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.     

Ceramic components manufacturing by selective laser sintering

In the present paper, technology of selective laser sintering/melting is applied to manufacture net shaped objects from pure yttria-zirconia powders. Experiments are carried out on Phenix Systems PM100 machine with 50 W fibre laser. Powder is spread by a roller over the surface of 100 mm diameter alumina cylinder. Design of experiments is applied to identify influent process parameters (powder characteristics, powder layering and laser manufacturing strategy) to obtain high-quality ceramic components (density and micro-structure).The influence of the yttria-zirconia particle size and morphology onto powder layering process is analysed. The influence of the powder layer thickness on laser sintering/melting is studied for different laser beam velocity V (V = 1250-2000 mm/s), defocalisation (−6 to 12 mm), distance between two neighbour melted lines (so-called “vectors”) (20-40 μm), vector length and temperature in the furnace. The powder bed density before laser sintering/melting also has significant influence on the manufactured samples density.Different manufacturing strategies are applied and compared: (a) different laser beam scanning paths to fill the sliced surfaces of the manufactured object, (b) variation of vector length (c) different strategies of powder layering, (d) temperature in the furnace and (e) post heat treatment in conventional furnace. Performance and limitations of different strategies are analysed applying the following criteria: geometrical accuracy of the manufactured samples, porosity. The process stability is proved by fabrication of 1 cm³ volume cube.     

A 3D dynamic numerical approach for temperature and thermal stress distributions in multilayer laser solid freeform fabrication process

This paper presents a 3D transient numerical approach for modeling the multilayer laser solid freeform fabrication (LSFF) process. Using this modeling approach, the geometry of the deposited material as well as temperature and thermal stress fields across the process domain can be predicted in a dynamic fashion. In the proposed method, coupled thermal and stress domains are numerically obtained assuming a decoupled interaction between the laser beam and powder stream. To predict the time-dependent geometry of the deposited material, once the melt pool boundary is obtained, the process domain is discretized in a cross-sectional fashion based on the powder feed rate, elapsed time, and intersection of the melt pool and powder stream area on the workpiece. Layers of additive material are then added onto the non-planar domain. Main process parameters affected by a multilayer deposition due to the formation of non-planar surfaces, such as powder catchment, are incorporated into the modeling approach to enhance the accuracy of the results. To demonstrate the proposed algorithm, fabrication of a four-layer thin wall of AISI 304 L stainless steel on a workpiece with the same material is modeled. The geometry of the wall, temperature, and stress fields across the modeling domain are dynamically predicted throughout the process. The model is used to investigate the effect of preheating and clamping the workpiece to the positioning table. Results show that preheating improves the process by reducing the thermal stresses as well as the settling time for the formation of a steady-state melt pool in the first layer. In addition, clamping the workpiece can also decrease thermal stresses at its critical locations (i.e. deposition region). In terms of geometrical aspects, the results show that the temperature and the thickness of the deposited layers increase at the end-points of layers 2–4. The reliability and the accuracy of the model are experimentally verified.     

Analysis and optimization of process parameters in Al–SiCp laser cladding

The laser cladding process parameters have great effect on the clad geometry and on dilution in the single and multi-pass aluminum matrix composite reinforced with SiC particles (Al/SiCp) coatings on ZE41 magnesium alloys deposited using a high-power diode laser (HPLD). The influence of the laser power (500–700 W), scan speed (3–17 mm/s) and laser beam focal position (focus, positive and negative defocus) on the shape factor, cladding-bead geometry, cladding-bead microstructure (including the presence of pores and cracks), and hardness has been evaluated. The correlation of these process parameters and their influence on the properties and ultimately, on the feasibility of the cladding process, is demonstrated. The importance of focal position is demonstrated. The different energy distribution of the laser beam cross section in focus plane or in positive and negative defocus plane affect on the cladding-bead properties.     

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.     

