【作者】 黄永俊；

【导师】 曾晓雁；

【作者基本信息】 华中科技大学 ， 物理电子学， 2009， 博士

【摘要】 激光—感应复合熔覆技术是将感应电磁加热与激光熔覆实时复合的新型熔覆技术。作为激光熔覆的升级技术，它能够弥补单纯激光熔覆能量不足及热源分布不利的缺点，具有熔覆速度快、熔覆沉积率高、工艺稳定性好、易于形成冶金结合、容易获得无裂纹的熔覆涂层等优点。激光—感应复合熔覆技术不是激光和感应热源的简单相加，感应加热的引入使熔覆过程中的物理化学变化更加复杂，因而对熔覆工艺、熔覆层的组织及性能等产生很大的影响。然而，国内外有关激光—感应复合熔覆技术的机理很少研究，阻碍了该技术的发展。本文对激光—感应复合熔覆的设备、工艺及机理进行了研究，获得的主要研究结果如下：构建了一个由高功率CO₂激光器、自动送粉器、数控工作台和高频感应器等部件组成的激光—感应复合熔覆系统。在此基础上，系统研究了激光—感应复合熔覆的基础工艺。结果表明：感应能量对熔覆层的宏观尺寸有明显的影响，随着感应能量的增加，熔覆层宽度W增大，高度H变化较小；复合熔覆层宏观尺寸与送粉速率、激光比能及感应能量密度等参数呈多元线性关系。系统研究了激光—感应复合熔覆中两种热源的作用方式。研究结果表明，对于氧乙炔热喷涂层、冷预涂层和自动送粉式激光—感应复合熔覆，激光和感应两种热源的作用方式是不同的。氧乙炔热喷涂层复合熔覆的感应涡流主要作用于熔覆层，随着感应加热的持续进行，感应涡流从熔覆层表面向下延伸，激光束的能量也是首先被熔覆层表面吸收，再通过热传导作用到熔覆层下部和基体；在冷预涂复合熔覆中，感应涡流主要作用于界面处基体表层，而激光直接作用于冷预涂层表面，激光热能通过热传导到达预涂层下部及基体；在自动送粉式复合熔覆中，感应涡流直接作用于界面处基体表层，一部分激光直接作用在粉末颗粒上，另一部分激光透过粉末云空隙作用于基材和熔融合金组成的熔池表面，它使基体和熔覆层更易熔化，改善了结合面及熔覆层表面的润湿性，提高了熔覆粉末材料的沉积率，是一种效率最高、对熔覆层结合性能最好的复合熔覆方式。通过引入无量纲参数激光—感应复合熔覆能量增益率Φ来定量评价各种工艺参数对复合熔覆效率的影响程度。定量分析结果表明，感应加热使复合熔覆能量显著增加。当其它工艺参数一定时，随着感应加热能量的增加，Φ值呈增大趋势，最大值Φ_max为20％，且存在一个获得Φ_max的极小感应能量值，这是由感应加热的特点及涡流的作用方式决定的；Φ值随送粉速率增大而明显增大，Φ_max达到110％。此外，与单纯激光熔覆相比，激光—感应复合熔覆效率大幅增加，Φ可达到341％以上，这意味着复合熔覆时的送粉速率和扫描速度可以大幅度提高。提出了适用于激光—感应复合熔覆的几何稀释率简化表达式，研究了激光—感应复合熔覆层的稀释率随工艺参数的变化规律。结果表明：随着扫描速度的增加，稀释率是一个呈马鞍形的变化过程；随着感应能量密度的增加，稀释率增大，当感应能量密度达到一定值时，稀释率基本保持不变。该研究结果对激光—感应复合层的稀释率控制及质量保证具有一定的实践指导意义。系统开展了激光—感应复合熔覆镍基自熔合金熔覆层的组织特征及性能研究。研究表明，复合熔覆层底部组织为树枝晶，随激光扫描速度加大及激光比能的减小，激光—感应复合熔覆层微观组织细小，显微硬度与抗拉强度提高，且快速扫描复合熔覆的显微组织结构和物相分布与单纯激光熔覆基本相同。另一方面，碳钢基材上单纯激光熔覆的热影响区为淬火组织。当感应能量较大时，激光—感应复合熔覆层中热影响区上部为过热区，热影响区下部为正火区；在一定的感应能量作用下，热影响区上部可得到正火组织。激光—感应复合熔覆层的热影响区中马氏体组织的消除，对于降低熔覆层的开裂程度十分有利。当激光—感应复合熔覆镍基自熔合金时，在合适的复合熔覆能量作用下，可以获得完全无裂纹的高性能熔覆层。更多还原

