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.     

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.     

On development of hypoeutectic aluminium–silicon powder metallurgy alloy

Abstract

Powder metallurgy allows for the rapid, automated and efficient production of many different types of automotive components. However, a drawback is the limited selection of readily available light alloy blends. Owing to the wide spread use of aluminium–silicon casting alloys for existing components it is logical to develop aluminium–silicon PM options. Therefore, an experimental hypoeutectic aluminium–silicon alloy was chosen for study and an optimum processing route developed. Tests were performed to determine the green strength and density as a function of compaction pressure. Sintering conditions were optimised based on sintered density, hardness and dimensional changes. Metallography, differential scanning calorimetry and energy dispersive X-ray spectroscopy analysis provided insight into post-sinter furnace cooling and heat treatment parameters. An appropriate T6 heat treatment was developed and samples were tested in tension. The alloy was able to achieve a high sintered density approaching 98% and a yield strength of 232 MPa under the T6 condition.     

Sintering behaviour of Al-6061 powder produced by rapid solidification process

Abstract

Complex aluminium alloy components fabricated by powder metallurgy (P/M) offer the promise of a low cost and high strength-to-weight ratio, which meets the demands of the automotive sector. This paper describes the die compaction and sintering response of an atomised Al-6061 alloy powder containing Mg and Si produced by rapid solidification. A design of experiments is used involving three levels for each of the die compaction pressure, sintering temperature, peak temperature hold time and heating rate. Three trials were used to obtain the optimum press sinter processing conditions. Besides the mechanical properties, phase transformation and microstructure are investigated. Supplemental insight is gained through thermogravimetric analysis, differential scanning calorimetry and SEM with energy dispersive spectroscopy. Analysis of variation is used to quantify the contribution of each design variable to the mechanical properties.     

EFFECT OF ADDITIONAL ALLOYING ELEMENTS ON THE PROPERTIES OF SINTERED MANGANESE STEELS

Abstract

Sintered alloys based on the Fe-Mn system have been investigated by using single-pressing and double-pressing techniques. Fe-Mn (Mn up to 8 wt.-%) and Fe-Mn-C (C up to 1·4 wt.-%) alloys were prepared both with manganese as an electrolytic powder and with a Fe-Mn master alloy. The influence of sintering temperature and sintering time on mechanical properties and homogenization is discussed. The effect of the additional alloying elements Cr, Mo, eu, and of their combinations on mechanical properties has been determined. Further investigations were carried out with a Fe-Mn-Cr-Mo-C master alloy. The optimum single-pressed and double-pressed alloy (Fe with Mn 0·8, Cr 0·8, Mo 0·8, and total C 0·6%) has a tensile strength (σ_B) of >700 N/mm². Optimum alloys of all investigated systems were hot-forged and their mechanical properties are compared with those of single- and double-pressing techniques. The alloys were heat-treated and their tempering behaviour determined. Jominy standard tests were carried out to determine hardenability of the porous sintered materials.     

铝材令汽车制造技术焕然一新

文章阐述了铝合金在汽车领域中越来越广泛地被应用的发展趋势,及各类铝合金在制造各种汽车零部件上的应用情况及其前景。     

PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids

Abstract

A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640^°C for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ≥340 MPa. Near full sintered densities of , ～98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys.     

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

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     

HIGH-STRENGTH SINTERED IRON-BASE ALLOYS BY USING TRANSITION METAL CARBIDES

Abstract

A simple method of producing alloy steels by mixing, pressing, and sintering iron powder with carbides of transition metals is described. The carbides must have a high carbon content and be soluble in iron. These characteristics are found in the carbides of chromium, tungsten, molybdenum, and vanadium. Tensile strengths of ～800 N/mm² have been achieved in annealed specimens having carbide additions of 1–12 wt.-%, depending on the carbide. The additions produce steels with partly air-hardening properties.

The effects of concentration and particle size of the carbides, sintered density, sintering conditions, and cooling rate on mechanical properties have been determined. The alloys are not very sensitive to the sintering atmosphere. The effect of heat-treatment on the mechanical properties of Fe-Cr₃C₂ alloys is reported. A tensile strength >1300 N/mm² was observed with 4 wt.-% Cr₃C₂. Dilatometric measurements were conducted with different carbide concentrations and heating conditions to study the sintering process. The important process of homogenization was investigated by electron microprobe analysis.     

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.     

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.     

Influence of powder characteristics on sintering behaviour and properties of PM Ti alloys produced from prealloyed powder and master alloy

Abstract

The use and development of titanium and titanium alloys have been strongly correlated to high technology industries where costs are not the most important aspect. Titanium could see its market grow by the application of lower cost and more efficient processing methods such as powder metallurgy. This work deals with the characterisation of two types of powders: commercial prealloyed powder and powder produced from master alloy combining mechanical milling and conventional blending to adjust the particle size. The characteristics of the powders, sintering behaviour and final properties of the parts indicate that the master alloy approach leads to better compressibility than the prealloyed powders and, therefore, to lower dimensional change during sintering. The most important result is that it is possible to obtain Ti alloys with properties similar to or better than alloys from prealloyed powders and to obtain homogeneous microstructures, which allows the composition to be adjusted to requirements.     

