Two-dimensional modeling of sintering of a powder layer on top of nonporous substrate

Selective laser sintering (SLS) of a two-component metal powder layer on the top of multiple sintered layers by a moving Gaussian laser beam is modeled. The loose metal powder layer is composed of a powder mixture with significantly different melting points. The physical model that accounts the shrinkage induced by melting is described by using a temperature-transforming model. The effects of the porosity and the thickness of the atop loose powder layer with different numbers of the existing sintered metal powder layers below on the sintering process are numerically investigated. The present work will provide a better understanding to simulate much more complicated three-dimensional SLS process.     

Direct Selective Laser Sintering of Hard Metal Powders: Experimental Study and Simulation

Direct selective laser sintering (SLS) technology can be used to produce 3D hard metal functional parts from commercial available powders. Unlike conventional sintering, it does not require dedicated tools, such as dies. Hence, total production time and cost can be reduced. The large shape freedom offered by such a process makes the use of, for example, sintered carbides components viable in domains where they were not applied before. Successful results have been obtained in the production of sintered carbide or hard metal parts through SLS. The investigation focuses on tungsten carbide–cobalt (WC-Co) powder mixture. This material is characterised by its high mechanical properties and its high wear resistance and is widely used in the field of cutting tools. This paper is devoted to the experimental study and the simulation of direct selective laser sintering of WC-Co hard metal powders.     

PA6粉末选区激光烧结多层应力分析

选区激光烧结成型过程中,激光束扫描时反复的加热、冷却带来不均匀的热应力,影响烧结过程和烧结件的质量。基于弹性变形原理,利用单轴应力作用下的变形梁的挠曲和应变与应力之间的关系,来分析烧结过程中的应力以及不同烧结层之间的相互影响,结果表明,在每个单层烧结时,烧结应力主要集中在烧结层的上表面和下表面的两端,且呈现为拉应力;在多层烧结时,烧结的热载荷对紧邻已熔接层的应力有直接影响,随着烧结层数增加,新烧结层对已熔接层的热影响逐渐减小,对应的随着温度变化较小,已熔接层的应力也较小且保持较低水平。     

粉末薄层选区激光烧结温度场数值模拟与实验研究

单层粉末的激光烧结温度场对三维结构熔结成型有着直接影响。通过对工程塑料粉末选区激光烧结过程能量作用形式的分析,构建了粉体与金属基板界面接触热阻的导热模型和扫描烧结的移动热源模型;利用有限元法对功率为10～25W,光斑直径为0.24mm的激光逐行扫描烧结过程的温度场进行了数值模拟;通过温度场分布模拟和激光烧结实验得到了单层厚度为0.5mm的PA6粉末行扫描间距的优化区间为0.3～0.4mm,获得了熔结质量较好、厚度均匀的平板形烧结物,为进一步优化工艺参数提供了基础。     

The application of Taguchi’s method in the experimental investigation of the laser sintering process

The selective laser sintering (SLS) of iron powder has been investigated through a number of experiments statistically planned as per Taguchi L8 design. Seven input parameters, namely, laser peak power density, laser pulse on-time, laser scan speed, stepping distance (distance traveled between pulses), interval–spot ratio (ratio of laser scan line interval and laser spot diameter), size range of iron powder particles, and powder layer thickness, were selected for the investigation. Density, porosity, and hardness were considered for the characterization of the sintered samples. Analysis of the results show that these properties are significantly affected by these factors. A discussion on the probable physical phenomena contributing to such dependence and an attempt towards the optimization of the process have also been included.     

Effect of delay time on part strength in selective laser sintering

Selective laser sintering (SLS) is one of the most popular layered manufacturing processes used for making functional prototypes of polymers and metals. It is a powder-based process in which layers of powder are spread and laser is used to sinter selected areas of preheated powder. In the present work, experimental investigations have been made to understand effect of delay time on SLS prototypes. Delay time is the time difference for laser exposure between any two adjacent points on successive scanning lines on a layer. Tensile specimens of polyamide material as per the ASTM standard are fabricated on SLS machine keeping delay time range constant for the entire specimen. Specimens are fabricated for different ranges of delay time and tested on universal testing machine for tensile strength. An optimum value of delay time range is obtained experimentally. As delay time depends on part build orientation, an algorithm has been developed and implemented to find out optimum part build orientation for improving tensile strength. The obtained results from developed code are validated experimentally for tensile specimen. Case study for a typical 3D part is also presented to demonstrate the capabilities of developed algorithm.     

