Sintering enhancement in dynamically compacted commercial iron powders

Powder metallurgy compacts of near theoretical density have been made from commercial sponge iron and atomized iron powders, the latter with and without admixed lubricant. Equivalent compacts were made by conventional quasi-static die pressing and by the dynamic powder compaction method to allow for comparative testing of mechanical properties. The compacts were sintered over a range of temperatures from 700 to 1120°C. Test specimens were cut from the compacts and tested to produce data on tensile strength, ductility (area reduction) and cantilever beam (Izod) impact strength.

Compacts made dynamically from both the sponge iron and atomized iron powders exhibited higher tensile strengths and ductilities than those made quasi-statically and sintered to the same temperature. However, there were marked differences in the impact strength. With the sponge iron powder, dynamic compacts had lower impact strength than equivalent quasi-static compacts, but the reverse result was obtained with the atomized powder. The atomized powder was of much higher purity than the sponge iron and the microstructural evidence indicated that the inferior impact strength of the dynamically compacted sponge iron was due to interaction between the shock waves used for compaction and the numerous brittle inclusions present in this material.

The results with the lubricated powder showed no sintering enhancement attributable to dynamic powder compaction. This suggests that the mechanism for the sintering enhancement that can be achieved with this consolidation technique is related to the friction processes at the particle boundaries, possibly coupled with the elevated temperatures present at these boundaries during dynamic compaction.     

Consolidation of nanoscale iron powders

The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.     

Dense pure binderless WC bulk material prepared by spark plasma sintering

The phases, microstructures and mechanical properties of binderless WC bulk materials prepared by the spark plasma sintering technique were investigated systematically. The addition of carbon was added to eliminate the impurity phase W₂C. The relative density, Vickers hardness and grain size increase obviously with increasing sintering temperature, but increase weakly with increasing pressure or sintering time. The high relative density of 99·1%, HV30 of 27·5 GPa and fracture toughness K_IC of 4·5 MPa m^1/2 of pure binderless WC bulk with a grain size of 400 nm was obtained by sintering the WC powders with a particle size of 200 nm and the addition of 0·63 wt-%C at 1800°C for 6 min under 70 MPa.     

Study of the sintering properties of plasma synthesized ultrafine SiC powders

A study on the sintering of ultrafine SiC powders synthesized from elemental Si and CH₄ using radio frequency (r.f.) induction plasma technology is reported. The powder had a particle size in the range of 40 to 80 nm and was composed of a mixture of α and β-SiC. It was subjected to pressureless sintering in an induction furnace in the presence of different sintering aids. With the addition of B₄C (2.0 wt% B) by mechanical mixing, the powders could only be partially densified, with the highest value of 84.5% of theoretical density being achieved at 2170 °C for 30 min. Through the use of “in-flight” boron doping of the powder during the plasma synthesis step (1.65 wt % B), the ultrafine powder obtained could be densified to above 90% of its theoretical density at 2050 °C for 30 min. The addition of oxide sintering aids (7.0 wt % Al₂O₃ + 3.0 wt % Y₂O₃) by mehanical mixing produced sintered pellets of 95% of theoretical density at 2000 °C for 75 min. The Vicker’s microhardness of the sintered pellets in this case was as high as 31.2 GPa. In order to improve our understanding of the basic phenomena involved, extensive microstructural (scanning electron energy microscopy: SEM), physical (shrinkage, weight loss, porosity, hardness) as well as chemical analysis (prompt gamma neutron activation analysis (PGNAA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA)) was carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and powder purity on the densification behaviour of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powder. This revised version was published online in November 2006 with corrections to the Cover Date.     

