40Cr轴面激光熔覆Ni60数值模拟

目的 针对激光熔覆过程中熔池内部复杂的传热和对流现象,分析激光功率和扫描速度对熔池内部温度场、流场演变和分布的影响.方法 采用双椭球热源模型,建立了40Cr轴面基体激光熔覆Ni60粉末过程的三维温度场流场数值模型,并进行试验验证.结果 熔覆过程形成了近似椭球体的熔池,最高温度位于移动光斑中心偏后方,达到了2080.4 ...     

The influence of convection on the homogeneity of laser-applied coatings

During laser alloying, a melt pool is created at the passage of a laser beam in which mixing occurs by convection because of surface tension gradients. This mixing process in the melt bath is reported here in different alloying elements on aluminium (manganese, nickel, cobalt) and steel (nickel) substrates and a homogeneous distribution did not appear to be evident. It is concluded that density differences, interface tensions and diffusion are the main factors affecting the mixing process.     

铍激光热传导对接焊对接间隙的数值处理方法

提出了一种虚拟材料法,用以消除铍激光热传导对接焊热过程数值分析中钎料与母材间的对接间隙所带来的能量边界条件.假定对接间隙被一种具有不同于母材和钎料的热物性参数的虚拟材料所填充,用指数旋转抛物线体热源模型预测了铍激光热传导对接焊缝横断面轮廓,并进行了试验研究.结果表明,在考虑对接间隙的情况下计算所得的熔池轮廓更接近于试验结果.虚拟材料的导热系数对熔池形状的影响较大,而密度和比热对熔池形状的影响较小.随着导热系数的增大,熔深和熔宽均逐渐减小.当导热系数取30 W/(m.K)时,计算所得的熔池形状与试验测得的值吻合良好.     

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Abstract

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB₂) was injected into a melt pool created by a CW–CO₂ laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min^–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min^–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.     

激光熔丝增材过程传热流动行为数值模拟

目的 研究激光熔丝增材制造过程的熔池流动特性,探究工艺参数对熔池流动与传热行为的影响.方法 建立了考虑运动丝材持续送进过程的激光熔丝增材熔池传热和流动行为数学模型.针对316L不锈钢的激光熔丝增材制造,开展了成形过程中丝材送进、熔化和凝固行为的实验和数值模拟研究.结果 模拟结果 显示在成形过程中,准稳态阶段激光辐照中心的最高温度约为2500 K.金属液主要由丝材端部向熔池尾部流去,并在熔池尾部凝固形成堆积体.同时,熔池表面最大速度可达0.8 m/s,并具有速度振荡特征.结论 基于激光熔丝增材制造过程数学模型的模拟结果 与实验吻合良好,结果 表明,减小送丝速度会增大熔池表面高温区面积,并导致熔池的速度振荡程度增加.     

Evolution of microstructure in laser clad Fe–Cr–Mn–C alloy

Abstract

The laser surface cladding technique was used to form in situ Fe–Cr–Mn–C alloys on AISI 1016 steel substrate. In this process, mixed powders containing Cr, Mn, and C in the weight ratio 10: 1 : 1 were delivered using a screw feed, gravity flow, carrier gas aided system into the melt pool generated by a 10 kW CO₂ laser. This technique produced an ultrafine microstructure in the clad alloy layer. The microstructure of the laser surface clad region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic transformation or retained austenite phase.

MST/356     

Simulation of temperature distribution in single metallic powder layer for laser micro-sintering

Simulation of temperature distribution in single metallic powder layer for laser micro-sintering (LMS) using finite element analysis (FEA) has been proposed, taking into account the adoption of ANSYS μMKS system of units, the transition from powder to solid and the utilization of moving laser beam power with a Gaussian distribution. By exploiting these characteristics a more accurate model could be achieved. The effects of the process parameters, such as laser beam diameter, laser power and laser scan speed on the temperature distribution and molten pool dimensions have been preliminarily investigated. It is shown that temperature increases with the laser power and decreases with the scan speed monotonously. For the laser beam diameter during single-track, the maximum temperature of the powder bed increases with the decrease in the laser beam diameter, but far from the center of the laser beam area, the temperature increases with the laser beam diameter. The molten pool dimensions in LMS are much less than that in classical selective laser sintering (SLS) process. Both molten pool length and width decrease with the laser beam diameter and the laser scan speed, but increase with the laser power. The molten pool length is always larger than the molten pool width. Furthermore, the center of molten pool is slightly shifted for the laser multi-track.     

