激光选区熔化增材制造中的粉体热动力学行为

激光选区熔化(SLM)可以直接成形近全致密、性能接近锻件的复杂结构金属零件, 是金属增材制造(3D打印)领域的热点技术之一. SLM成形过程中粉末颗粒的热/动力学行为复杂, 与零件成形缺陷及力学性能紧密相关. 本文介绍了离散单元法(DEM)与计算流体力学(CFD)联合建模在SLM中的创新应用, 结合粉末床原位测试及成形在线监测, 探索SLM粉末铺设和粉末床熔融两个工艺环节的复杂粉体热/动力学行为机制. 研究发现, 粉末铺设过程中: 粉体的黏结效应、壁面效应和渗流效应3种机制相互竞争、共同支配粉末动力学行为并最终决定粉末床铺设质量. 粉末床熔融过程中: 熔池喷发的高温金属蒸汽带动环境保护气体形成内旋涡流, 由此驱动散体粉末形成复杂流固耦合运动, 导致粉末床飞溅与剥蚀现象; 热浮力效应对粉末运动不起主导作用. 文中提出了DEM-CFD双向动态耦合模型, 可以充分考虑离散粉末与熔池蒸发气体之间的热力耦合作用, 为SLM粉体熔融热/动力学行为的仿真模拟提供了一种新途径.      

面向增材制造的熔池凝固组织演变的相场研究

激光增材制造(laser additive manufacturing, LAM)技术极适合复杂整体构件的近净成形和高附值损伤件的快速修复. 然而, 激光增材制造熔池内部复杂的动态凝固过程显著影响成形件的终态组织, 进而制约其服役性能. 本文针对激光直接能量沉积(direct energy deposition by laser, DED-L) Inconel 718过程, 构建宏观传热传质与多相场耦合的多尺度数学模型, 解决了熔池宏?微观温度场的直接耦合, 并基于MPI并行程序设计实现了熔池二维的全域定量模拟, 研究了凝固过程中的晶粒演变过程. 结果表明, 模拟的熔池尺寸、凝固界面与实验结果吻合较好. 熔池凝固界面形态和晶体择优取向是影响晶粒演变的重要因素. 在熔池横截面上, 凝固过程主要受温度梯度方向的驱使, 取向与温度梯度方向夹角越小的晶粒占优生长. 在纵截面上, 晶粒的生长表现出弯曲生长以及“上三角”的晶粒特征, 温度梯度方向的渐变导致了晶粒弯曲, 相邻晶粒的竞争行为决定了晶粒形貌. 本文阐明了金属激光增材制造晶粒演变的机理, 有助于厘清增材制造热物理、化学、冶金过程, 为凝固组织的预测和调控提供理论指导. 此外, 该多尺度数学模型也适用于其他金属材料的激光增材制造过程.      

增材制造微结构演化及疲劳分散性计算

为了预测增材制造中工艺参数?微结构?力学性能之间的关联规律, 提出了集成离散元、相场模拟、晶体塑性有限元和极值概率理论的计算方法, 揭示了激光扫描速度对微结构演化、屈服应力和疲劳分散性的影响. 首先, 采用离散元实现了重力作用下粉床在已凝固层表面上的逐层铺设; 其次, 通过热?熔体?微结构耦合的非等温相场模拟, 获得了熔体、气孔、晶界、晶粒取向等的时空演化以及最终形成的多晶微结构; 然后, 应用晶体塑性有限元计算了增材制造多晶微结构的宏观力学响应, 并得到表征疲劳裂纹萌生驱动力的疲劳指示参数(FIP); 最后, 采用极值概率理论分析了增材制造多晶微结构的FIP极值分布规律及疲劳分散性. 以316L不锈钢选区激光熔化增材制造为例的计算结果表明: 增材制造微结构的宏观屈服强度随激光扫描速度的增加而降低, 且呈各向异性; FIP极值符合Gumbel极值分布规律, 激光扫描速度增加可降低增材制造微结构疲劳分散性, 但会导致FIP极值升高, 使得疲劳裂纹萌生驱动力增加, 疲劳寿命降低.      

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响. 探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础. 选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点. 以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method, DEM),通过对比相应实验结果,标定了该模型的参数. 对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律. 研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强. 研究结果可指导SLS中铺粉工艺的优化.      

