Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

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.     

Modelling of multi-vortex convection of fine alloying components in the molten pool under the laser radiation

The work is devoted to the mathematical modelling and numerical solution of the problems of conjugate micro-convection, which arises under the laser radiation action in the metal melt with surface-active refractory disperse components added for the modification, hardening, and doping of the treated surface. A multi-vortex structure of the melt flow has been obtained, the number of vortices in which depends on the surface tension variation, on the temperature and power of laser radiation. Special attention is paid to the numerical modelling of the behavior in the melt of the substrate of disperse admixture consisting of the tungsten carbide particles. The role of microconvection in the distribution of powder particles in the surface layer of the substrate after its cooling is shown.     

Heat transfer modelling and stability analysis of selective laser melting

The process of direct manufacturing by selective laser melting basically consists of laser beam scanning over a thin powder layer deposited on a dense substrate. Complete remelting of the powder in the scanned zone and its good adhesion to the substrate ensure obtaining functional parts with improved mechanical properties. Experiments with single-line scanning indicate, that an interval of scanning velocities exists where the remelted tracks are uniform. The tracks become broken if the scanning velocity is outside this interval. This is extremely undesirable and referred to as the “balling” effect. A numerical model of coupled radiation and heat transfer is proposed to analyse the observed instability. The “balling” effect at high scanning velocities (above ∼20 cm/s for the present conditions) can be explained by the Plateau-Rayleigh capillary instability of the melt pool. Two factors stabilize the process with decreasing the scanning velocity: reducing the length-to-width ratio of the melt pool and increasing the width of its contact with the substrate.     

Single-step laser deposition of functionally graded coating by dual ‘wire-powder’ or ‘powder-powder’ feeding—A comparative study

The creation of iron-copper (Fe-Cu) alloys has practical application in improving the surface heat conduction and corrosion resistance of, for example, conformal cooling channels in steel moulds, but is difficult to achieve because the elements have got low inter-solubility and are prone to solidification cracking. Previous work by these authors has reported a method to produce a graded iron-nickel-copper coating in a single-step by direct diode laser deposition (DLD) of nickel wire and copper powder as a combined feedstock. This work investigates whether dual powder feeds can be used in that process to afford greater geometric flexibility and compares attributes of the ‘nickel wire and copper powder’ and ‘nickel powder and copper powder’ processes for deposition on a H13 tool steel substrate.In wire-powder deposition, a higher temperature developed in the melt pool causing a clad with a smooth gradient structure. The nickel powder in powder-powder deposition did not impart much heat into the melt pool so the melt pool solidified with sharp composition boundaries due to single metal melting in some parts. In wire-powder experiments, a graded structure was obtained by varying the flow rates of wire and powder. However, a graded structure was not realised in powder-powder experiments by varying either the feed or the directions. Reasons for the differences and flow patterns in the melt pools and their effect on final part properties of parts produced are discussed.     

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.     

Combining wire and coaxial powder feeding in laser direct metal deposition for rapid prototyping

Powder and wire deposition have been used separately in many laser-cladding, rapid prototyping and other additive manufacturing applications. In this paper, a new approach is investigated by simultaneously feeding powder from a coaxial nozzle and wire from an off-axis nozzle into the deposition melt pool. Multilayer parts are built from 316L steel using a 1.5 kW diode laser and different configurations of the powder and wire nozzles are compared in terms of surface roughness, deposition rate, porosity and microstructure. The parts are analysed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical microscopy techniques. Results show that deposition efficiency increased and surface roughness decreased with the combined process; some porosity was present in samples produced by this method, but it was 20-30% less than in samples produced by powder alone. Wire injection angles into the melt pool in both horizontal and vertical planes were found to be significant for attaining high deposition efficiency and good surface quality. Reasons for the final sample characteristics and differences between the combined process and the separate powder and wire feeding techniques are discussed.     

离焦量对空气中激光诱导铜等离子体光谱特性的影响

利用Nd: YAG脉冲激光在空气中烧蚀金属Cu靶,获得等离子体光谱;采用改变离焦量的方法,研究了离焦量的变化对谱线结构及谱线强度的影响;分析了离焦量分别为1mm、0mm和-2mm时,沿靶面法线方向不同空间距离处电子温度的演化规律;并对等离子体光谱的特性和产生机制进行了讨论. 结果表明,谱线结构、谱线强度和等离子体的电子温度都与离焦量的变化密切相关,聚焦点在-2mm处CuⅠ谱线相对强度出现峰值,电子温度数值最大;聚焦点在-0.5mm和-1.0mm附近谱线相对强度遽然降低的现象是由于等离子体的屏蔽效应造成的.     

