J. Duszczyk

Characteristics of rapidly solidified Al-Si-X preforms produced by the Osprey process

This paper describes some characteristics of an Al-20Si-X aluminium alloy, processed by the Osprey route, in terms of total oxygen content, porosity distribution and microstructure. A theoretical analysis of the solidification of the material, after a semi-liquid/semi-solid spray of atomized droplets-particles impacts the deposit, is presented. A heat flow calculation was conducted applying the forced convection method at quasi-steady conditions. Based on the calculation of the heat transfer coefficient the cooling rate was estimated within the range ∼102 to 104 K sec−1. Stroehlein OSA-MAT measurements showed that the total content of oxygen of the Osprey preform was 3.5 and 7 times lower than the corresponding values for argon (nitrogen) and air atomized Al-20Si-X powders, respectively. Light microscopic examination of the deposited material revealed a homogeneous microstructure with a porosity level as low as 1.3%. Microstructural features indicated that the Osprey process provided rapidly solidified material with an average cooling rate of 103 to 104 Ksec−1. This cooling range proves that the theoretical estimation presented in this work is sufficiently accurate.

Characteristics of rapidly solidified Al-Si-X powders for high-performance applications

Among the variety of new aluminium alloys, the Al-Si-X P/M system appears to be the most suitable for high-performance applications in the automobile industry. Our work concerns the research on the possible application of this system for products with enhanced wear and high-temperature resistance. This paper presents the characteristics of the air-atomized J1 (Al-20Si-3Cu-1Mg), J2 (Al-20Si-3Cu-1Mg-5Fe), J3 (Al-20Si-3Cu-1Mg-7.5Ni), K1 (Al-20Si-5Fe-2Ni), and the argon-atomized K2 (Al-20Si-5Fe-2Ni) powders, aimed at optimizing the processing conditions of the final products, in terms of production techniques, powder morphologies, powder sizes and size distributions, cooling rates, specific areas, surface oxide thicknesses and oxygen contents. Atomization in air (J1, J2, J3, K1) and atomization in argon (K2) resulted in morphologically different powders. Particle-size distributions were similar, indicating cooling rates of ∼104 to 106 K sec−1. This cooling range proves that the theoretical estimate presented in this work is sufficiently accurate. Al-Si-X P/M alloys consisted of primary and eutectic silicon crystals in an aluminium matrix (J1) plus intermetallic compounds (J2, J3, K1, K2). Air-atomized powders with different chemical composition showed an average oxide thickness of ∼30 to 40 nm. In powders with equal chemical composition, an inert atomization atmosphere produced powders with smaller surface area, lower amount of oxygen, and thinner total oxide thickness. The composition of surface oxides was strongly influenced by the chemical composition but the thickness was mainly influenced by the atomization atmosphere.

Microstructural features and final mechanical properties of the iron-modified Al-20Si-3Cu-1 Mg alloy product processed from atomized powder

The potential piston alloy Al-20Si-5Fe-3Cu-1Mg has been experimentally extruded from rapidly solidified powder, and subsequently heat treated. The effects of adding iron to the alloy on the microstructural evolution during the solution and ageing treatment subsequent to extrusion have been examined. The study shows that iron-bearing intermetallic particles modify the recrystallization behaviour of the present alloy during solution treatment at 470 °C in a complex way, through blocking the migration of recrystallized grain boundaries from particle-depleted areas, and pinning subgrain boundaries in particle-rich areas, thus leading to a partially recrystallized duplex structure in the final product. The observed two-fold role of the intermetallic particles is a consequence of their inhomogeneity in distribution, which in turn results from the processing history of the powdered alloy. It is also observed that, in the presence of the intermetallic particles, the excessive coarsening of the silicon particles dispersed in the α-Al matrix (as occurs to the base alloy during the heat treatment) is lessened. The retained subgrain boundaries provide heterogeneous nucleation sites for precipitation occurring during ageing. Most of the precipitates are characterized by being associated with iron, and the precipitating behaviour of copper and magnesium in the present alloy with the iron addition is accordingly altered. The resultant tensile properties of the alloy at room and elevated temperatures have been assessed, with reference to those of the base AI-Si-Cu-Mg alloy. The results indicate that the present alloy with the iron addition has a fairly high hot strength up to a temperature of 300 °C, which offers an important improvement ensuring its reliable application in automotive engines.

