金属焊接接头疲劳寿命延长技术综述

疲劳断裂是金属构件在循环或交变载荷作用下长期服役过程中的主要失效形式.焊接是重要的金属成型方法,焊接接头是同种金属或异种金属连接的部位,是焊接金属构件上组织结构和力学性能的渐变区.由于金属构件服役的环境越来越苛刻,长期在循环或交变载荷作用下服役时,焊接接头的疲劳问题也越来越突出.因此,如何延长金属构件焊接接头疲劳寿命已经成为广泛关注的重要科学问题.由于金属焊接工艺复杂,含有焊接接头的金属构件疲劳寿命受多种因素影响.本文首先总结了影响焊缝疲劳寿命的关键因素,包括组织变化、焊接缺陷、应力集中、残余应力等.由于焊接接头存在组织演变、几何结构变化以及焊接缺陷等问题,焊接接头疲劳性能通常较基体大幅下降.随后综述了多种焊缝疲劳延寿技术,如焊缝打磨技术、锤击技术、TIG熔修技术、高频机械冲击技术、低相变温度焊接材料技术等.根据焊缝疲劳延寿技术特征,大致分为焊缝形状修饰法、焊缝残余应力法和低相变温度材料法三类.     

TIG与FSW焊接工艺下2219铝合金疲劳裂纹扩展性能研究

目的 研究钨极惰性气体保护焊(TIG)和搅拌摩擦焊(FSW)对2219铝合金焊接接头疲劳性能的影响,并探究这2种不同焊接技术条件下焊接接头疲劳裂纹的产生与裂纹扩展原理,了解2种焊接接头的抗裂纹扩展能力,为工程实践应用提供数据参考。方法 采用疲劳裂纹扩展试验方法,测试上述2种焊接工艺条件下焊缝金属和热影响区组织的疲劳裂纹扩展速率da/dN和阈值,使用光学显微镜和扫描电子显微镜观察并分析金相组织和疲劳断口形貌特征。结果 疲劳裂纹倾向于沿裂纹处萌生,裂纹的存在成为主要的裂纹扩展源头,有利于加速裂纹向前延伸。热影响区由于组织结构不均匀,不同位置的晶粒尺寸存在明显差异,疲劳裂纹扩展路径倾向于沿靠近焊缝一侧向靠近母材区域扩展。TIG焊接工艺下焊缝金属和热影响区的裂纹扩展速率明显低于FSW焊接工艺下的焊缝金属和热影响区,与此同时,TIG焊接接头表现出优良的抗疲劳裂纹扩展性能。结论 通过此研究,建议2219铝合金焊接接头采用TIG焊接工艺,抗疲劳裂纹扩展效果更佳。     

发动机热端部件电子束焊修复接头的高温疲劳裂纹扩展行为

研究了力学不均匀性对电子束焊接修复接头疲劳裂纹扩展行为的影响规律，结果表明，由于力学不均匀性的影响，位于修复法接头焊缝和热潮影响区的疲劳裂纹垧母材偏转扩展，力学不均匀性与裂纹偏转导致裂纹扩展的驱动力下降，使得焊接区的表现疲劳裂纹扩展速率低于母材。     

FV520B不锈钢角焊缝接头的组织和断裂机理

通过对焊接接头疲劳断口的观察研究,讨论了控制接头疲劳失效的疲劳机理。实验研究了焊接接头的母材、焊缝及热影响区的微观组织形态。结果发现:母材组织为细小均匀的板条马氏体及弥散分布的第二相粒子,而焊缝区为粗大的板条马氏体及少量的第二相颗粒。组织特征的区别证明母材综合力学性能优于焊缝。     

2024-T4铝合金光纤激光填丝焊缝成形与组织性能的相关性

2024-T4铝合金光纤激光填丝焊缝横截面分为钉头形和近X形2种典型形貌,对比分析了该2种形貌的焊缝成形与组织形态、显微硬度和接头拉伸性能的相关性。结果表明:近X形横截面的焊缝在焊接过程中更加平稳,焊接飞溅更少。焊缝区的组织特征为垂直于熔合线相对生长的柱状晶组织和焊缝中心的等轴晶组织。钉头形焊缝中心晶粒的二次枝晶较发达,逐渐形成等轴树枝晶,而近X形焊缝中心晶粒相对细小,呈现为等轴胞状晶。与钉头形横截面的接头相比,近X形横截面的接头焊缝区析出的强化相θ(Al2Cu)相对较多,平均显微硬度值略高,热影响区的软化现象逐渐减弱,接头强度和塑性略低。     