316L不锈钢粉末光纤激光选区熔化特性

采用自行研制的光纤激光选区熔化快速成型设备,研究了选区激光熔化316L不锈钢粉末工艺参数、能量输入与样件致密度、表面形貌之间的关系以及微观组织特征。结果表明：扫描速度对成型效果影响最为显著;样件致密度随着激光能量密度提高有逐渐增大的趋势;能量密度作为选区激光熔化工艺的技术指标具有可操作性;表面形貌由激光功率与扫描速度比值所决定。深入探讨了能量输入、熔化凝固行为、激光功率与扫描速度比值与样件致密度、表面形貌的关系。结果表明：选区激光熔化凝固组织层内、层间熔合处为弧形,且为冶金结合,金相组织主要由柱状晶与等轴晶组成,层内靠近熔合线周围是柱状晶,而层间靠近熔合线附近主要是细小等轴晶,晶粒直径为1 μm左右。     

316L不锈钢粉末光纤激光选区熔化特性

采用自行研制的光纤激光选区熔化快速成型设备,研究了选区激光熔化316L不锈钢粉末工艺参数、能量输入与样件致密度、表面形貌之间的关系以及微观组织特征。结果表明:扫描速度对成型效果影响最为显著;样件致密度随着激光能量密度提高有逐渐增大的趋势;能量密度作为选区激光熔化工艺的技术指标具有可操作性;表面形貌由激光功率与扫描速度比值所决定。深入探讨了能量输入、熔化凝固行为、激光功率与扫描速度比值与样件致密度、表面形貌的关系。结果表明:选区激光熔化凝固组织层内、层间熔合处为弧形,且为冶金结合,金相组织主要由柱状晶与等轴晶组成,层内靠近熔合线周围是柱状晶,而层间靠近熔合线附近主要是细小等轴晶,晶粒直径为1μm左右。     

Innovative reconsolidation of carbon steel machining swarf by laser metal deposition

The concept of widespread recycling of metals in order to save cost, energy and ecological damage is gaining importance and this necessitates not simply disposing of machining waste. In this work a new way of reconstituting chips/swarf into a usable solid structure is explored by using them in place of metal powder in laser direct metal deposition. Samples of carbon steel machining swarf in three size ranges are reconstituted and the final structural characteristics like clad dimension, microstructure and physical properties are analysed. The results show that it is possible to reproduce a material that has full density, fine microstructure and no significant contamination from an unprecedented size and shape of particles. As general trends, individual deposition tracks become lower, wider and less hard as particle size increases. This work shows that the laser deposition process can be used with a larger range of particle geometries than previously considered and this could be the point leading to a new ‘local’ recycling method.     

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.     

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.     

Solute transport and composition profile during direct metal deposition with coaxial powder injection

Direct metal deposition (DMD) with coaxial powder injection allows fabrication of three-dimensional geometry with rapidly solidified microstructure. During DMD, addition of powder leads to the interaction between laser and powder, and also the redistribution of solute. The concentration distribution of the alloying element is very important for mechanical properties of the deposited clad material. The evolution of concentration distribution of carbon and chromium in the molten pool is simulated using a self-consistent three-dimensional model, based on the solution of the equations of mass, momentum, energy conservation and solute transport in the molten pool. The experimental and calculated molten pool geometry is compared for model validation purposes.     

Effects of different binders on microstructure and phase composition of hydroxyapatite Nd-YAG laser clad coatings

The laser clad coating technique can help to produce metallurgical bonding with high bonding strength between the coating layer and the substrate, which has been gradually applied for hydroxyapatite (HA) coating on metallic substrates. In this study, HA powder is mixed with two different binders, namely water glass (WG) and polyvinyl alcohol (PVA), respectively, and is then clad on Ti-6Al-4V substrates using an Nd:YAG laser system under various processing conditions. The microstructure, chemical composition and hardness of the coating layer and transition layer of the various samples are then systematically explored. The experimental results show that the coating layers of the various samples all contain both cellular dendrites and rod-like piled structures, while the transition layers contain only cellular dendrites. For all samples, the coating layer consists mostly of CaTiO₃, Ca₂P₂O₇, CaO and HA phases, whereas the transition layer contains primarily CaTiO₃, Ca₂P₂O₇, Ti₃P, Ti and HA phases. In addition, the transition layer of the WG samples also contains SiO₂ and Si₂Ti phases. In all of the specimens, the transition layer has a higher average hardness than the substrate or coating layer. Moreover, the transition layer in the WG sample is harder than that in the PVA sample.     

