Orientation preference and fractal character of short fatigue cracks in a weld metal

Short fatigue crack behaviour in a weld metal has been further investigated. The Schmid factor and the fractal dimension of short cracks on iso-stress specimens subjected to reversed bending have been determined and then applied to account for the distribution and orientation characteristics of short fatigue cracks. The result indicates that the orientation preference of short cracks is attributed to the large values of Schmid factor at relevant grains. The Schmid factors of most slip systems, which produced short cracks, are less than or equal to 0.4. Crack length measurements reveal that short crack path, compared to that of long crack, possesses a more stable and relatively larger value of fractal dimension. This is regarded as one of the typical features of short cracks.

Initiation and propagation of short fatigue cracks in a weld metal

Fatigue tests were performed using a purpose designed triangular shaped specimen to investigate the initiation and propagation of short fatigue cracks in a weld metal. It was observed that short fatigue cracks evolved from slip bands and were predominantly within ferrite grains. As the test progressed, the short crack density increased with minor changes in crack length. The growth of short cracks, in the early stage resulted mainly from coalescence with other existing cracks. The mechanism of short crack behaviour is discussed.

K418与42CrMo异种金属的激光穿透焊接

实验研究了连续波Nd：YAG激光焊接速度、侧吹保护气流量和离焦量等参量对激光穿透焊接K418和42CrMo焊缝成形的影响。结果表明，K418与42CrMo激光穿透焊接有X形和T形两种典型的焊缝形貌，且焊缝形貌是不对称的。随着焊接速度的提高，焊接线能量降低，焊缝尺寸变小，且焊缝上部尺寸变化比下部尺寸变化慢，焊缝形貌由X形过渡到T形。当离焦量在瑞利长度范围内时，焊缝正面宽度变化很小；当离焦量超出瑞利长度范围时，在足够高的激光功率密度下，焊缝正面宽度快速增加。在激光功率为3kW，侧吹保护气角度为35°条件下，通过优化焊接速度、侧吹保护气流量和离焦量等参量可以得到最佳焊缝质量。

Collective evolution characteristics and computer simulation of short fatigue cracks

Fatigue testing was conducted using a kind of triangular isostress specimen to obtain the short-fatigue-crack behaviour of a weld low-carbon steel. The experimental results show that short cracks continuously initiate at slip bands within ferrite grain domains and the crack number per unit area gradually increases with increasing number of fatigue cycles. The dispersed short cracks possess an orientation preference, which is associated with the crystalline orientation of the relevant slip system. Based on the observed collective characteristics, computer modelling was carried out to simulate the evolution process of initiation, propagation and coalescence of short cracks. The simulation provides progressive displays which imitate the appearance of experimental observations. The results of simulation indicate that the crack path possesses a stable value of fractal dimension whereas the critical value of percolation covers a wide datum band, suggesting that the collective evolution process of short cracks is sensitive to the pattern of crack site distribution.

平台用高强度钢多次补焊及TIG熔修的疲劳分析

海洋石油平台在服役期内，由于受到各种环境载荷的交变作用，其受力的焊接构件不可避免地会出现疲劳裂纹。对出现的疲劳裂纹进行补焊修复，可以延长平台的使用期，带来巨大的经济和社会效益。本文通过实验研究，考察了多次补焊对海洋平台用高强度钢焊接接头疲劳性能的影响，以及是否可以通过TIG熔修工艺的采用提高补焊构件的疲劳寿命，达到延长补焊后平台的使用寿命，减少补焊次数的目的。本文涉及的主要研究内容如下：1、考察了多次补焊对焊接接头疲劳寿命的影响。通过对平台用高强度钢焊接接头初焊试样，以及重复三次补焊试样的疲劳实验，得到了各次补焊试样在对数正态分布和威布尔分布下的寿命估算式和考虑了存活率的R-S-N曲线。对实验结果进行了比较和分析，从而考察了多次补焊对焊接接头疲劳性能的影响。2、在每次补焊后，由于焊趾外移造成了焊趾角的变化，使得焊趾处的应力集中系数发生了变化。本文采用有限元方法计算了试样的应力场分布，得到了焊趾处的应力集中系数与焊趾角的关系。考虑到应力集中系数的变化后，对多次补焊试样疲劳实验的实验数据进行了重新处理。3、考察了多次补焊熔修对焊接接头疲劳寿命的影响。本文通过对平台用高强度钢焊接接头补焊及补焊熔修试样的疲劳实验，得到了多次补焊和补焊熔修试样对数正态分布和威布尔分布下的寿命估算式和考虑了存活率的R-S-N曲线。对实验结果进行了比较和分析，从而考察了补焊后采用TIG熔修工艺对焊接接头疲劳性能的影响。4、一次补焊后进行熔修处理可以极大的改善试样的疲劳性能，提高试样的疲劳寿命。但这一结果是在平焊位条件下得出了。为了将这一结果用于实际，还要考察实际操作中大量采用的立焊位条件下熔修的效果。本文完成了立焊位条件下补焊及补焊熔修试样的疲劳实验，得到了相应的S-N曲线。5、疲劳实验数据的处理中，大量采用威布尔分布作为数据统计的分布类型。但由于威布尔分布具有三个参数，给求解带来一定的困难。本文提出一种以在威布尔坐标纸上，数据分布的线形相关系数作为判据的求解威布尔参数的方法，并通过程序加以实现。为了对疲劳实验数据的处理更加方便和直观，编制了“疲劳实验数据处理程序“，将对疲劳实验数据的编辑、分布参数的求解、不同存活率下的疲劳寿命估算和各种分布类型图以及R-S-N曲线的绘制工作包括在该程序中。