熔化焊接头“不均匀混合区”的形貌特征研究

通过对大量熔化焊接头的金相观察，研究了熔化焊接头焊缝底部附近“不均匀混合区”的形貌特征，归纳金相观察结果，发现不均匀混合区由小岛，后续熔化滞留层，扩散过渡层四种形貌构成，文中关于熔化焊接头“不均匀混合区”的形貌划分方法，能很好地解释熔化焊接头中焊缝底部附近的成分，组织．性能不均匀性的形成原因，有助于正确理解熔化焊接头的特征区域构成。     

静轴肩搅拌摩擦焊2A14-T4铝合金T形接头的组织和性能

采用静轴肩搅拌摩擦焊技术实现了8.5 mm厚2A14-T4铝合金T形接头的焊接,研究了接头的宏观成型、显微组织及力学性能.结果表明:焊缝表面呈现光滑无弧纹特征,焊缝外部和内部未发现焊接缺陷;SSFSW T形接头截面焊接区域形貌整体呈现两头宽、中间窄的"开口哑铃"状,焊核区晶粒表现为取向随机的等轴晶,焊核区平均晶粒尺寸表现为第二次焊核区最大,焊接重合区次之,第一次焊核区最小.焊核区再结晶机制以几何动态再结晶为主并伴随有部分连续动态再结晶;热机影响区较窄且晶粒被拉长变形,热影响区组织晶粒长大粗化;焊接过程中第二相粒子的析出粗化造成焊核区硬度降低,硬度最低点出现在第一道焊缝热影响区与热机影响区的交界处;接头的硬度较低区域和结构尺寸引起的应力集中导致T形接头底板与筋板容易萌生裂纹、发生断裂;断口中存在较多的撕裂棱以及大小不一的韧窝,在韧窝中存在尺寸不均匀的第二相颗粒,断口呈现韧性断裂特征.     

压铸镁合金AZ91D搅拌摩擦焊接头焊核演变机理研究

搅拌针旋转频率为1200r/min、焊接速率为40mm/min条件下,对4mm厚的压铸镁合金AZ91D进行连接,可获得无缺陷的焊缝接头。使用OM,SEM对焊缝接头微观组织进行观察。结果表明:压铸镁合金AZ91D搅拌摩擦焊接头焊核区域微观组织呈现出较大差异,顶部冠状区组织为均匀粗大、高致密度的再结晶晶粒,平均晶粒约为15μm;中心环形区域及焊核底部组织相对细小,均匀程度不如焊核冠状区;焊核区组织均为再结晶晶粒,晶粒形核模式与非连续动态再结晶模式类似。     

深潜器用Ti80厚板电子束焊接接头组织性能研究

采用电子束焊接方法焊接深潜器用56 mm厚Ti80合金,并对焊接接头的组织结构和力学性能进行研究。结果表明,焊接接头成形良好,无缺陷;焊缝组织为马氏体α相和残余β相组成的网篮组织;熔合区界线明显,过热区十分窄;热影响区组织由初生α相、马氏体α相和β相组成;焊接接头各区域显微硬度值分布不均匀,由焊缝至母材显微硬度值逐渐下降;拉伸断裂发生在远离焊缝的母材处,接头抗拉强度为935.3 MPa,大于原始母材的911.8 MPa;焊缝冲击吸收功为36.3 J,由焊缝至母材冲击吸收功值逐渐增大,接头各区域冲击断裂方式均为韧性断裂。     

