不锈钢光纤激光熔透焊接头微观结构对疲劳性能的影响分析

利用光纤激光器对不锈钢实施熔透焊试验,分析不锈钢光纤激光熔透焊接头微观结构对疲劳性能的影响。通过微观结构分析方法分析不锈钢光纤激光熔透焊接头微观结构对疲劳性能、硬度、拉伸强度的影响规律。试验结果表明：母材的疲劳寿命比焊接接头的高;在室温腐蚀试验下,循环次数越大焊接接头最大应力荷载下降越明显,腐蚀时间越长,对疲劳性能影响越大;室温疲劳试验中,同等循环次数下,光纤激光熔透焊接头疲劳强度随厚度增大而提高;在瞬断区显微组织对疲劳裂纹扩展断口形貌没有太大影响,母材比焊接接头在疲劳裂纹扩展时更容易出现疲劳条带;焊缝数量可提高焊接接头疲劳性能,在焊缝数量一样时,增加2倍焊缝长度,能够提高疲劳极限,但提升幅度不大;焊缝硬度高于母材的;焊接试样的拉伸测试指标均低于母材的拉伸测试指标。     

温度对625镍基高温合金焊接接头低周疲劳行为的影响

研究了Inconel 625镍基高温合金焊接接头在25和760℃下的低周疲劳行为,分析了2种温度下焊接接头的应变疲劳寿命数据和循环应力-应变数据,进而给出了合金焊接接头的应变疲劳参数.结果表明,Inconel 625镍基高温合金焊接接头在不同温度下的弹性应变幅和塑性应变幅与载荷反向周次的关系可分别用Basquin和Coffin-Manson公式来描述.合金焊接接头在25℃下疲劳变形时,主要发生循环软化,而在760℃下疲劳变形时则呈现循环硬化.焊接接头的低周疲劳裂纹以穿晶方式萌生于疲劳试样的自由表面,在25℃下疲劳裂纹以穿晶方式扩展,而在760℃下疲劳裂纹则以穿晶和沿晶混合方式扩展.     

TC17钛合金电子束焊接接头的疲劳裂纹扩展规律及疲劳剩余寿命

首先通过试验取得母材及焊接接头的疲劳裂纹扩展速率,然后结合TC17钛合金电子束焊接接头CTOD试验结果及裂纹容限计算值,以估算其疲劳剩余寿命.结果表明:在低应力水平或低△K下,TC17电子束焊缝的da/dN数据与母材的基本相当;然而随着应力水平的增加,焊缝的da/dN值越来越大.在初始裂纹尺寸相同的情况下,TC17合金电子束焊缝与母材疲劳裂纹扩展寿命曲线存在交叉点.当应力幅大于交叉点应力幅时,TC17母材疲劳裂纹扩展到临界裂纹尺寸的剩余寿命要高于相应焊缝的剩余寿命;当应力幅小于交叉点应力幅时,TC17母材扩展到临界裂纹尺寸的剩余寿命要低于相应焊缝的剩余寿命.     

Hastelloy C-276薄板激光焊接接头疲劳性能EI北大核心CSCD

对0.5 mm厚Hastelloy C-276薄板激光焊接接头进行疲劳试验,结合应力-寿命(S-N)曲线和疲劳断口形貌,研究母材及焊接接头的疲劳性能,分析母材和焊接接头的疲劳断裂机理。结果表明:0.5 mm Hastelloy C-276薄板焊接接头和母材的S-N曲线斜率基本相同,焊接接头疲劳性能和母材的基本相当;母材疲劳断口疲劳裂纹起源于试样侧表面,主要沿宽度方向扩展,随着应力的减小,疲劳源数目减少,疲劳裂纹扩展速率减小;焊接接头在母材和焊缝处随机断裂,焊接接头母材区断口形貌和母材断口形貌基本一致,而焊接接头焊缝区断口的疲劳裂纹起源于侧表面棱角处和焊缝表面,焊缝表面是主要疲劳源,裂纹主要沿厚度方向进行扩展,疲劳裂纹扩展区呈现出准解理断裂特征。     

