Investigation of warm compaction and sintering behaviour of aluminium alloys

Abstract

The effects of warm compaction on the green density and sintering behaviour of aluminium alloys were investigated. Particular attention is paid to prealloyed powders, i.e. eutectic and hypereutectic Al-Si alloys, regarding their potential applications in the automotive industry. The effects of chemical composition, alloying method, compacting temperature and the amount of powder lubricant were studied. The compaction behaviour was examined by an instrumented die enabling simultaneous measurement of density, die wall friction coefficient, the triaxial stresses acting on the powder during the course of compaction and ejection pressure. The sintering behaviour was studied via dilatometeric analysis as well as normal batch sintering. The results show that warm compaction could be a promising way to increase the green density of aluminium alloys, especially prealloyed powders, and to decreased imensional instability during sintering. Moreover, it reduces the sliding friction coefficient and the ejection force during the powder shaping process. This paper presents the significant advantages and drawbacks of using the warm compaction process for commercial PM aluminium alloys.     

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.     

DIMENSIONAL CONTROL IN HARD-METAL MANUFACTURE

Abstract

All hard-metal alloys contract ～50% by volume during sintering and, in addition, the continuous network of liquid phase makes them extremely weak. Hence, the two major problems in dimensional control are coping with this large contraction and at the same time preventing distortion during sintering.

Green compacts of closely controlled and very uniform density are essential to ensure that the contraction is both predictable and uniform. Methods of predicting contraction and of powder preparation and pressing to achieve this end are described.

Distortion results not only from mechanical weakness during sintering but also from the carburizing, decarburizing, oxidation, and reduction reactions that occur when the green compact is heated.

In conclusion, indications are given of the order of dimensional accuracy that can be expected in hard-metal manufacture.     

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.     

Novel technique for synthesis and consolidation of aluminium nitride nanopowders

Abstract

In this paper, the use of a microwave plasma method for the synthesis of aluminium nitride nanopowders is described. The powders were consolidated to near theoretical densities using the unique rapid consolidation technique, plasma pressure consolidation (P²C), developed by MMI. Rapid consolidation of nanopowders is an ideal requirement for better mechanical and thermal properties in the consolidated part, as it retains the fine microstructure preventing anomalous grain growth. Microwave plasma synthesis resulted in aluminium nitride nanopowders (85–200 nm), which were consolidated to near theoretical density using P²C in <5 min without sintering additives. The effect of yttria (3 wt-%) as a sintering additive on the thermal conductivity (TC) of aluminium nitride was also evaluated and compared with TC values obtained from additive free AlN consolidated samples.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Variability of powder characteristics and their effect on dimensional variability of powder injection moulded components

Abstract

In this study, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel. Three lots of gas atomised and three lots of water atomised powders were characterised and used to make six batches of PIM compound. These compound lots were injection moulded using a cavity pressure transducer and screw position regulation controls. The moulded geometry was measured in the green state and sintered state for dimensional variability. The general findings are that gas atomised powder produce less dimensional variability than the water atomised powder from lot to lot, however, the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering. Also, the lot to lot variation in the powder characteristics, such as particle size and pycnometer density, have an effect on dimensional stability whereas variations in powder characteristics such as surface area, tap and apparent density, and chemistry have little effect on dimensional stability.     

Effect of Mg on sintering phenomenon of aluminium alloy powder particle

Abstract

The effect of Mg on the sintering phenomenon of aluminium alloy powder particles has been examined using XPS analysis of the chemical reaction at the top most surface of the particle during heating. The relative density of the sintered material increases by 9% according to the increase of Mg content. The mechanical properties of the sintered material also increase remarkably as the Mg content in the particle increases. The ratio of the dimple patterns observed at the fractured surface after the tensile test also increases. It is considered that Mg acts to deoxidise the Al₂O₃ film that covers the particle surface as a barrier and helps sintering between the particles.     

