超声滚压对LZ50钢疲劳性能及短裂纹行为的影响EI北大核心CSCD

针对超声滚压(USRP)对材料短裂纹行为影响规律不明的问题,对LZ50钢开展USRP试验,对比分析USRP对材料表面形貌、表层显微组织、表层显微硬度、残余应力分布和疲劳寿命的影响,并在340MPa应力水平下开展旋转弯曲复型试验,研究USRP对疲劳短裂纹萌生和扩展的影响。结果表明:USRP能使材料表面发生塑性变形,有效改善材料表面状态,降低表面粗糙度;处理后的LZ50钢表层铁素体、珠光体组织显微硬度平均值分别从200 HV和240 HV提升至304 HV和357 HV,表层残余压应力从-103 MPa提升至-720 MPa,且表层显微硬度和残余压应力沿深度方向呈梯度分布,影响层深约为300μm。在340MPa应力水平下,USRP试样平均疲劳寿命提升385.54%,疲劳短裂纹突破晶界障碍、珠光体带状组织,出现第一、第二次显著降速时的平均寿命分数f分别从0.068和0.469延后至0.172和0.604,短裂纹的萌生和扩展得到明显抑制与延缓。研究结果有助于明确USRP在抑制材料疲劳裂纹萌生与扩展上发挥的积极作用,可为工程材料表面强化工艺的选择提供参考。     

TA15线性摩擦焊接头高周疲劳性能分析

在相同热处理制度下,开展了TA15钛合金母材与TA15线性摩擦焊接头高周疲劳性能试验,并对疲劳断口进行扫描电镜分析. 结果表明,从S-N曲线低应力区到高应力区,焊缝针状组织的疲劳强度高于母材双态组织,随着应力的增大,两种组织的疲劳强度差异逐渐缩小;接头针状组织的疲劳极限(463 MPa)高于母材双态组织(423 MPa). 断口分析结果表明,TA15母材的疲劳断口相对较平,扩展区二次裂纹较少,而线性摩擦焊接头中存在较多的二次裂纹,降低了裂纹扩展速度,说明针状组织的疲劳性能优于双态组织.     

金属表层梯度微结构对力学性能和裂纹扩展的影响

表层梯度结构广泛应用于工程材料,经表面强化技术处理后,零件表层结构呈梯度分布,可以改善零件的服役能力.综述了金属表层梯度微结构对力学性能和裂纹扩展的影响,从微观结构和力学性能的角度,结合非均匀材料中裂纹的扩展行为,考虑了微结构和力学性能之间的相互影响规律,分析了梯度结构中影响裂纹扩展的因素.经表面强化后,材料表面晶粒细化,晶粒尺寸沿纵深方向呈现梯度分布,改变裂纹萌生位置,同时对裂纹扩展起到阻碍,使零件疲劳寿命延长.梯度结构中金相组织由表层单一组织到芯部多相组织的转变,引起表层硬度和残余压应力向纵深方向逐渐减小,表面压应力可以减弱外力的作用,从而影响疲劳寿命.然而,梯度结构中微观组织和力学性能互相关联,像晶粒和位错能改变残余应力的分布,也会对裂纹扩展路径产生影响,如何定量表征他们的影响还有待进一步研究.     

2A97铝-锂合金薄板搅拌摩擦焊接接头组织特征及疲劳损伤特性

测试了搅拌摩擦焊接后2A97铝-锂合金薄板沿焊缝中心线以及垂直于焊缝方向的疲劳裂纹扩展速率,并且采用金相显微镜、扫描电镜、EBSD设备对焊缝组织特征以及断口形貌进行了观察。结果表明,搅拌摩擦焊后,无论是垂直焊缝还是沿焊缝宽度中线,疲劳裂纹的扩展方向都与预制裂纹方向一致,没有发生显著改变;与母材相比,搅拌摩擦焊接后两种类型的试样在稳态扩展区间内的扩展速率与母材的相当,但进入失稳扩展阶段的应力强度因子范围△K值较低,△K值分别为:母材,33.2 MPa·m~(1/2);裂纹平行焊缝试样,20.3 MPa·m~(1/2);裂纹垂直焊缝试样,22.3MPa·m~(1/2)。搅拌摩擦焊接过程中带来的组织改变是导致耐损伤性能恶化的主要原因。     

焊后热处理对308L奥氏体不锈钢焊缝蠕变-疲劳损伤机制的影响

对钨极氩弧焊308L奥氏体不锈钢焊接接头在高温下进行应变控制的蠕变—疲劳试验,使用光学显微镜和扫描电镜分别对焊态试样和焊后热处理试样进行观察,同时用能谱议(EDS)对析出相进行判定,分析微观结构演变和裂纹扩展机制,研究焊后热处理对焊缝金属微观组织以及焊接接头蠕变—疲劳破坏行为的影响.结果表明,经高温焊后热处理,焊缝金属中δ-铁素体的连续网状结构被打破,裂纹扩展模式由沿晶扩展向混合型扩展转变,从而提高了焊接接头的蠕变—疲劳抗力.     

