Finite element simulation of the temperature and stress fields in single layers built without-support in selective laser melting

Overhanging and floating layers which are introduced during the build in selective laser melting (SLM) process are usually associated with high temperature gradients and thermal stresses. As there is no underlying solid material, less heat is dissipated to the powder bed and the melted layer is free to deform resulting undesired effects such as shrinkage and crack. This study uses three-dimensional finite element simulation to investigate the temperature and stress fields in single 316L stainless steel layers built on the powder bed without support in SLM. A non-linear transient model based on sequentially coupled thermo-mechanical field analysis code was developed in ANSYS parametric design language (APDL). It is found that the predicted length of the melt pool increases at higher scan speed while both width and depth of the melt pool decreases. The cyclic melting and cooling rates in the scanned tracks result high VonMises stresses in the consolidated tracks of the layer.     

基于数值模拟的选区激光熔化成形残余应力演化研究

目的 以选区激光熔化成形（SLM）试件的残余应力为研究对象,研究残余应力对成形质量的影响,为SLM成形的产业化应用提供理论依据。方法 以316L不锈钢粉末为原材料,利用Altair Inspire软件的Print3D模块分析SLM成形中支撑、成形角对残余应力的影响及残余应力的演化规律,并进行实验验证。结果 SLM成形最大残余应力出现在零件与基板结合面,添加支撑可减小残余应力。零件不同位置残余应力的演化规律不同。顶部残余应力呈先增大后减小的趋势,底部两侧残余应力呈缓慢上升的趋势;底部中间残余应力的演化规律较为复杂：起先残余应力随温度的降低而增大,当温度降到最低点时达最大值;随后在热累积作用下,残余应力先减小后增大,当达到去应力退火温度时,残余应力又减小并在一定范围内波动。残余应力随着成形角的增大呈先增大后减小再增大的趋势,当成形角为60°时,残余应力较小。结论 在SLM成形时,在零件底部添加支撑可将最大残余应力位置转移到支撑上,从而减小成形件内部的残余应力,提高成形质量。成形零件不同位置残余应力的演化规律不同,成形角对残余应力的影响也不同,成形时应根据零件工况制定合适的打印策略。     

Phase-field model of interface migration and powder consolidation in additive manufacturing of metals

Consolidation phenomena are of fundamental importance for additive manufacturing since the quality of metal parts produced by selective laser melting (SLM) is greatly dependent on residual porosity. The most recent studies use the lattice Boltzmann method (LBM) to analyze conjugated multiphase flow, heat transport, and phase transitions in the molten zone. A phase-field approach suggested in this paper retains all advantages of LBM and provides an alternative tool for theoretical analysis of SLM. In case of consolidation of composite powders where wetting phenomena are crucial at the matrix-inclusion interfaces, the phase-field method is more suitable. In this paper, the problem of metallic powder consolidation is numerically studied and the dynamics of pore and gas bubbles evolution in the molten zone is described. A two-step mechanism of consolidation is proposed which differs from a single-step mechanism typical for additive manufacturing of polymer powders. The consolidation time for different particle orderings is studied and an algorithm for the parameter selection in the widely used viscoplastic model is derived.     

Hybrid Nanostructures and Stabilized Mechanical Properties of High-Entropy Alloy Induced by Warm Laser Shock Peening

Recently, the manufacture of high-entropy-alloy (HEA) parts by selective laser melting (SLM) has been extensively studied. However, the problems of high tensile residual stress, undesirable microstructure, and unstable mechanical properties in HEA parts caused by SLM process are difficult to be solved simultaneously. Herein, a warm laser shock peening (WLSP) process is used to obtain high strength, stable mechanical property, and favorable residual stress in SLMed FeNiCrCo HEA by regulating the microstructure in material. Experimental and simulation results show that WLSP treatment can induce high-density dislocations and nanotwins in the HEA, resulting in a 39.4% increase in the surface strength of the HEA. Moreover, because of the high atomic kinetic energy brought by high temperature in WLSP processing, WLSP-treated sample has higher density of dislocations and nanotwins than the sample treated by the LSP process at room temperature (RLSP), resulting in higher surface strength and better mechanical stability of the WLSP-treated HEA. Meanwhile, the WLSP treatment enables the tensile residual stress generated in the SLM process to be transformed into compressive residual stress, which can enhance the fatigue performance of the HEA. Therefore, WLSP has great potential in obtaining SLMed HEAs with excellent mechanical properties.     