复合级配17-4PH不锈钢粉末电磁先导阀阀芯顶杆组织及其性能

本研究分别以单一气雾化、气雾化∶水气联合雾化=3∶1复合级配17-4PH不锈钢粉末为原材料,采用粉末微注射成形方法制备电磁先导阀阀芯顶杆。对制备得到的电磁先导阀阀芯顶杆的表面质量、微观结构、收缩率及致密度进行测试,并对复合级配粉末阀芯顶杆烧结件进行热处理,分析热处理后样品的硬度以及磨损性能等指标。研究结果表明:复合级配粉末对具有微结构的阀芯顶杆的表面质量、微观结构、收缩率及致密度有着重要的影响。气雾化∶水气联合雾化=3∶1的复合级配17-4PH不锈钢粉末阀芯顶杆注射坯件表面质量相对较好,提高了粉末的烧结活性,烧结样品致密度达到98.86%,收缩率更为均匀,主要组织为马氏体,优于单一气雾化粉末的烧结性能。热处理后阀芯顶杆的组织为马氏体和少量的奥氏体,硬度显著提高,耐磨损性能得到了明显的改善。     

Investigations on Metal Injection Molding of 316L Stainless Steel

Metal Injection Molding (MIM) was performed with water atomized and gas atomized 316L stainless steel powders and powder blends thereof. Feedstocks were prepared using a thermoplastic binder system and subsequently molded into tensile test specimens. Different debinding procedures and sintering treatments were applied and their influence on carbon content in the product was compared. Chemical decomposition processes of the binder and the influence of powder morphology on debinding and sintering behaviour are discussed. Shrinkage of the MIM-fabricated parts was examined and correlated to the powder characteristics. As a result a procedure is suggested to achieve mechanical properties expected for 316L stainless steel.     

Densification behaviour of oxide-coated silicon nitride powders

Powder coating has been explored as a method of incorporating sintering additives into a ceramic powder. This procedure has been explored in the case of Si₃N₄ powders coated with thin layers of MgO.The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si₃N₄ powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.     

High voltage SnO<Subscript>2</Subscript> varistors prepared from nanocrystalline powders

In this study, SnO₂-based varistors were prepared from mechanically activated nanocrystalline powders. Nanocrystalline powders were derived by subjecting the initial powders to intensive high-energy activation with different times and ball to powder ratio. The effect of activation parameters on the powder properties and sintering temperature, as well as microstructural, micro-electrical and macro-electrical properties of the final specimens was evaluated. Varistors derived from high-energy mechanical activation exhibit a higher density (98.3% relative density) and more refined microstructure upon sintering at 1,300 °C in comparison varistors prepared from conventional powders. Breakdown voltage and nonlinear coefficient were increased up to 24 kV/cm and 45 respectively.     

Effect of Re on microstructural evolution and densification kinetics during spark plasma sintering of nanocrystalline W

In the present investigation, nanocrystalline W and W-xRe (x = 3, 5 wt.%) alloy powders were produced by mechanical milling/alloying using high energy ball milling. The nanocrystalline nature (∼50 nm) of these powders was validated by the Rietveld refinement of their respective X-Ray diffraction patterns. Subsequently, spark plasma sintering of the ball milled powders was carried out. It was observed that pure W was not able to densify completely (relative density of 93%) at a temperature of 1500 °C. However, the addition of 5 wt.% Re resulted in near complete densification (relative density of 97%) at the same sintering temperature. The enhanced densification of W-Re powders is mainly attributed to the ductilising effect of Re assisted by the nanocrystallinity of powders, and the application of pressure during sintering.     

Microstructural characterization of field assisted sintered bulk nanostructured V-4Cr-4Ti alloys

This article deals with synthesis of single-phase bulk nanostructured V-4Cr-4Ti alloy by mechanical alloying and field assisted sintering. Mechanically alloyed powders were sintered at 1050 and 1100°C with different sintering time. Both powder and sintered samples were subjected to structural and morphological characterization using x-ray diffraction and electron microscopy studies. x-ray diffraction and transmission electron microscopy studies of sintered samples confirm that field assisted sintering technique aids nanostructuring in V-4Cr-4Ti alloy. Microstructural parameters of sintered samples were calculated from x-ray diffraction patterns. Role of sintering variables on the microstrain evolution and subsequent domain size reduction in bulk V-4Cr-4Ti alloy are briefly discussed. Optimal sintering parameters to achieve nanostructured V-4Cr-4Ti alloy by field assisted sintering were deduced. Micro-hardness and relative density of sintered V-4Cr-4Ti alloy are found to increase with sintering temperature and sintering time.     

