粉末烧结体摆辗致密的规律以及压实体的性能分析

本文利用正交实验等方法在自行设计的实验装置上对粉末烧结体的摆辗致密规律及压实体的性能进行了研究，实验结果表明，摆辗力与高径比是影响致密程度的主要因素。通过数据的回归处理，得到了摆辗力与高径比对致密程度的影响规律，通过测定摆辗后压实体的性能，发现其密度分布与硬度分布具有一致性，都表现出沿轴向梯度较大而沿径向梯度较大而沿径向梯度较小的规律。     

Self-compacting high density tungsten–bronze composites

Green compacts of W–bronze were encapsulated in shells of bronze powder, placed in a ceramic mold and sintered in alumina tube furnace at 1150 °C. Throughout the sintering cooling stage the differential coefficient of thermal expansion ΔCTE of W–bronze was employed to induce an external compressive densification action. The process included simultaneous sintering, hot isostatic pressing (HIP) and infiltration act to enhance densification. By this technique, pilot sintered compacts of different W50–80 wt.%–pre-mix bronze of 97–99% theoretical density were produced. This process resulted in compacts of higher hardness, higher sintered density and better structure homogeneity as opposed to similar compacts densified by the conventional sintering process. The results showed a gain in hardness by 10–20% and in density by 5–15%. The impact of different cooling rates of 3, 4, 8 and 30 °C min^?1 on sintered density, microstructure and densification mechanisms was examined and evaluated. Low cooling rates of 3 and 4 °C min^?1 gave the best results.     

粉末烧结材料闭式模锻充满过程实验研究

通过烧结紫铜圆柱体试样闭式模锻实验,研究了充满模腔过程的变形与致密规律。得出了烧结紫铜闭式模锻充满模膜过程中,轴向应变与相对密度呈线性关系,模锻变形力与相对密度呈幂函数关系,为粉末烧结材料闭式模锻工艺设计提供了实验依据。     

合金元素P,Cu含量对铸铁短纤维结合剂组织和性能的影响

本文研究了合金元素磷和铜对铸铁短纤维烧结体显微组织和性能的影响。结果表明，合金元素磷可显著提高铸铁短纤维烧结体的密度、致密化系数和压强度。磷与铜的共同加入可进一步提高烧结体的压溃强度和硬度，同时，铜的加入可抑制由磷引起的烧结体的急剧。烧结机理随磷、铜含量的增加逐渐由固相烧结＋瞬态液烧结转变为液相烧结。     

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。     

铁基高密度烧结材料的摩擦磨损性能研究

采用烧结热锻工艺获得高密度和高硬度的铁基粉末烧结冶金材料,研究了铁基粉末中的铬含量和施加在烧结材料上的载荷对材料摩擦磨损性能的影响,并借助于扫描电镜观察和分析了粉末冶金烧结材料的磨损形貌、探讨其摩擦磨损机制.结果表明,在铁基粉末中加入0.6%铬时,该粉末材料的硬度、密度和耐磨性最好.主要磨损机制为氧化磨损、粘着磨损和磨粒磨损.     

Effect of Ni addition on the densification of TiC: A comparative study of conventional and microwave sintering

This study deals with the effect of conventional sintering and microwave sintering on the densification kinetics of Titanium Carbide (TiC) in the presence of Ni (1, 1.5, 2 wt%). TiC compacts were obtained after uniaxial pressing of powders synthesised by ball milling of Titanium and Carbon and sintering was done in the presence of Nickel. The samples prepared were subjected to conventional as well as microwave sintering. The XRD and SEM analysis were used for a study of the reaction of Ti and C powders upon addition of Ni, which reduced the sintering temperature to 1200 °C. The densification of TiC powders was due to the Ti-Ni eutectic system, the liquid phase formed at this temperature assisting the sintering process. The SEM images revealed the flake like structure of TiC in which the carbon diffused into Ti upon the addition of Ni, thereby supporting enhanced mass transfer. The XRD pattern showed the presence of Titanium Oxide (TiO₂) along with TiC which resulted in non-uniform distribution of hardness. Maximum hardness was achieved in the conventional sintered compacts which gradually increased with increase in Ni addition. The presence of the oxide phase and the formation of micro cracks resulted in non-uniform hardness for microwave sintered compacts. The maximum hardness of conventional sintered compact (375 HLD) was nearly 1.5 times more than the maximum hardness of the microwave sintered compact (250 HLD). The density of the microwave sintered compact was found to be higher by 8% than with the conventionally sintered compact.     

