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M Rappaz - One of the best experts on this subject based on the ideXlab platform.

  • Rappaz M. In situ observation of hot tearing formation in succinonitrile-acetone. Acta mater. 2001:1261–9. Applied Mechanics and Materials Vols
    2020
    Co-Authors: I Farup, -m J. Drezet, M Rappaz
    Abstract:

    Abstract-Hot tears have been induced during the Solidification of a succinonitrile-acetone alloy by pulling the columnar dendrites in the transverse direction with a pulling stick. The opening of the mushy zone (hot tears) always occurred at grain boundaries. At low volume fraction of solid, the opening can be compensated by leaner-solute interdendritic liquid (i.e., "healed" hot tears). At higher volume fraction of solid, hot tears directly nucleate in the interdendritic liquid or develop from pre-existing micropores induced by Solidification Shrinkage. Their surface (edge) is made of secondary dendrite arms, which have not yet bridged, but a few spikes have also been observed. These later spikes formed either by the necking of solid bridges established across the grain boundaries prior to pulling, or by the sudden break-up of the liquid film during pulling. Similar spikes have been found by SEM on the hot tear surface of an aluminium-copper alloy. 2001 Published b

  • Prediction of Hot Tear Formation in Vertical DC Casting of Aluminum Billets Using a Granular Approach
    JOM, 2013
    Co-Authors: M. Sistaninia, J.-m. Drezet, A. B. Phillion, M Rappaz
    Abstract:

    A coupled hydromechanical granular model aimed at predicting hot tear formation and stress–strain behavior in metallic alloys during Solidification is applied to the semicontinuous direct chill casting of aluminum alloy round billets. This granular model consists of four separate three-dimensional (3D) modules: (I) a Solidification module that is used for generating the solid–liquid geometry at a given solid fraction, (II) a fluid flow module that is used to calculate the Solidification Shrinkage and deformation-induced pressure drop within the intergranular liquid, (III) a semisolid deformation module that is based on a combined finite element/discrete element method and simulates the rheological behavior of the granular structure, and (IV) a failure module that simulates crack initiation and propagation. To investigate hot tearing, the granular model has been applied to a representative volume within the direct chill cast billet that is located at the bottom of the liquid sump, and it reveals that semisolid deformations imposed on the mushy zone open the liquid channels due to localization of the deformation at grains boundaries. At a low casting speed, only individual pores are able to form in the widest channels because liquid feeding remains efficient. However, as the casting speed increases, the flow of liquid required to compensate for Solidification Shrinkage also increases and as a result the pores propagate and coalesce to form a centerline crack.

  • three dimensional granular model of semi solid metallic alloys undergoing Solidification fluid flow and localization of feeding
    Acta Materialia, 2012
    Co-Authors: M. Sistaninia, A. B. Phillion, Jeanmarie Drezet, M Rappaz
    Abstract:

    A three-dimensional (3-D) granular model which simulates fluid flow within solidifying alloys with a globular microstructure, such as that found in grain refined Al alloys, is presented. The model geometry within a representative volume element (RVE) consists of a set of prismatic triangular elements representing the intergranular liquid channels. The pressure field within the liquid channels is calculated using a finite elements (FEs) method assuming a Poiseuille flow within each channel and flow conservation at triple lines. The fluid flow is induced by Solidification Shrinkage and openings at grain boundaries due to deformation of the coherent solid. The granular model predictions are validated against bulk data calculated with averaging techniques. The results show that a fluid flow simulation of globular semi-solid materials is able to reproduce both a map of the 3-D intergranular pressure and the localization of feeding within the mushy zone. A new hot cracking sensitivity coefficient is then proposed. Based on a mass balance performed over a solidifying isothermal volume element, this coefficient accounts for tensile deformation of the semi-solid domain and for the induced intergranular liquid feeding. The fluid flow model is then used to calculate the pressure drop in the mushy zone during the direct chill casting of aluminum alloy billets. The predicted pressure demonstrates that deep in the mushy zone where the permeability is low the local pressure can be significantly lower than the pressure predicted by averaging techniques.

  • a granular model of equiaxed mushy zones formation of a coherent solid and localization of feeding
    Acta Materialia, 2006
    Co-Authors: Stephane Vernede, Philippe Jarry, M Rappaz
    Abstract:

    Abstract The gradual transformation of a mushy zone during alloy Solidification, from a continuous liquid film network to a fully coherent solid, has been simulated using a granular model. Based on a Voronoi tessellation of a random set of nucleation centres, Solidification within each polyhedron is computed considering back-diffusion and coalescence. In the network of connected liquid films, a pressure drop calculation is performed assuming a Poiseuille flow in each channel, Kirchhoff’s conservation of flow at nodal points and flow losses compensating Solidification Shrinkage (KPL model). In addition to intergranular liquid pressure maps, the model shows the progressive formation of grain clusters, the localisation of the flow at very high solid fraction and thus natural transitions of the mushy zone.