Thermal Processing Optimization of Injection Molded Stainless Steel Powders

Stainless steels constitute the largest materials application group for powder injection moulding. Because of the importance of these alloys, much attention has been directed to the optimization of the thermal processing, including carbon contamination control during binder burnout and sintering. Densification in sintering has been mastered such that high final densities and competitive mechanical properties are available. Property optimization and attainment of precise final shapes depend on close control of the sintering temperature, since supersolidus liquid phase sintering is often employed to attain densification. Powder characteristics and processing details are given for obtaining optimal structures in 17-4 PH stainless steel by powder injection moulding. Much progress has occurred in working with lower cost water atomized powders that can be easily densified in spite of oxide surface layers.     

Solid-State Sintering of Tungsten Heavy Alloys

Solid-state sintering is a technologically important step in the fabrication of tungsten heavy alloys. This work addresses practical variables affecting the sinterability: powder particle size, powder mixing, and sintering temperature and time. Compositions containing 1 to 10 micrometer (μim) tungsten (W) powders can be fully densified at temperatures near the matrix solidus. Blending with an intensifier bar provided good dispersion of elemental powders and good as-sintered mechanical properties under adequate sintering conditions. Additional ball milling increases powder bulk density which primarily benefits mold and die filling. Although fine, 1 urn W powder blends have high sinterability, higher as-sintered ductilities are reached in shorter sintering times with coarser, 5 urn W powder blends; 10 urn W powder blends promise the highest as-sintered ductilities due to their coarse microstructural W.     

烧结温度对20%ZrO_2(3Y)/Al_2O_3复相陶瓷力学性能和微观结构的影响

20%纳米ZrO2(3Y)粉末加入到高纯亚微米Al2O3粉中,采用高压干压成型方法和恒速升温多阶段短保温烧结方法制备出不同烧结温度下的复相陶瓷。研究烧结温度对复相陶瓷力学性能的影响,通过XRD,EDS和SEM对复相陶瓷进行元素组成和微观结构分析。结果表明:烧结温度在很大程度上影响着复相陶瓷的力学性能和微观结构,常压烧结1600℃保温8h时,相对密度、维氏硬度和断裂韧性达到最大,分别为98.6%,18.54GPa和9.3MPa·m1/2,而基体晶粒尺寸为1.4~8.1μm,ZrO2相变量为34.6%。1600℃下复相陶瓷具有优质的微观结构,断裂方式为沿晶-穿晶混合断裂模式。ZrO2(3Y)粉体的加入,从相变增韧、内晶型颗粒增韧和裂纹偏转等多个方面提高了复相陶瓷的断裂韧性。     

The sintering process of a high permeability Ni-Fe-Cu-Mo alloy made by powder metallurgy

The magnetic permeability of a 77 Ni-14 Fe-5 Cu-4 Mo wt % alloy made by powder metallurgy is known to be improved by extending the sintering time considerably beyond that normally used. The room temperature measurement of resistivity during the sintering cycle of such an alloy clearly shows the overlapping stages of change that occur during the sintering process. The variation in resistivity and its relationship with the changes in density, in weight and in bend strength of compacts shows that de-oxidation of the constituent element powders occurs initially. De-oxidation is followed by sintering and alloying of the nickel and iron which is followed in turn by alloying of the molybdenum. The final stage involves the alloying of the copper and the elimination of pores.Electron microprobe analysis has shown that the copper does not alloy substantially until the copper particles melt, and that alloying is hindered if copper powder of large particle size is used. Sintering occurs more rapidly than alloying, but the rate of alloying is the most important factor in determining the electrical and magnetic properties of the alloy.     

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.     

