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你可能喜欢在铝孪晶中晶界对产生疲劳裂纹的影响--《轻合金加工技术》1980年04期
在铝孪晶中晶界对产生疲劳裂纹的影响
【摘要】：正 一前言在一般受到反复变形的金属表面的晶体内,皆能观察出固定的滑移带。对于铝及某些铝合金中,这个固定的滑移带扰如一个“钻出”和“钻入”的带子,随着这些滑移带的出现产生了应力集中进而导致疲劳裂纹。一方面,疲劳裂纹也可产生于晶界处,例如根据 Smith 对于铝,Kemsley 对于铜,Laird 等对于镍和 wood 等对于钢的研究,就可以用眼看到晶界的裂纹。但是这些滑移
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冶金型气孔对熔化焊接7020铝合金疲劳行为的影响
The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints
宋哲1, 吴圣川1,, 胡雅楠1, 康国政1, 付亚楠2, 肖体乔2
1 西南交通大学牵引动力国家重点实验室 成都 6100312 中国科学院上海应用物理研究所上海同步辐射光源 上海 201204
SONG Zhe1, WU Shengchuan1,, HU Yanan1, KANG Guozheng1, FU Yanan2, XIAO Tiqiao2
1 State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China2 Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
&Cite this article:
SONG Zhe, WU Shengchuan, HU Yanan, KANG Guozheng, FU Yanan, XIAO Tiqiao. The Influence of Metallurgical Pores on Fatigue Behaviors of Fusion Welded AA7020 Joints. Acta Metallurgica Sinica[J], ):
doi:10..448
国家自然科学基金项目No.,四川省科学技术计划项目No.,西安交通大学机械结构强度与振动国家重点实验室开放项目No.SV2016-KF-21;
中图分类号:
18)08-1131-10
Supported by National Natural Science Foundation of China (No.), Science and Technology Research and Development Project of Sichuan Province (No.), Open Research Project of State Key Laboratory for Strength and Vibration of Mechanical Structures of Xi'an Jiaotong University (No.SV2016-KF-21);
Received:&&&
基于同步辐射X射线三维高精度原位成像技术,识别和统计出工艺稳定的激光复合焊接7020铝合金接头中的气孔数量、形貌、尺寸和空间分布特征,结合气孔统计数据结果和焊缝晶粒大小,定义了7020铝合金激光复合焊接头中影响接头疲劳性能的气孔临界尺寸。利用同步辐射X射线三维原位疲劳实验数据和疲劳断口形貌,探讨了疲劳试样裂纹源处气孔尺寸、应力和疲劳寿命之间的定量关系。同时,基于有限元仿真分析,研究了不同位置下气孔处的应力场状态。最后,通过疲劳裂纹扩展速率实验,揭示了气孔对疲劳裂纹萌生、扩展和试样瞬断的影响。研究结果表明,激光复合焊接头临界气孔尺寸可定为30 μm;同步辐射X射线成像和疲劳断口显示,较大的表面气孔和近表面的气孔较容易萌生疲劳裂纹。仿真研究也表明,气孔周围的应力集中程度随着气孔位置由表面向内部移动呈现出先增大后减小最后趋于稳定的趋势;疲劳裂纹扩展速率数据分布趋势表明,气孔对长裂纹扩展过程的影响较小,可忽略不计,但一般认为对裂纹前缘形貌有较大影响。
同步辐射X射线成像
疲劳寿命评价
激光复合焊接
With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight structures is increasing, and aluminum alloys are widely applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the population, location and size of gas pores within AA7020 hybrid welded joints are firstly identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed through in situ fatigue life data via SR-μCT and fracture morphology. By using the finite element analysis, detailed works have been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the surface to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and sudden breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welded joint can be qualitatively identified as 30 μm; the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, and the fatigue crack propagation experiment further shows that the porosity has very little effect on the long crack growth but significant influence on the crack front.
Key words:
synchrotron radiation X-ray microtomography
welding defect
fatigue life assessment
laser hybrid welding
high-speed railway train
中强度和焊接性好的可热处理7020铝合金在室温下具有较高的强度、优良的耐蚀性和良好的时效强化能力,常被用于不便于进行热处理以及对焊接工艺要求较高的关键承载部件,如高速列车的车体底架、枕梁、受电弓和轴箱等。传统铝合金弧焊接头的组织粗大、裂纹较多、变形严重,因此铝合金接头成为整个焊接结构的薄弱部位[]。激光复合焊接方法具有高效率、小变形和优柔性的技术特点,在高速列车关键部件及结构制造中具有广阔的应用前景[]。
研究[]发现,铝合金焊接熔池金属在快速冷却过程中因过饱和氢的析出,会形成数量、尺寸、形状和位置等分布复杂的冶金型气孔,并且焊接时的热输入越小,气孔尺寸和气孔体积分数越小。焊接接头中冶金型气孔的存在降低了接头的有效承载面积,同时位于焊缝表面和近表面的形貌不规则气孔,在疲劳循环载荷作用下产生严重的应力集中,从而引起微裂纹的萌生,导致焊接结构的失效破坏。随着结构轻量化的迅速发展,铝合金结构越来越多地被用于高速列车中,由于服役条件的极端化,接头的载荷历程更加严苛,因此迫切需要对焊缝气孔与接头疲劳性能的关系进行深入系统的研究。
长期以来,各国学者普遍采用测量实验样品表面的裂纹长度和观测样品的疲劳断口等方法来揭示材料的疲劳损伤行为,并依此对结构部件的疲劳失效机制进行定量表征,从而建立了工程结构抗疲劳断裂设计的基础理论和评价体系。然而,金属材料中气孔类缺陷引起的疲劳开裂与无缺陷材料的疲劳损伤机理有较大不同,相关理论方法不能直接适用于焊接结构[]。高精度、高亮度、高准直、非破坏性的第三代同步辐射X射线成像技术(SR-μCT)使得学者可以深入到材料内部,可视化观测和追踪疲劳损伤过程及演变规律,已经成为表征材料内部缺陷和追踪裂纹演变的无可替代的科学工具[]。
疲劳裂纹萌生与材料特性、应力分布、表面状态、载荷大小、样品形状及环境条件等多种因素有关,但一般认为尺寸较大和形貌不规则的表面或近表面气孔缺陷,更容易引起疲劳裂纹萌生[,~]。尽管焊缝内部大尺寸气孔较少,但其对接头疲劳性能和寿命的影响仍不容忽视。同时,焊缝中链状小气孔对接头疲劳性能的影响也不能完全忽略,如2个距离很近的小气孔发生耦合成为裂纹[]。
对于含大基数冶金型气孔的熔焊铝合金或铸造铝合金来说,并不是所有气孔都会诱导疲劳裂纹的萌生,只有当气孔大到某种尺度时,才会对疲劳性能有影响。目前,国内外学者广泛开展了铸造合金气孔与疲劳性能之间关系的研究,获得临界气孔尺寸。例如在考察微观组织与气孔的关系中,发现当铸造357铝合金中的二次枝晶臂间距(SDAS)小于40 μm时,临界气孔尺寸应在晶粒尺寸范围内,否则应在SDAS尺寸范围内,据此得到引起疲劳损伤的临界气孔尺寸为155 μm []。类似的研究[]发现,气孔尺寸大于80~100 μm时,疲劳裂纹优先从气孔处萌生,反之则从近表面的较大共晶组织处萌生。一些学者还率先借助SR-μCT研究了热等静压铸造合金的疲劳行为,给出了铸造Al-Si7-Mg0.3和铸造A356-T6合金中的临界气孔尺寸分别为50 μm[]和25 μm[]。
