Analysis of martensite transformation behaviour in welded joint using low transformation temperature welding wire

Abstract

Low transformation temperature welding (LTTW) wire has been found to improve the fatigue strength in welded joints. In the present study, the temperature dependence of the mechanical properties in a dual phase microstructure of austenite and martensite was estimated using the properties of full austenite and full martensite in numerical analyses. A welding method effective for residual stress reduction and fatigue strength improvement was shown by applying a calculation method under transformation superplasticity and transformation induced plasticity in high strength steel welded joints. With this method, the influence of the welding pass sequence on the residual stress distribution and fatigue strength was examined in a boxing fillet welded joint using LTTW. The transformation tensile residual stress in the weld toe was decreased by sectioned welding, and the fatigue limit by sectioned welding with LTTW improved in comparison with the fatigue limit of a joint welded with conventional wire in the same process.     

3D finite element modeling of the thermally induced residual stress in the hybrid laser/arc welding of lap joint

In this study, a three-dimensional (3D) finite element model is developed to investigate thermally induced stress field during hybrid laser–gas tungsten arc welding (GTAW) process. In the hybrid welding case, we focus on the GTAW process sharing common molten pool with laser beam and playing an augment role in the hybrid welding system. An experiment-based thermal analysis is performed to obtain the temperature history, which then is applied to the mechanical (stress) analysis. A modified material model is used to consider the influence of face-to-face contact between the top and bottom metal sheets in the thermo-mechanical analysis of welding lap joints. Results show that the normal stress components prevail in the weld zone during hybrid welding process, and maximum thermal stress exceeding the yield point of material exists at the heat affected zone (HAZ) near the weld pool. Increasing the welding speed causes the penetration and width of weld bead to decrease, and the thermal stress concentration at the welded joint is also reduced accordingly. After welding and cooling down, longitudinal tensile stress (SZ) and transverse compressive stress (SX) are retained in the formed weld, and the higher longitudinal compressive stress exists around the weld bead. In addition, a series of experiments are performed to validate the numerical results, and a qualitative agreement is achieved. Compared to the welded joint obtained by GTAW and laser welding alone, the residual stress concentration in the weld joint obtained by hybrid laser–GTAW is the minimal one.     

Static tensile strength characteristics of fillet welding lap joints assisted with bonding

A series of tensile tests were carried out on fillet welded lap joints assisted with bonding for investigating the static tensile strength characteristics of the joints from the viewpoints of stress reduction effect around the welded part due to bonding. It was confirmed that the mechanical properties of epoxy resin bonding used in this study were not deteriorated by heating to less than 150°C. When the fillet welded lap joints with bonding were assembled, the bond layer 20 mm from the weld toe was subjected to heating to over 150°C. In other words, the mechanical properties in that region deteriorated. The strengths of the elastic limits of specimens with welding and bonding were higher than those of specimens with only welding by from 60 to 100 MPa. The ultimate tensile strengths of them were almost the same because they were broken at the base plate. The strains around the weld toe and the root of specimens with welding and bonding were smaller than those of specimens with welding by around 13% in the elastic region. The strengths of specimens with only bonding were 170 MPa, which could be explained by a theory of elastic stress distribution. Even if the bond layer 20 mm from weld toe of the specimens with welding and bonding was thermally damaged, the possibility was confirmed that the residual bond layer had around 100 MPa in strength. It could be concluded that the strength of the residual bonding assisted to decrease the stress around the welded part of the specimens with welding and bonding.     

NiTi coatings produced by the transferred plasma arc welding process and their wear characteristics

Summary

A physical interpretation is proposed for stress conditions at the toe of weld beads, the principal point of fatigue failure in welded structures. Global stress fields at the surface of joined sheets are composed of a structural stress state due to joint shape and a local stress state due to bead shape.

The proposed approach was applied to fillet welded joints, giving two different interpretations of the local stress field.     

