Effect of multiple impacts on high velocity pressed iron powder

High velocity compaction (HVC) process is a promising method for preparing high density powder metallurgy (PM) components with cost-effective. In the paper water atomized pure iron powder with 0.5 wt.% zinc stearate added was compacted by HYP35-7 High Velocity Compaction Machine. A computer controlled universal testing machine was used to measure the bending strength of samples. The effect of multiple impacts on the green density, the bending strength and the stress wave was discussed. The results show that at the identical total impact energy the green density and the bending strength of compacts pressed by double impact are the highest, whilst that of compacts prepared by triple impact is the lowest. In addition, the response time of stress wave is shortened gradually with increasing compacting numbers. Furthermore, the duration of stress wave of the second impact of double impact is the longest, but no obvious difference between that of single impact and the third impact of triple impact can be brought out. The comprehensive effect of the pressure and the duration results in the green density and the green strength increasing.     

(Ti-2B-60%Ni)/(3Ti-2BN-xNi)层状材料燃烧合成研究

研究了当燃烧波蔓延通过(Ti-2B-60%Ni)/(3Ti-2BN-xNi)(x=0,10%,20%,40%,均为质量分数,以下同)两层混合粉料时,3Ti-2BN-xNi层稀释剂Ni含量的改变及混合粉料相对密度的变化对燃烧波形态和燃烧波波速的影响规律.经实验测定,Ti-2B-60%Ni,3Ti-2BN,3Ti-2BN-10%Ni,3Ti-2BN-20%Ni单层混合粉料在生坯压制压力p＝60 MPa时其燃烧波波速分别为:28.35, 4.96, 4.43, 3.83 mm/s.在此条件下,燃烧波在3Ti-2BN-40%Ni混合粉料中不能自蔓延.上述研究结果为燃烧合成功能梯度材料奠定了基础.     

Effect of Agglomeration on Mechanical Properties of Porous Zirconia Fabricated by Partial Sintering

Porous ZrO₂ ceramics were fabricated by compacting a fine ZrO₂ powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO₂ specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO₂ grains. It was revealed that the ZrO₂ agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO₂ ceramics.     

The importance of the amount/thickness of die wall lubricant for UO2 pellets pressing

External lubrication is often used to complete compaction process of powder materials. The main goal of this method is generally to reduce the amount of admixed internal lubricant (Zinc stearate powder) within the raw material. The application of external lubricants enhances the density uniformity and the mechanical strength of the resulting compacts. This study investigates the effects of the external lubricant amount for UO₂ powder compaction and the properties of the corresponding green pellets (corresponding to the compacts before sintering) without any admixed lubricant in the raw powder in order to evaluate the feasibility of this route in the case of nuclear powder. Results show that there is a quantity or number of layers from which the external lubricant applied on the die wall becomes detrimental to the friction index and the ejection force measured during the pressing cycle. The quality (surface defects, mechanical strength) of the green pellets can also be affected by the amount of lubricant. Thus the quantity and the thickness of the die wall lubricant must be optimized in order to assure an efficient mixed lubrication mode corresponding to the better lubrication mode in our study case.     

Characterization and comparison of gray cast iron powder produced by target jet milling and high energy ball milling of machining scraps

Target jet milling and conventional ball milling were used to produce powders from gray cast iron scraps. Powders of similar size distribution produced by the two methods were pressed at different compacting pressures. Green compacts were made at the compacting pressures of 500, 600, 700 and 800 MPa. Jet milled powder showed good compaction behavior while ball milled powder showed very poor compressibility. Also, balanced compacts composed of 50% Hoganas SC100.26 iron powder and each of the cast iron powders produced in this work were made at 500 and 800 MPa and their green properties were determined. Results showed that, green properties of the jet milled powder were acceptable and superior compared to ball milled powder. The jet milling process proved to be a much more efficient process compared to ball milling in terms of time and production capacity.     

Effect of Blaine fineness of recycling brick powder replacing cementitious materials in self compacting mortar

The present study aims to investigate the use of recycling brick powder (RBP) as a fractional replacement of cement in self compacting mortar. Cement was replaced by recycling brick powder in different proportions (0, 5, 10, 15 and 20%) by weight of cement. Three Blaine fineness (SSB) were used in this study (3900, 4300, and 5200 cm²/g). The experimental results showed that the substitution of cement by recycling brick powder with higher content (20%) decreases the workability of self compacting mortar (about 3%) and reduces its passing ability (about 30%). On the other hand, the increase of Blaine fineness of (RBP) led to improve the Fresh behaviour of mortars when the content of recycling brick powder kept constant. Mechanical performances of mortars in terms of compressive and tensile strength were improved by 13 and 105% respectively. Economically, the substitution of cement with 10% of brick powder gives a win of 10% of cement with an increase of 13% in compressive strength and 107% in tensile strength.     