多次补焊对A6N01铝合金焊接接头疲劳性能的影响

通过对多次补焊条件下A6N01铝合金焊接接头进行疲劳试验,获得了不同补焊条件下焊接接头的条件疲劳极限值以及不同区域的裂纹扩展速率,并分析其疲劳断口形貌。结果表明,随着补焊次数增加,接头疲劳极限值减小,三次补焊后其由98.8 MPa下降到89.7 MPa,仍然满足使用要求;焊缝区和热影响区裂纹扩展速率均增大,但热影响区抵抗裂纹疲劳扩展能力优于焊缝区。随补焊次数增加,焊缝区气孔缺陷增多以及热影响区过时效现象加剧是造成焊接接头疲劳性能下降的主要原因。     

激光冲击强化对TA15钛合金双孔结构显微组织和疲劳性能的影响

对TA15钛合金进行激光冲击强化,研究了激光冲击强化对双孔结构试样显微组织、残余应力和疲劳性能的影响。结果表明,经激光能量25 J、圆形光斑?4 mm,冲击2次的激光冲击强化后,TA15钛合金晶体内部形成了大量的高密度位错和位错墙;同时在材料表面引入高达-500 MPa的残余压应力,可以平衡疲劳载荷作用下产生的拉应力,有效抑制疲劳裂纹萌生并减缓裂纹扩展速率。激光冲击强化可以大大提高钛合金双孔结构的疲劳寿命,相对于未强化试样提高了60%～89%,这是由于激光冲击强化引入的较大残余压应力使得裂纹尖端的有效应力强度因子大大减小,当有效应力强度因子小于材料的断裂韧性时,疲劳裂纹的扩展会被抑制或停止,从而提高疲劳寿命。     

7N01铝合金平滑焊接接头疲劳性能研究

主要对7N01铝合金平滑焊接接头疲劳性能进行研究,通过数据可靠性分析和模型选择,得到中值疲劳极限值为130 MPa。使用扫描电镜观察断口,发现疲劳裂纹起源于焊缝中近熔合线的液化微孔或微裂纹处。对比焊接试样、120及150 MPa应力水平下的疲劳试样的金相组织和纵截面形貌,焊接试件中熔合线近邻区、焊缝中心均存在液化微孔和液化裂纹;120 MPa疲劳应力水平下,熔合线近邻区、焊缝中心液化微裂纹长大,焊缝中心微裂纹线度在50~250μm之间,但裂纹线度均小于临界裂纹尺寸,试样未发生断裂;150 MPa疲劳应力水平下,熔合线近邻区的微裂纹扩展,形成宏观裂纹,其附近有部分微裂纹向焊缝中心扩展,焊缝中心的微裂纹也长大,线度在10~50μm之间。     

焊接区位置对焊接结构疲劳性能影响的定量分析

为了研究焊接区位置对结构疲劳性能的影响,本文对焊接结构连接件进行了疲劳试验,并进行了有限元分析及寿命预测。结果发现,焊接区与其它应力集中区会发生应力耦合效应,进而降低结构的疲劳寿命。焊缝位置越远离其他应力集中区,焊缝应力越小;当焊缝与其他应力集中区距离大于焊缝宽度的8倍时,耦合效应消失。在耦合区内,疲劳寿命与焊缝距离近似呈线性关系。     

电磁胀形对5052铝合金疲劳行为的影响（英文）

为了研究电磁胀形技术对铝合金疲劳行为的影响,对电磁胀形后的5052铝合金样件进行疲劳试验研究。疲劳应力-寿命曲线结果表明,相对于原始试样,电磁胀形后的疲劳试样疲劳强度有显著增加。采用扫描电子显微镜对断口形貌进行分析,结果表明,两种条件下,疲劳裂纹都萌生于存在应力集中的边角处,随着裂纹扩展都出现了典型的疲劳辉纹和韧窝结构。对比不同位置的疲劳辉纹宽度并计算裂纹扩展速率,由此得到的裂纹扩展速率曲线表明电磁胀形后试样疲劳裂纹扩展速率降低。理论分析表明疲劳强度的提高主要归因于电磁胀形引起的应变硬化和位错密度增加对裂纹尖端的屏蔽效应。     

Comparative study on fatigue properties of friction stir and MIG-pulse welded joints in 5083 Al-Mg alloy