Microstructure and mechanical properties of biocompatible high density Ti–6Al–4V/W produced by high frequency induction heating sintering

High frequency induction heating sintering method is used for sintering of the metal and ceramics powder. This technique has been used to produce high density compacts, containing as small grains as possible of powders. The alloy of Ti–6Al–4V was modified by addition of 2.5, 5, and 10 wt.% tungsten through powder metallurgy. Ti–6Al–4V/W was prepared by high-energy mechanical milling. The use of the high frequency induction heating sintering technique allows sintering to nearly full density at comparatively low temperatures and short holding times, and therefore suppressing grain growth. Different process parameters such as sintering temperature, and applied pressure have been investigated. The obtained compacts are characterized with respect to their densities, grain morphologies and pore distributions as well as hardness. Ti–6Al–4V/W powder precursors have been successfully compacted and consolidated to densities exceeding 98.8%. The maximum compressive strengths were obtained at sintering temperature 1000 °C for the samples containing 5% W, and at 1100 °C for the samples with 10% W. Maximum hardness was obtained 45 HRC at 1100 °C for 10% W.     

烧结温度对TiAl合金块体材料组织和性能的影响

采用机械合金化和等离子烧结工艺成功制备细晶TiAl合金,研究不同烧结温度(750~1050℃)对所得块体材料的显微组织和力学性能的影响。XRD研究发现:完全固结的块体材料主要由γ-TiAl和α2-Ti3Al相组成。采用SEM和TEM观察块体材料的显微组织。当烧结温度为950℃时,所得的块体材料致密度高,接近完全致密化,其压缩屈服强度为2106 MPa。当烧结温度大于950℃时,致密度未发生明显变化,块体材料的屈服强度有所下降。     

The effect of yttrium oxide in hydroxyapatite/aluminum oxide hybrid biocomposite materials: Phase,mechanical and morphological evaluation

This study presents the fabrication and characterization of composite materials of hydroxyapatite and aluminum oxide. Hydroxyapatite powder was obtained from bovine bones via conventional calcination routine. Although hydroxyapatite shows great biocompatibility with the human body, its applications are limited to non‐load bearing areas. For this purpose, fine powders of hydroxyapatite/alumina were admixed with 1 and 5 wt.% yittria. Powder‐compacts were sintered by two‐step sintering route by increasing temperature to 1550 °C for 2 h and then sintering at 1450 °C for 4 h. The effect of increasing yittria content on sintering behavior and mechanical properties was investigated in biocomposite hybrid materials.     

Ti50Cu25Ni20Sn5 bulk metallic glass fabricated by powder consolidation

A bulk metallic glass of Ti₅₀Cu₂₅Ni₂₀Sn₅ nominal composition was produced via a powder metallurgical route, namely the preparation of glassy powders by mechanical alloying followed by their consolidation by spark-plasma sintering. Samples were characterized with respect to their structure and thermal properties before and after sintering. A bulk glassy sample, having nearly full density and containing only a small fraction of the intermetallic NiTi₂ phase, could be obtained after careful selection of sintering parameters.     

基于制备钨钼复合靶材的SPS烧结连接EI北大核心CSCD

采用放电等离子烧结(SPS)制备钨(W)和钛锆钼(TZM)连接件。通过高能球磨和调节温度烧结出高致密度纯W块体,相对密度可达97.0%以上。在制备的纯W块体表面铺置TZM合金粉末,烧结TZM的同时对W和TZM进行连接,实现了异种金属块体与粉末的一步烧结连接。研究烧结温度和降温速率对W/TZM合金接头的微观组织和力学性能的影响。结果表明:W与TZM结合良好,烧结温度在1400~1600℃范围内时,W/TZM接头的剪切强度随烧结温度的升高而增大;在相同烧结温度下,采用快速冷却方式获得的接头剪切强度高于缓慢冷却接头的;当烧结温度为1600℃并采取快速冷却降温时,W/TZM接头的剪切强度达到最大,为159.7 MPa。