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1.
《粉末冶金学》2013,56(3):281-287
AbstractAn instrumented die was used to investigate the behaviour of metal powders during cold (ambienttemperature) and warm (up to 140°C) compaction. This instrument enables simultaneousmeasurement of density, die wall friction coefficient, the triaxial stresses acting on the powderduring the course of compaction and ejection pressure. Commercial iron, titanium, aluminium,316L stainless steel (SS) and aluminium–silicon powders were employed for investigation. Theresults demonstrated the advantages of powder preheating on the compaction behaviour of metalpowders concerning green density, dimensional changes, frictional behaviour, ejectioncharacteristics and compactibility. However, the outlines also determined that the response ofthe non-ferrous powders to powder preheating is somehow different from those of the ferrouspowders. In this context, the behaviour of prealloy aluminium–silicon powders during compactionwas found of particular interest, as their compactibility is strongly affected by powder preheating,whereas the dimensional changes after ejection decrease considerably. This article presents theeffect of cold and warm compaction on the consolidation and ejection characteristics of ferrousand non-ferrous metal powders. The influence of compaction condition (pressure andtemperature) with considering of the powder characteristics and densification mechanisms areunderlined. 相似文献
2.
《粉末冶金学》2013,56(1):12-19
AbstractThe cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ~1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol?. The activation energy for Ti hydride also appeared to be about 160 kJ mol?, but the model did not fit the experimental data well. 相似文献
3.
《粉末冶金学》2013,56(4):543-550
AbstractThe use and development of titanium and titanium alloys have been strongly correlated to high technology industries where costs are not the most important aspect. Titanium could see its market grow by the application of lower cost and more efficient processing methods such as powder metallurgy. This work deals with the characterisation of two types of powders: commercial prealloyed powder and powder produced from master alloy combining mechanical milling and conventional blending to adjust the particle size. The characteristics of the powders, sintering behaviour and final properties of the parts indicate that the master alloy approach leads to better compressibility than the prealloyed powders and, therefore, to lower dimensional change during sintering. The most important result is that it is possible to obtain Ti alloys with properties similar to or better than alloys from prealloyed powders and to obtain homogeneous microstructures, which allows the composition to be adjusted to requirements. 相似文献
4.
《粉末冶金学》2013,56(3):278-284
AbstractA new technological approach to the fabrication of high density powder metallurgy (PM) parts via single pressing sintering, allowing cold compaction to be performed without admixed lubricants, has been studied. The influence of in pore gas on the compacts' green density and their sintered properties were evaluated. A mathematical expression relating in pore gas pressure in the compacts to the green density was developed. The expression showed that in order to reduce the negative influence of gases trapped in the pores it is necessary to ensure effective air drainage from the compaction zone. In order to ensure sufficient air evacuation during cold compaction, a new design of porous die was developed. The behaviour of powder mixes with different lubricants during cold compaction in porous die was investigated. All the test conditions were evaluated in terms of green and sintered properties, including the ejection force, green and sintered densities, tensile strength and surface hardness. In the context of the experimental work, compaction in porous die promoted the improved combination of green and sintered properties compared with compaction in conventional dies. 相似文献
5.
An approximate master curve for the densification of cold-pressed titanium powder during vacuum sintering was published previously.
It was based on the combined results for three different titanium powders. The master densification curve model incorporates
the effects of particle size, compaction pressure, sintering time, and sintering temperature on densification. The collection
of a large amount of additional data now allows refinement of the model. Distinct curves are presented for three different
titanium powders, prealloyed Ti6Al4V, and Ti-Ni binary alloys. The master densification curve is sigmoidal, but deviates from
the ideal form at high sintered density; the relative sintered density saturates at 90 to 100 pct, depending on the particle
size of the titanium powder, and to a lesser extent the compaction pressure. The master densification curve below the saturation
level is slightly dependent on the compaction pressure. 相似文献
6.
