EURO PM2009 in cool Copenhagen

Abstract

The effect of additions of silicon powder on the sintering behaviour and microstructure of compacted 304L stainless powder has been studied. The shrinkage ratio increases substantially with silicon content. Silicon profoundly activates the sintering process through the formation of a eutectic and/or δ ferrite, which is pseudoperitectically formed during sintering. The sintering behaviour is closely related to the microstructures, which depend upon the amount of silicon addition. Ostwald ripening is encountered in the liquid phase sintered specimens (Si≤3 wt-%). The solid phase sintered materials (Si≥ wt-%) containing δ ferrite densify more rapidly than the liquid phase sintered ones. The densification kinetics are governed by the wetting characteristics of the eutectic liquid and the formation of ferrite. As a result of the silicon addition, the austenitic stainless steel powder aggregates are sintered into duplex stainless steels with austenite-ferrite structures. PM/0395     

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.     

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.     

Effect of copper infiltration on fracture mode in sintered steels

Abstract

The fracture mode of PM steels depends on features, such as pores, densification, diffusion of any added alloying elements, contact area between particles, microstructural homogeneity, and applied load conditions. Consequently, when a sintered steel is infiltrated, several factors that determine the fracture behaviour are affected. The density is raised (in absolute terms, because the infiltrant is usually of higher density, and in relative terms, because activated sintering is promoted by permanent liquid phase sintering). The liquid phase promotes a rounding of the pores which enhances stress transmission from the necks to the particles. Sintered steel with 0˙6%C was infiltrated with three amounts of copper to study the influences of the amount of infiltration and the resultant final density on the fracture mode. Specimens were first uniaxially pressed to 7˙0 g cm^–3 green density, then sintered and infiltrated simultaneously at 1120°C in a 90N₂–10H₂ atmosphere. The mechanical results and microstructures were analysed to evaluate the fracture behaviour and fracture surfaces. The mechanical behaviour was characterised by hardness, tensile and three point bending tests.     

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.     

CHARACTERIZATION OF THE DEGREE OF MIXING IN LIQUID-PHASE SINTERING EXPERIMENTS

Abstract

Homogeneity of mixing plays an important role in liquid-phase sintering. In order to describe quantitatively the dependence of shrinkage on the degree of mixing, six different tungsten-copper powder mixtures were prepared. These powder blends were either presintered or sintered in the presence of liquid phase. The degree of mixing in both the presintered and liquid-phase sintered samples was determined by quantitative microstructural analysis. The developed method is well able to characterize the different state of mixing in the six powder blends on a microscopic scale. It is shown that a close relation exists between the microscopically characterized degree of mixing and densification during liquid-phase sintering.     

Microstructure evolution and densification behaviour of powder metallurgy Al–Cu–Mg–Si alloy

ABSTRACT

An Al–Cu–Mg–Si alloy was prepared by conventional press-sintering powder metallurgy using elemental Al powder. The phase transformation process of Al–Mg, Al–Si alloy and Cu during the sintering process was investigated in details. It was found that a series of phase transitions take place in the alloy to disrupt the oxide film of Al particle and enhance the densification process. The relative density of the sintered samples reached 98%. A new Al–Mg–Cu–O compound was found at the grain boundaries except the MgAl₂O₄ phase, it is speculated that the disruption of the oxide film was also associated with the other alloy compositions except for Mg. Furthermore, no detectable AlN compound was found at the grain boundary region although sintering with flowing nitrogen atmosphere, which is benefit from the high density of the green compact and the excellent wettability between the liquid phase and the aluminium.     

Rapid sintering of iron powders under action of electric field

Abstract

A new rapid sintering technique for iron powders compacted under the action of an electric field with high current density has been advanced. The results show that the sintering densification of iron powder could be finished in less than 6 min at a temperature of 800^u C reached at a heating rate of 600 K s^?1, and the relative density of the sintered compact was over 95%. Moreover, the sintering densification was almost finished in the heating stage of the compact.     