304不锈钢管TIG焊接头凝固行为及热力耦合研究

目的 对2 mm厚不锈钢管的焊接工艺参数进行优化，并基于模拟仿真软件对接头的热应力场进行模拟，以解决薄壁管件接头应力测试不方便的问题。方法 以TIG焊对2 mm不锈钢管进行焊接，通过对接头宏观形貌、微观组织、显微硬度等结果进行优化进而得到最佳焊接工艺参数，采用双椭球热源和温度-位移耦合方法结合最优工艺参数进行数值模拟。结果 当焊接电流为150 A、焊接速度为66 cm/min时，焊接接头全部熔透，且正面及背面焊道均匀致密，成形良好。焊缝中心上部区域和下部区域均呈现等轴晶形貌，下部区域尺寸较上部区域尺寸略大，熔合线附近为柱状晶组织。焊接接头显微硬度整体分布呈现U形，其中热影响区显微硬度(197HV)大于焊缝区域硬度(162HV)，熔合线附近显微硬度达到最低值(145HV)。模拟结果显示，在焊接过程中，当纵向残余应力从母材向焊缝中心过渡时，由压应力逐步转化为拉应力;焊缝中心横向应力呈现为压应力，向两侧母材过渡时应力值逐渐趋近于0，径向应力值变化幅度较小，模拟数据变化趋势与实测数据变化趋势接近。     

氧化膜对6082铝合金搅拌摩擦焊接头疲劳性能的影响

对6 mm厚的6082-T6铝合金进行两种表面处理然后实施搅拌摩擦焊接,研究了对接面氧化膜对接头组织和疲劳性能的影响。结果表明,进行速度为1000 mm/min的高速焊接时,对接面未打磨和打磨的接头焊接质量都良好,接头强度系数达到81%;两种接头的疲劳性能基本相同,疲劳强度均为100 MPa;少数样品在焊核区外断裂,大部分样品在热影响区断裂。与接头相比,两种接头焊核区的疲劳性能有所提高,均为110 MPa,在疲劳测试中裂纹并未沿“S”线萌生和扩展。     

Mechanical and Structural Properties of Similar and Dissimilar Steel Joints

The mechanical properties of specimens from similar and dissimilar weld joints were examined. A ferritic steel (St37-2) and an austenitic stainless steel (AISI 304) were joined by the gas tungsten arc weld (GTAW) process using an austenitic filler metal. Mechanical and metallographic properties of the specimens were obtained by means of microhardness testing, tensile testing, bending fatigue testing, and light optical and scanning electron microscopy. The highest microhardness values were recorded on the ferritic–austenitic dissimilar weld joint, whereas the highest tensile strength and bending fatigue life were obtained with the austenitic–austenitic joints. Ferritic and pearlitic structures were observed in the microstructure of the ferritic–ferritic joint. The microstructures of austenitic–austenitic and austenitic–ferritic joints showed small recrystallization grains in addition to the typical austenitic and ferritic structures. Scanning electron microscopy was used to observe the fracture surfaces of the specimens and the origins of the fatigue cracks.     

Characterization of 2024-T351 friction stir welding joints

Characterization of macrostructure, microstructure, hardness, precipitate distribution, residual stress, and cyclic deformation behavior of 2024-T351 friction stir welded joints has been conducted. Inhomogeneous microparameters governing the nonuniform residual stresses and cyclic strength are discussed. The cyclic strength of the weld microregimes is controlled by grain size and distribution of precipitates achieved during the weld process. The comprehensive information of micro-and macromechanics is used to assist in understanding the mechanism that governed the fatigue crack initiation, propagation, and life of the welded joints.     

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve th     

Variation of fractographic appearance for different microstructures in welded joints having the same fatigue crack propagation properties

Fatigue fracture surfaces were examined with a scanning electron microscope to investigate the influence of the different microstructure between weld metal and heat affected zone. The specimens were centre-cracked type transverse butt welded joints. The relationship between macroscopic fatigue crack propagation rate and the stress intensity factor range is the same in spite of the difference in microstructure for both materials. It is shown that the fractographic appearance changes with microstructure even in the very low growth rate region near fatigue threshold. This suggests that fractographic appearance is not necessarily a guide to the rate of fatigue crack growth.     

