激光感应复合熔覆的稀释率分析模型及实验研究

在分析计算感应加热温度模型和激光感应复合熔覆能量作用的基础上,推导出了激光感应复合熔覆稀释率的表达式,定量反映了稀释率与激光、感应能量、材料特性和工艺参数之间的关系,并且通过实验进行了验证。研究结果表明：复合熔覆的稀释率随激光功率的增加而增大,随送粉率的增加而减小,随扫描速度的增大而减小;在其他参数恒定,感应能量增加时,稀释率显著增大。该稀释率分析模型有助于复合熔覆加工中稀释率的控制,为复合熔覆工艺参数的优化设计和熔覆层的质量控制提供了理论基础。     

激光熔覆中同轴送粉气体-粉末流数值模拟

应用FLUENT软件的离散相模块建立了激光熔覆中气体 粉末流的二维模型,研究了保护气和输送气流量及送粉量对粉末流浓度场和速度场的分布规律的影响以及对粉末流发散角和焦点的影响。计算结果表明:随着输送气流量的增大,粉末流速度增加,粉末流发散角逐渐减小;送粉量增加,焦点略微下移,焦点处粉末流浓度值增大。在相同的工艺参数下,使用单反相机拍摄粉末流分布,结果表明试验与计算结果基本吻合。     

激光熔覆中同轴粉末流温度场的数值模拟

在激光同轴送粉熔覆中,由于激光与粉末流相互作用,粉末流整体温度分布直接影响激光熔覆的质量.基于非预混燃烧模型,将激光相处理为连续性介质,粉末颗粒相看作离散相物质,建立了激光作用下粉末流的质量、动量和能量方程.用Fluent软件进行了不同激光功率和粉末流速度条件下粉末流整体温度场数值模拟,讨论了各种参数对温度场分布的影响.为了验证该模型的准确度,利用CCD比色测温方法测量了粉末流整体温度场分布.结果表明,数值模拟与CCD检测结果具有良好的一致性,数值模拟结果对激光熔覆具有指导意义.     

激光工业应用

使用2 kw半导体激光在工具钢表面熔覆高速钢粉末。在同轴送粉的粉末汇聚点与激光的聚焦点可获得无裂纹的熔覆层。随着激光功率的增加,熔覆层厚度和粉末利用率增加,同时基体对熔覆层的稀释率下降。获得的熔覆层的硬度达到800 Hvo.3,基体硬度200 Hvo.:,表明     

激光熔覆硬质合金强化化纤切断刀的研究

在机械压制法预置硬质合金WC/Co粉末的条形55Si2Mn弹簧钢上,用激光熔覆方法制备了化纤切断刀.调整熔覆层粉末配方中Al、TiC的加入量,结果表明加适量的Al粉能有效地抑制气孔,加TiC粉末能提高熔覆硬度.通过优化激光熔覆工艺参数,得到了无气孔缺陷、组织性能良好、硬度达到HV0.21250的激光熔覆层,达到了化纤切断刀的性能要求.     

激光熔覆中金属粉末粒子与激光相互作用模型

为了对同轴激光熔覆过程中运动的金属粉末粒子的速度和温度进行理论分析,并研究各工艺参量的影响,建立了运动中金属粉末粒子的运动模型和热模型.模拟结果表明,粉嘴几何尺寸、粒子直径以及气/粉两相流初始速度是影响粒子运动行为的重要因素;粉嘴几何尺寸、激光焦点位置、激光发散角、激光功率、粒子直径以及气/粉两相流初始速度是影响粒子热行为的重要因素.在相同的工艺参量下(粉嘴出口内径r=2 mm,粉嘴倾角α=60°,初始气流速度v0=0.8 m/s),基于数字粒子图像测速(DPIV)技术,对316L不锈钢粉末粒子运动模型进行了实验验证.结果表明,运动理论模型是可靠的.该模型是掌握同轴激光熔覆过程中金属粉末粒子运动行为的有效工具;同时,热模型也是分析粉末粒子温度随不同参量变化的重要工具.     

脉冲Nd:YAG激光熔覆修复塑料模具工艺参数的优化

 激光器的电压、电流、脉冲宽度、脉冲频率与激光的扫描速度、光斑直径等工艺参数直接影响着脉冲Nd:YAG激光熔覆质量。为了简化工艺调整过程，提出了重叠率的概念，并进行了理论推导，建立了重叠率与脉冲频率、激光扫描速度、激光光斑直径相互关系的方程式。利用预置法在低碳钢基体上熔覆Ni基合金粉末,获得了2组最佳工艺参数，即单脉冲能量、重叠率分别为6.7 J,97.4%和21 J,69.4%。研究表明：在单脉冲能量和重叠率一定的条件下，改变电流、脉宽、脉冲频率及熔覆速度不会影响熔覆质量。     

