Alumina Compacts Research Articles

In-Depth Comparison of an Industrially Extruded Powder and Ingot Al Alloys

An industrial press was used to consolidate compacted aluminum powder with a nominal diameter in the range of 1 µm. Direct and indirect hot-extrusion processes were used, and suitable process parameters were determined from heating conditions, ram speeds and billet temperatures. For comparison, a direct-extrusion press for hot extrusion of a conventional aluminum alloy AA 1050 was used. The extruded Al powder showed better mechanical properties and showed a thermal stability of the mechanical properties after annealing treatments. To increase the theoretical density of the directly extruded Al powder, single-hit hot-compression tests were carried out. Activation energies for hot forming were calculated from hot-compression tests carried out in the temperature range 300–580 °C, at different strain rates. Processing maps were used to demonstrate safe hot-working conditions, to obtain an optimal microstructure after hot forming of extruded Al powder.

Rapid microwave sintering of alumina ceramics with an addition of carbon nanotubes

Compacted alumina samples with an addition of 1.0 and 2.5 wt % carbon nanotubes (CNT) have been microwave sintered at heating rates 50 and 100°С/min to a maximum temperature of 1550–1600°С with zero hold time. The densification kinetics has been studied using optical dilatometry. No noticeable influence of CNT on the development of thermal instability during rapid microwave sintering of alumina has been detected. The relative densities of the samples containing 1.0 and 2.5 wt % CNT sintered at a maximum temperature of 1550 °C and zero hold time were 93.8 and 87.5%, respectively. Microwave sintering with a repeated development of thermal instability has resulted in an increase in the final density to 95.0%.

Uniaxial compression test on ceramic green compact with bending consideration using digital image correlation

A uniaxial compression test was performed on a green compacted alumina powder. Using Digital Image Correlation (DIC), displacement fields of two perpendicular surfaces of the specimen were measured showing unavoidable bending effect. A constitutive model was calibrated through three Finite Element Model Updating approaches. First, an analysis was performed using average axial strains assessed via DIC and resultant forces to identify the constitutive model. Then, a plane stress analysis was conducted out using the displacements of the boundaries of the regions of interest for each camera separately to obtain the sought parameters. Last, a 3D analysis was carried out using the multi-point constraint method to link displacement fields of both surfaces. Because of bending, the first two approaches give rise to two sets of different parameters. The analysis that considers the 3D nature of bending leads to the set of parameters with the highest degree of confidence.

Synthesis of graphene oxide and green properties of dry pressed alumina compacts with small addition of graphene oxide/graphite

In the present study, graphene oxide (GO) was synthesized by modified Hummers method, and the synthesized GO was characterized and confirmed by XRD, TEM and FTIR analysis. The effect of small (0–1.5 wt%) addition of synthesized GO on the properties of dry pressed green alumina (Al2O3) compacts was studied. The compaction and packing behaviour was correlated with the density and microstructure of green compacts. Observed results were compared with the properties of green samples prepared with small (0–3 wt%) addition of graphite (Gr). The experimental results revealed that the density of green alumina compacts increased with the addition of GO/Gr powder up to an optimum amount beyond which a decrease in density was observed at all compaction loads. The green densities of Al2O3-GO compacts and Al2O3-Gr compacts were in the range 75–85% theoretical density (TD) and 60–65% TD, respectively. Observed results were confirmed from the SEM micrographs of corresponding green compacts.

Effect of Sintered Alumina Blended Polyethylene Glycol Binder on the Pin on Disc Test Parameter

This study presents the effects of sintering temperature and binder content of polyethylene glycol (PEG) on wear properties of compacted alumina using pin on disc wear test under dry sliding condition. Wear rate was measured at 20N load and sliding speed of 200rpm at 0.28 m/s sliding velocity. The wear test sample was tested at 3 different sintering temperatures (1000°C, 1100°C, and 1200°C) to produce different microstructures. The results show that a dry pressed alumina ceramic with 5 wt. % of PEG content and sintering temperature at 1200°C recorded the lowest wear rate and volume lost compared to other samples. SEM was carried out to investigate the microstructure how it affects the wear property. It was found that at high sintering temperature of 1200° C the particle formation was strengthen to each other hence promote highest wear resistance.

