Densification of Al-2024 and Al-2024/Al2O3 Powders by Conventional P/M Route and ECAP: A Comparative Study

In this study, mechanically alloyed Al-2024 and Al-2024/Al₂O₃ powders are densified by conventional sintering and by equal channel angular pressing (ECAP) with and without back pressure. The powder was encapsulated in an aluminium can for consolidation through ECAP. The properties obtained in the compacts by conventional sintering route and by ECAP are compared. The effect of conventional sintering and ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. Room temperature back pressure aided ECAP results in nearly full denser (97?% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the consolidated compacts.     

Technology and properties of porous permeable aluminum-based materials

Results of the investigation of preparation conditions (pressure of powder pressing and time of sintering of compacts) and of the effect of aluminum additions (5–20 wt %) on the properties of a sintered porous material of an aluminum powder and its mixture with an Al₂O₃ powder are given. The possibility is shown of the production of porous (P = 30–35%), highly permeable (k = 2.0 to 2.5 × 10^?13 m²) aluminum-based sintered material with an addition of 20% Al₂O₃ and maximum size of pores equal to 1.3–1.5 μm in the following regime: p = 60 MPa, T = 723–823 K with an isothermal holding for 30–60 min.     

Growth of Mo grains around Al2O3 particles during liquid phase sintering

The growth behavior of Mo grains in contact with Al₂O₃ particles during liquid phase sintering was studied in 95.4 Mo-4 Ni-0.6 Al₂O₃ (wt%). Sintering of the powder compacts at 1460°C was repeatedly interrupted by cooling to 1300°C and reheating to 1460°C. Layers grown on Mo grains during each sintering interval between the interruptions were revealed by etch boundaries. The Mo grains grew around Al₂O₃ particles by solution-reprecipitation. In contact areas the Mo grains and the Al₂O₃-particles were separated by thin liquid films. Deposition or loss of material in contact areas resulting from material transport in the thin liquid film was found to be slow. During prolonged sintering some Al₂O₃ particles were trapped within growing Mo grains.     

The effect of small additions of disperse alumina inclusions on some characteristics of sintered nickel

Summary Materials containing 0.5, 1, 2, 3, 4, and 5 vol. % Al₂O₃ were obtained from mixtures of nickel oxide and alumina powder reduced with hydrogen. The apparent density of the reduced powder and the relative density of pressed compacts vary nonuniformly with increasing alumina content. The material with 1 vol. % Al₂O₃ has the maximum values of these characteristics. The sintering shrinkage and relative density of sintered specimens are minima for this composition. Disperse alumina inclusions appear to exert a dual effect on the shrinkage of contacts during sintering: On the one hand, they decrease shrinkage by disturbing contact between the nickel particles; on the other hand, they facilitate the evacuation of the water vapor forming during sintering. The disperse inclusions break up the nickel submicrograins and affect the microdistortions of the nickel lattice.     

Effect of Al2O3 and B4C particle additions on microstructure of PM copper based brake linings

Abstract

This paper highlights the effect of different ceramic particles on the structure of PM copper based brake linings. The copper based brake linings using a range of ceramic additives (1–6 wt-%) were prepared by powder metallurgy (PM). The optimum conditions for the production of brake linings were determined as compaction under 400 MPa and sintering at 805°C for 20 min in an argon atmosphere. The density of copper based brake linings decreased after sintering with the increase in ceramic powder contents for both Al₂O₃ and B₄C ceramic particles. The microstructural characterisation of produced samples showed that the lower boiling point elements in the as supplied powder vaporise during sintering from the structure and this leads to an increase in the porosity amount of the final component.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High 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.     

Reaction Pathways of Nanocomposite Synthesized in-situ from Mechanical Activated Al–C–TiO<Subscript>2</Subscript> Powder Mixture

In-situ Al matrix composite was synthesized from Al–TiO₂–C powder mixtures using mechanical alloying and heat treatment, subsequently. The effect of ball milling on reaction processes of the resulting nanocomposite was investigated. The evaluation of powder mixture without mechanical activation showed that at 900°C aluminum reduced TiO₂, forming Al₃Ti and Al₂O₃. After 20 h mechanical activation of powder mixture, Al₃Ti and Al₂O₃ were fabricated. After that, by increasing milling time up to 30 h, no new phases formed. The DTA analysis of 30 h milled powder indicated two peaks after aluminum melting at 730 and 900°C. The XRD results confirmed that at 730°C, molten Al reacted with TiO₂ and C, forming Al₃Ti, Al₂O₃ and Al₄C₃. After that, at 900°C, Al₃Ti reacted with Al₄C₃, causing TiC formation. This results proposed that the TiC formation is associated by a series of reactions between intermediate products, Al₃Ti and Al₄C₃ and the resultant nanocomposite was successfully synthesized after 30 h milling and heated by DTA analysis up to 1200°C.     

