Computer-aided experiment on arc welding

Fatigue properties of cast aluminium welded joints by friction stir welding (FSW) and MIG welding were investigated, comparing with that of the base plate. Fatigue crack propagation tests for the da/dN ? ΔK relation and bending fatigue tests for the S–N relation were carried out. Fatigue cracks in both FSW and MIG specimens were accelerated, when the fatigue crack tip reached the stir zone or the weld metal. This behaviour was discussed based on the crack closure induced by the crack surface roughness and the residual stress. In the S–N properties, the influence of specimen surface finishing on fatigue life was also examined. Fatigue lives of the FSW and MIG specimens in the ‘as weld’ condition were in the range of the largely scattered base plate fatigue lives, in spite of the different fatigue crack initiation sites in each specimen such as the porosity in the base plate, the tool mark bottom in the FSW and the weld toe in the MIG. The FSW specimens with the polished surface showed the particular improvement in fatigue strength for finite fatigue life.     

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.     

热点应力法评定焊接接头疲劳强度的影响因素

将三种方法确定的热点应力作为控制应力,引入焊接接头的疲劳切口系数,由Taylor的临界距离理论得到该系数.通过讨论接头类型、焊缝局部尺寸、板厚以及热点应力的确定方式对疲劳切口系数的影响,揭示热点应力法进行焊接接头疲劳评定的影响因素.结果表明,腹板厚度与焊缝局部几何尺寸对疲劳切口系数的分散性影响较小,但主板厚度的效应明显,由疲劳切口系数得到的板厚效应预测与试验结果接近;以两种外推法为基础构成的热点应力法能明显减少疲劳数据的分散性及其对接头类型与载荷形式的依赖性.     

板厚对不锈钢车体激光叠焊接头抗剪强度和疲劳强度的影响

为了研究板厚对不锈钢激光叠焊接头抗剪强度和疲劳强度的影响,该文针对0.8 mm+2 mm和2 mm+2 mm²种不同板厚搭配的不锈钢激光叠焊接头分别进行了拉伸试验和疲劳试验。结果表明,2 mm+2 mm接头的抗剪强度和疲劳强度均高于0.8 mm+2 mm接头。失效分析发现,2种接头的拉伸破坏由焊肉部位剪断引起的;2种接头的疲劳裂纹均萌生于2 mm未焊透板,裂纹起始位置在2个焊板之间靠近焊核附近未焊透板的热影响区,裂纹沿着焊核边缘向未焊透板外表面方向扩展,直至穿透未焊透板。对焊接接头部位的有限元受力分析可知,2种接头的应力集中程度的不同是造成它们抗剪强度和疲劳强度差异的主要原因。     

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.     

中低强度钢超声冲击处理焊接接头热点应力设计曲线

用ANSYS计算了对接接头、横向非承载十字接头和纵向非承载角接板的热点应力集中系数K_hs,根据Q235B和Q345B的超声冲击处理焊接接头的相关疲劳试验数据得出经过应力比、板厚和几何不平度修正后的超声冲击处理焊接接头的热点应力设计S-N曲线. 结果表明,经过超声冲击处理后,中低强钢(R_m≤355 MPa)对接接头、横向非承载十字接头和纵向非承载角接板的热点应力S-N曲线的分散性降低,与接头形式无关,可以用一条热点应力设计S-N曲线表达. 在采用热点应力法进行疲劳设计时,选取m=10.0,上述三种类型焊接接头在2×106次疲劳强度级别可统一采用FAT135来表示.     

基于结构应力的焊接接头疲劳分析

基于结构应力法对来自相关文献中的16Mn钢十字非承载焊接接头、十字承载接头和纵向立板角焊缝接头的疲劳强度的试验数据进行了分析.推导了适合有限元计算的离散结构应力计算公式,得到焊接接头的结构应力集中系数.证实了该方法具有网格划分不敏感性,给出了应用结构应力法进行疲劳评定时适合上述各类接头的结构应力S-N曲线.结果表明,同名义应力法相比较,采用结构应力表示的焊接接头疲劳试验数据其分散性较小.     

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.     

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.     

焊趾多裂纹的试验与仿真分析

焊接接头焊趾部位的疲劳裂纹多呈现为多裂纹形式.针对十字焊接接头,开展变幅载荷谱块下的拉伸疲劳试验,获得了表征裂纹瞬时前沿迹线的海滩条带的演变过程,验证了多裂纹的存在性,并且发现焊接接头焊趾处的多裂纹具有明显的不同步特性.采用有限元仿真技术,在焊趾部位分步插入多个半椭圆形初始裂纹,模拟了多裂纹的演变过程,获得了与试验断口一致的仿真结果.结果表明,裂纹融合导致裂纹前沿形状不断发生变化,最终有向半椭圆形状演变的趋势;焊接结构裂纹萌生和小裂纹扩展阶段在疲劳全过程中的寿命占比较大,具有不可忽略的影响.     

