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/d N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。     

Effect of welding processes on fatigue crack growth behaviour of AISI 409M ferritic stainless steel joints fabricated using duplex stainless steel fillers

The present investigation aims to study the effect of welding processes such as shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) on fatigue crack growth behaviour of the ferritic stainless steel (FSS) conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material and AISI 2209 grade duplex stainless steel (DSS) was used as filler metal, for preparing single pass butt welded joints. Centre cracked tensile (CCT) specimens were used to evaluate the fatigue crack growth behaviour. From this investigation, it is found that the GTAW joints showed superior fatigue crack growth resistance compared with SMAW and GMAW joints. The reasons for the superior performance were discussed in detail.     

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

The fatigue threshold and high growth rate region properties were investigated on several kinds of welded joints. These properties became unique in spite of the variation of steels (ferrite-pearite, martensite, austenite), welding method, heat input and stress ratio. It was revealed that the unique properties occurred from the fully opened fatigue crack due to the tensile residual stresses. Based on these results, the equation of the fatigue crack growth curve for the design and inspection of welded structures was proposed. It is also suggested that the inducement of compressive residual stress at the fatigue critical zone is effective in improving the fatigue properties of welded structures.     

Correlation of fatigue properties and microstructure in investment cast Ti-6Al-4V welds

The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.     

Rißfortschritt in ultrafesten Stählen und deren Schweißverbindungen bei dynamischer Beanspruchung

Crack propagation in ultra-high-strength steels and their welded joints under dynamic loading . Reported are results of investigation into the propagation of cracks in the base metal and weld metal of an ultra-high-strength steel. The material used in the investigations was a Ni? Co? Mo? alloy maraging steel with a yield point of 170 kp/mm². The steel was arc welded and TIG welded. The joints exhibited a drop of static strength in the range of 5 to 8 percent related to the base metal. Under zero-to-tension stress cycles the fatigue strength corresponded that of other high-strength steels, under tension-compression stress cycles the steel exhibited a higher fatigue strength. It was possible to show striations with the aid of scanning microscopy. Comparing the track propagation calculated in the microscopic range with the results obtained from the crack growth curves produced approximate agreement.     

TC4钛合金薄板激光焊接头疲劳性能研究

研究了TC4钛合金薄板母材及其激光焊接头的拉伸和疲劳性能.结果表明:与母材相比激光焊接头的强度升高,延伸率下降;拉伸试样均断在母材.激光焊接头的疲劳寿命在低应力水平时高于母材,而在高应力水平时低于母材.在疲劳扩展区,母材为韧性穿晶断裂,熔合区则呈现出韧性和脆性相混合的断裂形貌;在瞬断区,母材由等轴韧窝组成,而熔合区主要为粗大的穿晶解理平面.     

Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

Stress intensity factors for three‐dimensional weld toe cracks using weld toe magnification factors

In welded components, particularly those with complex geometrical shapes, evaluating stress intensity factors is a difficult task. To effectively calculate the stress intensity factors, a weld toe magnification factor is introduced that can be derived from data obtained in a parametric study performed by finite element method (FEM). Although solutions for the weld toe magnification factor have been presented, these are applicable only to non‐load‐carrying cruciform or T‐butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints, the currently used weld toe magnification factors do not adequately describe the behaviour of weld toe cracks. In this study, the weld toe magnification factor solutions for the three types of welded joints such as cruciform, cover plate and longitudinal stiffener joints were provided through a parametric study using three‐dimensional finite elements. The solutions were formed with exponents and fractions that have polynomial functions in terms of a/c and a/t – that is, crack depths normalised by corresponding half crack lengths and specimen thickness. The proposed weld toe magnification factors were applied to evaluate the fatigue crack propagation life considering the propagation mechanisms of multiple‐surface cracks for all welded joints. It showed good agreement within a deviation factor of two between the experimental and calculated results for the fatigue crack propagation life.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