New luminescent ceramics based on anion-deficient Al2O3–BeO for high-dose dosimetry

New Al2O3–BeO ceramics were synthesized by using high-temperature treatment of the initial compacts of nanostructured alumina in BeO crucible. The decomposition of the pulsed cathodoluminescence spectra into Gaussians revealed the luminescence bands related to oxygen vacancies and their aggregates in the ceramics. TL of the synthesized ceramics exposed to high-dose irradiation contained three peaks at 360 (A), 525 (B) and 730 K (C). It was found that optical stimulation at the wavelength of 470 nm causes optical bleaching of the traps responsible for peaks A and B, in contrast to the deep traps of the peak C, and gives rise to OSL. At the same time, there was a shift of the position of peaks A and B to the high-temperature region. This result shows the presence of the energy distribution of the traps responsible for TL peak A. It was found that the range of linearity of the TL dose response after gamma-irradiation is from 0.01 to 1.0 kGy. After irradiation with a pulsed electron beam (130 keV), the linear behavior of the OSL dose response was observed from 3 to 30 kGy.

Dynamic stress-strain response of high-energy ball milled aluminium powder compacts

Ball milling is a bulk powder manufacturing process used in the creation of dispersion strengthened and nanostructured materials. Fundamentally, these powders have not been dynamically characterized in a green state prior to hot consolidation. The understanding of high strain-rate compaction on void collapse and particle interaction for such systems can help the development of predictive models for impact events of porous metallic structures that may be employed as energy absorbers, reactive structures, and intermetallic materials. This study investigates high strain-rate impact of porous green compacts of as-received and high-energy ball milled (HEBM) aluminium powders characterized under dynamic compression using a split-Hopkinson pressure bar (SHPB) in a passive confinement configuration. The plastic deformation of the powder compacts and crush up were shown to be strain-rate insensitive within the strain rate range of 1000–2100 s−1 and as a result, were modelled adequately with a second order P-α model. The as-received aluminium and HEBM aluminium powders appear to have the same strain-hardening coefficient and strength index as solid aluminium after yielding. The respective stress-strain responses of green compacts follow the same trend but differ only in strength as result of porosity and pre-strain experienced prior to dynamic compression. The HEBM powder was found to be twice as strong as the untreated as-received aluminium powder.

Application of two-step sintering cycle to 20–30 nm α-Al2O3 compacts to investigate microstructure, hardness, strength, density and corundum formation

This paper discusses the hardness variation of 20–30 nm α-Al2O3 sintered specimens by two-step sintering (TSS) technique. This study mainly reveals the microstructure transformation of nano alumina into corundum grains. The uniaxial load of 4 ton was used to produce alumina compacts of 11 mm diameter and 3.5 mm thickness. In this two-step sintering process, high step temperature (T1) varied from 1250 °C to 1600 °C, whereas low step temperature (T2) was between 1100 °C to 1550 °C. To achieve hard nano alumina structure, seven TSS cycles were designed by varying the heating rate (°C/min), high step temperature (T1), high step temperature holding time (t1), low step temperature (T2), and low step temperature holding time (t2). After each TSS cycle, the Vickers hardness (HV), load-bearing capacity (KN), ultimate compressive strength (MPa), yield strength (MPa) and density (g cm−3) measurements of each sintered sample were carried out. The significant improvements observed in hardness and density of sintered nano alumina with an increase in high step temperature (T1) as well as holding time (t1). The microstructure transformation and material elemental analysis were carried out using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) respectively. It is evident from SEM micrographs that the specimen of sintered samples with high step temperature (T1) 1600 °C with 2 h holding time, low step temperature (T2) 1550 °C with 10 h holding time produced corundum grain transformation in the nano alumina matrix.

The effects of graphene nanoplatelet addition to in situ compacted alumina nanocomposites using ultra-high frequency induction sintering system

The aim of this study was to compact and sinter in situ graphene nanoplatelets (GNPs) with the addition of alumina (Al2O3) nanoparticles to form nanocomposites by using an ultra-high frequency induction sintering system with the assistance of applying a uniaxial load. To obtain the effect of the addition of GNPs to the compaction and to the mechanical properties of the nanocomposite, 1 to 5 wt.% GNPs was mixed to alumina nanoparticles by high-speed ball milling for 2 h at 350 rpm. Mixed Al2O3-GNP compositions were compacted in situ and sintered in a graphite die by induction-assisted heating at 1650 °C for 20 min with a uniaxial load applied in a vacuum chamber. The microstructure of as-sintered nanocomposites was observed using a scanning electron microscope (SEM) before and after thermal etching. Hardness and wear tests were carried out to determine mechanical properties. The results were compared with the properties of a pure alumina nanocompact. It was found that minimum grain size and maximum density, hardness, and wear resistance can be obtained by the addition of 1 wt.% GNPs to alumina nanoparticles. Higher amounts of GNP addition gradually decreased the density, hardness, and wear rates and increased the grain size of the alumina matrix.

