Low-temperature sintering process for UO2 pellets in partially-oxidative atmosphere

Low-temperature sintering（LTS） experiments of UO2 pellets and their results were reported. Moreover, a routine process of LTS for UO2 pellets was primarily established. Being sintered at 1 400 ℃ for 3 h in a partially-oxidative atmosphere, the relative density of the pellet can be up to around 94%. Pellets with such a high density are of benefit for following-up reduction-sintering processes. Orthogonal test indicates that the importance of factors affecting the density decreases in the sequence of partial-oxidative sintering temperature and time, reduction-sintering time and temperature, and sintering atmosphere. It is found that it is helpful to introducing a small amount of water vapor into the sintering atmosphere during the latter stage. It is believed that it is the key factor to raise the O/U ratio of original powder in order to improve the properties of the low-temperature sintered pellets.     

Processing and mechanical properties of porous 316L stainless steel for biomedical applications

Highly porous 316L stainless steel parts were produced by using a powder metallurgy process, which includes the selective laser sintering（SLS） and traditional sintering. Porous 316L stainless steel suitable for medical applications was successfully fabricated in the porosity range of 40%-50% （volume fraction） by controlling the SLS parameters and sintering behaviour. The porosity of the sintered compacts was investigated as a function of the SLS parameters and the furnace cycle. Compressive stress and elastic modulus of the 316L stainless steel material were determined. The compressive strength was found to be ranging from 21 to 32 MPa and corresponding elastic modulus ranging from 26 to 43 GPa. The present parts are promising for biomedical applications since the optimal porosity of implant materials for ingrowths of new-bone tissues is in the range of 20%-59% （volume fraction） and mechanical properties are matching with human bone.     

Sintering of WC-Co powder with nanocrystalline WC by spark plasma sintering

A 92WC-8Co powder mixture with 33 nm WC grains was prepared by strengthening ball milling and was then sintered by spark plasma sintering （SPS） at 1000-1200℃ for 5-18 rain under 10-25 kN, respectively. Movement of the position of low punch shown shrinkage of the sintered body began above 800℃. The shrinkage slowly rose as the temperature rose from 800 to 1000℃ and then quickly rose at above 1000℃ and then gradually rose at above 1150℃. The densities of the samples increased with an increase in sintering temperature, rapidly below 1100℃, and then gradually above 1100℃. WC grains grow gradually with increasing sintering temperature. The powder was sintered to near full density at 1100℃ for 5 rain under 10 kN. The best result of the sample with 275 nm WC grains and no pores was obtained at 1150℃ under 10 kN for 5 rain. The research found the graphite die had a function of carburization, which could compensate the sintered body for the lack of carbon, and had the normal microstructure.     

Low-temperature sintering method for NiCuZn ferrite and effect of Mn addition on electromagnetic properties

Low temperature sintering NiCuZn ferrite was employed at most cases due to its co-firability with Ag （below 960 ℃）. The NiCuZn ferrite sintered body with high-strength and high-frequency magnetic properties was fabricated. Firstly, NiCuZn ferrite powder was synthesized under CO2 atmosphere at 500 ℃from the mixed doxalate synthesized by liquid phase precipitation method. Then a small amount of boric acid （H3BO3） was added to the powder, and the NiCuZn ferrite powder compact was prepared with Newton press and CIP methods. Finally, NiCuZn ferrite sintered body was fabricated by sintering at 900 ℃ under CO2 atmosphere. The minimum sintering temperature （800 ℃） was determined by the study of high temperature shrinkage. By this method, NiCuZn ferrite sintered body with 0.5% （mass fraction） boric acid was obtained, which has the bending strength of 340 MPa. The effect of various Mn addition on electromagnetic properties were studied.     

Influence of Spark Plasma Sintering Temperature on the Densification~ Microstructure and Mechanical Properties of A1-4.5 wt.%Cu Alloy

The effect of sintering temperature on the densification mechanisms, microstructural evolution and mechanical properties of spark plasma sintered （SPS） compacts of a gas atomized Al-4.5 wt.%Cu alloy was investigated. The powder particles whose size varied between 10 to 500μm was subjected to SPS at 400, 450 and 500℃ at a pressure of 30 MPa. The compact sintered at 500℃ exhibited fully dense microstructure which was characterized by a uniform distribution of the secondary phase, free of dendrites and micro-porosity. Microscopy and the SPS data reveal that the events such as particle rearrangement, localized deformation and bulk deformation appear to be the sequence of sintering mechanisms depending on the size range of powder particles used for consolidation. The compact sintered at 500℃ exhibited the highest hardness and compression strength since the microstructure was characterized by fine distribution of precipitates, large fraction of submicron grains and complete metallurgical bonding.     

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 ℃.     

