Effect of compaction pressure and powder grade on microstructure and hardness of steam oxidised sintered iron

Abstract

Steam oxidation has proven to be an effective process to improve the properties of sintered iron components. The oxide formed on the surface and in the interconnected porosity strongly influences both the tribological and mechanical properties of these materials, for example through the extent of pore closure and the nature and morphology of the oxide produced. In this paper, the influences of compaction pressure and powder size on the microstructure, oxide content, hardness, and surface topography of steam treated sintered iron are analysed. Specimens prepared from atomised iron powders of different sizes (<65, 65–90, 90–125, and >125 µm) were compacted at four different pressures (300, 400, 500, and 600 MPa), sintered for 30 min at 1120°C and then subjected to a continuous steam treatment at 540°C for 2 h. A clear influence of the processing parameters on porosity was highlighted. Low porosity was always associated with high compaction pressure and greater powder size. Pore size was affected in the same way by compaction pressure, even though the effect of powder size acted in the opposite sense. Changes in compaction pressure and powder size had no significant effect on pore shape. Decreasing powder size always led to high hardness. The effect of compaction pressure on hardness is clear evidence of a compromise between porosity and blockage of the pore network by oxide. Samples produced with smaller powder sizes showed a continuous decrease in hardness as the compaction pressure increased, although for the large powder size there was a slight increase to a constant value of ultimate hardness. For the intermediate powder size a maximum hardness was obtained as the compaction pressure increased. X-ray diffraction showed that the oxide layer is composed of magnetite and haematite.     

Plasma densification of agglomerated Cr2O3 powder for thermal spray coating

Abstract

Double plasma flame treatments were carried out on spray dried Cr₂O₃ agglomerated powders to increase their apparent density. The powders that were subjected to the first densification treatment didn't show the entirely melted state, and were fully melted only after the second plasma treatment. Plasma densification resulted in powder size decreasing as well as apparent density of particles and also resulted in the fluidity increasing due to the powder melting and surface smoothing effects. However, some parts of the particles after the second treatment showed a hollowed structure, especially for a particle size above 30 µm. The influence of the thermal conductivity of powder and the gas pressure within aggregates exposed to the plasma flame in the particle densification process was discussed in detail. The powder density strongly influenced the structure of plasma sprayed coatings. The dense coatings with high hardness and high bond strength was achieved in the coatings produced from Cr₂O₃ powders after plasma densification.     

Development and characterisation of direct laser sintering multicomponent Cu based metal powder

Abstract

Recent advances in direct metal laser sintering (DMLS) have improved this technique considerably; however, it still remains limited in terms of material versatility and controllability of laser processing. In the present work, a multicomponent Cu based metal powder, which consisted of a mixture of Cu, Cu–10Sn and Cu–8·4P powder, was developed for DMLS. Sound sintering activities and high densification response were obtained by optimising the powder characteristics and manipulating the processing conditions. Investigations on the microstructural evolution in the laser sintered powder show that liquid phase sintering with partial or complete melting of the binder (Cu–10Sn), but non-melting of the cores of structural metal (Cu) acts as the feasible mechanism of particle bonding. The additive phosphorus acts as a fluxing agent to protect the Cu particles from oxidation and shows a concentration along grain boundaries owing to the low solubility of P in Cu and the short thermal cycle of laser sintering. A directionally solidified microstructure consisting of significantly refined grains is formed, which may be ascribed to laser induced non-equilibrium effects such as high temperature gradient and rapid solidification.     

Comparison of flow and shear band structures in oriented,columnar tungsten,single crystal tungsten-tantalum ands intered tungsten heavy alloy ballistic penetrators

Abstract

The penetration of long rods (L/D10) into standard RHA targets differs with rod failure and flow, which produces erosion and deceleration. Oriented, columnar grained W and [001] single crystal W-4%Ta rods exhibit dense shear/shear band flow phenomena consisting of overlapping bands ofpredominantly dynamically recrystallised (DRX) grain structures observed by optical metallography and transmission electron microscopy. This flow is contrasted withWHA (93%W, 5%Ni, 2%Fe) sintered rod penetration characterised by penetrator nose failure in blocks which move on narrow shear bands between the blocks and function like microstructural lubricants. From these comparative observations of residual, penetrated rods, strategies to promote rod penetration seem to involve the development of wide, overlapping bands or layers of equiaxed or preferentially oriented, refined microstructures, which facilitate material flow at high strain rates. Microstructural precursors, such as deformation twinning or the enhancement of recrystallisation or related microstructural issues, promoting frequent or overlapping shear bands, either through alloying or processing routes to control the shear instabilities, seem to provide the best strategies to enhance long rod penetration.     

Effect of particle composition on consolidation of hot briquetted iron

Abstract

The influence of the carbon concentration of directly reduced iron (DRI) powders on the compressibility and fracture strength of hot briquetted iron (HBI) has been studied. Industrially produced DRI, pure iron powder and Fe–C alloy powders (synthetic DRI) were used in the study. It was found that the mechanism of compaction could be attributed to pure yielding. The pressure required to attain a given density increased proportionally with the carbon content. The morphology and phases present in DRI powder had a significant influence on the compressibility. The fracture strength of the compacts increased with increasing carbon content of the DRI powder. These observations are discussed with reference to the current understanding of the mechanisms of compaction and fracture of compacted particulate materials.     

