Finite element analysis of temperature distribution in single metallic powder layer during metal laser sintering

The metal laser sintering (MLS) is used to make strong or hard metallic models for tools and dies directly from metallic powders. Thermal distortion is the serious problem after cooling of the solidified part rapidly. Uncontrolled temperature distribution in the metallic powder layer leads to thermal distortion of the solidified part. The study of temperature distribution within the metallic layer during MLS is important from the quality of the layer point of view.The high temperature generated in the powder layer leads to thermal distortion of the part and causes thermal as well as residual stresses in the part. In this paper the powder layer is assumed to be subjected to plane stress type of temperature variation and a transient finite element method-based thermal model has been developed to calculate the temperature distribution within a single metallic layer during MLS. A finite element code has been developed and validated with the known results from the literature.The obtained results of temperature distribution show the temperature and temperature gradient variation along X- and Y-axis. The effect of process parameters such as laser power, beam diameter, laser on-time, laser off-time and hatch spacing on temperature distribution within a model made of titanium during MLS is studied. The results computed by the present model agree with experimental results. Temperature increases with increase in laser power and laser on-time but temperature decreases with increase in laser off-time and hatch spacing.     

CALCULATION OF 3-D TRANSIENT TEMPERATURE FIELD DURING LASER TRANSFORMATION HARDENING PROCESS

1.IntroductionLasertransformationhardeningprocessisathreedimensiontransientheattransferproblemwhichinvolvesinphajsetransformation,heatconduction,heatconvectionandheatradiation.Thecalculationoftemperaturefieldoflasertransformationhardeningprocessisaprerequisitetotheoptimizationofparametersoflasertransformationhardening.Thisworkstartedintheearlyof1970',uptonow,alotofmodelsandcalculationmethodswereproposedtocalculatethetemperaturefieldoflasertransformationhardeningprocessll--gi.Allthesemodelsareb…     

Simulation on the Melt Flow in Laser Full Penetration Welding with a Model Including a Non-Rotational Symme-try Keyhole Based on Energy Balance

A three dimensional model including a non-rotational symmetry keyhole of which the geometry is calculated base on the energy balance at keyhole wall is created to simulate the heat transfer and fluid flow within the molten pool during laser full penetration welding of titanium alloy plate.In order to take into account the effects of multiple reflections within the keyhole,a ray tracing method is employed and the coordinate dependent heat flux obtained is exerted on keyhole wall during the simulation on the heat transfer and fluid flow within molten pool.Furthermore,the computed weld cross sectional profile is compared with the experimental result to verify the model and it is fond that the simulation result is consists with experimental result.     

二元合金激光快速熔凝过程的非Fourier模型

基于非Fourier定律，建立了脉冲激光加热条件下单相二元合金表面快速重熔和凝固过程的非平衡传热传质模型，并根据碰撞理论和Aziz的连续生长模型处理固／液界面，以解释快速熔凝过程界面动力学的非平衡效应，快速熔凝问题是涉及热质传输的移动界面问题，通过二阶精度的Von Neumann隐式差分格式和界面跟踪方法进行过程模型的数值求解。应用该模型，分析了Al-Cu二元合金的激光表面熔凝过程，结果表明，激光的高能量密度和非平衡界面动力学所引起的熔化过程和凝固过冷对于快速熔凝过程的影响很大，在快速熔凝过程中，界面速度的变化很大，且因基底材料和热流大小而不同，通过计算获得了界面温度，速度、溶质浓度及非平衡分配系数随界面位置的变化，结果显示，在凝固过程中界面速度和界面溶质浓度都存在着很大的波动。     

A predictive model and validation of laser cutting of nitinol with a novel moving volumetric pulsed heat flux

Nitinol alloys are widely used in manufacturing of cardiovascular stents due to excellent biomechanical properties. Laser cutting is the predominant process for stent manufacturing. However, laser cutting induces thermal damage such as heat affected zone (HAZ), micro cracks, and tensile residual stress, which detrimentally affect product performance. Laser cutting induced temperature distribution, stress development, and HAZ formation are critical process characteristics. However, they are difficult to measure experimentally due to the highly transient process. To better understand the process mechanics in laser cutting of nitinol, a three-dimensional finite element model of pulsed laser cutting was developed to incorporate a novel moving volumetric pulsed heat flux model with high spatial accuracy. A material subroutine was also incorporated to model superelasticity and shape memory of nitinol. The predicted kerf geometry and dimensions agreed well with the experimental data. Also, the effects of cutting speed, pulse power, and pulse width on kerf profile, temperature, and heat affected zone (HAZ) were investigated.     

