Numerical study on stress induced cambering distortion and its mitigation in welded titanium alloy sheet

Abstract

Arc welding process is widely used for fabricated thin structures in aerospace, shipyard, etc. Welding deformation is undesirable owing to the decrease in buckling strength and injures the beautiful appearance of structures. In addition, it causes errors during the assembly which result in more rework and productivity restriction. Welding deformation is complicated in real structure and is difficult to control. In the present study, numerical simulation by applying finite element has been performed using ANSYS to predict residual stress and cambering distortion in welded titanium alloy sheet. Furthermore, attempt was made to eliminate this distortion by applying dynamic spot cooling source technique behind the welding torch. The computed result revealed that the cambering could be eliminated under proper control of the parameters of this technique, such as the distance between the two sources. Transient residual stress through the thickness exhibit different behaviour and magnitude from that in conventional welding, and its cost is minimised by such technique. Explanation of this minimising mechanism was discussed.     

Arc pressure and weld metal fluid flow while using alternating shielding gases. Part 1: arc pressure measurement

Abstract

As part of an ongoing process to fully evaluate the effects of an alternating shielding gas supply on gas shielded welding processes, a comparison between the arc pressures generated using argon, helium, alternating shielding gases and pulsed gas tungsten arc welding (GTAW) has been conducted. Arc pressure variation and peaking are two of the fundamental phenomena produced during the alternating shielding gas process and are said to help create a stirring action within the liquid weld metal. However, there are no published data on arc pressure measurements during an alternating shielding gas supply, and consequently, these phenomena are based solely on theoretical assumptions. The experimental measurements made have shown that alternating shielding gases produce considerably higher arc pressures than argon, helium and pulsed GTAW due to a surge at weld initiation. The transient arc pressure measurements made when using alternating shielding gases are also considerably different from the theoretical assumptions previously reported.     

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.     

Investigation of low current gas tungsten arc welding using split anode calorimetry

Most previous split anode calorimetry research has applied high weld currents which exhibit pseudo Gaussian distributions of arc current and power density. In this paper we investigate low current arcs and show that both the current and power distributions have minima in the centre – varying significantly from the expected Gaussian profile. This was postulated due to the formation of the arc with the copper anode and the tungsten cathode. Furthermore, a number of parameters were varied including the step size between measurements, anode thickness and anode surface condition as well as cathode type and tip geometry. The step size between measurements significantly influenced the distribution profile and the anode thickness needed to be above 7?mm to obtain consistent results.     

几种施焊手法对管道钨极氩弧焊焊接质量的影响

钨极氩弧焊(GTAW)具有操作方便,焊接质量好等优点,在关键的焊接生产中应用较多,特别是在压力管道以及锅炉压力容器的打底焊中被普遍采用。在氩弧焊焊接过程中,焊工采用不同的焊接手法对焊接质量有较大的影响。以小口径管道氩弧焊为例,分析了三种施焊手法对焊道外观成型及焊接质量的影响,总结出管道现场焊接中合适的焊接手法,以提高压力管道现场焊接的一次合格率。     

A Study of Microstructure and Mechanical Properties for the Autogenous Single-Pass Butt Weldment of a Ferritic/Martensitic Steel Using Gas Tungsten Arc Welding

