Sintering of Doped, Nanocrystalline CeO_2 Powders Prepared under Hydrothermal Conditions

Nanocrystalline CeO2 powders (particle size ≈10-15 nm), doped with up to 20 at.-% of Mg,Ca or Y were prepared by chemical precipitation under hydrothermal conditions. The particle size and shape of the powders change slightly with the dopant concentrations. The the of the dopants on the sintering of the compacted powders was investigated during heating at a constant rate of 10℃/min. The elemental composition and the concentration of the dopant has significant efFect on the densification and grain growth. Compared to undoped CeO2, the dopants produce a shift in the densification curve to higher temperatures. For the same dopant concentration and under identical sintering conditions, the Ca doped samples reach nearly full density with the smallest grain size (≈50 nm), however, the Mg doped sample has the lowest density (≈95% of the theoretical) with the largest grain size (≈1 μm)

STUDY ON SELECTIVE LASER SINTERING OF METALLIC POWDERS

A series of experiments are carried out with selective laser sintering(SLS) for copper,copper-nickel powder matetials.The phenomena of the sintering process is ana-lyzed,the influence of techmological parameters is dis-cussed in detail,By means of SEM with energy spectrum system and X-ray diffraction spectrum,the morphology of the microstructure and the compositions of the different zones are analyzed,the mechanism of the laser sintering metallic powders is preliminarily explored,All those above lay the foundations for the forming metal parts by laser sintering metallic powders.

A comparison of the microstructure-dependent corrosion of dual-structured Mg-Li alloys fabricated by powder consolidation methods:Laser powder bed fusion vs pulse plasma sintering

In this study, powder metallurgy methods were used to fabricate Mg-7.5Li-3Al-Zn alloys from repowdered extruded alloys. Extruded alloys were powdered using ultrasonic atomization, and then laser powder bed fusion(LPBF) and pulse plasma sintering(PPS) were used to consolidate the bulk materials. A comparison of the properties of the fabricated alloys with those of a conventionally extruded one was carried out using methods that characterized the microstructure and corrosion resistance. When compared to their conventionally extruded counterpart, LPBF and PPS materials exhibited refined microstructures with low enrichment in Al Li and coarse Al, Zn, Mn precipitates. The main drawback of the LPBF alloy, printed for the needs of this study, was its porosity, which had a negative effect on its corrosion. The presence of unrecrystallized particle boundaries in the PPS alloy was also unbeneficial with regard to corrosion. The advantage of the LPBF and PPS processes was the ability to change the proportion of α(Mg) to β(Li), which when the complete consolidation of the material is achievable, may increase the corrosion resistance of dual-structured Mg-Li alloys. The results show that powder metallurgy routes have a wide potential to be used for the manufacture of Mg-Li based alloys.

IGNITING SHS BY LASER AND ITS APPLICATION TO SELECTIVE LASER SINTERING OF METALLIC POWDER MATERIAL

How to directly fabricate metallic functional parts with selective laser sintering (SLS) process is a potential technique that scientists are researching. Existent problems during directly fabricating metal part by use of SLS are analyzed. For the sake of solving the problems, a new idea of adding self-propagating high-temperature synthesis (SHS) material into metallic powder material to form new type of SLS metallic powder material is put forward. This powder material can release controllable amount of heat during its interaction with the laser beam energy to reduce the requirement to laser power during directly sintering metallic part, to prolong the time of metallic liquid phase existing, and to improve the intensity and accuracy of SLS part. For this reason, SHS material′s interaction with the CO2 laser beam energy is researched, which proves that CO2 laser beam energy may instantly ignite SHS reaction. On the basis of the above-mentioned researches, the effect of sintering the metal powder material mixing SHS material with CO2 laser is also researched, which shows: there is an optimal blending ratio of various material in the new metallic powder material. Under the optimal blending ratio and SLS process parameters, this new metallic powder material can indeed release amount of heat and SHS reaction may be controlled within the laser sintering. This research result makes it possible that the metallic part is directly sintered with small CO2 laser (less than 50W), which may greatly reduce the volume, cost and running expenditure of SLS machine, be propitious to application.

Selective laser sintering mechanism of polymer-coated molybdenum powder

A type of polymer-coated molybdenum powder used in selective laser sintering technology was prepared by coating polymer on molybdenum particles and frozen grinding techniques, with the maximum particle diameter of 71 μm. The laser sintering experiments of polymer-coated molybdenum powder were conducted by using the self-developed selective laser sintering machine (HLRP-350I). The method of microscopic analysis was used to investigate the dynamic laser sintering process of polymer-coated molybdenum powder. Based on the study, the laser sintering mechanisms of polymer-coated molybdenum powder were presented. It is found that the mechanism is viscous flow when the laser sintering temperature is between 100 ℃ and 160 ℃, which can be described by a two-sphere model; and the mechanism is melting /solidification when the temperature is above 160 ℃.

