ALBERT

Notes: Abstract The consolidation of an air-atomized Al-20Si-7.5Ni-3Cu-1 Mg alloy powder was performed utilizing hot extrusion, to determine its extrudability and understand its structural development in relation to process parameters. One of the main features exhibited by the material in this process was a high degree of softening over a peak extrusion pressure, which has been explained by the simultaneous onset of dynamic recovery and recrystallization during deformation. The peak extrusion pressure was shown to be strongly dependent upon the temperature applied, and this dependence has been described with temperature compensated strain rate. It was also observed that the process parameters had a fairly narrow range applicable to the extrusion of the powdered alloy and a significant influence on the deformation behaviour of the powder particles. The combination of heating and deformation, primarily used to convert the loose powder particles into an engineering material, resulted in the decomposition of the meta-stable aluminium matrix and transformations of constituent phases, initially formed in the rapidly solidified powder. Additionally, it was found that the extrusion temperature had an effect on the lattice size and perfection of the as-extruded matrix in the material. Three intermetallic dispersoids containing nickel were detected in the consolidated material, independent of extrusion temperature, and their formation was promoted by hot deformation. The silicon crystal phase in the extruded material was reshaped, and its size was insensitive to the extrusion temperature, which is thought to be caused by a high volume fraction of the coexistent dispersoids. The dispersions of the silicon crystals and intermetallic compounds with various sizes in the matrix substantially modified the deformation mode of the alloy. Evidence of dynamic recrystallization was found, which co-operated with dynamic recovery during deformation, giving rise to a duplex microstructure in the extruded material.

Notes: Abstract 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.

Notes: Abstract 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.

Notes: Abstract 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.

Notes: Abstract An attempt has been made to characterize a new, complicated Al-20Si-7.5Ni-3Cu-1Mg alloy powder produced by air atomization as a means of rapid solidification and its structural evolutions during continuous heating, in order to provide basic information for further investigations on its deformation behaviour and properties. The characterization consisted of size measurements, morphological observations, structural and thermal analyses of bulk powder, and microstructural examinations of individual powder particles. It was observed that the powder had a wide size distribution and irregular shapes, which were closely related to its varying internal structures. X-ray diffractometry (XRD) showed little shift of the diffraction line from the aluminium matrix of the powder, but a significant broadening, which has been attributed partly to the non-uniformity of supersaturation in the matrix of the powder and partly to the strains caused by the silicon crystals in the material. A differential scanning calorimetry (DSC) analysis revealed complex decomposition behaviour of the meta-stable aluminium matrix and transformations of nickel-bearing intermetallic compounds when heat was applied to the powder. XRD also showed that the meta-stable compounds formed in the powder did not match any known phases, and that they were transformed into Al3Ni, Al3(NiCu)2 and Al7Cu4Ni intermetallic dispersoids upon heating. The analyses also indicated that, due to the addition of nickel, some copper-containing phases, initially desired to create precipitation strengthening effects, no longer existed. This would diminish the ageing response of the alloy and probably change its category to be non-heat treatable — an important modification that has not yet been recognized by the alloy designers and users. Examinations on the powder particle sections showed variations in microstructure with powder particle size. Transitions in solidification mode within powder particles in accordance with local conditions of undercooling and heat extraction were also observed. The significant inhomogeneities in the microstructure of the powder have raised a problem to which special attention should be paid in both powder production and subsequent processing.

Notes: Abstract Hot torsion tests were performed to investigate the mechanical and microstructural responses of a quinary Al-20Si-7.5 Ni-3Cu-1Mg (wt %) alloy, consolidated from a rapidly solidified powder, to deformation at varying temperatures and strain rates. It was found that, under most of the deformation conditions applied, stress-strain curves were characterized by distinct stress peaks, which are usually absent from the curves shown by conventional aluminium alloys. Temperature and strain rate strongly influenced the stress and ductility of the material. Their combined influence on the peak stress has been expressed with a hyperbolic sine equation. The material also exhibited an extraordinarily high strain rate sensitivity,m, and a largem value variation with temperature. A relatively high value of activation energy for deformation was determined, which clearly reflects additional thermal barriers to metal flow, arising from a high volume fraction of multi-phase particles dispersed in the material. Additionally, the microstructure developed in the course of deformation was examined, which showed evidence of the co-operation of dynamic recovery and recrystallization. The initiation of local dynamic recrystallization is a result of a low level of dynamic recovery achievable in the material, which is again different from conventional aluminium alloys.

Notes: Abstract 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.

Notes: Abstract 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.

Notes: Abstract 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.

Notes: Abstract A study has been performed of the oxidation and degassing processes of aluminium-based alloy powders. Oxidation and hydration of gas-atomized metal powders take place during inflight solidification and cooling to room temperature, during collection and keeping in the powder collection box and during transport and storage before consolidation. Under the atomizing conditions, oxidation cannot be prevented. In contact with humid gases (air) the oxide layer on the powder surface takes up water vapour which is physically or chemically bound. A literature study shows that the oxide layer on atomized aluminium powder is amorphous and has a thickness of 2–10 nm depending on the atomizing conditions. The amount of water in the powder is sufficient to form a completely closed hydroxide layer on the outer surface of the powder. The thickness growth of the oxide layer is governed by cation diffusion. Degassing experiments were carried out by heating canned powders in vacuum. The partial pressures of evolved water vapour and hydrogen were registered as a function of temperature at a constant heating rate. Two different alloy powders were used: the first air atomized and containing 1% magnesium (Al-20Si-3Cu-1Mg-5Fe), and the second (Al-9Fe-2Mo-1Zr) magnesium-free powder, atomized by nitrogen. Much work has been done on degassing, but most of it is directed towards industrial applications. The quantitative theoretical description of the degassing phenomenon is still lacking. A new approach aiming at narrowing this gap is presented by employing Wagner's theory of high temperature oxidation of metals. The diffusion coefficient of aluminium cations through the amorphous aluminium oxide layer has been determined in the degassing temperature range by using the experimental data of Hayden et al. The diffusion coefficient of aluminium cations through the Al2O3 layer has also been evaluated from the degassing experiments. The values obtained directly from the degassing experiments are in reasonable agreement with those derived from the oxidation results. It has been concluded that extrapolation of the results obtained from diffusion experiments at high temperatures in aluminium oxides towards the temperature range of degassing cannot explain the formation of hydrogen during this process, even if the surface diffusion coefficient (much higher than lattice diffusion coefficient) is taken into account.