Numerical simulation of metallic powder flow in a coaxial nozzle in laser direct metal deposition

To investigate the influencing rule of deposited layer’s shape on coaxial powder feeding flow field in the metal forming process, gas–solid two-phase flow theory is used to analyze effect of deposited layers on powder concentration distribution and variation of focus distance from nozzle outlet to convergence point (the center of the convergent zone). Different height and width parameters of deposited layers were chosen to calculate the powder concentration distribution, consequently, and also their effect on additive height of single-trace cladding layer was studied by experimental investigations. The numerical results are in good agreement with experimental observations. The results indicated that additive height of cladding layer was non-uniform under uneven wall thickness of parts fabrication condition. Consequently, the surface of deposited layers with uneven thickness is not smooth, and hence affects surface forming quality.     

Optical focus control system for laser welding and direct casting

A non-intrusive optical sensor system has been developed for focus control of laser welding. This detects the light generated by the process through the laser delivery optics, and exploits the chromatic aberrations of these optics to detect any laser focal error at the workpiece. This system works for a wide range of materials and welding parameters, and example results are presented. The sensor has also been applied to laser ‘direct casting’, a process in which 3-D structures are built by flowing metal powder into a focused laser beam. In this case, melt pool temperature is also important, and so additional optics are incorporated into the sensor to provide a pyrometric temperature measurement which is used to control the laser power.     

Investigation into the direct laser forming process of steam turbine blade

In order to achieve high precision and unidirectional solidification microstructure of steam turbine blade fabricated by direct laser forming (DLF), this study is focused on the influence of the laser scanning speed, specific energy and powder feeding rate on the forming characteristics of single cladding pass, which is elementary units in DLF, and a new method of columnar crystal preparation is investigated. Results show that both the metal powder stream and the laser specific energy are the most important factors, that control the high temperature behavior of molten pool so as to determine the micro-morphology of laser scanning track. Based on the optimum processing conditions, high definition steam turbine blade of 316L is fabricated (surface roughness Ra of 10.08 to 26.51 μm or so). The microstructure is of fish scale type by natural cooling after DLF, but it is first found that columnar crystals can be formed on the non-columnar crystal substrate of 316L by intermittent cooling with liquid argon. Therefore, the above investigations will benefit the rapid development of steam turbine blade.     

Porosity formation and gas bubble retention in laser metal deposition

One of the inherent problems associated with laser metal deposition using gas-assisted powder transfer is the formation of porosity, which can be detrimental to the mechanical properties of the bulk material. In this work, a comprehensive investigation of porosity is carried out using gas atomised Inconel 718 powder. In the analysis, a clear distinction is made between two types of porosity; namely lack of fusion and gas porosity. The results show that the two types of porosity are attributed by different factors. The gas porosity, which is more difficult to eliminate than the lack of fusion, can be as high as 0.7%. The study shows that the gas porosity is dependent on the process parameters and the melt pool dynamics. The flotation of entrapped gas bubbles was analysed, showing that in a stationary melt pool the gas would be retained by Marangoni-driven flow. The overall Marangoni-driven flow of the melt pool is in the order of five times higher than the flotation effect, and this is the reason why the melt pool geometry would tend to dominate the flow direction of the gas bubbles. Through optimisation, the gas porosity can be reduced to 0.037%.     

原子光刻中驻波场与基片距离的判定方法研究

原子光刻实验中, 激光驻波场能起到原子透镜的效果, 实现原子汇聚. 激光驻波场与沉积基片间的距离对形成纳米条纹结构的质量具有重要影响. 利用高斯光束传播规律, 提出了一种能够定量判断激光驻波场与沉积基片相对位置的实验方法. 该方法通过调节装载有凸透镜和反射镜的精密位移台改变驻波场距基片的距离, 利用光电探测器接收反射光强的变化, 将位移改变量转变为接收器的电压信号. 利用驻波场激光束光斑直径值, 实现准确定位驻波场与基片的距离. 对上述实验过程进行数值模拟, 数值计算的结果和实验结果高度符合. 该方法实现了准确定位驻波场距基片的距离, 为后续深入研究驻波场和基片间距离对沉积纳米条纹结构质量的影响提供实验基础.     

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.     

A melt depth prediction model for quality control of laser surface glazing of inhomogeneous materials

This paper reports on an investigation into the development of an analytical model for the quality control of laser marking/engraving of clay tiles using a high-power diode laser (HPDL). An analytical model for the laser melting of inhomogeneous workpieces with parabolic melt pool geometry being assumed was developed. The theoretical results were compared with the experimental data. The predicted melt pool depth and the experimental values were in close correlation with the parameter for values less than 15 Wmm⁻¹ s^−1/2, in spite of simplifications introduced in the model. At the relatively large values of parameter the assumption of parabolic melt pool shape and one-dimensional heat transfer no longer holds true.     