铝合金的熔焊过程实际上是焊接母材和填充焊丝的重熔过程,微观组织和缺陷分布与铸造铝合金具有一定的相似性,因此可借鉴铸造铝合金疲劳行为的研究思路来揭示熔焊接头的疲劳机制。结果[]发现,铸造铝合金疲劳裂纹主要萌生于表面或近表面缺陷处;当不存在表面或近表面缺陷时,微观结构特性将成为诱导裂纹的重要机制。一般很难观察到铸造铝合金的内部气孔开裂现象,这主要是由于内部裂纹萌生需要更大的塑性区尺寸;另外内部气孔周围近似于真空环境,裂纹扩展速率相对较慢,当内部气孔与表面气孔同时萌生裂纹时,断裂主要由表面裂纹决定,因此很少观察到高周疲劳下内部气孔的开裂行为[,]。
有关气孔临界尺寸的研究不仅对工艺改进和性能预测具有重要意义,而且在进行仿真计算和无损探伤评价中还可以据此过滤掉那些数量众多的小气孔。前述实验和理论探索均表明,应该同时把合金组织特征长度和气孔尺寸作为表征材料疲劳行为的重要因素,相关研究[]结论也充分证明当气孔或者缺陷的尺寸接近甚至小于材料的平均晶粒尺度时,其对材料疲劳性能的影响可忽略不计。然而迄今为止,国内外针对焊缝气孔与接头疲劳行为之间关系的研究仍处于空白状态[,,]。近年来,3D打印技术(或者增材制造技术)的迅速发展及其在航空航天和轨道交通车辆关键结构中的广阔应用前景,使得人们对于增材制造材料中典型缺陷与损伤行为之间的相关性关系也产生了浓厚的兴趣。
铸造铝合金、熔焊接头和增材制造材料有着基本相似但略有差别的热循环过程,都存在气孔这一共性缺陷,如何对含缺陷部件进行损伤演变表征及服役性能预测研究,已成为当前材料与结构疲劳研究中的热点前沿课题。本工作采用第三代同步辐射X射线高精度三维成像技术,基于同步辐射原位加载试验机,分别在欧洲同步辐射光源(European synchrotron radiation facility,ESRF)和上海同步辐射光源(Shanghai synchrotron radiation facility,SSRF)系统开展了高速列车用中强度7020铝合金激光复合焊焊接接头的原位疲劳实验研究,对焊接接头的气孔数量、位置和尺寸进行定量统计表征,同时结合有限元仿真模拟,对比分析标准试样的疲劳实验数据,据此揭示气孔缺陷与铝合金焊缝疲劳行为之间的关系。
1 实验方法
焊接母材选用高速受电弓和枕梁用中强度铝合金,其成分(质量分数,%)为：Zn 4.22,Mg 1.21,Cu 0.10,Al 余量。为便于气孔成像研究,选取铝合金母材板厚为2 mm。选用直径为1.2 mm的高Mg含量的ER5356焊丝,利于补充焊接过程中接头Mg元素的烧损。在光纤激光-脉冲电弧复合焊接平台上,对经过化学清洗和钢丝刷打磨的Al板进行对接焊接。考虑到我国车辆结构中焊缝通常要进行光滑过渡处理,因此对焊板余高进行打磨,去除余高后自然时效3个月,然后对焊接接头进行力学性能测试,测得激光复合焊接铝合金的屈服强度和抗拉强度分别为209.8和255.9 MPa[]。
同步辐射X射线原位成像疲劳实验先后在ESRF-ID19和SSRF-BL13W1线站上开展,实验原理如a所示。用于原位疲劳成像实验的复合焊焊接接头试样,同样进行去余高处理,并进一步打磨至厚度约为1 mm,具体尺寸如b所示。以上海光源实验为例,实验操作流程为：首先对无加载试样进行原位成像,作为试样的初始损伤状态;然后对试样进行正弦疲劳加载,至一定循环周次,选择光子能量为19~21 keV的X射线对试样进行180°扫描,完成一次成像,一次成像共得到720张扫描照片(成像参数为：曝光时间0.1~2.0 s,空间分辨率6.5 μm);按照上述流程进行下一次加载和成像,直至试样断裂。基于ESRF-ID19的原位疲劳实验参数为：应力比R=0.1,频率f=20 Hz;基于SSRF-BL13W1的原位疲劳实验参数为：R=0.2,f=10 Hz[,]。之后,采用图像处理软件ImageJ统计焊接接头中气孔尺寸和数量,用三维图像重构软件Amira进行焊接接头气孔分布的三维重构。用Quanta FEG 250型扫描电镜(SEM)观察断口形貌,用HyperMesh软件开展网格模型剖分并基于ABAQUS软件开展气孔处应力状态的有限元模拟。
图1-8-1131/img_1.png图1
同步辐射X射线原位成像实验工作原理及原位疲劳试样尺寸示意图 Fig.1
Schematics of in-situ fatigue loading device based on synchrotron radiation X-ray tomography(a) principle diagram of operation (b) fatigue specimen size (unit: mm)
Fig.1-8-1131/img_1.png图1
同步辐射X射线原位成像实验工作原理及原位疲劳试样尺寸示意图 Fig.1
Schematics of in-situ fatigue loading device based on synchrotron radiation X-ray tomography(a) principle diagram of operation (b) fatigue specimen size (unit: mm)
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同步辐射X射线原位成像实验工作原理及原位疲劳试样尺寸示意图
Schematics of in-situ fatigue loading device based on synchrotron radiation X-ray tomography(a) principle diagram of operation (b) fatigue specimen size (unit: mm)
为更好地进行对比验证,依据HB 《金属材料轴向加载疲劳试验方法》开展激光复合焊接铝合金的高周疲劳实验,载荷形式为恒幅正弦波轴向应力加载,R=0.1,f=50 Hz,采用四级载荷进行实验,每个载荷水平下取2~3个试样,试样经过打磨余高平均厚度约2 mm,如b所示。
依据GB/T 《金属材料疲劳裂纹扩展速率试验方法》制备紧凑拉伸(CT)试样,开展激光复合焊焊接接头的疲劳裂纹扩展特性实验,试样宽度W=50 mm,双面打磨至厚度B=1.84 mm。使用线切割慢走丝方式加工切口,缺口尖端位于焊缝中心,缺口长度a0=10 mm,取3个试样进行实验。实验采取正弦波的加载方式加载,最大工作载荷为25 kN,f=10 Hz,R=0.1。
2 实验结果与分析
2.1 焊缝气孔特征描述
一般认为,气孔的尺寸、数量、形貌、分布和位置在一定程度上影响着材料的疲劳性能[],因此准确表征气孔尺寸及位置,对于揭示气孔与疲劳性能的关系至关重要。在上海光源与欧洲光源原位成像疲劳实验中,10个复合焊接头中气孔尺寸和数量的统计结果如所示。统计计算表明,气孔的平均直径约为11 μm,而且绝大多数气孔等效尺寸小于30 μm,占焊缝中气孔总数的97.1%;等效尺寸为30~50 μm的气孔仅占气孔总数的2.8%;等效尺寸大于50 μm的则更少,仅占气孔总数的0.1%。
图2-8-1131/img_2.png图2
上海光源与欧洲光源10个原位成像疲劳试样中气孔的统计结果 Fig.2
Statistical results of gas pores inside 10 in-situ imaging fatigue specimens at Shanghai synchrotron radiation facility (SSRF) and European synchrotron radiation facility (ESRF)
Fig.2-8-1131/img_2.png图2
上海光源与欧洲光源10个原位成像疲劳试样中气孔的统计结果 Fig.2
Statistical results of gas pores inside 10 in-situ imaging fatigue specimens at Shanghai synchrotron radiation facility (SSRF) and European synchrotron radiation facility (ESRF)
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上海光源与欧洲光源10个原位成像疲劳试样中气孔的统计结果
Statistical results of gas pores inside 10 in-situ imaging fatigue specimens at Shanghai synchrotron radiation facility (SSRF) and European synchrotron radiation facility (ESRF)
激光复合焊接头微观组织的电子背散射衍射(EBSD)结果[]表明,铝合金激光复合焊焊缝的平均晶粒尺寸为33 μm。正如前文所述,诱导疲劳裂纹萌生的气孔,其尺寸大小与材料的晶粒尺寸大小存在一定的相关性,当气孔尺寸小于材料的平均晶粒尺寸时,气孔对材料疲劳性能的影响可以忽略不计;而一旦突破晶界障碍,则短裂纹将迅速扩展,因此取略小于平均晶粒尺寸30 μm作为临界气孔尺寸。据此认为,等效直径小于30 μm的气孔对激光复合焊接头的疲劳性能没有影响。
详细地统计了10个焊接接头试样中,气孔与焊缝上自由表面之间的位置关系,其中横坐标代表气孔的等效直径,纵坐标则表示气孔中心距离焊缝上表面的距离。结果显示,等效直径较小的气孔集中分布于焊缝下部,而等效直径较大的气孔多位于距焊缝上表面400 μm的范围内。这是由于铝合金的化学性质活泼,表面极易形成氧化膜,焊接过程中母材中氧化物、焊丝表面污物以及空气和保护气中的水分在高能密度热源作用下分解出氢,在冷却过程中游离的氢又快速形成小气孔并上浮和长大,但由于熔池冷却速率极快,一部分气孔来不及逸出表面便遗留在焊缝中,形成气孔缺陷。统计发现,这种气孔大多为近球形,圆整度在0.65以上[]。后文中疲劳断口形貌分析均发现断面气孔的内壁光滑、洁净、无氧化夹杂物痕迹,是一类典型的冶金型氢气孔。
图3-8-1131/img_3.png图3