The effect of sintering temperature on densification of nanoscale dispersed W–20–40%wt Cu composite powders

Nanoscale dispersed particles of W–20–40%wt Cu were synthesized using a chemical procedure including initial precipitating, calcining the precipitates and reducing the calcined powders. The powders were characterized using X-ray diffraction and map analyses. The effect of sintering temperature was investigated on densification and hardness of the powder compacts. Relative densities more than 98% were achieved for the compacts which sintered at 1200 °C. The results showed that in the case of W–20%wt Cu composite powders, the hardness of the sintered compacts increased by elevating the sintering temperature up to 1200 °C while for the compacts with 30 and 40%wt Cu, the sintered specimens at 1150 °C had the maximum hardness value. The microstructural evaluation of the sintered compacts by scanning electron microscopy showed homogenous dispersion of copper and tungsten and a nearly dense structure. A new proposal for the variation of the mean size and morphologies of W-particles with volume percent of copper melt within the composites has been suggested.     

不同粒径Ti粉的高速压制行为和烧结性能

以平均粒径为150,75,48和38μm的4种Ti粉为原料(依次定义为A,B,C和D粉末),采用高速压制技术进行成形,考察粉末粒径对压坯密度、最大压制力和脱模力的影响,进一步研究粉末的高速压制特性和压坯的烧结性能.结果表明,高速压制的压坯密度与粉末粒径和松装密度有关.冲击能量较小时,压坯密度主要取决于松装密度,而冲击能量较高时,则主要取决于粉末粒径.在冲击能量≤761 J下成形时,具有最大松装密度的B粉末所获得的压坯密度最高;进一步增大冲击能量,平均粒径最大的A粉末所获得的压坯密度最高.粉末粒径对压坯密度和最大压制力具有相似的影响,并且4种粉末的最大压制力和压坯密度之间的关系均符合黄培云压制方程;但粉末粒径对脱模力无明显影响.试样的烧结密度随粒径的细化而增加,同时伴随着不同程度的晶粒长大.4种压坯经1250℃真空烧结后,最终均获得了近全致密的试样.     

Development of a Ti-based alloy: Design and experiment

We proposed the design methodology for titanium-based alloys based on a combination of literature survey, simulation, and experiment. We have selected and investigated the properties of novel Ti-Fe-Zr alloys specifically designed for densification via powder metallurgy techniques. Samples were produced by die compaction of mixed elemental powders with subsequent densification by sintering at 1,275°C in vacuum. Scanning electron microscopy and optical microscopy were used to examine the sintered microstructures to compliment hardness and tensile testing. The results show that density and mechanical properties increase with the iron and zirconium content. The best property combination was obtained with the addition of 5 wt.% iron and 5 wt.% zirconium when vacuum sintered at 1,275°C for 60 min.     

Mechanical properties of β-SiC fabricated by spark plasma sintering

The consolidation of SiC nanopowder synthesized by the mechanical alloying method was subsequently accomplished by spark plasma sintering of 1700 °C for 10 min under an applied pressure of 40 MPa. The SiC sintered compact with relative density of 98% consisted of nano-sized particles smaller than 100 nm. This phenomenon resulted in the ordering process of stacking disordered structure formed by mechanical alloying. In this work, the effect of grain size and relative density on the mechanical properties were studied. The mechanical properties of sintered compacts were evaluated and compared with the reference samples fabricated from the commercial SiC powder (β-SiC, 0.3 μm, IBIDEN Co., Gifu, Japan) with sintering additive (B–C mixture). The Vickers hardness and bending strength of those sintered compacts increased with the increment of the density. However, the mechanical properties were lower than those of reference samples in case of lower density, even though the mechanical property was close to that of reference sample in case of higher density. This phenomenon was considered for the difference of bond strength between grains because those sintered compacts were fabricated without any sintering additives, while those reference samples were fabricated by accelerating the grain bonding with a sintering additive of B–C mixture. In other words, those results indicated that the effect of sintering additive affected on mechanical properties directly. This paper was presented at the International Symposium on Manufacturing, Properties, and Applications of Nanocrystalline Materials sponsored by the ASM International Nanotechnology Task Force and TMS Powder Materials Committee on October 18–20, 2004 in Columbus, OH.     

