

A J-INTEGRAL ANALYSIS OF CRACKS IN OVERMATCHED WELDED JOINTS

The effects of material inhomogeneity and weld width of the overmatched welded joints with alongitudinal crack or flaw upon the J-interal,a characterizing parameter for elastoplasticfracture,have been studied.The overmatched weld is idealized to be a parallelly cracked andsandwiched hard layer,the yieht stress of which is higher than that of the base metal.Numeri-cal solutions were obtained by using elastoplastic finite element method under the assumptionof plane stress.The resuits show that the values of J-integral are remarkably affected by thematerial inhomogeneity ant the weld width especially when the nominal stress or nominalstrain is comparatively high,which,therfore,should be taken into account when evaluatingthe crack driving force for an overmatched joint.     

An introduction to physical phenomena in arc welding processes

As is well known, arc discharges have been applied to various processes such as welding, cutting, spray coating, melting and refining. Unlike electrodeless discharge methods such as high frequency discharge of inductive coupling type (eg. RF discharge), arc discharge is a polarized discharge in which the arc is generated between the positive and negative electrodes.¹ Accordingly, when the arc discharge is applied to welding processes, the material becomes one of the electrodes. In TIG welding, the material, that is, the molten pool, generally acts as the anode to the tungsten cathode. As shown in Fig. 1, TIG welding processes are based on the close energy balance between the 'electrode-arc plasma-molten pool'. On the other hand, for the formation of the molten pool, energy transfer from the arc is also important, but energy transfer in the molten pool after that is extremely important, too. In TIG welding of steels, in which energy transfer by convection current becomes dominant rather than thermal conduction,² the penetration at the weld joint has hugely different geometries according to the difference in convective current phenomena at the molten pool. As a driving force of convective current in the molten pool in TIG welding, four forces have been considered,^2-6 as shown in Fig. 1. They are the drag force (friction force) caused by plasma jet (cathode jet) generated by the arc, the buoyancy force induced by the density difference inside the molten pool, the electromagnetic force induced by the current flow inside the molten pool and Marangoni Force induced by the surface tension gradient of the molten pool. These four forces can be said to be also based on the close force balance of the 'electrode-arc plasma-molten pool', as with the energy balance.     

高匹配焊接接头裂纹的J积分研究

本文研究了含有纵向裂纹或缺陷的高匹配焊接接头材料不均匀性和焊缝宽度对弹塑性断裂表征参量J积分的影响规律.高匹配焊缝被理想化为含有一平行裂纹的一个硬夹层,其屈服应力高于母材.在平面应力条件下利用弹塑性有限元计算得到数值解.研究结果表明,材料不均匀性和焊缝宽度对J积分值有显著影响,特别当名义应力或名义应变较高时影响更大.因此,估算高匹配接头的裂纹推力时必须对此加以考虑.     

3D Numerical Simulation on Thermal Flow Coupling Field of Stainless Steel During Twin-Roll Casting

The surface crack and lateral crack of the AISI 304 stainless steel thin strip produced by twin-roll casting were observed. The temperature at the center of outlet during twin-roll-casting process was determined by infrared thermometer. In order to avoid the surface cracks of the casting strip, the thermal flow coupling field of AISI 304 stainless steel during twin-roll-casting process was simulated by a 3D fluid-structure coupling model. According to the simulation result, the effect of the casting speed on thermal flow field was analyzed and the process parameters were optimized. Moreover, by studying heat flux curves, the heat transfer mechanism between molten pool and roll was analyzed. The results show that, with the increase of the casting speed, the temperature of the molten pool increases and the solidification point moves toward the outlet. Meanwhile, the whirlpool above gets larger. Based on the solidification front position, the optimized process parameters are 1500 °C and 0.37 m/s. The heat transfer mechanism between molten pool and roll contains direct contacting heat transfer and air gap heat transfer.     

PREDICTION OF MACROSEGREGATION OF UNIDIRECTIONALLY SOLIDIFIED STEEL INGOTS Ⅰ. MATHEMATICAL MODEL

为了对定向凝固钢锭中的宏观偏析进行预测，本文对合金凝固过程的统一模型进行了修正，提出一个适用于低合金的凝固统一模型。该模型以Ｆｅ－Ｃ二元合金为基础，同时在凝固进程和固液相枢中枝晶间流动等方面引入其它合金元素的影响，更为全面地反映了碳偏析行为。     

Effect of strain rate on solidification crack susceptibility of different metals

In this paper,a slow bending speed varestraint test method with con-tinuously adjustable deformation rate for testing hot cracking sensitivity is de-veloped and its working principle is described.From great amounts of tests,itis known that strain rate affects the sensitivity of solidification crack because ofits influnce on material ductility in BTR directly.The smaller the strain rate,the better the ductility and the lower the solidification crack sensitivity.The ex-tent of this effect varies with different materials,for LF-6 aluminum alloy,it isgreat,but for 16Mn,19Mn5 and 20G steels,it is small.Through systematicinvestigation,it is concluded that strain rate of a metal during welding can belowered and the resistance to solidification crack can be enhanced by reducingstructrual restraint and by adjustment of welding parameters.     

残余应力对管线钢韧性断裂的影响

以弹塑性断裂力学理论为基础,应用ABAQUS有限元软件,采用固有应变法引入残余应力,并基于细观塑性损伤模型研究了残余应力对管线钢韧性裂纹扩展阻力曲线的影响.模型中选择了具有不同深浅裂纹的单边缺口弯曲（SENB）试样和DNV推荐准则中用于管线钢断裂评估的单边缺口拉伸（SENT）试样,并对这两种断裂力学试样在大范围屈服条件下的裂纹扩展阻力曲线（CTOD-R曲线）行为进行了对比分析.结果表明,对于深裂纹试样,残余应力降低了韧性裂纹扩展阻力曲线,而浅裂纹试样受残余应力的影响可忽略不计.     

力学失配对电子束焊接接头疲劳裂纹扩展行为的影响

研究了高温合金电子束焊接接头区的疲劳裂纹扩展行为,深入分析了接头区力学失配对疲劳裂纹扩展方向及疲劳裂纹扩展速率的影响规律。结果表明,由于力学不均匀性的影响,电子束焊接接头的疲劳裂纹扩展具有不稳定性,位于高匹配焊接区的疲劳裂纹偏离原裂纹扩展方向,跨越焊接区与母材,呈复合型扩展;焊接区疲劳裂纹的集团使得裂纹尖端有效应力强度因子减小,因而焊缝区裂尖局部有效裂纹扩展驱动力下降,导致接头的疲劳裂纹扩展速率下