Effect of FSW parameters on microstructure and mechanical properties of AM20 welds

This paper aims to demonstrate the successful friction stir welding (FSW) conditions of AM20 magnesium alloy. The maximum yield strength and ultimate tensile strength of weld were found to be 75% and 65% of the base metal strength, respectively. The maximum bending angle of the welded joint was 45°. Observations revealed that less plunging depth, high shoulder diameter, and low tool rotational speed and welding speed give better tensile properties. Maximum temperature was observed at 1?mm away from the tool shoulder toward the advancing side. Micro-hardness variation is found to be decreasing along the depth of the weld, and nugget zone (NZ) gives the higher hardness values when compared with base material (BM) and other welded zones. Needle-like grains of the BM became equiaxed grains due to grain recrystalized by the FSW process. The grains in the NZ were finer than thermo-mechanically affected zone and almost same size of grains observed at bottom, middle, and top of the NZ.     

BT20钛合金激光焊接接头的断裂韧性研究

对BT20钛合金及其激光焊接接头的断裂韧性进行了研究.同时分析了合金及激光焊接接头的硬度分布及显微组织.断裂实验表明,除了一个焊接接头紧凑拉伸(CT)试样是脆性启裂外,其它CT试样均在裂纹延性启裂并缓慢扩展后,发生脆性失稳断裂.母材的断裂韧性明显高于焊接接头,轧制方向对母材断裂韧性的影响不明显.焊接热影响区的断裂韧性介于母材和焊缝金属之间.本研究采用的焊后热处理没有改善焊接接头的断裂韧性,还有进一步恶化的趋势.添加活性剂对焊缝金属的断裂韧性没有明显作用,但对延性裂纹扩展长度有所改善.     

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.     

The effect of microstructures on fatigue crack growth in Q345 steel welded joint

It is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R‐ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R‐ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R‐ratio increasing. To the low R‐ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R‐ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse‐grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack‐tip stress shielding and then resulted in higher crack growth resistance.     

EFFECTS OF ENVIRONMENT AND HEAT TREATMENT ON THE TENSILE AND FATIGUE PROPERTIES OF Ti-24 at%A1-11 at%Nb

The α₂+β alloy Ti-24 at%A1-11 at%Nb was evaluated for mechanical properties in the as-received, mill annealed condition (nearly equiaxed) and in the β annealed condition (basketweave). The as-received condition had a moderately strong basal transverse α₂ texture which gave a higher yield strength, higher elastic modulus, and higher fatigue crack growth rate (FCGR) for loading in the transverse direction at room temperature. The β annealed material was stronger and more isotropic in both texture and properties. The fatigue crack growth rate is higher at 732°C in argon than at room temperature. A hydrogen environment results in a FCGR at 732°C comparable to or less than the FCGR in argon at 732°C for the as-received and β annealed conditions respectively. Although the hydrogen in solution at levels as high as several thousand ppm does not embrittle at elevated temperature, embrittlement is found after cooling to room temperature.     

Influence of HIP processing on microstructure and mechanical properties of superalloy Udimet 720LI

Abstract

Inert gas (high purity argon) atomised powder of composition conforming to that of the superalloy Udimet 720 of low interstitial grade was hipped at 1200°C/120 MPa/3 h. The hipped alloy has shown near theoretical density and consisted of equiaxed grains with an average diameter of ～45 μm. While primary γ′-Ni₃ (Ti, Al) precipitates with an acicular morphology were found at the grain boundaries, finer secondary γ′ precipitates with near cuboidal morphology were present in the austenite γ matrix. The yield strength (YS) of the as hipped alloy was found to be the same as that of the wrought alloy heat treated for creep applications (termed as creep resistant alloy) at room temperature (RT) as well as at 650°C. However, the ultimate tensile strength (UTS) and ductility were found to be higher than those of the wrought creep resistant alloy. On the other hand, the YS and UTS of the as hipped material were lower than those of the wrought alloy heat treated for high strength applications (termed as high strength alloy), although the ductility of the former was comparable to that of the latter. In order to improve the strength, the hipped alloy was subjected to a heat treatment consisting of solution treatment followed by two-step aging. Extensive precipitation of fine and coarse γ′ precipitates with cuboidal morphology during duplex aging treatment has led to a considerable improvement in YS and UTS of the alloy, although the corresponding ductility dropped moderately at RT and at 650°C. Fractography of the tensile tested specimens has shown ductile transgranular mode of fracture in the as hipped alloy at RT and at 650°C, while the hipped+heat treated alloy exhibited a mixed mode of fracture at those temperature. The stress rupture properties of the as hipped alloy compare well with those of the wrought alloy and have been found to improve significantly after heat treatment. The present investigation reveals that the hipped superalloy Udimet 720LI has substantial potential for use in the development of near net shaped components for aerospace applications.     