上海光源与欧洲光源原位成像疲劳试样中不同尺寸的气孔在焊缝中的位置分布 Fig.3
Position distribution of gas pores inside hybrid laser welded 7020 aluminum alloy from in situ imaging fatigue specimens at SSRF and ESRF
Fig.3-8-1131/img_3.png图3
上海光源与欧洲光源原位成像疲劳试样中不同尺寸的气孔在焊缝中的位置分布 Fig.3
Position distribution of gas pores inside hybrid laser welded 7020 aluminum alloy from in situ imaging fatigue specimens at SSRF and ESRF
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上海光源与欧洲光源原位成像疲劳试样中不同尺寸的气孔在焊缝中的位置分布
Position distribution of gas pores inside hybrid laser welded 7020 aluminum alloy from in situ imaging fatigue specimens at SSRF and ESRF
为一个典型激光复合焊7020铝合金接头的3D重构结果。由可以更直观地得出,在焊缝下部气孔密集并且尺寸较小;而焊缝上部气孔较为稀疏且尺寸较大,观测结果与中的统计结果基本一致。必须指出,如所示,焊缝上部存在形貌不规则或者小圆整度的大尺寸气孔,这主要是由于熔池前沿的液态金属局部熔体发生了扰动。该种气孔与因焊接工艺不匹配导致的气孔不同,它仍然属于冶金型气孔,这正是气孔越大圆整度越小的重要原因。极端的情况是,如果参数不匹配或者焊接过程不稳定,则形成工艺型气孔,如所示。这类气孔不仅是焊接质量不合格的直接证据,在疲劳加载中也极易成为裂纹源。
图4-8-1131/img_4.png图4
典型激光复合焊接7020铝合金接头的同步辐射X射线气孔分布 Fig.4
Synchrotron radiation X-ray based porosity imaging result of a typical 7020 aluminum alloy due to a hybrid laser welding process
Fig.4-8-1131/img_4.png图4
典型激光复合焊接7020铝合金接头的同步辐射X射线气孔分布 Fig.4
Synchrotron radiation X-ray based porosity imaging result of a typical 7020 aluminum alloy due to a hybrid laser welding process
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典型激光复合焊接7020铝合金接头的同步辐射X射线气孔分布
Synchrotron radiation X-ray based porosity imaging result of a typical 7020 aluminum alloy due to a hybrid laser welding process
图5-8-1131/img_5.png图5
激光复合焊接7020铝合金焊接接头中的工艺型气孔同步辐射X射线成像结果 Fig.5
Synchrotron radiation X-ray imaging result of process-induced pores inside a 7020 aluminum alloy due to a hybrid laser welding process
Fig.5-8-1131/img_5.png图5
激光复合焊接7020铝合金焊接接头中的工艺型气孔同步辐射X射线成像结果 Fig.5
Synchrotron radiation X-ray imaging result of process-induced pores inside a 7020 aluminum alloy due to a hybrid laser welding process
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激光复合焊接7020铝合金焊接接头中的工艺型气孔同步辐射X射线成像结果
Synchrotron radiation X-ray imaging result of process-induced pores inside a 7020 aluminum alloy due to a hybrid laser welding process
2.2 气孔对疲劳寿命的影响
同步辐射X射线三维成像疲劳实验发现,近表面单个大尺寸气孔、多个交互式气孔、角气孔以及小气孔聚簇往往成为应力集中和疲劳裂纹萌生的重要区域[,]。为了定量化表征气孔特征尺寸和几何形貌,Murakami[]最早提出采用缺陷影响区大小A来描述缺陷尺寸,A是三维缺陷(如气孔、夹杂、裂纹、缺口等)在垂直于最大主应力方向上投影面积的平方根。A的引入很好地解决了不规则形貌缺陷的尺寸定义问题,从而精巧地把缺陷尺寸大小与材料的疲劳性能联系了起来。与此同时,相关研究工作[,,]还表明,材料的疲劳性能主要与缺陷在垂直于载荷方向平面上投影面积的大小有关,而不是缺陷的3D形貌。因此采用A来对缺陷尺寸进行表征是一种较为理想的处理方式。
引入气孔位置参量h来表示裂纹萌生气孔边界至试样自由表面的最小距离。依据文献[]中的定义,当h=0时该类气孔缺陷被定义为表面缺陷(a);当h&A时此类气孔缺陷可认定为近表面缺陷,此时在气孔缺陷与试样自由表面之间的应力集中区也被认为是气孔缺陷的有效影响面积,因此常用外边界投影尺寸代替气孔边界投影尺寸,来表征气孔缺陷实际的影响区尺寸(b);当h&A时该类气孔缺陷则被称为内部缺陷,此时气孔缺陷边界与试样自由表面之间的应力集中区域不再考虑(c)。
图6-8-1131/img_6.png图6
焊接接头中气孔缺陷位置的分类示意图 Fig.6
Schematics of the pore position classification in the welded joint (A—the square root of the projection area perpendicular to the load direction, h—the minimum distance between the boundary of the crack initiation pore and the free surface of the specimen) (a) h=0, as surface defect (b) h&A, as sub-surface defect(c) h&A, as internal defect
Fig.6-8-1131/img_6.png图6
焊接接头中气孔缺陷位置的分类示意图 Fig.6
Schematics of the pore position classification in the welded joint (A—the square root of the projection area perpendicular to the load direction, h—the minimum distance between the boundary of the crack initiation pore and the free surface of the specimen) (a) h=0, as surface defect (b) h&A, as sub-surface defect(c) h&A, as internal defect
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焊接接头中气孔缺陷位置的分类示意图
Schematics of the pore position classification in the welded joint (A—the square root of the projection area perpendicular to the load direction, h—the minimum distance between the boundary of the crack initiation pore and the free surface of the specimen) (a) h=0, as surface defect (b) h&A, as sub-surface defect(c) h&A, as internal defect
根据上述气孔缺陷位置分类定义原则,对10个同步辐射X射线原位成像疲劳实验结果进行了统计,结果见。从表中可以得到,激光复合焊铝合金焊接接头疲劳裂纹均从表面或者近表面的气孔缺陷处萌生,气孔缺陷尺寸A分布范围为57.81~132.77 μm,均大于临界气孔尺寸所对应的A。
上海光源与欧洲光源原位疲劳实验疲劳寿命、缺陷位置和影响面积结果统计
Fatigue life data related with the defect location and affected area under in situ fatigue microtomography conducted at ESRF and SSRF