Structural development during the extrusion of rapidly solidified Al-20Si-5Fe-3Cu-1Mg alloy

An investigation concerning the changes of powder structure and microstructure during the extrusion of an important Al-Si-Fe-Cu-Mg alloy prepared from rapidly solidified powder has been carried out. The fragmentation of needle-shaped intermetallics in the alloy has been regarded as one of the main features of the process, which happens concurrently with the interparticle bonding and the shaping of the porous billets. The as-extruded microstructure is found to be mainly composed of the dynamically recovered α-Al matrix with numerous microcells, which are retained because of the inhibiting effect exerted by massive, fine second-phase particles on cell wall motion. Some recrystallized grains are also observed but their growth is effectively prevented. The refined intermetallics together with massive silicon particles and precipitates dispersed in the matrix can be expected to improve the thermal stability and high-temperature strength of the alloy to a great extent.

As-spray-deposited structure of an Al-20Si-5Fe Osprey preform and its development during subsequent processing

The application of the Osprey process to the fabrication of newly developed Al-20Si-X alloys is at present a subject of considerable interest. This paper reports the results of a study on the as-spray deposited structural characteristics of an Al-20Si-5Fe alloy preform and their development during subsequent hot extrusion and high-temperature exposure, by means of X-ray diffraction, differential scanning calorimetry and electron microscopy. It is shown that in the as-spray-deposited preform of the alloy an unusual increase in the lattice parameter of the aluminium matrix was detected, which persisted throughout the processing. No evidence of supersaturation in the preform aluminium matrix could be found, which is considered to be associated with the characteristics of solidification and subsequent decomposition of the hypereutectic alloy during spray deposition, this allows the extensive formation of second phases and thus a substantial decrease in the solute enrichment of the solution. A metastable intermetallic phase, identified as δ-Al4FeSi2, together with silicon phase, was present as the predominant dispersed phase in the preform, and its transformation into the equilibrium phase, β-Al5FeSi, through a peritectic reaction under the equilibrium conditions, must have been effectively suppressed during the Osprey process. The δ-phase initially with a platelet shape was fragmented into short rods during the extrusion subsequent to spray deposition, while a part of this phase was transformed into the equilibrium β-phase under the combined influence of heat and deformation. The refinement of the δ-phase, on the other hand, was found to decrease its metastability and thus to promote its decomposition during subsequent annealing at 400 °C. The coexistence of the high volume fractions of the intermetallic and silicon phases in the extruded material greatly modified its restoration kinetics, resulting in a partially recrystallized microstructure, after prolonged soaking at 400 °C for 100 H. Also shown is a peculiar microstructure of the as-spray-deposited material with numerous spherical colonies of 10–20 μm, characterized by the finer silicon particles and δ-phase platelets in their interior and occasionally decorated with micropores at their peripheries. These colonies are considered to originate from the remains of very fine droplets and particles in the larger droplets, which are presolidified in flight and then partially remelted at the deposition surface. The colonies were mixed up with the rest of the microstructure and the micropores closed by applying an extrusion operation.

Fracture features of a silicon-dispersed aluminium alloy extruded from rapidly solidified powder

The tensile fractography of an AI-20Si-3Cu-1 Mg alloy consolidated from rapidly solidified powder by extrusion has been investigated using optical and electron microscopy, and related to the processing conditions as well as the tensile behaviour of the alloy at room and elevated temperatures. The alloy studied shows distinct fracture features owing to the presence of dispersed silicon crystal particles with a bimodal distribution in size and of prior powder particle boundaries in the extrudate. It has been found that at room temperature cracks initiate by cracking the primary silicon crystal particles. Crack propagation occurs along the interfaces between the eutectic silicon crystal particles and the matrix and also between the prior powder particles, where microvoids are formed by the interfacial decohesion. At 300 °C, the fracture of the alloy involves microvoid nucleation, growth and coalescence at the interfaces between the silicon crystal particles and the aluminium matrix and between the prior powder particles. It has also been observed that the fractographic features of the alloy correspond well to the processing conditions including extrusion temperatures and subsequent heat treatment. The importance of minimizing the coarsening of the silicon crystals in processing in order to use the full strength potential of the alloy investigated is emphasized.

GAS ENTRAPMENT AND EVOLUTION IN PREALLOYED ALUMINIUM POWDERS

Part of a comprehensive research programme involving different aspects of degassing of powder metallurgy (P/M) aluminium alloys carried out in the P/M Group of the Delft University of Technology, is reported. The fundamental aspects of moisture and gas evolution during degassing of a porous billet are described in a semi-quantitative manner using a kinetic approach. During degassing of Al-20Si-X P/M alloys, at temperatures up to 550 °C, the partial pressures of moisture and hydrogen were within the range 10−4 to 10−7 mbar. The thermodynamics of gas desorption is mainly influenced by temperature which is the critical degassing parameter. It appears that the diffusion of aluminium through the oxide layer can explain, to a large extent, the kinetics of degassing of aluminium powders. A shift in the release of moisture and hydrogen towards higher temperatures is due to the presence of MgO in the surface layer, compared to the situation when only Al2O3 builds the oxide film. Thermodynamical data indicate that the reaction of magnesium with water vapour proceeds more intensely than that between aluminium and water vapour.

