Numerical Simulation and Experimental Investigation of Residual Stresses and Distortions in Pulsed Laser Welding of Hastelloy C-276 Thin Sheets

为了揭示0.5 mm厚度Hastelloy C-276薄板脉冲激光拘束焊接的热力学行为,借助ANSYS软件建立了三维有限元模型。实验测量了焊接温度历程和残余变形,验证了所建立有限元模型的可靠性。基于该有限元模型,采用改变夹具拘束条件的方法,进一步研究了拘束距离对Hastelloy C-276薄板焊接瞬态应力和塑性应变、残余应力及变形的影响规律。结果表明,温度历程和残余变形的模拟结果与实验结果吻合较好;拘束距离对瞬态塑性应变的大小有显著影响,进而改变了残余应力及变形的分布和大小。随着夹具拘束距离从20 mm减小到4 mm,除了纵向残余拉伸应力外,横向残余拉伸应力和位移的峰值以及角变形的大小都呈减小的趋势。相对较小的拘束距离可以作为抑制横向残余拉伸应力和角变形的高效低成本方法,但对纵向残余拉伸应力有不利影响。     

Characteristics of welding distortion/residual stress generated by fillet welding with laser beam and verification of generality of its highly accurate prediction

It was found that the characteristics of welding distortion and residual stress generated by laser beam welding (LBW) had the same tendency as those by arc welding but that the absolute value was small. Moreover, the process of LBW is quite different from that of arc welding, the validity and generality of the dominant factors to predict high accuracy welding distortion and residual stress generated by welding could be verified by the high accurate prediction of welding distortion and residual stress which had been proposed based on arc welding. Inherent force was proposed as a parameter which could indicate the magnitude of compressive residual stress, which might influence the load-carrying capacity. Although inherent forces with LBW were 40% that of arc welding, it was found that control of welding heat input was important so as to improve the load-carrying capacity.     

2A12T4铝合金焊接时拘束条件对热裂纹的影响

采用一种简便有效的热裂纹试验方法,在普通刚性焊接夹具上研究了拘束力和拘束距离对2A12T4铝合金焊接热裂倾向的影响.结果表明,保持拘束距离不变,随着拘束力的增加,焊接热裂纹率逐渐减小,当拘束力到达一定值后,热裂纹率趋于稳定;保持拘束力不变,随着拘束距离增加,焊接热裂纹率呈增加趋势.夹具对工件的拘束条件主要是通过影响焊件的回转变形程度和焊缝区金属的冷却收缩速度来影响焊接热裂倾向.

Abstract：

A simple and effective experimental method was adopted to investigate the effects of restraint force and restraint distance on hot cracking tendency during welding of 2A12T4 aluminum alloy on the common rigid welding fixture. Experimental results show that, under the condition of keeping restraint distance unchanged, the rate of hot cracks decreases gradually with increase of restraint force and finally well reaches a stable value when the reatraint force is great enough. Keeping the restraint force unchanged, the rate of hot cracks increases with increase of restraint distance. The influence of restraint condition on tendency of hot cracking through effecting the extent of externally-expanded deformation of weldtnent and the cooling speed of metals in and around the weld.     

Distortion control in welding by mechanical tensioning

Abstract

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.     

外拘束角接头旋转电弧焊接应力应变分析

为探究旋转电弧角焊焊接残余应变分布规律,控制工件变形,分析对比了三维多体耦合模型与外拘束力模型两种拘束模型下工件残余应力与变形.首先,设定模型间的接触关系,建立了由焊件、工作台和夹具组成的三维多体耦合模型;然后,通过设定工件上节点区域法向压力和位移约束,建立了外拘束力模型;再采用热—力直接耦合法,基于上述两种模型,运用ANSYS软件模拟了旋转电弧焊接变形,获得了两种模型下残余应变曲线,并进行试验验证.结果表明,三维多体耦合模型的焊件应变曲线更接近试验结果,并且其焊接残余应变范围及峰值均较平缓,为优化焊件装夹方式来控制旋转电弧角焊变形提供参考依据.     

焊接应力变形原理若干问题的探讨(二)

提出焊接残余应力形成和消除原理:焊接残余应力不是压缩塑性应变引起的,而是由于焊缝和近缝区金属在"力学熔点"及以下温度冷却收缩受阻产生的;消除焊接残余应力不是产生拉伸塑性应变以减少、抵消和补偿压缩塑性应变,而是将残余弹性应变转变为塑性应变;消除焊接残余应力并不是必须去除固有应变,部分去除或完全不去除固有应变也能完全消除残余应力.提出随焊后热精确控制应力变形焊接法,既可实现无应力焊接和无应力无变形焊接,也可实现适当压应力无变形焊接和较大压应力微变形焊接;并对传统方法与有限元法进行了分析比较.     