A 12%Cr ferritic/martensitic steel,HT-9,has been used as a primary core material for nuclear reactors.The microstructure and mechanical properties of gas tungsten arc butt welded joints of HT-9 in as-welded,and as-tempered conditions have been explored.In as-welded condition,the fusion zone (FZ) contained a fresh martensite matrix with delta (δ)-ferrite.Theδ-ferrite was rich in Cr and depleted in C compared with the matrix.The heat-aff ected zone (HAZ) could be divided into three areas as the distance from the fusion line increased:δ-ferrite/martensite duplex zone,fully recrystallized zone,and partly recrystallized zone.Prior austenitic grains did not coarsen in theδ-ferrite/martensite duplex zone due to the newly nucleatedδ-ferrite grains and incompletely ferritizing (δ-ferrite) during the welding thermal cycle.The weldment microhardness distributed heterogeneously with values above 600 HV _(1.0 )in the HAZ and FZ and 250 HV _(1.0 )in the base metal (BM).Solute C in the matrix,induced by the dissolution of carbide during the welding process,dominated the microhardness variation.Low toughness was observed in the FZ with a quasi-cleavage fracture tested from-80 to 20℃.The tensile fracture occurred in the relatively soft BM tested from 20 to 600℃.In as-tempered condition (760℃ for 1 h),M _(23 )C _6-type carbides precipitated within the martensitic laths,the lath boundaries,and theδ-ferrite/martensite interfaces.Moreover,V,Cr,Mo-rich nitrides with very small size also precipitated in theδ-ferrite/martensite interface.The tempering treatment improved the homogenous distribution of weldment hardness significantly.Tensile fracture still occurred in the BM of the weldment specimens tested from 20 to 600℃.The impact toughness improved significantly,but the ductile–brittle transaction temperature was-12℃ which was higher than that of the normalized and tempered (NT) BM.δ-ferrite was considered to be one of the major factors aggravating the impact toughness in the FZ.     

Numerical simulation and experimental investigation of temperature and residual stresses in GTAW with a heat sink process of Monel 400 plates

A 3D thermo-mechanical simulation model was developed to predict distributions of temperature and residual stresses during the gas tungsten arc welding (GTAW) process with a heat sink for Monel 400 plates using finite element method. The model was validated against the experimental measurements of both temperature and released strain in the welded plates. Effects of heat input, pipe diameter and water flow rate in the heat sink welding process were investigated. The results showed that in the GTAW process with a heat sink, the high temperature region was only limited to the vicinity of heat source and the maximum temperature of the sample was much lower than that of conventional GTAW process. This resulted in a lower residual stresses and even compressive stresses near the weld zone.     

镍基合金薄板焊缝组织与性能

研究了填丝、板厚以及退火工艺对镍基合金(C-276)薄板焊缝组织和性能的影响,结果表明,自熔焊接条件下,随着板厚增加,焊缝区晶粒依次增大2μm,金相组织更均匀且沿一定方向分布,析出相由焊缝区移向热影响区,焊缝拉伸强度低于基体拉伸强度,断口形状不规则,断口均发生明显塑性变形,热影响区硬度最高,板厚对腐蚀性能没有明显影响;填丝后,焊缝区金相组织排列有序,形状规则,有大量析出相产生,焊缝区、热影响区晶粒增大5μm～10μm,焊缝区拉伸强度降低,硬度高于热影响区,耐腐蚀性能较自熔焊有所提高;固溶退火后,析出相减少,晶粒粗化并出现再结晶,再结晶晶粒焊缝区较结晶前晶粒减小2μm,热影响区晶粒增大15μm～20μm,焊缝区拉伸强度降低,但抗腐蚀性能明显提高。     

单电源交流双钨极氩弧焊接铝合金工艺机理分析

以6061铝合金为研究对象,进行单电源交流双钨极氩弧焊接工艺试验.采用金相显微镜、CCD摄像头、电流检测系统对焊接接头的组织、电弧形态以及电流变化过程进行观察与检测分析.结果表明,大电流下双钨极可获得均匀美观的铝合金焊缝成形,但钨极串行优于钨极并行.电弧电流检测显示焊接过程中流经两个TIG焊枪的电流大小呈现非均匀分配形式,单电源单钨极电流密度沿着焊接方向呈现单峰分布而双钨极非单峰分布.在电弧作用区域双钨极耦合电弧的电流密度值要小于单钨极.另外双钨极能扩展耦合电弧的弧根,增加熔池受热面积,在保证焊缝成形的同时提高焊接效率.     

Fuzzy neural networks for arc welding quality control

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