Study on typical hole defects in AZ91D magnesium alloy prepared by low pressure lost foam casting

The hole defects can easily occur in magnesium alloy castings that are prepared by low pressure lost foam casting(LP-LFC)process when the process parameters such as vacuum,pouring temperature and f illing velocity are not properly selected.In this study,the forming mechanism of the hole defects in AZ91D magnesium castings by LP-LFC process was investigated.The shape,location and surface appearance of the hole defects were observed using optical microscopy and scanning electron microscopy,and the chemical composition on the surface of the holes was analyzed using energy spectrometer.The result indicates that there are two types of hole defects,i.e.,the pyrolysis products related hole defects,including concentrative hole and blow hole defects,and slag related hole defects.The concentrative hole and the blow-hole defects were formed either by the liquidEPS degradation products entrapped in the molten metal under the condition that the pouring temperature is equal to or lower than 730℃,or by the hindered transport of EPS pyrolysis products.Some irregular shape hole defects were caused by slag or by coating slough entrapment when the pouring temperature is equal to 750℃and the f illing velocity is equal to or greater than 100 mm·s-1.To reduce or eliminate the hole defects,the vacuum and f illing velocity must be properly chosen to ensure that the metal front prof ile exhibits convex shape and in laminar current state,and the pouring temperature should be just high enough to ensure that the molten melt has adequate heat energy to complete the foam pyrolysis and to fully occupy the mould.For AZ91D magnesium castings in this study,the parameters should be 730℃pouring temperature,0.02-0.03 MPa vacuum and 80mm·s-1f illing velocity.

Role of oxides in the formation of hole defects in friction stir welded joint of 2519-T87 aluminum alloy

The role of oxides in the formation of hole defects in friction stir welded joint of 2519-T87 aluminum alloy has been investigated by using optical microscope, scanning electron microscope, electron backscatter diffraction and electron probe microanalyzer to examine the distribution of oxides and the features of hole defects, and using ABAQUS 3D thermo-mechanical coupling finite element model based on arbitrary Lagrangian-Eulerian method to simulate the material flow behavior. Oxides exist at the edge of tunnel hole and in the micropores in the joint. Based on distribution of oxygen and material flow behavior, it is believed that the oxides on the surface of the alloy tend to flow down into the bulk along the flow direction of plastic material during friction stir welding, aggregate in the weak region of material flow at the intersection of the shoulder affected zone and the stir pin-tip affected zone, and consequently prevent the material from contacting and diffusing. Due to the insufficient material flow and therefore the small plastic deformation,the pressure is not high enough to compress the accumulated oxides, resulting in hole defects.

Lamb wave topological imaging for blind hole defects in isotropic plates

The research aims at validating the ability of topological imaging to blind holes in isotropic plates using Lamb waves. Due to the defect is not symmetric around the mid- plane of the plate, the effect of Lamb mode conversion will have to be taken into account. The imaging method is based on two computations of ultrasonic fields, one forward and one adjoint, performed for the defect-free reference medium. The excited signal and scattered Lamb waves caused by the blind hole, are used as emitting sources to compute the forward problem and the adjoint problem, respectively. With the help of the finite element simulations, the natural refocusing process of the multimode Lamb waves at the defect location is visually demonstrated by the transient acoustic field snapshots at the different moments to strengthen the physical mechanism of the topological imaging method. The numerical results demonstrate that topological imaging has relatively stronger applicability to the blind hole in contrast to classical Delay And Sum （DAS） method and Time Reversal （TR） method. The topological imaging could handle complex Lamb wave signals containing mode conversions without the imaging quality being affected. The proposed imaging method presents a certain developing potential for detecting and imaging asymmetric defects in plate-like configurations using Lamb waves.

