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.     

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.     

Heat transfer and diffusion-controlled kinetics of liquid–solid phase in titanium matrix composite during selective laser melting

This study describes a self-consistent theoretical model of simulating diffusion-controlled kinetics on the liquid–solid phase boundary during high-speed solidification in the melt pool after the selective laser melting (SLM) process for titanium matrix composite based on Ti–TiC system. The model includes the heat transfer equation to estimate the temperature distribution in the melt pool and during crystallization process for some deposited layers. The temperature field is used in a micro region next to solid–liquid boundary, where solute micro segregation and dendrite growth are calculated by special approach based on transient liquid phase bonding. The effect of the SLM process parameters (laser power, scanning velocity, layer thickness and substrate size) on the microstructure solidification is being discussed.     

The influence of laser and powder defocusing characteristics on the surface quality in laser direct metal deposition

To investigate the influencing rules of the variations of powder and laser defocusing distance on surface quality and obtain the smooth surface of parts in laser direct metal deposition, the thin-walled metal parts were fabricated under three different powder defocusing distances and three different laser defocusing distances conditions. The experimental results show that a high surface quality can be obtained with the powder focussed below the substrate and laser focussed above the substrate process, and the variation in which the powder focus moves from above to below the melt pool plays a leading role and the variation in which the laser focus moves from above to below the melt pool plays a supplementary role in the influence on the surface quality. To explain the experimental results, a simple model of the track height is established.     

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.     

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.     

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.     

Determination of geometrical factors in Layerwise Laser Melting using optical process monitoring

Layerwise Laser Melting (LLM) is a layerwise production technique enabling the production of complex metallic parts. In the process a thin layer of powder is first deposited on a base plate. With the energy of a scanning laser beam this layer is melted at selected places, according to a predefined scanning pattern. After scanning, a new layer of powder is deposited on top of the previous layer and selectively melted. This sequence of depositing and scanning is repeated until the complete part is built. The local geometry surrounding the melt pool has a large influence on the processing behavior. For process control issues, this influence must be known and quantified, in order to determine a priori optimal processing conditions and to interpret measured melt pool radiation. In order to study the melt pool behavior, optical process monitoring of LLM has been applied using a high speed near-infrared CMOS camera and a large area silicon photodiode sensor. Data processing rates up to 10 kHz and real-time process monitoring are achieved using image and signal processing on a Field Programmable Gate Array (FPGA). Several case studies will be presented showing that the geometric influencing factors can be studied and quantified by analyzing the melt pool sensor output.     

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.     

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.     

Analysis and prediction of single laser tracks geometrical characteristics in coaxial laser cladding process

Direct Laser Fabrication is a promising new manufacturing technology coming from laser cladding process. From a coaxial nozzle, powder is fed through a laser beam on a substrate. The powder melting and solidification processes lead to the fabrication of a part layer by layer. In this work 316L stainless steel powder is used to form laser tracks on a low carbon steel substrate. The layer geometry is an important process characteristic to control the final part of fabrication. This paper presents analytical relationships between the laser tracks geometrical characteristics (width, height, area, penetration depth) and the processing parameters (laser power P, scanning speed V and powder mass flow Q_m). Three values of each processing parameters are fixed and so 27 different experiments have been made and analyzed. The validity of these results is discussed studying the correlation coefficient R, the graphical analysis of the residuals and the uncertainty evaluations. Two kinds of models are studied to predict the form and the geometrical characteristics of the single laser tracks cross sections. The first one is an analytical model in which the distribution of the powder in the feed jet is supposed to govern the laser clad geometry. Three distributions are proposed: Gaussian, uniform and polynomial. In the second model the general form of the clad cross section is supposed to be a disk due to the surface tension forces. Analytical relationships are established between the radius and the center of the disk in one hand and the process parameters in the other hand. This way we show that we can reproduce the laser track geometry in all the area experimentally explored.     

CO2 laser heating of surfaces: Melt pool formation at surface

The melt pool formation during the heating of titanium and steel surfaces by a moving CO₂ laser beam is examined. The repetitive pulses are introduced in the simulations and the Marangoni effect in the melt pool is incorporated in the model study. The influence of laser scanning speed and the laser intensity parameter on the melt pool size is also considered. The enthalpy–porosity method is adopted to account for the phase change in the irradiated spot. It is found that the influence of laser scanning speed on the melt pool size is considerable, which is more pronounced for laser beam parameter β=1. The melt pool size is smaller for stainless steel as compared to that corresponding to titanium.     

Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy

This paper reports on laser surface remelting experiments performed on a Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO₂ laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the microstructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210~mm/s. An implicit expression of the critical solidification velocity for the cellular--planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M--S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS was successfully achieved in the present experiments.     