Sintering behaviour and properties of manganese-doped alumina

The effect of manganese (0.1, 0.5 and 1.0 wt%) on the sintering and mechanical properties of alumina was studied. Sintering was carried out by the conventional heating method in a box furnace and in a hybrid multimode microwave furnace. XRD analysis revealed the precipitation of a spinel second phase (MnAl2O4) in manganese-doped samples as a result of manganese limited solubility in the corundum lattice. The addition of 0.1 wt% manganese was most beneficial in enhancing the densification of alumina (97.5% relative density when compared to 94.2% for the undoped sample), hindered grain growth, and improved the hardness of the ceramic when sintered at 1500 °C. The study also revealed that microwave sintering was effective in suppressing grain growth of alumina. In addition, the hardness was dependent on the sintered bulk density and that grain coarsening ensued as the density of the sintered alumina exceeded 95% of theoretical.

The grain growth in alumina compacts sintered under high pressure

Abstract In this study, alumina ceramics were produced with high density and nanograin size without additives from nano-size γ-Al2O3. The effects of sintering pressure (3 GPa and 7 GPa) and temperature (600 °C and 1 200 °C) on the grain size of sintered samples were studied. It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase. Pressure-assisted densification decreases grain size, so that nano-scale grains can be achieved in the sintered structure. The effect of various sintering conditions on grain growth in alumina was investigated during high-pressure sintering for better understanding of pressure and temperature effects on microstructural properties.

Effect of calcination temperature on friction and wear behavior of α–alumina (α‐Al2O3) for biomedical applications

AbstractIn this study, structural evolution, sliding friction, and wear behavior of the nano‐sized alpha‐alumina (α‐Al2O3), synthesized by calcination of gibbsite (Al2(OH)6) at different temperatures (273‐1473 K) are reported using X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and ball‐on‐disk wear tests. The effect of calcination temperature and the correlation between grain size, mechanical properties, and tribological behavior of the compounds are investigated. After calcination at 1473 K, the crystallite size of calcined and sintered alumina (α‐Al2O3) compacts is as small as 10 nm. Also, it is observed that the average particle and grain size significantly affect the tribological properties of the compounds. At all the applied loads of 2, 8, and 16 N, respectively, the sliding wear rate and coefficient of friction were lower in the alumina compacts calcined at 1473 K. The improved tribological properties are attributed to the finer microstructure resulting in enhanced hardness of the sintered compacts calcined at 1473 K.

Grain Size and Grain Boundary Analysis of Aluminum Compacts with/without 10 wt.% of Nickel Powder Processed by Equal Channel Angular Pressing

Grain Size and Grain Boundary Analysis of Aluminum Compacts with/without 10 wt.% of Nickel Powder Processed by Equal Channel Angular Pressing

A Study on Fatigue Characteristics of Al-SiC Metal Matrix Composites Processed Through Microwave Energy

Aluminium based Metal Matrix Composites are the very promising light materials with enhanced mechanical properties. Aluminium Metal Matrix composites (AMC) are identified as one of the very good materials for aerospace and automobile applications because of their lightweight, cost effectiveness and high stiffness. Metal Matrix Composites can be produced by various methods such as liquid infiltration, stir casting, spray deposition and powder metallurgy technique. Microwave processing of metals is a rapid developing technology which will offer significant time saving and can be cost effective. It is a volumetric process and very efficient to obtain high heating rates due to the interaction of microwave with the material in molecular level. The present work is an attempt made to study the fatigue characteristics of microwave processed Al-SiCP metal matrix composite. Sintering of the compacted Aluminium powder with 4% SiC is carried out using microwave energy. The sintered metal matrix composite is subjected to secondary processing of Extrusion. The fatigue characteristics and micro hardness properties are investigated.

Fragmentation and mechanical performance of tailored nickel-aluminum laminate compacts

The fragmentation of materials is a complex sequence of physical processes in which the kinetic energy is converted into deformation and fracture energy. The incorporation of reactive mixtures adds a third form of energy, chemical energy. The fragmentation and mechanical performance of nickel-aluminum compacts was examined under dynamic conditions using mesostructured powder compacts in which the interfaces between the powders (having initial sizes between 355 and 500 µm) were tailored during the swaging fabrication process. Fragmentation was created in ring samples of this material through explosively driven expansion (generating velocities around 100 m/s) and analyzed through high-speed photography, laser interferometry and soft capture of fragments. Quasi-static and dynamic mechanical testing was conducted to examine the mechanical performance and to provide parameters for the constitutive description. Experimental results are compared with fragmentation theories to characterize the behavior of reactive powders based on the material’s mesostructure by introducing the fracture toughness of the compacts, following the principal elements of the earlier work on tailored aluminum compacts. The fracture toughnesses, which ranged from 0.17 to 0.67 MPa m1/2, are related to the interfacial cohesion between particles and the fragmentation is a direct consequence of these parameters. The mean fragment size is calculated using a modified form of Mott’s theory and successfully compared with experimental results; it ranges from 10 µm to 40 µm. Finite element simulations on Al compacts confirm that the fragmentation increases (smaller fragment sizes) with a decrease in fracture toughness. The methodology developed here can be applied for tailoring the fragmentation of reactive munitions.

