D.P. Bishop

On enhancing the mechanical properties of aluminum P/M alloys

Abstract Sintered aluminum alloys are an attractive material for the automobile industry, both because of the low specific gravity and high strength-to-weight ratio of aluminum itself, and the fabrication advantages associated with a powder metallurgy process. However, properties such as impact, stiffness, corrosion and wear resistance are often poor, thereby restricting the widespread use of these materials. Recent work by the authors has shown that hardness, wear resistance and tensile properties of a P/M Al–Cu–Mg ternary master alloy can be improved using a novel diffusion/supersolidus liquid phase sintering process. Improvements were due to in-situ microalloying during sintering, in particular, the influence of Ag and Sn. To complement this work, the present investigation addresses the response of a commercial alloy, AA2014, to the microalloying process. Results show that sintered densities for the commercial alloy were relatively unaffected by the presence of either Ag or Sn, and were superior to the ternary master alloy. Hardness and tensile properties were also improved relative to those obtained for the ternary, and were comparable to wrought 2014. Examination of final microstructure of Ag modified AA2014 using TEM showed the presence of Ω as the principal precipitate, but only after extended sintering times. This particular precipitate is believed to contribute to enhanced hardness. The apparent absence of Ω for short sintering times was due to the presence of silicon in the commercial product. However, the corrosion behavior of the P/M AA2014 was superior to the wrought product and thus the process is presented as a potential P/M alternative to using ingot metallurgy techniques for microalloying.

On enhancing the interfacial chemistry of a simulated AA2014-SiCp composite material

The use of aluminum alloys in automotive applications has increased significantly in recent years due to the need for more fuel-efficient vehicles. These alloys alone do not enjoy the strength offered by traditional ferrous products. However, the development of new alloys through micro/macroalloying and the incorporation of load-bearing materials such as SiC into the matrix have enhanced their popularity. Unfortunately metal matrix composites such as AA2014-SiC often fail catastrophically due to fibre or particulate pullout in service. Such failures are difficult to predict and are often a result of poor wetting at the metal/reinforcement interface. In the present work Sn and Ni were examined as potential sintering/wetting aids. In particular, Sn or Ni (0.5–2 w%) were added to a simulated AA2014 alloy (Al-4Cu-0.5Mg) with and without 14.5 w% SiC following standard powder metallurgy techniques. Because the distribution of blend constituents is of critical importance various dispersants were evaluated. Best particle dispersion was obtained using oleic acid while mixing for 8 h. Sintering temperatures ranged from 605–620°C and both green and final densities were determined using mercury densitometry. Resulting microstructures were examined using scanning electron microscopy and electron probe microanalysis with particular attention directed to the SiC-alloy interface. Nickel was found to enhance the wetting of SiC by AA2014 and the interfacial region was found to be chemically superior to a commercial copper-coated SiC product. Tin contributed to an increase in intermetallic formation. It is believed that the improved interfacial region was due to the presence of a small amount of liquid phase at the AA2014-SiC interface giving a chemical rather than the usual mechanical bond between reinforcement and alloy.

Diffusion-based microalloying via reaction sintering.

As a possible means of reducing the costs associated with the production of metal matrix composites, the use of inexpensive, naturally occurring minerals as a reinforcing agent is one alternative currently being considered. In such efforts, the occurrence of extensive chemical reaction between the minerals and matrix alloy has been noted. In an effort to utilize the reaction products from such reactions, a novel technique known as core/shell processing was developed. Core/shell and bulk alloy samples were prepared through powder metallurgy techniques (blending, cold isostatic pressing, and sintering) followed by hot swaging and finally machining as required. Sintered samples were examined by means of mercury densitometry, optical/scanning electron microscopy, electron microprobe analysis, and mechanical testing (tensile and impact). Microprobe analysis of sintered core/shell samples indicated the occurrence of extensive chemical reactions between the alloy and mineral particles in the shell region, resulting in a rejection of calcium from the mineral into the surrounding matrix followed by eventual migration into the intergranular regions of the core. Mechanical testing revealed core/shell processed samples had significantly improved impact properties while maintaining tensile properties similar to bulk alloy samples.

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.

Diffusion-based microalloying of aluminium alloys by powder metallurgy and reaction sintering

A diffusion-based technique of microalloying aluminium powder metallurgy products was examined to expand the range of feasible alloying additions. Thermodynamic calculations and diffusion rates for several elements suggested that tin and silver were the most promising; these elements were successfully alloyed into AA 2014 on both a macroscopic and a microscopic scale. The final microstructures were examined using X-ray diffraction, X-ray mapping and energy-dispersive electron probe microanalysis. Silver additions were homogeneous throughout the alloy microstructure, whereas tin was concentrated in intergranular regions only. The results suggested that the technique was viable for a variety of microalloying elements. Also, the extent of alloying was predicted reasonably well using a mathematical mass balance model.

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.

