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.     

An in-process method for the inverse estimation of the powder catchment efficiency during laser cladding

An estimation of the heat loss by conduction can be obtained from measurements of the surface temperature and an overall heat balance at the clad laser interaction zone. Through an inverse calculation of the boundary temperature from observed surface temperatures the powder catchment efficiency can be estimated along with the variation in the clad height expected during laser cladding. This method shows a possible way to monitor and control the clad height and profile as required by near net shape manufacturing methods based on laser cladding.     

Coaxial laser cladding on an inclined substrate

This paper describes an experimental and theoretical study of the cladding mode of coaxial laser cladding on an inclined substrate. Based on the image analysis of the powder stream and clad profile measurements in coaxial laser cladding, it was found that irregular clad profiles always formed on an inclined surface and the location of the peak profile shifted away from the clad center. This phenomenon is caused by uneven distributions of powder concentration and laser beam intensity. A modified Gaussian mode for powder stream and laser beam was proposed to estimate the clad profiles on an inclined plane under laser beam irradiation. The effects of the inclined steel substrate on the CO₂ laser beam absorption and stainless-steel powder catchment were examined experimentally. The results show that both the laser absorption and the powder catchment on the mild steel decrease with increasing the cladding angle. From the analysis of laser beam mode, the clad width is equivalent to the beam spot size on the inclined substrate. However, the clad height correlates well with the distribution of the powder concentration. The results show that the Gaussian cladding mode could be adopted in various laser cladding applications such as rapid prototyping and butt welding to predict the clad profiles precisely.     

Finite element analysis of laser irradiated metal heating and melting processes

Laser technology has shown fast growth due to its demands in material processing and manufacturing. Laser material processing includes various applications like cutting, welding, drilling, cladding and surface treatment. In laser surface treatment, the material properties at the surface are altered through surface alloying and transformation hardening. In this study, an enthalpy-based computational model is developed for analyzing laser heating and melting of metals. The solution to the problem is obtained by using a finite element method and validated by comparing the results with that given by an analytical solution to a limiting case problem. A solution algorithm and a computational code are developed to estimate the temperature distribution, solid-liquid interface location and shape and size of the molten pool. The computational model is validated by comparing results with a limiting case analytical model. The study is conducted to analyze the heating rate, the heat affected zone, and the shape and size of the molten pool using a Gaussian laser beam.     

Modelling of heat and mass transfer in the laser cladding during direct metal deposition

A physical and mathematical model has been proposed for computing the thermal state and shape of the individual deposited track at the laser powder cladding. A three-dimensional statement of the two-phase problem of Stefan type with curved moving boundaries is considered. One of the boundaries is the melting-crystallization boundary, and the other is the boundary of the deposited layer, where the conservation laws are written from the condition of the inflow of the additional mass and energy. To describe the track shape the equation of kinematic compatibility of the points of a surface is used, the motion of which occurs at the expense of the mass of powder particles supplied to the radiation spot. An explicit finite difference scheme on a rectangular nonuniform grid is used for numerical solution of equations. The computations are carried out by through computation without an explicit identification of curved boundaries by using a modification of the immersed boundary method. The computational results are presented for the thermal state and the shape of the surface of the forming individual track depending on physical parameters: the substrate initial temperature, laser radiation intensity, scanning speed, powder feeding rate, etc.     

In-time motion adjustment in laser cladding manufacturing process for improving dimensional accuracy and surface finish of the formed part

This paper presents in-time motion adjustment in laser cladding manufacturing process as a means to improve dimensional accuracy and surface finish of the built part. Defects occurring during laser cladding degrade the part quality such as dimensional accuracy and surface finish. In this paper, in-time motion adjustment strategy was presented to remedy and eliminate defects occurring during laser cladding to improve the dimensional accuracy and surface finish. Based on the relationship between the motion of laser head relative to the growing part and other parameters in effects on clad profile, the laser traverse speed, stand-off distance and laser approach orientation to the existing clad layer were adjusted by instructions from a close-loop control system in real time to remedy and eliminate defects. The results of the experiments verified the effects of in-time motion adjustment on dimensional accuracy and surface finish.     