  • transition of the mushy zone from continuous liquid films to a coherent solid
    Philosophical Magazine, 2006
    Co-Authors: Stephane Vernede, M Rappaz
    Abstract:

    While studies of Solidification microstructures have focused mainly on the tips of the dendrites, the last stage Solidification is equally important from the point of view of defect formation (porosity, hot tearing), mechanical strength build-up and precipitation of phases. In particular, the transition from continuous liquid films to a coherent solid in low concentration alloys is of crucial importance for hot tearing formation, and more generally speaking for liquid feeding ability and coherency development. Based on a fairly recent theoretical model of coalescence which will be recalled briefly, new results obtained for a population of equiaxed grains will be presented. A granular-type model based on a Voronoi tessellation has been used for the description of the gradual disappearance of liquid films and the clustering of equiaxed grains. This percolation-type approach has been used then to calculate the pressure drop in the mushy zone on the assumptions of a Poiseuille flow in between the grains and a Kirchhoff model for the connectivity of the liquid films including the Losses associated with Solidification Shrinkage (i.e, PKL model). Comparison with a standard average pressure drop calculation based on Carman-Kozeny’s relationship will be presented.

Stephane Vernede - One of the best experts on this subject based on the ideXlab platform.

  • Computational Materials Laboratory, Ecole Polytechnique Fédérale de Lausanne,
    2013
    Co-Authors: Stephane Vernede
    Abstract:

    The gradual transformation of a mushy zone during alloy Solidification, from a continuous liquid film network to a fully coherent solid, has been simulated using a granular model. Based on a Voronoi tessellation of a random set of nucleation centers, Solidification within each polyhedron is computed considering back-diffusion and coalescence. In the network of connected liquid films, a pressure drop calculation is performed assuming a Poiseuille flow in each channel, Kirchhoff’s conservation of flow at nodal points and flow Losses compensating Solidification Shrinkage (KPL model). In addition to intergranular liquid pressure maps, the model shows the progressive formation of grains clusters, the localisation of the flow at very high solid fraction, and thus natural transitions of the mushy zone.

  • a granular model of equiaxed mushy zones formation of a coherent solid and localization of feeding
    Acta Materialia, 2006
    Co-Authors: Stephane Vernede, Philippe Jarry, M Rappaz
    Abstract:

    Abstract The gradual transformation of a mushy zone during alloy Solidification, from a continuous liquid film network to a fully coherent solid, has been simulated using a granular model. Based on a Voronoi tessellation of a random set of nucleation centres, Solidification within each polyhedron is computed considering back-diffusion and coalescence. In the network of connected liquid films, a pressure drop calculation is performed assuming a Poiseuille flow in each channel, Kirchhoff’s conservation of flow at nodal points and flow losses compensating Solidification Shrinkage (KPL model). In addition to intergranular liquid pressure maps, the model shows the progressive formation of grain clusters, the localisation of the flow at very high solid fraction and thus natural transitions of the mushy zone.

  • transition of the mushy zone from continuous liquid films to a coherent solid
    Philosophical Magazine, 2006
    Co-Authors: Stephane Vernede, M Rappaz
    Abstract:

    While studies of Solidification microstructures have focused mainly on the tips of the dendrites, the last stage Solidification is equally important from the point of view of defect formation (porosity, hot tearing), mechanical strength build-up and precipitation of phases. In particular, the transition from continuous liquid films to a coherent solid in low concentration alloys is of crucial importance for hot tearing formation, and more generally speaking for liquid feeding ability and coherency development. Based on a fairly recent theoretical model of coalescence which will be recalled briefly, new results obtained for a population of equiaxed grains will be presented. A granular-type model based on a Voronoi tessellation has been used for the description of the gradual disappearance of liquid films and the clustering of equiaxed grains. This percolation-type approach has been used then to calculate the pressure drop in the mushy zone on the assumptions of a Poiseuille flow in between the grains and a Kirchhoff model for the connectivity of the liquid films including the Losses associated with Solidification Shrinkage (i.e, PKL model). Comparison with a standard average pressure drop calculation based on Carman-Kozeny’s relationship will be presented.

Ivar Farup - One of the best experts on this subject based on the ideXlab platform.

  • in situ observation of hot tearing formation in succinonitrile acetone
    Acta Materialia, 2001
    Co-Authors: Jeanmarie Drezet, Ivar Farup, M Rappaz
    Abstract:

    Abstract Hot tears have been induced during the Solidification of a succinonitrile-acetone alloy by pulling the columnar dendrites in the transverse direction with a pulling stick. The opening of the mushy zone (hot tears) always occurred at grain boundaries. At low volume fraction of solid, the opening can be compensated by leaner-solute interdendritic liquid (i.e., “healed” hot tears). At higher volume fraction of solid, hot tears directly nucleate in the interdendritic liquid or develop from pre-existing micropores induced by Solidification Shrinkage. Their surface (edge) is made of secondary dendrite arms, which have not yet bridged, but a few spikes have also been observed. These later spikes formed either by the necking of solid bridges established across the grain boundaries prior to pulling, or by the sudden break-up of the liquid film during pulling. Similar spikes have been found by SEM on the hot tear surface of an aluminium–copper alloy.