基于半解析VOF-DEM的激光直接沉积多尺度过程模拟

与传统铸造技术相比, 基于金属粉末的增材制造技术因其生产周期短、可操作性强而在航空航天、生物医学等领域具有很好的优越性. 尤其是激光直接沉积技术, 因其自由度高, 在复杂构件制造、部件修复中有着广泛的运用. 但是该激光直接沉积过程涉及多物理场、跨尺度、极端高温高压环境和相变问题, 仅靠实验不能很好地研究其中的机理. 已有数值模拟技术一般通过预设或者射入拉格朗日点作为颗粒输入, 不能做到同时考虑环境气体、颗粒碰撞和相变过程. 本文在近期发展的基于核函数近似背景流场的半解析CFD-DEM耦合方法中引入了流体体积分数法(VOF), 发展了可以同时模拟含热、刚体颗粒、相变和自由液面及相变界面的半解析VOF-DEM (或半解析CFD-DEM-VOF)方法, 从而首次实现了真实物理环境下激光直接沉积技术的数值模拟. 其中, VOF中的气相为环境气体, 液相为熔融和凝固的金属相, 界面通过iso-Advector重构, DEM为未熔化的金属粉末, 且流体网格可解析离散元颗粒形状. 这一模拟框架可以有效复现颗粒之间的碰撞、粘结、熔化、融合, 以及熔池熔道的形成, 为激光直接沉积技术的数值模拟提供了开拓性的范式, 并可以被应用到其他带相变的颗粒系统中.      

Simulation and experimental analysis of melt pool evolution in laser engineered net shaping

In this work, the evolution of melt pool under single-point and single-line printing in the laser engineered net shaping (LENS) process is analyzed. Firstly, the basic structure of the melt pool model of the LENS process is established and the necessary assumptions are made. Then, the establishment process of the multi-physical field model of the melt pool is introduced in detail. It is concluded that the simulation model results are highly consistent with the online measurement experiment results in terms of melt pool profile, space temperature gradient, and time temperature gradient. Meanwhile, some parameters, such as the 3D morphology and surface fluid field of the melt pool, which are not obtained in the online measurement experiment, are analyzed. Finally, the influence of changing the scanning speed on the profile, peak temperature, and temperature gradient of the single-line melt pool is also analyzed, and the following conclusions are obtained: With the increase in scanning speed, the profile of the melt pool gradually becomes slender; The relationship between peak temperature and scanning speed is approximately linear in a certain speed range; The space temperature gradient at the tail of the melt pool under different scanning speeds hardly changes with the scanning speed, and the time temperature gradient at the tail of the melt pool is in direct proportion to the scanning speed.     

Instability of sintering of porous bodies

Sintering models are discussed and used to analyze flow instabilities that may arise during preliminary compaction of powders. These instabilities can be at the origin of heterogeneities in the densification. The material is modeled as a viscoplastic thermal sensitive porous material. The modeling includes the limit case of a linear viscous material. The effects of sintering conditions (temperature and pressure in the case of pressure sintering) and the effects of material characteristics such as porosity, heat capacity, theoretical density, surface tension, particle size and creep parameters on stability of sintering are investigated. The heat release associated with the plastic flow is shown to sometimes have an important role. Stability criteria are derived and applied to the analysis of sintering and hot isostatic pressing, using various sintering models. These stability criteria can be used to optimize the densification process; one can control, for example, temperature so as to avoid any instability. Stability maps enabling an optimization of temperature–pressure regime in hot isostatic pressing are built for sample metal (nickel) powder.     

Direct reduction of hematite powders in a fluidized bed reactor

Ultrafine hematite powder was reduced to produce ultrafine iron powder in a 50%Ar–50%H₂ atmosphere at 450–550 °C in a fluidized bed reactor. The ultrafine hematite powder shows the typical agglomerating fluidization behavior with large agglomerates fluidized at the bottom of the bed and small agglomerates fluidized at the upper part of the bed. It was found that defluidization occurred even at the low temperature of 450 °C with low metallization rate. Defluidization was attributed mainly to the sintering of the newly formed iron particles. Granuation was employed to improve the fluidization quality and to tackle the defluidization problem, where granules fluidized like a Geldart's group A powder. Granulation was found to effectively reduce defluidization during reduction, without however sacrificing reduction speed. The as-reduced iron powders from both the ultrafine and the granulated hematite exhibited excellent sintering activity, that is, fast sintering at temperature of as low as ～580 °C, which is much superior as compared to that of nano/ultrafine iron powders made by other processes.     

