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摘要:
真空环境下激光焊接熔深得到显著提高，焊缝成形及气孔等缺陷得到极大改善，可以获得常规激光焊接方法难以获得的显著效果。近些年，有关真空激光焊接过程机理，低真空甚至局部负压环境激光焊接装置的研究日益完善，真空激光焊接技术在船舶、核电及压力容器等领域大厚板焊接中展现出良好的应用前景。本文首要概述环境压力变化对激光焊缝熔深，焊缝表面成形及气孔等缺陷的影响规律，从焊接过程等离子体羽辉及匙孔、熔池的动态行为特性方面总结国内外学者有关真空激光焊接机理的研究成果，并介绍了真空焊接技术在工业领域的应用情况，最后对目前已报道的研究中存在的问题进行分析并对真空焊接技术的发展前景进行展望。
Abstract:Compared with conventional laser welding, the welding quality was improved significantly while the laser welding was conducted under vacuum. The penetration depth of the weld seam increased sharply. The welding formation was improved and the porosity defects were suppressed effectively. In recent years, numerous researches on the mechanism of welding process, low vacuum and local subatmospheric pressure laser welding equipments have been undertaken. The laser welding under vacuum exhibits the wonderful application prospects to weld the thick plates in the shipbuilding, nuclear instrument and pressure vessel industries. The influences of ambient pressure on the laser welding penetration depth, surface formation and porosity defect were summarized. The domestic and overseas research findings on mechanism of laser welding under vacuum were elaborated from the aspects of plasma plume, keyhole and molten pool behaviors. In addition, the applications of laser welding under vacuum in the industry were introduced. Finally, the problems of reported researches were analyzed and the prospects of the technology were discussed.
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Key words:
- vacuum /
- laser welding /
- plasma plume /
- keyhole and molten pool behaviors
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Compared to electron beam welding, the penetration depth of high power laser welding is smaller due to theattenuation effect of the plasma plume on laser power deposition. To date, numerous researches on the suppression oflaser plasma plume had been undertaken, such as pulse laser welding, weaving laser welding, change the shielding gascompositions, and apply the side-assistant gas and electromagnetic fields. However, the increase of the penetration depthand the improvement of the weld quality were inconspicuous. Compared with conventional laser welding, the weldingformation and quality was improved significantly while the laser welding was conducted under vacuum. In this work,the influences of ambient pressure on the laser welding penetration depth, surface formation and porosity defect weresummarized. The domestic and overseas research findings on mechanism of laser welding under vacuum were elaborated from the aspects of plasma plume, keyhole and molten pool behaviors. In addition, the applications of laser welding under vacuum in the industry were introduced. Finally, the problems of reported researches were analyzed and theprospects of the technology were discussed. The previous researches on the laser welding under vacuum indicated thatthe penetration depth of the weld seams increased sharply, the welding formation was improved and the porosity defectswere suppressed effectively. Critical vacuum degree enough for improving the weld penetration depth and quality wasdetected for aluminum alloy, titanium alloy, nickel-base alloy and steel. The laser welding characteristics under vacuumwas related to the plasma plume, keyhole and molten pool flow behaviors. The planet wheel carrier in the power stationhad been welded successfully by applying this technology. The laser welding under vacuum exhibits the wonderful application prospects to weld the thick plates in the shipbuilding, nuclear instrument and pressure vessel industries. In thefuture works, the authors suggest that the laser power deposition mechanism should be investigated systematically byconsidering the physical properties of the materials and the collision characteristics of the ions in the plasma plume. Besides, the heat and mass transfer characteristics, solidification behavior of the molten pool should be studied. In order toexpand the application fields of laser welding under vacuum, the low vacuum and local subatmospheric pressure laserwelding equipments should be developed towards higher adaptability and integration. Local subatmospheric pressurelaser welding equipments with the excellent pressure maintaining property should be developed. Moreover, the feasibility of laser welding with filler and laser hybrid welding under vacuum is of vital interest for the development this technology.
