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.     

Numerical Simulation and Experimental Investigation of Residual Stresses in the Circumferential Butt GTAW of Incoloy 800H Pipes

The residual stresses developed during the circumferential butt gas tungsten arc welding (GTAW) process of Incoloy 800H pipes were simulated using the finite element method. A decoupled thermostructural model was developed in three dimensions. The element birth and death technique was used for the addition of filler material in the weld pool. The Goldak double ellipsoidal model was used to simulate the distribution of arc heat during welding. The plastic behavior of the material was described by Von Mises yield function and the bilinear kinematics hardening was assumed. To validate the thermostructural model, both temperature and residual stress distributions within the pipes were measured using thermocouples and strain gages, respectively. Good agreements were found between the experimental and simulation results. The model was then used to predict distribution of residual stresses during the GTAW of Incoloy 800H pipes and to study effects of process parameters on the residual stresses.     

Studies on multipass welding with trailing heat sink considering phase transformation

A two pass butt welding of 6 mm mild steel plates was simulated using 3D finite element model using temperature and phase dependent material properties. Material phase transformations were simulated using suitable phase transformation kinetic models. Mechanical analysis is carried out using nodal temperature and phase proportions as input. Experiments were carried out using liquid nitrogen (LN2) as trailing heat sink. Trailing heat sink helped to reduce the residual stress in the fusion zone (FZ) and heat affected zone (HAZ) although distortions were found be increasing. A parametric study was conducted to study the effect of distance between weld arc and trailing heat sink. The heat sink closer to weld arc reduced both distortions and residual stresses.     

Experimental Investigation into the Effects of Weld Sequence and Fixture on Residual Stresses in Arc Welding Process

This study concentrates on the effects of weld sequence and welding fixtures on distribution and magnitude of induced arc welding residual stresses built up in butt-joint of Gas Tungsten Arc Welding (GTAW) AA5251 plates. Aluminum plates have been welded under different welding conditions and then, longitudinal and transverse residual stresses were measured in different points of the welded plates employing hole-drilling technique. The results indicate that welding sequence significantly alters the distributions of both longitudinal and transverse residual stresses while the changing in the weld sequence leads to 44% decrease in longitudinal residual stress. Besides, both the geometry of weld pool and distribution of residual stresses are affected by the welding fixtures while implementation of fixture causes about 21 and 76% reductions in the depth of weld pool and transverse residual stress, respectively, for the material and welding conditions used in this research.     

钛合金带热沉钨极氩弧焊中热沉作用

采用数值模拟和实验相结合的方法研究了钛合金TC4薄板常规及带热沉的钨极氩弧焊焊接过程中温度及应力应变的分布,考察了热沉对温度场和应力应变场的影响规律,探讨了使用该技术实现应力和变形控制的机理.结果表明:带热沉的钨极氩弧焊焊接过程中,紧随热源之后热沉急冷作用使得试件形成马鞍形温度场,而热沉作用部位温度最低.热沉作用部位的急冷收缩对周围金属产生拉伸作用,使得焊缝及近缝区金属升温过程中产生的压缩塑性应变减小,冷却过程中产生的拉伸塑性应变增大,接头中不协调应变减小,残余应力降低.实验测量与有限元模拟结果吻合良好,证实了采用热沉控制应力与变形的有效性和有限元模型的正确性.     

Effect of trailing heat sink on residual stresses and welding distortion in friction stir welding Al sheets

Abstract

This paper investigates a trailing heat sink, which was designed and applied to friction stir welding (FSW) in order to control the residual stresses and welding distortion. Residual stresses, residual plastic strains and welding distortion of 2024-T3 and 5083-H321 Al sheets welded by FSW with and without the trailing heat sink were compared. The optimal placement of the heat sink was discussed. The results revealed that the reductions in peak tensile stresses were 66% for 2024-T3 and 58% for 5083-H321 by application of the trailing heat sink in FSW. In addition, the welding distortion could be reduced drastically by this method. The 5083-H321 sheet with a size of 1000×100×3·5 mm welded by this method was very flat and had almost no distortion. This method achieved in-process control of stresses and welding distortion, without additional complicated work before or after welding operation.     