Effects of rare earth addition on sintering process and dielectric property of cordierite based glass-ceramics

The effects of rare earth oxide on the sintering and dielectric property of cordierite-based glass-ceramics with non-stoichiometric composition prepared by quenching of molten droplets were investigated. The results show that the addition of rare earth oxide can lower the sintering temperature of cordierite glass-ceramics, improve the densification process and obviously reduce sintering activation energy. It is found that the densification of cordieritebased glass-ceramics is a liquid phase sintering process. The dielectric constant of the sintered compacts enhances with the increase of the density. When the sintering temperature is identical, the rare earth addition is found to have a noticeable effect on the dielectric loss of glass-ceramics. The properties of the glass-ceramics containing rare earth oxide appear to be correct for low firing temperature substrates.     

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 baUing 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.     

New Steel Making Process through Reduction and Sintering of Cementite Powder

A new steel making process through reduction and sintering treatment of cementite powder was proposed, and microstructure of the material obtained by this process was examined by scanning electron microscopy and EBSP analysis.The cementite powder obtained by mechanical alloying was compacted under high pressure up to 1000MPa, and then reduced during sintering in a hydrogen (H2) gas atmosphere at elevated temperatures. The cementite compacts were completely reduced by the sintering at 1073K to 1273K for 18ks. The relative density of sintered material became 95% or more when the temperature was elevated above 1173K. The SEM observation and EBSP analysis indicated that the obtained steel has equiaxed ferrite-pearlite structure with grain size of around 5μm and the crystallographic orientations of ferrite grains are randomly distributed.     

Microstructure and properties of liquid-phase sintered tungsten heavy alloys by using ultra-fine tungsten powders

The microstructure and properties of liquid-phase sintered 93W-4.9Ni-2.1Fe tungsten heavy alloys using ultra-fine tungsten powders (medium particle size of 700 nm) and original tungsten powders (medium particle size of 3um) were investigated respectively. Commercial tungsten powders (original tungsten powders) were mechanically milled in a high-energy attritor mill for 35 h. Ultra-fine tungsten powders and commercial Ni, Fe powders were consolidated into green compacts by using CIP method and liquid-phase sintering at 1465℃ for 30 rain in the dissociated ammonia atmosphere. Liquid-phase sintered tungsten heavy alloys using ultra-fine tungsten powders exhibit full densification (above 99% in relative density) and higher strength and elongation compared with conventional liquidphase sintered alloys using original tungsten powders due to lower sintering temperature at 1465℃ and short sintering time. The mechanical properties of sintered tungsten heavy alloy are found to be mainly dependent on the particles size of raw tungsten powders and liquid-phase sintering temperature.     

Warm compacted NbC particulate reinforced iron—base composite（I）——Effect of fabrication parameters

NbC was used as reinforced particles in the fabrication of iron-base composite.Baill milling was introduced to overcome the problems of agglomeration and powder separation during powder mixing.After ball milling,the fine NbC particles are embedded on the surface of iron particles and evenly distributed in the mixed powders.Warm compaction was used not only to increase the green density but also to improve the formability of the mixed powder and to improve the compact‘s green strength to facilitate handling.The influences of fabrication parameters such as ball milling time,annealing temperature and time,warm compaction temperature,sintering temperature and sintering time were studied.Compacts with a relative sintered density of 97% and a tensile strength of more than 800MPa can be obtained by using a ball milling time of 5h,an annealing temperature of 800℃,a compaction pressure of 600MPa,warm compaction temperature of 120℃，sintering temperature of 1280℃,and sintering time of 80min,The shrinkage at this sintering condition was approximately 4.3%。     

Preparation of multi-walled carbon nanotube-reinforced TiNi matrix composites from elemental powders by spark plasma sintering

Carbon nanotube (CNT)-reinforced TiNi matrix composites were synthesized by spark plasma sintering (SPS) employing elemental powders.The phase structure,morphology and transformation behaviors were studied.It was found that thermoelastic martensitic transformation behaviors could be observed from the samples sintered above 800 ℃ even with a short sintering time (5 min),and the transformation temperatures gradually increased with increasing sintering temperature because of more Ti-rich TiNi phase formation.Although decreasing the sintering temperature and time to 700 ℃ and 5 min could not protect defective MWCNTs from reacting with Ti,still-perfect MWCNTs remained in the specimens sintered at 900 ℃.This method is expected to supply a basis for preparing CNT-reinforced TiNi composites.     

Effects of sintering pressure and temperature on microstructure and tribological characteristic of Cu-based aircraft brake material

A novel Cu-based P/M aircraft brake material was prepared and the effects of sintering pressure and temperature on microstructure and tribological characteristic were investigated. For the constant sintering temperature, when the sintering pressure increases from 0.5 MPa to 1.5 MPa, the porosity, wear loss and friction coefficient decrease remarkably. When the sintering pressure increases from 1.5 MPa to 2.5 MPa, the porosity further decreases but in a little degree and wear behaviors are improved slightly. However, once the sintering pressure is larger than 2.5 MPa, it has no obvious effect on microstructure and tribological characteristic. For the constant sintering pressure, when the sintering temperature increases from 900 ℃ to 930 ℃, the sintered density remarkably-increases, and wear behaviors are obviously improved. For further increasing sintering temperature to l 000 ℃, the density keeps on increasing, but wear behaviors change slightly.     