System development,formability quality and microstructure evolution of selective laser-melted magnesium

A selective laser melting (SLM) system, which consisted of a fibre laser, a three-dimensional motion platform and a motion control system, was developed in this study. The effect of process parameters on the microstructure evolution of SLMed magnesium parts was investigated. The results revealed that under an irradiation of laser energy density <3.0?J/mm, the powder remained in the discrete state. At a laser energy density 3.0–6.0 J/mm, the powder partially melted and sintered together, yielding incompact tracks. As the energy density increased to 6.0–12.0 J/mm, the powder fully melted forming continuous and smooth tracks. With a further increase in the laser energy density evaporation of the powder occurred. Dense magnesium parts free of pores and cracks were successfully fabricated with the optimal energy density of 10.0 J/mm. The immersion experiment revealed that the degradation product was mainly consisted of Mg(OH)₂, which slowed down the degradation rate acting as a protective layer.     

激光选区熔化316L不锈钢凝固特征与微观组织结构

对不同工艺参数下激光选区熔化(Selective Laser Melting, SLM)成形316L不锈钢微观组织结构进行表征,研究不同工艺参数下SLM成形316L不锈钢微观组织结构演化规律、单熔化道凝固特性。结果表明,SLM成形316L不锈钢具有跨尺度、非均质凝固组织特征,包括微米尺度柱状晶粒、小角晶界、熔池界面和纳米尺度亚结构。单熔化道的稳定成形是三维块体成形的基础,熔化道稳定性由激光工艺参数与金属粉体物理特性共同决定。不同的激光工艺参数显著影响SLM成形316L不锈钢微观组织结构,通过改变激光参数可实现微观组织结构的调控,在不同的激光逐层旋转角度下,SLM成形316L不锈钢晶粒尺寸随着扫描间距的增大而增大。强制定向热流使得外延生长机制主导凝固晶粒的生长,在不同的激光工艺参数下,沿增材方向的柱状晶粒形貌普遍存在。     

Microstructure and inclusion of Ti–6Al–4V fabricated by selective laser melting

Selective laser melting (SLM) was used in fabricating the dense part from pre-alloyed Ti-6Al-4V powder. The microstructural evolution and inclusion formation of as-fabricated part were characterized in depth. The microstructure was characterized by features of columnar prior β grains and acicular martensite α'. High density defects such as dislocations and twins can be produced in SLM process. Investigations on the inclusions find out that hard alpha inclusion, amorphous CaO and microcrystalline Al₂O₃ are three main inclusions formed in SLM. The inclusions formed at some specific sites on melt pool surface. The microstructural evolution and inclusion formation of as-fabricated material are closely related to the SLM process.     

选区激光熔化不锈钢全流程关键问题研究现状与进展

选区激光熔化(SLM)是制造精度最高的金属增材制造工艺,用于制造复杂几何形状的金属零件。316L不锈钢具有面心立方结构,在从熔融态冷却至室温的过程中通常不发生固态相变,基于这种特性,316L不锈钢成为SLM中应用最广泛的金属材料。与传统工艺相比,SLM工艺虽然能够生产高致密、高性能的零件,但是它无法避免孔洞、空隙等缺陷的出现,且存在力学性能差异和需要后处理加工等问题。为了解决这些问题,需揭示SLM制造工艺参数对性能的影响规律。综述了SLM-316L制备全流程前、中和后期在原始粉末、工艺参数及后处理方面的研究现状,首先讨论了粉末质量指标及制粉工艺对不锈钢制件的影响机理;其次总结了激光输入功率、扫描速度等工艺参数对制件性能影响的研究现状;最后对表面机械磨损处理、电解抛光等后处理方式及制件性能影响规律做了简要总结。阐明了通过SLM影响因素预测不锈钢成形零件力学性能的学术观点,以期为获取高质量零件、促进不锈钢材料的实际应用提供一些参考。     