考虑到疲劳裂纹萌生的不确定性和复杂性,同步辐射X射线三维成像技术在准确捕捉疲劳裂纹萌生过程的应用中仍存在局限性,因此实验时仍然需要借助扫描电子显微镜(SEM)对疲劳断口进行观测,两者结合最终给出疲劳源区的特征性质。a给出了疲劳试样断口的整体微观形貌。从图中可以清楚地分辨出疲劳断口的3个典型区域,裂纹萌生于表面较大的气孔缺陷处。由b可知,疲劳源区裂纹呈放射状非线性扩展,当裂纹一侧到达试样自由表面之后,表现为1/4椭圆形长裂纹扩展。c为稳定扩展区内疲劳条带的局部图,由于焊缝组织结构和应力状态复杂,疲劳条带发生攀移。d为瞬断区微观断口形貌。从图中可以观察到少量的韧窝结构,同时断面上分布着较大尺寸的气孔,这进一步说明试样发生瞬断时总是沿着材料的最弱截面。
图7-8-1131/img_7.png图7
表面气孔萌生疲劳裂纹且寿命为77340 cyc的试样的断口形貌 Fig.7
Specimen fracture morphologies at Nf =77340 cyc and crack initiation from a surface pore(a) macro morphology of fracture surface (b) fatigue source morphology(c) fatigue striation of stable propagation zone (d) final fracture region
Fig.7-8-1131/img_7.png图7
表面气孔萌生疲劳裂纹且寿命为77340 cyc的试样的断口形貌 Fig.7
Specimen fracture morphologies at Nf =77340 cyc and crack initiation from a surface pore(a) macro morphology of fracture surface (b) fatigue source morphology(c) fatigue striation of stable propagation zone (d) final fracture region
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表面气孔萌生疲劳裂纹且寿命为77340 cyc的试样的断口形貌
Specimen fracture morphologies at Nf =77340 cyc and crack initiation from a surface pore(a) macro morphology of fracture surface (b) fatigue source morphology(c) fatigue striation of stable propagation zone (d) final fracture region
所统计的原位疲劳实验结果是在不同的加载条件下得到的,为了方便10组数据之间的分析和比较,需要对实验结果进行标准化或者等效化处理。Walker[]提供了一种不同应力比实验结果之间标准化转化的方法,应力等效公式为：
式中,σeff表示应力比R=-1时施加的最大等效应力,σmax则表示给定应力比时实验中作用在试样上的最大应力,n为拟合常数。
由于熔焊与铸造相似的热过程,故常数n可采用铸造铝合金数据进行拟合[],拟合结果n=0.47,其值满足Walker公式拟合常数数据范围[]。对中所有数据进行等效化处理,并绘制出最大等效应力与循环周次之间的关系,如所示。同时为了便于比较,激光复合焊铝合金焊接接头2 mm厚标准试样的疲劳寿命实验结果也一并绘制在中。中利用疲劳实验数据带进行描述,结果发现,同步辐射原位疲劳实验结果与标准疲劳实验结果具有基本一致的疲劳寿命规律,即随着施加应力的增加,疲劳寿命逐渐降低。值得注意的是,疲劳源气孔的大小和位置分布不同,也是造成试样疲劳寿命存在较大离散性的原因;另外考虑到同步辐射X射线机时极其有限和宝贵,平均每年最多48 h,使得原位疲劳实验的数据量非常有限,加之实验条件、试验机状态和试样尺寸等方面存在差异,在一定程度上加剧了同步辐射X射线原位疲劳试样寿命的离散程度,尽管如此其仍具有材料疲劳寿命的一般规律。
图8-8-1131/img_8.png图8
疲劳实验和同步辐射X射线原位疲劳实验最大等效应力与循环周次的S-N关系 Fig.8
S-N plot of the maximum effective stress (σeff) and Nf in fatigue test and synchrotron radiation X-ray in situ fatigue test
Fig.8-8-1131/img_8.png图8
疲劳实验和同步辐射X射线原位疲劳实验最大等效应力与循环周次的S-N关系 Fig.8
S-N plot of the maximum effective stress (σeff) and Nf in fatigue test and synchrotron radiation X-ray in situ fatigue test
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疲劳实验和同步辐射X射线原位疲劳实验最大等效应力与循环周次的S-N关系
S-N plot of the maximum effective stress (σeff) and Nf in fatigue test and synchrotron radiation X-ray in situ fatigue test
2.3 气孔处应力状态
前文阐述了疲劳源区气孔缺陷尺寸、气孔缺陷位置及等效应力与焊接接头疲劳寿命之间的关系。本工作参考相关文献[,,],采用有限元仿真分析,进一步探讨了气孔缺陷位置对气孔缺陷处应力状态的影响。假设焊接接头试样为均匀连续体,冶金型气孔等效为空心球体,然后建立体积为3.0 mm×1.0 mm×1.0 mm的长方体含缺陷模型,长方体模型尺寸大小的选取,与同步辐射实验试样关键部位的尺寸一致。开展含气孔接头CAD模型的四面体网格自动划分,气孔等效直径尺寸大小取30 μm,气孔周围的网格尺寸为0.5 μm,网格划分完成后导入有限元软件进行计算。激光复合焊7020铝合金焊接接头的仿真性能参数为：弹性模量E=73.399 GPa,Poisson比v=0.3,边界条件和载荷施加方式参考文献[],如所示。施加的远场载荷大小为σ =200 MPa,该值略低于激光复合焊焊接接头的屈服强度,从而能够模拟弹性过程。考虑到气孔尺寸远小于长方体仿真模型体积,因此认为应力是均匀地施加在气孔上的。
图9-8-1131/img_9.png图9
仿真模型中气孔与焊缝上表面的位置关系示意图及焊缝有限元仿真边界约束条件的施加 Fig.9
The relationship between the position of the pore and the upper surface of the weld in the simulation model and the boundary conditions of the finite element simulation (s—the distance from the pore center to the free surface (X-Y plane);r—S1—X-Z
L1—the intersection line of the X-Z plane
U1, U2 and U3—the translational degrees of freedom along coordinate axis X direction, along coordinate axis Y direction, along coordinate axis Z direction, respectively, σ—the applied stress)
Fig.9-8-1131/img_9.png图9
仿真模型中气孔与焊缝上表面的位置关系示意图及焊缝有限元仿真边界约束条件的施加 Fig.9
The relationship between the position of the pore and the upper surface of the weld in the simulation model and the boundary conditions of the finite element simulation (s—the distance from the pore center to the free surface (X-Y plane);r—S1—X-Z
L1—the intersection line of the X-Z plane
U1, U2 and U3—the translational degrees of freedom along coordinate axis X direction, along coordinate axis Y direction, along coordinate axis Z direction, respectively, σ—the applied stress)
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仿真模型中气孔与焊缝上表面的位置关系示意图及焊缝有限元仿真边界约束条件的施加
The relationship between the position of the pore and the upper surface of the weld in the simulation model and the boundary conditions of the finite element simulation (s—the distance from the pore center to the free surface (X-Y plane);r—S1—X-Z
L1—the intersection line of the X-Z plane