铝合金超声辅助搅拌摩擦焊接接头组织性能研究

目的探究超声振动对铝合金搅拌摩擦焊的作用效果。方法分别采用普通搅拌摩擦焊和超声辅助搅拌摩擦焊方法,对7075铝合金进行焊接试验,并对焊接接头的微观组织、力学性能、断口形貌进行分析。结果普通搅拌摩擦焊焊缝中生成了隧道型缺陷,施加超声振动后,缺陷消失,形成了无缺陷的良好接头,且与普通搅拌摩擦焊相比,超声辅助搅拌摩擦焊焊缝热影响区晶粒长大程度较小,焊核晶粒细化。接头强度明显提高,达到铝合金母材强度的71.5%,接头断裂模式为韧窝和准解理的混合断裂形式。结论超声振动促进了塑性金属的流动,能有效抑制孔洞、隧道型缺陷等的形成,同时超声振动能在提升金属塑性的同时,降低焊缝的热输入。     

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.     

Fatigue behavior of friction plug welds in 2195 Al–Li alloy

The focus of this paper will be on the fatigue behavior of friction stir welded 2195 Al–Li plates that contain friction plug welds. Tensile tests were performed for specimens containing base metal, friction stir welded 2195-T8, and friction stir welded 2195-T8 containing a friction plug weld consisting of a 2195-T8 plug. The ultimate strength was determined for base metal, friction stir welded material, and friction plug welded material. Fatigue properties were determined for both the friction stir weld and friction plug welded specimens in the medium to high cycle regimes. Comparison of the results show that the friction plug weld reduced both the UTS and fatigue life as compared to specimens containing only friction stir weld. The reduction in fatigue life is most likely due to the complication of weld geometry, interacting heat affected zones, and strength mismatch between base metal, friction stir weld, and plug material.     

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.     

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。     

Joint properties of dissimilar Al6061-T6 aluminum alloy/Ti–6%Al–4%V titanium alloy by gas tungsten arc welding assisted hybrid friction stir welding

Hybrid friction stir butt welding of Al6061-T6 aluminum alloy plate to Ti–6%Al–4%V titanium alloy plate with satisfactory acceptable joint strength was successfully achieved using preceding gas tungsten arc welding (GTAW) preheating heat source of the Ti alloy plate surface. Hybrid friction stir welding (HFSW) joints were welded completely without any unwelded zone resulting from smooth material flow by equally distributed temperature both in Al alloy side and Ti alloy side using GTAW assistance for preheating the Ti alloy plate unlike friction stir welding (FSW) joints. The ultimate tensile strength was approximately 91% in HFSW welds by that of the Al alloy base metal, which was 24% higher than that of FSW welds without GTAW under same welding condition. Notably, it was found that elongation in HFSW welds increased significantly compared with that of FSW welds, which resulted in improved joint strength. The ductile fracture was the main fracture mode in tensile test of HFSW welds.     

Macro and microscopic observations of fatigue crack growth in friction stir welded aluminum joints

This paper presents the macro and microscopic fractography performed on fractures from fatigue cracks through friction stir welded joints. The welds were placed under different angles in the various specimens to study the influence of the yield strength and residual stress on fatigue crack growth. As a result, different behavior was observed at the macro level, depending on the type of alloy and orientation of the weld. The variations in rotation of the crack plane raised a number of questions regarding the mode of loading, i.e. mode I or mode II. The purpose of this study was to investigate the fracture surfaces at microscopic level to find explanations for the local macro behavior. Special focus was placed on the fracture surfaces on which features were observed indicating mode II fatigue crack growth.     

Analysis of sensitization phenomenon in friction stir welded 304 stainless steel

This study evaluates the degree of sensitization (DOS) of 304 stainless steel joined by friction stir welding (FSW). Single-loop electrochemical potentiokinetic reactivation tests were performed using a 0.5 mol/L H₂SO₄ + 0.01 mol/L KSCN solution. Sensitization was promoted by exposition of the stainless steel at temperatures between 400°C and 850°C. The microstructure was characterized using optical microscopy to identify the weld zone and the base metal. The samples treated at 550°C showed the most severe intergranular corrosion. The DOS was lower in the weld zone than in the base metal after heat treatments. This reduction in the DOS for the weld zone indicates that FSW is a beneficial process in joining stainless steel.     