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.     

镁对氢化钛烧结致密化的影响

以氢化钛粉末和镁粉为原料,利用模压工艺制备纯钛,主要研究了镁对氢化钛粉末烧结致密化的影响。结果表明,少量镁的添加能够促进氢化钛粉末的烧结致密化,而过量镁则降低烧结密度,镁添加量为0.5%(质量分数,下同)时效果最优,将烧结体相对密度由96.5%提高到98.7%。通过对烧结过程动力学、镁还原氧化钛反应的热力学和动力学,以及烧结体成分进行分析,发现少量镁促进氢化钛烧结致密化的机理为:真空烧结时镁从坯体中挥发,形成的镁蒸气对粉末颗粒表面的氧起到净化作用,提高了烧结活性,从而提高烧结密度。     

Synthesis and consolidation of γ-Ni-Fe nanoalloy powder

The present work studies the synthesis and consolidation of γ-Ni-Fe nanoalloy powder by the mechano-chemical process comprising high-energy ball-milling of NiO-Fe₂O₃ powder and a subsequent hydrogen reduction process. To examine the formation mechanism of the nanoalloy powder, the effect of the oxide powder char-acteristics on the reduction process and alloying was investigated by varying the ball-milling time. The reduction process and the alloying of the γ-Ni-Fe phase proved to accelerate as the ball-milling time increased. However, prolonged milling (for 30 hours) retarded the reduction of Fe₂O₃ as well as the alloying process. The densification process of the Ni-Fe nanoalloy powder strongly depended on the degree of agglomeration which results in enhancing homogeneous sintering. The limited densification of the nanoalloy powder originates from the high degree of particle agglomeration. While intra-agglomerate porosity is easily eliminated in the course of sintering, it is found to resist densification. The limitation of the sintered density could be overcome by increasing the green density of the powder compacts. Full density was achieved by starting with a green density of 72% theoretical density.     

Rheological, mechanical and corrosive properties of injection molded 17-4PH stainless steel

The feedstock based on the binder 65％PW-30％ EVA-5％SA has the best general rheological properties for the 17-4PH stainless steel powder. The 17-4PH stainless steel compacts sintered at 1380℃ for 90 min have the best mechanical properties and the good microstructure with homogeneously distributed pore structure and the moderate-sized grains. Whereas the compacts sintered for 60 min and 120 min show an inadequate and an over-sintered microstructure respectively. The compacts sintered at 1380℃ for 90 min have the density of 7.70 g/cm^3 , the strength of 1 275 MPa, the elongation of 5％, and hardness of HRC36. With the increase of sintering temperature, the density, strength and hardness increase, while the elongation decreases. The 17-4PH stainless steel has good corrosion resistance, showing an activation-passivation polarization curve. But the passivation potential range is narrow and the spot corrosion potential is low, indicating a low anti-spot corrosive properties.     

An analysis of the powdered metal compaction process in solid cylind cal parts

In this study, the friction effect on the powdered metal compaction process has been analyzed by applying finite element methods. A plasticity theory applicable to powdered metal compaction is summarized and a variational form for finite element analysis is described. The compaction processes of the axisymmetric solid cylinder are simulated for different compact geometries in single-action pressing. Efforts are focused on the pressure transmitted between the upper and lower punches through the compact and density distributions within the compacts. The numerical results show that: (1) the friction condition between the metal powder and the dies can be determined simply from the force data transmitted by a single such action compaction test and the simulation results; (2) the density variations within the compacts rely on such compact geometry as the height to diameter ratio and the frictional conditions between powder and dies; the (3) additional useful information obtained includes the Min/Max density ratio within the compacts.     