In situ formation of FeAl–Al2O3 nanocomposite at different conditions of milling and subsequent annealing

Abstract

FeAl–Al₂O₃ nanocomposite powder was synthesised under different conditions of milling and annealing. The structure, morphology and microstructure of the milled powders were monitored by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively. Results showed that the formation of FeAl and Al₂O₃ took place in explosive mode during milling with the cup speed of 600 rev min^?1. However, at the cup speed of 500 rev min^?1, FeAl and Al₂O₃ were synthesised only during annealing. Formation of the FeAl and Al₂O₃ was completed after 120, 270 and 360 min of milling at the ball to powder weight ratios (BPRs) of 5∶1, 15∶1 and 10∶1 respectively. Maximum microhardness of 8·8 GPa was obtained in the 270 min milled sample with the BPR of 15∶1 and cup speed of 600 rev min^?1. Mean grain size of 30 nm was calculated in the annealed FeAl that was in consistent with TEM results.     

The influence of milling parameters on the characteristics of milled and spray-dried NiCoCrAlY–Al2O3 composite powders

Spray-drying process was selected to agglomerate ball milled NiCoCrAlY–Al₂O₃ composite powders. The effect of the starting alloy powder size on the morphology of composite powder was studied. The parameters of milling were optimised by orthogonal experiment to improve the powder’s flowability and apparent density. Then the optimised powder was sprayed by air plasma spray to prepare NiCoCrAlY–Al₂O₃ composite coating. The results showed that the size distribution of starting particles decided the deformation of alloy particles and the characteristics of agglomerated powders eventually. With the decreasing size range of the starting alloy particles, the sphericity of agglomerated powders increased. The optimised milling parameters were as follows: solid content, 60?wt-%; BPR, 4:1; the rotating speed, 350?rev?min^?1; and milling time, 5?h. And the contribution of solid content was the largest. The Al₂O₃ splats showed good adhesion with alloy matrix when the composite powder melted in good condition.     

Production of NiAl–matrix composites by reactive sintering

Abstract

This work was devoted to the development of NiAl–matrix composite and its production by reactive sintering powder metallurgy. Various types of reinforcement (aluminium oxide, silicon and tungsten carbides, titanium silicide) were tested. The best chemical compatibility and the highest hardness and wear resistance were achieved by Al₂O₃ fibres. Electroless nickel plating pretreatment of Al₂O₃ fibres improves both distribution of fibres and hardness of the composite. However, it strongly reduces the wear resistance, probably due to phosphorus content in the nickel coating. In situ formation of NiAl–Al₂O₃ composites by reactive sintering of a pressed powder mixture of Ni, Al and NiO was unsuccessful. Only a small amount of cubic γ-Al₂O₃ was detected after reactive sintering and hence no significant hardness increase was observed.     

Synthesis of γ-TiAl by Reactive Spark Plasma Sintering of Cryomilled Ti and Al Powder Blend: Part II: Effects of Electric Field and Microstructure on Sintering Kinetics

The current study shows the dramatic effect of an electric field (EF) and use of nanosized cryomilled grains on accelerating sintering kinetics during spark plasma sintering of blended elemental powder compacts of Ti₅₃Al₄₇ targeted to produce γ-TiAl intermetallic compounds. The EF had the dominating effect since it reduced the activation barrier for diffusion through Al₃Ti leading to faster growth of Al₃Ti; the precursor to γ-TiAl. The Avrami exponent (n) determined for the micrograin compact lies between 1.0 and 1.5, which indicates that reaction sintering is controlled by bulk diffusion in these compacts, while for cryomilled compacts this is between 0.7 and 1.0 suggesting the important role of dislocations and grain boundaries on the transformation during reaction sintering. The activation energies were found to be in increasing order as: cryomilled compacts with EF (182 kJ/mol); micrograin compacts with EF (290 kJ/mol); cryomilled compacts without EF (331 kJ/mol); and micrograin compacts without EF (379 kJ/mol). The cryomilled microstructure also enhanced the sintering kinetics because of the availability of faster diffusing paths in Al and Ti including larger grain boundary area and dislocation density.     

