J. M. Montes

Thermal and electrical conductivities of sintered powder compacts

Abstract A simple new equation for calculating thermal and electrical conductivities of powder sintered compacts is proposed. In this equation, the effective conductivity of the sintered compact is a function of the solid material conductivity, the degree of compact porosity and the packing porosity of the starting powder. The latter parameter determines to a great extent the compact pore structure, since it depends on powder particle size, shape and distribution. The new equation is physically applicable from zero porosity to the packing porosity. The conductivity equation has been evaluated using sintered compacts of iron, nickel, titanium and aluminium. The compacts were prepared by cold pressing and sintering. Results are in very good agreement with theoretical predictions.

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.

A One-Dimensional Model of the Electrical Resistance Sintering Process

In this work, a theoretical model for the processing technique usually known as electrical resistance sintering (ERS) under pressure is proposed and validated. This technique consists of the consolidation of a metallic powder mass under compression by means of a high-intensity electrical current that passes through the aggregate. The electrical current heats the powder mass by the Joule effect. The model is numerically solved by the finite differential method, in which the added difficulty of the thermal–mechanical coupling of the process has been taken into account. To simplify the numerical resolution, a one-dimensional scheme has been considered for both the powder densification mechanical problem and the heat generation and transmission problem. Furthermore, the theoretical predictions obtained after solving the model are compared with the data recorded by the ERS equipment sensors during electrical consolidation experiments with iron and titanium powders. The reasonable agreement between the theoretical and experimental curves suggests that the model, despite its simplifications, reproduces the main characteristics of the process.

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 metal powder aggregates and sintered compacts

New equations for computing the electrical conductivity of powder aggregates and sintered compacts are proposed. In both cases, the effective or apparent conductivity is a function of the bulk material conductivity, the porosity of the sample and the tap porosity of the starting powder. Additional parameters are required for powder aggregates, such as the conductivity of the oxide covering the particles, the thickness of the oxide layers and the ease of descaling them. The new equations are valid from zero porosity to the tap porosity. Links between the equations and the percolation conduction theory are stated. Measurements of electrical resistance on sintered compacts and powder aggregates subjected to different pressures were performed. The proposed equations have been validated with these data. The electrical conductivity of both sintered compacts and powder aggregates of aluminium, bronze, iron and nickel was determined and compared to the equation predictions, resulting in notably good agreement.

Nanocrystalline Al Composites from Powder Milled under Ammonia Gas Flow

The production of high hardness and thermally stable nanocrystalline aluminium composites is described. Al powder was milled at room temperature in an ammonia flow for a period of less than 5 h. NH3 dissociation during milling provokes the absorption, at a high rate, of nitrogen into aluminium, hardening it by forming a solid solution. Controlled amounts of AlN and Al5O6N are formed during the subsequent sintering of milled powders for consolidation. The pinning action of these abundant dispersoids highly restrains aluminium grain growth during heating. The mean size of the Al grains remains below 45 nm and even after the milled powder is sintered at 650°C for 1 h.

Densification rate of metal powders during hot uniaxial compaction

Abstract A model to describe the densification rate of a metal powder aggregate undergoing constant uniaxial pressure and temperature conditions is proposed. The model is based on the power law creep equation, and it is obtained by using the equivalent simple cubic system, a theoretical tool proposed by the authors in previous work. This theoretical tool assumes that it is possible to predict the evolution of the densification under the pressure of an actual powder system via the study of the same problem in a system of deforming spheres packed into a simple cubic lattice. The proposed model is validated with the help of experimental data obtained from uniaxial hot compaction experiments carried out with aluminium, tin and lead powders. The agreement obtained between theoretical curves and experimental data is reasonably good.

Thermal Conductivity of Powder Aggregates and Porous Compacts

A new equation for calculating the thermal conductivity of metal powder aggregates and sintered metal powder compacts is proposed. In this equation, the effective conductivity of the powder system is a function of the conductivity of the fully dense material, the porosity of the system, and the tap porosity of the starting powder. The new equation is applicable to powder systems, from the tap porosity to zero porosity, as well as to consolidated powders. The proposed equation has been experimentally validated by fitting to data from other authors. The results confirm a good agreement with theoretical predictions.

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.

New creep law

Abstract In the present paper, a new creep law describing the high temperature strain rate of materials, which is applicable to both high and low values of stresses, is proposed. In a similar way to the known hyperbolic sine law, the proposed equation guarantees a potential behaviour for low stress values (in accordance to the power law of creep) and a pure exponential behaviour for high stress values, according to the power breakdown law of creep. The differences and possible advantages of the proposed equation are analysed in comparison with the hyperbolic sine law.