Fatigue strength improvement of stainless steel using weld toes dressing with low transformation temperature welding wire

In the present study, low transformation temperature welding wire (LTTW), which can induce residual compressive stress around 304L welded joints, has been developed to improve the fatigue performance of welded joints. Procedure of design and chemical composition of weld metal and deposited metal of LTTW are introduced. The microstructure of weld metal of LTTW is composed of low carbon martensite and residual austenite. The residual stress distribution of a single welding bead of LTTW was measured and the result shows that compressive residual stress is generated. In addition, fatigue test was also carried out on no-load cruciform welded joints of 304L stainless steel under three conditions: as welded, 308L dressing and LTTW dressing. The result shows that the fatigue life of LTTW dressing joints (2×10⁶ cycles) is improved by 14–23 times and 3–6 times compared with that of as welded joints and 308L dressing joints.     

分段退焊对角接接头焊接残余应力及变形的影响

基于热-弹塑性有限元理论,以Abaqus软件为平台进行角接接头焊后残余应力及变形的分析,采用分段移动热源模型并利用Fortran语言开发热源子程序,分别采用直通焊和分段退焊两种方式进行角接接头焊接温度场、残余应力及变形的数值模拟分析.结果表明:横向变形是角接接头最主要的变形;角接接头焊接在焊缝端口处的残余应力为压应力,...     

斜十字接头三维焊接残余应力的数值模拟

为研究全熔透焊接十字接头残余应力空间分布特点,分析十字接头焊缝形式对焊接残余应力分布状态的影响,基于温度场和应力场间接耦合方式,对全熔透焊接十字接头残余应力开展了有限元数值模拟研究. 采用ANSYS有限元软件,选择Q345C钢材典型热力学参数,构建全熔透焊接十字接头有限元模型,分析得到焊接过程结构温度场分布. 将焊接十字接头温度场作为输入条件,基于ANSYS热–力耦合分析得到全熔透焊接斜十字接头三维残余应力场分布. 结果表明,全熔透焊接十字接头残余应力峰值主要分布在焊趾和焊根处,焊缝角度变化会对焊缝处残余应力分布带来较大影响.     

Hybrid laser/arc welding process for controlling bead profile

Abstract

A laser hybrid welding process in which a defocused laser beam is applied beside a gas metal arc weld (GMAW) pool to modify the bead shape was studied. The present paper aims to produce welds with improved toe geometry and better fatigue life than those made with GMAW alone and to apply a numerical simulation to help configure the hybrid process. First, stationary hybrid welds were made to validate weld bead shape predictions and to characterise the spreading of the arc weld deposit to the laser heated spot. Next, the travelling hybrid process was configured with the aid of simulations and fatigue test specimens were welded. Proper application of the laser heat input induced molten metal to spread to the laser heated area, increasing the fillet weld leg length. This produced a larger weld toe angle that decreased the stress concentration and increased the fatigue life of the welds relative to standard mean values.     

Angular distortion of fillet welded T joint using low transformation temperature welding wire

Abstract

A newly developed low transformation temperature welding wire, of which the transformation start temperature is lower than that of conventional welding wires, was applied to fabrication of fillet welded T joints. The welding angular distortion and the temperature profile of the weld metal were continuously measured during the welding process. The angular distortion of the fabricated T joint was reduced when the weld metal reached the martensitic transformation start temperature. The residual angular distortion was less with the low transformation temperature welding wire than that with the conventional welding wires. The welding distortion of T joints was calculated by a numerical simulation with consideration of the effect of phase transformation under weld thermal cycles. The welding distortion was reproduced with high accuracy in the numerical simulation. Results of the numerical simulation also determined that there was a direct correspondence between the transformation expansion of the weld metal and the angular distortion.     

An estimation of factors influencing residual stress characteristics of fillet welded lap joints

The residual stress behaviours in fillet welded lap joints of sheet metal have been researched in a systematic testing procedure with varied steel types, steel thicknesses and welding wires having different transformation points. Consequently, under the simulated fabrication welding conditions (with a constant amount of deposited metal), the transverse residual stress at the weld toe, which is deemed critical in fatigue strength, has been found almost invariable to a change in steel type and thickness, but it has been clarified to become compressive to a greater extent as the wire’s transformation point is lower. Moreover, as for the residual stress inside the weld metal, the compressive residual stress area has been found to expand as the welding wire’s transformation point reduces, from the results of the thermo-elastic-plastic analysis.     