Effects of Powder Processing on the Green Compacts of High-Purity BaTiO3

Two processing methods have been used to prepare different green compacts of high-purity BaTiO₃. The substructures of agglomerates have been examined by SEM, and the strength of the agglomerates was interpreted in a plot of the logarithm of the pressure versus the relative density, which is consistent with the structures of cut surfaces of green compacts. The Hg-penetration results have been used to compare the uniformity of the powder compacts. The wer method (wet milling and subsequent wet pressing) was used to produce uniform green compacts.     

Effect of Internal Lubricants on Defects in Compacts Made from Spray-Dried Powders

The role of internal lubricants in the closure of large intergranular pores during dry-pressing was investigated. Alumina was spray-dried with and without an internal lubricant to yield granules with similar characteristics other than lubricant effects. Green and sintered microstructures were evaluated at different compaction pressures. The defects were quantified by evaluating the fracture surface of ∼90% dense sintered compacts. The samples that contained an internal lubricant had higher green densities and fewer defects at comparable compaction pressures. The internal lubricant did not cause any significant reduction in green strength or increase in springback.     

改性黑索今装药的起爆规律研究

为了研究钝化RDX（黑索今）装药对冲击波响应剧烈程度的变化规律,在改变钝化RDX装药的密度,装药的轴向间隙和径向间隙后,用改进的小隔板实验装置进行测定。结果表明：冲击波响应剧烈程度随着装药密度的增大而增大;在钝化黑索今药装药结构中,轴向间隙和径向间隙都会在不同程度上降低冲击波响应的剧烈程度。     

Powder Processing for the Fabrication of Si3N4 Ceramics: I, Influence of Spray-Dried Granule Strength on Pore Size Distribution in Green Compacts

The effect of spray-dried granule strength on the micro-structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si₃N₄ powder made with ultrafine A1₂O₃ and Y₂O₃ powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray-drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.     

Powder processing of ultra-high molecular weight polyethylene I. Powder characterization and compaction

Powders of ultra-high molecular weight polyethylene from three suppliers were characterized for density, crystallinity, particle size, particle size distribution and particle morphology. The powders had molecular weights in the range 2-5 million. Bulk powder behavior, compressibility, green strength and springback were evaluated and explained in terms of particle characteristics. The green densities of the powders were found to reach a plateau at pressures of about 100 MPa. These plateau density levels were found to depend upon powder, characteristics and to lie between 80 and 90 percent relative density. Green strength is shown to be a unique function of a densification parameter.     

Influence of Viscous Deformation at the Contact Point of Primary Particles on Compaction of Alkoxide-Derived Fine SiO2 Granules under Ultrahigh Isostatic Pressure

Viscous deformation and the adhesion force at the contact point between amorphous silica particles under ultrahigh isostatic pressure (up to 1 GPa) are important in the densification of powder compacts. The amount of viscous deformation and the strength of adhesion force have been changed in the present study by altering the calcination temperature and particle diameter, and the new values have been determined successfully using a diametral compression test. The diameter of spherical and monosized alkoxide-derived silica powders has been controlled within the range of 10–400 nm. Close-packed granules of these powders have been produced by spray drying. Because of viscous deformation, as-spray-dried ultrafine silica powders without calcination could be consolidated into highly dense compacts (>74% of theoretical density) by applying ultrahigh isostatic pressure (1 GPa). Relatively high temperature in the calcined particles (400°C) causes viscous deformation at the contact point to disappear almost completely and clearly increases the adhesion force, because of neck growth that has resulted from viscous sintering. At temperatures >200°C, the green density of the calcined powders decreases to 65% of theoretical density, even under 1 Gpa pressure. The relationship between green density and viscous deformation in silica particles at the point of contact has been analyzed quantitatively by the Hertz and Rumpf model. On the other hand, if relatively low isostatic pressure ( P c < 100 MPa) is applied, the green density and intergranular pore volume depend on the strength of the spray-dried granules. The relationship between granule strength and neck growth at the contact point with calcination has been estimated quantitatively.     

Sintering of Nanosized MnZn Ferrite Powders

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha-Fe₂O₃ in the spinel lattice.     

Study of Jet Interaction with Interlayer Material of Bulging Armor

The paper presents an analytical approach for determining the effects of different properties of inert material of bulging armor sandwich on its efficiency against a shaped charge jet. The crater and shock wave parameters have been investigated. A theoretical relation for calculating the energy of the shock wave, produced by the jet impact, in the inert material has been derived on the basis of continuity of pressure and particle velocity at the interface of jet and inert material. This relation reveals that the higher the velocity of the jet the greater is the amount of energy in the shock wave. The energy of the shock wave also increases with increase of density and decrease of strength of the inert material. The shock energy has been found to further depend on the constants of equation of state of the inert material, (u_s=a+b u).     