A Multiscale Understanding of the Thermodynamic and Kinetic Mechanisms of Laser Additive Manufacturing

Selective laser melting (SLM) additive manufacturing (AM) technology has become an important option for the precise manufacturing of complex-shaped metallic parts with high performance. The SLM AM process involves complicated physicochemical phenomena, thermodynamic behavior, and phase transformation as a high-energy laser beam melts loose powder particles. This paper provides multiscale modeling and coordinated control for the SLM of metallic materials including an aluminum (Al)-based alloy (AlSi10Mg), a nickel (Ni)-based super-alloy (Inconel 718), and ceramic particle-reinforced Al-based and Ni-based composites. The migration and distribution mechanisms of aluminium nitride (AlN) particles in SLM-processed Al-based nanocomposites and the in situ formation of a gradient interface between the reinforcement and the matrix in SLM-processed tungsten carbide (WC)/Inconel 718 composites were studied in the microscale. The laser absorption and melting/densification behaviors of AlSi10Mg and Inconel 718 alloy powder were disclosed in the mesoscale. Finally, the stress development during line-by-line localized laser scanning and the parameter-dependent control methods for the deformation of SLM-processed composites were proposed in the macroscale. Multiscale numerical simulation and experimental verification methods are beneficial in monitoring the complicated powder-laser interaction, heat and mass transfer behavior, and microstructural and mechanical properties development during the SLM AM process.     

Implementation of a thermomechanical model for the simulation of selective laser melting

Selective laser melting (SLM) is an additive manufacturing process in which multiple, successive layers of metal powders are heated via laser in order to build a part. Modeling of SLM requires consideration of both heat transfer and solid mechanics. The present work describes continuum modeling of SLM as envisioned for eventual support of part-scale modeling of this fabrication process to determine end-state information such as residual stresses and distortion. The determination of the evolving temperatures is dependent on the material, the state of the material (powder or solid), the specified heating, and the configuration. Similarly, the current configuration is dependent on the temperatures, the powder-solid state, and the constitutive models. A multi-physics numerical formulation is required to solve such problems. This article describes the problem formulation, numerical method, and constitutive parameters necessary to solve such a problem. Additionally, various verification and example problems are simulated in the parallel, multi-physics finite element code Diablo, and the results presented herein.     

Selective Laser Melting under Variable Ambient Pressure: A Mesoscopic Model and Transport Phenomena

Recent reports on the selective laser melting (SLM) process under a vacuum or low ambient pressure have shown fewer defects and better surface quality of the as-printed products. Although the physical process of SLM in a vacuum has been investigated by high-speed imaging, the underlying mechanisms governing the heat transfer and molten flow are still not well understood. Herein, we first developed a mesoscopic model of SLM under variable ambient pressure based on our recent laser-welding studies. We simulated the transport phenomena of SLM 316L stainless steel powders under atmospheric and 100 Pa ambient pressure. For typical process parameters (laser power: 200 W; scanning speed: 2 m∙s⁻¹; powder diameter: 27 μm), the average surface temperature of the cavity approached 2800 K under atmospheric pressure, while it came close to 2300 K under 100 Pa pressure. More vigorous fluid flow (average speed: 4 m∙s⁻¹) was observed under 100 Pa ambient pressure, because the pressure difference between the evaporation-induced surface pressure and the ambient pressure was relatively larger and drives the flow under lower pressure. It was also shown that there are periodical ripple flows (period: 14 μs) affecting the surface roughness of the as-printed track. Moreover, the molten flow was shown to be laminar because the Reynolds number is less than 400 and is far below the critical value of turbulence; thus, the viscous dissipation is significant. It was demonstrated that under a vacuum or lower ambient pressure, the ripple flow can be dissipated more easily by the viscous effect because the trajectory length of the ripple is longer; thus, the surface quality of the tracks is improved. To summarize, our model elucidates the physical mechanisms of the interesting transport phenomena that have been observed in independent experimental studies of the SLM process under variable ambient pressure, which could be a powerful tool for optimizing the SLM process in the future.     