Welding distortion investigation in fillet welded joint and structure based on iterative substructure method

Abstract

In this paper, prediction and controlling angular distortion in fillet welded joint and structure were investigated. First, two methods to reduce angular distortion in fillet welded joint were investigated by experimental and numerical analysis. One was to apply a constant external force in-process and the other was rigid clamping. In numerical analysis, a new in-house finite element code has been developed based on the idea of iterative substructure method (ISM) to calculate welding distortion in rational time. During experimental analysis, the constant external force was designed and applied in-process to reduce angular distortion of fillet welded joint. The results showed that the distortion can be efficiently predicted by ISM, which were in good agreement with the experimental ones. Applying constant external force in process was a more effective method to reduce distortion than using rigid clamping. In addition, with a constant load distance from weld bead, the locations of the applied constant external force and rigid clamping along the longitudinal direction (welding direction) have little influence on the magnitude of welding angular distortion. Finally, the angular distortion of a large fillet welded structure was predicted with ISM and also controlled with applying a constant external force based on the simulation results of the fillet welded joint.     

基于折合应力正面角焊缝T形焊接接头的尺寸设计

角焊缝接头在工程中应用广泛,由于焊缝接头应力分布不均匀,它成为结构的薄弱环节.焊缝折合应力是国际焊接学会推荐用于计算焊缝静载强度的,该应力公式反应折合应力与焊缝内部切应力与正应力的关系.文中以正面角焊缝T形焊接接头为分析对象,假设角焊缝断裂发生在折合应力最大值所在的截面,分析得出Q345正面角焊缝T形焊接接头最大折合应力与承载力和焊缝尺寸的关系.基于承载能力和等强度准则设计焊缝尺寸,并通过有限元软件对T形焊接接头进行数值模拟,其数值模拟结果与理论计算结果基本一致,说明基于折合应力设计正面角焊缝T形焊接接头焊缝尺寸是可行的.     

换热器管子与管板焊接接头残余应力数值模拟

利用有限元软件ABAQUS,对换热器管子与管板焊接残余应力进行数值模拟,获得了焊接接头残余应力的分布规律,比较了伸出角接头和内角接头的优劣.计算结果表明,内角接头残余应力比伸出角接头小.最大径向应力出现在管板表面的热影响区,对管板表面裂纹有主要影响.最大环向应力出现在焊缝根部,对管子与管板连接失效影响较大.相邻两换热管之间,由于后面换热管的焊接加热作用,使前面管子焊缝局部应力值下降,有利于降低应力腐蚀开裂的敏感性.研究结果为优化换热器管子与管板的焊接工艺、控制残余应力提供了理论依据.     

Fatigue characteristics of patch plate joints by fillet welding assisted with bonding

A series of fatigue experiments and elastic analysis were carried out for investigating fatigue characteristics of patch plate joints assembled by fillet welding assisted with bonding. In the case that fatigue cracks occurred at the weld toe by the four-point bending fatigue experiment, the fatigue life of joints assembled by welding and bonding (WB) specimens were almost the same as those by only welding (W) specimens. The elastic analysis simulating the four-point bending loaded situation on W and WB specimens was performed for elucidating its reason. The stress concentration at the weld toe was even high in the WB specimens. Therefore, the fatigue life of WB specimens was not longer than that of W specimens. On the other hand, the stress around the weld root of WB specimen was around 30% of that of W specimen. The possibility of stress reduction effect by bonding was indicated around the weld root rather than around the weld toe. In order to verify this possibility, the four-point bending fatigue experiment was performed by setting the specimens so that the tensile stress was applied on the weld root. It was confirmed that the fatigue cracks occurred from the weld root in both W and WB specimens. The fatigue life defined in this study of WB specimens was from 4 to 8 times longer than that of W specimens when the applied nominal stress range was under 175 MPa. The fatigue life defined in this study of WB specimens was from 2 to 3 times longer than that of W specimens when the applied nominal stress range was over 200 MPa. The results indicated the fatigue life improvement of patch plate joints by fillet welding assisted with bonding when the fatigue cracks occurred at the weld root.     

超声冲击处理对7A52铝合金焊接应力影响的数值模拟

利用有限元软件ABAQUS,建立了7A52铝合金双丝MIG焊的热力耦合数值模型,得到焊接残余应力场;随后利用数值传递方法,对焊接接头处进行超声冲击处理,建立焊接和超声冲击处理耦合模型,得到冲击后的残余应力场,分析了冲击前后残余应力分布特点和大小,并且通过改变冲击针移动速度分析其对残余应力的影响规律.旨在探讨超声冲击对7A52铝合金焊接残余应力改善的影响.计算结果表明,超声冲击处理能够显著改善焊缝和热影响区的表面残余应力,且随着冲击移动速度的增加,得到的焊接接头处压应力值逐渐减小.     

