【作者】 张菁；

【导师】 蔡艳；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2012， 硕士

【摘要】 光致等离子体对CO₂激光能量的吸收、折射、反射等降低到达小孔的激光能量密度,改变激光与工件的能量耦合,进而造成激光焊接缺陷,如气孔、未熔透等。侧吹混合He-Ar气体是常用由于抑制等离子体的实验手段。基于热传导率、电负性、解离能、电离能等方面的考虑,He是最为理想的保护气体。在工业生产中,目前常常采用加大侧吹气体流量来抑制等离子体,但是我国氦气资源紧缺,价格昂贵,因此,研究侧吹气体He-Ar配比的对等离子体形态及行为的影响机理,对于提高焊接质量、节约生产成本具有重要意义。论文基于流体动力学理论,建立了用于分析侧吹保护气体流场的二维非稳态模型。通过模型,以纯He作为侧吹气体,改变喷嘴高度,喷嘴角度,喷嘴半径,以及气体流量等工艺参数,计算出流场中激光入射点处的静压力,横向流速以及He所占的比率,并以上述三个参数作为优化工艺参数的标准。通过计算,计算表明喷嘴角度的最佳范围为40°～55°,将喷嘴高度控制在6～12 mm较为合适,喷嘴在X方向上正对激光入射点最好,喷嘴在X方向上的最佳工作区间为-1.5mm⁰.5mm,喷嘴半径为3mm⁴mm比较利于流场控制。基于此优化区间,选定工艺参数,只改变侧吹He-Ar气体配比,分别为纯He, 75%He25%Ar和50%He50%Ar。计算三种不同侧吹气体He-Ar配比条件下激光入射点处的静压力,横向流速和He所占的比率。结果表明,随Ar含量的增加,静压力和横向流速随之增大,He所占的比率减小。论文建立了基于不同侧吹He-Ar气体配比下的激光-等离子体模型,综合考虑了等离子体对激光的吸收和折射作用。计算了不同成分等离子体在不同温度下的粒子密度,吸收系数,以及热力学性质和组分输运性质,这对于保证模型计算的准确性起到了很大的作用。计算了纯He,75%He25%Ar以及40%He60%Ar侧吹气体配比下等离子体的温度场、浓度场和速度场。结果表明,随Ar含量的增加,等离子体的高度降低,面积增大。同时,随着Ar含量的增加,整体流场速度增大,该速度的沿X方向的分量急剧增大,而在Z方向,速度分量变化不大。横向流速增大有利于流场对流散热,即可以解释为什么等离子体高度随Ar含量的增加而降低。同时,在不同He-Ar配比的等离子体温度场中,在x=0mm处沿z轴方向超过70%的区域温度低于16000K,也就是说等离子体中电子数密度和吸收系数主要受到Fe成分的影响,相对的He-Ar配比对其影响不大。进而在温度低于16000的区域对激光的作用差别不大。也可以解释为什么较低的He含量依旧能够获得较好的焊缝成型。论文采用国产船用CCSB钢板,基于高速摄影系统和Labview图像处理平台,对其进行激光深熔焊。试验结果表明,等离子体的高度随侧吹气体中Ar含量的增大而降低,面积随之增大。激光功率增大时,等离子体高度和面积均增大。此结果与激光-等离子体模型的结果一致。同时,随着侧吹气体中He含量的减少,焊缝的表面熔宽增大,熔深有所减小。更多还原

【Abstract】 Laser-induced plasma reduces the energy density by keyhole through absorbing, refracting and reflecting the CO₂ laser energy then changes the laser energy coupling with the workpiece, which will then cause welding defects such as pores and no penetration, etc. Mixture of He-Ar as side-blown gas is often used in inhibiting plasma. Based on thermal conductivity, electron negativity, dissociation energy, ionization energy and other considerations, He is the ideal shielding gas. In industrial production, it is often to inhibit plasma by increasing side-blown gas flow rate. However, He resources are scarce and expensive, it is of great importance to do some research on the influence of side-blow gas with different He-Ar ratios on plasma for improving welding quality and reducing producing cost.In this paper, based on fluid dynamics theory, a two-dimensional unsteady flow model has been built for analyzing flow field under different side-blown gas He-Ar ratios. Through the model, pure He is used as side-blown gas and nozzle height, nozzle angle, nozzle radius and gas flow rates are changed to calculate the static pressure, horizontal velocity and He volume fraction at the laser focusing point under each condition. Besides, the above three parameters are used to optimize process parameters. Calculation result shows that the optimum nozzle angle range is 40°～55°, the optimum nozzle height range is 6～12 mm, the optimum nozzle radius range is 3mm⁴mm and the best range for nozzle in X direction is -1.5mm⁰.5mm. Based on the optimum ranges, process parameters are selected. Only change He-Ar ratios among pure He, 75%He25%Ar, 50%He50%Ar and calculate the static pressure, horizontal velocity and He volume fraction at the laser focusing point. The result shows that with the increase of Ar content, static pressure and horizontal velocity are increase whereas the volume fraction of He is reduced.In this paper, based on a comprehensive consideration of plasma’s absorption and refraction for laser energy and laser beam, a laser-plasma model has been built under different side-blown gas He-Ar ratio. Particle densities, absorption coefficients, thermodynamic properties and transport properties under different temperatures and within different plasma components are calculated, which have played a significant role in ensuring the accuracy of the model. The temperature field, velocity field and volume fraction field of the plasmas under different He-Ar ratios within pure He, 75%He25%Ar, 40%He60%Ar are calculated. The results show that with the increase of Ar content, the plasma height reduces and area increases. Meanwhile, with the increase of Ar fraction, the whole velocity magnitudes increase which show great velocity increase in X direction but little change in Z direction. The growth of horizontal velocity improves heat conductivity and it can help explain why the plasma height reduced along with the increase of Ar fraction. CCSB steel plates are used in the experiment. High speed camera system and Labview-based image processing platform are used in capturing plasma images and image processing. The experiment results show that plasma height reduced with the increase of Ar fraction while its area grows. The plasma height and area increase when laser power grows. Meanwhile, along with the reduction of He fraction of side-blown gas, the weld width increases while depth reduces.更多还原