Experimental and numerical studies of powder\ud flow during the die filling stage in powder metallurgy cold\ud compaction processes are presented. An experimental setting\ud consisting of a horizontal pneumatically activated shoe,\ud a vertical die and high-speed video system has been designed.\ud The experiments show the existence of three flow regimes:\ud continuous, transitory and discrete, which are identified in\ud terms of the particle size, the morphology and the speed of\ud the shoe. In the continuous regime the powder flows in a progressive\ud manner but in the discrete one some perturbations\ud appear as a consequence of a shear band formation that forms\ud discrete avalanches. A numerical model, based on a ratedependent\ud constitutive model, via a flow formulation, and in\ud the framework of the particle finite element method (PFEM)\ud is also proposed. For the purpose of this study, the use of the\ud PFEM assumes that the powder can be modelled as a continuous\ud medium. The model, provided with the corresponding\ud characterisation of the parameters, is able to capture the two\ud fundamental phenomena observed during the filling process:\ud (1) the irreversibility of most of the deformation experienced\ud by the material and (2) the quick dissipation of the potential\ud gravitatory energy of the granular system through the inter-particle friction processes, modelled by the plastic dissipation\ud associated with the material model. Experimental and\ud numerical results have been compared in order to study the\ud viability of the proposed model.Peer ReviewedPreprin
Aparte de sus numerosas ventajas, la pulvimetalurgia presenta también algunas restricciones. Algunas de ellas dependen del propio proceso y afectan principalmente al diseño de la pieza. Los requerimientos geométricos y los que conciernen a la tenacidad limitan las posibilidades de esta tecnología. Otro tipo de limitaciones, también bien establecidas, están asociadas a la etapa de compactación: la fricción entre las partículas y las herramientas induce en el compacto una distribución no uniforme de densidad. El estado de tensiones heterogéneo que se desarrolla, especialmente durante la eyección del molde, produce, frecuentemente, grietas en la preforma e incluso la fractura del molde.Todos estos problemas se han resuelto tradicionalmente mediante métodos de trial and error. Sin embargo, el desarrollo reciente de nuevas y eficientes herramientas de cálculo puede contribuir a reducir el coste del diseño de los procesos de fabricación y contribuir a mejorar la calidad del producto final. A este respecto, el principal objetivo de la simulación numérica es determinar la forma óptima de producir piezas sin de- La industria pulvimetalúrgica está interesada en extender las aplicaciones y mejorar la calidad de sus productos. Para ello, necesita un conocimiento detallado de sus procesos que permita su control. La representación del comportamiento mecánico de estos materiales se ha realizado utilizando diversos tipos de modelos; en la actualidad, la mayoría de los grupos de trabajo considera a los polvos metálicos como materiales granulares. Los modelos de plasticidad definidos para los materiales geológicos se están aplicando a los agregados metáli-cos. Sin embargo, ninguno de los modelos conocidos puede representar adecuadamente su comportamiento mecánico, especialmente en estados de fallo. Palabras ClavePlasticidad. Materiales granulares. Compactación de polvos metálicos. Modelización. Modelling of the plasticity in cold compaction of metal powders AbstractPowder metallurgical industry is very interested in spreading its applications and improve the quality of the PM products. Therefore, a detailed knowledge of their processes, in order to control them, is necessary. Many different types of models have been used to represent the mechanical behaviour of these materials; however, nowadays, most of the groups working in this field admit that metallic powders and green compacts have to be considered as granular materials. Plasticity models specially defined for geological materials are being applied on metallic aggregates. Nevertheless, none of the known ones can represent adequately their mechanical behaviour, specially in states of failure. KeywordsPlasticity. Granular materials. Compaction of metal powders. Modelling.
Resumen En el proceso de conformado pulvimetalúrgico, la consolidación del polvo se inicia con el llenado del molde en el que se fabricará la preforma porosa, el cuál está caracterizado por la distribución de densidades del polvo dentro del molde y depende tanto de las propiedades del mismo como del método de llenado utilizado. A continuación, presentamos los primeros resultados obtenidos mediante un montaje experimental que reproduce el llenado de moldes y que, empleando un sistema de video, permite analizar el flujo de las partículas durante el llenado. El objetivo es determinar el efecto del tamaño, naturaleza y morfología del polvo, así como el de la velocidad del cargador y la geometría del molde en la distribución de la densidad aparente en moldes industriales de geometría compleja. Palabras clave Materiales granulares. Flujo granular. Distribución de densidades. Llenado de moldes.
Centrifugal atomization is a rapid solidification technique for producing metal powders. However, its wide application has been limited to the production of common metal powders and their corresponding alloys. Therefore, there is a lack of research on the production of novel materials such as metallic glasses using this technology. In this paper, aluminum-based glassy powders (Al86Ni8Y4.5La1.5) were produced by centrifugal atomization. The effects of disk speed, atomization gas, and particle size on the cooling rate and the final microstructure of the resulting powder were investigated. The powders were characterized using SEM and XRD, and the amorphous fractions of the atomized powder samples were quantified through DSC analysis. A theoretical model was developed to evaluate the thermal evolution of the atomized droplets and to calculate their cooling rate. The average cooling rate experienced by the centrifugally atomized powder was calculated to be approximately 7 × 105 Ks−1 for particle sizes of 32.5 μm atomized at 40,000 rpm in a helium atmosphere. Amorphous fractions from 60% to 70% were obtained in particles with sizes of up to 125 μm in the most favorable atomization conditions.
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