U1, U2 and U3—the translational degrees of freedom along coordinate axis X direction, along coordinate axis Y direction, along coordinate axis Z direction, respectively, σ—the applied stress)
为了准确描述模型中球形气孔与焊缝自由表面的相对位置关系,假设气孔中心位于与Z-Y面平行的试样中部截面上,设气孔中心到焊缝上自由表面(X-Y平面)的距离为s,气孔的半径为r,使用s/r的值对气孔位置进行描述。中详细给出了气孔和焊缝自由上表面之间的位置关系。
为相同加载条件下,试样气孔缺陷处第一主应力σ1的分布规律。a~c分别对应于s/r=-0.2、0和2.7时,平行于X-Z平面过气孔中心切面的主应力场。由图可知,3种位置下应力的最大值基本接近,局部应力超过了焊接接头的屈服强度和抗拉强度,因此在实际的试样内部,气孔缺陷周围会存在相应的局部塑性变形区,塑性变形区的存在成为焊接接头的薄弱位置,也是疲劳裂纹萌生的优先选择区域。仿真结果同时也说明,应力集中出现在垂直于加载方向且过气孔缺陷中心的截面上,分布于气孔缺陷周向部位。该结果一定程度上表明了采用面积参数A来表示缺陷尺寸的合理性。
图10-8-1131/img_10.png图10
平面X-Z上的主应力分布 Fig.10
Principal stress fields across the pore parallel to X-Z plane(a) s/r =-0.2 (b) s/r =0 (c) s/r =2.7
Fig.10-8-1131/img_10.png图10
平面X-Z上的主应力分布 Fig.10
Principal stress fields across the pore parallel to X-Z plane(a) s/r =-0.2 (b) s/r =0 (c) s/r =2.7
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平面X-Z上的主应力分布
Principal stress fields across the pore parallel to X-Z plane(a) s/r =-0.2 (b) s/r =0 (c) s/r =2.7
利用应力集中系数Kt=σ1/σ0来表征气孔导致的应力集中程度(σ0为远端名义应力),对于给定尺寸的气孔,应力集中系数与气孔深度之间的关系如所示。由图可知,当s/r=1,即气孔和自由面相切时,应力集中系数迅速增加,应力集中严重;当s/r&1时,随着气孔深度的增加,应力集中系数迅速减少,直至s/r&1.6之后趋于稳定值;当s/r&1时,随着气孔深度的减小,气孔与自由表面之间交点的过渡角变大,应力集中程度减小,应力集中系数逐渐降低。依据内部气孔的定义,计算可知当s/r=2.7时,可认为气孔缺陷属于内部缺陷,此时气孔所对应的应力集中系数为平稳值1.55。前文论述和实验结果均表明疲劳试样很少从内部气孔开裂,即可认为当存在较大应力集中表面或近表面缺陷时,应力集中系数为1.55的内部气孔是不会引起裂纹萌生的。综上,对于激光复合焊7020铝合金焊接接头内部,应力集中系数小于1.55且直径小于30 μm的内部气孔,对焊接接头的疲劳性能没有影响。为真实地模拟气孔周围的应力集中程度,同时给出了考虑应力-应变关系的弹塑性仿真结果。应力集中引起气孔周围局部的塑性变形,造成应力释放,从而减小了气孔周围应力的集中程度,塑性变形区成为疲劳裂纹萌生的主要选择点。
图11-8-1131/img_11.png图11
弹性和弹塑性仿真模拟不同位置时气孔周围应力集中系数的变化曲线 Fig.11
Elastic and elastic-plastic simulations of stress concentration curves near the pores
Fig.11-8-1131/img_11.png图11
弹性和弹塑性仿真模拟不同位置时气孔周围应力集中系数的变化曲线 Fig.11
Elastic and elastic-plastic simulations of stress concentration curves near the pores
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弹性和弹塑性仿真模拟不同位置时气孔周围应力集中系数的变化曲线
Elastic and elastic-plastic simulations of stress concentration curves near the pores
相关研究也发现与类似的变化规律[,,],虽然在材料种类、气孔大小、气孔形貌以及外加应力上存在差异,但总体变化趋势基本一致。这些分析虽然是基于静力学仿真得到,但一般认为裂纹萌生与严重的应力集中有关,因此仿真分析结果仍然具有普遍的适用性。
2.4 气孔与疲劳裂纹扩展
气孔周围的材料单元容易产生局部塑性变形,从而引起短裂纹扩展加速,一旦穿过气孔,短裂纹前缘又会因为处于三向应力状态而减速[]。与长裂纹不同,气孔对疲劳短裂纹扩展的影响更为复杂,并且短裂纹行为也与材料微观结构和外部载荷类型密切相关[],例如晶粒尺寸、晶界、第二相、夹杂、应力状态、环境氛围等都会对短裂纹扩展产生很大的影响[],因此很难精准描述和定量表征短裂纹前缘扩展特性,可见关于气孔对短裂纹扩展过程的影响仍有待深入研究。
对于疲劳长裂纹扩展而言,当裂纹萌生于气孔且气孔尺寸较大时,裂纹尖端循环塑性变形区属于小范围屈服,故仍可采用经典的线弹性断裂力学理论和方法研究其扩展行为。Paris-Erdogan[]提供了一种比较简单的长裂纹扩展寿命的估算方法,它把裂纹尖端场的应力强度因子幅与扩展速率建立起了联系。
基于同步辐射X射线三维成像的原位疲劳实验结果表明,疲劳裂纹扩展中出现了切割气孔的现象[]。本课题组的相关研究[]发现,铝合金激光复合焊接头疲劳裂纹前缘的微小气孔,对裂纹扩展局部形貌和局部扩展速率影响较大,但对裂纹总体扩展速率的影响似乎并不明显。为了进一步探讨这一现象,选择3组激光复合焊铝合金焊接接头试样,开展疲劳裂纹扩展速率实验,见。从中可以清楚地看出,焊缝中气孔的存在,使得裂纹萌生区存在比较显著的扰动现象(同一焊接参数的接头的扩展速率变化较大),这表明气孔影响着焊接接头疲劳裂纹的萌生行为,充分表明对萌生裂纹的气孔进行深入研究的必要性;而在稳定扩展区,3组接头表现出基本一致的扩展行为,表明一旦裂纹扩展,可以忽略焊缝中气孔对整体裂纹扩展速率的影响作用;此外当处于瞬断区时,由于外部载荷不变,材料减薄明显,裂纹会沿着焊缝中存在较大尺寸气孔的薄弱部位扩展,从而导致裂纹扩展出现明显的偏离,疲劳断口的瞬断区出现明显的台阶型断口特征,这一现象也可用d来解释。当裂纹逼近气孔时,在材料局部区域会发生反遮蔽效应,裂纹与气孔之间的距离越小反遮蔽效应越强,从而诱导裂纹转向,并进入气孔[]。一旦裂纹前缘进入气孔时,疲劳裂纹的局部扩展路径和形貌会发生明显变化,而由于气孔的钝化作用,使得裂纹扩展速率降低[]。然而,当主裂纹前缘气孔尺寸较大、分布较为密集或者链状气孔时,疲劳裂纹具有较高的扩展速率,这表明气孔的存在会加速裂纹的扩展[]。因此气孔对裂纹扩展的影响,取决于裂纹前缘至气孔的距离以及裂纹扩展路径上气孔的位置、数量和分布。
图12-8-1131/img_12.png图12
3个激光复合焊7020铝合金的裂纹扩展速率 Fig.12
Fatigue crack growth rates of three laser hybrid welded 7020 aluminum alloy joints under stress ratio R=0.1 and mean thickness t=1.84 mm
Fig.12-8-1131/img_12.png图12
3个激光复合焊7020铝合金的裂纹扩展速率 Fig.12
Fatigue crack growth rates of three laser hybrid welded 7020 aluminum alloy joints under stress ratio R=0.1 and mean thickness t=1.84 mm
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3个激光复合焊7020铝合金的裂纹扩展速率
Fatigue crack growth rates of three laser hybrid welded 7020 aluminum alloy joints under stress ratio R=0.1 and mean thickness t=1.84 mm
综上所述,在相同的微结构特征、外部加载和环境条件下,当主裂纹前缘气孔较少时,裂纹一旦开始稳定扩展,气孔对整体裂纹扩展速率几乎没有显著的作用和影响。另一方面,在焊接结构的疲劳设计中,更应该关注气孔的位置和尺寸,尤其是位于表面或者近表面的气孔。
(1) 激光复合焊7020铝合金中等效尺寸为30~50 μm的气孔仅占气孔总数的2.8%;等效尺寸大于50 μm的则更少,仅占气孔总数的0.1%;焊接接头焊缝区平均晶粒尺寸为33 μm,依据气孔尺寸和微观晶粒尺寸之间的关系,选取略小于焊缝区平均晶粒尺寸(30 μm),作为临界气孔尺寸。
(2) 同步辐射X射线原位疲劳实验结果表明,疲劳裂纹多萌生于表面和近表面较大气孔处或者近表面簇集的小气孔处;焊接接头的疲劳性能符合一般材料的疲劳性能规律,且随着气孔尺寸和外加应力的增大,疲劳寿命逐渐减小。
(3) 仿真分析结果显示,对于激光复合焊接7020铝合金内部,应力集中系数小于1.55且直径小于30 μm的内部气孔,对焊接接头的疲劳性能没有影响;当接头中气孔大于临界尺寸30 μm时,疲劳裂纹常常萌生于气孔引起的塑性变形区部位。
The authors have declared that no competing interests exist.
