F. G. Cuevas

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.

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 5u2009h. 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 45u2009nm and even after the milled powder is sintered at 650°C for 1u2009h.

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.

Improvement in the Mechanical Behavior of Mechanically Alloyed Aluminum Using Short-Time NH3 Flow

In order to study the influence of a short-time ammonia gas flow during mechanical alloying (MA) of aluminum powders, samples were prepared using a simple press and sinter method. All milling experiments were performed at room temperature for a total of 10xa0hours. A short-time ammonia flow was incorporated into the milling process, allowing for the appearance of nitrogen-rich second phases, mainly oxycarbonitride and oxynitride aluminum (Al3CON and Al5O6N, respectively), during powder sintering. Testing of the sintering parts showed that the use of a short-time ammonia gas flow during vacuum milling substantially improved the mechanical properties at room and high temperatures.

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.

Modelling of three powder compaction laws for cold die pressing

Abstract Three models to describe the relationship between the green porosity of compacts and the applied external pressure during the compaction of metal powders are proposed in order of growing complexity. Only in the third model do all of the parameters have a clear physical meaning and become measurable. These parameters are related to the plastic behaviour of the powder particle material, the friction coefficient between the powder and die walls, as well as the interparticle friction. The three proposed models include a parameter representing a certain value related to the initial porosity of the powder mass; however, only in the third model is this value clearly defined by the tap porosity of the powders. The proposed models have been experimentally verified by compressibility curves obtained from metal powders of different types. The agreements between the second and third models and the experimental data are reasonable over the tested pressure range.

On the compressibility of metal powders

ABSTRACT A new equation relating the porosity of green compacts and the applied external pressure during the cold die compaction of metal powders is proposed. All of the parameters in the model have a clear physical meaning. These parameters are those related to the plastic behaviour of the material, as well as to the ‘structural resistance’ of the powder mass. Also the friction between the powders and die walls is considered, as a kind of constraint that diminishes the local pressure borne by the fully dense material. The model includes, as a key parameter, the tap porosity of the powders (an extremely useful property that contains the morphometric information of the powder). The proposed model has been experimentally checked with the compressibility curves obtained with five metal powders of different types. The agreement between the model and experimental data is reasonable over the tested pressure range.

In Situ Synthesis of Al-Based MMCs Reinforced with AlN by Mechanical Alloying under NH3 Gas

Aluminum matrix composites (AMCs) reinforced by aluminum nitride were prepared by mechanical alloying followed by a simple press and sintering method. Milling began under vacuum and after a period of between 1 and 4 h, NH3 gas flow (1 cm3/s) was incorporated until the total milling time of 5 h was reached. Results show that in addition to the strain hardening taking place during mechanical alloying, NH3 plays an additional role in powder hardening. Thereby, the properties of the sintered compacts are strongly influenced by the amount of N incorporated into the powders during milling and the subsequent formation of AlN during the consolidation process. The obtained AMC reaches tensile strengths as high as 459 MPa and hardness much higher than that of the as-received aluminum compact.