Amorphous Al-Ti Powders Prepared by Mechanical Alloying and Consolidated by Electrical Resistance Sintering

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.Ministerio de Economía y Competitividad (Spain) / Feder (EU) DPI2015-69550-C2-1-PMinisterio de Economía y Competitividad (Spain) / Feder (EU) DPI2015-69550-C2-2-

Fracture toughness of cemented carbides obtained by electrical resistance sintering

The unique combination of hardness, toughness and wear resistance exhibited by WC-Co cemented carbides (hardmetals) has made them a preeminent material choice for extremely demanding applications, such as metal cutting/forming tools or mining bits, in which improved and consistent performance together with high reliability are required. The high fracture toughness values exhibited by hardmetals are mainly due to ductile ligament bridging and crack deflection (intrinsic to carbides). In this work two WC-Co grades obtained by using the electric resistance sintering technique are studied. The relationships between the process parameters (cobalt volume fraction, sintering current and time, die materials, etc.), the microstructural characteristics (porosity, cobalt volume fraction, carbide grain size, binder thickness and carbide contiguity) and mechanical properties (Vickers hardness and fracture toughness) are established and discussed. Also the presence of microstructural anisotropy and residual stresses is studied. The sintering process at 7 kA, 600 ms and 100 MPa, in an alumina die, followed by a treatment of residual stress relief (800 °C, 2 h in high vacuum), allows to obtain WC-Co pellets with the best balance between an homogeneous microstructure and mechanical behaviour.EU for funding this research with in the framework of the EU 7th Framework FoF.NMP.2013-10 608729 EFFIPRO Projec

Medium-frequency electrical resistance sintering of oxidized C.P. iron powder

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.Ministerio de Economía y Competitividad DPI2015-69550-C2-1-PMinisterio de Economía y Competitividad DPI2015-69550-C2-2-

Crystallization Process and Microstructural Evolution of Melt Spun Al-RE-Ni-(Cu) Ribbons

The crystallization process, both at the initial and subsequent stages, of amorphous Al88-RE4-Ni8 alloys (RE = Y, Sm and Ce) has been studied. Additionally, the consequences of adding 1 at.% Cu replacing Ni or Al were studied. The stability of the amorphous structure in melt spun ribbons was thermally studied by di erential scanning calorimetry, with Ce alloys being the most stable. The e ect of Cu to reduce the nanocrystal size during primary crystallization was analyzed by transmission electron microscopy. This latter technique and x-ray di raction showed the formation of intermetallic phases at higher temperatures. A clear di erence was observed for the Ce alloy, with a simpler sequence involving the presence of Al3Ni and Al11Ce3. However, for the Y and Sm alloys, a more complex evolution involving metastable ternary phases before Al19RE5Ni3 appears, takes place. The shape of the intermetallics changes from equiaxial in the Ce alloys to elongate for Y and Sm, with longer particles for Sm and, in general, when Cu is added to the alloy.Ministerio de Economía y CompetitividadUnión Europea DPI2015-69550-C2-2-P

Polvos de Al-Al 3Ti obtenidos mediante aleado mecánico y tratamiento térmico

Polvos mezclados de aluminio y titanio ( 10 % en peso) han sido aleados mecánicamente en un molino Attritor, obteniéndose una solución metaestable de titanio en la matriz de aluminio. Se han estudiado los cambios producidos en la forma y tamaño de las partículas, estructura y microestructura, al variar el tiempo de molienda entre 2 y 10 h. El procesado final se realiza para un tiempo de 10 h, habiéndose disuelto aproximadamente un 9 % en peso de titanio. Finalmente, se realiza un tratamiento térmico a diversas temperaturas, hasta un máximo de 625 °C, lo que produce la precipitación de diversas fases, como distintas estructuras de AI3TÍ y AI4C3. La aparición de estas segundas fases es caracterizada en función de la temperatura de tratamiento utilizada

Ceramic dies selection for electrical resistance sintering of metallic materials

Processing metallic powders by electrical resistance sintering requires the use of insulating ceramics dies. Selecting the appropriate ceramic material according to the electrical, thermal and mechanical properties is a need. Dies produced with several ceramic materials have been tested during the production of cemented carbide in order to check their behaviour in the process and final product properties. Tialite/mullite, zircon/mullite, zirconium phosphate based ceramic, yttria-stabilized zirconia and sialon, in most cases with modified compositions and shaping processes in order to achieve a high density, have been tested. Dry powder processing by cold isostatic pressing and furnace sintering resulted to be the better process for dies production. The effect of die properties on the produced cemented carbide, and the behaviour and life of the die during the production have been analysed. Very smooth die surface increases the number of cycles withstood during metallic parts production, because of lower extraction stresses, as checked for sialon dies. Zirconium phosphate based dies, with low thermal conductivity, show the most densified hard metal parts surface.Pproject EFFIPRO (EU) FP7-2013-NMP-ICT-FoF GRANT AGREEMENT N° 6087

