Application in powder metallurgy of CVD carbon nanofibres: microstructure and mechanical properties CNF reinforced Distaloy AQ

Powder metallurgy technique has been extended in automobile industry and other engineering applications. Competitive manufacturing costs have led to important innovations in different fields of powder metallurgy. Recently, research has been focused on reaching higher performance at a lower cost. Carbon nanostructures as carbon nanofibres (CNF) have also entered in the material science as reinforcing composites. In this study, different materials formed by low alloy steel Fe-Ni-Mo powder (Distaloy AQ) with 0.4% of graphite and 0.5% CNFs (carbon nanofibres) synthesised at 723, 773, 823 and 873?K have been studied. All the materials were compacted at 700?MPa and sintered at 1393?K and 90N₂–10H₂ atmosphere. The influence of CNFs synthesis temperature has been analysed for its application in powder metallurgy. The microstructure, mechanical properties such as tensile strength, elongation, hardness, density and wear behaviour were investigated. The CNFs synthesised at 873?K promoted better mechanical properties.     

Thermal property measurements of metal injection moulded Inconel 625 and Inconel 718 using combined thermal analysis techniques

ABSTRACT

The high-temperature thermal properties of powder metallurgy derived superalloys have not been reported in the literature. In this study, the coefficient of thermal expansion (CTE), specific heat and thermal diffusivity of metal injection moulded (MIM) Inconel 625 and Inconel 718 were measured. Measurements of wrought Nickel 200 were also made to verify the methods. These thermal property measurements were made in the range of room temperature to 1000–1200°C using dilatometry, differential scanning calorimetry and laser flash analysis. Thermal conductivity of all three materials was calculated using the measured diffusivity, specific heat and CTE. All the MIM results were compared to published data for the wrought form of these alloys and found to be in close agreement outside of phase transition regions.     

THE PRODUCTION OF CERMETS CONTAINING A RELATIVELY LARGE AMOUNT OF DISPERSED PHASE

Abstract

The preparation and related properties of dispersions of ceramics in metallic matrices are reviewed, with particular reference to recent British work in this field. There have been substantial advances in the fabrication of cermets by powder metallurgy, particularly those based on UO₂ or PUO₂ and stainless steel, it being now possible to manufacture almost perfect dispersions in bar or strip form. Some closer understanding of the mechanical properties of cermets or porous matrices has been achieved. These theoretical and technological developments are capable of wide application to non-nuclear materials.     

Characterisation of functionally graded and VC/steel surface alloyed composites produced using powder metallurgy

Abstract

In this study, low carbon steel specimens with surface alloyed composites were produced by means of powder metallurgy. Vanadium carbide, graphite (1·2 wt-%) and Fe were used for the surface alloyed composite, while Fe and graphite (0·2 wt-%) were used for the low carbon steel side. The powder mixtures were compacted together in the same mould. On the surface alloyed side the vanadium carbide content was changed from 5 to 25 wt-%. Microstructural investigations including EDX and X-ray, hardness measurement and abrasive wear tests were performed. The results showed that V₈C₇ formed in the alloyed surface and carbon diffusion from the alloyed surface to the parent metal created a functionally graded material. The hardness values decreased towards the parent metal. Wear resistance increased as the vanadium carbide increased in the surface alloyed composite. Thus, a functionally graded steel having a surface composite that is resistant to abrasive wear can be obtained via the powder metallurgy route.     

Driving force for low temperature sintering of interstitial and intermetallic powders and induced applications

Abstract

Intermetallic as well as (carbides and nitrides) interstitial compounds present functional and structural properties which make these materials necessary for advanced technologies. Meanwhile fabrication routes based on melting and casting or plastic deformation, are usually far to compete with powder metallurgy processes likely to provide near net shaped parts and components. The present study is devoted to a model system exemplified by nitrided iron and steel powders for which the thermal stability of nitrides is severely decreasing when temperature exceeds critical values during densification treatments. Thanks to the analysis of the thermal treatment induced transformations mainly characterised by Mössbauer spectroscopy, the consequences and impact on driving forces for densification under critical thermal conditions are discussed in order to achieve an optimised sintering process and an actual development.     

Simulated effects of martensite start temperature,thermal conductivity and pore content on end quench cooling rate

Abstract

Process modelling, based on finite difference methods, is used to show that the thermal conductivity increases, which typically attend the martensite transformation in steel, affect the cooling rate in the Jominy end quench test. A one-dimensional model, which includes the effects of material property variations, is presented that predicts slightly increased cooling rates with increases in the M _s temperature for fully dense steels and significantly increased rates for powder metallurgy (PM) steels. The model is based on earlier studies of the end quench test that initially showed increased cooling rates in PM steels versus fully dense ones and then went on to show water penetration of the pores as a causative mechanism. In the present study, it is shown that by combining a simple theory of this mechanism with the aforementioned M _s effects, it is possible to obtain cooling curves that display a marked resemblance to the experimentally observed ones of these earlier studies.     