用旋转挤压方法控制薄板的焊接变形

提出了旋转挤压控制薄板焊接变形的新方法,它足利用特定形状的挤压头对焊缝及近缝区金属进行旋转挤压,产生拉伸塑性应变,减小甚至抵消残余压缩塑性应变,以此来消除焊接残余变形.该方法处理后的焊道表面平整光滑,减小了应力集中.结果表明,旋转挤压法能将薄壁构件焊接变形降到非常低的水平,选择适当的工艺参数,该方法能将焊接残余变形控制在常规焊接状态的3%以下.旋转挤压控制薄板焊接变形的效果与多个工艺参数有关,只有当各个工艺参数取值适当且匹配良好时,才能取得良好的焊接变形控制效果.     

Controlling welding residual stress and distortion by a hybrid technology of transient thermal tensioning and trailing intensive cooling

ABSTRACT

A new method of hybrid technology for controlling welding residual stress and distortion is proposed, which contains the transient thermal tensioning with two symmetrical heat sources ahead and the trailing intensive cooling with a cooling source behind the welding heat source during the welding process. Welding temperature, stress and distortion were compared and discussed under different welding processes by experimental and numerical methods. The results reveal that the hybrid technology adjusted the temperature field during welding and redistributed the longitudinal residual stress of welding joint. Thus, the welding residual distortion was reduced. Compared with conventional metal inert gas welding, the reductions of the welding residual stress and distortion of joint by the hybrid technology reached 65% and 58%, respectively.     

夹具拘束模型在焊接过程有限元分析中的建立及应用

夹具及垫板的拘束作用对被焊平板试样残余应力与变形的演变历程具有重要影响。将夹具和垫板作为弹塑性体包含到焊接数值模拟模型中,采用4种模型模拟夹具及垫板与被焊平板试样之间的相互作用,能够提高焊接数值模拟研究方法的科学性和准确性.数值模拟结果表明,夹具拘束作用的不同处理方式对平板试样焊后残余应力分布与变形趋势有重要影响,不同拘束条件下的平板试样残余变形模拟结果,与平板试样在自由状态下和夹具夹持状态下焊接实验获得的变形趋势均一致,在焊接过程初期阶段,平板试样上下表面的横向应力分布差异,对决定平板试样的残余变形趋势有重要影响。     

电渣焊接头焊接残余应力与变形的数值模拟

为研究电渣焊接头焊接残余应力的分布规律及变形特征,基于有限元分析软件MSC. Marc,开发了用于模拟电渣焊接头温度场、残余应力与焊接变形的热-弹-塑性有限元计算方法. 在所开发的计算方法中,采用全椭球等密度体积移动热源模型来模拟电渣焊的热输入,并以生死单元技术考虑焊缝成形,数值模拟了箱型柱中的腹板与隔板处电渣焊接头的焊接残余应力与变形. 同时,采用试验方法实测了电渣焊接头的横向收缩及角变形. 结果表明,横向收缩和角变形的计算结果都与试验值比较吻合,验证了所开发的数值计算方法的妥当性. 此外,利用数值模拟方法详细研究了电渣焊接头焊接残余应力的分布特征.     

随焊高速气流场辅助高强铝合金薄板焊接应力演变及控制变形机理

从力学角度出发提出了“随焊高速气流场”柔性控制高强铝合金薄板焊接失稳变形的新方法,研究了该方法控制2A12高强铝合金薄板焊接变形的有效可行性,分析了随焊高速气流载荷对薄板应力演变规律的影响,阐明了其控制焊接残余应力及变形机理.基于有限元法分析了温度场及应力场,确定了气动载荷与热源作用距离这一关键因素,并获得了气体压力的合理有效范围.在自行研制的随焊高速气流场装置上进行试验验证.结果表明,当气动载荷作用距离为20 mm、气动载荷为30 MPa时,焊接失稳变形基本消失,焊缝中截面上的纵向残余拉应力峰值较常规焊下降了77.73%,残余压应力峰值下降了69.23%,板边变形最大挠度仅为0.9 mm,较常规焊的8.5 mm下降了89.41%.试验结果与模拟结果吻合良好,验证了随焊控制模型的正确性.     

Brazing of metals – consumables,technologies and standardisation

For predicting welding distortion and residual stress generated by laser–arc hybrid welding, a series of experiments and analyses were carried out. A bead-on-plate welding was performed on SM490 steel by using a fibre laser and CO₂ arc welding by changing their heat input ratio. The experiment was simulated by thermal elastic–plastic analysis with the proposed simulation model considering the penetration shape by laser and arc separately. By using this model, the experimental results could be simulated with high accuracy. Therefore, the validity and generality of the numerical simulation model could be verified. The tendency and magnitude of angular distortion varied with the heat input ratio of laser and arc. The results indicated the possibility of the ideal heat input ratio of laser and arc for controlling angular distortion generated by hybrid welding. On the other hand, it was confirmed that the heat input ratio of laser and arc did not affect residual stress generated by hybrid welding.     