激光定向能量沉积的粉末尺度多物理场数值模拟

激光定向能量沉积技术作为一种同轴送粉式金属增材制造技术, 以其制造效率高、成形尺寸大等优势在航空、航天、交通等领域具有广阔的应用前景. 然而, 该技术在金属零件的尺寸精度和形状精度控制方面存在诸如尺寸偏差大、表面不平整等控形问题, 亟需发展高效高精度预测熔覆层成形尺寸形貌的数值模拟方法. 针对该问题, 本文建立了考虑激光-粉末-熔池交互过程的高保真多物理场数值模型. 其中, 采用高斯面热源等效激光光束, 利用拉格朗日质点法求解粉末输送及其与激光交互的过程, 进一步结合有限体积法和流体体积法求解粉末-熔池的交互及其流动凝固过程, 并通过TC17合金单道熔覆层实验结果进行了验证. 基于该模型, 首先预测了不同工艺参数下单道熔覆层形貌尺寸, 并对熔覆层形貌的变化趋势及其内在的物理机理进行了深入分析. 结果表明, 依赖于工艺参数的粉末温度分布和粉末基板能量分配比例对熔池流场和熔覆层尺寸有显著的影响. 本文所建立的数值模型可辅助激光定向能量沉积增材制造技术控形工艺参数优化, 所得结论可为成形件尺寸和形状精度控制提供理论指导.      

环形浅池内硅熔体中的热流体波

为了了解工业Czochralski炉内硅熔体表面轮型的基本特征,对环形浅池内硅熔体的热毛细-浮力对流进行了三维数值模拟,硅熔池内径为15 mm,外径为50 mm,深度为3 mm,熔池外壁被加热,内壁被冷却,底部固壁和顶部自由表面均绝热或者允许一个垂直方向的传热。模拟结果表明,当径向温差较小时,熔池内会产生稳定的单胞热毛细-浮力流动,随着温差的增大,流动将转变为三维振荡流动,在熔体自由表面会出现沿周向运动的轮型,小的垂直方向的热流密度(3W/cm2)对这种振荡流动没有大的影响。同时,讨论了流动和温度波动的特征,并确定了振荡流动的临界条件。     

The role of viscoelasticity in polymer sintering

An experimental study for polymer sintering has been carried out using pairs of powder particles. Although in many cases Newtonian sintering models successfully describe polymer sintering, they predict a faster coalescence rate than that observed with the polypropylene copolymer resins used in this study, indicating that factors other than the surface tension and the viscosity play a role in polymer sintering. Observations of coalescence under the microscope and rotational molding experiments suggest that melt elasticity slows down the process. Based on these findings, a mathematical model describing the complete polymer sintering process for viscoelastic fluids has been developed. The approach was similar to that of Frenkel (1945) and the convected Maxwell constitutive equations were used together with the quasi-steady state approximation. The proposed viscoelastic sintering model is capable of predicting the sintering rate slowdown observed in this study. Received: 18 August 1997 Accepted: 30 March 1998     

Modeling the evolution of stress due to differential shrinkage in powder-processed functionally graded metal–ceramic composites during pressureless sintering

Pressureless sintering of powder-processed functionally graded materials is being pursued to economically produce metal–ceramic composites for a variety of high-temperature (e.g., thermal protection) and energy-absorbing (e.g., armor) applications. During sintering, differential shrinkage induces stresses that can compromise the integrity of the components. Because the strength evolves as the component is sintered, it is important to model how the evolution of the differential shrinkage governs the stress distribution in the component in order to determine when the strength will be exceeded and cracking initiated. In this investigation, a model is proposed that describes the processing/microstructure/property/performance relationship in pressurelessly sintered functionally graded plates and rods. This model can be used to determine appropriate shrinkage rates and gradient architectures for a given component geometry that will prevent the component from cracking during pressureless sintering by balancing the evolution of strength, which is assumed to be a power law function of the porosity, with the evolution of stress. To develop this model, the powder mixture is considered as a three-phase material consisting of voids, metal particles, and ceramic particles. A micromechanical thermal elastic–viscoplastic constitutive model is then proposed to describe the thermomechanical behavior of the composite microstructure. The subsequent evolution of the thermomechanical properties of the matrix material during sintering is assumed to obey a power law relationship with the level of porosity, which is directly related to the shrinkage strain, and was refined to account for the evolving interparticle cohesion of the matrix phase due to sintering. These thermomechanical properties are incorporated into a 2-D thermomechanical finite element analysis to predict the stress distributions and distortions that arise from the evolution of differential shrinkage during the pressureless sintering process. Differential shrinkage results were verified quantitatively through comparison with the shape profile for a pressurelessly sintered functionally graded nickel–alumina composite plate with a cylindrical geometry, and the stress distribution results verified from qualitative observations of the absence or presence of cracking as well as the location in specimens with different gradient architectures. The cracking was mitigated using a reverse gradient at one end of the specimen, and the resulting distortions associated with the shape profile were determined to be no more than 15% reduced from the predictions. The effects of geometry were also studied out-of-plane by transforming the plate into a rod through an increase in thickness, while in-plane effects were studied by comparing the results from the cylindrical specimen with a specimen that has a square cross-sectional geometry. By transforming from a plate to a rod geometry, the stress no longer exceeds critical levels and cracks do not form. The results from the in-plane geometric study indicated that critical stresses were reached in the square geometry at temperatures 100 °C less than in the cylindrical geometry. Additionally, the location of primary cracking was shifted towards the metal-rich end of the specimen, while the stress distribution associated with this shift and the lower temperature for the critical stress resulted in secondary cracking.     