Effect of MnS powder addition and sintering temperature on the corrosion resistance of sintered 303LSC stainless steels

As the sintered parts are to be machined after sintering the MnS powder is usually added to improve the machinability. Effect of the MnS powder content on the corrosion resistance of the sintered 303LSC alloys was investigated. Experimental results show that the addition of MnS powder decreases the green density of the 303LSC compacts. The sintered density of the admixed powder compacts slightly increase with increasing MnS content. The weight loss rate of the sintered specimens being immersed in the 10% FeCl₃ corrosion test solution increases with increasing MnS content. The corrosion resistance is improved with increasing sintering temperature. The metallographic evolution of the corroded surface was studied.     

Microstructural evolution during sintering of near-monosized agglomerate-free submicron alumina powder compacts

Colloidally processed near-monosized, agglomerate-free submicron alumina powder compacts were sintered under different conditions to study the evolution of pore structures during sintering. The results showed that even though the compacts were agglomerate-free to start with, agglomeration took place during sintering due to local densification of the particles with different co-ordination numbers. Inter-agglomerate channel-like pores were formed as a result, which eventually evolved into an isolated pore on further sintering. The densification rate was controlled by mass transport via grain boundary diffusion before the formation of isolated inter-agglomerate pores, after which it was controlled by the sinterability of the pores. From this stage on, grain growth was required to bring about further sintering. At low sintering temperatures, grain growth was sluggish, probably a result of impurity controlled grain growth. This resulted in an abrupt drop in the densification rate and the phenomenon of end density at low sintering temperatures. The present work shows that an initial agglomerate-free green structure of fine, monosized particles is essential to resist particle agglomeration and grain growth during sintering, so as to achieve a low sintering temperature and a fine grain size sintered product.     

3D FE modeling of cold rotary forging of a ring workpiece

Cold rotary forging is an advanced but much complex incremental metal forming process with multi-factors coupling interactive effects. Previous researches concentrated mainly on studying the cold rotary forging process of the cylindrical workpiece based on the analytical and experimental methods. In the current work, in order to better investigate and understand the cold rotary forging process of the ring workpiece, a 3D elastic–plastic dynamic explicit FE model of the process is developed under the ABAQUS software environment. Some key technologies of modeling methods are dealt with reasonably and some key forming conditions are also determined properly. The reliability of the proposed 3D FE model is verified experimentally. Through simulation, the distributions and histories of different field-variables such as stress, strain and force and power parameters are investigated in detail. The research results provide valuable guidelines for better understanding the deformation characteristics of cold rotary forging of the ring workpiece. Furthermore, the modeling methods presented in this paper have the general significance to study other rotary forging processes, such as the hot rotary forging process, the rotary forging process of workpiece with complex profile and so on.     

溶胶-喷雾干燥超细/纳米晶W-20Cu复合粉末的烧结行为

采用喷雾干燥-氢气还原法制备超细/纳米晶W-20Cu(质量分数,%)复合粉末,粉末压坯直接从室温推入高温区烧结不同时间后直接取出水淬,研究其烧结致密化和显微组织的变化。结果表明,超细/纳米晶W-20Cu粉末在1000～1200℃烧结时发生迅速致密化。粉末压坯在1200℃烧结60min,其材料致密度已达到96.4%。1420℃烧结90min时致密度达到99%以上。1100～1420℃烧结时其烧结致密化活化能不断减小,从1100℃时的276.3kJ/mol减小到1420℃时的29.1kJ/mol。当温度低于1200℃时,W晶粒长大不明显,当温度超过1300℃时,W晶粒开始有明显长大。随温度的升高W晶粒发生显著球形化,1420℃烧结时发现其晶粒长大符合G3=kt的Ostwald机制,此时晶粒长大动力学系数K仅为0.024μm3/min。