非承载角焊缝十字接头疲劳性能

文中基于结构应力方法对非承载角焊缝接头的疲劳性能进行了数值分析与试验研究。首先根据不同焊脚尺寸的十字接头焊趾截面结构应力,发现焊脚尺寸越大,接头疲劳性能反而越低,疲劳试验也证明了这一现象。同时,试验数据与BS 7608标准中F级S-N曲线的对比说明了疲劳评估时进行应力修正的必要性。然后,分别利用含装配间隙和熔深的有限元模型计算接头结构应力,证明两者对接头的疲劳性能影响很小。文中还通过与承载角焊缝接头的对比,证明细节特征对接头疲劳性能的影响程度与接头受力状态直接相关。在定义焊缝缺欠质量等级时,应考虑接头在结构中的受力状态,制定合理的控制参数,从而降低结构的制造成本。     

未焊透焊缝疲劳强度分析及工程应用

本文针对工程中焊接结构大量采用单面焊对接焊缝这一事实,对含有未焊透缺陷的单面焊对接焊缝试件进行了疲劳试验,得到了疲劳强度曲线。由于单面焊对接焊缝中的未焊透缺陷位于试件根部表面,在缺陷率相同的条件下,其疲劳强度损失比双面焊未焊透焊缝更严重。试件母材为低碳钢钢板(σ_b≥450MPa)。本试验结果应用于钢瓶焊缝的缺陷容限分析,为制订我国民用在役液化石油气钢瓶判废标准提供了实验依据。     

厚板铝合金FSW和MIG焊接接头疲劳性能

对厚度10 mm的6082-T6铝合金搅拌摩擦焊(FSW)和MIG焊接接头的疲劳强度进行了试验研究,并与6082-T6母材疲劳性能进行了对比分析.结果表明,6082-T6母材的疲劳S-N曲线最高、MIG焊接接头S-N曲线度最低,而FSW接头的疲劳S-N曲线近似位于两者之间;在高应力区FSW疲劳强度低于MIG焊接接头、而在低应力区高于MIG焊接接头.大部分FSW试样疲劳裂纹启始于焊缝根部的"弱连接"缺陷,采用机械加工去掉1.4 mm厚度焊缝根部材料后,FSW疲劳强度明显提高并接近母材数据.厚板6082-T6铝合金FSW焊缝根部质量控制是影响疲劳性能的关键因素.     

Comparison of the Stress Intensity Factor of Load-Carrying Cruciform Welded Joints with Different Geometries

The investigation of fatigue strength needs an accurate solution and reliable values of the stress intensity factor (SIF). In this study, SIF of load-carrying cruciform welded joints has been evaluated using finite element method (FEM), and compared with the available solutions from literature. Load-carrying cruciform welded joints with isosceles triangles and non-isosceles triangle fillet weld shapes were considered and have been analyzed by the FEM-based simulator FRANC2D program. Moreover, the effects of plate thickness and penetration depth have been considered. The aim of this work was to study the effects of these geometrical variables on fatigue SIF of the load-carrying welded joints with lack of penetration. The ability of FRANC2D to find an appropriate SIF solution is shown and compared with available solutions.     

Friction stir welding of aluminium lap joint by tool without probe

A friction stir welding process, with a rotating tool without a probe, was employed and applied to a lap joint of aluminium plate. The thickness of the aluminium plates was 0.5 mm. New tool shapes were developed. The tops of the tool were dome shaped. In this process, the rotating tool was plunged into the aluminium plate. The tool-rotating axis was vertical to the specimen surface, and then moved in the welding direction at a speed of 20 mm/s. Tool rotation speed was 18,000 rpm.

At tool plunge depths of 0.1 mm or over, it was possible to weld the two plates. At tool plunge depth of 0.1 mm, its joint was fractured at the weld interface. At tool plunge depth of 0.2 mm or over, the joints were fractured at the stir zone of the upper plate or the heat affected zone of the lower plate. Based on observation of the hardness profiles and the thickness change of the weld area, controlling factors of the joint strength are discussed.     