40Cr钢表面激光熔覆层的磨损性能

为研究模具钢熔覆层的磨损性能,采用铁基粉在40Cr钢表面进行激光熔覆,以激光熔覆层为上试样,GCr15钢珠为下试样,采用HT－500磨损试验机进行摩擦磨损试验,并与40Cr基体的磨损性能相对比。利用表面形貌仪测量磨痕深度和宽度。研究结果表明：载荷小于250 g时,相同载荷下基体的摩擦系数大。载荷小于300 g时,随磨损时间延长,熔覆层、基体的摩擦系数都随着载荷增加而减小。当载荷为300 g时,基体的摩擦系数在0.563～0.589之间变化,平均值为0.576,且随时间逐渐升高,耐磨性变差;熔覆层的磨擦系数在0.431～0.457之间变化,平均摩擦系数为0.444,磨痕深度和宽度分别是0.65 mm和1.096 μm,且随时间逐渐下降,表现了良好的耐磨性能。当载荷增加到500 g时,平均摩擦系数和磨痕深度比300 g时分别增加了75％和47倍,且摩擦系数逐渐升高,磨损性能下降。     

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.     

Optimization of the UV CuII laser

The cw output power of the uv CuII laser has been optimized with respect to the hollow cathode geometry, the discharge current, the fill gas pressure and the resonator mirror parameters. A maximum laser output power of 900 mW for multiline operation at 248.6, 259.1, 260.0, and 270.3 nm was achieved with 100 A discharge current, 260 V voltage and 16 mbar fillgas pressure, when a hollow cathode of 1.2 m length and 2×6 mm² cross section was employed. The single-pass pain gl has been estimated to 7%. A hollow cathode cross section of 1.5×4.5 mm² is suggested as an optimum geometry. In addition, some investigations on the mechanism of the laser power decay in Ne–Cu-discharges are presented.     

Semiconductor intersubband laser/detector performance optimization using a simulated annealing algorithm

A numerical method for global optimization of semiconductor intersubband laser/detector performance parameters is presented. The single-band effective-mass Schroedinger equation is solved by employing the argument principle method (APM) to extract both the bound (B) and quasibound (QB) eigen-energies of the quantum heterostructure. APM is combined with a simulated annealing (SA) algorithm to determine a set of device design parameters such as potential barrier height V_i, layer thickness d_i, applied biasV_Bias , for which the intersubband device performance is within a predetermined convergence criterion. The method presented incorporates the energy-dependent effective mass of electrons in nonparabolic conduction bands. The performance of the method is evaluated for the design of an asymmetric Fabry–Perot electron-wave interference filter (laser structure) and a dual-band quantum well infrared photodetector (QWIP). Results with and without nonparabolic effects are presented. In addition, results from the present method are compared to results obtained via the optimization technique based on super-symmetric quantum mechanics (SUSYQM) for the case of an optically-pumped quantum cascade (QC) laser. The present method is shown to improve the device performance beyond that obtained via SUSYQM optimization. Further, the present model can handle many optimization parameters and can incorporate fabrication constraints to achieve physically realizable devices.     

Parametric analysis of the selective laser melting process

Selective laser sintering/melting (SLS/SLM) technology is used for manufacturing net-shaped objects from commercial Inox 904L powder with ≤20 μm particle size. Experiments were carried out on PHENIX-PM100 machine equipped with a 50 W cw fiber laser. Powder is layered by a roller over the surface of a 100 mm-diameter build cylinder. Optimal parameters of layer thickness and power input per unit speed for SLM were determined. It was shown that the greater the value of P/V ratio is, the larger is the remelted line (called as “vector”). Influence of the shifting of consecutive single vectors on the process of forming the first layer was studied. Different strategies for forming objects with less than 1 mm-sized inner structures were tested, as, for example, forming a 20 mm × 20 mm × 5 mm box with 140 μm-thick inner compartment walls.     

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。     

激光熔覆TiC陶瓷涂层的组织和摩擦磨损性能研究

采用激光熔覆技术在TC4合金表面上制备了TiC陶瓷涂层,分析了熔覆层的微观组织,测试了熔覆层的硬度和摩擦磨损性能。结果表明:TiC激光熔覆层分为熔覆区和稀释区两个区域,熔覆区未受到基底的稀释,由TiC颗粒和TiC树枝晶组成;稀释区受到了基底的稀释,由TiC树枝晶和钛合金组成;TiC激光熔覆层的显微硬度在HV700~1500之间,明显地改善了TC4合金表面的摩擦和磨损性能。     

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.