Influences of Friction Stir Welding and Post‐Weld Heat Treatment on Al–Cu–Li Alloy

In this paper, the influences of friction stir welding (FSW) and post‐weld heat treatment (PWHT) on the microstructures and tensile properties of Al–Cu–Li alloy are investigated. After FSW, strengthen loss occurred in the welding area. Remarkable softening occurs in the thermo‐mechanically affected zone (TMAZ) resulting from dissolution of Al₃Li (δ′) phases. Recrystallization and precipitation of ultra‐fine δ′ phases take place in the nugget zone (NZ) that lightens the softening degree of this zone. A noteworthy enhancement in the hardness and tensile strength of the joint is achieved after T8 re­aging treatment (3% ? pre‐deformation, 30 h at 152 °C). However, re‐solution treatment coupled with re‐aging treatment leads to ductility deterioration in the joint because coplanar slip of coarse Al₃Li phases induces severe stress concentration during plastic deformation.

     

R‐curve behaviour of friction stir welds in aluminium‐lithium alloy 2195

ABSTRACT The fracture resistance of friction stir welds in 2195‐T8 is described in this paper. R‐curves were produced for several crack planes, parallel to the welding direction and situated at varying distances from the weld centreline. The friction stir weld was also characterized by hardness and tensile testing. Fracture resistance of the friction stir weld is compared to the base metal and to a variable polarity plasma arc weld. Results show that the material in and around the friction stir weld is tougher than the base metal and the variable polarity plasma arc weld. The friction stir weld fracture performance is discussed in the light of the observed hardness and fractographic data presented.     

Elucidation of fatigue characteristics and fracture mechanism of friction stir spot‐welded tension–shear joint steels

The fatigue life and fracture mechanism of friction stir spot welded tension–shear joints using 590‐MPa class steel as a base material under constant‐amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.     

Fatigue crack growth property of laser beam welded 6156 aluminium alloy

Fatigue crack growth behaviours in different welding zones of laser beam welded specimens were investigated using central crack tension specimens for 6156 aluminium alloy under constant amplitude loading at nominal applied stress ratio R = 0.5, 0.06, ?1. The experimental results showed that base metal (BM) exhibited superior fatigue crack resistance compared to weld metal (WM) and heat‐affected zone (HAZ). Crack growth resistance of WM was the lowest. The exponent m values for BM and HAZ at different stress ratios are close and around 2.6, while m for WM at different stress ratio is around 4.7. The discrepancy between crack growth rates for WM and BM is more evident with increasing stress ratio, while it is a little change for HAZ and BM. Change of the microstructure in WM deteriorates the resistance of fatigue crack growth compared to BM. It was mainly due to grain boundary liquation and dissolving of second‐phase particles in the weld region. It was also found that the variety of fatigue crack resistance for different welding zones is in conformity with the change of hardness. BM with the highest hardness exhibited the maximum resistance for fatigue crack, and WM with the lowest hardness exhibited the minimum fatigue crack resistance.     

Oxidation and fatigue crack propagation in the range of low stress intensity factor in relation to the microstructure in P122 Cr–Mo steel

Fatigue strength and life of weldment at high temperatures are important for the materials in power plants. The fatigue crack growth rate is accelerated by oxidation. Similarly, the high-temperature fatigue life is influenced by oxidation. The base metal, the weld metal and the heat-affected zone (HAZ) of the P122 (Cr–Mo steel) weldment were oxidized between 600 °C and 700 °C for up to 500 h in air, and their oxidation behavior was examined. The oxidation resistance increased in the order of HAZ, base metal and weld metal. The scales were mainly Fe₂O₃. Fatigue tests were performed to measure the fatigue crack growth rate in the range of low stress intensity factor, and the results are discussed from the viewpoint of different microstructures and oxidation.     

Fracture toughness and fatigue crack growth in Ti‐6Al‐4V friction stir welds

Friction stir welding of titanium holds the promise for producing joints with microstructures and mechanical properties that are more comparable to wrought material than traditional fusion welding processes. Extensive data exist on the microstructure and static mechanical properties of titanium friction stir welds, but very little are available on the durability (fatigue) and even less on the damage tolerance (fracture toughness and fatigue crack growth). This paper presents the results of an investigation into the damage tolerance of friction stir welds made in 6 mm thick Ti‐6Al‐4V after a post‐weld heat treatment. It was found that the apparent fracture toughness was lower than the wrought base material, 7–25% depending on the crack orientation relative to the weld, but the crack growth performance (ΔK vs. da/dN) of the weld in the absence of weld‐induced residual stresses was identical to the base material.     