Fatigue properties of arc-welded lap joints with weld start and end points

Fatigue properties of arc‐welded lap joints with weld start and end points were investigated through experiments with 2.3‐mm and 3.2‐mm thick 440 MPa‐class steel sheets. Macroscopic fatigue crack‐initiation sites depended on the length of the weld bead to the specimen width. In joints with shorter weld beads, cracks mainly initiated at the toe of the weld start points, while joints with longer beads had initial cracks at the toe of the bead centre. Crack‐propagation analyses, taking stress distribution around the weld toe and residual stress into account, suggested that residual stress distribution could move crack‐initiation sites from the weld start point to the bead centre, although the applied stress at the toe of the weld start point remains the highest.     

纵肋与横隔板交叉构造细节穿透型疲劳裂纹扩展特性及其加固方法研究

纵肋与横隔板交叉构造细节是正交异性钢桥面板最易发生疲劳开裂的构造细节,通过建立有限元数值模型,采用断裂力学方法,研究栓接角钢加固方式对该处疲劳易损细节穿透型裂纹的加固效果。基于疲劳试验足尺节段模型相对应有限元模型,建立了纵肋与横隔板焊接处穿透型疲劳裂纹模型,针对栓接角钢和纵肋外侧栓接钢板两种加固技术的加固效果进行评估。研究结果表明:钢桥面板纵肋与横隔板交叉构造细节的疲劳裂纹扩展至一定长度后将发展成穿透型裂纹,裂纹面受力复杂,纵肋腹板内外侧疲劳裂纹扩展特性表现的不一样,但是随着裂纹扩展的逐步进行,裂纹尖端的开裂模式均以复合型开裂为主;栓接角钢加固方式主要抑制纵肋与横隔板交叉构造细节易损部位疲劳裂纹的I型开裂,因此能很好地抑制短裂纹的扩展,但对于该细节处以复合形式扩展的穿透型疲劳裂纹的加固效果并不显著;在纵肋外侧栓接半U形钢板的加固方法能有效改善穿透型疲劳裂纹的等效应力强度因子,并且加固之后均保持在裂纹扩展阈值以下,表明该加固方式对穿透型疲劳裂纹有良好加固效果。     

Effect of Al foils interlayer on microstructures and mechanical properties of Mg–Al butt joints welded by gas tungsten arc welding filling with Zn filler metal

Gas tungsten arc butt welding of Mg–Al filling with Zn filler metal without and with Al foils in different thicknesses was carried out. Additional Al element was introduced into the fusion zone to accurately modulate microstructure and composition of the welding seam. Microstructures and mechanical properties of the welded joints were examined. Results show that the addition of appropriate quantity of Al element increases the content of Al-based solid solution in the fusion zone near the Mg base metal. The solid solution can eliminate the stress concentration and hinder crack propagation, so the tensile strengths of the joints are improved. However, the immoderate quantity of Al element will lead to the formation of partially Al-rich zones and deteriorate the mechanical property of the joints.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

Influence of the submerged arc welding in the mechanical behaviour of the P355NL1 steel—part II: analysis of the low/high cycle fatigue behaviours

A normalized fine grain carbon low alloy steel, P355NL1 (EN10028-3), intended for service in welded pressure vessels, where notch toughness is of high importance, has been investigated. Applications with this steel usually require the intensive use of welds. One of the most common welding processes that are used in the manufacturing of pressure vessels is the submerged arc welding. This welding process is often automated in order to perform the main seam welds of the body of the vessels. The influence of the automated submerged arc welding, in the mechanical performance, is investigated. In this paper (Part II) the low and high cycle fatigue and crack propagation behaviours are compared between the base and welded materials. Several series of small and smooth specimens as well as cracked specimens made of base, welded and heat affected materials, respectively, were fatigue tested. Strain, stress and energy based relations for fatigue life assessment, until crack initiation, are evaluated based on experimental results and compared between the base and welded materials. Finally, the fatigue crack propagation behaviours are compared between the base, welded and heat affected materials.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