Analysis of grain growth in hybrid weld HAZ based on the coupled thermo-fluid mode

Accurate calculation of thermal cycles is a prerequisite to model grain growth in the heat-affected zone (HAZ). T improve the computation precision of thermal field and HAZ geometry, a coupled model of heat transfer and fluid flow is developed for laser + GMAW-P hybrid welding of TCS stainless steel. Utilizing computed temperature fields from the coupled model, the evolution of grain structure in the HAZ of TCS stainless steel in hybrid welding is numerically simulated by using a three dimensional Monte Carlo model. Simulation results show that more accurate HAZ grain structure can be obtained based on the coupled model of fluid flow and heat transfer, and the computed grain size distribution agrees well with the corresponding experimental results.     

Three-dimensional transient finite element analysis of the selective laser sintering process

A transient three-dimensional finite element model is developed to simulate the phase transformation during the selective laser sintering process; taking into account the thermal and sintering phenomena involved in this process. A bi-level structure integration procedure is chosen, in which the temperature dependent thermal conductivity, specific heat, and density are integrated at the outer level then used as material constants for the integration of the heat equation in the inner level. Results for temperature and density distribution, using a polycarbonate powder, are presented and discussed.     

Experimental and computational appraisal of the shape accuracy of a thin-walled virole aero-engine casing manufactured by means of laser metal deposition

Laser metal deposition (LMD) of metallic powders, especially of high-strength nickel based alloys, allows for the manufacturing of components of high shape complexity and load capacity. However, high temperature gradients, induced during laser processing may have an impact on the product quality, especially when it comes to the geometrical accuracy of thin-walled components. This paper aims to provide a modelling approach of the heat effects during LMD manufacturing of a thin-walled virole aero-engine structure, in order to calculate possible shape deviations compared to the target CAD geometry. Hereby, a model reduction method is facilitated which allows the finite element analysis of such larger components in reasonable time. Major process characteristics as heat input, molten region geometry, material deposition (i.e. layer thickness), temperature dependent material and powder properties, phase transformation, process sequence and convection effects are taken into account. The proposed model aims to decrease time consuming trial-and-error testing effort during process design and development by providing reliable results on the shape accuracy of components. The computed final shape of the final product was compared to 3D measurements on a real demonstrator virole component.     

Finite element simulation of magnesium alloys laser beam welding

In this paper, a three-dimensional finite element model is developed to simulate thermal history magnesium-based alloys during laser beam welding. Space–time temperature distributions in weldments are predicted from the beginning of welding to the final cooling. The finite element calculations were performed using Cast3M code with which the heat equation is solved considering a non-linear transient behaviour. The applied loading is a moving heat source that depends on process parameters such as power density, laser beam dimensions and welding speed, and it is associated to moving boundary conditions. Experiments were carried out to determine temperature evolution during welding and to measure the laser weld width. By comparing the thermal model answers with the measurements, it is found that numerical simulations results are in a good agreement with the experimental data.     

基于有限元模拟研究激光除锈时金属基底表面温度场

目的 根据激光除锈后金属基底表面熔池直径,通过有限元模拟,表征激光除锈时金属基底表面温度场分布。方法 考虑到激光除锈时,使用的脉冲激光具有功率小、重复频率大和脉宽短的特征以及金属具有对激光反射率大的特征,采用高斯面热源和有限元网格划分策略,可以较好地实现描述激光除锈时金属基底表面温度场分布。采用白光干涉仪和扫描电子显微镜测量和观察激光除锈后金属基底表面熔池的尺寸。结果 经过反复模拟计算和实验结果对比,最终确定高斯面热源参数为：热源效率48%,高斯系数1,热源半径30 mm。根据金属基底表面单个节点温度与时间的关系,分析表明在激光除锈过程中,金属基底表面形成了一个极快速的加热冷却温度场。结论 根据激光除锈后金属基底表面熔池直径,确定热源模型参数,进而通过模拟计算得到了较为准确的金属基底表面温度场的分布。     