Characteristics of central visual field defect after macular hole surgery

AIM: To investigate the characteristics of postoperative central visual field defect(cVFD) in patients with macular hole(MH).METHODS: Eighteen eyes from 18 MH patients were involved in this retrospective study which reviewed square root of loss variance(sLV) and mean defect(MD) of the visual field test in all subjects. The relationship between cVFD and MH stage, as well as the postoperative ellipsoid zone disruption were evaluated using Spearman's correlation test. RESULTS: Our analysis determined Spearman coefficient is 0.705 for the correlation between sLV and MH stage(P<0.01), 0.877 for the correlation between sLV and postoperative ellipsoid zone disruption(P<0.01) and 0.721 for the correlation between MD and postoperative ellipsoid zone disruption(P<0.01). A significant relationship was also detected between postoperative ellipsoid zone disruption and MH stage(r=0.470, P<0.05). Univariate regression analysis indicated that sLV and MD were associated with postoperative ellipsoid zone disruption(P<0.01, P<0.01, respectively). CONCLUSION: Postoperative cVFD is highly correlated with MH stage and postoperative ellipsoid zone disruption in patients with MH.

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue（VHCF） properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz.Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation.For smooth specimens,a fine granular area（FGA） is observed near the grain boundary in the fracture surface of the specimens broken after 10~7 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime,and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens.At last,an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.

不同成形方式对WE43镁合金组织和力学性能的影响

采用激光选区熔化成形(SLM)、铸造及挤压方式制备了WE43镁合金试样,通过维氏硬度计、密度测试计、光学显微镜、扫描电镜以及拉伸试验机等设备分析了不同制备方式下WE43镁合金的宏微观组织和力学性能变化规律;设计了基于指数函数的模型对不同成形方式的WE43应力–应变曲线进行统一拟合,为WE43材料未来增材、减材以及等材工艺复合制造复杂零件打下基础。结果表明,SLM成形WE43有明显的各向异性,铸态和挤压态不明显。SLM成形WE43镁合金的强度最高,抗拉强度达到313 MPa,是铸态的183%;挤压态WE43镁合金塑性最好,伸长率达到10.2%,是铸态的232%;此外,SLM态镁合金密度只有1.731 g/cm^(3),仅为挤压态的85.7%和铸态的95.2%。在断裂特性上,SLM态和挤压态为韧性断裂,而铸造态为脆性断裂。在内部存在20μm左右孔洞形缺陷的情况下,SLM成形镁合金依然具有最高的强度,主要原因是SLM成形WE43镁合金平均晶粒尺寸仅为2.6μm,基体内存在大量的稀土相沉淀以及纳米级亚稳相。由此可知,通过进一步的后处理方法焊合SLM态镁合金内部孔洞形缺陷后,材料力学性能可以大幅提高。

改进FFT算法在频域全聚焦超声成像中研究

针对钢块中孔洞缺陷的超声成像检测,目前普遍采用时域超声成像算法。该方法中的延时叠加过程需要进行重复、繁琐的迭代运算,无法满足高质量实时成像的需求,频域超声成像算法具有更高的成像分辨率和更快的成像速度。提出了一种针对任意非0值输入的基2-FFT输入分级截断算法,将该算法应用在频域FMC-PSM算法成像中,得到改进的FMC-PSM算法,用于钢块中孔洞缺陷检测超声成像。实验结果证明,相较于标准FMC-PSM算法,改进的频域FMC-PSM算法能够在呈现更高质量图像的同时将成像速度提高了15%,有望解决超声成像中成像质量和成像效率难以兼顾的问题。

孔洞缺陷对TC4切削加工影响的仿真研究

为研究孔洞缺陷对TC4材料切削加工的影响,使用有限元切削仿真方法对切削过程的应力应变、切屑形态和切削力特点和规律进行研究。通过热力耦合仿真、数据分析、时频分析等方法对含缺陷材料切削过程中的参数的研究,研究结果表明:位于切削路径上的缺陷,在切削完后缺陷处表面出现应力集中,位于切削路径上的缺陷处表面应力值相比无缺陷时明显减大;缺陷位于表层区域时,由于缺陷导致材料内部结构变化,切屑断裂,切屑形态剧烈变化;当缺陷位于切削路径上和表层区域时,切削至缺陷处时切削力发生突变,切削状态发生失稳,并且位于表层区域的缺陷对切削力的突变影响最大;最后通过Choi-Williams时频分析对含不同位置缺陷切削仿真得到的切削力信号进行分析,观察到不同位置缺陷情况下切削力波动的时频信号,判断缺陷的位置。

