[en] A particular kind of hot pressing, in which the powder mass, subjected to pressure, is simultaneously heated by passing through a high-intensity electrical current is described and analysed. Commercial iron powder was used for the experimental work. Special emphasis is given to the densification kinetics. A qualitative microscopic interpretation of densification rate is suggested, based on the hypothesis of partial formation and rapid solidification of liquid phase. This interpretation is supported by the evolution of global porosity and electrical resistance of specimens during the electrical sintering process. The microstructural characteristics of compacts consolidated in the conventional way (cold pressing and furnace sintering) and by electrical resistance sintering are compared. (Author) 18 refs

[en] Several consolidation procedures have been developed during the last fifteen years to process mechanically alloyed (MA) powders at the Metallurgy and Materials Engineering Group (University of Seville). MA powders were processed by conventional cold pressing and vacuum sintering. In addition, several densification promoters were used. The resulting parts, with second phases precipitated during the consolidation, showed good tensile strength, both at room and high temperature. Nowadays, nano structured and amorphous MA alloys are being processed by electrical resistance sintering (ERS), which prevents microstructure evolution during consolidation. (Author) 49 refs

[en] To compare the utility of helical computed tomographic (CT) angiography and digital subtraction arteriography in the detection of intracranial aneurysms in the presence of acute subarachnoid hemorrhage (SAH) or suspicion of intracranial aneurysm. Thirty-one patients, 28 with SAH confirmed by CT or lumbar puncture and three with signs of cranial nerve compression, were studied prospectively between March 1997 and March 1998 by two different radiologists blinded to each others findings. One radiologist performed helical CT after intravenous injection of 100 cc of iohexol (240 mg/ml), followed by 2 mm-tick reconstructions of the circle of Willis taken every 0.5 mm. These images were processed in a workstation equipped with a 3 D shaded surface display (3 D-SSD). In five cases, maximum intensity projection (MIP) reconstructions were also carried out. Arteriography was performed subsequently by the other radiologist, who was not informed of the results of CT angiography. He selectively catheterized both carotid and vertebral arteries, obtaining anteroposterior, lateral and oblique images, with matrix of 512 x 512 pixels. CT angiography identified 21 aneurysms in 18 patients. Twelve patients presented no abnormal findings and one was found to have a subdural-subarachnoid hematoma. We had one false negative and one false positive. The minimum size of correctly diagnosed aneurysms was 4 mm and the maximum size was 34 mm. The aneurysm in the case of false positive measured 2,5 mm. The sensitivity of this technique for the detection of aneurysms was 95,2% and the specificity was 92.8%. We consider helical CT angiography with 3 D-SSD reconstruction to be a highly valuable tool in the diagnosis of aneurysms of the circle of Willis. In selected or extremely urgent cases, it may even eliminate the need to perform preoperative convectional angiography. (Author) 16 refs

[en] In the present study, mechanical alloying (MA) of Al75Fe25 elemental powders mixture was carried out in argon atmosphere, using a high energy attritor ball mill. The microstructure of the milled products at different stages of milling was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results showed that the amorphous phase content increased by increasing the milling time, and after 50 hours the amorphization process became complete. Heating the samples resulted in the crystallization of the synthesized amorphous alloys and the appearance of the equilibrium intermetallic compounds Al₅Fe₂

[en] The effect of intensity and duration of the electrical resistance sintering process on the phase stability, porosity distribution and microstructural evolution of Al₅₀Ti₅₀ amorphous powders is studied. The phase transformations during the consolidation process were determined by X-ray diffraction. The porosity distribution was observed by optical and scanning electron microscopy. The amorphous phase is partially transformed to the crystalline phase during the sintering process, and formation of AlTi and AlTi₃ intermetallic compounds occurs for temperatures higher than 300 °C. Finally, it is observed that the compacts core have lower porosity and a higher tendency to the amorphous-crystalline phase transformation than the periphery

[en] A new model undertaking the densification kinetics of uniaxially pressed metallic powders at constant temperature is proposed. This model is developed according to the power law of creep, and the expression of the ‘net pressure’ derived by the authors in a previous work. This net pressure describes the ‘geometrical hardening’ experienced by the powder mass, during compression. In order to validated the model three different powders were uniaxially pressed, aluminium, tin and lead, being obtained data from hot compaction experiments. The similarity between the model predicted curves and the experimental data is quite acceptable. In addition, the goodness of the model is contrasted with two other theoretical models addressing the same problem. The approach developed can be useful to model hot uniaxial pressing and electrical consolidation processes, which start with loose powders, i.e., not previously cold compacted powders.

[en] In this paper, the process known as Electrical Resistance Sintering under Pressure is modelled, simulated and validated. This consolidation technique consists of applying a high-intensity 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.