铝合金薄板焊接应力三维有限元模拟

采用热弹塑性有限元方法,对铝合金薄板脉冲TIG焊接接头的焊接应力进行了三维数值模拟,考虑材料性能随温度的变化,对焊接残余应力进行测量.结果表明,铝合金薄板中的焊接应力产生、发展很快,加热结束后不久应力便趋于稳定.热源前缘和两侧区域存在数值很高的纵向和横向动态压应力,焊缝中心的纵向残余拉应力低于母材的屈服强度,距焊缝中心10 mm处的最大纵向残余拉应力达到母材的屈服强度,并且拉应力区较宽,远离焊缝区域的纵向残余压应力数值较大,因此铝合金薄板焊接结构易发生动态和焊后失稳,横截面上纵向残余应力的数值模拟结果与实测结果基本一致.     

Thermal analysis of horizontal fillet joints by considering bead shape in gas metal arc welding

Abstract

In gas metal arc (GMA) welding, the weld size, that is, the locally melted area of a workpiece, is one of the most important factors determining the strength of a welded structure. Variations in the welding power and the welding heat flux may affect the weld pool formation and ultimately the size of the weld. Therefore, an accurate prediction of the weld size requires a precise analysis of the weld thermal cycle. In the present study, a model that can estimate the weld bead geometry and a method for thermal analysis, including the model, are suggested. To analyse the weld bead geometry, a mathematical model was developed with transformed coordinates to apply to horizontal fillet joints. A heat flow analysis was performed using a two-dimensional finite element model that was adopted to compute the base metal melting zone. The reliability of the proposed model and the thermal analysis was evaluated through experiments, and the results showed that the proposed model was highly effective in predicting the weld bead shape, and that the predicted melting zone of the base metal also corresponded well with the experimental profile.     

铝合金同轴式等离子-MIG焊接工艺

使用同轴式等离子-MIG焊枪,对防锈铝LF5进行工艺试验,研究其工艺参数对焊缝成形及组织的影响,并检验了组织中的缺陷.结果表明,同轴式等离子-MIG焊接铝合金时能够获得较为理想的焊缝;与偏置式等离子-MIG焊接低碳钢相比,用同轴等离子-MIG焊接铝合金时等离子电流的增加对焊缝影响更大,而对熔宽的影响相对较小;焊接过程中,焊丝与母材在高温时熔敷共同形成焊缝中过渡;等离子弧清理范围变小,易形成氧化物夹渣.     

双相不锈钢管道全位置焊接残余应力三维有限元数值模拟

以热弹塑理论为基础，利用ANSYS非线性分析有限元程序，对双相不锈钢管道接头环焊缝残余应力进行三维数值模拟。建立了管道全位置焊接瞬态温度场和应力场三维移动热源模型，获得了环焊缝焊接接头轴向和环向残余应力的分布规律：在管道接头内表面的焊缝及近缝区的轴向和环向残余应力均为拉应力，随着离开焊缝距离的增加，由拉应力逐渐过渡为压应力。在管道接头外表面焊缝中心处的轴向残余应力为压应力，而环向残余应力为拉应力。从环向位置上的应力变化规律可以看出正半周和负半周的应力分布具有明显的对称性。研究结果为优化生产工艺，控制残余应力提供了理论依据。     

超声冲击对焊接结构残余应力的影响

以Q345钢结构箱型柱为对象，研究了超声冲击工艺对焊接残余应力的影响。对电渣焊和埋弧焊两种焊缝进行了超声冲击试验，其中埋弧焊焊缝采用了全覆盖冲击和焊趾冲击两种冲击工艺。残余应力测量表明，采用冲击工艺，可以在焊缝表面一定深度(小于3mm)下产生压应力，最高测得-134MPa；焊趾冲击不但使焊趾表面产生压应力，也降低了焊缝的残余应力。对非熔透埋弧焊和熔透埋弧焊焊缝的测量结果显示，在盲孔法测量的深度范围内，超声冲击可降低焊缝最大主应力约34％～55％。