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 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.     

Analysis of fatigue crack growth behavior of SS 316(N) welds using the unified approach

Fatigue crack growth (FCG) behavior of SS 316(N) weld has been evaluated at different R‐ratios at room temperature and compared with that of the base metal. The FCG resistance of weld is better than that of the base material and is due to the residual stresses developed during the welding. The data were analyzed using the unified approach that considers the two‐parametric (ΔK and K_max) nature of fatigue. The R‐ratio effects in both the base and weld metals are accounted for without invoking the extrinsic parameters, such as plasticity‐induced crack closure. Since the residual stresses are of the monotonic type, they affect the crack growth via the K_max‐parameter. The crack growth trajectory plots were developed, and they show how the two crack tip driving forces, ΔK and K_max, change to overcome the FCG resistance of the weld in relation to that of the base metal. The results also show that the effects from the compressive residual stresses are more dominant at low R‐values and occur via the K_max parameter.     

Effects of stress ratio and temperature on fatigue crack growth in a Ti–6Al–4V alloy

Fatigue thresholds and fatigue crack growth (FCG) rates in corner notched specimens of a forged Ti–6Al–4V aero-engine disk material were investigated at room temperature and 350 °C. The threshold stress intensity range, ΔK_th, was determined by a method involving a step change in stress ratio (the ‘jump in’ method). It was found that for three high stress ratios (R=0.7–0.9), where crack closure effects are widely accepted to be negligible, there were similar ΔK_th values at room temperature and 350 °C under the same R. For a given temperature, ΔK_th was observed to decrease from 3.1 to 2.1 MPam with R increasing from 0.7 to 0.9. The fatigue crack growth rate was influenced by increasing temperature. For high stress ratios, FCG rate at 350 °C was higher than that at room temperature under the same ΔK. For a low stress ratio (R=0.01), higher temperature led to higher FCG rates in the near-threshold regime, but showed almost no effect at higher ΔK. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a K_max effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the K_max and fatigue threshold, is proposed to describe FCG rates in the near-threshold and Paris regimes for both temperatures. The predictions compare favourably with experimental data.     

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.     

Effects of temper condition and corrosion on the fatigue performance of a laser-welded Al–Cu–Mg–Ag (2139) alloy

The effects of temper condition and corrosion on the fatigue behavior of a laser beam welded Al–Cu–Mg–Ag alloy (2139) have been investigated. Natural aging (T3 temper) and artificial aging (T8 temper) have been applied prior to welding. Corrosion testing has been performed by exposing the welded specimens to a salt spray medium for 720 h. Aging influences the corrosion behavior of laser welds. In the T3 temper, corrosion attack is in the form of pitting in the weld area, while in the T8 temper corrosion is in the form of pitting and intergranular corrosion in the base metal. In the latter case corrosion is attributed to the presence of grain boundary precipitates. Corrosion degrades the fatigue behavior of 2139 welds. The degradation is equal for both the T3 and T8 tempers and for the corrosion exposure selected in this study corresponds to a 52% reduction in fatigue limit. In both cases fatigue crack initiation is associated with corrosion pits, which act as stress raisers. In the T3 temper, the fatigue crack initiation site is at the weld metal/heat affected zone interface, while for the T8 temper the initiation site is at the base metal. Fatigue crack initiation in uncorroded 2139 welds occurs at the weld toe at the root side, the weld reinforcement playing a principal role as stress concentration site. The fatigue crack propagates through the partially melted zone and the weld metal in all cases. The findings in this paper present useful information for the selection of appropriate heat treatment conditions, to facilitate control of the corrosion behavior in aluminium welds, which is of great significance for their fatigue performance.     

Fatigue crack growth resistance of gas tungsten arc,electron beam and friction stir welded joints of AA2219 aluminium alloy

AA2219 aluminium alloy square butt joints without filler metal addition were fabricated using gas tungsten arc welding (GTAW), electron beam welding (EBW) and friction stir welding (FSW) processes. The effect of three welding processes on fatigue crack growth behaviour is reported in this paper. Transverse tensile properties of the welded joints were evaluated. Microstructure analysis was also carried out using optical and electron microscopes. It was found that the FSW joints are exhibiting superior fatigue crack growth resistance compared to EBW and GTAW joints. This was mainly due to the formation of very fine, dynamically recrystallised grains and uniform distribution of fine precipitates in the weld region.     