Injection moulding of 316L stainless steels reinforced with nanosize alumina particles

Abstract

This study aims to compare the effect of Al₂O₃ nanoparticle additions on the densification and mechanical properties of the injection moulded 316L stainless steels. The 316L stainless steel and Al₂O₃ nanoparticles were dry mixed and moulded using a wax based binder. The critical powder loading for injection moulding were 60 vol.-% for all samples. Debinding process was performed in solvent using thermal method. After the debinding process, the samples were sintered at 1405°C for 60 and 120 min under vacuum. Metallographic examination was conducted to determine the extend of densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all the samples after wear testing. All the powders, fracture surfaces of moulded and sintered samples were examined using scanning electron microscope. The sintered density of straight as well as Al₂O₃ nanoparticles reinforced injection moulded 316L stainless steels increases with the increase in sintering time. The additions of Al₂O₃ nanoparticles improve the hardness and wear resistance with the increase of sintering time.     

A study on the microstructural evolution of Al-25 at. pct V-12.5 at. pct M (M = Cu,Ni, Mn) powders by planetary ball milling

The alloying behavior of Al-25 at. pct V-12.5 at. pct M (M = Cu, Ni, Mn) by planetary ball milling of elemental powders hours as been investigated in this study. In Al₃V binary system, an amorphous phase was produced after 6 hours and the amorphous phase was mechanically crystallized after 20 hours. The large difference in the diffusivities between Al and V atoms in Al matrix results in the formation of the amorphous phase when the homogeneous distribution of all the elements in a powder was achieved at 6 hours. According to thermal analyses, the amorphous phase in the binary Al₃V was crystallized at 350 °C. The addition of ternary elements (Cu, Ni, Mn) increased the activation energy for the crystallization to D0₂₂ phase by interfering with the diffusion process. Therefore, ternary element addition improved the thermal stability of the amorphous structures. The amorphous phase in the 12.5 at. pct Ni added Al₃V was crystallized to D0₂₂ phase at 540 °C. The mechanical crystallization of the amorphous phase in the ternary element-added Al-V system either occurred later or was not observed during ball milling up to 100 hours. It is thought that the amorphous intermetallic compacts could be produced more easily in ternary element-added alloys by using an advanced consolidation method.     

Nanocrystalline Powders Based on ZrO2 for Biomedical Applications and Power Engineering

An 80 mass% ZrO₂ ― 20 mass% Al₂O₃ powder was produced using a complex method which integrates sol-gel technology and hydrothermal synthesis. The specific surface areas of the powder varied from 39 to 5.3 m²/g depending on the thermal treatment conditions. Metastable F-ZrO₂ formed after powder annealing at 400°C. The phase transformation F-ZrO₂ → T-ZrO₂ (traces of M-ZrO₂) occurred under powder thermal treatment from 700 to 1000°C. Only Θ-Al₂O₃ was detected under experimental conditions. The powder was characterized by sintering activity. Operating the processes under powder thermal treatment in the ZrO₂ ― Y₂O₃ ― CeO₂ ― Al₂O₃ system will allow one to produce a variety of ceramic microstructures from fine-grained to “self-reinforced.” These powders can be used in manufacturing surgical cutting tools as well as in ceramic passive bioimplants and solid electrolytes for fuel elements.     

SOME ASPECTS OF THE MILLING OF CEMENTED CARBIDE POWDER

Abstract

Cemented carbide powders milled for various times have been studied with respect to the shape and internal structure of the grains. The shrinkage of powder compacts during heat-treatment was recorded dilatometrically. The major effects of milling seem to be to alter the grain size and morphology of the powder and to form a fine dispersion of small particles of Co₃O₄.     

Experimental measurements and modelling of viscosity in CaO–Al2O3–MgO slag system

Abstract

The viscosity of the CaO–Al₂O₃–MgO slag system has been measured by the rotating cylinder method up to 1823 K. The MgO content was between 0·39 and 11·33 wt-%, and the mass ratio of CaO/Al₂O₃ was between 0·60 and 1·28. The results indicated that both the MgO content and the mass ratio of CaO/Al₂O₃ have an influence on the viscosity of the slag. The effect of the mass ratio of CaO/Al₂O₃ is larger than that of the MgO content. The viscosity decreased with increasing MgO content and then increased. The effect of the mass ratio of CaO/Al₂O₃ on the viscosity was similar. The main mineralogical compositions of the slag were determined by X-ray diffraction, and their effects on viscosity were investigated. The measured results were in good agreement with those given by the Iida model.     

