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

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

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-

Nickel Porous Compacts Obtained by Medium-Frequency Electrical Resistance Sintering

A commercially pure (c.p.) nickel powder was consolidated by Medium-Frequency Electrical Resistance Sintering (MF-ERS). In this consolidation technique, a pressure and the heat released by a high-intensity and low-voltage electrical current are concurrently applied to a metal powder mass. A nickel powder with a high tap porosity (86%) and a low applied pressure (only 100 MPa) is chosen in order to be able to obtain compacts with different levels of porosity, to facilitate the study of the porosity influence on the compact properties. The influence of current intensity and heating time on the global porosity values, the porosity and microhardness distribution, and the electrical conductivity of the sintered compacts is studied. The properties of the compacts consolidated by MF-ERS are compared with the results obtained by the conventional powder metallurgy route, consisting of cold pressing and furnace sintering. A universal equation to describe the porosity influence on all the analyzed properties of powder aggregates and sintered compacts is proposed and validated

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

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

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

Production and processing of ultra-fine grained, nanostructured and amorphous alloys by mechanical alloying

El Grupo de Metalurgia e Ingeniería de los Materiales de la Universidad de Sevilla ha desarrollado, durante los últimos quince años, diversas técnicas para la consolidación de aleaciones de aluminio obtenidas por aleado mecánico (AM) de polvos. Las aleaciones, reforzadas por segundas fases precipitadas durante el procesado, han sido prensadas y sinterizadas por procedimientos convencionales y se ha estudiado la contribución de ayudas a la sinterización, obteniéndose elevadas resistencias y buenas propiedades en caliente. En la actualidad se desarrollan, también por aleado mecánico, aleaciones nanométricas y amorfas en las que, gracias al uso de la sinterización por resistencia eléctrica (SRE), puede preservarse en gran medida la microestructura de los polvos en las piezas consolidadas.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, nanostructured and amorphous MA alloys are being processed by electrical resistance sintering (ERS), which prevents microstructure evolution during consolidation.CICY

Influence of Milling Atmosphere on the Controlled Formation of Ultrafine Dispersoids in Al-Based MMCs

Properties of compacts made from aluminium powder, milled under different atmospheres, were evaluated. The duration of all the milling processes was 10 h, although different atmospheres were tested: vacuum, confined ammonia, and vacuum combined with a short-time ammonia gas flow (5 min). Milled powders were consolidated by cold uniaxial pressing and vacuum sintering.The nature and content of the second phases change with the milling atmosphere, allowing the modification of the mechanical properties of the compacts. Results showed that hardness and tensile strength were highly dependent on the milling atmosphere. Milling carried out in vacuum with a short-time ammonia gas flow notably improved compacts’ properties, as compared with 10 h vacuum milling. Hardness increased from 96 to 150 HB, and ultimate tensile strength rose from 247 to 476 MPa.Ministerio de Economía y Competitividad DPI2015-69550-C2-1-P DPI2015-69550-C2-2-

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 5 h. 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 45nm and even after the milled powder is sintered at 650∘C for 1h