【作者】 孙宇；

【导师】 芦凤桂；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2023， 博士

【摘要】 针对核电转子用厚壁结构件的高质量、高效率焊接制造,高功率激光先进制造方法具有良好的应用前景。然而,在激光深熔焊接过程中维持匙孔的稳定性非常重要,其动态行为直接影响熔池流动和凝固过程,特别是在厚板结构激光焊接中,易产生气孔、裂纹等缺陷。因此,系统研究光纤激光厚板焊接过程中匙孔、熔池动态行为对气孔、裂纹等缺陷的影响机制,进而提出缺陷抑制方法,对厚板激光焊接质量提升具有重要的意义。本文针对NiCrMoV核电转子用钢厚板,采用高功率激光深熔焊接方法,系统研究厚板高功率单、双光束激光深熔焊接过程中激光功率、焊接速度、窄间隙坡口拘束等因素对裂纹和气孔缺陷的影响规律,揭示厚板激光深熔焊接接头中周期性裂纹缺陷的形成机理,并提出抑制方法。建立多场耦合的激光深熔焊接数值模型,模拟熔池流动、匙孔波动、气泡形成及演化过程,阐明气孔分布与匙孔稳定性和熔池流动、能量传输与累积以及熔池形状特征的关联性,诠释了激光深熔焊接过程中周期性链状气孔形成机制及窄间隙坡口拘束对气孔的影响机理,提出了在超窄间隙坡口拘束下抑制气孔的方法。本文的主要研究结果如下:首先,开展了板厚25 mm的激光深熔焊接试验,发现焊缝中存在的主要缺陷为周期性凝固裂纹,裂纹分布平行于底部熔合线,距离熔合线约1.5 mm。结合金相表征,澄清了受热输入影响变化的焊缝裂纹敏感区域深宽比对凝固裂纹敏感性的影响规律:在一定范围内热输入增加使焊缝裂纹敏感区域深宽比小于4.0后,焊缝中凝固裂纹敏感性极低,裂纹缺陷被消除。周期性凝固裂纹缺陷产生于向焊缝中心生长的柱状晶前端,凝固裂纹的产生与自底部熔合线生长的柱状晶凝固前沿结晶状态相关,凝固前沿液态金属结晶潜热累积并周期性释放造成了晶粒生长暂时停顿和重新开始,使凝固裂纹的产生具有周期性。其次,为了研究板厚对焊接缺陷敏感性的影响,开展了板厚10 mm的激光焊接试验,发现焊缝缺陷转变为以气孔为主,出现了周期性链状气孔缺陷。结合考虑超微细金属颗粒羽烟行为对激光削弱作用的激光热源和多重反射效应,建立了激光深熔焊接数值模型,模拟结果表明:熔池中匙孔后壁产生的顺时针主涡流造成的能量传输与积累使熔池后壁形成中部向内凹陷的形状,在一个周期内,受这一特殊形状影响,频繁产生的气泡在顺时针涡流带动下迁移,并易于依照它们产生的顺序逐个在熔池中部、下部和底部的不同凝固位置被捕获,最终演化为链状气孔缺陷;超微细金属颗粒羽烟周期性波动引发激光能量的相应改变,使得匙孔振荡频率、熔池后壁下部热量积累、顺时针主涡流传热以及熔池后壁中部凝固潜热积累等因素同步周期性变化,导致熔池形状发生周期性变化,从而使链状气孔表现为随着底部熔合线的周期性变化而周期性排布。同时分析了不同焊接速度对气孔行为的影响规律:当焊接速度降低时,熔池后壁中部没有向内凹陷,这种熔池形状和强烈的流体流动模式使气泡倾向于迁移到焊缝中较高的位置逸出或形成气孔;当焊接速度增加时,顺时针主涡流向上部分的流动减弱,结合较高的冷却速度导致熔池后壁中部向内凹陷的熔池形状较早形成,气孔更容易在近熔合线底部相同高度处形成。最后,研究了窄间隙坡口拘束下不同热输入的单光束和不同能量分配比的双光束等对气孔行为的影响。实验结果表明:采用串行双光束焊接方法,能量分配比1:1既能有效抑制气孔,熔深损失程度小,且焊缝裂纹敏感区域深宽比减小,产生凝固裂纹缺陷极少。模拟结果表明:气孔的分布主要受熔池后壁相对焊接方向向后隆起区域的位置和高度上的范围影响,能量分配比3:1的情况下,隆起区域相对于单光束情况下更高且高度范围上更大,故气孔分布范围更广;能量密度相对较低的跟随光束在匙孔后壁上制造局部凸起并向下流动,增大了匙孔的不稳定性,不利于抑制气孔。串行双光束抑制气孔的关键在于跟随光束使匙孔前后壁之间距离增加,能量分配比1:1大幅降低匙孔长径比从而减小了匙孔振荡和坍塌频率,有助于抑制气孔;引导光束和跟随光束分别作用在匙孔前壁和后壁,极大拓宽匙孔前后壁距离,降低匙孔坍塌的可能性,且熔池流动稳定,气孔得到有效抑制。更多还原