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1. 引言
激光焊接由于其能量密度大自动化程度高，具备高质量、低变形、高精度、高效率及高柔性等特点，已被广泛应用于航空航天、核电、船舶、汽车、电子等领域，并被称为“21世纪最有发展潜力的高效智能化焊接方法之一”。近年来，随着大功率、高光束质量激光器制造技术的快速发展，其在船舶、核电、压力容器等工业领域厚板深熔焊方面的应用备受关注。但是，大功率激光焊接过程中，等离子体羽辉对激光的屏蔽作用增强，一方面，致密等离子体羽辉对激光的吸收和散射降低了激光能量的传输效率；另一方面，致密等离子体羽辉对激光的折射影响焊缝的成形。此外，铝合金材料激光深熔焊过程中匙孔的不稳定性和熔池的流动行为对焊缝气孔的影响也是学者们的研究焦点。
目前，学者们抑制等离子体羽辉采取的方法有：
1) 脉冲激光法[1]，调整脉冲激光的工艺参数，使脉冲激光作用于等离子体消散时，抑制等离子体对激光的屏蔽作用；
2) 摆动激光法[2]，通过激光束沿焊接方向上的来回摆动可有效“规避”等离子体羽辉；
3) 改变保护气成分(采用He和Ar混合保护气)[3]，采用He和Ar混合保护气时，等离子羽辉的温度相比纯Ar气时有明显的降低，He气的加入对等离子体羽辉有一定的抑制作用；
4) 加侧吹气流[4]，采用侧吹保护气的方法可以吹散部分等离子体，降低等离子体对激光的折射效应，提高激光能量的传输效率；
5) 外加电磁场[5, 6]，激光焊接过程中，外加适当的电磁场可以有效“驱散”等离子体，增大激光能量的吸收率。
上述方法虽然在一定程度上增大了激光深熔焊过程中能量的传输效率，但是针对铝合金等材料，对熔深的增大及焊缝成形的改善效果并不明显。
近年来有研究表明，在真空环境下，激光焊缝的熔深显著增大，焊缝气孔缺陷和焊缝成形得到极大的改善。日本的大阪大学，国内的哈尔滨工业大学、上海交通大学、华中科技大学、西北工业大学等单位研究了不锈钢、铝合金和高温合金等材料在真空环境下激光焊接成形特性。基于等离子体形貌及物理特性分析了真空环境下激光能量的传输特性，解释了焊缝熔深增大的原因。基于图像监测、数值模拟的方法分析了环境压力对激光深熔焊过程中匙孔及熔池动态行为的影响。
本文首先简要概述真空激光焊接特性，包括环境压力变化对焊缝熔深、焊缝表面成形及气孔等缺陷的影响规律，从焊接过程等离子体羽辉及匙孔、熔池的动态行为特性方面总结国内外学者有关真空激光焊接机理的研究成果，并介绍了真空焊接技术在工业领域的应用情况，最后对目前已报道的研究中存在的问题进行分析及真空焊接技术的发展前景进行展望。
2. 真空环境下激光焊接特性
大阪大学Arata[7]在1985年率先开展真空激光焊接的研究，作者发现随着环境压力的减小，钢的激光焊熔深显著增大，当环境压力为10-3 Torr(约0.1 Pa)，激光功率为11 kW，焊接速度为10 cm/min时，焊缝熔深可达到40 mm，如图 1所示。基于高速摄像和X射线透射结果可知，低真空环境下(低于几Torr)激光致等离子体几乎完全被抑制，并且匙孔特征与电子束焊接几乎相同。此后真空激光焊接技术的研究未见报道，直到2001年，大阪大学Katayama[8]研究了环境压力为30 Pa～30000 Pa时不锈钢和铝合金的激光焊接特性，同样，作者发现随着环境压力的减小，焊缝的熔深增大，熔宽变小。
低真空环境下，激光焊接的熔深可大幅增大，在实现大厚板的深熔焊接方面展现出较强的优势。Katayama[9, 10]在2011年建立一套可实现大功率真空激光焊接的装置，如图 2所示，真空舱内压力可达到0.1 kPa，组合两台YAG激光器使激光输出功率最大可达26 kW，通过对不锈钢进行真空激光焊接试验(环境压力为0.1 kPa)，发现焊缝熔深最大可达到73 mm，如图 3所示。当激光功率较大为16 kW时，环境压力降低可以有效抑制焊缝表面的飞溅缺陷，但是，当环境压力低于1 kPa时，焊缝出现驼峰缺陷。