Effect of boron on microstructure and mechanical properties of PM sintered and nitrided steels

Abstract

PM Distaloy and Astaloy alloys have many applications in the automotive industry and are used in structural elements with different wear resistance values. Their main features are adequate density, hardness, tensile strength, and good ductility. For the purpose of the experiment presented here, Distaloy SA and Astaloy Mo powders, alloyed with various amounts of elemental boron powder, were used. The Distaloy and Astaloy alloys were produced through mixing, compacting, and sintering at t=1393 and 1473 K, and, after the completed sintering process, they were plasma nitrided at 793 K. Experimental results showed that if boron was added, while sintering, the shrinkage phenomena increased ( 1473 K) and some parameters of those alloys (density, hardness, and tensile strength) were improved. Upon the ion nitriding treatment of the surface of base Astaloy Mo samples, a surface layer was created composed of the ε solution and γ' nitride, whereas the surface layer on the Distaloy SA base was mainly composed of a γ' compound. Boron activates the sintering process of Distaloy SA and Astaloy Mo samples but it has no significant impact on the surface layer's thickness of Distaloy SA alloys as opposed to Astaloy Mo alloys in which boron promotes a greater thickness of surface layers along with a reduced depth of nitrogen diffusion.     

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     

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.     

THE DEVELOPMENT OF VACUUM-SINTERED STEELS

Abstract

The work described is concerned with the development of vacuum-sintered ferrous materials that can be used for high-strength structural parts. The reasons for choosing this production route are given, and the development of the materials is traced from the simple iron–carbon alloys to the more complex iron–carbon–nickel–chromium alloys. The metallography of the various series of alloys is included and the mechanical properties are fully reported. Tensile strengths > 70 tons in ^?2 were obtained after heat-treatment. An indication of the dimensional changes that took place during sintering is also given.     

Consolidation of diamond tools using Cu–Co–Fe based alloys as metallic binders

Abstract

The present work reports the results obtained on the evaluation of both the response of commercially available Cu–Fe–Co pre-alloyed powders during processing for the consolidation of diamond tools under several fabrication routes, and the performance of the tools during in field cutting operations. The new metallic binders look attractive since they combine good sinterability with adequate values of hardness and wear resistance, as observed from the cutting tests carried out on granite and marble. The consolidation routes under research were conventional hot pressing, pressureless sintering, and hipping after sintering. The amount of copper contained in these alloys leads to fully dense parts by hot pressing at temperatures approximately 150°C lower than those used for cobalt. Pressureless sintering experiments showed that full densification requires the application of hipping post-sintering treatments. During processing under non-reducing atmospheres, the presence of metallic oxides contained in the powder was observed to exert a direct influence on the degradation of the diamond grits.     

Reactions between ferrous powder compacts and atmospheres during sintering – an overview

ABSTRACT

This overview paper describes the interaction of powder metallurgical iron-base alloys with the atmosphere during sintering. The methods of thermal analysis serve to clarify the processes that take place especially during the heating stage of the sintering cycle. After a discussion of the physical and chemical fundamentals of the sintering process, the methods of thermal analysis are explained. The differences between plain iron and alloyed systems are discussed in detail. Classical PM low alloy steels with alloying elements, such as Cu, Ni and Mo, react in a similar way as unalloyed carbon steels. The situation changes dramatically, when oxygen sensitive elements as chromium, manganese and even more silicon come into play. The removal of the surface oxygen is much more crucial, and there are several competing reactions, which have to be considered when these systems should be sintered in industrial scale to reach the desired mechanical and dimensional properties.     

Unidirectional Solidification of Eutectic Cast Iron

Abstract

High intrinsic damping in alloys finds many applications in the elimination of unwanted vibrations and reduction of acoustic noise. The increasing interest in these materials has resulted in the development of a new class of engineering alloys, the HIDAMETS (high-damping metals), specifically tailored for use in vibration-resistant structures. Numerous different physical mechanisms (usually controlled by the alloy microstructure) can be responsible for the level of internal friction or damping in a candidate alloy for a particular application. Consequently, alloy selection depends mainly upon the conditions to be encountered in service, i.e. temperature, frequency of vibration, amplitude of vibration, magnetic field, etc. Characterization of the internal friction in high damping alloys is described and some of the mechanisms involved are discussed. Results obtained for two different alloys are presented. The first is a commercial Mn-Cu alloy used for marine propellers and the second is a die-cast Zn-Al alloy with possible application in the automotive industry.