BT20钛合金电子束热处理残余应力计算

利用三维有限元分析软件，模拟了BT20钛合金薄板焊态和焊后电子束局部热处理的实际焊接温度场以及残余应力的分布。结合数值计算，讨论了不同的热处理方式以及热处理工艺参数对焊接接头残余应力分布的影响。结果表明，在钛合金薄板焊缝的背面进行电子束局部热处理，可以显著降低焊缝中心处的残余拉应力。数值计算结果还表明，在其它工艺参数相同的情况下，随着局部热处理加热宽度和加热时间的增加，焊缝及近缝区的纵向残余应力随之降低，同时产生残余应力的范围也随之增大。     

铝合金搅拌摩擦焊过程热力演变的三维数值模拟

对搅拌摩擦焊过程中搅拌头速度变化进行分析,建立了考虑搅拌摩擦焊过程中焊缝产热的热源模型.对2024铝合金搅拌摩擦焊温度场和应力场进行了三维有限元模拟,表明焊缝两侧温度和应力分布的不对称现象不明显,主要由于焊接速度远小于搅拌头转速所致,但随着焊接速度加快,这种不对称现象逐渐加强.焊接过程中焊缝中心温度低于搅拌头边缘温度,焊接前方和两侧均为压应力,后方为拉应力;焊接结束后与搅拌头接触区的横向和纵向残余应力为较大拉应力,远离焊缝残余应力较小;沿厚度方向上,横向和纵向残余应力均逐渐降低.有限元计算结果与短波长X射线应力测试结果进行对比,结果表明,二者趋势基本吻合.     

K-TIG焊接接头的应力与变形

采用SYSWELD软件对Q345低合金钢板的匙孔型钨极气保焊（keyhole gas tungsten arc welding,K-TIG）焊接过程进行了模拟,选用了3种形式的组合热源对K-TIG焊接过程的温度场进行数值模拟,并与实际焊缝轮廓进行对比,发现采用上半部分双椭球热源和下半部分3D高斯热源的组合热源所得温度场与实际情况较为相似.并通过K-TIG焊接数值模拟,分别研究板厚、间隙和焊接速度对K-TIG焊接接头变形和应力的影响.结果表明,减小焊接板厚有利于减小焊后z向变形和横向残余应力,留出适当的间隙有利于减小焊后残余应力,增大焊接速度有利于减小焊后变形,但不利于控制焊后残余应力.     

Trailing heat sink effects on residual stress and distortion of pulsed laser welded Hastelloy C-276 thin sheets

A three-dimensional finite element model (FEM) was established to reveal the thermal-mechanical behaviors of pulsed laser welding (PLW) with and without trailing heat sink. Experiments were carried out to measure the welding temperature histories, residual distortions and solidification profiles. The simulation results agree well with the corresponding experimental measurements. The peak values of the temperature and transient longitudinal tensile stresses valleys in the weld increase as the cooling intensity increases from 5000 to 15,000 W/(m² K), while those of the temperature and transient longitudinal compressive stresses near the weld decrease. The peak values of the longitudinal residual compressive stresses and plastic strains, and the maximum deflections in longitudinal and transverse direction decrease as the cooling intensity increases from 5000 to 15,000 W/(m² K). The magnitudes of the transverse shrinkage distortions increase as the cooling intensity increases from 5000 to 15,000 W/(m² K). The proper cooling intensity to reduce the residual stresses and distortions of the PLW with the trailing heat sink is detected at 10,000 W/(m² K). The trailing heat sink is technically feasible for actual pulsed laser restraint welding in Hastelloy C-276 thin sheet structures.     