Influence of Granule Character on Strength and Weibull Modulus of Sintered Alumina

Alumina specimens were prepared by compacting well-characterized spray-dried alumina granules of excellent deformability, followed by sintering. The strength and Weibull modulus were measured and microstructure was evaluated for specimens prepared from granules with different density and morphology. Compacts of spheroidal granules with lower density and lower granule strength sintered to 98% relative density and had fewer microstructural defects than compacts prepared from spheroidal or hollow granules of higher density. Flexural strength of sintered material was found to depend on initial granule size but not density or morphology of the granules. Weibull moduli of 18–20 were independent of particular granule characteristics.     

Fabrication and characterization of silica ceramic compact prepared using Aloe-Vera mucilage as a binder

In this study, silica compacts were fabricated through a powder processing route at different compaction pressure, using Aloe-Vera (AV) mucilage as a binder. The silica compacts were prepared at 90, 100, and 110 MPa compaction pressure using 0%–16 wt% of AV binder. The optimum amount of AV binder was 14 wt% for both 90 and 100 MPa and 12 wt% for 110 MPa. The maximum achieved green density and green strength of silica compacts at the optimum binder amount were 62.3% and 4 MPa, respectively at 110 MPa compaction pressure. The green silica compacts prepared at 110 MPa compaction pressure exhibited a minimum porosity of 21% and maximum flexural strength of 15 MPa after sintering at 1400°C. The green silica compacts with the optimum amount of binder were strong enough for machining. The Fourier transform infrared spectroscopy analysis revealed the functional groups present in AV mucilage. The binder burnout characteristic of AV mucilage in the silica compact was determined by thermogravimetric analysis and differential thermal analysis. Additionally, AV gel acted as a binder and solvent simultaneously for ceramic compaction.     

Influence of solid loading and particle size distribution on the porosity development of green alumina ceramic mouldings

Alumina ceramic mouldings with different solid contents ranging from 55 to 70 vol% and different ratios of coarse/fine powders, i.e. 0.4 μm (fine) and 3 μm (coarse), respectively, were prepared by compression moulding at 75 °C under a compressive stress of 10 MPa. The porous parameters, such as porosity, pore size and pore size distribution, of the green compacts were evaluated after removal of organic vehicles. Experimental evidence showed that the green density, as well as the sintered density, of the moulded alumina increased linearly with increased solid loading to an optimum of 65 vol% and decreased roughly linearly with increased coarse/fine ratio. Further increase in solid loading reduced particle packing efficiency, resulting in lower green and fired densities. No considerable improvement in green and sintered density of the moulded alumina was achieved by adjusting the coarse/fine ratio, which is due to the fact that coarse particles suppress the driving force of densification. The green compacts generally showed a bimodal pore size distribution character which may be the most important factor in dominating the densification of the powder compacts. The peak frequency at larger pore region is approximately 20–35 μm in diameter and at the smaller pore region is ˜50–95 nm in diameter. The larger pores are believed to be due to the presence of internal voids originating from entrapped gas and are probably caused by the removal of organic vehicles.     

Influence of Various Consolidation Techniques on the Green Microstructure and Sintering Behavior of Alumina Powders

Submicrometer alumina powders, of both standard and narrow particle-size distributions, were consolidated by dry-pressing and colloidal-forming techniques. The resulting green compacts were characterized in terms of pore-size distribution, green density, shrinkage behavior, and sintered density. The interrelationships between powder characteristics, green compact microstructure, and sintering behavior are discussed. There are distinct differences between the characteristics of compacts that were directly consolidated from powder suspensions and those that were dry pressed. Dense microstructures can be achieved at low temperatures using narrow-sized, well-dispersed powders with colloidal-forming techniques.     

The use of hardness in the study of compaction behaviour and die loading

An experimental relationship between density, hardness and compacting pressure is obtained for the isostatic compaction of Alcoa grade 1202 atomised aluminum powder. These results are used to evaluate the use of hardness measurement in the determination of density contours within compacts and pressure distributions at the compact—die interface. Density contours and pressure distributions are presented for closed die compacts; the results are in general agreement with those reported in the literature for more complex techniques. The technique is shown to be suitable for use in many situations.     

Net shaping nanopowders with powerful ultrasonic action and methods of density distribution control

Abstract

Two methods of compacting dry poly- and nanodisperse powders into compacts of a complicated shape with uniform density distribution in the volume have been developed: pressing under powerful ultrasonic action and collector pressing by the control of friction forces redistribution. Experimental measuring and simulation of the process of deformation of a powder body by different methods show that in the case of ultrasonic pressing, the dispersion of the density distribution has decreased by 20%, and the relative differential of density along the pressing height has decreased by 35% as compared to the case of the conventional uniaxial pressing. Collector pressing allows reducing the dispersion of the density distribution (has decreased by a factor of 10), while the relative density differential along the green compact height has decreased by 60%.