Fatigue strength prediction of ultra high strength steel butt‐welded joints

First, fatigue tests were performed on butt‐welded joints made of novel direct quenched ultra high strength steel with high quality welds. Two different welding processes were used: MAG and Pulsed MAG. The weld profiles, misalignments and residual stresses were measured, and the material properties of the heat‐affected zone were determined. Fatigue tests were carried out with constant amplitude tensile loading both for joints in as‐welded condition and for joints after ultrasonic peening treatment. Finally, in fatigue strength predictions, the crack initiation phase was estimated using the procedures described by Lawrence et al. [Lawrence F V, Ho N J and Mazumdar P K (1981) Predicting the fatigue resistance of welds. Annu. Rev. Mater. Sci, 11, 401–425]. The propagation phase was simply estimated using S–N curves for normal quality butt welds, which may contain pre‐existing cracks or crack‐like defects eliminating the crack initiation stage.     

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.     

Establishing criteria for root and toe cracking of load carrying cruciform joints of pressure vessel grade steel

In this investigation an attempt has been made to establish a criterion to forecast the possible crack initiation region (toe or root) in double fillet welded load carrying cruciform joints and also to know the probable failure mode. Cruciform joints were fabricated from pressure vessel grade (ASTM 517 ‘F’ grade) steel using shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes. Fatigue crack growth experiments were carried out in a mechanical resonance vertical pulsator (SCHENCK 200 kN capacity) with a frequency of 30 Hz under constant amplitude loading (R=0). It was found that the fatigue crack initiation lives (root crack or toe crack) were relatively lower in the joints fabricated by FCAW process than the joints fabricated by SMAW process.     

焊接方法对10Ni5CrMoV钢接头疲劳性能的影响

针对屈服强度为785MPa级别的10Ni5CrMoV钢,分别采用焊条手工焊、气体保护焊和埋弧焊三种方法在其表面堆焊单层焊并设计成板状疲劳试样,研究三种焊接方法对10Ni5CrMoV钢接头疲劳性能的影响。结果表明:对于堆焊单层焊道试样,焊址处应力集中程度对疲劳启裂寿命起主要作用,随应力集中程度的增大,疲劳启裂寿命减小。焊条手工焊、埋弧焊和气体保护焊三种焊接方法中,气体保护焊焊趾处应力集中程度最大,疲劳启裂寿命最低;焊条手工焊焊趾处应力集中程度最小,疲劳启裂寿命最高。     

Characterization of failure modes for different welding processes of AISI/SAE 304 stainless steels

Weld joints manufactured with a welding electrode type 308L and by three different arc welding processes shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and flux cored arc welding (FCAW) in a AISI/SAE 304 were studied in order to compare the failure mechanisms associated with their mechanical and microstructural properties. Chemical compositions were analyzed by optical emission spectroscopy and the ferrite numbers (FN) of the welds were also identified. Relevant microstructural characteristics of the different processes were analyzed by microscopy techniques. Finally, fatigue tests were performed to study the variations in the mechanical properties of each process and to analyze their most probable failure modes by means of a fractographic study, in which the characteristic morphologies of each one (nucleation, propagation, final fracture) were identified by means of optical stereoscopy and scanning electron microscopy (SEM). Three different fracture modes were found at the welding joints that showed correlations with microstructural changes produced during the welding process. The first failure mode displayed that the nucleation of the crack was at the weld root. The second failure mode was generated at the heat affected zone (HAZ), where the crack nucleated due to a variation in the grain size produced by the process and then further propagated through the edge of the weld. The third failure mode appeared due to the presence of exogenous inclusions generated by the welding process, which acted as stress concentrators in the weld and produce the initiation and further propagation of the crack. Lastly, some welding processes presented a combination of the previous failure modes and consequently multiple sites of crack nucleation.