A study on an atomized Al-Fe-Mo-Zr powder to be processed for high temperature applications

The performance of a powder metallurgy material in processing and service depends very much upon the initial characteristics of the atomized powder. An investigation on the characterization of an Al-8.5Fe-2Mo-1Zr alloy powder, to be further processed for high temperature applications, has, therefore, been carried out. Analyses have been performed of its composition, morphology, size and microstructure by means of chemical and metallographic methods. Results show that although the powder was atomized in nitrogen, its oxide and hydrogen contents are high enough to be comparable to those of other aluminium alloy powders atomized in air. Auger spectroscopy indicates the presence of discrete oxide particles at a depth of 100 nm below the powder particle surface, which corresponds to the topography of the powder particles revealed by SEM. The oxidation during atomization, plus further oxidation and moisture adsorption during subsequent handling, is considered to be mainly responsible for the analysed results, in addition to the contributing factor that the present powder has a relatively high specific surface area. Applying an appropriate degassing process is, therefore, of particular importance for obtaining the desired properties of the material processed from the powder. In order to simplify the process and to minimize the coarsening of microstructure, an on-line degassing technique has been proposed. X-ray diffractometry shows that the lattice parameter of the α-Al matrix in the present powder is altered in a complicated way, and that the diffraction spectrum of intermetallic particles does not match any established phase, presumably due to the involvement of molybdenum in a metastable phase to form an Al-Fe-Mo intermetallic. The microstructure of the powder particles finer than 10 μm is featureless, and larger ones exhibit a distinctive microstructure which is composed of a featureless zone, a transitional zone and a cellular zone. The relative percentage of the three zones is strongly dependent upon the size of individual powder particles and thus their cooling rates. Solidification processes responsible for the formation of the three zones are described in this paper. It is also found that as a result of microstructural inhomogeneity, microhardness within and between the powder particles differs significantly, and this difference can be retained after consolidation and influence the properties of the final engineering material. It is thus thought that creating an extremely fine powder particle size (smaller than 10 μm) with an overwhelming featureless microstructure may not be commercially feasible at present, while producing a fairly homogeneous microstructure by narrowing the scattering range of powder particle size could be more important for obtaining uniform deformation and oxide break-up during consolidation, and desired mechanical properties of the final product.

Heating sequence and hydrogen evolution in alloyed aluminium powders

The results reported here, showing the effect of a non-continuous degassing sequence on the Al-20Si-3Cu-1 Mg powder, are a complement of previous work concerning the continuous degassing of the same powder. The degassing experiments were carried out, under high vacuum, in the temperature range 20 to 550 °C in a horizontal furnace heated at a uniform heating rate of 2.5 °C min−1. The partial pressures of the released gases were monitored and analysed during the heating phase by a computerized Edwards EQ80F residual gas analyser (RGA). RGA measurements indicate that water and hydrogen are the main degassing products. A complete degassing can only be achieved if the sample is heated up to a temperature where the chemical reactions are finished in the applied time. Thermodynamical equations alone are not enough to explain the kinetics of degassing of aluminium powders. The diffusion of aluminium through its surface oxide layer (Al2O3), described by the self-diffusion of aluminium, can explain to a large extent the kinetics of degassing aluminium powders.

Relationship between degassing conditions and tensile properties of Al-20Si-X P/M products

Results are reported which show the effect of different degassing modes on the properties of the Al-20Si-3Cu-1 Mg powder. The paper complements previous papers [1–3] concerning the conventional and modified degassing of the same powder. This research was mainly directed to study the influence of temperature on the tensile properties, ultimate tensile strength, σUTS, and elongation, ɛ, of extrudates obtained of Al-20Si-3Cu-1Mg compacts non-degassed, conventionally degassed, and treated by a modified process, namely degassing assisted by flushing with a depurative gas such as argon or nitrogen. The processing of the Al-20Si-3Cu-1Mg P/M powder must include a degassing step which significantly improves the tensile properties, at room and elevated temperatures, of the products of compacted powder with respect to those of the products whose compacts were non-degassed. It is apparent that degassing assisted by flushing with argon or nitrogen gives products with higher tensile properties than those of the products conventionally degassed under optimal conditions of temperature and time and much higher than those of the non-degassed products. The tensile results are in agreement with the theoretical approach to the gas entrapment and evolution of the aluminium powders presented in previous papers.