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.

Commercially pure (C.P.) iron powders with a deliberate high degree of oxidation were consolidated by medium-frequency electrical resistance sintering (MF-ERS). This is a consolidation technique where pressure, and heat coming from a low-voltage and high-intensity electrical current, are simultaneously applied to a powder mass. In this work, the achieved densification rate is interpreted according to a qualitative microscopic model, based on the compacts global porosity and electrical resistance evolution. The effect of current intensity and sintering time on compacts was studied on the basis of micrographs revealing the porosity distribution inside the sintered compact. The microstructural characteristics of compacts consolidated by the traditional cold-press and furnace-sinter powder metallurgy route are compared with results of MF-ERS consolidation. The goodness of MF-ERS versus the problems of conventional sintering when working with oxidized powders is analyzed. The electrical consolidation can obtain higher densifications than the traditional route under non-reducing atmospheres.

A novel processing method for amorphous Al50Ti50 alloy, obtained by mechanical alloying and subsequently consolidated by electrical resistance sintering, has been investigated. The characterisation of the powders and the confirmation of the presence of amorphous phase have been carried out by laser diffraction, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The amorphous Al50Ti50 powders, milled for 75 h, have a high hardness and small plastic deformation capacity, not being possible to achieve green compacts for conventional sintering. Moreover, conventional sintering takes a long time, being not possible to avoid crystallisation. Amorphous powders have been consolidated by electrical resistance sintering. Electrically sintered compacts with different current intensities (7–8 kA) and processing times (0.8–1.6 s) show a porosity between 16.5 and 20%. The highest Vickers hardness of 662 HV is reached in the centre of an electrically sintered compact with 8 kA and 1.2 s from amorphous Al50Ti50 powder. The hardness results are compared with the values found in the literature.

In this paper, the process known as Electrical Resistance Sintering under Pressure is modelled, simulated and validated. This consolidation technique consists of applying a highintensity electrical current to a metallic powder mass under compression. The Joule effect acts heating and softening the powders at the time that pressure deforms and makes the powder mass to densify. The proposed model is numerically solved by the finite elements method, taking into account the electrical-thermal-mechanical coupling present in the process. The theoretical predictions are validated with data recorded by sensors installed in the electrical resistance sintering equipment during experiments with iron powders. The reasonable agreement between the theoretical and experimental curves regarding the overall porosity and electrical resistance suggests that the model reproduces the main characteristics of the process. Also, metallographic studies on porosity distribution confirm the model theoretical predictions. Once confirmed the model and simulator efficiency, the evolution of the temperature and the porosity fields in the powder mass and in the rest of elements of the system can be predicted. The influences of the processing parameters (intensity, time and pressure) as well as the die material are also analyzed and discussed.

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