A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints

The effects of various surface treatment techniques on the fatigue crack growth performance of friction stir welded 2195 aluminum alloy were investigated. The objective was to reduce fatigue crack growth rates and enhance the fatigue life of welded joints. The crack growth rates were assessed and characterized for different peening conditions at a stress ratio (R) of 0.1, and 0.7. The surface and through-thickness residual stress distribution were also investigated and presented for the various regions in the weld. Tensile residual stresses introduced during the welding process were found to become significantly compressive, particularly after laser peening. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the welds. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material.     

Characterization of incubation time on creep crack growth for weldments of P92

Most structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. In such weldment, it is known that the multi-axial stress fields appear due to the plastic constraint induced by the differences in material micro-structure between the weld metal, heat affected zone (HAZ) and base metal. In the present study, the experiments of creep crack growth tests and the structural mechanical analyses of weldment were conducted to understand the effects of stress multi-axiality of weldment on the characteristics of creep crack growth, creep deformation and creep ductility. Additionally, to characterize the time of creep crack initiation up to the start of a brittle type creep crack growth, the distribution of stress multi-axiality (TF) through the base metal, fine-grain HAZ, coarse-grain HAZ to weld metal were investigated.     

Effects of various environments on fatigue crack growth in Laser formed and IM Ti–6Al–4V alloys

The fatigue crack growth behaviors of Laser formed and ingot metallurgy (IM) Ti–6Al–4V alloys were studied in three environments: vacuum, air and 3.5% NaCl solution. Taking the Unified Fatigue Damage Approach, the fatigue crack growth data were analyzed with two intrinsic parameters, stress intensity amplitude ΔK and maximum stress intensity K_max, and their limiting values ΔK^* and . Fatigue crack growth rates da/dN were found increase with stress ratio R, highest in 3.5% NaCl solution, somewhat less in air and lowest in vacuum, and higher in IM alloy than in Laser formed one. In 3.5% NaCl solution, stress corrosion cracking (SCC) was superimposed on fatigue at R=0.9 for where K_max>K_ISCC, the threshold stress intensity for SCC. This and environment-assisted fatigue crack growth were evidenced by the deviation in fatigue crack growth trajectory (ΔK^* vs. curve) from the pure fatigue line where . Furthermore, the fractographic features, identified along the trajectory path, reflected the fatigue crack growth behaviors of both alloys in a given environment.     

Comparative study of fatigue and fracture in friction stir and electron beam welds of 24 mm thick titanium alloy Ti‐6Al‐4 V

In this study, friction stir welding of Ti‐6Al‐4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well‐established electron beam welding method.     

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.     

Role of nonlinear fracture mechanics in assessing fracture and crack growth in welds

An experimental study was conducted to assess the structural performance of repair welds in an ex-service 1Cr-1Mo-0.25V steam turbine casing material. Material from two weld techniques, one involving a post-weld heat-treatment that produced undermatched welds and the other involving a temper bead welding technique that produced overmatched welds were tested. Both welding techniques were implemented in two base metal conditions giving rise to four different welds and two different base metal conditions. The tests conducted included tensile tests, creep tests, fracture toughness tests, fatigue crack growth tests, creep crack growth tests, and creep-fatigue crack growth tests on the base metal, weld metal and the weldment region.The yield strength of the weld metal in the undermatched condition was approximately 10% lower than the base metal, while the weld metal in the overmatched condition had a yield strength that was 30% higher than the base metal at 565 °C. The creep deformation rates in the undermatched welds were 60 times faster than the base metal at a stress of 207 MPa. In the overmatched welds, the creep rates at 207 MPa were about 2.8 times faster in one case and 2.8 times slower in the other.The crack path in fracture toughness specimens followed the interface between the transition layer and the weaker of the weld metal and the base metal. The J-resistance curves for the weldments at 565 °C showed significant variability among duplicate samples from the same welds. This scatter was caused by the variability in the location of the precrack with respect to the fusion line and the location of the low fracture toughness region in the weldment. This behavior was explained using a novel approach for characterizing the fracture of welds. The creep-fatigue crack growth rates at equivalent (C_t)_avg values in undermatched welds was higher than the crack growth rates in the overmatched weld samples. In all cases under creep-fatigue, the crack appeared to grow in the weaker of the base metal and the weld metal. Recommendations for future work are provided to enhance the theoretical underpinnings of the nonlinear fracture mechanics frame-work to rigorously address fracture and crack growth in welds.     

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.