Influence of process strategy on distortion and residual stresses for the powder based laser metal deposition process

Laser metal deposition (LMD) induces a complex 3‐axis residual stress state, which superimposes the external service stresses and can cause unpredicted in‐service failures. To optimize the process it is important to know the mechanisms and the opportunities to influence the occurrence of residual stresses. From the mathematical point of view, laser metal deposition represents a free boundary value problem. The track geometry is part of the solution. Previously the authors developed a thermal model that is used to calculate the time‐ and space resolved temperature distribution and the track geometry. The thermal model encompasses the powder stream, its interaction with the laser radiation, the shadowing of the laser radiation by the particles, the heating of the particles and the melt pool computation. In this publication, the evolution of residual stresses for overlapping tracks for single and multilayer processing for different powder mass rates is described.     

Three-dimensional finite element analysis of temperatures and stresses in wide-band laser surface melting processing

During laser surface melting of steel components, obtaining the desired distribution of microstructure and residual stresses with minimum distortion is essential for production goals and reliable service performance. In this study, a three-dimensional finite element based model, which is integrated into commercial finite element analysis (FEA) software SYSWELD by means of user subroutines, has been developed to simulate the wide-band laser surface melting (LSM) processing and predict temperature history and stress field with different laser scanning speed. In the proposed computational procedure, thermal, metallurgical transformation and mechanical aspects were taken into account, and the heat transfer analysis, the temperature dependent on material properties and a coupled transient thermo-mechanical analysis were used. Effects of laser scanning speed on melting temperature field and residual stress were investigated. The simulation results show that laser scanning speed changes have significant effects on melting residual stress. For experimental verification, laser surface melting of thin plate 42CrMo4 steel was achieved by a 5 kW continuous wave CO₂ laser with laser scanning speed from 10 m/s to 30 m/s. The computational results are in good agreement with experimental measurements.     

金属激光3D打印过程数值模拟应用及研究现状

数值模拟可以高效、有针对性地对金属激光选区熔化成型过程中的温度场、熔池形状、残余应力和变形、凝固过程微观组织演变等过程建立相应的模型并对成形件的相关性能做出准确预测,为工艺优化提供科学的依据,显著降低工艺开发成本和缩短工艺开发周期,有力推动金属增材制造向工业级应用的转变。本文综述了金属激光增材制造过程中温度场、熔池动力学、成形件内部残余应力和变形、显微组织变化4个方面数值模拟的最新研究进展,概述了金属SLM过程数值模拟所取得的最新进展,分析了金属SLM数值模拟领域的研究热点和所存在的计算时间长、成本高等问题,最后提出金属SLM过程数值模拟应将3D打印过程中快速凝固、微熔池等特征与大数据、人工智能、深度学习等技术相结合,进一步提高数值模拟精度,拓宽金属激光增材制造加工窗口,为个性化产品开发提供指导。     

Long fatigue crack growth in Inconel 718 produced by selective laser melting

Growth of long fatigue cracks is investigated in Inconel 718 superalloy produced by selective laser melting (SLM). The fatigue crack growth curve and the threshold value of the stress intensity factor are experimentally determined on compact-tension specimens fabricated using a RENISHAW A250 system and the recommended processing parameters.The crack propagation curve and the crack propagation threshold of this SLM material are compared with literature data describing the behavior of conventionally manufactured Inconel 718. The fatigue crack growth is discussed in terms of the specific microstructure and residual stresses produced by selective laser melting.     