Atomic force microscope study of WC-10Co cemented carbide sintered from nanocrystalline composite powders

In order to compare the spark plasma sintedng （SPS） process plus hot isostatic press （HIP） with vacuum sintedng plus HIP, an investigation was carried out on the topography, microstructure and gain size distribution of nanocrystalline WC-10Co composite powder and the sintered specimens prepared by SPS plus HIP and by vacuum sintering plus HIP by means of atomic force microscopy （AFM）. The mechanical properties of the sintered specimens were also investigated. It is very easy to find cobalt lakes in the specimen prepared by vacuum sintering plus HIP process. But the microstructure of the specimen prepared by SPS plus HIP is more homogeneous, and the grain size is smaller than that prepared by vacuum sintering plus HIP. The WC-10Co ultrafine cemented carbide consolidated by SPS plus HIP can reach a relative density of 99.4%, and the transverse rupture strength （TRS） is higher than 3540 MPa, the Rockwell A hardness （HRA） is higher than 92.8, the average grain size is smaller than 300 nm, and the WC-10Co ultrafine cemented carbide with excellent properties is achieved. The specimen prepared by SPS with HIP has better properties and microstructure than that prepared by vacuum sintering with HIP.

Numerical simulation of temperature field during selective laser sintering of polymer-coated molybdenum powder

The technology of length-alterable line-scanning laser sintering was introduced. Based on the research of laser heating property, powder thermal physics parameters and laser sintering process, a numerical model of the temperature field during length-alterable line-scanning and laser sintering of polymer-coated molybdenum powder was presented. Finite element method (FEM) was used to simulate the temperature field during laser sintering process. In order to verify the simulated results, a measuring system was developed to study the laser sintering temperature field. Infrared meter was introduced to measure the surface temperature of sintering powder; the temperature of its inside part was measured by thermocouple. The measured results were compared with the numerical simulation results; the conformity between them is good and the relative error is less than 5%.

Microstructural evolution during direct laser sintering of multi-component Cu-based metal powder

A multi-component Cu-based metal powder was chosen for direct laser sintering. The powder consists of a mixture of high-purity Cu powder, pre-alloyed CuSn and CuP powder. Liquid phase sintering with complete melting of the binder (CuSn) but non-melting of the cores of structural metal (Cu) proves to be a feasible mechanism for laser sintering of this powder system. The microstructural evolution of the sintered powder with variation of laser processing parameters was presented. High sintering activities and sound densification response were obtained by optimizing the laser powers and scan speeds. Using a high laser power accompanied by a high scan speed gives rise to balling effect. At a high laser power with a slow scan speed the sintering mechanism may change into complete melting/solidification, which decreases the obtainable sintered density. The role of additive phosphorus in the laser sintering process is addressed. Phosphorus can act as a fluxing agent and has a preferential reaction with oxygen to form phosphatic slag, protecting the Cu particles from oxidation. The phosphatic slag shows a concentration along grain boundaries due to its light mass as well as the short thermal cycle of SLS.

Selective laser sintering of polymer-coated Al_2O_3/ZrO_2/TiC ceramic powder

A type of polymer-coated Al2O3/ZrO2/TiC ceramic powder was prepared. The laser sintering mechanism of polymer-coated Al2O3/ZrO2/TiC powder was investigated by studying the dynamic laser sintering process. It is found that the mechanism is viscous flow when the sintering temperature is between 80 ℃ and 120 ℃, and it is melting/solidification when the temperature is above 120 ℃. The process parameters of selective laser sintering were optimized by using ortho-design method. The results show that the optimal parameters include laser power of 14 W, scanning velocity of 1 400 mm/s, preheating temperature of 50 ℃ and powder depth of 0.15 mm. A two-step post-treatment process is adopted to improve the mechanical properties of laser sintered part, which includes polymer debinding and high temperature sintering. After vacuum sintering for 2 h at 1 650 ℃, the bending strength and fracture toughness of Al2O3/ZrO2/TiC ceramic part reach 358 MPa and 6.9 MPa·m1/2, respectively.

Advances in selective laser sintering of polymers

Polymers are widely used materials in aerospace,automotive,construction,medical devices and pharmaceuticals.Polymers are being promoted rapidly due to their ease of manufacturing and improved material properties.Research on polymer processing technology should be paid more attention to due to the increasing demand for polymer applications.Selective laser sintering(SLS)uses a laser to sinter powdered materials(typical polyamide),and it is one of the critical additive manufacturing(AM)techniques of polymer.It irradiates the laser beam on the defined areas by a computer-aided design three-dimensional(3D)model to bind the material together to create a designed 3D solid structure.SLS has many advantages,such as no support structures and excellent mechanical properties resembling injection moulded parts compared with other AM methods.However,the ability of SLS to process polymers is still affected by some defects,such as the porous structure and limited available types of SLS polymers.Therefore,this article reviews the current state-of-the-art SLS of polymers,including the fundamental principles in this technique,the SLS developments of typical polymers,and the essential process parameters in SLS.Furthermore,the applications of SLS are focused,and the conclusions and perspectives are discussed.

Microstructure of Selective Laser Sintered Polyamide

The selective laser sintering (SLS) was used to prepare components from modified polyamide (PA) powder.The behaviour of the sintering process was analyzed.The influences of the fill laser power,powder bed temperature and powder thickness were discussed in detail.By means of SEM,the morphology and the heat influence were analyzed.Results show that the powders were fused thoroughly which allowed a more dense structure to be built at a powder bed temperature of 98℃,fill laser power of 12W,slice thickness of 0.10mm and a default scanning speed of 1700mm/s.