  • two phase modeling of mushy zone parameters associated with hot tearing
    Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2000
    Co-Authors: Ivar Farup, Asbjorn Mo
    Abstract:

    A two-phase continuum model for an isotropic mushy zone is presented. The model is based upon the general volume-averaged conservation equations, and quantities associated with hot tearing are included, i.e., after-feeding of the liquid melt due to Solidification Shrinkage is taken into account as well as thermally induced deformation of the solid phase. The model is implemented numerically for a one-dimensional model problem with some similarities to the aluminium direct chill (DC) casting process. The variation of some key parameters that are known to influence the hot-tearing tendency is then studied. The results indicate that both liquid pressure drop due to feeding difficulties and tensile stress caused by thermal contraction of the solid phase are necessary for the formation of hot tears. Based upon results from the one-dimensional model, it is furthermore concluded that none of the hot-tearing criteria suggested in the literature are able to predict the variation in hot-tearing susceptibility resulting from a variation in all of the following parameters: Solidification interval, cooling contraction of the solid phase, casting speed, and liquid fraction at coherency.

Kah Fai Leong - One of the best experts on this subject based on the ideXlab platform.

  • Modeling temperature and residual stress fields in selective laser melting
    International Journal of Mechanical Sciences, 2018
    Co-Authors: Yingli Li, Pengfei Tan, Chee Kai Chua, Shu Beng Tor, Kun Zhou, Kah Fai Leong
    Abstract:

    The paper investigates the temperature and residual stress fields in the selective laser melting (SLM) process. A three-dimensional thermo-mechanical coupling model is developed to simulate a multi-track multi-layer SLM process using the finite element method. The model considers the temperature-dependent material properties which consist of thermal conductivity, density, enthalpy, yield stress, thermal expansion coefficient and Young's modulus. The simulated process includes the heating, melting, vaporization, Solidification, Shrinkage and cooling phenomena in the powder bed. The SLM scanning laser beam can be described as a moving volumetric heat source that is able to penetrate through the powder layers. The modeling results show that the residual stress component of the built part in the direction of the layer height increases with the number of the printed layers. It is found that at a given point, the residual stress component in the scanning direction is generally larger than the other two components, and the maximum von Mises stress occurs in the middle plane of the printed part. The temperature evolution and residual stress distribution predicted by the model can serve to provide guidance for SLM process parameter optimization.

M. Sistaninia - One of the best experts on this subject based on the ideXlab platform.

  • Prediction of Hot Tear Formation in Vertical DC Casting of Aluminum Billets Using a Granular Approach
    JOM, 2013
    Co-Authors: M. Sistaninia, J.-m. Drezet, A. B. Phillion, M Rappaz
    Abstract:

    A coupled hydromechanical granular model aimed at predicting hot tear formation and stress–strain behavior in metallic alloys during Solidification is applied to the semicontinuous direct chill casting of aluminum alloy round billets. This granular model consists of four separate three-dimensional (3D) modules: (I) a Solidification module that is used for generating the solid–liquid geometry at a given solid fraction, (II) a fluid flow module that is used to calculate the Solidification Shrinkage and deformation-induced pressure drop within the intergranular liquid, (III) a semisolid deformation module that is based on a combined finite element/discrete element method and simulates the rheological behavior of the granular structure, and (IV) a failure module that simulates crack initiation and propagation. To investigate hot tearing, the granular model has been applied to a representative volume within the direct chill cast billet that is located at the bottom of the liquid sump, and it reveals that semisolid deformations imposed on the mushy zone open the liquid channels due to localization of the deformation at grains boundaries. At a low casting speed, only individual pores are able to form in the widest channels because liquid feeding remains efficient. However, as the casting speed increases, the flow of liquid required to compensate for Solidification Shrinkage also increases and as a result the pores propagate and coalesce to form a centerline crack.

  • three dimensional granular model of semi solid metallic alloys undergoing Solidification fluid flow and localization of feeding
    Acta Materialia, 2012
    Co-Authors: M. Sistaninia, A. B. Phillion, Jeanmarie Drezet, M Rappaz
    Abstract:

    A three-dimensional (3-D) granular model which simulates fluid flow within solidifying alloys with a globular microstructure, such as that found in grain refined Al alloys, is presented. The model geometry within a representative volume element (RVE) consists of a set of prismatic triangular elements representing the intergranular liquid channels. The pressure field within the liquid channels is calculated using a finite elements (FEs) method assuming a Poiseuille flow within each channel and flow conservation at triple lines. The fluid flow is induced by Solidification Shrinkage and openings at grain boundaries due to deformation of the coherent solid. The granular model predictions are validated against bulk data calculated with averaging techniques. The results show that a fluid flow simulation of globular semi-solid materials is able to reproduce both a map of the 3-D intergranular pressure and the localization of feeding within the mushy zone. A new hot cracking sensitivity coefficient is then proposed. Based on a mass balance performed over a solidifying isothermal volume element, this coefficient accounts for tensile deformation of the semi-solid domain and for the induced intergranular liquid feeding. The fluid flow model is then used to calculate the pressure drop in the mushy zone during the direct chill casting of aluminum alloy billets. The predicted pressure demonstrates that deep in the mushy zone where the permeability is low the local pressure can be significantly lower than the pressure predicted by averaging techniques.