The objective of this investigation was to compare the fatigue properties of friction stir welds with those of MIG-pulse welds. The 5083 Al-Mg alloy was welded by single pass friction stir welding（FSW） and double-sided MIG-pulse welding. The results show that friction stir（FS） welds have a better appearance than MIG-pulse welds for the lack of voids, cracks and distortions. Compared with the parent plate, FSW welds exhibit similar fine grains, while MIG-pulse welds display a different cast microstructure due to the high heat input and the addition of welding wire. The S-N curves of FSW and MIG-pulse joints show that the fatigue life of FS welds is 18 - 26 times longer than that of MIG-pulse welds under the stress ratio of 0.1 and the calculated fatigue characteristic values of each weld increase from 38.67 MPa for MIG-pulse welds to 53.59 MPa for FSW welds.     

载荷方向对TC4钛合金激光焊缝疲劳性能的影响

针对不同载荷方向对TC4钛合金激光焊缝的疲劳性能的影响展开对比试验分析,对激光直、斜焊缝的疲劳寿命和疲劳断口进行了对比计算与分析.结果表明,直焊缝试件的疲劳寿命相比斜焊缝的分散性小,斜焊缝试件的中值疲劳寿命是直焊缝的1.98倍;直、斜焊缝试件疲劳裂纹均起始于显微硬度最低的热影响区表面,直焊试件裂纹在焊缝区扩展至断裂,疲劳裂纹扩展区有脆性解理特征,而斜焊试件裂纹穿过焊缝进入母材扩展至断裂,扩展区有典型韧性扩展特征;在瞬断区,斜焊缝试件比直焊缝表现出更好的韧性.     

Microstructure and fatigue crack growth behaviour of electron beam welding in 30CrMnSiNi2A steel

The effects of two post-weld heat treatment processes on the microstructure and fatigue properties of the electron beam welded joints of 30CrMnSiNi2A steel were studied. Electron beam local post-weld heat treatment ( EBLPWHT) , in a vacuum chamber, immediately after welding and a traditional furnace whole post-weld heat treatment (FWPWHT) were accepted. The experimental results show that, after EBLPWHT, the main microstructure of weld is changed from coarse acicular martensite into lath martensite, and base metal is changed from ferrite and perlite into upper bainite and residual austenite, however the microstructures of different zones of joints in FWPWHT conditions are tempered sorbite. The fatigue crack growth rate da/dN of welds and base metal are not obviously changed among EBLPWHT, FWPWHT test and as-welded (AW) test, as the mechanical properties of materials have a certain but not large effect on the da/dN of welded joints. The resistance to near threshold fatigue crack growth data of welded joints can be largely improved by EBLPWHT and it is related to microstructure and crack closure effect.     

Effects of dilution and baseplate strength on stress distributions in multipass welds deposited using low transformation temperature filler alloys

Transformation plasticity can be utilised to control residual stresses in steel welds. This requires special filler alloys that transform at a sufficiently low temperature to compensate for accumulated thermal contraction strains. However, the welding parameters needed to optimise the effect in multipass joints have yet to be established. This topic has been investigated by characterising the residual stress distribution in multipass welds fabricated with different welding alloys and baseplates using neutron diffraction to assess the effects of dilution and baseplate strength. While the use of richly alloyed weld metal does enhance fatigue performance in single pass joints, the extent of stress relief that can be derived from transformation plasticity is reduced due to incomplete martensitic transformation when further layers are deposited. For all cases studied, compressive stresses were measured in the weld metal with balancing tensile stress in the heat affected zone of the plate. The magnitude of the tension was observed to be a function of the strength of the baseplate. Recommendations are also presented for the combination of welding and material parameters that lead to the optimum exploitation of transformation plasticity as a method for boosting the fatigue performance of multipass welded joints.     

Microstructure and fatigue resistance of high strength dual phase steel welded with gas metal arc welding and plasma arc welding processes

This study presents the microstructure and high cycle fatigue performance of lap shear joints of dual phase steel (DP590) welded using gas metal arc welding (GMAW) and plasma arc welding (PAW) processes. High cycle fatigue tests were conducted on single and double lap joints under a load ratio of 0.1 and a frequency of 20 Hz. In order to establish a basis for comparison, both weldments were fabricated to have the same weld depth in the plate thickness. The PAW specimens exhibited a higher fatigue life, a gentle S-N slope, and a higher fatigue limit than the GMAW specimens. The improvement in the fatigue life of the PAW specimens was primarily attributed to the geometry effect that exhibited lower and wider beads resulting in a lower stress concentration at the weld toe where cracks initiate and propagate. Furthermore, the microstructural constituents in the heat-affected zone (HAZ) of the PAW specimens contributed to the improvement. The higher volume fraction of acicular ferrite in the HAZ beneath the weld toe enhanced the PAW specimen’s resistance to fatigue crack growth. The double lap joints displayed a higher fatigue life than the single lap joints without changing the S-N slope.     