Influence of preparation process on sintering behavior and mechanical properties of ultrafine grained Ti（C, N）-based cermets

The influences of forming and sintering processes on distortion, cracking as well as mechanical properties of sintered bodies of ultrafine grained Ti（C, N）-based cermets were investigated. The results show that lubricant is indispensable to fabrication of ultrafine Ti（C, N）-based cermets, however, with low binder content in powder mixture, the lubrication action of paraffin is attenuated. A appropriate level of 2% （mass fraction） paraffin is determined for a cermet with binder content of 36% （mass fraction）. It is also found that the influence of compaction pressure on distortion and cracking of sintered bodies presents a complex relationship. A relatively lower or higher compaction pressure, less than 100 MPa and more than 400 MPa respectively, favors uniform density distribution in green compact. The heating rate of sintering should be strictly controlled. Too fast heating rate results in enclosed pores to burst and forms large size pores in sintering body. A heating rate of 3 ℃/min is recommended.     

Change in relative density of WC-Co cemented carbides in spark plasma sintering process

The relative density of WC-Co cemented carbides during spark plasma sintering (SPS) was analyzed.Based on the change in displacement of the ram in the SPS system,the relative densities in the sintering process can be achieved at different temperatures.The results indicated that densification of the samples started at near 900℃,the density rapidly reached its maximum at the increasing temperature stage,in which the temperature was lower than the sintering temperature of 1200℃,and most of the densification took place in the stage.Besides,the theoretcal values were consistent with the experimental results.     

Preparation of oriented linear copper fiber sintered felt and its performance

Long metal fibers were manufactured in horizontal lathe with a multi-tooth tool. Based on the coarse antler surface structure of copper fibers, a new sintering technology was put forward to manufacture a kind of oriented linear copper fiber sintered felt. The sintering mechanism of oriented linear copper fiber sintered felt was studied. Compared with sintered copper-wire felt, the characteristics of sintered copper-fiber felts were analyzed in details. Owing to the coarse antler surface structure of copper fibers, oriented linear copper-fiber felt was sintered under the condition ofmicro/nano scale range, and copper fibers easily bonded together in the sintering process. Microchannels with micro-scale coarse antler surface structure were constructed. These characters give oriented linear copper fiber felt some new merits： high filtration accuracy, high flow capability, low resistance loss, good capability to resistance pressure, stable and uniform pore, high specific surface area. The properties of oriented linear copper fiber sintered felt were analyzed.     

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%.     

Selective laser sintering of polymer-coated Al2O3／ZrO2／TiC ceramic powder

A type of polymer-coated Al2O3/ZrO2/TiC ceramic powder was prepared. The laser sintering mechanism of polymer-coated Al2 O3/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 posttreatment 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.     

Characterizations and mechanical properties of impregnated diamond segment using Cu-Fe-Co metal matrix

Diamond impregnated Cu-Fe-Co based saw-blade segments are directly processed by vacuum and pressure-assisted sintering at different temperature,with the purpose of reducing the cobalt content in diamond tools.Copper and iron are used as the bonding elements and cobalt-chrome pre-alloyed powder is used as the hardening phase.Effects of sintering temperature on microstructures and mechanical properties of the sintered matrix and diamond graphitization were investigated by X-ray diffraction analysis,electron probe micro-analyzer,universal testing machine,digital Rockwell hardness tester and Raman scattering analyzer.Results showed that microstructures of the sintered matrix were refined and porosities in the sintered matrix were closed to a more spherical-like shape with the increase of the sintering temperature.Densification,hardness and tensile strength of the matrix sintered at 820 ℃ were 12.75％,2.72％ and 156.38％ higher than that of the matrix sintered at 740 ℃,respectively.Diamond graphitization was not occurred at 820 ℃.The hardness and the tensile strength rose 32.8％ and 13.5％,respectively,after 7.5 h ageing treatment.The matrix densification ascent and the dispersed distribution of Co-Cr pre-alloyed powders contributed a hardness improvement and a tensile strength improvement to the Cu-Fe based matrix.     

Spark plasma sintering on mechanically activated W-Cu powders

Mechanically activated W-Cu powders were sintered by a spark plasma sintering system (SPS) in order to develop a new process and improve the properties of the alloy. Properties such as density and hardness were measured. The microstructures of the sintered W-Cu alloy samples were observed by SEM (scanning electron microscope). The results show that spark plasma sintering can obviously lower the sintering temperature and increase the density of the alloy. This process can also improve the hardness of the alloy. SPS is an effective method to obtain W-Cu powders with high density and superior physical properties.