Effect of mechanical processing on thermal and mechanical properties of fibrous fumed alumina compacts

ABSTRACTIn the present study, the fibrous fumed alumina compact with low thermal conductivity is prepared with a mechanical processing technique. The thermal conductivity of the compact remains low up to a temperature of 350°C. For the fibrous compact, the mechanical processing parameters play important role on controlling the thermal insulation capability. The thermal conductivity of compact increases from 0.036 to 0.042 W/m∙K by increasing the rotating speed of rotor. On the other hand, the value remains low by increasing processing time. The strength is decreased due to the increase of processing time. Thus, a lower rotating speed and a shorter processing time could result in a compact with favorable thermal insulation and strength. The results can be related to the surface structure of the coated alumina fibers and granulated structure within the compacts.

New magnetic aluminum matrix composites (Al-Zn-Si) reinforced with nano magnetic Fe3O4 for aeronautical applications

Aluminum metal matrix composites (AMMCs) are light-weight materials are widely used in structural applications due to their favorable properties such as lightweight, high specific strength and modulus, wear resistance and low coefficient of thermal expansion. Today, new developed magnetic aluminium based composites (Al-Zn-Si) became very attractive due to their improved magnetic, physical and mechanical properties. These composites, depending on final desired properties with different reinforcement types, are very suitable for electrical motors and other usage in aircraft engineering. These promising properties of AMMCs are attributed to the size and distribution of the reinforcement, as well as to the grain size of the matrix. Process that was used in this work is called ‘Sinter-Forging’ during which compacted aluminium specimens were forged just after sintering. A novel near-net shape, low-cost sinter-forging approach to processing particle-reinforced metal matrix composites allow manufacturing parts for high performance applications with short processing time and low energy consumption. The aim of this work is to briefly present the efficient ‘Sinter-Forging Process’ and to summarize properties of new composites developed for aeronautical applications.

A validation procedure for numerical models of ceramic powder pressing

This paper describes an experimental procedure to validate numerical models used to simulate powder pressing. It consists mainly of two steps: closed die uniaxial pressing followed by isostatic pressing. The uniaxial pressing causes a non-homogeneous density distribution in the pressing direction as a consequence of friction between die walls and powder. In the isostatic pressing, less compacted regions have a larger volumetric strain, resulting in a non-trivial shape of the re-compacted part, which computes indirectly the previous density distribution. Experimental data from both steps are compared to the results from finite element models. The Drucker-Prager/Cap constitutive model was used to represent the compaction of alumina powder. Several simulations covering a range of parameters obtained from the literature were performed to calibrate the model, through an inverse analysis. The developed procedure sheds a light in the methods to calibrate and/or validate constitutive models used for powder pressing.

Fabrication and Machining Performance of Powder Compacted Alumina Based Cutting Tool

This study focuses on the fabrication of alumina based cutting tool for machining application. Specific weight of alumina powders were pressed in a mold using hydraulic press at 8 tons before compressed through Cold Isostatic Press (CIP) at 30000 psi. Green body then was dried and sintered at 1700 °C at 4 hours sintering time to form solid cutting tools. These cutting tools were then tested in turning operation to machine AISI 1045 with different cutting speeds. The results shows that CIP pressed alumina cutting tool obtained the hardness of 83.2 HRA which is considered adequate to machine AISI 1045 steel. The shrinkage of alumina powders recorded about 7% form green compact to sintered body. In terms of flank wear, the alumina cutting tool demonstrated decreased wear rate as the cutting speed increased from 150 m/min to 225 m/min. Wear area focused at the edge of cutting tool due to small nose radius with the minimum wear rate recorded at 0.0025 mm/s for 225 m/min cutting speed. The newly fabricated cutting tools can be improved if finer or secondary reinforced particles were used.

Production of Wire Mesh Reinforced Aluminium Composites through Explosive Compaction

In this study, aluminum based composites with stainless steel wire-mesh as reinforcement is fabricated by explosive compaction technique. Stacks containing four layers of alternatively positioned aluminum sheets and stainless steel wire-meshes are explosively compacted at varied explosive masses and the results are reported. Microstructure of explosive compacted aluminum composite reveal a smooth interface at lower explosive mass, while formation of reacted products are observed at higher energetic conditions. Though the hardness of the post clad composite is higher than pre-clad materials, the maximum hardness is observed at the first interface.