CONSIDERATIONS ON THE PRACTICAL EFFECTS OF LUBRICANTS AND BINDERS COMMONLY USED IN COMPACTING METAL POWDERS

Abstract

Data acquired from practical industrial experience with several commonly used compacting lubricants have been investigated from the viewpoints of: (1) thermal decomposition in relation to sintering conditions (atmospheres, vacuum, temperature); (2) influence on the compactibility of iron powder (improved density) and the reduction of ejection force.

On the basis of the results obtained, the paper concludes with several points of guidance regarding the use of lubricants in iron powder metallurgy.     

Characterisation of powder metallurgy H13 steels prepared from water atomised powders

ABSTRACT

In this paper, powder metallurgy (PM) H13 steels were investigated based on the characterisation of water atomised H13 powders. Vacuum sintering was carried out from 1150 to 1265°C for sintering densification. The relative density of the as-sintered sample reached to 95% and mechanical properties were released above 1200°C. To eliminate the residual pores, subsequent forging was performed. The relative density increased to above 99%. As the concentration of Si elements on the powder surface was higher than the nominal composition, there was Si segregation in the form of silicon oxides (1–3?μm). Although oxide particles such as SiO₂ were inevitable, the mechanical performance of PM H13 steels was still comparable to that of ingot metallurgy products. The tempered PM H13 steels exhibited high tensile strength of 1460?MPa in YS, 1737?MPa in UTS and 8.7% in elongation. Besides, the impact toughness was as high as 14.7?J?cm^?2.     

Book Review

Abstract

Currently available compaction-ready aluminium powders enable sintered preforms to be readily produced by the powder metallurgy route. Aluminium bearing materials with good sliding properties can be produced by sintering-in abrasion-resistant particles or by using alloy powders with homogeneously distributed lead additions. Reactively ground and mechanically alloyed granulates with dispersoid particles of oxides, carbides, and inter-metallic compounds provide high-temperature PM materials with improved properties. New techniques for powder production provide aluminium alloy powders with extraordinary metallurgical effects within the particles and controlled properties. The consolidation of rapidly solidified aluminium alloy powders into high-strength PM semiproducts has considerably enlarged the potential of aluminium powder metallurgy. The aims of numerous worldwide development projects in powder metallurgy are to improve conventional aluminium alloys and develop new alloys which cannot be produced by the . traditional melting route. PM/0253     

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.     

Consolidation and green body characteristics of gelcast metallic powder

Abstract

A preliminary study concerning the compaction of metallic powder was carried out in order to investigate a rarely explored route in powder metallurgy, to form complex geometry parts, known as gelcasting. Green bodies produced with as supplied stainless steel powder showed a tendency to form foam, which affected the surface finishing. The mechanical behaviour of green compacts was also affected by the processing additives present in the metallic powder. Organics in the as supplied powder were removed by thermal treatment at 500°C and additional samples were produced. Although no difference in green density was observed, these samples displayed better surface finish and mechanical characteristics, as a result of improved adhesion between the polymer network and particle surface. The results showed that the gelcasting process is able to produce green parts suitable for subsequent thermal treatment.     

Bulk high hardness Al90Ce2Mn8 alloy prepared by powder metallurgy

Abstract

Using powder metallurgy, bulk high strength Al₉₀Ce₂Mn₈ alloy 25 mm in diameter by 10 mm with near nil porosity has been obtained under certain pressing and heating conditions. The conditions for the best mechanical properties are a pressing temperature of 753-793 K, a pressing time of 30 min, and pressing stress of 1·2 GPa. The compression strength reaches 895 MPa with a hardness of 26 HRC when the alloy is pressed at 753 K. The strength increase is attributed to second phase strengthening and fine grain strengthening.     

Improving pore morphology of PM 316L stainless steels by prealloyed powder prepassivation in 20% nitric acid

Abstract

The effect of prepassivation of prealloyed powder of 316L stainless steels on pore morphology and powder particle shape was investigated. Image analysis technique was used to study the effect of prepassivation, compaction and sintering temperature on the pore morphology of powder metallurgy 316L stainless steels. Porosity, dimension and morphology of the pores were characterised by means of four basic parameters: fraction of surface porosity, equivalent circle diameter, shape factor and elongation factor. In addition, SEM macrographs of powder particles were also investigated by applying the image analysis technique. The Feret’s average diameter and elongation factor were employed to describe the size and roundness of powder particles respectively. Annealing treatment reduced the equivalent circle diameter of pores and simultaneously improved f_shape and f_elong towards higher values. It was proposed that the prepassivation treatment reduced irregularity of powder particles through elimination of sharp corners of powder particles by exposure to acid environment.     