激光表面纹理化过程中的Rayleight-Taylor不稳定性

 用表面纹理化的热模型,分析了固体表面的熔化和重固化的物理过程,计算了在纹理化过程中的表面温度,穿透深度,熔化深度,相面移动速度等物理量,提出了Rayleight-Taylor不稳定性形成周期化结构的机理。     

激光立体成形中熔池凝固微观组织的元胞自动机模拟

本文通过采用自适应网格技术, 将激光立体成形的宏观温度场模型和凝固微观组织的低网格各向异性元胞自动机模型(cellular automaton, CA)结合, 建立了适用于激光立体成形的集成数值模型. 模型包括基材的温度场分布, 熔池形貌和熔凝过程的凝固微观组织. 模拟了激光扫描速度为15 mm/s时, 激光作用在Fe-C单晶基材上形成熔池的形状以及熔池内凝固微观组织. 计算结果揭示了熔池内固液界面从平界面失稳到胞\枝晶的非稳态凝固过程, 并得到了平界面组织形成的白亮带. 白亮带上方形成了外延生长的枝晶列.     

Laser produced melt pool: Influence of laser intensity parameter on flow field in melt pool

The flow field developed in the laser produced melt pool is investigated and the influence of the Marangoni effect on temperature field is examined. The experiment is carried out to trace the solidified melt pool geometry and the heating is simulated in line with the experimental conditions to predict the melt size in the irradiated region. In the simulations, the control volume approach is used incorporating the Marangoni effect. The enthalpy-porosity method is adopted to account for the phase change in the irradiated region. The study is extended to include the influence of the laser intensity parameter (β) on temperature and the flow field in the melt pool. It is found that the melt pool geometry and the flow field in the melt pool is influenced by the laser intensity parameter. In this case, the number of circulation cell formed in the melt pool is doubled for the intensity parameter 0.4≤β≤0.6. The predictions of the melt pool geometry agree well with the experimental data.     

Micromachining of microchannel on the polycarbonate substrate with CO2 laser direct-writing ablation

Low-power CO₂ laser direct-writing ablation was used to micromachine a microchannel on the polycarbonate substrate in this work. The influence of the process parameters (the laser power, the moving velocity of the laser beam and the scanning times) on the micromachining quality (the depth, the width and their aspect ratio) of the microchannel was experimentally studied. The depth and width of microchannel both increase with the increase of the laser power and the decrease of the moving velocity of the laser beam. When higher laser power and slower moving velocity were used, the polycarbonate surface bore more heat irradiated from the CO₂ laser for longer time which results in the formation of deeper and wider molten pool, hence the ability to fabricate bigger microchannel. Because of the effect of the laser power on the depth and width of microchannels, higher aspect (depth/width) ratio could be achieved using slower moving velocity and higher laser power, and it would reach a steady state when the laser power increases to 9.0 W possibly caused by the effect of laser power on the different directions of microchannel. The polycarbonate–polycarbonate chip was bonded with hot-press bonding technique.     

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.     

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%.     

液氮冷却条件下激光快速熔凝Ni-28 wt%Sn合金组织演变

研究了在液氮冷却条件下激光快速熔凝Ni-28 wt%Sn亚共晶合金的组织演化过程. 结果显示, 熔池从上至下可以分为三个区域: 表层为平行激光扫描方向的α-Ni转向枝晶区; 中部为近乎垂直于熔池底部外延生长的α-Ni柱状晶区; 底部为少量的残留α-Ni初生相和大量的枝晶间(α-Ni+Ni₃Sn) 共晶组织. 激光熔凝区组织受原始基材组织的影响很大, 熔池中的α-Ni枝晶生长方向受到了热流方向和枝晶择优取向的双重影响. 与基材中存在的层片状、棒状和少量离异(α-Ni+Ni₃Sn)共晶的混合组织相比, 熔池内的共晶组织皆为细小的规则(α-Ni+Ni₃Sn)层片状共晶, 皆垂直于熔池底部外延生长, 并且从熔池顶部至底部, 共晶层片间距逐渐增大. 分别应用描述快速枝晶生长的Kurz-Giovanola-Trivedi 模型和描述快速共晶生长的Trivedi-Magnin-Kurz模型对熔池表层凝固界面前沿的过冷度进行估算, 发现熔池表层α-Ni 枝晶和(α-Ni+Ni₃Sn)层片共晶的生长过冷度在50.4-112.5 K 之间, 远大于相应深过冷凝固(α-Ni+Ni₃Sn) 反常共晶生长的临界过冷度20 K, 这说明文献报道的临界过冷度并不是反常共晶出现的充分条件.