F.G. Cuevas

Influence of PCA Content on Mechanical Properties of Sintered MA Aluminium

Aluminium powder has been mechanically milled using different amounts of process control agent (PCA). Mechanically alloyed aluminium powder (MA Al) was prepared by attrition milling in the presence of 1.5 and 3wt.% of an EBS wax. Milling was carried out in vacuum during 10 h. Milled powders were consolidated by a press-and-sintering method. This consolidation method is not usually employed with MA Al powders. The amount of dispersed carbides formed in the Al powder increases with the percentage of PCA. These carbides restrain Al grain growth during sintering, resulting in consolidated compacts with a grain size of about 550 nm. Thus, these PM materials can be considered ultrafine grained materials. Due to grain refinement and dispersion strengthening, the tensile strength of MA Al specimens is increased remarkably.

Production of Al–Al3Ti powders by mechanical alloying and annealing

Abstract Mixed powders of Al and Ti (10 wt-%) have been mechanically alloyed in an attritor mill under vacuum or nitrogen atmosphere. Pure aluminium powders have also been prepared, in the same conditions, for comparison. After milling for 10 h, a metastable solution of Ti in an Al matrix is obtained, with ∼9 wt-%Ti dissolved in the matrix. The evolution of these powders during milling is reported. Their thermal stability has been studied using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), identifying the observed changes by X-ray diffraction (XRD). Annealing of these powders at different temperatures, up to a maximum of 625°C, produces the precipitation of new phases, such as Al4C3 and different structures of Al3Ti, as well as grain growth. The appearance of these second phases, and their influence on powder microhardness, has been characterised as a function of the selected heat treatment temperature.

Electrical conductivity of sintered powder compacts

Abstract Some equations for calculating the electrical conductivity of porous materials are reviewed and their applicability to sintered powder compacts is discussed. A previous equation proposed by the authors, in which the effective conductivity of a sintered compact is composed of a function of the fully dense material conductivity, the compact porosity degree and tap porosity of the starting powder, is reviewed. The primary aim of the present work is to establish a connection between such equation and the percolation conduction theory.

On selection of milling atmosphere for production of Al–10%Ti powders

Abstract A mixture of aluminium and 10 wt-% titanium powders was attrition milled for 10 h under air, nitrogen and vacuum atmospheres; pure aluminium powders were also prepared in a like manner. Particle size distribution, morphology and microstructure of the powders were studied by laser diffraction, scanning electron microscopy (SEM) and X-ray diffraction (XRD); special attention was paid to the influence of the milling atmosphere. There were differences in powder particle size obtained from pure Al powders that were not observed for Ti containing powders, however the same homogeneous morphology and microstructure was attained for the different milling atmospheres. The effect of milled powder annealing on microstructure was studied by differential scanning calorimetry (DSC) and XRD. New phases and their crystallite size were characterised as a function of annealing temperature, milling atmosphere, and powder microhardness. In short, the studied milling atmospheres for the production of Al–10%Ti powders do not affect the properties of the obtained powders, and in general, low cost atmospheres could be used.

Intermetallic Effects on the Ductility of Sintered Aluminium

Mechanically alloyed aluminium powder was prepared by attrition-milling for 10 hours in the presence of a wax. Milled powders were annealed in vacuum at different temperatures (500, 575, 600, 625 and 650°C). Compacts were consolidated starting from unannealed and from 600°Cannealed powders. Studies by SEM microfractography and quantitative metallography, to investigate the influence of Fe-Al intermetallics on compacts fracture, have been carried out. It is concluded that fracture takes place at regions where the area occupied by the intermetallics is high and intermetallics particles are big. Intermetallic particle size can be controlled by an appropriated heat treatment.

La Fe Victoriosa Casting Features

As the first step in the restoration process of a Renaissance bronze statue, a comprehensive study of the metallic product was carried out. This paper reports metallographic features and their possible relation to the casting process. Tensile properties are also discussed as they depend on porosity and lead content, what can be useful in the restoration process.

Metallic Powders Amorphization by Mechanical Alloying and Subsequent Electrical Sintering

Al-base and Fe-base powders have been amorphized by a high energy milling process in an Attritor miller. Microstructural evolution in powder particles has been analyzed by XRD, DSC, SEM and TEM. The conventional route of cold pressing and sintering applied to these powders does not result adequate to preserve their amorphous or nanometric character. An additional disadvantage of this route appears during the cold pressing stage, as a consequence of the insufficient green strength of the compacts, due to the high hardness of the milled powders. In order to avoid these difficulties a new consolidation technique, electrical resistance sintering (ERS), has been successfully employed. ERS consolidated compacts have been microstructurally characterized by optical microscopy and XRD, showing that compacts preserve their amorphous and/or nanometric character.

Electrical Resistance Sintering of M.A. Al-5AlN Powders

In this work, mechanically alloyed Al-5AlN powders have been sintered by the Electrical Resistance Sintering (E.R.S.) technique. A die of alumina-base refractory concrete has been employed. Several electric intensity currents and passage times through the compact have been tested during the consolidation process. Compacts have been mechanically characterized by their hardness distribution and by an indirect tensile test. The obtained results are compared with the corresponding values of compacts prepared with the same powders by the conventional route of cold pressing and furnace sintering. Finally, for all the electrically consolidated compacts, the final porosity, as well as the average hardness and the strength in the indirect tensile test are empirically related to the electric energy supplied during the process. This energy is a function of the electric intensity current and passage time. The aforementioned empirical relationships are useful to select the best process conditions.

Effective stress during biaxial compression of powder aggregates

Abstract An equation to calculate the effective stress acting on interparticle contacts during the biaxial compression of a powder aggregate is proposed. This equation is obtained by a new method in which the study of the stress–strain relationship is not necessary. This new method does not require the use of sophisticated numerical computation software. The proposed expression satisfies upper and lower boundary conditions, and its range of application is greater than that of other equations, which are often limited to very small deformations. In this work, initially, simple cubic packing of spheres is studied, and then results are extrapolated to real powder systems using a normalised porosity. The proposed equation can be useful in compaction and extrusion processes.