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Resistance sintering represents a recent approach in the field of powder metallurgy. Classified as an electrical conductive assisted sintering (ECAS) process, its ultrafast characteristics enable fast densification, in only hundreds of milliseconds, which is much faster than conventional sintering, which requires hours. This is obtained from the electro-thermomechanically coupled phenomena, resulting from the simultaneous application of the compaction pressure and electrical current. In conventional sintering, these phases are made separately. The resistance sintering approach was investigated by adaptation of machines originally designed for resistance welding. Two different machines were used, based on alternating current and middle-frequency direct current. The main prerequisite of the sintering powder is to be electrically conductive, to permit the flow of the electrical current inside the specimen, while the die is electrically insulated. The heat is therefore generated by the Joule effect. Two different materials, commercially pure titanium and a neodymium alloy used to produce permanent magnets, were investigated and successfully sintered. Two different geometries, discs and rings, were studied and proper tooling solutions were designed for the purpose. Post-processing of the sintered samples involved the evaluation of the achieved densities, mechanical properties and microstructures. Magnets were tested also in terms of magnetic strength. Results were correlated with the adopted process parameters, which are electrical current density, sintering time and compaction pressure. The electrical current density proved to be the most influencing parameter. The magnetic properties were evaluated and compared to commercial Nd magnets, both sintered and bonded. Results proved to be compatible with the magnetic properties given by the powder producer. As a part of the investigations concerning the resistance sintering, the tribological effects of several lubricants were tested by applying them as die-wall lubrication prior to filling the powder inside the die. The efficiency of the lubricants in facilitating the ejection of the sintered sample by decreasing the required ejection force was investigated. Zinc stearate proved to be the most effective and was therefore used in all the experimental cases. Finally, numerical simulations were exploited by modelling the resistance sintering process with a continuum-based approach adopting a porous formulation. An indication of the process temperatures was achieved by numerical results, owe to the impossibility to obtain direct experimental measurements in resistance sintering, by using conventional measuring methods as thermocouples or pyrometers.
|Place of Publication||Kgs. Lyngby|
|Publisher||Technical University of Denmark|
|Number of pages||196|
|Publication status||Published - 2019|