随焊激冷减小铝合金薄板的焊接变形

研究了随焊激冷减小铝合金薄板焊接变形的效果及夹具的散热条件对该效果的影响,研究表明,随焊激冷能够减小铝合金薄板的焊接变形,其效果受夹具自身冷却作用影响很大。随着夹具散热条件的降低,随焊激冷所起的作用增强,但降低焊接变表的效果仍不够明显,适当预热夹具本身可以减小焊接变形,但更重要的是预热使激冷造成了温差拉伸,因此获得了最小的焊接变形。     

线能量对TC4钛合金激光焊接残余应力和变形的影响

利用有限元分析和实验测试，研究了TC4钛合金平板激光焊接线能量对变形和残余应力的影响规律，并通过焊缝金相实验分析了线能量与焊接残余应力和变形的内在关系。结果表明：钛合金激光焊接产生的纵向残余拉伸应力约700MPa～850MPa，而横向残余拉伸应力只有50MPa～80MPa。激光焊接线能量增加时，纵向残余应力拉伸区域变宽，峰值应力降低，而横向残余应力随线能量的增加而升高。在临界焊透规范以上焊接时，角变形随线能量的增大而减小，但横向收缩变形增大。试件被完全穿透焊接时，线能量对角变形的影响作用降低。     

多束流电子束薄板焊接应力变形数值模拟

为了减小薄板结构的焊接变形,基于电子束高频偏转扫描技术在焊缝两侧添加辅助扫描热源实现了多束流电子束焊接及焊前预热. 建立了矩形均匀加热辅助热源模型,采用热弹塑性有限元分析方法对1.5 mm厚304不锈钢薄板进行多束流电子束焊接数值模拟,并进行了试验验证. 结果表明,焊后残余应力和变形的实测结果与模拟结果吻合良好,多束流电子束焊接方法不仅可以改变熔池前方材料的受力状态,而且可以减小熔池形成瞬间熔池前方材料的压应力峰值,有利于减小熔池的前方压缩塑性应变,进而减小薄板结构的焊接变形.     

Stress and distortion mitigation technique for welding titanium alloy thin sheet

Abstract

A stress and distortion mitigation technique for Gas Tungsten Arc Welding (GTAW) of titanium alloy Ti–6Al–4V thin sheet is presented. The proposed welding technique incorporates a trailing heat sink (an intense cooling source) with respect to the welding torch, and it is also called the Dynamically Controlled Low Stress No-Distortion (DC-LSND) technique. The development of this mitigation technique is based on both detailed welding process simulation using the advanced finite element technique and systematic laboratory experiments. The finite element method is used to investigate the detailed thermomechanical behaviour of the weld during conventional GTAW and DC-LSND GTAW. With detailed computational modelling, it is found that by the introduction of a heat sink at some distance behind the welding arc, a saddle shaped temperature field is formed as a result of the cooling effects of the heat sink; the lowest temperature exists in the zone where the heat sink is applied. High tensile action on the surrounding zone is generated by abrupt cooling and contraction of the metals beneath the heat sink, which increases the tensile plastic strain developed during the cooling process and decreases the compressive plastic strain developed in the heating process, and therefore mitigates the residual stresses and plastic strains within and near the weld. The experimental results confirmed the effectiveness of the DCLSND technique and the validity of the computational model. With a proper implementation of the DC-LSND technique, welding stress and distortion can be reduced or eliminated in welding titanium alloy Ti–6Al–4V thin sheet, while no appreciable detrimental effects are caused on the mechanical properties of welded joints by applying the heat sink in the GTAW process.     

换热器管与管板接头焊接残余应力的有限元分析

基于ANSYS平台对换热器伸出管板角接头和平焊接头的焊接残余应力进行数值模拟,获得了焊接接头的残余应力分布.有限元分析结果表明,接管环焊缝的残余应力分布具有局部性的特点,最大等效应力出现在焊根处.本文焊接条件下管接头环焊缝在焊接热影响区位置上径向为压应力,环向为拉应力,这种径向受压,环向受托的应力状态下更容易引起径向裂纹的萌生和扩展.     

超声滚压加工对X80管线钢焊接残余应力的影响

建立超声表面滚压加工（ultrasonic surface rolling process,USRP）的三维有限元模型,开发了模拟焊接的移动双椭球热源子程序,利用有限元软件ABAQUS模拟了X80管线钢焊缝不同方向的焊接残余应力,在此基础上叠加USRP的超声振动与静载荷的综合作用,模拟了表面塑形变形、应力和应变,耦合后分析了USRP前后残余应力的变化规律.结果表明,经过USRP处理,X80管线钢表面焊缝区由三向残余拉应力变为三向残余压应力,随着USRP次数的增加,残余压应力数值不断增大,残余应力σ_x,σ_y,σ_z变化规律基本相同.     

Effects of distance between arc and heat sink on stress and distortion in DC-LSND welding technology

The characteristics of temperatures, stresses and strains fields have been studied numerically for a titanium alloy sheet welded with an improved gas tungsten arc welding method, in which a trailing spot heat sink is introduced to control the welding stress and distortion. The impinging jet model is employed to describe the internee heat transfer between the cooling media and the top suufuce of the workpiece. The influcnee of the distance between arc and heat sink is investigated. Results show that there is an ideal range of distance. Using the ideal distance, a low stress and no distortion welding structure can be derived.