【Abstract】 For the high-quality and efficient welding and manufacturing of thick wall structural components for nuclear power rotors,the advanced manufacturing method of high-power laser has good application prospects.However,it is important to maintain the stability of the keyhole during laser deep penetration welding,as its dynamic behavior directly affects the flow and solidification process of the melt pool,and porosity and cracking defects tend to occur especially in laser welding of thick plate structures.Therefore,it is of great significance to systematically study the influence mechanism of the keyhole and melt pool dynamic behavior on defects such as porosity and cracking during the fiber laser welding process of thick plate,and then propose defect suppression methods for improving the quality of laser welding on a thick plate.In this paper,high-power laser deep penetration welding method is adopted for the thick plate of NiCrMoV steel for nuclear power rotors,and the influence of factors such as laser power,welding speed,and narrow gap groove restraint on cracking and porosity defects in high-power single and dual beam laser deep penetration welding of thick plates are systematically studied.The formation mechanism of periodic crack defects in thick plate laser deep penetration welding joints is revealed and suppression methods are proposed.A multi-field coupled numerical model for laser deep penetration welding is established,and the processes of melt pool flow,keyhole fluctuation,bubble formation and evolution are simulated.The correlation between porosity distribution and keyhole stability,melt pool flow,energy transfer and accumulation,as well as melt pool shape characteristics are elucidated,and the mechanism of periodic chain-like porosity formation and the influence of narrow gap groove restraint on porosity formation during laser deep penetration welding are interpreted,then methods to suppress porosity under narrow gap groove restraint conditions are proposed.The main research contents and results in this paper are as follows:Firstly,the laser deep penetration welding experiments with the plate thickness of25 mm are conducted,it is found that the main defects present in the weld joint are periodic solidification cracks,which are distributed parallel to and approximately 1.5mm away from the bottom fusion line.Based on metallographic characterization,the influence of weld aspect ratio on solidification cracking susceptibility under the effect of heat input change is clarified: after an increase in heat input within a certain range causes the weld aspect ratio to be less than 4.0,the solidification cracking susceptibility in the weld is extremely low,and crack defects are eliminated.The periodic solidification crack defects occur at the front end of columnar crystals growing towards the center of the weld,and the formation of periodic solidification crack defects is related to the status of solidification front crystallization during the growth process of columnar crystals originating from the bottom fusion line.The accumulation and periodic release of crystallization latent heat in the liquid metal at the solidification front cause the periodic pause and restart of crystallization,which leads to the periodicity in the formation of solidification cracks.Secondly,in order to study the influence of plate thickness on the susceptibility of welding defect,the laser welding experiments with the plate thickness of 10 mm is carried out.It is found that the weld defects are converted to be dominated by porosity,and periodic chain-like porosity appears.A numerical model for laser deep penetration welding is established by considering the laser heat source of weakening effects on fiber laser by ultra-fine metal particle plume behavior and the multiple reflection effects.The simulated results show that,the profile of molten pool concave inward at the middle part depends on the heat accumulations transferred by the clockwise primary vortex from the keyhole rear wall.In a single period,affected by this special profile,the frequently generated bubbles migrate with the clockwise vortex and tend to be successively trapped according to the order of their production,at different solidification positions such as the middle part,the lower part and the bottom part of the molten pool,respectively,finally evolve to be chain-like porosity;the periodic fluctuations of ultra-fine metal particle plumes trigger corresponding changes in laser energy,causing periodic changes in factors such as keyhole oscillation frequency,heat accumulation in the lower part of the rear wall of the melt pool,and clockwise main vortex heat transfer,the melt pool profile undergoes periodic changes,resulting in chain-like porosity exhibiting periodic distribution with the changes of bottom fusion line.Meanwhile,the influences of varied welding speeds on porosity behavior are analyzed: when the welding speed decreases,there is no concave inward at the middle of the molten pool rear wall,and this molten pool profile with a violent fluid flow pattern makes bubbles tend to migrate to higher positions in the weld to escape or form porosity;when the welding speed increases,the upper flow of the clockwise main vortex is weak,and the higher cooling rate leads to earlier formation of the concave inward profile at the middle part of the molten pool rear wall,making it easier for porosity to form at the same height near the bottom of the fusion line.Finally,the effects of single-beam with varied heat inputs and dual-beam with different energy distribution ratios on porosity behaviors under narrow gap groove restraint conditions are investigated.The experimental results show that using the serial dual-beam welding method with an energy distribution ratio of 1:1 can effectively suppress porosity,obtain a low loss degree of weld penetration,and reduce the aspect ratio,resulting in very few solidification crack defects.The simulation results show that the distribution of porosity is mainly affected by the location and height range of the bulge region that protrudes backwards relative to the welding direction on the molten pool rear wall,when the energy distribution ratio is 3:1,the bulge region is higher and the height range is larger than in the case of a single-beam welding,so the porosity distribution range of which is wider;trailing beam with relatively low energy density create local humps flowing downward on the keyhole rear wall,increasing the instability of the keyhole,which is not conducive to suppressing porosity.The key to suppressing porosity with serial dual-beam depends on the increase of the distance between the front and rear walls of the keyhole by the trailing beam,and serial dualbeam welding with the energy distribution ratio of 1:1 can greatly reduce the lengthdiameter ratio of the keyhole,thus,reducing the oscillation and collapse frequency of the keyhole,which helps to suppress the porosity;the leading beam and the trailing beam act on the front and rear walls of the keyhole,respectively,greatly broadening the distance between them,significantly reducing the possibility of keyhole collapse,and the molten pool flow is stable,which effectively suppress the porosity.更多还原