直接金属激光烧结成形的机理及实验研究

探讨了直接金属激光烧结成形机理，分析了烧结参数与激光输入能量之间关系。在室温惰性气体环境下，进行了不同匹配参数下314不锈钢金属粉末的直接激光烧结成形实验，测试了成形件的微观组织结构和机械性能，研究了工艺参数与直接金属激光烧结成形件的微观组织结构及宏观机械性能的关系。为制定合理的激光烧结工艺参数提供了依据。     

Temperature and stress analysis and simulation in fractal scanning-based laser sintering

A model for calculating the evolution of temperature and thermal stresses within a single metallic layer formed on the powder bed using different scanning patterns in selective laser sintering (SLS) was proposed. The model allows for the non-linear behavior of thermal conductivity, specific heat, and elastic modulus due to temperature changes and plastic deformation capabilities with a bilinear isotropic hardening behavior. The effect of laser scanning patterns on temperature, residual thermal stresses and distortion were investigated. It was shown that the distortion and transient stresses of a layer processed by a moving laser beam is decreased with fractal scanning pattern.     

Development and application of copper–nickel zirconium diboride as EDM electrodes manufactured by selective laser sintering

The production of electrical discharge machining (EDM) electrodes by conventional machining processes can account for over 50 % of the total EDM process costs. The emerging additive manufacturing (AM) technologies provide the possibility of direct fabrication of EDM electrodes. Selective laser sintering (SLS) is an alternative AM technique because it has the possibility to reduce the tool-room lead time and total EDM costs. The main difficulty of manufacturing an EDM electrode using SLS is the selection of an appropriate material. This work investigated the direct production of EDM electrodes by means of the SLS using a newly developed non-conventional metal–matrix composite material composed of a metallic matrix (CuNi) and an advanced ceramic (ZrB₂). The influence of important SLS parameters and material content on the densification behavior and porosity of the electrodes was investigated. EDM experiments were conducted to observe the electrodes behavior and performance. It was found that the ZrB₂-CuNi electrodes could be successfully manufactured by SLS. Interlayer bonding and porosity are directly influenced by the layer thickness. Smaller layer thicknesses improved bonding between layers and decreased the porosity of the parts. The laser scan speed has a significant effect on the densification behavior. The scan line spacing affects the pore structure by means of overlapping. The surface morphology of the samples was not affected by varying the scan line spacing. The ZrB₂-CuNi electrodes presented a much superior performance than SLS copper powder electrodes, but inferior to solid copper electrodes.     

Direct metal laser fabrication: machine development and experimental work

A novel modification of the freeform technique selective laser sintering machine via system integration to develop a direct metal laser fabrication machine suitable for both nonmetal and metal materials is put forward in this paper, the aim of which is to establish an experiment platform for studying the direct metal laser fabrication (DMLF) as a variant of selective laser sintering (SLS). The system integration of two machines is realized by use of the low-power SLS Machine (CO₂, 50 W) and high-power laser processing machine (CO₂, 2,000 W) in our institute after the improvement or rebuilding of light route system, functional units, machinery and electrical system, and control software in the primary SLS machine, which means that direct metal laser fabrication machine with the laser power up to 1,000 W has been developed successfully. Functional tests of two machines and DMLF experiments of a large number of metal powders including Cu-based mixture and 316L-based mixture of powder have been done in detail. The results show that the material suitable for direct metal laser fabrication machine ranges from nonmetal powders to metal powders, including polystyrene, polyamide, polycarbonate, sands, and a large number of metal powders. At the same time, the primary functions or performances of both low-power SLS Machine and high-power laser processing machine are preserved entirely. Metal samples based on copper and 316L powders can be fabricated with the relative density of about 80% and 100%, respectively, by use of developed DMLF machine. The macro appearance and microstructure and processing mechanisms of DMLF are analyzed minutely. The academic application of DMLF machine for metal powders with high melting point has been making its mark.     