Manufacturing three-dimensional nickel titanium articles using layer-by-layer laser-melting technology

Specific features of layer-by-layer synthesis of three-dimensional (3D) nickel titanium (NiTi, nitinol) articles by selective laser melting (SLM) technology have been studied. Nonporous 3D nitinol articles have been obtained for the first time in a single technological cycle. A necessary condition was that the NiTi powder medium was heated to 500°C during sintering. The structure and composition of intermetallic phases in SLM-synthesized samples have been studied by optical metallography, microhardness measurements, scanning electron microscopy, X-ray diffraction, and energy-dispersive x-ray analysis techniques. Optimum SLM regimes for manufacturing NiTi articles and promising medical applications of this material are considered.     

Evaluation of residual stress in stainless steel 316L and Ti6Al4V samples produced by selective laser melting

Selective laser melting (SLM) has great potential in additive manufacturing because it enables the production of full-density complex parts with the desired inner structure and surface morphology. High temperature gradients as a result of the locally concentrated energy input lead to residual stresses, crack formation and part deformation during processing or after separation from the supports and the substrate. In this study, an X-ray diffraction technique and numerical simulation were used for investigating the residual stress in SLM samples fabricated from stainless steel 316L and Ti6Al4V alloy. Conclusions regarding directions and values of stresses in SLM objects are given.     

Experimental Investigation on Selective Laser Melting of Bulk Net-Shape Pure Magnesium

In this study, bulk net-shape pure magnesium was fabricated by selective laser melting (SLM). Effects of Mg particle size and processing parameters on the properties of SLM specimens were investigated. The scan speed and laser power were optimized based on macro-observation of the forming process and resulted specimens. Through the comparison analysis of topology, density, and micro-hardness, it was found that the bulk magnesium specimen made of ?250 mesh powder showed better results than that made of ?400 mesh powder. The relative densities of both SLM specimens were over 95%, and their micro-hardness values were higher than that of as-cast magnesium. It was also revealed that the scanning time interval (STI), as a relatively new processing parameter, had an obvious effect on the properties of SLM bulk magnesium in terms of surface morphology, roughness, and micro-hardness.     

Parametric studies of multi-material laser densification

Fabrication of multi-material components via a laser-assisted layer-by-layer fabrication process has been numerically simulated and analyzed using a three-dimensional thermo-mechanical model. Effects of the chamber preheating temperature, laser scanning rate, initial porosity and thickness of each powder layer on the out-of-plane warping and residual thermal stresses of a nickel/porcelain workpiece have been investigated. It is found that warping and residual thermal stresses of the laser-densified multi-material workpiece are more sensitive to the chamber preheating temperature and the thickness of each powder layer than to the laser scanning rate and the initial porosity of the powder layer. The major mechanism responsible for these phenomena is identified to be related to the change of the temperature gradient induced by these laser processing parameters.     

Infrared thermal imaging for melt pool analysis in SLM: a feasibility investigation

ABSTRACT

Melt pool dimension can help to relate process parameters and build part quality in selective laser melting (SLM) process. In this study, a near-infrared thermal imager (about 670?nm spectral range) was employed to collect powder layer thermal signal in SLM machine using nickel-based alloy as raw powder material. Radiant temperature distribution at different build heights has been acquired and melt pool sizes have been analysed. The major findings are as follows: (1) It is possible to estimate melt pool dimension based on the identified radiant liquidus temperature and appropriate thermal imager setting, but it is difficult to obtain true temperature. (2) At nominal process conditions of 600?mm/s beam speed and 180?W beam power for Inconel 718 powder, the melt pool has a length of about 0.36?mm and a width of about 0.21?mm. Build height seems to have little effect on melt pool dimensions.