Development of Wear-Resistant Cu-10Cr-3Ag Electrical Contacts with Nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Dispersion by Mechanical Alloying and High Pressure Sintering

Cu-10Cr-3Ag (wt pct) alloy with nanocrystalline Al₂O₃ dispersion was prepared by mechanical alloying and consolidated by high pressure sintering at different temperatures. Characterization by X-ray diffraction and scanning electron microscopy or transmission electron microscopy shows the formation of nanocrystalline matrix grains of about 40 nm after 25 hours of milling with nanometric (<20 nm) Al₂O₃ particles dispersed in it. After consolidation by high pressure sintering (8 GPa at 400 °C to 800 °C), the dispersoids retain their ultrafine size and uniform distribution, while the alloyed matrix undergoes significant grain growth. The hardness and wear resistance of the pellets increase significantly with the addition of nano-Al₂O₃ particles. The electrical conductivity of the pellets without and with nano-Al₂O₃ dispersion is about 30 pct IACS (international annealing copper standard) and 25 pct IACS, respectively. Thus, mechanical alloying followed by high pressure sintering seems a potential route for developing nano-Al₂O₃ dispersed Cu-Cr-Ag alloy for heavy duty electrical contact.     

Synthesis and characterisation of Al2O3/Ni-type composites obtained by spark plasma sintering

This paper presents the influence of sintering on the structure, morphology and compressing strength of alumina/nickel composite compacts obtained by spark plasma sintering (SPS). Al₂O₃/Ni composites were prepared by SPS in argon atmosphere at temperatures in the range of 1000–1200 –C with a holding time of 2, 5 and 10?minutes. The heating rate was 200 C?min^?1. These composites have been characterised by X-ray diffraction, SEM and EDX. The relative density and compressive strength of the as-obtained compacts were determined. The results showed that the alumina particles are uniformly dispersed in a quasi-continuous Ni network, and there was no sign of phase changes during sintering. The maximum strength of the alumina/nickel composite with a content of 75 vol. ? Al₂O₃ and 25 vol. ? Ni was about 240?MPa for the samples sintered at 1200?C for 10?minutes.

Special block from the conference RoPM2017 guest edited by Ionel Chicinas, Technical University, Cluj-Napoca.     

Crystal structure of calcium and aluminium silicoferrite in iron ore sinter

Abstract

The crystal structures of real and synthetic iron ore sinters were compared. In the Fe₂O₃-CaO-Al₂O₃- SiO₂ system, the crystal structure of a synthetic solid solution of CaO.2Fe₂O₃ (CF₂) was studied by X-ray powder diffraction. As the Al₂O₃ content in pure CF₂ increased, the formation of hemicalcium ferrite increased. On the other hand, CF₂ content decreased and hematite increased as the SiO₂ content increased. The crystal structure of CF₂ with Al₂O₃ and SiO₂ (calcium and aluminium silicoferrite, also known as silicoferrite of calcium and aluminium, SFCA) changed from monoclinic to triclinic, and the unit cell volume decreased when the Al₂O₃/SiO₂ ratio increased. The solubilities of Al₂O₃ SiO₂ in CF₂ were 5-7 wt-% and 2-4 wt-%, respectively at 1250°C.     

Isothermal reduction behaviour of MnO2 doped Fe2O3 compacts with H2 at 1073–1373 K

Abstract

Pure Fe₂O₃ and Fe₂O₃ doped with 2, 4, and 6 mass% of MnO₂ (>99%) compacts annealed at 1473 K for 6 h were isothermally reduced with H₂ at 1073–1373 K. The O₂ weight loss resulted from the reduction of compacts was continuously recorded as a function of time using thermogravimetric analysis (TGA). High pressure mercury porosimeter, optical and scanning electron microscopes, X-ray phase analysis and vibrating sample magnetometer were used to characterise both the annealed and reduced samples. In MnO₂ containing samples, manganese ferrite (MnFe₂O₄) was identified. The rate of reduction of pure and doped compacts increased with temperature and decreased with the increase in MnO₂ content. Unlike in pure compacts, the reduction of MnO₂ containing samples was not completed and stopped at different extents depending on MnO₂ (mass%). At initial reduction stages, the decrease in the rate was due to the presence of poorly reducible manganese ferrite (MnFe₂O₄) phase which was partially reduced to iron manganese oxide (FeO_0.899, MnO_0.101) at the final stages. The reduction mechanism was predicted from the correlation between the reduction kinetics and the structure of partially reduced samples at different temperatures. The reduction of pure and doped samples was controlled by a combined effect of interfacial chemical reaction and gaseous diffusion mechanism at their initial stages. At final stages, the interfacial chemical reaction was the rate controlling mechanism.