AbstractIn order to investigate the friction behaviour of powder during compaction, a new method has been developed. Compaction is a complicated process and direct and continuous measurement of the coefficient of friction is not easy, because the coefficient of friction varies due to changes in such process parameters as pressure distributions, powder surface deformation etc. In this paper, a new device for measuring the coefficient of friction between metal powder particles in contact with the die wall during compaction is presented. Using the conventional methods for direct measurement of the radial pressure during compaction is very difficult. The new device offers the possibility of investigating the normal pressure on the powder particles directly and continuously by keeping the green density constant. The measurements are performed using strain gauges mounted on the upper punch. The upper punch surface in the new device corresponds to the die wall in a conventional press. The sliding velocity, compaction velocity, normal load and temperature can be monitored and controlled. Measurement of the coefficient of friction at low densities is one of the advantages and possible applications of this apparatus. The investigation shows that the powder compaction is controlled by a combination of powder rearrangement and elastic and plastic deformation of particles. At densities below 4g cm-3 the dominant process is particle rearrangement. No plastic deformation occurs at such low values of density. At densities above 4·5g cm-3 the plastic deformation of the powder surface in contact with the die wall seems to be completed and the coefficient of friction is more or less constant. 相似文献
7.
《粉末冶金学》2013,56(3):240-252
AbstractA comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640°C for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ≥340 MPa. Near full sintered densities of , ~98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys. 相似文献
8.
《粉末冶金学》2013,56(4):323-327
AbstractAlthough powder metallurgy (PM) material is dominated by ferrous alloys, there is a growing interest in Al PM. The usage of Al PM in automotive applications depends on the development of higher density and improved dynamic properties. Several approaches have been proposed to increase density of sintered parts. Warm compaction process of Al powder was used to achieve high density. In this study the authors focused on the effect of warm compaction on Alumix 123 L (ECKA Granules) powder blend. It has been found that warm compaction at 110°C led to a reduction in the ejection force by 27·9%, increased green density to 94% of theoretical density and increased sintered strength to 315 MPa as compared to those pressed at room temperature. 相似文献
9.
10.
AbstractStainless steel 316L MIM components can be made from either prealloyed powders or from master alloys blended with carbonyl iron powder. In this study these two techniques were compared using prealloyed and master alloyed gas atomised powders of ? 16 μm and ? 22 μm sizes. Four different compounds were prepared, characterised and injection moulded into tensile bars. The bars were compared for green strength, green defects, sintered strength and microstructure. The green components are stronger when carbonyl iron powder is used with the gas atomised master alloy. This material also seems to be less susceptible to moulding defects. The sintering strength of the material produced using the pre-alloyed powder was higher than the master alloyed prepared material. Little difference in mechanical properties existed between the materials fabricated from gas atomised prealloyed ? 16 μm and the ? 22 μm powders. Also, the viscosity of the mixtures was higher for the ? 16 μm material and the master alloy mixtures than for the –22 μm gas atomised prealloyed powders. 相似文献
11.
《粉末冶金学》2013,56(2):163-167
AbstractThe 7000 series aluminium alloys processed using elemental powder mixtures are prone to distortion, which is manifest as hourglassing or waisting in cylindrical specimens. By characterising the density distribution using hardness measurements, it is shown that the green density is not evenly distributed through a part, even though aluminium is relatively soft and readily compacted. Because the density equilibrates during sintering, the non-uniform green density leads to distortion. The cause of this distortion is a result of differential shrinkage, which occurs during sintering as well as on solidification during cooling from sintering. Distortion can be controlled by increasing the compaction pressure, which homogenises the green density and does not affect the tensile properties. 相似文献
12.
13.
《粉末冶金学》2013,56(1):68-72
AbstractHigh density Fe3Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe2Al5/FeAl2 (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe3Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min- 1 heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.SEM microscopy suggests that agglomerated Fe2Al5/ FeAl2 particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe2Al5/FeAl2 components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity. 相似文献
14.
粉末冶金温压的致密化机理 总被引:4,自引:2,他引:4
通过对铁粉的动态压制曲线、脱模力曲线、 X射线衍射、摩擦和润滑的研究, 揭示了温压的致密化机理: 温压成形过程可分为3个阶段, 在初期阶段, 粉末的颗粒重排过程占主导地位, 颗粒重排份额与冷压相比提高15%~31%; 而在后期, 温压致密化以塑性变形为主, 铁粉塑性变形程度的改善又为粉末颗粒的二次重排起了协调作用, 使铁粉获得最大程度的颗粒填充密度; 其间, 温压润滑剂对致密化起了重要作用, 它降低了摩擦因数, 改善了粉末和模壁、粉末和粉末之间的润滑条件, 有效地降低了粉末成形的摩擦阻力, 有利于粉末致密化的顺利进行. 相似文献
15.
