【作者】 李凯；

【导师】 高洪明；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2014， 博士

【摘要】 伴随着海洋油气资源的开发，石油平台、海底管道等大量的结构件被用于水下。而这些结构件长期工作在海水侵蚀和水流冲击的环境中，随着服役年限的增加，这些结构件的水下焊接修复工作量将日益增加。高压干法熔化极气体保护焊（高压干法GMAW）由于其良好的焊缝性能、较深的水深适用范围，被认为是一种具有现实意义的深水焊接方法。本文以高压干法GMAW实验为基础，在环境压力小于2MPa，即等效水深为200m的范围内，针对高压干法GMAW电弧行为、熔滴过渡特性、飞溅产生机理等问题进行了深入研究；从焊接过程的物理本质入手，揭示高压环境中GMAW工艺特点；在此基础上，进行了80m水深条件下X65管线用钢的对接焊试验。首先根据高压干法焊接过程特点，研制了陆上模拟压力环境的焊接实验装备，该装备包含高压环境模拟系统和自动焊接试验系统，能够满足环境压力5MPa以内的自动焊接要求。建立了典型的高压干法GMAW电弧形态模型。采用高速摄像机对电弧形态进行采集，通过分析发现，环境压力的增加使弧柱区电场强度增加；压力环境中，焊接电流的增加使电极斑点和弧柱区尺寸扩展；焊接电压与弧长的变化仍近似为线性关系。直流正接阴极弧根沿焊丝侧面上爬高度随环境压力的增加而减小，当环境压力高于0.4MPa后，直流正接的电弧稳定性有明显的提升。压力环境中，直流正接时，阴极斑点在熔滴下表面始终存在，而熔池表面未见阳极斑点；直流反接时，阳极斑点在熔滴下表面形成，熔池一侧可以形成一至若干个阴极斑点。高压干法GMAW的电弧静特性曲线仍为上升特性，且随环境压力的升高而曲线上移；焊丝熔化速度与干伸长仍近似为线性关系；低于0.4MPa时，直流正接的焊丝熔化效率更高，而超过0.6MPa后直流反接与直流正接的熔化效率区别不明显。采用激光背光辅助的高速摄像系统对高压干法GMAW的熔滴过渡特征进行了观察和分析。直流反接的典型熔滴过渡可分为：大滴排斥过渡、射滴排斥过渡、高压射流过渡。前两者主要区别是熔滴尺寸和过渡频率，它们的熔滴均偏离焊丝轴线方向过渡，易产生与熔滴尺寸相近的飞溅；高压射流过渡时，熔滴尺寸很小，电弧弧根包裹着焊丝末端燃烧，熔滴在电弧内部完成过渡，几乎无飞溅产生。压力环境中采用直流正接时，根据焊接电压的不同，熔滴过渡形式主要有短路过渡和排斥过渡。通过分析发现阳极弧根沿熔滴一侧上爬是形成直流反接排斥过渡的前提条件。压力环境中电弧弧根收缩，弧根不再包裹着熔滴四周，而是在熔滴一侧燃烧，导致了斑点力、电磁力方向发生改变，对熔滴产生不对称的侧向力的作用，使熔滴偏离焊丝轴线方向，从而形成排斥过渡。分析了阳极弧根沿熔滴上爬现象产生的原因。阐述了直流反接时熔滴过渡形式转变的临界环境压力，通过实验得出了不同焊接参数下的熔滴过渡形式分布图。随环境压力的增加，高压干法GMAW直流反接的熔滴过渡形式最终均转变为排斥过渡。分析了高压干法GMAW焊接飞溅产生机理，其主要包含两种飞溅形式：熔滴偏离型飞溅和熔滴反弹型飞溅。前者是排斥过渡过程中形成的，熔滴脱离焊丝端部时具有一定水平速度，其不能落入熔池中从而形成的飞溅，可出现在环境压力大于0.2MPa的直流正接与直流反接时，飞溅尺寸与熔滴尺寸接近。后者是熔滴接触母材后由于向上的电磁力作用反弹至自由空间而形成的飞溅，飞溅尺寸一般小于熔滴尺寸，只出现在环境压力大于0.4~0.6MPa的直流反接时。提出形成熔滴反弹型飞溅必须要满足的两个基本条件：第一，下落的熔滴接触到母材后，电弧弧根在其表面燃烧相比在熔池表面更容易；第二，环境压力需达到一定程度，电弧弧根收缩至接触到母材的熔滴表面燃烧，使流过熔滴的电流达到一定值。最后，进行了等效水深为80m条件下X65钢板的对接焊工艺试验研究。同参数时，直流正接的平均电流相比反接有所升高，其幅度约为20~40A。因此，规划了第一层打底焊与第二层填充焊采用直流反接、第三层填充焊和第四层盖面焊采用直流正接的工艺，试验结果表明，焊缝成形良好，焊接接头内部无缺陷，拉伸性能与母材接近，能够满足性能要求。更多还原