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The stress field around a pore was analyzed as a function of the pore position in depth in the surface of a linear elastic solid using finite element modeling. It was found that the pore depth dominated the stress field around the pore on the surface and that the maximum stress was increased sharply when the pore intercepted with the surface at its top. Given the applied nominal stress, the magnitude of the maximum main stress only depended on the relative depth of the pore, while the pore size affected the stress distribution in the surface. An elastic-plastic model was also used to account for the yielding effect in the region where stress was over the yield strength. The results still indicated a significant maximum stress concentration when the pore was just buried underneath the surface, but with a lowered value than that of the linear elastic model. These results were consistent with the experimental observations that fatigue cracks were preferably initiated from pores and particles, which were just intercepted at their top with the sample surface or just buried beneath the surface.
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Tammas-Williams S, Withers P J, Todd I, et al.The influence of porosity on fatigue crack initiation in additively manufactured titanium components[J]. Sci. Rep., 2017, 7: 7308
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Li P, Lee P D, Maijer D M, et al.Quantification of the interaction within defect populations on fatigue behavior in an aluminum alloy[J]. Acta Mater., 2009, 57: 3539
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Walker K.The effect of stress ratio during crack propagation and fatigue for 2024-T3 and 7075-T6 aluminum [A]. Effects of Environment and Complex Load History on Fatigue Life[C]. West Conshohocken, PA: ASTM, 1970, 462: 1
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Dowling N E, Calhoun C A, Arcari A.Mean stress effects in stress-life fatigue and the Walker equation[J]. Fatigue Fract. Eng. Mater. Struct., 2009, 32: 163
ABSTRACT Mean stress effects in finite-life fatigue are studied for a number of sets of experimental data for steels, aluminium alloys and one titanium alloy. Specifically, the agreement with these data is examined for the Goodman, Morrow, Smith–Watson–Topper and Walker equations. The Goodman relationship is found to be highly inaccurate. Reasonable accuracy is provided by the Morrow and by the Smith–Watson–Topper equations. But the Morrow method should not be used for aluminium alloys unless the true fracture strength is employed, instead of the more usual use of the stress-life intercept constant. The Walker equation with its adjustable fitting parameter γ gives superior results. For steels, γ is found to correlate with the ultimate tensile strength, and a linear relationship permits γ to be estimated for cases where non-zero mean stress data are not available. Relatively high-strength aluminium alloys have γ≈ 0.5, which corresponds with the SWT method, but higher values of γ apply for relatively low-strength aluminium alloys. For both steels and aluminium alloys, there is a trend of decreasing γ with increasing strength, indicating an increasing sensitivity to mean stress.
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Borbély A, Mughrabi H, Eisenmeier G, et al.A finite element modelling study of strain localization in the vicinity of near-surface cavities as a cause of subsurface fatigue crack initiation[J]. Int. J. Fract., 2002, 115: 227
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Dezecot S, Maurel V, Buffiere J Y, et al.3D characterization and modeling of low cycle fatigue damage mechanisms at high temperature in a cast aluminum alloy[J]. Acta Mater., 2017, 123: 24
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Gerard D A, Koss D A.The influence of porosity on short fatigue crack growth at large strain amplitudes[J]. Int. J. Fatigue, 1991, 13: 345
The influence of pore microstructure (0% to 6% rounded porosity, isolated vs. interconnected) on short crack growth during low cycle fatigue has been evaluated using powder-processed titanium as a model. The presence of porosity enhances short crack propagation adjacent to the pores as a result of localized, pore-induced plasticity. Observations of short cracks using surface replicas indicate that once a crack extends beyond a plastic zone of an isolated pore it decelerates to propagation rates similar to those observed for fully dense materials of similar grain sizes. For low levels of porosity in which the pores are isolated, the vast majority of short crack growth occurs outside the pore-induced plastic zones, and thus the overall short crack growth rates are not significantly affected. However, when the pores are interconnected, short cracks propagate at higher rates of growth than cracks of comparable size in the dully dense material. The enhanced growth rates appear to be a result of the continuous nature of the plastic zones which provide a high strain amplitude propagation path.
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(莫德锋, 何国求, 胡正飞等. 孔洞对铸造铝合金疲劳性能的影响[J]. 材料工程, 2010, (7): 92)
Mo D F, He G Q, Hu Z F, et al.Effect of porosity on fatigue property in aluminum cast alloys[J]. J. Mater. Eng., 2010, (7): 92
研究了铸造铝合金中孔洞在裂纹萌生、扩展过程中发挥的作用,孔洞的尺寸、体积分数、分布位 置、形貌率等因素对材料的疲劳性能有重要影响。从试验现象和疲劳寿命模型两个方面综述了国内外学者研究的最新进展,指出了应力-寿命模型与线弹性断裂模型 之间的内在联系,并讨论了存在的问题及研究方向。
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Paris P C, Erdogan F.A critical analysis of crack propagation laws[J]. J. Basic Eng., 1963, 85: 528
ABSTRACT Mass and enthalpy balances are necessary for the calculation and evaluation of process plants. The economical and ecological results cannot be optimal because the different values of energy, according to the second law of thermodynamics, and the influence of the environment are not included. This is accomplished by analysis of exergy and evaluation of any process plant through combination with a flowsheeting system. A suitable environmental model in regard to the above-mentioned objectives is proposed, since the exergy depends on the choice of a state of reference.
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Zhang H, Toda H, Hara H, et al.Three-dimensional visualization of the interaction between fatigue crack and micropores in an aluminum alloy using synchrotron X-ray microtomography[J]. Metall. Mater. Trans., 2007, 38A: 1774
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Tsay L W, Shan Y P, Chao Y H, et al.The influence of porosity on the fatigue crack growth behavior of Ti-6Al-4V laser welds[J]. J. Mater. Sci., 2006, 41: 7498
The effect of porosity––a common welding defect––on the fatigue crack growth rate (FCGR) in Ti–6Al–4V laser welds was investigated. The experimental results reveal that porosity was present in partial penetration welds over a narrow fusion zone (FZ) with martensite structure. The FCGR of the FZ was lower than that of the base plate. The fracture surface morphology of weld metal was much rougher as compared to that of the base plate. Randomly oriented martensite in the FZ led to local cleavage fracture along a preferred plane, thus, altering the crack growth direction significantly out of the primary crack plane. The zigzag crack path in the FZ resulted in a reduced FCGR at a given Δ K compared to the base plate. Besides, the porous weld showed a serration on the crack growth curve, and behaved the similar crack growth characteristics as the defect free one. SEM fractography revealed that the deflection of crack path around porosity together with local notch blunting as the crack tip pierced into porosity, balanced the increased FCGR for the occurrence of instant crack advance as the crack front reached the porosity at a low stress ratio. In contrast, the serration and drop in FCGR occurred sparingly at a high stress ratio as the crack front met the porosity.
[本文引用: 1]
铝合金熔焊微气孔的三维同步辐射X射线成像
... 中强度和焊接性好的可热处理7020铝合金在室温下具有较高的强度、优良的耐蚀性和良好的时效强化能力,常被用于不便于进行热处理以及对焊接工艺要求较高的关键承载部件,如高速列车的车体底架、枕梁、受电弓和轴箱等.传统铝合金弧焊接头的组织粗大、裂纹较多、变形严重,因此铝合金接头成为整个焊接结构的薄弱部位[1].激光复合焊接方法具有高效率、小变形和优柔性的技术特点,在高速列车关键部件及结构制造中具有广阔的应用前景[2]. ...