Numerical modelling and experimental investigation on welding residual stresses in large‐scale tubular K‐joints

This paper is devoted to the experimental and numerical assessment of residual stresses created by welding in the region surrounding the weld toe of tubular K‐shaped joints (i.e. region most sensitive to fatigue cracking). Neutron‐diffraction measurements were carried out on K‐joints cut from large‐scale truss beams previously subjected to high cycle fatigue. Tri‐axial residual stresses in the transverse, longitudinal and radial direction were obtained from the weld toe as a function of the depth in the thickness of the tube wall. In addition, thermomechanical analyses were performed in three‐dimensional using ABAQUS and MORFEO finite element codes. Experimental and numerical results show that, at and near the weld‐toe surface, the highest residual stresses are critically oriented perpendicularly to the weld direction, and combined with the highest externally applied stresses. Based on a systematic study on geometric parameters, analytical residual stress distribution equations with depth are proposed.     

热处理温度对TC17(α+β)/TC17(β)钛合金线性摩擦焊接头组织及力学性能的影响

对TC17(α+β)/TC17(β)钛合金线性摩擦焊接头进行热处理实验,采用光学显微镜(OM),扫描电子显微镜(SEM)和显微硬度仪等检测手段,研究不同热处理温度对焊接接头微观组织及力学性能的影响。结果表明:焊态下,接头焊缝区发生再结晶,界面处为亚稳定β相组织,显微硬度低于母材,接头高周疲劳强度为345 MPa。TC17(α+β)侧热力影响区因焊接速率过快,残留了大量的初生α相。经过焊后热处理,亚稳定β相分解,焊缝析出弥散的(α+β)相。随着热处理温度的升高,细小的次生α相长大,部分发生球化。热处理后,因亚稳定β相分解,焊缝及热力影响区的显微硬度大幅度升高,接头疲劳强度平均提高65 MPa;随着热处理温度的升高,接头热力影响区的断裂韧度增加。     

Fatigue behaviour of post weld heat treated electron beam welded AA2219 aluminium alloy joints

This paper reports the effect of post weld heat treatment on fatigue behaviour of electron beam welded AA2219 aluminium alloy. An attempt has been made to enhance the fatigue strength of the electron beam welded joints through post weld heat treatment methods such as solution treatment, artificial aging, solution treatment and artificial aging. Electron beam welding machine with 100 kV capacity has been used to fabricate the square butt joints. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN has been used to evaluate the fatigue life of the welded joints. Of the three post weld heat treated joints, the solution treated and aged joints are enduring higher number of cycles under the action of cyclic loads.     

Effects of ultrasonic impact treatment on pre-fatigue loaded high-strength steel welded joints

Comparative fatigue tests were conducted on as-welded, weld toe peened specimens before and after fatigue loading. Fracture surface, residual stress, microstructure and hardness were determined. The test results showed that as the pre-fatigue loading period extended, deeper cracks may have initiated and propagated and the fatigue life improvement decreased. The processes of ultrasonic peening on welded joints with existing cracks were modeled by finite element analysis. The numerical results indicated that the mechanism of UIT improving fatigue performance included two factors: compressive residual stress and the change of crack orientation. Both effects reduced as the crack became deeper.     

Low-cycle fatigue of 1Cr–18Ni–9Ti stainless steel and related weld metal under axial, torsional and 90° out-of-phase loading

The fatigue behaviour of base metal and weld joints of 1Cr–18Ni–9Ti stainless steel has been studied under uniaxial, torsional and 90° out‐of‐phase loading. A significant degree of additional hardening is found for both base metal and weld metal under 90° out‐of‐phase loading. Both base metal and weld metal have the same cyclic stable stress–strain relationship under torsional cyclic loading and 90° out‐of‐phase cyclic loading. Base metal exhibits higher cyclic stress than weld metal under uniaxial loading, and Young's modulus and yield stress of weld metal are smaller than those of base metal. Weld metal exhibited lower fatigue resistance than base metal under uniaxial and torsional loading, but no significant difference was found between the two materials under 90° out‐of‐phase loading. A large scatter of fatigue life is observed for weld metal, perhaps because of heterogeneity of the microstructure. The Wang–Brown (WB) damage parameter and the Fatemi–Socie (FS) damage parameter, both based on the shear critical plane approach, were evaluated relative to the fatigue data obtained.