Properties and microstructural evolution of W-Ni-Fe alloy via microwave sintering

The mechanical properties and microstructure evolution of 93W-4.9Ni-2.1Fe (wt.%) alloys were investigated via microwave sintering. The microwave sintering promoted the dissolution and diffusion of tungsten atoms in the matrix phase and strengthened sintering activity. With the increase of microwave sintering temperature, pores in the alloy were reduced and gradually eliminated, tungsten grains coarsened, the distribution of tungsten grains and matrix phase became more homogeneous, and the fracture mode transformed from intergranular fracture to tungsten transgranular cleavage fracture, respectively. The W-matrix interfacial bond strength of 93W-4.9Ni-2.1Fe was enhanced and the mechanical properties were significantly improved with the increase of sintering temperature.     

微波烧结工艺对钨基合金挤压棒坯显微组织及力学性能的影响

研究了微波烧结温度和时间对钨基合金挤压棒坯显微组织及力学性能的影响。采用高倍SEM和光学金相显微镜分别对合金断口和显微组织进行了形貌观察;对合金相对密度、抗拉强度、延伸率和硬度进行了测定和分析。结果表明:当微波烧结时间为30min时,随着烧结温度的升高,合金性能呈峰值变化,烧结温度为1550℃时,合金的力学性能达到极大值,相对密度、抗拉强度、延伸率和HRC硬度分别为98.3%、920MPa、9.7%和30.5;当微波烧结温度为1550℃时,随着烧结时间的延长,合金的力学性能先上升后下降;随着微波烧结温度的升高及微波烧结时间的延长,钨晶粒的尺寸逐渐增大。     

Microwave sintering of hardmetals

Application of microwave radiation as a heat source for sintering of hardmetal is described. Sintering of hardmetal with microwaves leads to a finer microstructure because of lower sintering temperatures and shorter processing times. A further variant is the microwave reaction sintering of a powder mixture of metallic tungsten, carbon and cobalt to obtain finer microstructures than by the conventional route. Moreover, this process offers a great potential for simplifying and shortening the process sequence in hardmetal production. The in-situ formation of WC-platelets during microwave reaction sintering was observed.     

Effect of heating mode and sintering temperature on the consolidation of 90W-7Ni-3Fe alloys

The present study compares the sintering response of 90W-7Ni-3Fe alloys consolidated in a 2.45 GHz microwave furnace and a conventional furnace. The W-Ni-Fe compacts were sintered in a temperature range of 1200-1500 °C corresponding to solid-state as well as liquid phase sintering. The compacts were successfully sintered in a microwave furnace with about 80% reduction in the overall processing time. For both the heating modes, the W-Ni-Fe alloys exhibited significant densification prior to melt formation through solid-state sintering. The in situ dilatometric studies revealed that the contribution to densification from solid-state sintering is higher at lower heating rates. In comparison to conventional sintering, microwave sintered compacts showed relatively refined microstructure and higher hardness and flexural strength.     

Microstructure and mechanical properties of functionally gradient cemented carbides fabricated by microwave heating nitriding sintering

A new method is presented for the fast preparation of functionally graded cemented carbide materials by microwave heating nitriding sintering. The influence of composition and sintering temperature on the mechanical properties, microstructure, and phase composition of the materials was studied. Results showed that functionally graded cemented carbides with the desired mechanical properties can be obtained rapidly by microwave heating nitriding sintering. A gradient layer with a Ti(C, N)-enriched surface layer, and underneath a Co-enriched layer formed on the top of the hard alloy substrate. The nitriding process had little effect on the microstructure of the matrix. A lower surface roughness, and the similar layer thickness as seen in conventional heating nitriding was obtained by microwave heating nitriding sintering in a short period of time. The thickness of the gradient layer increased with increasing temperature. The high Ti content in the raw material was beneficial to the formation of the gradient layer; however, the Co content had little effect on the gradient layer thickness when it increased from 6% to 10%.     