【Abstract】 Underwater structures such as oil platforms, subsea pipelines are used widely with developing offshore oil and gas resources. These structures are working in the long-term under the environment with water flow and sea erosion. As time goes on, underwater welding repair work for the structures will be increased. Due to its good performance and wide depth of application, dry hyperbaric gas metal arc welding (GMAW) is considered to be a deep-water welding method with practical significance. In this thesis, based on dry hyperbaric GMAW experiments within2MPa ambient pressure which equals to the water depth of200m, the arc behavior, metal transfer characteristics and spatter generation mechanism are the research emphasis. And then the butt welding of X65plate at the water depth of80m using dry hyperbaric GMAW method is studied.Based on the characteristics of underwater welding process, experimental equipment is developed for onshore simulating. The equipment consists of high pressure environment system and automatic welding test system, which can satisfied the welding test requirements within5MPa ambient pressure.A typical arc shape model of dry hyperbaric GMAW is established. As ambient pressure increased, electric field intensity of arc column increases. At high ambient pressure, electrode spot and arc column area expanded with increasing welding current. The relationship between welding voltage and arc length is approximate to linear, which is consistent with GMAW at normal pressure. Rising height of cathode arc root along the wire side in direct current electrode negative (DCEN) decreases with the increasing ambient pressure. The arc stability with DCEN has obvious improvement at ambient pressure more than0.4MPa. In pressure environment, with DCEN, the cathode spots can be seen under the droplet and the anode spots is not generated on the molten pool. This phenomenon is different from direct current electrode positive (DCEP) welding process, whose anode spots and one or several cathode spots can be seen. Arc static characteristic curve of dry hyperbaric GMAW is still rising characteristic. The curve moves downward with increasing ambient pressure. The relationship between wire melting rate and wire extension is approximate to linear. The wire melting efficiency with DCEN is higher than that with DCEP at ambient pressure below0.4MPa. After0.6MPa, the wire melting efficiency with DCEN is nearly the same to that with DCEP.The metal transfer mode in dry hyperbaric GMAW has been investigated by using high speed camera system with infrared laser as backlight. The metal transfer mode with DCEP can be classified to large droplet repelled transfer, projected repelled transfer and hyperbaric streaming transfer. Droplet size and transfer frequency are mean differences of the former two. The transfer tracks of the former two deviate from the axial direction of welding wire and the spatter with near droplet size is easy to generation. With hyperbaric streaming transfer, welding arc is burning around the wire end and the droplet size is very small which transfer inside the welding arc. The process is nearly no spatter generated. According to the different welding voltage, the metal transfer mode with DCEN is short-circuit transfer and droplet repelled transfer. Through the analysis, it has been found that anode arc root rising along the droplet surface is the precondition of droplet repelled transfer forming with DCEP. In the pressure environment, arc root burning contracts to one side of the wire instead of wrapping around the droplet. It results in the direction changing of spot force and electromagnetic force, which cause the asymmetric side force on the droplet. So that the droplet deviates from the axial direction of the wire, and the droplet repelled transfer formed. In addition, the reason of anode arc root rising along the droplet has been analyzed. The critical ambient pressure for metal transfer mode changing has been expounded. Distribution diagram of metal transfer modes with different welding parameters has been obtained by the experiments. Metal transfer mode of dry hyperbaric GMAW with DCEP will be changed into droplet repelled transfer as the increasing ambient pressure.The spatter generation mechanism of dry hyperbaric GMAW has been investigated. Two spatter types, droplet deviated spatter and droplet rebounded spatter, are observed. The former is generated in the process of droplet repelled transfer, which is due to the high horizontal speed of the droplet detachment. This spatter type begins to appear at the ambient pressure reached0.2MPa. In both DCEP and DCEN process, droplet deviated spatter can be generated and its size is close to the droplet. The latter, droplet rebounded spatter, is a new spatter generated process. The spatter is always generated after the cathode root burning on the droplet. And then the droplet rebounds to the free space and changes into the spatter instead of adhering to the workpiece. Droplet rebounded spatter is generated at the ambient pressure over0.4~0.6MPa with DCEP only. The reason that the electromagnetic force is the driving force of droplet rebounded was analyzed. Two basic conditions of droplet rebounded spatter generation is proposed. First, after the droplet contacting with the workpiece, welding arc root burning on the droplet surface is easier than that on the molten pool. Second, the current flowing through the drop reaches a certain value, which caused by the arc root contracting to the droplet surface at more than a certain ambient pressure.Finally, Process test research on the butt welding of X65plate at the water depth of80m is studied. With the same welding parameters, average welding current of DCEN is20-40A higher than DCEP. Therefore, the parameters of multi-layer welding process have been planned. DCEP method is employed for the fist layer, backing weld, and second layer, filling weld. DCEN method is employed for the third layer, filling weld, and fourth layer, cosmetic weld. The welding joint without defect is gained. The tensile testing results show that the tensile property is close to the base metal. Therefore, the butt welding of X65plate with dry hyperbaric GMAW process can be satisfy the requirements更多还原