镍基非晶复合涂层激光制备及其纳米压痕测试

采用大功率半导体激光熔覆和重熔的工艺在低碳钢表面制备Ni-Fe-B-Si-Nb合金非晶复合涂层,并对所得涂层进行了纳米压痕性能测试。研究结果表明,当激光熔覆时激光功率为0.8kW,熔覆速度为0.36m/min,送粉速度为12g/min,重熔时激光功率为3.5kW,熔覆速度为8m/min,在低碳钢表面成功制备了Ni40.8Fe27.2B18Si10Nb4非晶复合涂层,涂层主要由非晶相和NbC颗粒相组成。纳米压痕测试结果表明经激光重熔后所得非晶复合涂层的显微硬度和弹性模量远远大于未重熔的熔覆层,并且也大于同成分大块非晶。     

停顿时间对多道熔覆镍基合金温度场的影响

根据激光熔覆的温度场理论,利用ANSYS有限元模拟软件,模拟了多道熔覆镍基合金上表面的温度场,重点研究了激光道次间的停顿时间对激光熔覆镍基合金温度场的影响。模拟在不同停顿时间条件下的激光熔覆镍基合金的温度场,得到每一道初始位置上表面的最高温度值,以及激光基底和熔覆层交界处不同位置的温度和温度梯度值。模拟结果表明,在多道熔覆的过程中,道次间停顿1.5s对比无停顿和停顿1s时熔覆效果好,并做了相应的实验。实验结果表明,道次间停顿1.5s时,熔池深度变小,熔覆的表面平整度较好,熔覆质量较高。数值模拟和实验一致性较好,模拟和实验结果都表明,停顿时间对多道熔覆镍基合金温度场的影响较大,研究成果对激光多道熔覆镍基合金选择合适的工艺参数、降低温度梯度、提高多道次激光熔覆质量具有重要意义。     

激光熔覆层开裂行为的影响因素及控制方法

激光熔覆层开裂是影响覆层质量最主要的因素,特别是厚覆层。选用５ｋＷ 横流ＣＯ２ 激光器对各种不同材料和零件进行厚覆层激光熔覆,实验、检测和使用的效果说明：激光熔覆层开裂主要与激光系统参数、工艺处理条件、覆层材料、基体状况等四个方面有关。分析了熔覆层的开裂行为,并介绍了几种控制方法。     

Effects of processing parameters on structure of Ni-based WC composite coatings during laser induction hybrid rapid cladding

The relationships between the processing parameters (i.e. laser specific energy, powder density, preheated temperature of substrate and types of substrate) and the structure characteristics of Ni-based WC composite coatings during laser induction hybrid rapid cladding (LIHRC) were investigated systematically. The results show that laser specific energy, cladding height and contact angle have a linear relation with powder density, as can provide the predictions of laser processing parameters according to the geometrical characteristics of a single laser track (i.e. cladding height, cladding width). Moreover, dilution of composite coating increases with the increasing of laser specific energy and the preheated temperature of substrate, while reduces with the increasing of powder density. The types of substrate also have an important effect on dilution of composite coating, as has a strong dependence on the thermophysical properties of substrate (i.e. melting point, resistivity and permeability).     

Prediction of clad angle in laser cladding by powder using response surface methodology and scatter search

Currently, laser cladding is an important process that allows the deposition of thick protective coatings on substrates. The article presents an experimental investigation of the influence of processing parameters on clad angle in laser cladding by powder (LCP). The clad angle is determined from the mathematical expression relating to the clad height and clad width. The cladding angle model was developed in terms of laser power, scanning speed, and powder mass flow rate by means of response surface methodology. A first-order equation covering a narrow range of the variables and a second-order equation covering a wide range of the variables are presented. An optimization technique, Scatter Search, is used to determine optimal processing parameters. The adequacy of the predictive model was tested by analysis of variance and found to be adequate.     

3-D finite element modeling of laser cladding by powder injection: effects of laser pulse shaping on the process

This paper introduces a 3-D transient finite element model of laser cladding by powder injection to investigate the effects of laser pulse shaping on the process. The proposed model can predict the clad geometry as a function of time and process parameters including laser pulse shaping, travel velocity, laser pulse energy, powder jet geometry, and material properties. In the proposed strategy, the interaction between powder and melt pool is assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is obtained, it is assumed that a layer of coating material is deposited on the intersection of the melt pool and powder stream in the absence of the laser beam in which its thickness is calculated based on the powder feedrate and elapsed time. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The process is simulated for different laser pulse frequencies and energies. The results are presented and compared with experimental data. The quality of clad bead for different parameter sets is experimentally evaluated and shown as a function of effective powder deposition density and effective energy density. The comparisons show excellent agreement between the modeling and experimental results for cases in which a high quality clad bead is expected.     