FACTORS AFFECTING THE RADIAL AND AXIAL SHRINKAGE IN SOFT-METAL POWDER COMPACTS

Abstract

The variables affecting the radial: axial (R/A) shrinkage ratio in copper-powder compacts have been investigated. The value of R/A is linearly dependent on compacting pressure, green density, and sintering temperature, and also increases with decrease in the particle size of the powder. The observed variation of R/A is attributed to the differences in density in the green compacts, which result in anisotropic stresses in sintering. Surface-tension forces or residual stresses introduced during compaction cannot alone be regarded as the main driving forces responsible for shrinkage; anisotropic stresses also play an important role in the densification of metal-powder compacts. By proper control of these variables, parts can be produced from the compacts to close dimensional tolerances.     

The role of pore evolution during supersolidus liquid phase sintering of prealloyed brass powder

ABSTRACT

The aim of this research is to study the pore structure as well as to assess the liquid phase sintering behaviour of Cu-28Zn powder specimens at different green density levels and temperatures. For this purpose, samples were compacted to obtain six different green densities and then sintered at 870°C, 890°C and in part at 930°C for 30?min. The results revealed that the spherical pores which are formed inside the grains can be swept by grain boundaries due to grain growth and join to primary pores so that secondary intragranular pores are eliminated and intergranular pores enlarged at higher temperatures. Also, the pores move upwards to the top of sample due to buoyancy forces. The role of pore structure in distortion is more tangible at higher temperatures (930°C) so that O-shape and X-shape distortions were observed at high and low green density samples, respectively.     

Combined effects of time and temperature on strength evolution using integral work-of-sintering concepts

Sintered materials have significantly higher strength than green compacts. The evolution of that strength during the sintering cycle involves a combination of annealing, thermal softening, and sintering events. The dynamic interplay between heating rate, sintering time, and sintering temperature controls the in situ strength and determines the final sintered strength. Although sintered strength is a well-explored subject, the dynamic evolution of strength requires new models. This research has measured both the sintered and in situ strengths as functions of heating rate, hold times, and temperature for die-compacted prealloyed bronze powder. A core concept is the use of an integral work of sintering to determine the effective strengthening due to sintering. The model is used to map strength evolution vs the key processing parameters. It is concluded that, during solid-state sintering of bronze, the key sources of distortion are the density and thermal gradients.     

Modeling of distortion after densification during liquid-phase sintering

Liquid-phase sintering (LPS) is a technique widely used to sinter hard and heavy metals such as tungsten carbide and tungsten heavy alloys. LPS involves formation of a liquid phase during sintering that promotes fast densification. However, the ratio of liquid to solid, microstructure and external forces (gravity, component/substrate friction) act to promote distortion as a function of sintering time and temperature. To understand and control distortion during LPS, a numerical model is being developed to solve continuity and momentum equations using a finite-element technique. In this article, transient distortion under gravity is calculated as a function of surface tension, density, and viscosity of the material. The effect of the friction force due to the component support during isothermal sintering is also evaluated and compared with experimental data acquired by in-situ recording of distortion during sintering.     

New insights into influencing variables of water atomisation of iron

Abstract

Trace amounts of surfactants have an acute influence on measured surface tension of melts and may influence viscosity. A water atomisation experiment was performed to investigate if variations of these elements could affect quality. Effects of water pressure, melt superheat, and sulphur content, iron scrap oxygen content, and aluminium content were studied. Responses studied were particle size distribution, apparent density, flow, powder chemistry, morphology, green density, and dimensional change. A large sulphur addition reduced the particle size, as a result of a reduction of surface tension, but the largest effect came from changing water pressure. Higher water pressures also resulted in powders with lower apparent density, lower flowrate, and reduced swelling during sintering. An empirical water atomisation model is proposed. Aluminium additions reduced the powder size standard deviation and increased the carbon content of the powder. A reduced powder size standard deviation was seen also for melts with raised superheating.     