选择性激光烧结中铺粉过程离散元模拟

铺粉过程是选择性面激光烧结中重要的工艺过程之一,为研究铺粉过程中不同工艺参数对粉末床铺粉质量的影响,研究并设计了一种辊子-刮板式的铺粉结构,以PES为烧结粉末材料,通过EDEM软件对铺粉过程进行离散元仿真。模拟分析了辊子-刮板的移动速度V和刮板端部倾斜角度α对粉末床铺粉质量的影响。结果表明:辊子-刮板的移动速度V越大,铺粉质量越差;刮板端部倾斜角度α对粉末床铺粉质量影响很小。

细晶钨铜复合材料制备工艺的研究

将W 2 0 %Cu混合粉末在行星式高能球磨机中机械合金化 (MA)。经过一定时间球磨后可以得到W晶块尺寸 30nm左右的纳米粉末。测定了粉末晶粒尺寸、粉末的粒度、比表面、松装密度和振实密度等性能。粉末的晶块尺寸用XRD分析得出。研究了MAW 2 0 %Cu粉末烧结后的显微组织。研究表明 ,球磨后粉末在 12 0 0～ 130 0℃下烧结即可达到近全致密 ,相对密度在 99 5 %以上 ,拉伸强度达到 780MPa以上 ,伸长率大于 3 5 % ,钨晶粒尺寸在 1～ 2 μm左右。

α-Al_2O_3纳米粉的烧结动力学

采用等速升温烧结及常规烧结方法对平均粒径约10nm的α-Al2O3纳米粉的烧结动力学进行了研究．等速升温烧结实验表明，坯体致密化主要发生在1200—1450℃范围内．粉体经1400℃，2h的常规烧结后，相对密度达到98．85％．理论分析表明，提高生坯密度，改善其显微结构均匀性有可能大幅度降低该粉体的烧结致密化温度．

选择性激光烧结技术的发展现状

介绍了选择性激光烧结技术的原理、特点及其研究发展状况 ,简述了选择性激光烧结金属粉末的两种典型成型工艺 ,并简要分析讨论了选择性激光烧结技术成型金属零件所存在的一些问题。最后 。

多组分铜基金属粉末选区激光烧结致密化机理

研究了选区激光烧结专用多组分铜基金属粉末(组分包括纯Cu,预合金CuSn,CuP)的烧结性能。结果表明,通过合理控制激光工艺参数(特别是激光功率和扫描速率),能顺利实现粉末烧结成形,且无明显的“球化”效应和翘曲变形。扫描电镜和X射线衍射分析证实,此组粉末体系的激光烧结是基于液相烧结机制,其中熔点较低的CuSn充当粘结金属,熔点较高的Cu充当结构金属;而添加元素P则起稀释剂的作用,能避免Cu颗粒表面氧化。研究了粉末体系中粘结金属含量对粉末烧结致密化和烧结件微观组织的影响。结果表明,在一定范围内粘结金属含量的提高有利于改善烧结致密度;但若粘结金属过量,则会因“球化”效应而降低致密度。

聚苯乙烯粉末选择性激光烧结成型机理的研究

分析了聚苯乙烯粉末材料选择性激光烧结成型机理,并通过填充改性方法提高其成型性能。结果表明,添加无机填料可提高聚苯乙烯粉末材料烧结性能,而且,多种填料同时添加,效果更明显。

纳米复合WC-6Co粉末的快速烧结

以液相复合-连续还原碳化方法制备的纳米复合WC-6Co粉末为原料,采用放电等离子烧结(SPS),制取了超细硬质合金。利用扫描电镜、维氏硬度仪、洛氏硬度仪、密度测试仪、MTS陶瓷测试系统等,观察烧结体显微结构,测试其硬度、密度、断裂强度、矫顽磁力、磁饱和度。结果表明采用放电等离子烧结获得的烧结体的硬度HV1≥19500MPa,断裂强度TRS≥2800MPa,平均晶粒度150nm^300nm。制备了高强度、高硬度的超细WC-6Co硬质合金。

金属粉末激光选区烧结过程的特征探讨

采用激光选区烧结的方法 ,对Ni基合金粉末进行了一系列激光烧结成形试验 .在试验的基础上 ,阐述了用激光直接熔化金属粉末 ,烧结成形金属零件的一些基本特征 ,这主要包括在激光烧结成形过程中的熔池球化特征、熔体流动特征和动态凝固组织特征 .分析了熔池球化现象对烧结成形的不利影响 ,提出了减少粉层厚度和加大激光功率 ,可避免熔池球化现象的出现 .进一步探讨了烧结过程的基本规律和烧结成形的基本机理 。

纳米WC-Co复合粉末的制备工艺及其烧结特性

综述了纳米WC -Co复合粉末制备工艺的国内外研究进展 ,总结了纳米WC -Co硬质合金复合粉末的烧结特性 ,并对目前国内外的研究现状进行了分析 。