基于三维激光点云数据的电力电缆绝缘缺陷识别

针对电力电缆绝缘缺陷识别精准度较低的问题,提出一种基于三维激光点云数据的电力电缆绝缘缺陷识别方法。利用剪裁法增强原始三维激光点云数据,通过区域生长法与最小二乘法完整获取电缆绝缘材料的三维结构面信息。借助Canny边缘检测方法求解电缆绝缘表面缺陷与内部缺陷边缘信息,自动识别出电力电缆绝缘的缺陷位置及缺陷类别。结果表明,所提方法可以精准识别电缆绝缘表面的划痕缺陷、电缆外屏蔽表面起泡和孔洞缺陷,识别耗时短,鲁棒性较优,具有较高实际应用价值。

超声相控阵检测与CIVA仿真对比研究

锅炉制造、安装、检验过程中,复杂结构焊缝一般可利用超声相控阵检测技术实施有效检测,其检测工艺的选择对检测结果影响很大,针对特定工件及工艺,结合CIVA仿真可快速分析成像效果,对于工艺优化具有重要意义。采用CIVA软件实施相控阵A型试块特定区域缺陷孔的相控阵检测仿真,同时采用相同工艺开展实际相控阵检测,对比分析各缺陷孔定位定量精度。结果表明:CIVA仿真与实际检测结果差异较小,实际检测精度稍优于CIVA仿真。

SiC基GaN上多晶金刚石散热膜生长及其影响

通过微波等离子体化学气相沉积(MPCVD)法,在SiC基GaN高电子迁移率晶体管(HEMT)异质结构材料上生长多晶金刚石散热膜,采用光学显微镜(OM)、拉曼光谱、非接触霍尔测试系统、X射线衍射(XRD)和扫描电子显微镜(SEM)对生长样品进行表征,研究了生长温度、多晶金刚石散热膜厚度对GaN HEMT异质结构材料性能的影响。测试结果表明,当多晶金刚石生长温度为625℃,散热膜厚度为20μm时,GaN材料载流子迁移率降低9.8%,载流子浓度上升5.3%,(002)衍射峰半高宽增加40%。生长温度越高,金刚石散热膜的生长速率越快。当金刚石散热膜厚度相差不大时,生长温度越高,GaN所受拉应力越大,材料电特性衰退越明显。多晶金刚石高温生长过程中,金刚石引入的应力未对GaN结构产生破坏作用,GaN材料中没有出现孔洞等缺陷。

空位缺陷调控策略构筑高能量密度六方孔洞MXene水系超级电容器

利用碳空位缺陷有序化策略构筑了多层六方孔洞MXene电极材料,在软包超级电容器中实现了高比容量和高能量密度水系钾离子存储。该电极材料具有较大比表面积的三维六方孔洞结构,为储钾提供了更多的活性位点。结合六方孔洞内壁新暴露的钛原子的化合价变化引起的赝电容效应,阐明了多层六方孔洞MXene水系钾离子超级电容器比容量提高的内在原因。通过密度泛函理论计算多层六方孔洞MXene对钾离子的吸附能,并结合电化学储钾性能实验及动力学分析,确定了钾离子被吸附的最佳位置,得出了钾离子的吸附规律。通过定量分析多层六方孔洞MXene中电子的能带结构和差分电荷密度等电子传输规律,揭示了其水系钾离子超级电容器具有高电导率和良好倍率性能的内在机理。