Native oxide buried InAlAs ridge waveguide laser diode

InAlAs wet thermal oxidation process was adopted to fabricate ridge waveguide laser diodes. First, applying the oxidation process on the whole etched surface of an InAlAs upper cladding layer, we formed a current blocking and optical confining layer of ridge waveguide laser diode. This is a self-aligned structure, which makes the laser diode fabrication steps much simpler and etching-depth control less important. Assessment of the fabricated laser diode revealed that current-voltage characteristics and slope efficiency did not worsen, and threshold current was reduced by the oxidation process. PACS 85.35.Be; 42.55.Px     

激光冲击对E690高强钢激光熔覆修复微观组织的影响

为研究激光冲击对E690高强钢激光熔覆修复层微观组织的影响,选用专用金属粉末对E690高强钢试样预制凹坑进行激光熔覆修复,并使用脉冲激光对激光熔覆层进行冲击强化处理,同时采用扫描电镜、透射电镜和X射线应力分析仪分别对激光冲击前后激光熔覆层的微观组织和表面残余应力进行检测。结果表明:激光熔覆修复后,激光熔覆层组织为等轴晶,熔覆层与E690高强钢基体之间冶金结合良好,其表面残余应力为均匀分布的压应力。经激光冲击后,激光熔覆层截面晶粒得到细化,并观察到大量的形变孪晶,互相平行的孪晶界分割熔覆层粗大晶粒,在激光熔覆层的晶粒细化过程中发挥着重要作用;试样表层位错在{110}滑移面上发生交滑移,在晶界周围形成了位错缠结。经激光冲击后,激光熔覆层冲击区域表面残余压应力数值相较于冲击前提升了1.1倍。     

激光熔敷WC-TiN-SiC-Co高硬质合金及机理初探

叙述了用一台输出功率为2kW的连续CO₂激光器,在碳钢表面实现了激光熔敷WC-TiN-SiC-Co高硬质合金的实验方法,并对激光处理后的样品,从相的组成,显微组织,硬度分布及耐磨性能等方面作了综合分析。结果表明:样品表面的化学成分,显微组织和机械性能都发生了根本性转变和很大的提高,同时对激光熔敷形成高硬质合金的机理作了初步的探讨。 关键词：     

光纤布拉格光栅及其应用

近年来新型的紫外激光写入的光纤布拉格光栅元件（ＦＢＧ）以其具有直接写人光纤芯区、插入损耗小、易于全光集成及波长选择性好、传感信息对波长绝对编码等优点,成为国内外光纤技术领域的研究热点。该论文综述了ＦＢＧ制作技术的进展,介绍和比较了各种制作方法的优缺点,对ＦＢＧ在光纤通讯、光纤传感器等方面的应用前景进行了分析和说明,结果表明ＦＢＧ将对整个光纤技术领域产生重要的影响。     

Microstructure and interface interaction in laser induction hybrid cladding of Ni-based coating

Based on the powder feeding laser induction hybrid cladding experiments by means of three different laser and induction energy, the microstructure of Ni-based coating and interface characteristics between the Ni-based coating and steel substrate were investigated. The results show that the hybrid cladding energy including laser and induction energy has an important influence on the formation of the interface and the microstructure of the Ni-based coating characterized by the dendrite. In addition, the laser and induction energy can complement each other. For high hybrid cladding energy, the single phase Fe-Ni-Cr solid solution is formed at the interface between the coating and substrate, while the microhardness of the Ni-based coating decreases. For low hybrid cladding energy, the solid solution phases of Fe-Ni-Cr and Ni-Fe-Cr are respectively obtained on both sides of the interface and microhardness of the Ni-based coating is relatively high. During laser induction hybrid cladding, the metallurgical bond characterized by the white light layer is achieved between the coating and substrate, and the extent of metallurgical reaction can be controlled by adjusting the laser energy and induction energy appropriately.     

Plunging method for Nd : YAG laser cladding with wire feeding

The effects of wire feeding direction and position, cladding time, and cladding speed on the quality of cladding layer for laser cladding with wire feeding are studied. Experiment results indicate that the wire feeding direction and position are important for wire laser cladding. By adopting the correct wire feeding direction and position, the wire can be plunged into the melt pool and can be melted by the heat of the molten metal. With the increase of cladding speed, the dilution of clad layer can be reduced and the growth of the grain size of heat-affected zone of the base metal can be limited. If proper cladding parameters are used, the clad layers will have a good surface shape, sound metallurgical bonding with base metal, low dilution, and the effect of heating on heat-affected zone metal can be limited. The hardness of the clad layer and heat-affected zone are checked. It indicates that with the increase of cladding speed, the hardness of the clad layer and heat-affected zone increases.     

Depressed cladding, buried waveguide laser formed in a YAG:Nd3+ crystal by femtosecond laser writing

Depressed cladding waveguides have been formed in laser crystals by a tightly focused beam of a femtosecond laser. A laser based on a depressed cladding waveguide in a neodymium-doped YAG crystal has been demonstrated for what is believed to be the first time.     