42CrMo激光焊焊缝组织

42CrMo合金钢C含量高、合金元素含量多、淬硬倾向大,焊接性能差。激光焊接具有功率密度高、焊接变形小等优点,适合焊接传统工艺难焊的同种或异种金属。通过额定功率为3kW的Nd∶YAG固体激光器焊接42CrMo,采用光学显微镜(OM)、扫描电镜(SEM)、X衍射、硬度仪分析了焊缝区域组织、成分和硬度变化。研究结果表明:焊缝区域组织为马氏体,热影响完全淬火区的组织为马氏体+贝氏体组织,热影响不完全淬火区的组织为贝氏体。从焊缝正面到焊缝背面的硬度分布表明:由于焊缝正面有保护气体的作用提高了焊缝正面熔池的冷却速度,使焊缝背面的硬度低于焊缝正面的硬度。

Corrosion fatigue of welded joints of steel for marine platform

Experiments on the corrosion fatigue behaviour of welded joints of the steel for marine platform in air and seawater, and of the joints in seawater with cathodic protection, yielded data for linear regression to obtain fatigue life curves (Delta S-N-f). The laws of corrosion fatigue in welded joints of test steel are discussed with reference to those of A(587) and A(131) steel. In these experiments, the fatigue damage occurring at all welded joints around the weld interface resulted in the cracks and fractures. The fatigue life of the specimens in seawater with cathodic protection is longer than that in seawater Without protection.

基于人工神经网络的钛合金焊接接头机械性能预测

采用多层前向反馈神经网络模型,对钛合金钨极氩弧焊的焊接接头机械性能进行了模拟和预测。其中,输入参数包括钛合金成分、冷却速度和热处理参数;输出参数包括5个重要的机械性能,即极限抗拉强度、延伸率、断面收缩率、屈服强度和硬度。详细分析了铝和钒这2种元素对机械性能的影响。

大型激光叠焊系统研究与开发

介绍一种新型激光叠焊系统,利用该系统可将尺寸范围为12.0m×1.5m厚度不等的两块不锈钢板重叠焊接在一起。针对大尺寸不锈钢板尺寸大、易变形等特点提出了主—辅式压紧机构,有效地控制了焊接过程中大尺寸板材的变形并消除了板材间隙;设计了ufxyz型四轴联动方案:u轴与x轴同向,u轴驱动钢板进行大范围运动,x轴驱动激光头小范围运动。u轴与x轴的复合运动提高系统精度的同时提高了系统焊接动态响应速度。对实际焊接板材进行试验验证,结果表明使用该系统进行焊接的钢板可承受压强最大能达到6.1MPa。

结构光视觉传感器误差分析与结构分析

结构光视觉传感器是视觉焊缝跟踪系统获得焊缝信息的重要组成部分,其测量误差与性能对焊缝跟踪系统的总体测量精度及可靠性有着直接影响。本文对应用于焊缝跟踪的结构光视觉传感器进行误差分析,包括传感器硬件系统结构误差、激光散斑噪声误差及镜头畸变误差等,并对不同结构方式下的视觉传感器建立了数学模型,具体分析了结构参数对其误差的影响,提出结构光视觉焊缝跟踪传感器优化设计方法,并依据仿真结果给出结构优化设计参数,最后通过实验验证了该优化设计方法的正确性。

基于数学形态学的激光拼焊熔池图像处理研究

基于拼板激光焊接工程中的实时检测系统,应用数学形态学中的相关图像处理技术,提出了一种新的熔池图像处理流程模型,并通过实验得到了三种清晰的熔池边缘图像,同时验证了该模型的正确性。将算法与经典算法进行比较,证明了该算法对于激光拼焊的适用性和鲁棒性。

焊缝图像处理技术在激光拼焊中的应用

基于拼板激光焊接工程中的视觉跟踪系统,应用相关图像处理技术,建立了一种焊缝跟踪的图像处理流程模型,并通过实验验证了该模型的正确性。