Pressing vacuum sintering of multipowders for manufacturing novel engine valve seat on Gleeble 1500 simulator

Abstract

Patented Fe based multipowders for manufacturing novel engine valve seat were compacted and sintered using a Gleeble 1500 thermal simulator system. Microstructures and properties of the sintered alloy have been studied by hardness test, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) analysis. Results indicate that a sintered alloy with a hardness of 38 HRC and a density of 7·23 g cm^?3 can be obtained when the green compacts are sintered at 1240°C for 30 min. It is found that lattice parameters of the alloy matrix increase with increasing sintering temperature. Additionally, the investigation of green compacts' shrinkage during sintering shows that there is a threshold value for the density of the green compacts. When the density of the green compacts is lower than that value, the density of the sintered alloy remains almost constant with increasing density of green compacts. An equation to calculate the threshold density has been suggested as νρ² = C ₁ ρ² + C ₂ m² + C ₃ m ρ.     

Wear resistant sintered aluminium parts for automotive applications

Abstract

The automotive industry's focus on weight saving has increased interest in optimised process routes and alloy compositions for light alloy PM components. Conventional pressing, sintering and sizing of aluminium alloys containing about 16%Si has been applied to produce components with high wear resistance and mechanical strength. CISIZE^®, a novel continuous isostatic pressure sintering process, combined with sizing, produces aluminium alloys with finely dispersed Si particles having excellent ductility. Combining parts obtained by these two process routes can give interesting tribological systems. Complete sintered aluminium cam phaser systems, including the sprocket wheel, are being produced in series using this approach, which also shows promise for automotive parts such as oil pumps and rotors.     

THE EFFECT OF INTERPARTICLE CONTACT AREA ON THE STRENGTH OF COLD-PRESSED ALUMINIUM POWDER COMPACTS

Abstract

Measurements of the tensile strength of spherical cold-pressed aluminium powder, pressed to various densities up to the theoretical maximum, have shown that compaction is a two-stage process. At some high, intermediate pressure, interparticle sliding occurs in a way that does not itself increase densification but makes it easier for further deformation to occur. It is likely that the pressure at which this sliding takes place is dependent on the work-hardening rate of the powder as well as the powder size and morphology. In any case, it is shown to be important to the densification and strength reached by the compact.

It is concluded that the strength of a green compact is dependent upon the interparticle metallic contacts made during compaction. However,the green strength is well below that of wrought aluminium, probably due to the presence of broken-up oxides,which act as stress-concentrators at the interparticle boundaries.     

Phase transformations in reactive sintering of tungsten

Abstract

It has been demonstrated recently that tungsten (T _m = 3410±20°C) can be sintered by reactive sintering in a reductive atmosphere such as hydrogen. This alternative technique to the conventional sintering (T _s>2000°C) makes use of a small amount of aluminium addition which acts as a sintering aid and hence lowers the sintering temperature significantly (T _s1200°C). This study explores the phase transformations that take place during reactive sintering of tungsten in view of the mechanisms involved. DSC, SEM and TEM have been used for a fundamental understanding of this system.     

Formation of aluminium nitride during sintering of powder injection moulded aluminium

Abstract

A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed.     

THE FORMATION OF URANIUM AND OF BERYLLIUM ALLOYS BY THE SOLID-STATE SINTERING OF MIXED ELEMENTAL POWDERS

Abstract

Large volume expansions accompany the formation of binary alloys of beryllium with uranium, thorium, iron, copper, zirconium, titanium, and vanadium, and of uranium with aluminium, during the sintering of the mixed, cold-compacted elemental powders. No expansion was detected during the sintering of binary mixtures of beryllium with aluminium, silicon, and magnesium, or mixtures of uranium with zirconium, molybdenum, iron, nickel, manganese, and chromium.

When it occurs, expansion is anisotropic, being greatest in the direction of compacting; the degree of anisotropy varies with the constituents and the composition of the alloys. In systems undergoing expansion, the volume expansion/composition graphs exhibit maxima. For a given system the magnitude of the maximum is a function of the shape of compact, the particle size of the powders, and the sintering time and temperature; the composition at which the maximum occurs is sensibly unaffected by these latter variables.

These experimental observations, together with those of other investigators, can be satisfactorily interpreted on the hypothesis that volume expansion is due to the formation of diffusional porosity during sintering.