选择性激光烧结金属件精度和密度的研究

金属件成形精度和密度是快速成形技术在工业应用中的关键问题之一。通过对覆膜金属粉末选择性激光烧结 ,研究了激光烧结工艺参数激光功率、扫描速度、扫描间距、单层层厚 ,烧结体厚度对金属件成形精度和密度的影响关系 ,并得出了一个分析 X、Y轴方向尺寸误差的定性公式 ,给出了烧结工艺参数优化的方法 ,而且优化了一组烧结工艺参数 ,用此工艺参数成功烧制出金属原型件     

A macroscale FEM-based approach for selective laser sintering of thermoplastics

A numerical approach to model the selective laser sintering (SLS) of polypropylene is proposed. A 3D thermal model was developed and thus enables the prediction of the temperature fields and the extension of the sintered area in the powder bed taking into account the phase change during multiple laser passes. Powder–liquid, liquid–solid and solid–liquid phase changes were modelled during the SLS and the subsequent cooling processes. Then, a 3D thermomechanically coupled model was set up based on the temperature results of the thermal model in order to predict the distortion of the produced parts after cooling down. Different pre-heating temperatures were considered, highlighting their influence on the final part properties.     

激光直接烧结成形多层金属薄壁件的温度场有限元模拟

为掌握多层粉末烧结过程中激光熔池的加热冷却规律以及各烧结层之间的相互影响,综合考虑热传导、热辐射和热对流以及材料的高度非线性,基于ANSYS平台建立了多层金属薄壁件的三维温度场有限元模型,利用APDL语言编程实现模拟中激光热源的移动,采用"单元生死"技术描述粉末材料动态增长过程。模拟结果表明:在现有工艺参数下,烧结获得的熔深在0.15mm以上,熔宽在0.61mm左右,烧结成形件与基体以及层层之间搭接牢固;成形件中,与x方向的热梯度相比,z方向热梯度占绝对优势,这说明成形件在冷却过程中热量的散失以堆积方向为主。将模拟结果与实验结果进行了对比,实验结果较好地验证了模拟结果。     

Microstructural evolvement and formation of selective laser melting W–Ni–Cu composite powder

W–Ni–Cu alloy (90 wt% W, 7.5 wt% Ni, and 2.5 wt% Cu) parts were successfully fabricated via selective laser melting method. Phases, microstructure, compositions, and laser forming parameters of laser melted samples were investigated. It was found that the W–Ni–Cu powder system was based on the mechanism of liquid solidification. This process was realized through full melting of W, Ni, and Cu particles under high laser energy input. However, using relatively lower energy input, particle bonding was realized through liquid phase sintering with complete melting of Ni–Cu acting as binder and nonmelting of W acting as structure. Due to the Ni–Cu solid solution phase that appeared in a wide range from 1,084 to 1,455 °C, a coherent matrix interface can be observed after solidification. The microhardness of laser-fabricated specimens varied with different powder layer thicknesses, resulting from the laser-treated condition and ability of trapped air in the loose powder bed to escape. The metallurgical mechanisms were also addressed.     

Numerical simulation of the thermal history multiple laser deposited layers

Multilayer direct laser metal deposition is a fabrication process in which the parts are fabricated by creating a molten pool into which metal powder particles are injected, and a layer is laid down by moving the pool. Height is added by creating additional layers on top of the first layer. During fabrication, a complex thermal history is experienced in different regions of the build. The thermal history includes the reheating process for previously deposited layers caused by subsequently deposited layers. The objective of this study is to provide insight into the thermal history during the direct laser deposition process. Using the commercial ABAQUS/CAE software, a thermomechanical 3D finite element model was developed. This work presents a 3D heat transfer model that considers the continuous addition of powder particles in the front of a moving laser beam using ABAQUS/CAE software. The model assumes the deposit geometry appropriate to each experimental condition and calculates temperature distribution, cooling rates, and remelted layer depth which can affect the final microstructure. Model simulations were qualitatively compared with experiments results acquired in situ using a K-type thermocouple.     

Selective laser melting W–10 wt.% Cu composite powders

Tungsten–Copper (W–Cu) alloys are promising materials for electrical and thermal applications. However, its forming method still remains limited in conventional powder metallurgy technique which is not suitable for manufacturing parts with intricate shapes. In this work, selective laser melting technology was introduced for fabricating W–10 wt.% Cu alloys parts. The morphological feature of a single molten track was analyzed. The results show that liquid phase sintering with complete melting of the binder (Cu), but nonmelting of the structural metal (W), acts as the main mechanism in this process. The melting conditions of single layers in different processing parameters were investigated. The results show that a moderate melting zone can be acquired from an established process map. Moreover, investigations on multilayers forming show that the final density increases with the decrease of scan speed until it reaches a plateau due to the insufficient rearrangement in liquid phase sintering and the balling effect.     