Effect of residual carbon on the sintering process of M2 high speed steel parts obtained by a modified metal injection molding process

In this article, we present the evolution of the microstructure during sintering of M2 high speed steel (HSS) parts obtained by a modified powder injection molding (PIM) process, which uses a new binder system based on a thermosetting resin. The most important characteristics of this process is that molding is carried out at room temperature by pouring the slurry (resin and tool steel previously mixed) directly into the mold. The mold is then heated to the curing temperature of the resin. The best mixture of polymer and steel powders was 60 pct volume of metal powder. The resin was removed by thermal debinding. The sintering process was carried out under vacuum atmosphere. We tested different debinding temperatures in order to retain residual carbon in the samples coming from the thermal degradation of the polymer. The best results were obtained at low debinding temperature (300 °C). In this case, residual carbon had a beneficial effect, extending the sintering temperature range by 100 deg, making it possible to reach very high density at temperatures as low as 1100 °C. The mechanism of this densification seems to be via supersolidus liquid phase (SPLS). The microstructural study of sintered parts revealed a homogeneous distribution of carbides that change their morphology with increasing temperature. Besides spherical M₆C carbides, which appear in all the temperature ranges studied, a new rodlike M₂C carbide appears.     

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.     

Diary

Abstract

The addition of amorphous Fe–Si–B particles to Fe powder increases the shrinkage of sintered components resulting in higher densification rates. Consequently, several research groups worldwide have studied the properties of such systems in an attempt to produce superior structural alloys. In the present work, Fe₇₅Si₁₀B₁₅ ribbons obtained by melt spinning were milled in a high energy Spex mill for times varying from 2 to 32 h. The resulting powders were characterised by differential thermal analysis and X-ray diffraction. The results showed that the amorphous characteristics of the ribbons persisted after the milling process. Next, samples consisting of a mixture of Fe powder and 4 wt-% milled amorphous phase were uniaxially pressed and sintered following a series of thermal cycles. High temperature microstructures were obtained for compacts subjected to rapid cooling from the sintering temperature. The results of scanning electron microscopy and energy dispersive spectroscopy revealed substantial precipitation of fine Fe₂B particles before α → γ allotropic transformation. In addition, an oxide phase was observed in the interface between Fe and the additive particles. Preliminary analysis suggested that the oxide particles can be easily reduced by adding small amounts of carbon to the system. PM/0765     

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.     

Grain growth and densification during liquid phase sintering of W-Ni

Mixtures of single crystalline spherical W particles of 200–250 μm dia., fine W powder of 10 μm size and fine Ni powder were sintered at 167°C. Depending on the amount of liquid phase present complete densification is obtained after different sintering times and this is always accompanied by pronounced growth of the large W spheres at the expense of the smaller grains. With limited amounts of liquid phase present, under the influence of the densification force arising from the pores, grain growth occurs preferentially away from the areas where the grains are in close contact. Thus grain shape accommodation occurs during growth and this enables complete densification. In order to achieve rapid densification during liquid phase sintering, conditions of rapid grain growth are therefore favourable.     

Characterisation of sintering of alumina matrix–stainless steel dispersion composite and interaction between chromium,carbon and alumina during powder metallurgy process

Abstract

This paper aims to study the sintering of 316L stainless steel and alumina composites. Compositions range from 0 to 100 vol.-% steel, and the experimental procedures involve density and microstructure analysis of the samples, as well as dilatometric measurements. In this study, it is shown that reducing atmosphere debinding can lead to carbon residues. These have a negative effect on alumina densification by delaying the sintering onset. For metal–ceramic composites, densification is modified by a complex interaction involving carbon (which lowers alumina density), chromium oxide (which is documented in literature to diminish alumina densification) and stainless steel phase. Chromium carbide formation is possible for some experimental conditions (1–30% stainless steel and hydrogenated argon debinding); this mechanism, locking both carbon and chromium outside alumina phase, leads to higher sintered densities.     

