Abstract The overlapping friction taper plug welding (FTPW) process influences the overlapped welds and bonding quality of the friction stitch weld, therefore, flaws, microstructure and hardness evolutions were observed with the progress of overlapping FTPW process in a simulated wet condition to study the forming mechanism of underwater friction stitch welding. DH36 plugs were used to repair the arc welded DH36 joint. Six burn-off distances (namely consumptions) of plug were chosen to observe the intermediate status of flaws, microstructure and hardness. With the increase in burn-off distance, bonding was firstly formed at the bottom, then at the sidewall and finally at the rounded corner. In the overlapping FTPW process, the microstructure was coarse lath martensite and hardness up to 410 HV10 in the center of stir zone while acicular ferrite and grain boundary ferrite with hardness from 290 to 320 HV10 was observed at the rounded corner of stir zone; fine lath martensite was observed and hardness was about 370 HV10 in the forged zone in upper part of the overlapping friction taper plug weld; the overlapped friction taper plug welds were tempered and the hardness was reduced with a minimum about 250 HV10. The overlapping FTPW process study shows increase in heat input at the location of flaws at the beginning of the process, improving material flow at the location of flaws, and good plug material selection will promote the weld quality in underwater friction stitch welding.

中文翻译：

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在模拟潮湿条件下观察摩擦缝焊修复过程——重叠焊缝中的缺陷、微观结构和硬度演变

摘要 重叠摩擦锥形塞焊 (FTPW) 工艺影响摩擦缝焊缝的搭接焊缝和结合质量，因此，在模拟湿条件下观察重叠 FTPW 工艺过程中的缺陷、显微组织和硬度演变，以研究其成形过程。水下摩擦缝焊机理[J]. DH36 插头用于修复弧焊 DH36 接头。选取了6个塞子的燃烧距离（即消耗量）来观察缺陷、微观结构和硬度的中间状态。随着烧掉距离的增加，结合首先在底部形成，然后在侧壁，最后在圆角。在重叠的FTPW过程中，搅拌区中心显微组织为粗大板条马氏体，硬度可达410 HV10，而在搅拌区圆角处观察到针状铁素体和晶界铁素体，硬度为290～320 HV10。在重叠摩擦锥塞焊缝上部的锻造区观察到细小的板条马氏体，硬度约为370HV10；对重叠摩擦锥塞焊缝进行回火，硬度降低至最低约 250 HV10。重叠 FTPW 工艺研究表明，在工艺开始时缺陷位置处的热量输入增加，改善缺陷位置处的材料流动，良好的塞子材料选择将提高水下摩擦缝焊的焊缝质量。