Applications of excimer laser in nanofabrication

This paper addresses novel applications of an excimer laser (308 nm wavelength, 20 ns pulse duration) in nanofabrication. Specifically, laser assisted nanoimprint lithography (LAN), self-perfection by liquefaction (SPEL), fabrication of metal nanoparticle arrays, and the fabrication of sub-10-nm nanofluidic channels are covered. In LAN, a polymeric resist is melted by the laser pulse, and then imprinted with a fused silica mold within 200 ns. LAN has been demonstrated in patterning various polymer nanostructures on different substrates with high fidelity and uniformity, and negligible heat effect on both the mold and the substrate. SPEL is a novel technology that uses selective melting to remove fabrication defects in nanostructures post fabrication. Depending on the boundary conditions, SPEL is categorized into three basic types: Open-SPEL that takes place with surface open, Capped-SPEL where a cap plate holds the top surface of the nanostructures and Guided-SPEL where a plate held a distance above the structure guides the molten materials to rise and form a new structure with better profile. Using SPEL (in less than 200 ns), we have achieved a reduction of line edge roughness (LER) of Cr lines to 1.5 nm (3σ) (560% improvement from the original), which is well below what the previous technologies permit, and a dramatic increase of the aspect ratio of a nanostructure. We have used SPEL to make sub-25-nm smooth cylindrical NIL pillar molds and smoothing Si waveguides. Excimer laser is also used to make metal nanoparticles. Monolayers of particles are fabricated on various substrates (silicon, fused silica and plastics) by exposing thin metal films to a single laser pulse. Periodic nanoparticle arrays have been fabricated by fragmentation of metal grating lines. The periodicity of these nanoparticles can be regulated by surface topography such as shallow trenches. Finally, an excimer laser pulse has been used to melt the top portion of 1D and 2D Si gratings to seal off the top surface, forming enclosed nanofluidic channel arrays. The channel width has been further reduced to 9 nm using self-limited thermal oxidation. DNA stretching using 20 nm wide self-sealed channels is also demonstrated.     

Investigation on the direct laser metallic powder deposition process via temperature measurement

The direct laser metallic powder deposition process was investigated with the aid of a radiant thermometer by building thin walls. The measured infrared (IR) temperature signal showed good correlation with the deposition process and the quality of the deposited samples. The influence of the powder particle size and the z-increment on the quality of the deposited samples and the IR-temperature signal was examined. It was found that the particle size of the powders shows no significant influence on the measured IR-temperature signal and the deposition process. However, both the deposition process and the measured temperature signal depended strongly on the z-increment. The variation of the melt pool temperature and cooling rate resulted in an inhomogeneous dimension accuracy, microstructure and hardness of the deposited sample. An abnormal deposition process can be recognized by the IR-temperature signal.     

Birth and evolution of wave-front dislocations in a laser beam passed through a photorefractive LiNbO3: Fe crystal

We report on an experimental and numerical investigation of the process of spontaneous optical vortices nucleation in a wave front of a laser beam passed through a photorefractive LiNbO₃ : Fe crystal with self-induced nonlinear lens. The complex lens structure produces mainly defocusing of the beam passing through the crystal due to a negative variation of the refractive index, whereas side parts of the lens have a positive sign of refractive-index variation and partially focus the beam. The resulting wave-front distortions lead to a phase bifurcation occurring at a certain distance after the crystal when the amplitude of the light wave becomes zero. We study in detail the process of edge dislocation nucleation and its decay in the near field producing a pair of unity-charged opposite-sign screw dislocations. After birth, they spread along dislocation axes as stable objects.     

Heat Transfer and Thermocapillary Convection during the Laser Deposition of Metal Powders Implemented in Additive Technologies

Heat-transfer- and thermocapillary-convection macroprocesses observed during direct laser metal deposition (DLMD) with coaxial powder injection are examined. The study is performed using the 3D mathematical model incorporating self-consistent equations for free surface evolution, heat transfer, and hydrodynamics, which allow for powder-particle embedding into the thermocapillary convection zone under DLMD. The processes under consideration refer to the main ones underlying additive laser technologies, which determine the microstructural properties and quality of synthesized parts. The convection-diffusion equations are numerically solved using the final volume method. Calculations are carried out for the thermocapillary convection of H13 steel powder. The influence of laser-radiation characteristics (power, scanning rate, intensity distribution in the beam) and the powder-mass flow velocity on temperature fields, the structure of convective melt flow (including a maximum melt velocity), and the geometric characteristics (height and width) of the object formed is investigated.     

高压熔渗生长法制备金刚石聚晶中碳的转化机制研究

在6 GPa和1500 ℃的压力和温度范围内, 利用高压熔渗生长法制备了纯金刚石聚晶, 深入研究了高温高压下金刚石聚晶生长过程中碳的转化机制. 利用光学显微镜、X-射线衍射、场发射扫描电子显微镜检测, 发现在熔渗过程中金刚石层出现了石墨化现象, 在烧结过程中金刚石颗粒表面形貌发生了变化. 根据实验现象分析, 在制备过程中存在三种碳的转化机制: 1)金属熔渗阶段金刚石颗粒表面石墨化产生石墨; 2)产生的石墨在烧结阶段很快转变为填充空隙的金刚石碳; 3)金刚石直接溶解在金属溶液中, 以金刚石形式在颗粒间析出, 填充空隙. 本文研究碳的转化机制为在高温高压金属溶剂法合成金刚石的条件下(6 GPa和1500 ℃的压力和温度范围内)工业批量化制备无添加剂、无空隙的纯金刚石聚晶提供了重要的理论指导.