从Katayama的试验结果可以看出，真空环境下激光焊缝的熔深并非随环境压力的减小线性增大，而是当环境压力低于某一数量级时，焊缝熔深大小趋于稳定，即真空度对焊缝成形和质量的影响存在临界值，德国学者Borner[11]也发现同样的现象，但是作者并没有对此进行深入的研究。哈尔滨工业大学李俐群[12, 13]自主研制一套可实现高真空环境的装置(极限真空度可达到10-4 Pa数量级)，并对真空激光焊接的极限真空度展开系统的研究，作者首先研究了铝合金激光焊缝熔深随焊接环境压力的变化规律，发现当环境压力低于101 Pa时，焊缝熔深基本不再发生变化，如图 4所示。并且作者也认为焊缝熔深的变化与光致等离子体羽辉特性有关，并观察了铝合金、钛合金、镍基合金和不锈钢真空激光焊接过程中等离子羽辉形貌，发现当舱内压力低于101 Pa时，等离子体羽辉的截面面积变化不明显，如图 5所示。作者认为真空激光焊接存在临界真空度，不需要极高的真空度就可以获得熔深较大且气孔缺陷极少的焊缝，并且材料的种类和激光参数对临界真空度的影响不大。
但是，在真空舱内焊接无法满足大尺寸结构件的焊接要求。基于临界低真空的观点，上海交通大学唐新华[14-17]设计了局部“真空室”，如图 6所示，通过快速抽气在密封腔内产生局部的负压，可使焊接工件熔池上方局部范围内达到低真空。作者通过试验发现当环境压力达到20 kPa时，激光焊缝的熔深会突然增大，并且焊缝的熔深随着局部“真空室”内的压力的减小呈现指数型增大的规律。但是焊接过程中局部“真空室”的移动会导致内部压力的增大，作者对比激光点焊和和连续焊(局部“真空室”随激光头一起运动)焊缝成形发现，连续焊的熔深较点焊有所减小，这是因为缝焊过程中，局部“真空室”的移动造成漏气现象从而导致气压的增大[16, 17]。
通用中国科学研究院Yang[18]采用局部真空辅助激光焊的方法得到汽车用镀锌钢板无间隙搭接的高质量接头。焊接过程中匙孔保持敞开且更加稳定有助于锌蒸汽及时逸出，此外，等离子体羽辉偏向局部真空的抽气装置，有助于提高熔池的稳定性及激光能量的吸收率。
西北工业大学包海涛[19, 20]对真空环境下不锈钢和镁合金的激光焊缝缺陷和力学性能进行了研究，发现真空环境下镁合金激光焊缝的裂纹缺陷得到有效控制，并且与大气环境下得到的焊缝相比，真空环境下焊缝中心附近位置的显微硬度值有所增大，这主要是因为真空环境下等离子羽辉得到抑制，等离子体的热转导作用减弱，焊缝晶粒未明显长大。
3. 等离子体羽辉特性、匙孔及熔池动态行为
学者们普遍认为真空环境下激光焊接熔深增大的一个主要原因是等离子体羽辉得到有效抑制。日本大阪大学Katayama[9, 10, 21]通过观察探测激光穿过等离子体羽辉后光斑位置及折射角的变化(如图 7所示)，认为真空环境下焊接激光束与等离子体羽辉的相互作用减弱是熔深增大的原因，并且真空环境下焊接飞溅得到有效抑制，匙孔的开口尺寸减小且更加稳定。
基于等离子体的光谱结果及理论计算，法国Fabbro[22]和国内哈尔滨工业大学李俐群[12, 23, 24]分析了真空激光焊接过程中等离子体羽辉的物理特性，激光焊接过程中等离子体的电子密度由环境压力所决定，等离子体的电子密度随着环境压力的降低而减小，等离子体对产生的透镜效应及对激光能量的逆韧制吸收可完全被抑制，激光能量的吸收模式以菲涅尔吸收为主，大气环境下激光焊缝的“钉子状”截面形状消失，熔宽变小，熔深变大。上海交通大学唐新华[14, 25]分析不同环境压力下探测激光透过等离子体羽辉的光谱结果，建立探测激光衰减的三维分布规律，由此得到了环境压力与等离子体羽辉被抑制的关系，即等离子体外侧扩散型羽辉、中部压缩型羽辉和匙孔附近驻留型羽辉被完全抑制时对应的压力值分别为80 kPa、20 kPa和3 kPa。