Study of microstructural and mechanical properties of weld heat affected zones of 2024-T3 aluminium using Gleeble simulation

Abstract

The effect of the microstructural properties on the mechanical properties of welding thermal cycles and post-weld heat treatment of the heat affected zone (HAZ) in 2024-T3 aluminium alloy has been investigated. Gleeble HAZ simulation, differential scanning calorimetry, TEM and tensile test have been utilised to investigate the regions representative of HAZ microstructures. The decay of strength in the weld HAZ is primarily due to the precipitation and coarsening of stable S phases. The welded HAZ in the region at peak temperature of 414°C has the lowest strength after natural aged temper. Post-weld T81 artificial aging (PWAA-T81) heat treatment at 190°C for 12 h has no effect on improving the HAZ strength; the HAZ strength of 2024-T3 alloy obtained by PWAA-T81 treatment is less than that obtained by natural aging, and its lowest strength is shifted to the region of the peak temperature, which is 452°C. Scanning electron microscopy observation reveals that the fracture mode changes from transgranular to intergranular failure when the 2024 specimen is exposed to a thermal cycle up to a peak temperature of 550°C. This is caused by the liquation of grain boundary segregates or formation of a eutectic structure while the specimen is subjected to high temperature thermal cycles during welding, which results in a decrease in the strength and ductility of the grain boundary. It is also shown that the decrease in ductility in this high temperature HAZ cannot be improved using the PWAA-T81 heat treatment.     

焊接接头中的裂端应力三轴性

采用弹塑性有限元方法研究了平面应变条件下裂位于焊缝中心不同强度匹配和裂纹深度焊接接头的裂端应力三轴性。结果发现,与均匀材料相比,同等载荷水平下,高匹配接头裂端应力三轴性降低,低匹配接头裂端应力三轴性升高;随裂纹变浅,应力三轴性降低,强度匹配的影响也更为显著。从焊缝和母材塑性变形与应变硬化交互作用方面进行了解释。     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Characterisation of fatigue fracture surfaces of friction stir channelling specimens tested at different temperatures

The fatigue fracture surfaces of friction stir channelling specimens tested at room temperature, 120 °C and 200 °C were observed in a scanning electron microscope (SEM) in order to analyse their morphology and the crack propagation mechanisms. Three different friction stir channelling conditions were tested and analysed. For all specimens tested the developing fatigue-crack has always initiated at the advancing side, namely on the boundary between the nugget and the thermo mechanically affected zone (TMAZ) into the interior of the specimen. The crack has propagated through the channel nugget with a path tangential to the advancing side. After the crack has reached the processed surface, a second crack initiated at the channel bottom. The fracture surfaces have shown a semi-elliptical shape crack front. This second crack has propagated uniformly through the base material. Fatigue crack propagation on the TMAZ was mainly characterised by fatigue striations. It was found, on most of the surfaces observed, a clear coexistence of the intergranular fracture mode and the transgranular fracture mode. A relationship between the fatigue testing temperature and the roughness of the fracture surfaces was found. The fracture surfaces roughness was considerably lower at a testing temperature of 200 °C for the three friction stir channelling conditions analysed.     

钛/铝异种合金脉冲激光焊接接头裂纹产生机理

目的 对0.8 mm厚的Ti6Al4V钛合金和2 mm厚的AA6060铝合金薄板进行脉冲激光焊接,分析异种轻合金激光焊接裂纹产生的机理及界面结合机理。方法 采用扫描电镜、EDS能谱以及显微硬度计等微观表征分析方法,对焊接接头的形貌特征、成分以及显微硬度进行分析,探索焊接接头处裂纹产生的原因。结果 钛/铝脉冲激光焊接性较差,接头存在严重的裂纹缺陷,裂纹多集中在焊缝与铝母材交界处以及焊缝中心区域位置,主要以热裂纹为主;接头焊缝可能存在大量的Ti-Al金属间化合物以及少量未熔的钛,其界面层主要成分推测为层状TiAl和外层锯齿状的TiAl3;接头整个焊缝区域的平均显微硬度为HV0.1420,其硬度水平远远高于焊缝两侧铝合金母材,也高出钛合金母材很多。结论 钛铝金属间化合物使钛铝焊接接头焊缝区脆性增大,另外接头焊缝区存在较大的组织应力、热应力、拉压应力、拘束应力等复杂应力,致使焊缝内存在较严重的裂纹缺陷。