New strategy of solid/fluid coupling during numerical simulation of thermo-mechanical processes

In this study, numerical methods are developed to simulate thermomechanical processes, taking into account both the fluid flows in the molten pool and the deformations of the solid parts. The methods are based on a new strategy of solid/fluid coupling. They allow to simulate the formation of the molten pool by taking into account the fluid flows through both effects of the surface tension (“curvature effect” and “Marangoni effect”) and the buoyancy. An ALE approach is used to follow the evolution of the free surface. The effects of the deformations in the base metal on the fluid flows in the molten pool (solid/fluid interaction) is ensured by imposing the velocities of the solid nodes during the thermo-fluid simulation. As an application, a thermo-fluid-mechanical simulation of laser welding is carried out. It is found that the solid/fluid interaction has a minor effect on simulation results.     

Characteristics of surface oscillation in thermocapillary convection

The characteristics of surface oscillation in a rectangular pool of silicone oil have been investigated experimentally. The horizontal cross-section of the pool is 52 mm × 36 mm, and the depth of the silicone oil layer is in the range of 1.1-4.8 mm. The applied temperature difference between the two sidewalls leads to shear flow along the free surface from hot to cold and a back flow in the underlying layer. With the increase of the temperature difference, the original steady flow will become unstable to unsteady flow. A CCD laser displacement-sensor with high resolution is used to measure the position of the liquid surface dynamically. And the Hilbert-Huang transform is chosen to analyze the experiment data which is nonlinear and non-stationary. The characteristics of surface oscillation have been obtained. And the relationship of the characteristics with the temperature difference and liquid layer depth has been discussed in details.     

Conditions of bubble formation in the intergranular layer of a glass phase in ceramics sintering

Calculation results for the evolution of the gas-concentration distribution in the intergranular layer of a glass phase cooling from the sintering temperature to room temperature are presented. The calculations are performed for two stages of cooling of the glass phase. The calculation results suggest that gas dissolution in the intergranular layer of the glass phase leads to substantial softening of the conditions of bubble formation in it, and the second (low-temperature) stage of cooling makes a major contribution to the saturation of the glass phase with the gas. Institute of Mechanics and Applied Mathematics, Rostov State University, Rostov-on-Don 344090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 3, pp. 175–181, May–June, 1999.     

Characterization of strain rate effects in sheet laser forming

This work presents numerical simulations and experimental validation of sheet laser forming processes using a single-step straight path with different laser beam powers (four levels ranging from 30 W to 120 W) and scanning speeds (four levels ranging from 5 mm/s to 20 mm/s) in graphite-coated AISI 304 stainless steel 0.6-mm-thick sheets. The numerical simulations of these cases are performed via a coupled thermomechanical finite element formulation accounting for large strains, temperature-dependent material properties and convection–radiation phenomena. Firstly, a rate-independent plastic model is used. Although this model adequately predicts the final bending angle for the cases achieving relatively low maximum temperatures, i.e. cases with low laser beam powers and high scanning speeds, it fails in describing the deformation pattern for the cases with higher maximum temperatures, i.e. cases with high laser beam powers and low scanning speeds. Secondly, in order to overcome this drawback, a rate-dependent viscoplastic model including a stress-dependent viscosity law is proposed to simulate the same cases. The final bending angles provided by this model are found to be in good agreement with the experimental measurements for the whole ranges of laser beam power and scanning speed studied in this work. Therefore, the use of this viscoplastic model in the simulation of sheet laser forming allows us to conclude that the strain rate effects, which mainly play a relevant role at high temperatures, can be adequately characterized.     