Study on stress and deformation of keyhole gas tungsten arc-welded joints

Keyhole gas tungsten arc welding(K-TIG)of Q345 low alloy steel plates was simulated by using SYSWELD software.The temperature field of the K-TIG welding process was simulated with three different combined heat sources and was compared with the weld profile that was obtained experimentally.The temperature field that was obtained by a combination of a double ellipsoid heat source on the upper half and a three-dimensional Gauss heat source on the lower half was similar to the real situation.The effects of plate thickness,gap and welding speed on the deformation and stress of the K-TIG welded joints were investigated by K-TIG welding numerical simulation.A reduction in the thickness of the weld plates reduced the z-direction deformation and transverse residual stress;an appropriate gap reduced the residual stress and an increase in the welding speed reduced deformation after welding,but did not help to control the residual stress after welding.     

Numerical modelling and experimental determination of temperature distribution during manual metal arc welding

Abstract

Welding is a highly reliable and efficient metal joining process. Manual metal arc (MMA) welding is very widely used in industry. The temperature distribution that occurs during welding affects the material microstructure, hardness, and the residual stresses present in the material after welding. In the present work, the temperature distribution during bead on plate welding using MMA welding was experimentally determined for AISI type 304 stainless steel plates and low carbon steel plates of thickness 6 and 12 mm. A three-dimensional computer model based on the control volume method has been developed to predict the temperature distribution in the heat affected zone (HAZ) and in the base plate region of the bead on plate welds, using the weld parameters as input data to the computer model. In this computer model, the heat energy used to melt the electrode is considered as a separate heat flux term and the remaining heat supplied by the welding arc is considered as another heat flux term. A good match between the experimental results and the theoretical predictions was obtained. Using the computer model, the time taken to cool from 800 to 500°C in the coarse grained HAZ (close to the fusion line) of low carbon steel specimens was calculated. From this cooling time and the chemical composition of the material, the maximum hardness in the coarse grained HAZ was predicted. Microhardness measurement in the same region of the welded plates was carried out. The experimentally measured values and predicted results match closely.     

热沉影响钛合金薄板焊接残余应力的试验分析

采用切条应力释放法测量了钛合金TC4薄板常规钨极氩弧焊(GTAW)和动态控制低应力无变形GTAW对接试件中的纵向残余应力和纵向残余塑性应变的分布。测量结果表明,钛合金常规GTAW缝中残余拉应力峰值小于其母材屈服强度,焊缝附近存在残余压缩塑性应变;动态控制低应力无变形GTAW焊技术中热沉的冷却作用使得热源与热沉之间的高温金属承受强烈的拉伸作用,产生拉伸塑性变形,部分抵消了焊接过程中已产生的缩短的塑性变形,使得试件中纵向残余塑性应变减小,焊接残余拉应力峰值降低,残余压应力水平降低。切条应力释放法是一种简便有效的薄板焊后残余应力测量方法,能够满足工程应用的精度要求。     

Welding of ASTM A106 Gr. B steel pipes for high-temperature service – part 1 – residual stress analysis

Welding is the principal process of manufacture used in the fitting and repair of tube sections. However, there is a lack of information about the alterations mechanically/metallurgically caused by the welding heat cycle, especially about the behaviour of the residual stresses. The objective of the first part of this work was to evaluate the welding residual stress in small diameter pipes used in oil refineries. Two-inch diameter pipes were welded using the manual GTAW process. AWS ER 70 S3 filler rods with diameters of 2.5 and 3.25 mm were employed. An electronic power supply was used, together with data acquisition systems to control the welding parameters. Stress measurements were carried out with an X-ray mini diffractometer. The axial residual stress profiles determined in the outer surface of the pipes were formed by compressive stresses in the weld region (the fusion zone and heat affected zone) and for tension stresses in the areas more distant from the weld bead. The evidence suggested that on the inner surface of the pipes, the stress profile is the opposite from that observed for the outer surface, with tension stress in the welding zone and compressive stress in the region further from the weld bead.     

TIG熔修后T形接头的焊接残余应力及消除

用红外热成像仪测量了P355NL1钢T形接头在焊接过程中的温度场分布,得到焊接过程的温度循环曲线,并且与有限元方法模拟的温度场结果吻合,验证模拟计算的准确性;使用盲孔法,测量了T形板焊接MAG焊后、TIG熔修后以及热处理后的残余应力;采用间接法利用温度场计算结果计算了T形接头试板焊接残余应力场,模拟结果与实际测量的残余应力趋势相符.结果表明,TIG熔修能一定程度的降低焊缝中心的残余应力,但增大了熔修区域的残余应力;而通过整体热处理能有效地消除熔修带来的大部分残余应力,但仍有部分残余应力存在.     

