摘要
针对MARK III型液化天然气(LNG)船围护系统中的304L不锈钢波纹板搭接接头进行了圆形扫描激光焊接工艺研究。对于圆形扫描激光焊接的搭接接头,其下板焊缝边缘存在咬边,且在较大扫描幅度和较高扫描频率下咬边现象尤其明显。通过高速摄影发现,焊接过程中圆形扫描轨迹内部存在未熔化区域。激光斑点除自身的扫描运动外还叠加了沿焊接方向的移动,随着扫描路径的重复叠加,在扫描激光焊接熔池的内部,未熔化区域逐渐减小,最终达到稳定状态。当扫描幅度或频率提高时,单位长度母材吸收的激光能量减小,熔池温度降低,当前扫描周期下形成的熔池前沿快速凝固,从而导致未熔化区尺寸无法进一步缩小。当扫描幅度降至1.5 mm或者扫描频率降至50 Hz时,未熔化区域较小甚至完全消失,咬边现象也随之消失。
Objective The 304L stainless steel membrane sheets in the MARK III LNG containment system are extensively applied, which should be connected by welding in the form of fillet joint. In practical production, plasma arc welding(PAW) is more employed in the joint production. However, the production efficiency is low. So it is considered to apply laser welding in the production to increase the welding speed and enhance the production efficiency. Thus, several investigations are necessary for the application.In this research, the flow of molten pool and weld formation in 304L stainless steel fillet welding by the circular scanning laser are investigated. Thus, the reason why undercut occurs and how various welding parameters affect its appearance are highlighted, which contributes to preventing undercut from occurring. Methods In this study, 304L stainless steel sheets whose depth is 1.2 mm are used. A single factor experiment is carried out to study the influence on the formation of the weld by different welding parameters. Among all of the welding parameters, scanning amplitude and scanning frequency are considered to have a substantial effect during the welding process, so the high-speed photography is employed to study the impact of these parameters on the molten pool’s fluid flow, which is employed to explain the formation of weld with the molten pool solidification and the energy distribution in circular scanning.Results and Discussions The molten pool’s dynamic process from the establishment to stabilization during circular scanning laser welding is observed using high-speed photography. The experimental findings reveal an undercut on the bottom plate welded by circular scanning mode, particularly with larger scanning amplitude, and larger undercut. The scanning laser moves along the preset trajectory, and the high-speed photographic findings reveal a molten pool forming along the moved path of the laser spot during the first scanning period, while unmelted solid remains inside the circular area. In a single scanning cycle, there is a melted loop and an unmelted round area(Fig. 5). The moving of the laser spot superimposes the moving along the welding direction except for its scanning movement, and then the unmelted area gradually decreases because of repeated stacking of the molten pool and eventually reaches a steady-state(Fig. 6). In aggregate, the laser energy is concentrated on both sides of the weld, while the energy in the central region is low(Fig. 10). Under small scanning amplitudes, the unmelted area disappears while it is always there under large scanning amplitudes. Under the scanning laser’s agitation, molten metal of the upper plate spatters at the molten pool’s front due to the impetus from the laser(Fig. 7), which also leads to the lower plate’s molten metal flowing to the upper at the molten pool’s trailing end(Fig. 8). Meanwhile, as the scanning amplitude increases, the absorbed laser energy per unit length decreases, leading to the lower molten pool temperature. Then the molten pool’s front solidifies fast, and thus the unmelted area can not be further reduced. Therefore, the lower plate’s molten metal cannot not be supplemented, which leads to the undercut’s occurrence.Conclusions The following conclusions can be drawn from the above experiments. First, the penetration depth and the degree of undercut of the circular scanning laser overlap joint are negatively correlated with scanning frequency. When other welding parameters are fixed, the higher the scanning frequency is, the smaller the penetration depth is. While welding, the proposed scanning frequency is below 200 Hz, and the scanning amplitude is below 2 mm.Second, there is base metal loss including spatters or evaporation during welding, which requires melted metal from the upper sheet to supplement. When the scanning amplitude is large or the frequency is high, the molten pool’s solidification speed is fast and the solidification time is short, and then the downward flow channel of molten metal at the molten pool’s front becomes narrower, which leads the downward flow of molten metal to be reduced. Thus, the molten metal at the lower side is not enough and the undercut occurs. Third, the unmelted area in the molten pool is caused by the energy distribution in the circular scanning. The laser energy is concentrated on both sides of the weld, while the energy in the central region is low. When the scanning amplitude is large(more than 1.5 mm) or the frequency is high(200 Hz), the molten pool’s temperature is low, so the molten pool solidifies fast, creating a large unmelted zone, and then the undercut intensifies.
作者
杨晖
李芳
华学明
陈科
Yang Hui;Li Fang;Hua Xueming;Chen Ke(Shanghai Key Laboratory of Materials Laser Processing and Modification,School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai 200240,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2022年第22期69-76,共8页
Chinese Journal of Lasers
基金
工业和信息化部高技术船舶科研计划(2018函473号)
工业和信息化部高技术船舶科研计划(2020函313号)。
关键词
激光技术
扫描激光焊接
咬边
搭接焊
圆形扫描
304L不锈钢
laser technique
scanning laser welding
undercut
overlap welding
circular scanning
304L stainless steel sheet