基于深度学习的兰姆波SCF-TFM超分辨率成像

腐蚀和裂纹是结构板常见的缺陷形式,兰姆波在非贯穿型损伤处发生模式转换是制约兰姆波成像质量的主要因素。此外,声波衍射遵循瑞利准则,超声成像存在分辨率极限。本文设计了一个全卷积神经网络对接收信号进行分割与重构,实现目标模态的自动拾取,抹除杂波和模式转换的干扰。提出符号相干因子全聚焦成像法(SCF-TFM),在全矩阵聚焦成像过程中施加符号相干因子,抑制非目标区域散射波对成像结果的干扰,同时考虑散射信号的幅值及相位信息,可以一定程度上突破瑞利准则的限制,实现超分辨率成像。实验结果表明:对于单个盲孔缺陷,该方法成像结果的横向分辨率比全聚焦提高62.41%,信噪比提升58.23%;而对于多个非对称盲孔缺陷,当缺陷间距大于瑞利准则分辨率极限时,该方法的信噪比提高了92.89%;缺陷间距小于瑞利准则分辨率极限时,该方法可以实现超分辨率成像。

湿型砂铸造中的旧砂微粉回用工艺

针对湿型砂铸造中旧砂循环混配再利用时型砂综合性能欠佳,铸件砂眼、夹砂结疤的缺陷率高,铸件质量不稳定等问题,对型砂进行了检测和数据分析,通过对双盘冷却机旋风分离后的微粉进行充分回用,调节了型砂粒度分布,改善了型砂中微粉含量比例,保证了型砂性能相对稳定受控,降低了混砂辅料用量比例,节约了生产成本,降低了型砂对水分的敏感性,促进了改善型砂流动性,提升了型砂综合性能和铸型质量,有效降低了铸件产生砂眼、夹砂结疤缺陷的风险。

HOLE基因外显子2区SNP与先天性心脏病的关联研究

目的:研究HOLE基因外显子2区SNP与人类先天性心脏病(CHD)的相关性。方法:以病例组179例CHD患者和对照组183例门诊体检健康人为研究对象,酚-氯仿法提取外周血DNA,PCR扩增产物经变性丙烯酰胺电泳鉴定基因型,应用卡方检验分析组间基因型频率及等位基因频率差异。结果:(1)粤黔两地汉族人群中存在HOLE基因SNPrs10569304的插入缺失(GCC/-)多态性,对应为等位基因A/B。(2)对照组和病例组的基因型频率及等位基因频率符合Hardy-Weinberg平衡,两组AA、AB和BB三种基因型频率分别为21.31%、54.09%、24.59%和16.75%、46.36%、36.87%,经卡方检验基因型分布差异有统计学意义(χ2=6.51,P<0.05),与对照组相比,病例组中BB基因型频率升高,AA、AB基因型频率降低。(3)对照组与病例组A和B等位基因频率分别为48.36%,51.64%和39.94%,60.06%,病例组B等位基因频率显著高于对照组,分布差异有显著性(χ2=5.20,P<0.05)。结论:粤黔两地汉族人群中HOLE基因SNPrs10569304存在插入缺失(GCC/-)多态性,具有BB基因型的个体罹患CHD的危险度增高。

904L不锈钢荒管开裂原因

某904L不锈钢荒管在穿孔工艺后,其内壁发生开裂现象。采用宏观观察、化学成分分析、力学性能测试、扫描电镜及能谱分析、金相检验等方法分析了该荒管开裂的原因。结果表明:荒管穿孔前近内表面存在疏松缺陷,在穿孔工艺应力的作用下,沿疏松缺陷处萌生裂纹,最终导致荒管发生开裂。

裂纹-孔洞缺陷下PVC涂层织物类膜材的撕裂行为

膜结构节点和边界处往往需要通过膜面开孔来实现关键部位的连接,孔洞边缘的应力集中会影响膜面的撕裂行为。为探究聚氯乙烯(polyvinyl chloride,PVC)涂层织物类膜材中孔洞对裂纹扩展行为的影响,开展了含有裂纹-孔洞缺陷的PVC涂层织物类膜材的单轴拉伸试验,分析了裂纹扩展全过程和撕裂机理,提出了适用于含裂纹-孔洞多缺陷PVC涂层织物类膜材撕裂强度的预测模型,并验证了模型的适用性。研究结果表明:含裂纹-孔洞缺陷的PVC涂层织物类膜材在中心撕裂过程中存在裂纹端扩展和孔端再扩展两阶段;定义了裂纹端临界撕裂荷载、裂纹端峰值撕裂荷载、孔端临界撕裂荷载和孔端峰值撕裂荷载4项撕裂特征指标,以衡量含裂纹-孔洞缺陷膜材的抗撕裂性能;孔洞的存在导致试样在孔端再扩展阶段的承载力提升,但随着孔洞尺寸增大,孔端的承载力提升效应减弱,裂纹端峰值撕裂荷载逐渐变为控制试样破坏的极限荷载;所提出的二次函数-指数函数应力场模型能有效地预测膜材的裂纹端临界撕裂应力和孔端临界撕裂应力。