Liquid phase sintering of ferrous powder by carbon and phosphorus control

Abstract

Powder mixtures composed of liquid forming master alloy powder and coarse iron powder were sintered to near full density by having a high amount (20 wt-%) of liquid phase during sintering. This was made possible by the use of the Fe-P-C system with or without Cu. Without post-sintering treatment, a brittle microstructure was obtained. By means of altered C and P control and decarburisation heat treatment of the as sintered material, the final non-brittle microstructure was achieved. Using the open porosity and liquid phase as a diffusion path, rapid decarburisation is created and the local combination of carbon and phosphorus in the microstructure is avoided. In this way, iron phosphide is not formed on grain and/or particle boundaries. Presence of pores is confirmed to be beneficial for grain growth control.     

Starch consolidation of M3/2 high speed steel powder – influence of microstructure on mechanical properties

Abstract

The influence of microstructure on the mechanical properties of starch consolidated super solidus liquid phase sintered AISI type M3/2 high speed steel powder has been evaluated. Hardness measurements, Rockwell C indentation and scratch testing were used to evaluate the mechanical properties and light optical microscopy and scanning electron microscopy were used for post-test characterisation. The results show that it is possible to starch consolidate and sinter large particle size high speed steel powder to obtain microstructures with high mechanical strength. However, the results show a strong correlation between the as sintered microstructure and the resulting mechanical properties and illuminate the importance of having a dense and isotropic microstructure in order to meet engineering requirements in demanding applications. Consequently, the failure mechanisms observed during indentation and scratch testing can be related to residual pores, present in the low temperature sintered samples, and a coarse microstructure with eutectic carbides, present in the high temperature sintered samples.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

SLIP-CASTING TUNGSTEN CARBIDE/COBALT POWDER MIXTURES

Abstract

By utilizing a correctly constituted suspending medium and binder, stable slip-casting powder mixtures of tungsten carbide and cobalt were prepared. The stability of the suspension was governed by the pH of the medium; casting rate was dependent on the formulation of the slip, and particularly on the solid concentration. Close control of these variables permitted the use of plaster of Paris moulds for casting sound, dense bodies of the powder mixtures, which were sintered for final densification without the appearance of any defects.     

The founding of EPMA

Abstract

Liquid phase sintering is commonly used in powder metallurgy to improve physical properties through densification enhancement. With the aim of combining the advantages of liquid phase sintering and the use of promising alloying elements such as Mn and Si, liquid promoters with complex compositions were designed to provide a low melting point to form a liquid phase below the common sintering temperatures. The properties of these liquid phases were characterised in terms of contact angle, spreading evolution and infiltration. Using a Krüss drop shape analysis system, both wetting angle experiments and infiltration experiments were performed by changing the substrate characteristics from sintered to green iron specimens respectively. The discussion is based on the different features found for these liquids compared with copper, which is a well known liquid phase former used for improving the properties of low alloy steels. Simulations of the thermodynamic and kinetic processes taking place were performed by combining ThermoCalc and DICTRA software analysis.     

Supersolidus liquid-phase sintering of prealloyed powders

A model is derived for the sintering densification of prealloyed particles that form internal liquids when heated over the solidus temperature. The model considers the powder size, composition, and microstructure, as well as the processing conditions of green density, heating rate, maximum temperature, hold time, and atmosphere. Internal liquid forms and spreads to create an interparticle capillary bond that induces densification during sintering. Densification is delayed until the particles achieve a mushy state due to grain boundary wetting by the internal liquid. This loss of rigidity and concomitant densification of the semisolid particles depends on the grain size and liquid quantity. Viscous flow is the assumed densification mechanism, where both viscosity and yield strength vary with the liquid content and particle microstructure. Densification predictions are compared to experimental data, giving agreement with previously reported rapid changes in sintered density over narrow temperature ranges. The model is tested using data from steels and tool steels of varying carbon contents, as well as boron-doped stainless steel, bronze, and two nickel-based alloys.