... 图3详细地统计了10个焊接接头试样中,气孔与焊缝上自由表面之间的位置关系,其中横坐标代表气孔的等效直径,纵坐标则表示气孔中心距离焊缝上表面的距离.结果显示,等效直径较小的气孔集中分布于焊缝下部,而等效直径较大的气孔多位于距焊缝上表面400 μm的范围内.这是由于铝合金的化学性质活泼,表面极易形成氧化膜,焊接过程中母材中氧化物、焊丝表面污物以及空气和保护气中的水分在高能密度热源作用下分解出氢,在冷却过程中游离的氢又快速形成小气孔并上浮和长大,但由于熔池冷却速率极快,一部分气孔来不及逸出表面便遗留在焊缝中,形成气孔缺陷.统计发现,这种气孔大多为近球形,圆整度在0.65以上[1].后文中疲劳断口形貌分析均发现断面气孔的内壁光滑、洁净、无氧化夹杂物痕迹,是一类典型的冶金型氢气孔. ...
高速列车6005A铝合金型材焊接热裂纹分析
... 中强度和焊接性好的可热处理7020铝合金在室温下具有较高的强度、优良的耐蚀性和良好的时效强化能力,常被用于不便于进行热处理以及对焊接工艺要求较高的关键承载部件,如高速列车的车体底架、枕梁、受电弓和轴箱等.传统铝合金弧焊接头的组织粗大、裂纹较多、变形严重,因此铝合金接头成为整个焊接结构的薄弱部位[1].激光复合焊接方法具有高效率、小变形和优柔性的技术特点,在高速列车关键部件及结构制造中具有广阔的应用前景[2]. ...
... 研究[3]发现,铝合金焊接熔池金属在快速冷却过程中因过饱和氢的析出,会形成数量、尺寸、形状和位置等分布复杂的冶金型气孔,并且焊接时的热输入越小,气孔尺寸和气孔体积分数越小.焊接接头中冶金型气孔的存在降低了接头的有效承载面积,同时位于焊缝表面和近表面的形貌不规则气孔,在疲劳循环载荷作用下产生严重的应力集中,从而引起微裂纹的萌生,导致焊接结构的失效破坏.随着结构轻量化的迅速发展,铝合金结构越来越多地被用于高速列车中,由于服役条件的极端化,接头的载荷历程更加严苛,因此迫切需要对焊缝气孔与接头疲劳性能的关系进行深入系统的研究. ...
... 长期以来,各国学者普遍采用测量实验样品表面的裂纹长度和观测样品的疲劳断口等方法来揭示材料的疲劳损伤行为,并依此对结构部件的疲劳失效机制进行定量表征,从而建立了工程结构抗疲劳断裂设计的基础理论和评价体系.然而,金属材料中气孔类缺陷引起的疲劳开裂与无缺陷材料的疲劳损伤机理有较大不同,相关理论方法不能直接适用于焊接结构[4].高精度、高亮度、高准直、非破坏性的第三代同步辐射X射线成像技术(SR-μCT)使得学者可以深入到材料内部,可视化观测和追踪疲劳损伤过程及演变规律,已经成为表征材料内部缺陷和追踪裂纹演变的无可替代的科学工具[5]. ...
... 疲劳裂纹萌生与材料特性、应力分布、表面状态、载荷大小、样品形状及环境条件等多种因素有关,但一般认为尺寸较大和形貌不规则的表面或近表面气孔缺陷,更容易引起疲劳裂纹萌生[4,6~9].尽管焊缝内部大尺寸气孔较少,但其对接头疲劳性能和寿命的影响仍不容忽视.同时,焊缝中链状小气孔对接头疲劳性能的影响也不能完全忽略,如2个距离很近的小气孔发生耦合成为裂纹[9]. ...
... 长期以来,各国学者普遍采用测量实验样品表面的裂纹长度和观测样品的疲劳断口等方法来揭示材料的疲劳损伤行为,并依此对结构部件的疲劳失效机制进行定量表征,从而建立了工程结构抗疲劳断裂设计的基础理论和评价体系.然而,金属材料中气孔类缺陷引起的疲劳开裂与无缺陷材料的疲劳损伤机理有较大不同,相关理论方法不能直接适用于焊接结构[4].高精度、高亮度、高准直、非破坏性的第三代同步辐射X射线成像技术(SR-μCT)使得学者可以深入到材料内部,可视化观测和追踪疲劳损伤过程及演变规律,已经成为表征材料内部缺陷和追踪裂纹演变的无可替代的科学工具[5]. ...
... 对于含大基数冶金型气孔的熔焊铝合金或铸造铝合金来说,并不是所有气孔都会诱导疲劳裂纹的萌生,只有当气孔大到某种尺度时,才会对疲劳性能有影响.目前,国内外学者广泛开展了铸造合金气孔与疲劳性能之间关系的研究,获得临界气孔尺寸.例如在考察微观组织与气孔的关系中,发现当铸造357铝合金中的二次枝晶臂间距(SDAS)小于40 μm时,临界气孔尺寸应在晶粒尺寸范围内,否则应在SDAS尺寸范围内,据此得到引起疲劳损伤的临界气孔尺寸为155 μm [10].类似的研究[7]发现,气孔尺寸大于80~100 μm时,疲劳裂纹优先从气孔处萌生,反之则从近表面的较大共晶组织处萌生.一些学者还率先借助SR-μCT研究了热等静压铸造合金的疲劳行为,给出了铸造Al-Si7-Mg0.3和铸造A356-T6合金中的临界气孔尺寸分别为50 μm[5]和25 μm[6]. ...
... 疲劳裂纹萌生与材料特性、应力分布、表面状态、载荷大小、样品形状及环境条件等多种因素有关,但一般认为尺寸较大和形貌不规则的表面或近表面气孔缺陷,更容易引起疲劳裂纹萌生[4,6~9].尽管焊缝内部大尺寸气孔较少,但其对接头疲劳性能和寿命的影响仍不容忽视.同时,焊缝中链状小气孔对接头疲劳性能的影响也不能完全忽略,如2个距离很近的小气孔发生耦合成为裂纹[9]. ...
... 对于含大基数冶金型气孔的熔焊铝合金或铸造铝合金来说,并不是所有气孔都会诱导疲劳裂纹的萌生,只有当气孔大到某种尺度时,才会对疲劳性能有影响.目前,国内外学者广泛开展了铸造合金气孔与疲劳性能之间关系的研究,获得临界气孔尺寸.例如在考察微观组织与气孔的关系中,发现当铸造357铝合金中的二次枝晶臂间距(SDAS)小于40 μm时,临界气孔尺寸应在晶粒尺寸范围内,否则应在SDAS尺寸范围内,据此得到引起疲劳损伤的临界气孔尺寸为155 μm [10].类似的研究[7]发现,气孔尺寸大于80~100 μm时,疲劳裂纹优先从气孔处萌生,反之则从近表面的较大共晶组织处萌生.一些学者还率先借助SR-μCT研究了热等静压铸造合金的疲劳行为,给出了铸造Al-Si7-Mg0.3和铸造A356-T6合金中的临界气孔尺寸分别为50 μm[5]和25 μm[6]. ...