碳纤维复合材料PCD刀具低温和干式制孔试验研究

目的针对CFRP开展二氧化碳低温冷却和干式条件下的PCD刀具制孔试验研究,从而提高CFRP材料的切削性能、制孔精度及制孔质量。方法在传统钻孔和低温冷却条件下,采用常规和织构PCD刀具开展制孔试验,分析不同切削参数下的轴向推力、总切削力、入口直径和相对误差、表面黏结、孔壁形貌及缺陷的差异性。结果在干式和低温冷却条件下,CFRP钻入过程产生的轴向推力和总切削力均随着进给速度的增大而不断增大;在试验条件下,当进给速度增大时,PCD刀具+干式、PCD刀具+低温及PCD织构刀具+低温3种方式下获得的入口直径相对误差范围分别为0.29%~0.42%、0.57%~1.15%、0.38%~0.89%;与干式加工相比,低温冷却有效抑制了刀具切屑黏结;总体来说,在低温冷却下,孔壁表面有明显的纤维断裂,形成较多的纤维断口坑、微裂纹,树脂表面涂覆均匀性较差。结论在CFRP钻孔过程中,相对于干式钻孔,在相同切削参数下,低温冷却下可以获得更大的切削力;与低温冷却相比,采用干式钻孔能够提高入口直径尺寸的制孔精度,有效提高抗黏结效果;总体来说,相同切削参数下,干式制孔获得的孔壁表面更加平整和均匀。

含空洞缺陷桩基超声波透射法多物理场数值模拟

随着中国建筑行业的快速发展,桩基运用也越来越广泛,桩基的完整性检测至关重要。超声波无损检测技术具有检测深度大、范围广、灵敏度高、速度快等优点,已成为桩基检测中运用最多的检测方法。超声波在混凝土中衰减严重,且在混凝土这种集合型复合材料中的传播过程十分复杂,对缺陷的判断只能定性分析,无法进行定量分析,不能准确的检测出缺陷的具体位置。为了准确有效的检测桩基缺陷的位置,通过Comsol Multiphysics有限元分析软件建立二维桩基无损检测模型,利用压力声学、固体力学模块和压电效应进行多物理场耦合,分析单孔空洞和双孔空洞在不同位置中超声波的传播规律。模拟结果表明:不管单孔还是双孔空洞,超声波在空洞后面形成缺口,与发射换能器、空洞圆心在一条直线上;通过对横向和纵向不同位置的首波声压值分析,能够有效的判断出缺陷的具体位置。

摆臂类零件工艺分析与优化

针对摆臂类零件的凸包和翻孔在成形时易开裂的问题,提出了摆臂类零件改进设计的方法。首先对复杂结构的摆臂类零件常见的冲压工艺缺陷进行分析,通过CAE分析结合实物样件对零件设计进行验证;其次对于凸包区域的开裂问题,根据零件装配要求,在尽可能减少修模量的前提下,通过调整圆角半径和拔模角度改善凸包区域的成形缺陷;对于对向翻孔开裂的问题,将弹性元件的压力作为优化目标,确定最佳的圆角半径、翻边高度和拔模角度参数组合,此外,通过增加局部导向结构解决弹性元件压入摆臂时产生的刮痕缺陷;最后以试制方式加工出相应状态的样件,对不同的优化方案进行验证。结果表明:对于摆臂类零件的优化,若CAE分析凸包位置存在开裂风险且减薄率不大于28%时,圆角半径增大不超过2 mm、拔模角度增加不超过5°;增大对向翻孔的压料区域和压料力,并在结构上增加相应的倒角可提升零件的压装性能。