【Abstract】 The laser-induction hybrid cladding represents a novel surface treatment technology, which combines the induction heat source and laser beam simutaneously. This technology not only can overcome the weakness of heat distribution by laser cladding with single heat source, but also has the advantages such as higher scanning speed, bigger powder deposition rates, better processing stability and better metallurgical bond state. Especially, it is easy to fabricate the free-cracking cladding coating with high performance. However, laser-induction hybrid cladding processing is not a simple plus of laser and induction heat source. Compared with the individual laser cladding or pure induction cladding, the microstructures and properties of the hybrid coatings have been changed greatly due to the complexity of physical and chemical reactions in the processing of hybrid cladding. However, there have been few reports about the mechanisms of laser-induction heating hybrid cladding up to now. In this dissertation, the equipment, technology and mechanisms of laser-induction hybrid cladding have been studied systematically. The following are the main results:Firstly, a system of laser-induction hybrid cladding was set up, which was comprised of a CO₂ laser system, a powder auto-feeding apparatus, a 4-aixis working table with CNC controller and a high frequency induction heater, etc. Based on the system, the effects of processing parameters on the quality of the hybrid cladding coatings were studied systematically. The results demonstrate that the induction energy has big influence on the dimensions of hybrid cladding coating. With the increase of the induction energy, the coating width increases and the height keeps almost unchanged. Furthermore, the dimensions of hybrid coatings have a linear relationship with the powder feeding rate, laser specific energy and inducton energy density.It is significant to study the interaction mode of two heat sources between the laser and induction heating source in the laser-induction hybrid cladding processing. The results demonstrate that the interaction modes of two heat sources are totally different from each others for different cladding methods, i.e. pre-placed coating by thermal spraying, pre-placed coating by organic binder and powder feeding in laser-induction hybrid cladding. As far as the laser-induction hybrid cladding with thermal spraying pre-placed coatings, the induction eddy current is mainly produced in the coatings. The induction eddy current moves from the top to bottom of the coating in the course of induction heating, while the laser is directly absorbed by the surface of coating and then is conducted to the substrate subsequently. As far as the laser induction hybrid cladding with the pre-placed coatings by organic binder, the induction eddy current generates primarily on the surface of substrate, while the laser is absorbed by the surface of coating. As far as the laser induction hybrid coating with powder feeding, however, the induction eddy current is entirely produced on the surface of substrate, while the laser beam irradiates on the surface of molten pool simultaneously. This energy action mode improves the wetting capability of interface and increases powder deposition rates greatly, and thus has the highest cladding efficiency and the best bond strength with the substrate among above three hybrid cladding processing methods.A quantitative analysis method was developed to express the interaction extent of the laser and indution heat source during the laser-induction hybrid cladding processing. A dimensionless parameterΦwas introduced to denote the changes of the effective cladding energy. The dimensionless parameterΦwas defined as energy increase rate of laser-induction hybrid cladding. The biggerΦ, the stronger influence extent of cladding parameter on hybrid cladding energy. The results of quantitative analysis demonstrate that the induction heat enhances the cladding deposition rates distinctly. For low induction energy, the parameterΦand surface temperature of workpiece increase with the increasing of induction energy. When the inducton energy exceed a critical value, the parameterΦand surface temperature of workpiece will keep unchanged. On the other hand, the parameterΦincreases evidently with the increasing of powder feeding rate. When the powder rates exceed a critical value, the parameterΦwill be kept unchanged. In addition, compared with the individual laser cladding, the hybrid cladding energy is greatly enhanced and theΦcan exceed 341 %, which means that the powder feeding and scanning speed are obviously increased.The dilution expression of laser-induction hybrid cladding was put forward. Moreover, the relationship between the dilution and process parameters of hybrid cladding was investigated. The dilution is a saddle shape with the increasing of scanning speed. In addition, the dilution increases with the increasing of induction energy density. For high induction energy density, the dilution retains invariable. The above conclusions are crucial for controlling the dilution and dominating the quality of cladding coating in laser-induction hybrid cladding.The microstructures and properties of Ni-based coating prepared by laser-induction hybrid cladding were studied in detail. The results show that the basic microstructures of hybrid cladding coating are dendrites. The microstructures of the hybrid coating become finer with the increasing of scanning speed and the decreasing of laser specific energy. At the same time, the microhardness and tensile strength increase. Furthermore, the microstructures of hybrid coating are similar to Ni-based coating prepared by individual laser cladding. One very important experimental phenomena is that for individual laser cladding performed on steel substrate, the microstructures of the heat affected zone are martensite as the cooling speed is beyond the critical quenching speed. However, only normalization zone can be found in the heat affected zone in laser-induction hybrid cladding, which is highly helpful to reduce the cracking of cladding coating.Usually, the crack susceptibility is a serious problem in laser cladding due to the high heating and cooling speed. In laser-induction hybrid cladding of Ni-based coating, cracking is relatively low due to the preheating effect by induction energy. Furthermore, the appropriate hybrid cladding energy can fully eliminate the cracking of Ni-based coating in laser-induction hybrid cladding.更多还原