固定电弧脉冲TIG焊接熔池流体流动与传热模型

在浮力、电磁力和表面张力梯度共同作用下对固定电弧脉冲ＴＩＧ焊接熔池中的流体流动与传热的动态过程建立了数学模型。在控制方程组强烈耦合的情况下，将处理导热问题边界条件的附加源项法应用于处理动量边界条件，同时采用交替方向隐式迭代法及双块修正技术，使求解非稳态控制方程组的迭代收敛速度大为提高。计算结果展示了脉冲ＴＩＧ焊接过程中，随着焊接电流的周期性变化，熔池流场与热场的周期性变化规律以及熔池形成的动态发展过程。根据该模型得出的计算结果与试验测定结果吻合程度良好。     

A transient finite element simulation of the temperature and bead profiles of T-joint laser welds

Laser welding is a high power density welding technology, which has the capability of focusing the beam power to a very small spot diameter. Its characteristics such as high precision and low and concentrated heat input, helps in minimizing the micro-structural modifications, residual stresses and distortions on the welded specimens. In this study, finite element method (FEM) is adopted for predicting the bead geometry in laser welding of 1.6 mm thick AISI304 stainless steel sheets. A three-dimensional finite element model is used to analyze the temperature distribution in a T-joint weld produced by the laser welding process. Temperature-dependent thermal properties of AISI304 stainless steel, effect of latent heat of fusion, and the convective and radiative boundary conditions are included in the model. The heat input to the model is assumed to be a 3D conical Gaussian heat source. The finite element code SYSWELD, along with a few FORTRAN subroutines, is employed to obtain the numerical results. The T-joint welds are made using a Nd:YAG laser having a maximum power of 2 kW in the continuous wave mode. The effect of laser beam power, welding speed and beam incident angle on the weld bead geometry (i.e. depth of penetration and bead width) are investigated. Finally, the shapes of the molten pool predicted by the numerical analysis are compared with the results obtained through the experimentation. The comparison shows that they are in good agreement.     

Mass transfer in the process of pulsed laser alloying of titanium in nitrogen vapors

Thermocapillary mass transfer accompanying pulsed laser alloying of titanium in vapors of liquid nitrogen is studied. The impurity concentration fields are determined by the methods of numerical modeling. The numerical and experimental results are compared.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 3, pp. 375–380, March, 1990.     

A three dimensional model for direct laser metal powder deposition and rapid prototyping

A three-dimensional model for direct laser metal powder deposition process and rapid prototyping is developed. Both numerical and analytical models are addressed. In the case of numerical modeling, the capabilities of ANSYS parametric design language were employed. The model calculates transient temperature profiles, dimensions of the fusion zone and residual stresses. Model simulations are compared with experimental results acquired on line using an ultra-high shutter speed camera which is able to acquire well-contrasted images of the molten pool, and off-line using metallographical and x-ray diffraction analyses. The experiments showed good agreement with the modeling. The results are discussed to provide suggestions for feedback control and reduction of residual stresses.     

LASER SURFACE TREATMENTS FOR PROMOTING CORROSION RESISTANCE OF A CARBON STEEL

The effect of laser treatment and laser alloying on corrosion resistance of 1045 steel has been studied. Various ways of surface alloying have been investigated by using continuous CO₂ laser beam: i) irradiation of chromium painted surfaces; ii) irradiation of Ni and Cr-electroplated surfaces; iii) direct injection of Ni and Cr powders into the melt pool. A high and uniform level of alloying in the surface layer can be achieved in the cases of electroplated surfaces and direct powder injection. It requires, however, an appropriate choice of irradiation conditions (such as beam power, beam traverse speed, beam defocusing and degree of overlapping) that provide remelted layer of a limited and fairly uniform thickness. It has been found that in order to achieve corrosion behavior of laser treated surfaces similar to that of austenite type 304 stainless steel chromium and nickel contents in the alloyed layer are to be higher than those of 304 steel.     