陶瓷材料微波烧结研究进展与工业应用现状

本文主要介绍了微波烧结原理、烧结设备和微波技术在陶瓷粉末合成、硬质合金、金属陶瓷、功能陶瓷材料烧结及涂层制备方面的应用;微波烧结新技术在溶胶凝胶、自蔓延高温合成、水热法、电热法、涂层溶解、陶瓷材料干燥、连接等领域的最新发展。微波烧结具有加热速度快、烧结坯体温度分布均匀;活化烧结、烧结时间短、抑制晶粒长大、组织结构可控、高效节能等优点。探讨微波烧结产业化的现状和存在的问题,烧结材料介质特性数据缺乏和设备的缺乏、昂贵,是阻碍微波烧结技术发展产业化最主要的两大障碍。对解决微波烧结工业化的难题阐述了观点,微波场的优化设计、陶瓷材料的介电性能的数据库建设及理论发展、微波设备和烧结工艺的联合开发等是目前微波技术研究主要的努力方向。     

Improving sinterability of ceramics using hybrid microwave heating

Microwave processing, as a new method for sintering ceramics, has key advantages such as increased heating rate, uniform heating and reduced cost compared to conventional methods. It is generally accepted that microwave sintering can improve the macroscopic mechanical performances of ceramics, however, the performances of microwave-sintered ceramics on the microscopic scale are rarely investigated. In the present study, the ceramics are sintered by hybrid microwave sintering (HMS), which combines the characteristics of microwave heating and conventional heating. To evaluate the homogeneous performance of the sintered ceramics, the behaviors of thermal residual stress distribution in the microwave-sintered and conventionally sintered ceramics were investigated by X-ray diffraction technique. The thermal residual stress investigation shows microwaves can sinter ceramics in entire volume while offering improved mechanical properties. Subsequently, the distribution behaviors of pore ratio and hardness in the ceramics were investigated, respectively. The experiment results confirm that the sinterability of ceramics is homogenously improved by hybrid microwave sintering.     

Microwave hybrid fast sintering of porcelain bodies

Microwave heating offers many advantages over conventional heating methods, such as saving energy, very rapid heating rates and considerably reduced processing times. However, few studies have used microwave energy to sinter traditional ceramics. Thus, the aim of this work is microwave hybrid fast sintering of porcelain bodies. Bodies of sanitary ware, dental and electrical porcelain were sintered quickly. The control of the heating cycle was the main factor in achieving success in microwave hybrid fast sintering of porcelain bodies. Heating cycles lower than 60 min with soaking times ranging from 8 to 19 min were used for the sintering of the porcelain bodies. The modulus of rupture of the microwave-sintered sanitary ware and electrical porcelain bodies were similar to those of conventionally sintered samples, despite the short sintering cycle. The modulus of rupture of the microwave-sintered dental porcelain bodies was higher than those of the conventional sintered samples.     

Sintering response of austenitic (316L) and ferritic (434L) stainless steel consolidated in conventional and microwave furnaces

This study compares the effect of heating mode on the densification, microstructure, strength and hardness of austenitic and ferritic stainless steel. The compacts were sintered in a radiatively heated (conventional) and a 2.45 GHz microwave furnace. Both 316L and 434L compacts couple with microwaves and heat up to the sintering temperature rapidly (45 °C/min). The overall processing time was reduced by about 90% through microwave sintering. While the microwave sintered compacts exhibit a finer microstructure, there is no corresponding improvement in densification and mechanical properties. This has been correlated with elongated and irregular pore structure.     

Using microwaves to synthesize pure aluminum and metastable Al/Cu nanocomposites with superior properties

The processing of aluminum using powder metallurgy techniques is a challenging task due to the presence of thin oxide films on the surface of the metal particles which prevent strong bonding between particles during sintering. In this study, an innovative hybrid microwave sintering technique is utilized to process pure aluminum and Al/Cu nanocomposites without the need of any protective atmosphere. Hybrid microwave sintering involves the heating of aluminum compacts using both microwave energy and radiant heating. Significant savings in time and energy can be achieved using the fabrication methodology. Results revealed that processing methodology used in this study provides a simple and inexpensive option to existing processing methodologies to synthesize pure Al and Al/Cu nanocomposites with superior combination of properties.     