数值模拟和实验研究Ti6Al4V合金激光熔透焊接残余应力

采用热-弹塑性三维有限元法研究激光熔透焊接Ti6Al4V合金的残余应力,并采用小孔法测量焊接残余应力以和计算结果进行比较.有限元计算时,建立了以焊缝形貌尺寸为参数的统一锥形热源模型来模拟不同热输入时的焊接温度场,并讨论了边界条件和有限元网格大小的确定.研究结果表明:采用焊缝轮廓尺寸作为热源参数能准确模拟焊缝横截面轮廓;钛合金激光熔透焊接的纵向残余应力分布梯度陡;在焊件表面和内部残余应力分布趋势不同,采用小孔法测量的残余应力分布和计算的焊接件内部残余应力分布相似.     

Residual stresses simulation for friction stir welded joint

Abstract

As a solid state joining technique, friction stir welding (FSW) can produce high strength, low distortion joints efficiently. Compared to fusion welding, residual stresses in FSW joints are expected to be low due to a relatively low heat input. However, apart from the heat input, the force from the tool also plays an important role in the development of welding stresses. In the present paper, a semicoupled thermomechanical finite element model containing both thermal load and mechanical load was established to simulate the development of welding stresses during FSW process; an autoadapting heat source model was employed in the thermal analysis; the fixture was also included in the mechanical analysis model. The simulation results showed that due to the effect of the tool force, the longitudinal residual tensile stresses became smaller and were asymmetrically distributed at different sides of the weld centre; the peak of the tensile residual stresses at the retreating side was lower than that at the advancing side. Calculated and experimental results were compared.     

316不锈钢超薄板脉冲激光焊残余应力及变形

在超薄金属板焊接过程中,残余应力及变形对产品质量有重要影响.文中研究了316不锈钢超薄板（厚度为70 μm）脉冲激光焊接过程的残余应力和焊接变形.采用热-弹-塑性有限元法和半椭球移动热源模型,考虑模型的几何和材料非线性因素,采用顺序耦合的方法对超薄板结构的温度场、应力-应变场进行模拟.采用光纤激光器对70 μm的316不锈钢板进行焊接,用红外测温仪对特征点热循环进行测量,用激光位移传感器测量了焊接变形,用X射线衍射应力测试仪测试了残余应力.结果表明,温度场、残余应力、变形的模拟计算结果与试验结果吻合.     

数值计算模拟工艺参数对电子束焊接残余应力的影响

电子束焊接残余应力的实测需要花费大量成本,因此采用数值模拟其焊接残余应力的大小和分布具有重要意义.利用三维有限元分析程序,建立了TC4钛合金板电子束焊接温度场和残余应力场的有限元分析模型,着重分析了高压和中压两种工艺参数对其接头焊接残余应力的影响.旨在探讨不同工艺参数对电子束焊接过程的影响规律,从而优化工艺,降低成本.计算结果表明,采用中压参数焊接的电子束焊接接头残余应力的峰值比采用高压参数的接头残余应力峰值高;而且其残余应力分布更集中于焊接接头中段.     

钢管相贯节点焊接残余应力与热损伤的非线性有限元分析

分析钢管相贯节点的残余应力是研究其极限承载力和疲劳寿命不能回避的问题.针对K形钢管相贯节点焊接时的热作用特点,通过对焊接过程中加热与冷却温度场的正确描述,并采用相应的热传导数学和物理模型,进行温度场和应力场的耦合计算,考虑钢材热物理、力学参数随温度变化的非线性性能,得出的残余应力分布规律与残余变形,比较理想地与节点试验的破坏模式相吻合.针对相贯节点试验中支管屈曲的破坏形式,提出了焊接热损伤的概念和机理.