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

In the work an approach to avoid a circumferential temperature distribution existing during nucleate pool boiling on a horizontal cylinder within low heat flux densities is presented. The idea of the approach is local heat transfer enhancement by a porous layer application on a part of the heating surface. An experiment on nucleate pool boiling heat transfer from horizontal cylinders to saturated R141b and water under atmospheric pressure is reported. Experiments have been conducted using stainless steel tubes with the outside diameter between 8 mm and 23 mm with the active length of 250 mm. The outside surface of the tubes was smooth or partially coated with a porous metallic layer. In particular, measurements of inside circumferential temperature distribution have been performed.     

Experimental study of the temperature field generated during orthogonal machining of an aluminum alloy

During the machining of metals, plastic deformation and friction lead to the generation of heat in the workpiece, which results in thermomechanically coupled deformation. Recently, several numerical models of this highly coupled process have been produced in response to increased interest in high speed machining. It is important to characterize the thermal field in the cutting zone in order to completely verify these models of high speed machining and to direct further advancement in this area. In this work, HgCdTe infrared detectors are used to experimentally measure the temperature distribution at the surface of a workpiece during orthogonal cutting. From these temperature measurements, the heat generated in the primary shear zone and the friction zone can be examined and characterized. A modified Hopkinson bar technique has been developed to perform orthogonal machining at speeds ranging from 10 to 100 m/s. In the present work, a cutting velocity of 15 m/s is employed in all the tests in order to demonstrate the capability of the apparatus and characterize thermal fields during low speed machining. Temperature fields are obtained during the orthogonal cutting of aluminum as a function of depth of cut. It is seen that depth of cut can vary both the maximum temperature as well as the distribution of the temperature field in the aluminum workpiece. the maximum temperature increased with depth of cut (238°C for 1.5 mm cut, 207°C for 1.0 mm cut and 138°C for 0.5 mm cut) and the temperature field extended further beneath the cut surface with decreasing depth of cut.     

Factors influencing particle agglomeration during solid-state sintering

Discrete element method(DEM) is used to studythe factors affecting agglomeration in three-dimensionalcopper particle systems during solid-state sintering.A newparameter is proposed to characterize agglomeration.Theeffects of a series of factors are studied,including particlesize,size distribution,inter-particle tangential viscosity,temperature,initial density and initial distribution of particleson agglomeration.We find that the systems with smallerparticles,broader particle size distribution,smaller viscosity,higher sintering temperature and smaller initial densityhave stronger particle agglomeration and different distributions of particles induce different agglomerations.This studyshould be very useful for understanding the phenomenon ofagglomeration and the micro-structural evolution during sintering and guiding sintering routes to avoid detrimental agglomeration.     

增材制造中滚筒铺粉工艺参数对尼龙粉体铺展性的影响研究

铺粉工艺是基于粉床的增材制造(additive manufacturing, AM)技术中的关键工序之一. 滚筒铺粉工艺参数包括铺粉层厚H, 滚筒直径D, 滚筒的旋转速度ω和平移速度V, 对增材制造工艺中的粉体铺展性具有重要影响. 本文以尼龙粉体为研究对象, 采用离散元法(discrete element method, DEM)模拟其滚筒铺展过程, 建立沉积分数、覆盖率和沉积速率3个铺展性指标. 采用中心复合设计(central composite design, CCD)生成30组仿真案例, 通过响应曲面法(response surface methodology, RSM)拟合了3个铺展性指标的回归模型. 采用方差分析证明了回归模型的准确性和预测的有效性, 并详细分析了工艺参数对粉体铺展性指标的影响规律. 结果表明, 铺粉层厚H是最大的影响因素, 平移速度V是次要的影响因素, 滚筒直径D和滚筒的旋转速度ω对粉体铺展性指标影响较小, HV和DV为影响粉体铺展性指标的主要交互因素. 以3个铺展性指标为优化目标, 采用期望值法对滚筒铺粉工艺参数进行多目标优化, 获得了预测的最优铺粉工艺参数和粉体铺展性指标组合, 并通过实验验证了粉体铺展性指标的预测结果与实验结果吻合良好. 本文的研究结果可指导增材制造中滚筒铺粉工艺参数的优化.