Effect of heating mode on sintering of ferrous compacts through powder metallurgy route

Abstract

Microwave heating is recognised for its various advantages, such as time and energy saving, very rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved properties. The present paper focuses on preliminary work carried out with the use of microwave radiation applied to sintering of ferrous compacts. The ferrous alloy compacts were sintered in a multimode microwave furnace of 2·45 GHz and 6 kW nominal power at 1120°C for 60 min in forming gas. Results of densification, mechanical properties and microstructural evaluation of the microwave sintered samples are reported and compared with conventionally sintered ones. In general, it is observed that the microwave radiation generally enhances the properties of the sintered material when compared with conventionally sintered material.     

Transient Liquid-Phase Sintering Characteristic of W-Ni-Fe Alloy via Microwave-Assisted Heating

研究了W-Ni-Fe合金在2.45 GHz微波炉中瞬时液相烧结的致密化行为和力学性能。结果表明:微波辅助热场下的93W-Ni-Fe合金显示出优异的力学性能和快速的致密化过程,其压缩试样在1500℃下烧结5 min后,拉伸强度、延伸率、相对密度和硬度(HRC)分别是1200 MPa,16.6%,98.6%和42.0;在微波辅助热场下,试样烧结可以减少80%的烧结时间;微波辅助热场下的瞬时液相烧结有利于减少烧结时间,加快致密化过程,并且有利于钨晶粒的细化,获得组织均匀和综合性能高的W-Ni-Fe合金。     

微波烧结超细WC-8Co硬质合金中的脱碳及其改进(英文)

通过微波烧结技术制备超细WC-Co硬质合金。烧结过程中在试样的表层形成脱碳相W3Co3C。在混料过程中添加炭黑,研究碳含量与合金力学性能之间的关系。结果表明:当碳含量为0.45%时,合金的硬度和断裂强度达到最大值,分别为HRA93.2和3396MPa。SEM观察发现在微波烧结超细硬质合金过程中,WC晶粒的长大主要为初期的合并长大。     

The modeling of electric-current-assisted sintering to produce bulk nanocrystalline tungsten

Nanocrystalline tungsten has the potential to have superior strength and hardness properties versus conventional tungsten. While tungsten nanopowders are becoming more commonly available, processing through conventional press and sinter techniques induce unacceptable grain growth. As an alternative, electric-current-assisted sintering (commercially known as SPS or PPC) allows extremely high heating rates (>1,000°C/min.) to be achieved which accelerates the consolidation process and preserves the nanocrystalline structure of the material. The high heating rates can, however, lead to non-uniform density and compromised properties. This work employs numerical simulations of the process as a means to understand and reduce these gradients and optimize the process.     

机械合金化和添加微量Y2O3对制备细晶钨合金

采用机械合金化、添加微量Y2O3和冷等静压、液相烧结工艺制备Ф25mm的晶粒度为3～4μm的细晶93W-4.9Ni-2.1Fe(质量分数%,下同)合金棒材,研究粉末机械合金化、添加微量Y2O3、烧结温度和保温时间对合金棒材烧结致密化和显微组织的影响。结果表明:在1480℃液相烧结时钨晶粒发生明显球化,在此温度下降低保温时间对控制钨晶粒长大有较大影响,保温时间为30min时,钨晶粒尺寸为5～8μm;保温时间为60min时,钨晶粒为8～10μm。添加微量稀土氧化物Y2O3可以进一步有效地抑制晶粒的长大,降低合金的钨晶粒尺寸和提高组织均匀性,在1480℃烧结60min时,钨晶粒为3～4μm,而且晶粒尺寸分布更均匀。     

Microwave sintering of copper–graphite composites

Microwave processing is a distinctive and alternative technique when compared with the available processes such as material synthesis, sintering, and processing utilizing the conventional heating sources. Owing to microwave–material molecular interaction, microwave heating is of internal and faster. This results in improved quality of the product with time and energy savings. Metal at its bulk form reflects microwaves; however in its powder form, it couples with microwaves. This work emphasizes on the development of copper–graphite metal matrix composite for electrical sliding contact applications through microwave hybrid heating (2.45 GHz, 3.2 kW). The fabricated composites were tested for their mechanical properties such as porosity, relative density and hardness. Microstructural aspects were studied through SEM.     