Heating and material removal process in hybrid laser-waterjet ablation of silicon substrates

A hybrid laser-waterjet micromachining technology has recently been developed for near damage-free micro-ablation. It uses a laser to heat and soften the target material and a waterjet to expel the laser-softened elemental material to decrease thermal damages and increase the material removal. A computational model for the hybrid laser-waterjet micro-grooving process for single crystalline silicon is presented in this paper using an enthalpy-based finite difference method. Laser heating and waterjet cooling and expelling with the temperature-dependent silicon properties are considered in the model to predict the temperature profiles of silicon and groove characteristics under different machining conditions. The simulation results show that the introduction of a high pressure waterjet enables to remove material at its soft-solid status much below its melting temperature, while the waterjet cooling effect can reduce the workpiece temperature during the laser non-pulse period and eliminate the effect of heat accumulation, so that the thermal damage induced by laser heating is minimized. The temperature field model is also used to predict the groove depth and profile, and it is found that the model can reasonably represent the machined groove characteristics when comparing to the experimental data.     

基于ANSYS的活塞环激光热处理温度场计算及其实验研究

应用有限元分析软件ANSYS计算了圆柱状模型活塞环环槽激光热处理的温度场,从环槽的边缘处开始,将截面上径向坐标每次减少0.1 mm,依次确定同一径向坐标处深度方向最大温升大于相变温度(AC1)的点,并用曲线将其连接起来近似硬化层的形状,在此基础上讨论了激光功率、扫描速度与形成硬化层深度的关系.将激光束偏转一定的角度照射到活塞环的环槽面上,转动工件来实现整个环面的热处理.根据硬化层和基体耐腐蚀性不同的特点腐蚀后显现出截面上硬化层的形状,测量沿硬化层宽度和深度方向的硬度值测定硬化层的尺寸.     

Heat and Mass Transfer Within an Evaporating Solution Droplet in a Plasma Jet

Solution precursors have been injected into the plasma gases to produce finely structured ceramic coatings with nano- and sub-micrometric features. The trajectory history and heat and mass transfer within individual solution droplets play a very important role in determining the coating microstructure. A mathematical model is developed to analyse the thermal behavior of individual precursor droplets travelling in the high temperature plasma jet. This model involves the motion and evaporation of the precursor droplet in a DC plasma jet and the heat and mass transfer within the evaporating droplet. The influence of Stefan flow, as well as the variable thermo-physical properties of the solution and the plasma gas, is considered. The internal circulation due to the relative velocity between the droplet and the plasma jet, which may be approximated by the Hill vortex, is considered as well. The trajectory, temporal droplet surface temperature, and radius variation are predicted. The temporal temperature and concentration distributions within the evaporating droplet are presented for different injection parameters.     

激光深熔焊接热循环的研究

焊接热循环对焊接接头的性能影响至关重要,考虑等离子体的作用,采用点热源和线热源叠加的数学模型对激光焊接低碳钢的热循环进行了求争,并将计算结果与激光焊接热循环计算机检测系统实测的结果进行了对比,结果表明：计算结果与实示检测结果都显示激光焊接有极快的加热和冷却速度,而且计算结果与实测结果有较好的近似性。     

Finite element simulation of laser spot welding

Abstract

The present work reports on a two-dimensional axisymmetric finite element analysis of heat flow during laser spot welding, taking into account the temperature dependence of the physical properties and latent heat of transformations. An analysis based on conduction heat transfer alone, but using the 'double ellipsoidal' representation of the laser beam, seems to be sufficient to estimate the transition to keyhole formation during laser spot welding, although the 'double ellipsoidal' representation requires an a priori knowledge of the expected weld pool dimensions. Transient temperature isotherms and the weld pool dimensions are predicted using the model; the latter are found to compare well with measurements of weld bead dimensions. The results show that the keyhole mode is stimulated using either a high laser power and low on-time or a low laser power and high on-time. The outcomes are found to be sensitive to the assumed absorptivity and the assumed weld pool depth used to define the 'double ellipsoidal' heat source.     