Microstructure and Fatigue Properties of Laser Welded DP590 Dual-Phase Steel Joints

In this paper, cold-rolled DP590 dual-phase steel sheets with 1.5 mm thickness were butt-welded by a fiber laser, and the evolution and effect on microhardness, tensile property and fatigue property of the welded joint microstructure were studied. The results showed that the base metal is composed of ferrite and martensite, with the martensite dispersed in the ferrite matrix in an island manner. The microstructure of the weld zone was lath-shaped martensite that can be refined further by increasing the welding speed, while the heat-affected zone was composed of ferrite and tempered martensite. The microhardness increased with increasing welding speed, and the hardness reached its highest value—393.8 HV—when the welding speed was 5 m/min. Static tensile fracture of the welded joints always occurred in the base metal, and the elongation at break was more than 16%. The conditional fatigue limits of the base metal and the weld joints were 354.2 and 233.6 MPa, respectively, under tension–tension fatigue tests with a stress rate of 0.1. After observation of the fatigue fracture morphology, it was evident that the fatigue crack of the base metal had sprouted into the surface pits and that its expansion would be accelerated under the action of a secondary crack. The fatigue source of the welded joint was generated in the weld zone and expanded along the martensite, forming a large number of fatigue striations. Transient breaking, which occurred in the heat-affected zone of the joint as a result of the formation of a large number of dimples, reflected the obvious characteristics of ductile fracture.     

Development of friction stir spot welding using rotating tool without probe and its application to low-carbon steel plates

A friction stir spot welding process, in which a rotating tool without a probe was employed, was applied to a lap joint of low carbon steel plates with 0.5 mm thickness. In this process, the rotating tool of 3.6 mm diameter, rotating at 18,000 rpm, was plunged into the upper plate at a rate of 0.2 mm/s, and then kept at a maximum plunged depth of 0.05–0.25 mm for 0–1 s (dwell time). In the weld obtained by this process, a hole due to the impression of the plunged tool probe was not formed, although a slight depression by the tool plunging remained. At tool plunge depths of 0.05 mm or less, it was impossible to weld the plates. At tool plunge depths from 0.1 to 0.14 mm, joints were fractured at the interface in tensile test, and the failure load increased with tool plunge depth. At tool plunge depths from 0.16 to 0.22 mm, joints were fractured at an almost constant load along the periphery of the depression, leaving a part of the upper plate on the bottom plate surface. The maximum tensile failure load of 1.8 kN was obtained at a plunge depth of 0.2 mm. Based on the observation of the weld microstructure and measurement of the thermal cycle at various spots in the weld, controlling factors of the joint strength are discussed.     

HXD2机车转向架拉杆座角焊缝疲劳失效机理

针对HXD2型电力机车在某穿越秦岭山脉线路服役中多台次拉杆座角焊缝出现裂纹的问题,进行了试验与仿真分析。发现裂纹表面有明显疲劳辉纹,表明角焊缝服役中发生疲劳失效;焊缝根部是疲劳起裂位置,有多源起裂形貌特征;起裂后裂纹从中心向两侧扩展、从根部向焊缝表面扩展,在接头横截面上裂纹路径与水平方向夹角近似为45°;统计分析和仿真计算表明,角焊缝裂纹的发生与拉杆座/基板之间的间隙值有关联性,根部间隙越大,则角焊缝抗疲劳性能越差;分析其主要机理是间隙较小时,焊缝受压后拉杆座底面与构架基板表面更容易发生接触,接触后基板表面分担部分载荷。因此,间隙较小时,焊缝应力幅较小;而间隙较大时,不仅应力幅较大,而且在焊根处易出现末端尖劈形貌。 创新点： 分析了某穿越秦岭山脉线路HXD2电力机车拉杆座角焊缝疲劳失效行为,揭示了电力机车车架角焊缝疲劳裂纹与角焊缝根部间隙的相关性,阐明了根部间隙影响拉杆座角焊缝抗疲劳性能的机理。     

不锈钢冷金属过渡焊角接接头应力及变形规律研究

在高速列车轻量化课题中,不锈钢薄板作为车体结构的关键部分越来越受重视。针对异种厚度的SUS301L不锈钢薄板,采用冷金属过渡技术进行角接试验,通过热电偶采集焊接时距焊缝不同位置的温度变化。用3D激光扫描仪测量焊接变形,由于采用焊缝悬空的约束方法,底板焊后发生了较大的呈波浪状的变形,最大变形量达到6.545 mm。用μ-X360分析仪检测接头残余应力,应力呈"M"型分布。     

基于不同应力法的焊接构架纵向角接头疲劳累积损伤评估

在焊接构架结构应力计算整体有限元模型基础上,通过考虑接头角焊缝和1 mm焊趾虚拟缺口半径,对转臂定位座板与箱形侧梁下盖板的组焊纵向角接头进行逐步细化,构建热点应力和有效缺口应力计算局部模型.在动态疲劳试验载荷等效的9级载荷谱作用下,分别基于名义、热点及有效缺口三种不同应力参量,结合ⅡW建议的疲劳设计S-N曲线进行累积损伤计算.结果表明,有效缺口应力的累积损伤远高于名义和热点应力,极有可能引起评估结果过于保守.为充分发挥有效缺口应力法的优势,需要以更加符合结构实际的缺口几何数据和强度数据作为支撑.