温压技术是由在加热的阴模中压制预热的粉末组成[1],已知温压有助于零件密实,从而改进烧结件的性能[2,3]。温压需要在适合温压的温度范围内进行。特别是,粉末混合粉应具有好的流动性,同时对阴模模壁有良好润滑性,以减小脱模力。在试验室和工业生产中都研究了用粘结剂处理的和未经粘剂处理的用温压技术制造的材料的性状与性能。为了确定和定量各种关键生产参数,诸如压制压力,粉末温度与阴模温度,生产速率及零件大小对生坯和烧结件特性和零件脱模力的影响,进行了专门的试验研究。依照粉末流动性与松装密度的稳定性,压制压力与温度以及压制零件的重量与密度讨论了温压的工艺性。 相似文献
16.
温压压制压力强化因子及压制曲线的唯象分析 总被引:7,自引:1,他引:6
介绍了一种使用钢铁粉末室温压制回归方程来建立其温压压制方程的分析方法。粉末和模具温度、装粉高度通过压力强化因子影响温压生坯密度。温压压制方程描绘的压制曲线与实验数据相符合。曲线表明装粉高度增加时,粉末和模具加热温度应当降低。对于某些装粉高度温压失去有效性。 相似文献
17.
Investigations on friction behaviour of treated and coated tools with poorly lubricated powder mixes
《粉末冶金学》2013,56(3):224-228
AbstractFriction at the interface between tools and powder plays a significant role in the cold die compaction process in both compression and ejection steps. In this work, the influence of tools materials and coatings as well as lubricant content in the powder is investigated with a sliding piece device. Three iron powders with different amounts of lubricant, from 0 wt-% to 0·6 wt-% (all percentages are expressed in weight), are tested on eleven parallelepipedic slabs (different couples of material and coating), which simulate tools surfaces. Results are presented in terms of friction coefficient evolution, friction coefficient values and surface analysis. Then they are compared with previous work to check their validity and consistency. Finally an assessment of the friction aptitudes of each slab will be made to open prospects of improving industrial processing conditions. 相似文献
18.
AbstractThe possibilities of the production of aluminium-base copper and/or silicon alloys by conventional powder compaction and sintering methods have been studied. The effects of various lubricants, pressing, and sintering conditions on the behaviour of Al–Cu and Al–Cu–Si alloys were evaluated systematically. The role of copper and silicon additions during compaction and sintering and their advantages or disadvantages are discussed. All alloys underwent large dimensional changes (sudden swelling followed by rapid contraction) during sintering at temperatures greater than Al–Cu eutectic temperature and it is suggested that a process of particle rearrangement is largely responsible for this behaviour. The mechanical properties of the alloys were highly dependent on the sintering temperature. PM/0215 相似文献
19.
AbstractIn the powder shaping process, friction is often considered as one of the main limiting factors since it can be the cause of bad green density distributions as well as ejection defects. Therefore, knowledge of this is important for better controlling the compaction stage. This paper makes an attempt to bring out the different process parameters (density, normal stress, sliding velocity, temperature, and displacement) which influence the friction between a standard iron powder and a tool in tungsten carbide. With this view, a sliding piece device has been built and is described. Several tests have been performed; from the results emerge a decrease of the friction coefficient with increasing density or normal stress, conversely the velocity increases the friction while temperature has no significant role. In this work, particular attention is given to the amplitude of the displacement showing that it can highly affect friction and wear of green parts. An attempt is made at explaining the observed phenomena and results are correlated with an analytical expression. 相似文献
20.
《粉末冶金学》2013,56(2):139-142
AbstractThe study of the influence of temperature on the performance of admixed lubricants is important since higher densities are desired while keeping the ejection force at a reasonable level. Therefore, three lubricants admixed with iron powder were evaluated during compaction at 25, 65, and 110°C. An instrumented die permitting the measurement of the applied and transmitted pressures through the compact lead to the evaluation of the slide coefficient. This empirical parameter is related to the stress ratio and to the friction coefficient characterising the friction of the compact on the die wall. The evolution of the slide coefficient revealed a different behaviour at the beginning of compaction, where a higher shear resistance is desirable, compared with the end of compaction, which was more influenced by the amount of lubricant at the interface between the compact and the die wall. A too low shear resistance at that stage could however lead to stick–slip phenomenon. 相似文献