Finite element analysis of single layer forming on metallic powder bed in rapid prototyping by selective laser processing

A method for calculating the distribution of temperature and stress within a single metallic layer formed on the powder bed in rapid prototyping with the selective laser melting method is proposed. The solidified layer is assumed to be subjected to plane-stress deformation and the two-dimensional finite element methods for heat conduction and elastic deformation are combined. In the simulation, the finite element mesh is constructed on the surface of the powder bed. The heat caused by laser irradiation is given to the elements under the laser beam. Shrinkage due to solidification is assumed to result in only the change of the layer thickness. In the elastic finite element simulation, the Young's modulus of the solidified part is expressed as a function of temperature. To simplify the calculation, the whole area is treated to be continuous, and the powder bed and the molten part are assumed to have a very small Young's modulus. The heat conduction and the elastic finite element calculations are carried out alternately. The obtained results of deformation and tensile stress distribution show the possibility and places of cracking of the layer during forming.     

激光熔覆熔池二维准稳态流场及温度场的数值模拟

本文建立了二维准稳态激光熔覆熔池流场及温度场的数值模型,除考虑对流换热外,模型还考虑了局部大变形自由表面．在贴体正交曲线坐标系下采用了非交错网格SIMPLE算法离散求解动量方程,计算出了激光熔覆熔池自由表面形状和温度场、速度场及局部特征凝团参.数值结果表明,表面张力温度系数和扫描速度μ0对熔池自由表面形状及熔池内温度分布、速度分布有重要影响．同时进行激光熔覆的工艺实验,实测枝晶二次臂间距（SDAD）的实验结果和数值结果吻合良好．     

An Analytical Model for Evaluation of Bending Angle in Laser Forming of Metal Sheets

In this study, an analytical model is developed to evaluate the bending angle in laser forming of metal sheets. The model is based on the assumption of elastic-bending theory without taking into account plastic deformation during heating and cooling phases. A thermal field is first established, then the thermal component of deformation is calculated and it is used in the strain balance to evaluate the bending angle. The basic idea is that it is possible to use a two-layer model whereas the heated layer thickness depends on the effective temperature distribution along the sheet thickness. A comprehensive experimental study is carried out and the main process parameters, i.e., laser power, scanning speed, sheet thickness, were varied among several levels to evaluate the accuracy of the developed model. Model predictions were confirmed by experimental measurements especially on materials with low conductivity. The established analytical model has demonstrated to provide a great insight into the process parameters effects onto the deformation mechanism within pure temperature gradient mechanism and bucking to temperature gradient transition conditions.     

Tool temperatures and crack development in milling cutters

This paper presents a theoretical and experimental study of the dynamic temperature field on a milling cutter tooth. A finite element model is developed to simulate the heat cycle of the cutting tooth by a time-varying heat flux. Forced convection effect has been investigated at exit. Experiments on simulating thermal process of face milling by heating cemented carbide teeth with laser pulses have also been conducted; the dynamic temperature field on a cutting tooth was measured. It has been shown that when the tooth is disengaged its temperature does not decrease to a level which will induce tensile stresses on the rake face. Thus, thermal cracks are not initiated during this stage of cutting. In this paper the authors propose that thermal cracks develop cyclically with increases in the temperature gradient at each entry of the cutter into the workpiece.     

Micro scale 3D FEM simulation on thermal evolution within the porous structure in selective laser sintering

A micro scale 3D finite element model (FEM) with consideration of powder arrangements was developed by selective laser sintering (SLS), where a multi-layer powder stacking model with cubic powders interlaced each other was established to describe the porous powder bed. All the powders directly exposed to laser irradiations were loaded with the Gaussian function of heat flux, and the non-linear thermal conductivity and specific heat owing to temperature change and phase transformation were considered. Comparison between the modeling and experiment indicated that both the simulated length of the sintered piece and shrinkage depth of the powder bed agreed well with the experiment data. The temperature field of laser sintering was intermittent in the micro scale due to the discretely distributed particles with maximum temperature produced in the top layer of the powder bed, and two primary ways of bonding were found in the powder bed during laser sintering.