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。     

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.     

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.     

Laser,tungsten inert gas,and metal active gas welding of DP780 steel: Comparison of hardness,tensile properties and fatigue resistance

The microstructural characteristics, tensile properties and low-cycle fatigue properties of a dual-phase steel (DP780) were investigated following its joining by three methods: laser welding, tungsten inert gas (TIG) welding, and metal active gas (MAG) welding. Through this, it was found that the size of the welded zone increases with greater heat input (MAG > TIG > laser), whereas the hardness of the weld metal (WM) and heat-affected zone (HAZ) increases with cooling rate (laser > TIG > MAG). Consequently, laser- and TIG-welded steels exhibit higher yield strength than the base metal due to a substantially harder WM. In contrast, the strength of MAG-welded steel is reduced by a broad and soft WM and HAZ. The fatigue life of laser-and TIG-welded steel was similar, with both being greater than that of MAG-welded steel; however, the fatigue resistance of all welds was inferior to that of the non-welded base metal. Finally, crack initiation sites were found to differ depending on the microstructural characteristics of the welded zone, as well as the tensile and cyclic loading.     

Comparison of fatigue properties between friction stir welds and TIG welds

对比分析了搅拌摩擦和氩弧焊两种工艺方法对铝合金焊接接头疲劳性能的影响，建立了焊接接头的S-N曲线，结果表明：在相同的载荷条件下，搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头。搅拌摩擦焊接头疲劳寿命N=106次的疲劳强度值约为59~65MPa之间。对焊接接头显微组织的分析表明：搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区，阻碍了滑移带的形成和裂纹的扩展，从而提高了接头的疲劳性能。TIG焊接接头疲劳端口分析显示，焊接缺陷是主要的疲劳裂纹源。     

Effects of thickness on fatigue properties of investment cast Ti-6Al-4V alloy plates

The present study is concerned with the effects of plate thickness on high-cycle fatigue properties and fatigue crack propagation behavior of investment cast Ti-6Al-4V alloys having Widmanstätten structure. High-cycle fatigue test and fatigue crack propagation test were conducted on three cast plates having different thickness, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The high-cycle fatigue results indicated that fatigue strength of the three cast plates was quite similar because of their similar tensile strength. In the case of the fatigue crack propagation, the thicker cast plate composed of thinner platelets had the slightly faster crack propagation rate than the other plates. The effective microstructural factor determining the fatigue crack propagation rate was found to be the thickness of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold K regime.     

Effect of microstructural features on the failure behavior of hybrid laser welded AA7020

Detailed investigations of microstructural feature, mechanical property, fatigue strength, and damage mechanism were conducted on hybrid laser welded 7020‐T651 aluminum alloys used into high‐speed railway vehicles. The results show that the hybrid laser welding process can induce significant changes of microstructures and alloying elements, together with numerous gas pores. Such local modifications degrade the fatigue performance. The tensile strength of welded joints was approximately 74% with respect to the base metal, thus satisfying the design standard. The fatigue property was determined in the low and high cycle regimes. It was found that the fatigue strength of welded joints was fairly inferior to that of the base metal, but far higher than the IIW recommended value. Furthermore, welding defects were well believed to contribute to the shorter fatigue life. The small fatigue crack growth presented highly discontinuous and inhomogeneous due to microstructure and porosity. By contrast, the crack stable growth stage was less sensitive to microstructural features of hybrid welded joints.     

Microstructural and mechanical characterization of electron beam welded Al-alloy 7020

The electron beam (EB) welding process is used to weld any metal that can be arc welded with equal or superior weld quality. EB welding is carried out in a high-purity vacuum environment, which results in freedom from impurities such as oxides and nitrides. Thus, pore-free joints can readily be achieved in metallic materials, such as Al-alloys and Ti-alloys. However, autogenous EB welding of some aluminium alloys leads to a significant strength reduction (undermatching) in the fusion zone due to the loss of strengthening phases. For such Al-alloys, the local microstructure-property relationships should be established to satisfy the service requirement of a welded component with strength undermatching. Autogenous EB welding was performed on 5 mm thick aluminium alloy 7020 plate. Microstructural characterization of the weld metals was made by optical and scanning electron microscopy. Extensive microhardness measurements were conducted in the weld regions of the joints which exhibited a hardness loss in the fusion zone due to the loss of strengthening phases. Tensile properties of the joints were determined by testing flat transverse tensile specimens at room temperature without machining the weld profiles. Furthermore, elastic-plastic fracture toughness tests (CTOD) were carried out on the base material and welded joints at room temperature.     

服役条件下螺旋焊接管线钢疲劳裂纹扩展机理分析

对服役条件下螺旋焊接管线钢裂纹扩展机理进行了分析,通过试验测定了两种母材区和两种焊缝区的疲劳裂纹扩展速率,通过扫描电子显微镜分析了螺旋焊接管线钢中裂纹扩展速率不同的原因。结果表明:焊缝疲劳裂纹扩展速率快于母材中的疲劳裂纹扩展速率;焊缝组织中分布着许多孔洞和表面裂纹,导致组织疏松,使焊缝金属的韧性降低,从而使管线钢承载能力降低,在循环波动载荷下易发生螺旋焊缝疲劳断裂失效。     

In-situ SEM study of short fatigue crack propagation behavior in a dissimilar metal welded joint of nuclear power plant

In-situ Scanning Electron Microscopy (SEM) fatigue experiments were carried out to study short fatigue crack propagation (FCP) behavior of various regions (weld zone, interface region and heat affected zone (HAZ)) in a domestic dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short fatigue crack initiation and propagation behavior was investigated with its influence on both material fatigue and structure fatigue analyzed. Considering material fatigue, in the weld region, crack grows along δ ferrites when propagating parallel to the dendrite, and deflects or branches along δ ferrite, γ austenite dendrite, δ/γ interface and grain boundaries when propagating perpendicular to the dendrite; in safe ends, the crack grows along slip lines and coalesces with secondary cracks; in A508 HAZ, the crack propagates or branches along martensite transgranularly. In terms of structural fatigue, the crack tends to deflect when propagating across the weld/A508 interface or weld/316 L interface with the influence of local microstructure, and the weld/A508 interface region has a resistance to FCP due to its high strength. The fatigue crack propagation rate of each region was compared and analyzed. The fatigue fractography was also characterized under SEM to analyze the crack propagation process.