Consolidation of iron powder by electrical discharge

Capacitor electrical discharge consolidation (CEDC) is a technique that uses the heat of the Joule effect of a high intensity electric current to consolidate powders. In this study, the effect of the precompaction pressure and the number of discharges on the porosity, microstructure and hardness of the compacts is analysed. Furthermore, the sintering results of iron powders obtained through the conventional route (cold pressing and furnace sintering) and by CEDC are compared. Experiments show that at low initial pressures the powder column has the necessary resistance to produce the joule heat necessary for powder consolidation. At an initial pressure of 200 MPa the porosity of the specimens decreases from 0.32 to 0.24, and the Vickers microhardness increases from HV10 29 to HV10 51 after 50 discharges.The authors also wish to thank the technicians M. Sánchez (University of Seville, Spain), C. Cantero and C. Lara (University of Huelva, Spain) for experimental assistance. This research was funded by University of Seville Research Funding Programme (project code 2020/00000647). Funding for open access charge: Universidad de Huelva / CBU

Medium-Frequency Electrical Resistance Sintering of Soft Magnetic Powder Metallurgy Iron Parts

The fabrication of soft magnetic Fe parts by the medium-frequency electrical resistance sintering (MF-ERS) technique is studied in this paper. This consolidation technique involves the simultaneous application to metallic powders of pressure and heat, the latter coming from the Joule effect of a low-voltage and high-intensity electric current. Commercially pure iron powder was used in the consolidation experiences. The porosity distribution, microhardness, electrical resistivity and hysteresis curves of the final compacts were determined and analysed. The results obtained were compared both with those of compacts consolidated by the conventional powder metallurgy (PM) route of cold pressing and vacuum furnace sintering, and with fully dense compacts obtained by double cycle of cold pressing and furnace sintering in hydrogen atmosphereFinancial support of the Ministerio de Economía y Competitividad (Spain) and Feder (EU) through the research projects DPI2015-69550-C2-1-P and DPI2015-69550-C2-2-P is gratefully acknowledged The authors also wish to thank the technicians J. Pinto, M. Madrid and M. Sánchez (University of Seville, Spain) for experimental assistanc

Fabricación y caracterización de núcleos magnéticos de aleaciones amorfas mediante ruta pulvimetalúrgica simple y económica

La fabricación de núcleos amorfos (tanto para motores eléctricos, como transformadores) es una tarea compleja que hasta ahora ha requerido la fabricación del material amorfo en forma de cintas de fino espesor (mediante enfriamiento muy severo, melt spinning) y su posterior apilado y/o plegado para la formación de la pieza final. El proceso puede resultar costoso, y las propiedades de la pieza, a menudo, se resienten por el hecho de poseer demasiadas fronteras. Aunque se han ensayado diversos métodos para obtener materiales amorfos en bloque, ninguno, por el momento, está exento de dificultades y está explotándose industrialmente. El objeto de esta investigación es mostrar una ruta alternativa de fabricación de núcleos amorfos (o parcialmente nanocristalinos, embebidos en matriz amorfa), que permite obtener bloques de material (no formados por unión de cintas) con la forma definitiva, sustituyendo la técnica de melt spinning por una ruta pulvimetalúrgica consistente en la amorfización del polvo mediante molienda mecánica de alta energía y posterior consolidación rápida por vía eléctrica (técnicas FAST, abreviatura de Field Assisted Sintering Techniques). Esta combinación permite obtener piezas masivas de material amorfo (o parcialmente nanocristalino) con la forma definitiva.Manufacturing of amorphous cores (for electric motors and transformers) is a complex task that until now has required the manufacture of amorphous material in the form of thin strips (by very rapid cooling, melt spinning) and subsequent stacking and / or folded to form the final piece. The process can be expensive, and properties of the piece often resent to have too many borders. Although various methods have been tried for amorphous materials block, none, for the moment, is exempt from difficulties and is exploited industrially. The object of this research is to show an alternative route of manufacture of amorphous cores (or partially nanocrystalline, embedded in an amorphous matrix), giving material blocks (not formed by bonding tape) with the final form, replacing the technique of melt-spinning consisting of a powder amorphization by mechanical high energy milling and subsequent rapid consolidation by (FAST, Field Assisted Sintering Techniques) powder-metallurgical route. This combination allows to obtain massive pieces of amorphous material (or partially nanocrystalline) with the final form