TC4钛合金薄板及焊缝低周疲劳的损伤规律

对TC4钛合金薄板母材及其激光焊缝进行了低周疲劳损伤规律的理论分析和试验研究。在对应力疲劳不同周次后试样的拉伸性能进行测定及分析表明,残余伸长率对该材料的疲劳损伤较为敏感,可以作为表征其低周疲劳损伤的有效物理参量。结果表明,对TC4钛合金薄板母材,以残余伸长率定义的损伤变量初始值D0为0.037,损伤变量D随着循环周次的增加缓慢增加,当循环周次达寿命的90%后才进入快速增加阶段。连续损伤力学理论分析结果可以很好的解释TC4薄板的低周疲劳损伤规律,理论模型与实验结果吻合很好。而激光焊接接头试样中由于存在组织不均匀性及焊接缺陷,使得焊缝的低周疲劳损伤规律偏离了理论模型,由于焊缝中有气孔等缺陷的存在使得焊缝试样循环到寿命的1%时,初始损伤变量值即达到0.4,而且由于焊缝中心晶粒粗大使得循环周次达寿命的50%时损伤变量即开始快速增加。     

铜铝复合接触线用银铜合金疲劳试验

通过采用不同应变幅控制,对不同变形量冷拉银铜合金进行室温低周疲劳试验.结果表明,随着应变幅的增大,滞后环面积也随之增大;随着循环周次的增加,循环应力逐渐降低,从而产生疲劳软化;加工率为38%的银铜合金循环周期大于加工率为19%的银铜合金循环周期;试样断裂后存在三个明显疲劳特征区:裂纹源、裂纹扩展区、瞬断区;银铜合金的裂纹扩展具有穿晶和沿晶两种方式.     

Hastelloy C-276薄板激光焊接接头疲劳性能

对0.5 mm厚Hastelloy C-276薄板激光焊接接头进行疲劳试验,结合应力-寿命(S-N)曲线和疲劳断口形貌,研究母材及焊接接头的疲劳性能,分析母材和焊接接头的疲劳断裂机理。结果表明:0.5 mm Hastelloy C-276薄板焊接接头和母材的S-N曲线斜率基本相同,焊接接头疲劳性能和母材的基本相当;母材疲劳断口疲劳裂纹起源于试样侧表面,主要沿宽度方向扩展,随着应力的减小,疲劳源数目减少,疲劳裂纹扩展速率减小;焊接接头在母材和焊缝处随机断裂,焊接接头母材区断口形貌和母材断口形貌基本一致,而焊接接头焊缝区断口的疲劳裂纹起源于侧表面棱角处和焊缝表面,焊缝表面是主要疲劳源,裂纹主要沿厚度方向进行扩展,疲劳裂纹扩展区呈现出准解理断裂特征。     

镍基高温合金电子束焊接接头疲劳性能

对GH4169高温合金板材电子束焊接试样进行了金相分析和高、低周疲劳试验,分析GH4169合金的高、低周疲劳性能及疲劳断口形貌.结果表明,GH4169合金电子束焊接接头焊缝区的特点是树枝晶比较明显,为比较发达的树枝晶和明显的二次枝晶.在较高放大倍数下发现,母材和热影响区也为树枝晶结构.接头低周疲劳性能不低于母材,随应变幅降低,相同应变幅下焊接接头疲劳寿命变得略高于母材.接头高周中值疲劳强度比母材高约80 MPa.     

TC4-DT钛合金电子束焊接接头的疲劳断口研究

采用单边缺口试样,用扫描电镜观察了TC4-DT钛合金母材及电子束焊接接头焊缝区与热影响区的疲劳断口,分别分析了疲劳裂纹起裂源、扩展区及瞬断区的断口形貌特征.结果表明,疲劳裂纹均起裂于试样缺口根部,并由试样的一侧扩展到另一侧,裂纹扩展初期均沿一定的结晶面扩展,属于脆性穿晶断裂;母材和热影响区的瞬断部位均有明显的韧窝出现,属于塑性沿晶断裂,而焊缝区断口上韧窝不明显;热影响区疲劳断口有疲劳条带出现,但在母材和焊缝区断口上很难看见.     