Application of Koistinen and Marburger’s athermal equation for volume fraction of martensite to diffusional transformations obtained on continuous cooling 0·13%C high strength low alloy steel

Abstract

Koistinen and Marburger’s (KM) equation for the variation of volume fraction of athermal martensite y with temperature Tbelow the M _s has been applied to continuous cooling diffusional data. The data consisted of dilatometer curves obtained on continuous cooling of a 0·13%C high strength low alloy steel. The KM equation takes the form, ln(1 - y) = α(M _s - T ). Plots of -ln (1 - y) against temperature for what are thought to be grain boundary ferrite, intragranular ferrite, proeutectoid ferrite, and pearlite give a series of straight lines of increasing slope α. Intersections of these lines give the temperature of transformation points in good agreement with those on the dilatometry curves. Values of α obtained for each transformation are compared with those previously obtained for martensite in plain carbon and alloy steels and ferrite in Fe–9%Ni.     

Mechanical alloying of FeAl intermetallic powder for metal injection moulding process

Abstract

The Fe–48 at.-%Al powder used for powder injection moulding (PIM) was prepared by mechanical alloying in a high energy planetary ball mill and subsequent vacuum annealing. The effects of stearic acid (SA) as the process control agent on powder characteristics were investigated using X-ray diffractometer, laser particle size analysis and scanning electron microscopy. The dependence of solvent debinding efficiency of PIM feedstock on the powder characteristics was also studied. The results indicate that a low SA content for ball milling helps to prepare near spherical, coarse powder particles that exhibit good solvent debinding efficiency. On the contrary, a high SA content leads to thin layered, small particles with poor PIM solvent debinding efficiency. An intermediate SA content such as 1 wt-% makes the ball milled powder combine the advantageous characteristics for PIM process.     

Metal matrix composite reinforced with co-continuous Al–Al2O3 fibres produced in situ by reaction of an aluminium matrix with fossil SiO2 spicules

Abstract

Aluminium metal matrix composite (MMC) reinforced with fossil silica fibres was produced by a powder metallurgy extrusion route. The fibres, silica rich spicules known as spongilites, come from abundant natural geological deposits in Brazil. Further processing the MMC by heat treating at high temperatures converts the silica fibres into an interlaced (Al–Si)/Al₂O₃ microcomposite structure, retaining the original fibre morphology. The new co-continuous microstructure of the fibres is a result of a reduction or displacement reaction, where the silicon released from the silica reduction forms at first a liquid Al–Si phase around the fibres and later on diffuses into the solid matrix. The fine internal microstructure of the fibres was studied by FEG-SEM and optical microscopy both on polished and fractured surfaces. Tensile properties of the MMC before and after heat treatment were measured, showing a considerable increase in UTS. Analysis of the fracture surface of the heat treated ruptured specimens showed necking (bridge formation) in the metallic phase of the fibre and no visible pullout.     

High temperature low cycle fatigue properties of two cast gamma titanium aluminide alloys with refined microstructure

Abstract

Two different cast gamma titanium aluminide alloys with refined microstructures were studied, Ti–45Al–2Mn–2Nb (at.-%) containing 0·8 vol.-%TiB₂ (XD45) and Ti–47Al–2Mn–2Nb (at.-%) containing 0·8 vol.-%TiB₂ (XD47). The fine grained, nearly lamellar microstructure of XD45 shows superior low cycle fatigue (LCF) properties compared with the coarser duplex structure of XD47. The lifetime for both alloys can be attributed to the amount of inelastic strain in each cycle and the difference in life between XD45 and XD47 increases as the strain range decreases. Overall, XD45 exhibits better LCF properties due to a combination of higher yield strength, lower elastic modulus, and smaller sized lamellar colonies. In both XD45 and XD47, a majority of the fracture initiations occurred at the surface or subsurface at or near weak spots in the microstructure. Such weak spots can be defects such as pores/cavities or surface damage but also TiB₂ laths, lamellar colonies oriented perpendicular to the loading direction and debonded gamma grains or grain clusters acting as stress raisers. Both alloys exhibit multiple crack initiation and stable crack growth.     

Friction and wear behaviour of rapid prototype parts by direct metal laser sintering

Abstract

Rapid prototyping and manufacturing (RP/M) is one of the proven tools for product development owing to its advantages such as short product cycle, high quality product and possibility of fabricating functionally gradient materials. Several RP/M techniques do exist. Among them, direct metal laser sintering (DMLS) method is quite popular as near net shape components with high dimensional accuracies can be manufactured. Meager information is available as regards the tribological behaviour of laser built parts, although this is vital in judging the suitability of built-up parts for applications where sliding is inevitable. In the light of the above, the present investigation is aimed at building parts by DMLS technique of RP/M using 50 μm iron powder and characterising its density, microstructure, microhardness, tensile strength, friction and wear behaviour under sliding conditions. The effect of laser speed on the above properties of built-up parts has been studied. The build layer thickness was maintained at 50 μm. Laser diameter of 0˙4 mm, laser power of 180 W and hatch spacing of 0˙2 mm were adopted. However, the laser speed was varied from 50 to 125 mm s^–1 in steps of 25 mm s^–1. Laser speed had a profound influence on density, microstructure, microhardness, tensile strength, friction and wear behaviour of built-up parts. Lesser laser speed resulted in higher density, microhardness, tensile strength and wear resistance while higher laser speed promoted lowering of coefficient of friction.