焊缝成形良好及气孔缺陷消失与匙孔的稳定性及熔池的流动行为密切相关。日本大阪大学Katayama[8]发现环境压力减小后焊缝的气孔缺陷得到有效改善，当环境压力为400 Pa时，在不锈钢和铝合金焊缝中均未观察到气孔，作者首次通过X射线透射系统观察了真空激光焊接过程中匙孔及熔池的动态行为，发现与大气环境下不同，当环境压力为400 Pa时，在匙孔的尖端并未发现气泡的产生，并且熔池液态金属沿着匙孔壁向上流动(大气环境下液态金属沿着熔池底部向后方流动，如图 8所示)，有助于气泡快速逸出熔池，如图 9所示。
上海交通大学唐新华[14]从一侧透明玻璃观察了真空焊接过程中小孔内壁的动态行为，发现随着环境压力的降低，匙孔的深度增大，匙孔后壁上由于匙孔内局部不稳定的金属蒸汽产生的不稳定波动得到改善，当环境压力低于10 kPa时，匙孔后壁变得平滑且稳定，从而可以有效抑制气孔缺陷的产生。
法国Fabbro和国内华中科技大学庞盛永[26-28]对匙孔壁蒸汽反作用力计算模型进行改进，首次计算了环境压力对激光深熔焊过程中匙孔动态行为的影响。作者发现真空环境下匙孔壁的平均温度较大气环境下有所降低，如图 10所示，导致真空环境下焊缝熔深增大。并且作者认为匙孔表面温度的不同会影响匙孔附近液态金属的流动行为，相比之下，大气环境下匙孔壁温度较高导致匙孔附近热传输作用明显使熔池流动行为更加复杂，从而使焊缝截面形貌发生变化，如图 11所示。此外，作者还发现随着焊接速度的增大，真空环境下焊缝的熔深的增大幅度逐渐变小，这是因为随着焊接速度的增大匙孔壁的温度升高。当焊接速度较高(6 m/min)时，真空环境下焊缝熔深和大气环境下趋于一致，这是因为高速焊接时，环境压力的变化不会改变金属的蒸发温度[28]。
4. 真空激光焊接的应用
德国Borner[29]自制一套真空激光焊接设备，如图 12所示，真空舱尺寸为1.5 m3，极限真空度可以达到10-1 Pa，作者对3 mm，10 mm和15 mm不同厚度的钢的真空激光焊接特性进行研究，发现焊接过程飞溅缺陷得到有效抑制，焊缝的几何形状得到改善。作者认为真空环境下激光焊缝质量得到有效改善，拓宽了大功率固态激光器在工业领域的应用范围。
2015年，德国PTR公司[30]与亚琛工业大学合作开发了一套真空激光焊接设备，激光头置于真空舱内部，如图 13所示。将真空激光焊接技术应用于电站大型齿轮焊接上，获得了表面光滑、无缺陷的焊缝，如图 14所示。该研究为真空激光焊接技术在工业领域的应用提供宝贵的参考。
5. 总结及展望
自1985年提出真空激光焊接技术以来，学者们普遍发现在低真空或者局部负压环境下，激光焊接焊缝熔深得到大幅提高，焊接过程飞溅及焊缝气孔等缺陷得到有效改善，并且环境压力对激光焊缝熔深及缺陷的影响存在临界值。真空环境下焊接特性的变化与等离子行为及熔池流动行为密切相关。随着大型真空舱及局部真空装置的出现，真空激光焊接技术已在电站大型齿轮焊接中得到应用，并在船舶、核电、压力容器等工业领域大厚板焊接中展现出较强的应用前景。
但是目前对真空环境下激光能量传输特性的研究尚不完善。对于真空环境下熔池液态金属流动行为发生改变缺乏科学的解释。真空激光焊接时，合金元素的蒸发、熔池液态金属的流动行为对焊缝金属凝固特性及接头性能的影响也有待进一步明确。在工程应用方面，真空激光焊缝“窄而深”的特点对工件装配的适应性较差，对真空激光焊接在工业领域的广泛应用有一定的限制。
未来真空焊接技术的发展需要从以下方面突破：
1) 有关真空环境下激光能量传输特性的研究，还需考虑真空环境下材料物性的变化、离子碰撞特性对等离子体羽辉团聚及扩散的影响。需借助图像在线监测、数值模拟、匙孔壁受力理论分析等方法系统的研究真空环境下熔池传热传质特性，并进一步阐明真空环境下焊缝金属的凝固行为。
2) 真空或局部真空装置应向高适应性和高度集成化的方向发展。针对大尺寸复杂结构件的焊接，需研制保压性能更好且结构适应性强的局部真空装置。并且，要在真空环境下实现激光填丝甚至激光复合焊接，对真空焊接舱或局部真空装置的设计和制造提出了更高的要求。
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