金属陶瓷复合涂层的激光熔覆与无裂纹的实现

鉴于传统的激光熔覆金属陶瓷复合涂层技术主要存在2方面不足：其一,熔覆效率低,导致大面积熔覆时成本昂贵;其二,由于激光熔覆本身的特点,即快速加热与快速凝固,在激光熔覆过程中,热应力极易诱导熔覆层开裂。基于此,综述了国内外激光熔覆金属陶瓷复合涂层的研究进展,指出其存在的主要问题,并提出了激光感应复合快速熔覆的新方法,即感应预热基材的同时快速激光熔覆。该方法不仅可使熔覆效率大大提高而且获得了无裂纹的金属陶瓷复合涂层。     

Study on modes of energy action in laser-induction hybrid cladding

The shape and microstructure in laser-induction hybrid cladding were investigated, in which the cladding material was provided by means of three different methods including the powder feeding, cold pre-placed coating (CPPC) and thermal pre-placed coating (TPPC). Moreover, the modes of energy action in laser-induction hybrid cladding were also studied. The results indicate that the cladding material supplying method has an important influence on the shape and microstructure of coating. The influence is decided by the mode of energy action in laser-induction hybrid cladding. During the TPPC hybrid cladding of Ni-based alloy, the laser and induction heating are mainly performed on coating. During the CPPC hybrid cladding of Ni-based alloy, the laser and induction heating are mainly performed on coating and substrate surface, respectively. In powder feeding hybrid cladding, a part of laser is absorbed by the powder particles directly, while the other part of laser penetrating powder cloud radiates on the molten pool. Meanwhile, the induction heating is entirely performed on the substrate. In addition, the wetting property on the interface is improved and the metallurgical bond between the coating and substrate is much easier to form. Therefore, the powder feeding laser-induction hybrid cladding has the highest cladding efficiency and the best bond property among three hybrid cladding methods.     

Effects of process variables on laser direct formation of thin wall

In this paper, effects of process variables on wall thickness, powder primary efficiency and speed of forming a thin metallic wall in single-pass coaxial laser cladding are investigated, and some resolution models are established and testified experimentally. With some assumptions, each of wall thickness, powder primary efficiency and formation speed can be defined as a function of the process variables. Wall thickness is equal to width of the molten pool created in single-pass laser cladding and determined by laser absorptivity, laser power, initial temperature, scanning speed and thermo-physical properties of clad material. Powder primary efficiency and formation speed are both dependent on an exponential function involving the ratio of melt pool width, which is decided by the process variables, to powder flow diameter. In addition, formation speed is influenced by powder feed rate. In present experiment, a 500 W continual-wave (CW) CO₂ laser is used to produce thin-wall samples by single-pass coaxial laser cladding. The experimental results agree well with the calculation values despite some errors.     

Study on cross-section clad profile in coaxial single-pass cladding with a low-power laser

In this paper, a model of cross-section clad profile on the substrate in coaxial single-pass cladding with a low-power laser was studied. The static model of powder mass concentration distribution at cold-stream conditions was defined as a Gaussian function. In coaxial single-pass cladding with a low-power laser, since the influence of surface tension, gravity and gas flow on the clad bead could be neglected, the cross-section profile of the clad bead deposited by a low-power laser on the substrate was dominated by the powder concentration at each point on the pool and the time when the material was liquid at this point. The height of each point on the cross-section clad profile was defined as a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left the melt pool. In the presented experiment, powder of Steel 63 (at 0.63 wt% C) was deposited on a substrate of Steel 20 (at 0.20 wt% C) at the laser power of 135 W. The experimental results testified the model.     

Investigation of temperature and stress fields in laser cladded coatings

Temporal and spatial distributions of temperature and strain-stress have been modelled and investigated experimentally for the laser cladding process. The model corresponded to experimental conditions where the multilayer protective coatings were prepared by direct laser cladding of stellite SF6 powder on X10Cr13 chromium steel by means of a 1.2 kW CO₂ laser. For calculations the effect of base preheating, temperature dependent material properties, and also influence of time-break between cladding of the consecutive layers were taken into account. The calculated temperature fields indicated good bonding of the substrate and coating, which was in agreement with the micro-analytical test results. A decrease of the number of microcracks in the coating with an increase of substrate preheating temperature was concluded from stress calculations and confirmed in the experiment. Moreover, an increase of the cracking susceptibility with an increase of the time delay between cladding of the consecutive layers was evidenced by modelling. The best technological results were obtained for the case of single-layer coatings prepared on a preheated substrate and for higher coating thickness required the processing of consecutive layers with a possibly short time delay is advisable due to effective usage of laser beam energy for preheating and lower temperature gradients.     

Analytical modeling and experimental investigation of laser induction hybrid rapid cladding for Ni-based WC composite coatings

Laser induction hybrid rapid cladding (LIHRC) cannot only increase the cladding efficiency, but can also eliminate porosity and cracking of ceramic–metal composite coatings. In order to obtain a deep understanding of LIHRC with rapid cladding speed and high powder deposition rate, an analytical model of LIHRC for Ni-based WC composite coatings is proposed in the paper. The predictions of cladding height and powder efficiency obtained with this model are in good agreement with experimental results. Injection angles at which the attenuation rate of laser power is relatively low are identified and crack-free composite coatings with smooth surface, good profile and metallurgical bonding to substrate can be obtained. The calculated results for the temperature of the powder particles are compared to experimental data of the microhardness profiles and show a similar trend.