Thermal behavior and densification mechanism during selective laser melting of copper matrix composites: Simulation and experiments

Simulation of temperature distribution and densification process of selective laser melting (SLM) WC/Cu composite powder system has been performed, using a finite volume method (FVM). The transition from powder to solid, the surface tension induced by temperature gradient, and the movement of laser beam power with a Gaussian energy distribution are taken into account in the physical model. The effect of the applied linear energy density (LED) on the temperature distribution, melt pool dimensions, behaviors of gaseous bubbles and resultant densification activity has been investigated. It shows that the temperature distribution is asymmetric with respect to the laser beam scanning area. The center of the melt pool does not locate at the center of the laser beam but slightly shifts towards the side of the decreasing X-axis. The dimensions of the melt pool are in sizes of hundreds of micrometers and increase with the applied LED. For an optimized LED of 17.5 kJ/m, an enhanced efficiency of gas removal from the melt pool is realized, and the maximum relative density of laser processed powder reaches 96%. As the applied LED surpasses 20 kJ/m, Marangoni flow tends to retain the entrapped gas bubbles. The flow pattern has a tendency to deposit the gas bubbles at the melt pool bottom or to agglomerate gas bubbles by the rotating flow in the melt pool, resulting in a higher porosity in laser processed powder. The relative density and corresponding pore size and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.     

Laser weld pool management through diffractive holographic optics

Abstract

Conduction laser welding involves initiating a melt pool by exposure to high power laser induced light and controlled thermal conduction. Existing welding techniques generally provide enough energy to join the component but have no real control over the melt pool. This process can invariably lead to overheating in adjacent areas or even the melt pool itself, often causing unavoidable effects, such as ‘burn through’. The present work presents a procedure in which a desired melt pool shape is conceived, and a bespoke beam irradiance distribution is designed to match. The beam is shaped not by conventional lenses but by a diffractive holographic optical element (DHOE). The DHOE utilises holography to wholly create highly complex three-dimensional energy distributions through constructive and destructive interference. This technique allows novel beam irradiance distributions to be applied to conduction mode laser welding, with the melt pool transverse profile being shaped to a specific design. Holographic conduction laser welding has been shown to be successful and represents a significant step forward in the industry, as demonstrated in this case in both mild and stainless steels. The fusion zone is shown to be particularly influenced by the shape of the illuminating laser beam profile, and many of the welds demonstrate a highly novel weld profile because of this. The use of a bespoke beam irradiance distribution allows control of the heat flow to the workpiece, and this allows greater control over material migration due to surface tension effects. Many of the welds demonstrate unique surface solidification patterns directly linked to the beam profile used. The DHOE also presents a number of additional advantages, such as an increased usable depth of field, allowing for less stringent set-up tolerances. Comprehensive metallography has been performed on samples of these welds through the use of optical microscopy, electron microscopy, electron backscatter diffraction and energy dispersive (X-ray) spectroscopy. These techniques offer in depth analysis of crystal size, shape, orientation and phase. By incorporating DHOEs into a laser welding process, not only does the melt pool shape become controllable, but also the crystal growth is highly influenced. Many of the undesirable attributes of a conventional laser weld are reduced by using a beam distribution created by a DHOE, bringing the microstructure of the weld pool closer to that of the parent material.     

Numerical aperture of optical fibers based on high-purity As2S3 glass: Unsteady-state modal distribution

The numerical aperture of core-clad multimode optical fibers with the core made from high-purity As₂S₃ glass is measured by a far-field technique using a cw CO laser (5.3-6.3 μm). The lengths at which a steady-state modal distribution is reached are determined for optical fibers with metallic coatings, tetrafluoroethylene/difluoroethylene copolymer coatings, or immersion. Various factors affecting the numerical aperture of fibers with an unsteady-state modal distribution are examined.     