Spark Plasma Sintering and Upsetting of a Gas-Atomized/Air-Atomized Al Alloy Powder Mixture

Al-Zn-Mg-Cu alloy powder, Alumix 431D, was modified by replacing the native air-atomized pure Al particles with gas-atomized pure Al. Samples were sintered using spark plasma sintering (SPS), and upset forging was applied to the sintered samples by SPS. Densities over 98 and 99% of theoretical were obtained for the sintered and forged samples, respectively. Microstructural analysis and characterization of all samples were done using energy-dispersive spectroscopy and x-ray diffraction. Mechanical properties were evaluated using microhardness and flexural strength and strain measurements. The microhardness value of the T6 tempered sample was comparable to that of its wrought counterpart AA7075. Particle bonding after sintering was incomplete and reveals that composite oxide layer of Al-Zn-Mg-Cu alloy powder is difficult to disrupt, and it is necessary to apply a secondary process like forging to improve particle bonding. The loss in ductility following T6 tempering is ascribed to void formation due to the dissolution of the secondary phases, remaining undissolved precipitates, and a localized lack of cohesion between particles.

Effects of Fe and Ni additions on an emerging Al–4·4Cu–1·5Mg powder metallurgy alloy:

Abstract The effects of Fe and Ni additions to an Al–Cu–Mg powder metallurgy alloy were studied. The transition elements were incorporated through two different approaches, admixed elemental powders and prealloying of the base Al powder. Prealloying proved to be the superior alloying technique as it did not invoke any negative effects on the general sintering behaviour of the baseline alloy. The microstructures of prealloyed materials also exhibited a refined distribution of the aluminide phases formed. These included Al7Cu2Fe, Al7Cu4Ni and Al9FeNi as identified through a combination of SEM/EDS and XRD. The most promising alloy was identified as Al–4·4Cu–1·5Mg–1Fe–1Ni as it exhibited tensile properties that exceeded those of the baseline material. This advantage was also found to persevere after thermal exposure. On a étudié les effets d’additions de Fe et de Ni sur un alliage d’Al–Cu–Mg obtenu par la métallurgie des poudres. On a incorporé les éléments de transition au moyen de deux approches différentes – poudres élémentaires ajoutées ou préalliage de la poudre d’Al de base. Le préalliage était supérieur comme technique d’alliage puisqu’il n’invoquait pas d’effets négatifs sur le comportement général de frittage de l’alliage de référence. La microstructure des matériaux de préalliage exhibait également une distribution raffinée des phases d’aluminures formées. Celles-ci incluaient Al7Cu2Fe, Al7Cu4Ni et Al9FeNi, telles qu’identifiées au moyen d’une combinaison de SEM/EDS et de XRD. L’alliage le plus promettant était l’Al–4·4Cu–1·5Mg–1Fe–1Ni, puisqu’il exhibait des propriétés de traction qui excédaient celles du matériau de référence. On a également trouvé que cet avantage persistait après une exposition thermique.

Effect of processing variables on production of powder metallurgical titanium

Abstract Hydride/dehydride titanium powder has been processed using a conventional die compaction powder metallurgical approach. The effects of various processing parameters have been assessed in terms of the microstructure and mechanical behaviour, including lubricant type and amount, compaction pressure, delubrication atmosphere, and sintering conditions. While low lubricant contents can result in high sintered densities, a minimum lubricant content is required to maintain die life. It was also demonstrated that a compaction pressure of 300 MPa is sufficient to provide high sintered densities, while again minimising potential for die damage. Densities close to 99% of theoretical could be achieved when sintering at 1450°C for 4 h. Tensile strengths in excess of 750 MPa were obtained when sintering at 1300 or 1400°C. The mechanical behaviour was strongly influenced by a combination of intrinsic powder impurities (i.e. oxygen), residual carbon contamination from the lubricant, grain growth and retained porosity. On a traité de la poudre d’hydrure/dé-hydrure de titane en utilisant une approche métallurgique conventionnelle de compaction de la poudre en matrice. On a évalué l’effet des divers paramètres de traitement sur la microstructure et sur le comportement mécanique, incluant le type et la quantité de lubrifiant, la pression de compaction, l’atmosphère de délubrification et les conditions de frittage. Bien que de faibles teneurs en lubrifiant puissent résulter en de hautes densités de frittage, une teneur minimale en lubrifiant est requise pour assurer la survie de la matrice. On a également démontré qu’une pression de compaction de 300 MPa était suffisante pour atteindre de hautes densités de frittage, tout en minimisant encore une fois le potentiel d’endommagement de la matrice. On pouvait obtenir des densités de près de 99% de la valeur théorique lorsque le frittage était effectué à 1450°C pendant 4 heures. On a obtenu des valeurs de résistance à la traction de plus de 750 MPa lorsque le frittage était effectué à 1300 ou à 1400°C. Le comportement mécanique était fortement influencé par une combinaison des impuretés intrinsèques de la poudre (i.e. oxygène), de la contamination de carbone résiduel du lubrifiant, de la croissance de grain et de la porosité retenue.

Consolidation of aerospace grade aluminum 7055 powder through SPS-forge processing

ABSTRACT The objective of this research was to assess the SPS-forge response of fully pre-alloyed aerospace grade 7055 (Al–8Zn–2.1Mg–2.3Cu–0.2Zr) powder. The core variables investigated were the sintering temperature, time, and atmosphere employed, as well as the average particle size and the manner of uni-axial loading. Samples sintered in vacuum at 500°C for 40 min with delayed loading offered the most desirable combination of density, hardness, and bend properties but remained relatively brittle. Hot forging of the sintered preforms was found to impart sizable gains in mechanical properties. The SPS-forge product exhibited tensile yield strength of 603 MPa coupled with a ductility of 9.1% and a hardness of 93 HRB. All such properties were largely equivalent to those measured for the wrought counterpart.