Investigating surface roughness of parts produced by SLS process

Selective laser sintering (SLS) has been recognized as one of the best rapid prototyping (RP) technique for producing solid models, directly from computer-aided design data by fussing together different layers with the help of laser light. Further, RP has traditionally been used for producing a solid model for visualization purpose and assessing kinematic functionality. So, the model is required to have superior mechanical integrity and surface quality for handling and model testing. This study investigates surface roughness (SR) of parts produced by SLS process. The empirical models have been purposed to predict the feasibility of different process parameters viz., laser power, scan spacing, bed temperature, hatch length, and scan count on SR. Further, these parameters have been optimized using face-centered central composite design with response surface methodology. The optimized parameters have been verified by conducting confirmation experiments.     

氧化锆零件激光选区烧结/冷等静压复合成形技术

氧化锆陶瓷材料以其优异的性能在工业生产中具有极大的应用前景,但由于脆性大、硬度高等原因,复杂形状氧化锆零件往往难以成形和加工。为了获得复杂形状氧化锆陶瓷零件,通过溶剂沉淀法将粘接剂尼龙12覆膜至纳米氧化锆粉末的表面,然后对覆膜后的粉体进行激光选区烧结(Selective laser sintering, SLS)成形,并通过传统的冷等静压(Cold isostatic pressing, CIP)技术对SLS零件进行致密化处理,同时满足氧化锆初坯成形时形状复杂度和密度的要求。通过试验得出在激光能量密度为0.415 J/mm2时,获得的SLS陶瓷件密度较大,对不同激光能量密度制备的SLS陶瓷件进行保压压力为200 MPa的冷等静压致密化处理,根据热脱脂机理以及粘接剂的TG曲线,分别制定了SLS/CIP试样的热脱脂工艺,最后对脱脂试样进行高温烧结,在后续处理的各环节,氧化锆零件的密度仍受SLS成形的影响,但该影响逐渐减弱,SLS/CIP/FS成形件最大相对密度和维氏硬度分别达到了97%和1180 HV1,已接近“模压-烧结”的致密氧化锆陶瓷的性能,在试样断口的扫描电子显微镜(SEM)分析基础上,对氧化锆复合成形的微观演变进行了研究。虽然最终烧结件密度和硬度仍有待提高,但是提出了一种极具潜力的氧化锆零件近净成形工艺方法,为制造高性能复杂形状的陶瓷零件奠定了基础。     

碳化硅零件的激光选区烧结及反应烧结工艺

为了获得高密度、高性能、复杂结构的碳化硅陶瓷件,提出采用机械混合法制备含有黏结剂和乌洛托品固化剂的碳化硅复合粉体,对复合粉体进行激光选区烧结（SLS）形成陶瓷素坯,并对素坯进行气氛烧结和渗硅处理,使其与基体发生反应烧结,最终形成复杂陶瓷异形件。实验证明：若激光功率为8.0 W、扫描速率为2 000 mm/s、扫描间距为0.1 mm、单层厚度为0.15 mm,获得的 SLS 陶瓷样品密度和强度最好。对SLS试样进行合理的中温碳化和高温渗硅,所得碳化硅陶瓷烧结体的抗弯强度最高可达 81 MPa,相对密度大于86%。     

Investigation into manufacturing injection mold via indirect selective laser sintering

Cooling system of an injection mold is important for the promotion of production rate and the quality of injection plastic components. Conformal cooling channels are newly developed temperature-adjusting method to promote the efficiency of cooling system. They can be made in the injection mold inserts via the method of indirect selective laser sintering combined with traditional powder metallurgy. This work discussed some processes such as thermal transmission, powders removing, and metal melt infiltration during the manufacturing of the mold inserts in detail. The result showed that redundant powders outside of laser-scanned areas might be sintered together with the sintered parts owing to the accumulation of laser energy during sintering process. This was solved by switching the temperature to initial level after one layer had been sintered. A limit length corresponding to some certain power vacuum system was found when the removal of unsintered powders in the cooling channels was carried out. Therefore, some subsidiary channels leading the cooling channels outside were made to help the removal of powders within the cooling channels. Dripping method was adopted during metal melt infiltration process, which was proven to be relevant for maintaining of the final shape of infiltrated inserts.