Parametric study of surface morphology for selective laser melting on Ti6Al4V powder bed with numerical and experimental methods

A three-dimensional (3D) selective laser melting (SLM) model comprising coupled heat transfer and flow behavior is proposed. The free surface of the melt pool is calculated by the volume-of-fluid (VOF) method, which is a means of acquiring the surface morphology. In this research, laser powers and laser scanning speeds were normalized to characterize the influence on the surface morphology. Results showed that when the scanning speed was increased, the surface morphology initially became flatter, but then roughness developed again at high speed case. Further, as the laser power was increased, the surface morphology gradually roughened. To better describe the surface-morphology phenomenon according to different laser parameters, the melt pool volume and melt pool lifetime were also investigated. With these two factors constrained, a fine surface could be obtained with a low melt pool volume and proper lifetime (approximately 100 to 130 μs). The surface morphology and the width of the melt track were experimentally acquired, and are in a good agreement with the results predicted by simulation.     

Modification of metal surfaces by the laser melt-particle injection process

The laser melt-particle injection process was used to accomplish diverse modifications in the structure and chemistry of metal surfaces. This process consists in melting a shallow pool on the surface of a metal with a high power laser beam and injecting fine particles into the melt. In this paper we discuss wear-resisting surfaces produced by injecting TiC particles into the surfaces of 5052 aluminum and by surface alloying the same base metal with silicon. The operating conditions employed in the surface-alloying experiments were very similar to those used for carbide injection, but nearly complete dissolution of the injected silicon particles occurred during alloying while the injected carbide particles did not dissolve to any detectable degree.Previous experiments with this process have all been conducted at reduced pressure in an inert gas. The surface alloying and some of the carbide injection work described here were done at atmospheric pressure using a powder injection nozzle with inert gas shielding.     

Optimization of liquid cooling fins in microelectronic packaging

The present study investigates the heat transfer from a fin of the combination of cone and frustum of a cone immersed in boiling FC-72. The temperature distribution within the fin is determined with the help of a three-dimensional finite element computation technique using linear axisymmetric elements. From the temperature distribution, the fin base temperature gradients at various base temperature excesses are obtained. The numerical results are found to be in good agreement with published experimental data. Parametric studies have been carried out using this numerical method. An artificial neural network is used to consolidate additional data from the parametric studies. The genetic algorithm is then applied to the case of a fixed volume fin, in order to determine the dimensions which optimize the heat transfer per unit volume. It is observed that the heat transfer per unit volume decreases with fins of increasing volumes. Conclusively, the technique used in the analysis is able to provide a fast and accurate guideline to select and design the cooling for microelectronic systems.     

Melt pool vorticity in deep penetration laser material welding

In the present study, the vorticity of melt motion in the keyhole and weld pool has been evaluated in case of high power CO₂ laser beam welding. The circulation of vorticity is obtained as a function of Reynolds number for a given keyhole volume which is linked to Mach number variation. The shear stress and thermal fluxes present in the turbulent pool are linked to diffusivity and Prandtl number variation. It was shown that below a critical value of Rayleigh number, the conduction mode of melt transfer signifying beam absorption becomes dominant. Above this value, convective heat transfer indicates melting and evaporation occurring in the weld pool during laser welding. The evaporative recoil pressure expels the liquid while surface tension and hydrostatic pressure help to retain the melt in the keyhole cavity in this high power laser beam welding. The understanding of several hydrodynamic phenomena occuring in the weld pool is valuable not only for understanding basic mechanistic aspects but also for process optimization involved in laser beam welding.     

等离子+缆式焊丝脉冲GMAW复合焊熔池流体行为研究

目的 研究等离子+缆式焊丝脉冲GMAW复合焊过程的熔池流体行为.方法 综合考虑传热学以及流体动力学,建立Fluent数值分析模型.使用双椭球–锥体热源模型代表等离子弧焊传热模型,用双椭球热源表征GMAW电弧传热并考虑熔滴传热,同时考虑熔池受到的电磁力、浮力、表面张力、等离子流力等作用力.基于Fluent软件,对复合焊过...