Nd:YAG纳秒激光诱导硅表面微结构的演化

利用Nd:YAG纳秒激光(波长为532和355 nm)对单晶硅在真空中进行了累积脉冲辐照,研究了表面微结构的演化情况.在激光辐照的初始阶段,532和355 nm激光脉冲均在硅表面诱导出了波纹结构,后者辐照硅表面后形成了近似同心但稍显混乱的环形波纹结构.随着脉冲数的增加,波纹结构逐渐演化为一种类似珠形的凹凸结构,最后形成准规则排列的微米量级锥形结构,该微结构的生长依赖于表面张力波和结构自组织.分析发现,形成的交叉环形结构主要是在355 nm激光辐照硅的过程中,表面张力波导致波纹结构部分叠加的结果.     

激光工艺参数对钛合金表面NiCrBSi合金熔覆层组织及硬度的影响

在TC4合金表面进行了激光熔覆NiCrBSi合金涂层的试验 ,利用SEM和XRD等对熔覆层的微观组织进行了分析 ,测试了熔覆层的显微硬度。结果表明 ,激光工艺参数对熔覆层的组织和硬度有极大的影响 ,随稀释率的增加 ,激光熔覆层中形成了TiB2 和TiC等颗粒增强相 ,熔覆层的硬度明显提高。     

Numerical simulation of powder transport behavior in laser cladding with coaxial powder feeding

Laser cladding with coaxial powder feeding is one of the new processes applied to produce well bonding coating on the component to improve performance of its surface. In the process, the clad material is transported by the carrying gas through the coaxial nozzle, generating gas-powder flow. The powder feeding process in the coaxial laser cladding has important influence on the clad qualities. A 3D numerical model was developed to study the powder stream structure of a coaxial feeding nozzle. The predicted powder stream structure was well agreed with the experimental one. The validated model was used to explore the collision behavior of particles in the coaxial nozzle, as well as powder concentration distribution. It was found that the particle diameter and restitution coefficient greatly affect the velocity vector at outlet of nozzle due to the collisions, as well as the powder stream convergence characteristics below the nozzle. The results indicated a practical approach to optimize the powder stream for the coaxial laser cladding.     

激光熔覆TiC-Ni复合涂层的摩擦磨损性能和磨损机制研究

利用光学显微镜和扫描电镜观察了钛合金表面TiC-Ni激光熔覆层的宏观形貌和微观组织,测试了激光熔覆层的硬度、摩擦系数和磨损量。利用SEM观察了磨损的表面形貌和磨屑的形貌,分析了激光熔覆层的磨损机制。结果表明:激光熔覆层组织致密,无气孔和裂纹,硬度为基材的3倍;激光熔覆层的摩擦系数随环境压力的降低而提高,磨损量随环境压力的降低、法向载荷的增加而增加;低载时为轻微的磨粒磨损,高载时为严重的剥层磨损。     

Theoretical model for the transverse interference pattern of GRIN optical fiber using a laser sheet of light

A new technique (El-Ghandoor et al., Opt. Laser Technol. 31(7) (1999) 481–488) has been applied to study the shape of transverse interference fringes, instead of multiple beam Fizeau fringes (Marhic, Stein, Appl. Phys. Lett. 35 (1975) 1678–1682), from a GRIN optical fiber. In this technique, a laser light sheet is used to illuminate and pass directly through the optical fiber. Theoretical expressions are derived for the optical path differences of three groups of interference beams. The first one passes through the cladding, the core, and then the cladding once again; the second passes through the cladding only, and the third passes through the surrounding air. Theoretical expressions for the shape of transverse interference fringes formed across isotropic non-absorbing optical fibers are also calculated.     

不锈钢表面激光熔覆层与喷焊层耐磨性对比研究

本文研究在１Ｃｒ１８Ｎｉ９Ｔｉ基体上采用激光熔覆和离子喷焊二种工艺形成的涂层对耐磨性的影响。使用５ｋＷ横流ＣＯ２激光器对预置在基体上的Ｃｏ基自熔合金粉末进行单道或多道扫描，得到的熔层与等离子焊层对比结果是：激光熔层缺陷率低，成品率高，其结构致密均匀，晶粒细小，成分稀释率更小，对基体热影响小，熔层硬度与强韧性更高。性能试验证明：激光熔层具有更高的抗擦伤磨损和抗冲击滑动高温磨损性能，耐磨性提高了一倍左右。     

Formation of cross-sectional profile of a clad bead in coaxial laser cladding

In order to determine a cross-sectional profile of a clad bead in coaxial laser cladding, its formation mechanism is investigated theoretically and experimentally. In laser cladding, every point at the back edge of a melt pool is contributed to a cross-sectional profile of the clad bead to be formed, and points at the same pool edge but on different cross sections are located at different cross-sectional profiles of the clad bead. A cross-sectional profile of a clad bead is composed of points of intersection between the cross section and a series of pool edges. Model of the cross-sectional clad profile in single-pass coaxial laser cladding is developed. A 500 W CO₂ laser is used in the experiment. The experimental result agrees well with the calculated cross-sectional clad profile.