Structuring and Mechanical Properties of Composite Ceramics SiC(C) ― Si3N4, Sintered under High Pressure

We have studied the kinetics of high-pressure sintering of a composite SiC(C) ― Si₃N₄ powder of a certain phase composition. We consider structuring and mechanical properties of the ceramics obtained on the basis of this powder.

     

Review of the Magnetic Properties of Sintered Iron

Abstract

Throughout the past decade a number of new powder metallurgy processes have appeared which offer con siderable promise for superior aeroengine combustor, blade, and disc alloys. Furthermore, several features of these processes can be exploited for improved material utilization such that total manufacturing costs are contained to combat steep increases in basic alloying element prices. The processes include gas atomization of superalloy powder for critical rotating parts, rotating electrode atomization for titanium powder in similar components, controlled thermomechanical processing of attrited powders to produce oxide dispersion strengthened superalloy sheet and airfoil parts, and finally plasma-sprayed gas-atomized powder for advanced hot-section overlay and thermal barrier coatings. Considerable alloy and process development work has already been undertaken on all these systems by aeroengine material suppliers and advantages accruing in terms of superior properties and/or lower processing costs demonstrated at laboratory level and for some cases also in engine service. PM/0186     

Sintering parameters and mechanical properties of injection moulded aluminium powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded aluminium powder. Gas atomised aluminium powder was injection moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for feedstock. Binder debinding was performed in solvent and thermal method. After debinding, the samples were sintered at different temperatures and times in high purity N₂. Metallographic studies were conducted to determine the extent of densification and the corresponding microstructural changes. The results show that gas atomised aluminium powder could be sintered to a maximum 96·2% of theoretical density. Maximum density, tensile strength and hardness were obtained when sintered at 650°C for 60 min.     

Investigation of Microstructure,Mechanical and Wear Behaviour of B<Subscript>4</Subscript>C Particulate Reinforced Magnesium Matrix Composites by Powder Metallurgy

In the present study, magnesium and magnesium matrix composites reinforced with 10, 20 and 30 wt% B₄C particulates were fabricated by powder metallurgy using hot pressing technique. The microstructure, mechanical properties and wear behaviour of the samples were investigated. Microstructure characterization showed generally uniform distribution of B₄C particulates. XRD investigations revealed the presence of Mg, B₄C and MgO phases. The mechanical properties of the investigated samples were determined by hardness and compression tests. Hardness and compressive yield strength significantly increased with increasing B₄C content. The reciprocating wear tests was applied under loads of 5, 10 and 20 N. Wear volume losses decreased with increasing B₄C content. Abrasive and oxidative wear mechanisms were observed.     

On development of press and sinter Al–Ni–Mg powder metallurgy alloys

Abstract

The objective of this research was to initiate the development of powder metallurgy alloys based on the Al–Ni–Mg system. In doing so, binary (Al–Mg) and ternary (Al–Ni–Mg) blends were prepared, compacted and sintered using elemental and master alloy feedstock powders. Research began with fundamental studies on the sintering response of the base aluminium powder with additions of magnesium. This element proved essential to the development of a well sintered microstructure while promoting the formation of a small nodular phase that appeared to be AlN. In Al–Ni–Mg systems a well sintered structure comprised of α aluminium plus NiAl₃ was produced at the higher sintering temperatures investigated. Of these ternary alloys studied, Al–15Ni–1Mg exhibited mechanical properties that were comparable with existing commercial 'press and sinter' alloys. The processing, reaction sintering and tensile properties of this alloy were also found to be reproducible in an industrial production environment.     

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H₂. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.     

Mapping complex microstructures in powder metallurgy steels

Abstract

New methods of characterising multiple phase powder metallurgy steels have been investigated. Mapping of microhardness with a lateral resolution of as little as 5 μm and with up to 10⁴ indents can produce distributions of mechanical properties which are characteristic of different steel grades. Maps of these properties can be correlated with the different phases observed in etched microstructures, and quantification of size and interconnectivity of the regions with, for example, different hardness or stiffness is then possible. Electron backscatter diffraction can also produce simultaneously highly detailed maps of the different phases in a powder metallurgy steel with chemical composition information. Percentages of the different phases present can be determined from the crystallography and morphology of the grains observed, but the technique is limited by the speed with which maps can be acquired.