TC4钛合金薄板激光焊接接头的疲劳寿命及断口分析

研究了TC4钛合金薄板激光焊接接头及母材的疲劳性能,并对其疲劳断口进行了观察.结果表明,接头的疲劳寿命在低应力水平时高于母材,在高应力水平时低于母材.母材疲劳裂纹萌生于试样表面;在裂纹扩展区有平行排列的弯曲的二次裂纹和期间更细的疲劳辉纹,瞬断区为细小的等轴韧窝.焊缝疲劳裂纹起源于表面的气孔,源区有笔直且平行排列的二次裂纹;在裂纹扩展区,断口形貌与组织有关,细晶区为韧窝,在细晶与柱状晶交界处为敞口浅韧窝,柱状晶和粗晶区为晶粒大小的刻面,上有大量的微剪切滑移带,断裂机理为滑移带形成及开裂.     

Effects of the Electron Beam Welding Process on the Microstructure,Tensile, Fatigue and Fracture Properties of Nickel Alloy Nimonic 80A

The purpose of this study was to evaluate rotary bending high-cycle fatigue properties and crack growth of Nimonic 80A-based metal and electron beam-welded joints. All the tests were performed at room temperature. Fracture surfaces under high-cycle fatigue and fatigue crack growth were observed by scanning electron microscopy. Microstructure, hardness and tensile properties were also evaluated in order to understand the effects on the fatigue results obtained. It was found that the tensile properties, hardness and high-cycle fatigue properties of the welded joint are lower than the base metal. The fracture surface of the high-cycle fatigue shows that fatigue crack initiated from the surface under the high stress amplitude and from the subsurface under the low stress amplitude. The effect of the welding process on the statistical fatigue data was studied with a special focus on probabilistic life prediction and probabilistic lifetime limits. The fatigue crack growth rate versus stress intensity factor range data were obtained from the fatigue crack growth tests. From the results, it was evident that the fatigue crack growth rates of the welded are higher than the base metal. The mechanisms and fracture modes of fatigue crack growth of welded specimens were found to be related to the stress intensity factor range ΔK. In addition, the effective fatigue crack propagation thresholds and mismatch of welded joints were described and discussed.     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

The low cycle fatigue(LCF) tests were carried out using symmetrical cyclic loading under total strain amplitude control conditions.The present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded(EBW) joint in the following aspects,i.e.,cyclic deformation behavior,fatigue life and fatigue fracture behavior.The results show that the softening of the joint is significant at larger strain ranges,while not obvious at smaller strain ranges.The joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal.A fatigue crack of the joint not only originates at the surface or subsurface,but also at defects in the fusion zone(FZ).The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism,while the joint shows brittle fracture mechanism.In all the fatigue fracture zones many dimples appear,showing the typical ductile fracture.     

AZ31B镁合金及其焊接接头的疲劳断裂机理

对AZ31B镁合金进行疲劳实验,在2×106循环次数下,母材、对接接头、横向十字接头和侧面连接接头的疲劳强度分别为66.72,39.00,24.38和24.40MPa。采用光学显微镜对裂纹扩展特征进行分析,结果表明,AZ31B母材的疲劳裂纹宏观扩展路径平滑,但微观观察发现疲劳裂纹扩展方向曲弯,有些裂纹分成两岔;裂纹尖端扩展均为沿晶扩展。焊接接头裂纹均在焊趾部位起裂,对接接头和横线十字接头的裂纹沿着热影响区扩展;侧面连接接头的裂纹起裂位于焊脚部位。采用扫描电子显微镜对疲劳断裂机理进行分析。疲劳断口由准解理或解理台阶组成,均为脆性断裂,断口中存在二次裂纹,对接接头中存在疲劳条纹,其间距约为5μm。     

Fatigue-life prediction of SiC aluminum composite using a Weibull model

The feasibility of the acoustic emission technique in predicting the residual fatigue life of 6061-T6 aluminum matrix composite reinforced with 15 vol.% SiC particulates (SiC_p) is presented. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were induced at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. The number of cumulative AE events increased exponentially with the increase in strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion. Cumulative events during post fatigue tensile tests reduced with a decrease in the residual fatigue life. Based on the high cycle fatigue damage accumulation model, a Weibull probability distribution model is developed to explain the post fatigue AE activity of specimens during tensile tests. Using the model, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change significantly with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.     