航空发动机薄壁机匣疲劳裂纹修复焊接变形控制

分别采用恒定直流TIG焊、脉冲直流TIG焊、微束等离子弧焊等方法对航空发动机材料K4169试板进行了焊接试验,经测试脉冲直流TIG焊接头力学性能最优,采用该方法对机匣支板头通气孔附近的裂纹进行了修复试验,测试了裂纹所在支板头对应的内外环安装边的径向距离和距基面的距离,分析了热处理对焊接变形的影响.结果表明,机匣支板头通气孔附近的疲劳裂纹修复,导致机匣发生了径向伸缩和周向扭曲变形.热处理对于薄壁机匣疲劳裂纹修复后焊接应力的消减起到重要作用,可有效地减小焊接变形,保证装配精度.     

含焊接缺陷的30CrMnSiNi2A钢焊接件的疲劳寿命估算

本文分析了焊接缺陷对超高强度钢３０ＣｒＭｎＳｉＮｉ２Ａ焊接件疲劳寿命的影响，给出了定理评估含焊接缺陷的焊接件疲劳性能的方法，焊接缺陷可分成类裂纹型和非裂纹型两类，前者的作用与裂纹相当，含这类缺陷的焊接件寿命近似等于裂纹扩展寿命；后者的作用则类似缺口，含这类缺陷的焊接件寿命为裂纹起始与扩展寿命之和，裂纹扩展寿命可用断裂力学的方法计算，始裂寿命则可用材料的始裂寿命公式计算，但在计算中要考虑表面状态修正     

错边对钢悬链立管横焊接头疲劳性能的影响

采用填丝TIG方法进行了不同错边量的立管环缝横焊试验,对其接头试样在80,120和175 MPa应力范围下进行了疲劳试验,研究错边对横焊环缝接头疲劳性能的影响. 结果表明,错边方式对横焊接头疲劳裂纹起裂位置有重要影响,“上错边”接头的疲劳裂纹主要从根部焊趾上侧起裂,无错边接头的疲劳裂纹主要从根部焊趾下侧起裂,而“下错边”接头的疲劳裂纹则全部从根部焊趾下侧起裂;在相同应力范围下,试样的疲劳循环周次随错边量的增大而降低,当错边量超过0.60 mm后,疲劳循环周次会显著下降.     

Fatigue characteristics of patch plate joints by fillet welding assisted with bonding

A series of fatigue experiments and elastic analysis were carried out for investigating fatigue characteristics of patch plate joints assembled by fillet welding assisted with bonding. In the case that fatigue cracks occurred at the weld toe by the four-point bending fatigue experiment, the fatigue life of joints assembled by welding and bonding (WB) specimens were almost the same as those by only welding (W) specimens. The elastic analysis simulating the four-point bending loaded situation on W and WB specimens was performed for elucidating its reason. The stress concentration at the weld toe was even high in the WB specimens. Therefore, the fatigue life of WB specimens was not longer than that of W specimens. On the other hand, the stress around the weld root of WB specimen was around 30% of that of W specimen. The possibility of stress reduction effect by bonding was indicated around the weld root rather than around the weld toe. In order to verify this possibility, the four-point bending fatigue experiment was performed by setting the specimens so that the tensile stress was applied on the weld root. It was confirmed that the fatigue cracks occurred from the weld root in both W and WB specimens. The fatigue life defined in this study of WB specimens was from 4 to 8 times longer than that of W specimens when the applied nominal stress range was under 175 MPa. The fatigue life defined in this study of WB specimens was from 2 to 3 times longer than that of W specimens when the applied nominal stress range was over 200 MPa. The results indicated the fatigue life improvement of patch plate joints by fillet welding assisted with bonding when the fatigue cracks occurred at the weld root.     

马氏体含量对双相钢焊点拉拉疲劳性能的影响

焊点的疲劳性能是决定车身安全性和可靠性的重要因素. 对不同强度的DP780,DP980和DP1180双相钢焊点进行了检验,在疲劳试验机上进行了拉剪疲劳试验,获得了焊点的疲劳寿命曲线,分析了焊点显微组织、硬度与疲劳性能的关系. 并对焊点的疲劳失效进行了分析. 结果表明:母材性能对其焊点的疲劳性能有较大影响. 母材强度越高即马氏体含量越多,其疲劳性能越优异,三种材料的失效模式均是沿着焊点圆周断裂,裂纹在熔核与母材交界的热影响区萌生,且先贯穿板厚,贯穿板厚的裂纹作为二次裂纹源向板宽方向扩展直至断裂.