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.     

An investigation of the fabrication of tungsten carbide–alumina composite powder from WO3, Al and C reactants through microwave-assisted SHS process

Possibility of synthesis of tungsten carbide–alumina composite powder from WO₃–Al–C mixture via microwave-assisted SHS process in a domestic microwave oven has been investigated. By comparison of the results of thermodynamic calculations with experimental findings, it was found that during microwave heating of WO₃:2Al:C mixture, synthesis process initiates by vigorous exothermic reaction of WO₃ with Al which results in a great deal of heat. Major portion of tungsten carbide phase in the product is W₂C, whose formation is supposed to be related to the high thermodynamic stability of this compound at high temperatures. W₂C formation could also be related to carbon loss phenomenon in the mixture, as a consequence of some carbon burn. It has been concluded that addition of excess carbon to the initial mixture together with extension of the microwave processing time, increase the amount of WC phase in the product in expense of W₂C. Experimental results showed that only small amounts of W₂C remain in the product with around 80 mol% excess initial carbon and about 10 min of microwave heating time.     

高密度活化钨粉低温烧结近全致密化行为

采用物理化学方法制备超细高密度活化钨粉(W-0.1%Ni复合粉末,质量分数),研究球磨时间对活化钨粉形貌及其物理性能的影响,探讨球磨处理对该高密度活化钨粉烧结致密化行为的影响,并与超细纯钨粉末的烧结致密化行为进行对比。结果表明:微量活化元素镍的添加及球磨处理能明显加速钨粉的低温烧结收缩速率,显著促进钨粉的烧结致密化程度;球磨5 h后,活化钨粉在1 600℃下烧结即可达到近全致密化(致密度为99.4%),此外,镍元素的添加和球磨处理也能显著促进钨晶粒的长大。     

堇青石表面13X-SiC吸附涂层微波法快速制备及评价

目的 利用微波烧结技术实现在堇青石表面快速制备整体分子吸附层.方法 在13X分子筛粉体中添加具有较强微波吸收能力的碳化硅(SiC)粉末介质,通过浸渍粘结,在堇青石蜂窝陶瓷表面形成13X-SiC复合涂层,利用微波烧结快速制备整体式分子吸附层.根据微波加热机理设计微波烧结时间,通过宏观观察、称量法和热循环试验判定堵孔率、负载率、涂层与基底的结合能力.采用SEM和XRD分析复合涂层的微观形貌与组织结构,采用氮吸附法表征复合涂层的气体吸附能力.结果 微波烧结2.1 min可在堇青石蜂窝陶瓷表面获得灰黑色吸附涂层,蜂窝堵孔率低于2.14％,复合涂层负载率约为18.69％,热循环脱落率约为0.47％.微波烧结未影响13X分子筛分子结构,仍存在5 nm左右的介孔.烧结后,SiC与13X分子筛堆积粘结在一起,形成了具有堆积孔隙的复合涂层,大孔和中孔占比增加.复合涂层中SiC的引入减少了13X分子筛占比,整体比表面积由517 m2/g降低至131 m2/g,仍具有一定吸附能力.可通过合理设计13X分子筛与SiC的比例,调节复合涂层的吸附能力.结论 利用微波法烧结制备出与堇青石基底结合力强、负载率高的13X分子筛与SiC灰黑色复合涂层,缩短了堇青石整体吸附器的生产时间,降低了物理吸附载体制造成本,将有助于密闭环境中气体污染问题解决,并为航天器在轨分子污染物控制提供了新途径.