... 对于含大基数冶金型气孔的熔焊铝合金或铸造铝合金来说,并不是所有气孔都会诱导疲劳裂纹的萌生,只有当气孔大到某种尺度时,才会对疲劳性能有影响.目前,国内外学者广泛开展了铸造合金气孔与疲劳性能之间关系的研究,获得临界气孔尺寸.例如在考察微观组织与气孔的关系中,发现当铸造357铝合金中的二次枝晶臂间距(SDAS)小于40 μm时,临界气孔尺寸应在晶粒尺寸范围内,否则应在SDAS尺寸范围内,据此得到引起疲劳损伤的临界气孔尺寸为155 μm [10].类似的研究[7]发现,气孔尺寸大于80~100 μm时,疲劳裂纹优先从气孔处萌生,反之则从近表面的较大共晶组织处萌生.一些学者还率先借助SR-μCT研究了热等静压铸造合金的疲劳行为,给出了铸造Al-Si7-Mg0.3和铸造A356-T6合金中的临界气孔尺寸分别为50 μm[5]和25 μm[6]. ...
... 疲劳裂纹萌生与材料特性、应力分布、表面状态、载荷大小、样品形状及环境条件等多种因素有关,但一般认为尺寸较大和形貌不规则的表面或近表面气孔缺陷,更容易引起疲劳裂纹萌生[4,6~9].尽管焊缝内部大尺寸气孔较少,但其对接头疲劳性能和寿命的影响仍不容忽视.同时,焊缝中链状小气孔对接头疲劳性能的影响也不能完全忽略,如2个距离很近的小气孔发生耦合成为裂纹[9]. ...
... [9]. ...
... 同步辐射X射线三维成像疲劳实验发现,近表面单个大尺寸气孔、多个交互式气孔、角气孔以及小气孔聚簇往往成为应力集中和疲劳裂纹萌生的重要区域[9,19].为了定量化表征气孔特征尺寸和几何形貌,Murakami[20]最早提出采用缺陷影响区大小A来描述缺陷尺寸,A是三维缺陷(如气孔、夹杂、裂纹、缺口等)在垂直于最大主应力方向上投影面积的平方根.A的引入很好地解决了不规则形貌缺陷的尺寸定义问题,从而精巧地把缺陷尺寸大小与材料的疲劳性能联系了起来.与此同时,相关研究工作[21,22,23]还表明,材料的疲劳性能主要与缺陷在垂直于载荷方向平面上投影面积的大小有关,而不是缺陷的3D形貌.因此采用A来对缺陷尺寸进行表征是一种较为理想的处理方式. ...
... 对于含大基数冶金型气孔的熔焊铝合金或铸造铝合金来说,并不是所有气孔都会诱导疲劳裂纹的萌生,只有当气孔大到某种尺度时,才会对疲劳性能有影响.目前,国内外学者广泛开展了铸造合金气孔与疲劳性能之间关系的研究,获得临界气孔尺寸.例如在考察微观组织与气孔的关系中,发现当铸造357铝合金中的二次枝晶臂间距(SDAS)小于40 μm时,临界气孔尺寸应在晶粒尺寸范围内,否则应在SDAS尺寸范围内,据此得到引起疲劳损伤的临界气孔尺寸为155 μm [10].类似的研究[7]发现,气孔尺寸大于80~100 μm时,疲劳裂纹优先从气孔处萌生,反之则从近表面的较大共晶组织处萌生.一些学者还率先借助SR-μCT研究了热等静压铸造合金的疲劳行为,给出了铸造Al-Si7-Mg0.3和铸造A356-T6合金中的临界气孔尺寸分别为50 μm[5]和25 μm[6]. ...
... 铝合金的熔焊过程实际上是焊接母材和填充焊丝的重熔过程,微观组织和缺陷分布与铸造铝合金具有一定的相似性,因此可借鉴铸造铝合金疲劳行为的研究思路来揭示熔焊接头的疲劳机制.结果[10]发现,铸造铝合金疲劳裂纹主要萌生于表面或近表面缺陷处;当不存在表面或近表面缺陷时,微观结构特性将成为诱导裂纹的重要机制.一般很难观察到铸造铝合金的内部气孔开裂现象,这主要是由于内部裂纹萌生需要更大的塑性区尺寸;另外内部气孔周围近似于真空环境,裂纹扩展速率相对较慢,当内部气孔与表面气孔同时萌生裂纹时,断裂主要由表面裂纹决定,因此很少观察到高周疲劳下内部气孔的开裂行为[11,12]. ...
... 铝合金的熔焊过程实际上是焊接母材和填充焊丝的重熔过程,微观组织和缺陷分布与铸造铝合金具有一定的相似性,因此可借鉴铸造铝合金疲劳行为的研究思路来揭示熔焊接头的疲劳机制.结果[10]发现,铸造铝合金疲劳裂纹主要萌生于表面或近表面缺陷处;当不存在表面或近表面缺陷时,微观结构特性将成为诱导裂纹的重要机制.一般很难观察到铸造铝合金的内部气孔开裂现象,这主要是由于内部裂纹萌生需要更大的塑性区尺寸;另外内部气孔周围近似于真空环境,裂纹扩展速率相对较慢,当内部气孔与表面气孔同时萌生裂纹时,断裂主要由表面裂纹决定,因此很少观察到高周疲劳下内部气孔的开裂行为[11,12]. ...
... 铝合金的熔焊过程实际上是焊接母材和填充焊丝的重熔过程,微观组织和缺陷分布与铸造铝合金具有一定的相似性,因此可借鉴铸造铝合金疲劳行为的研究思路来揭示熔焊接头的疲劳机制.结果[10]发现,铸造铝合金疲劳裂纹主要萌生于表面或近表面缺陷处;当不存在表面或近表面缺陷时,微观结构特性将成为诱导裂纹的重要机制.一般很难观察到铸造铝合金的内部气孔开裂现象,这主要是由于内部裂纹萌生需要更大的塑性区尺寸;另外内部气孔周围近似于真空环境,裂纹扩展速率相对较慢,当内部气孔与表面气孔同时萌生裂纹时,断裂主要由表面裂纹决定,因此很少观察到高周疲劳下内部气孔的开裂行为[11,12]. ...
... 有关气孔临界尺寸的研究不仅对工艺改进和性能预测具有重要意义,而且在进行仿真计算和无损探伤评价中还可以据此过滤掉那些数量众多的小气孔.前述实验和理论探索均表明,应该同时把合金组织特征长度和气孔尺寸作为表征材料疲劳行为的重要因素,相关研究[13]结论也充分证明当气孔或者缺陷的尺寸接近甚至小于材料的平均晶粒尺度时,其对材料疲劳性能的影响可忽略不计.然而迄今为止,国内外针对焊缝气孔与接头疲劳行为之间关系的研究仍处于空白状态[14,15,16].近年来,3D打印技术(或者增材制造技术)的迅速发展及其在航空航天和轨道交通车辆关键结构中的广阔应用前景,使得人们对于增材制造材料中典型缺陷与损伤行为之间的相关性关系也产生了浓厚的兴趣. ...
... 引入气孔位置参量h来表示裂纹萌生气孔边界至试样自由表面的最小距离.依据文献[13]中的定义,当h=0时该类气孔缺陷被定义为表面缺陷(图6a);当h&A时此类气孔缺陷可认定为近表面缺陷,此时在气孔缺陷与试样自由表面之间的应力集中区也被认为是气孔缺陷的有效影响面积,因此常用外边界投影尺寸代替气孔边界投影尺寸,来表征气孔缺陷实际的影响区尺寸(图6b);当h&A时该类气孔缺陷则被称为内部缺陷,此时气孔缺陷边界与试样自由表面之间的应力集中区域不再考虑(图6c). ...
... 由于熔焊与铸造相似的热过程,故常数n可采用铸造铝合金数据进行拟合[13],拟合结果n=0.47,其值满足Walker公式拟合常数数据范围[25].对表1中所有数据进行等效化处理,并绘制出最大等效应力与循环周次之间的关系,如图8所示.同时为了便于比较,激光复合焊铝合金焊接接头2 mm厚标准试样的疲劳寿命实验结果也一并绘制在图8中.图8中利用疲劳实验数据带进行描述,结果发现,同步辐射原位疲劳实验结果与标准疲劳实验结果具有基本一致的疲劳寿命规律,即随着施加应力的增加,疲劳寿}