AZ31镁合金激光焊件的力学性能和应力腐蚀行为

采用Nd-YAG激光对AZ31 HP镁合金进行激光自熔焊接。显微组织分析表明,使用或不使用填料（焊料）AZ61镁合金得到的激光焊接接头的平均晶粒尺寸大约为12μm,显微硬度和拉伸强度与母材相近。然而,慢应变速率拉伸表明,在ASTM D1384溶液中两种焊接接头的抗应力腐蚀性能比母材略差。可观察到应力腐蚀裂纹在焊缝金属萌生并向热影响区（HAZ）扩展。然而,在以AZ61镁合金为填料（焊料）获得的焊接接头中,观察到裂纹起源及扩展出现在热影响区（HAZ）。在慢应变速率拉伸试验中,由于试样表面暴露在腐蚀环境中,在氢氧化镁/氧化镁层形成局部损伤,从而导致应力腐蚀裂纹的生成。     

Effect of dwell condition on fatigue behavior of a high-Nb TiAl alloy at 750 °C

The strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of a newly developed Ti-45Al-8Nb-0.2W-0.2B-0.02Y (at.%) alloy were carried out at 750 °C in air. The hysteresis loop, cyclic stress response and life modeling as well as failure mechanism of the alloy were investigated in detail. It was revealed that the tensile and compressive mean stresses would generate when the dwell condition was introduced at minimum and maximum strain, respectively. In addition, the dwell condition, especially for the compressive dwell condition, would significantly decrease the fatigue life. The typical continuum damage accumulation(CDA) and modified CDA life models proposed in the present study were employed to predict both LCF and CFI life of the alloy, which showed that the modified CDA life model had a higher accuracy than the typical CDA life one. Moreover, only single crack initiation source was observed at 92% (i.e. 11/12) of LCF fracture while multiple crack initiation sources at 84% (i.e. 31/37) of CFI fracture. Apparently different from LCF specimen showing more transgranular appearance, CFI specimen shows more intergranular appearance.     

不锈钢在扭转/拉伸复合载荷下的低周疲劳裂纹扩展

研究了从SUS316NG不锈钢圆捧试样的环状预裂纹开始的低周疲劳裂纹扩展行为，分析了扭转／拉伸复合加载时不同载荷比对弹塑性疲劳裂纹扩展的影响．用J积分的指数方程表达了简单加载及复合加载的裂纹扩展速率．对于同样的J积分范围值，Ⅰ型载荷下的裂纹扩展速率最高，而Ⅲ型载荷下的速率最低．裂纹断口在高应力水平下为宏观平坦，在复合加载条件下的疲劳断口上观察到了疲劳条痕，同Ⅰ型加载条件下的相同，疲劳条痕间距与裂纹扩展速率二者数值相等．     

AZ31镁合金及其TIG焊接接头断裂机理研究

对AZ31镁合金及其焊接接头进行拉伸、冲击和疲劳试验,分析了镁合金的断裂机理及疲劳裂纹扩展方向.母材拉伸试验结果表明,试样几乎没有缩颈,抗拉强度为236.29 MPa;焊接接头的抗拉强度为185.68 MPa,拉伸断裂从焊接接头焊趾部位启裂,抗拉强度为母材的78%.冲击试验在-80～340 ℃进行,结果表明,在较低温度下AZ31镁合金冲击韧性较小,断口为准解理形貌的脆性断裂;随着温度的增加,断裂形式由准解理+韧窝形貌的混合断裂过渡为韧性断裂;在常温下焊缝中心的冲击韧性比母材的高,但热影响区的冲击韧性较差.AZ31B镁合金母材的疲劳强度为66.72 MPa,对接接头的疲劳强度为39.00 MPa;母材疲劳断口由解理台阶组成,为脆性断裂;焊接接头疲劳断口由解理和准解理台阶组成,为脆性断裂.