References of "Rassili, Ahmed"
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See detailThixoforming of Steel: New Tools Conception to Analyse Thermal Exchanges and Strain Rate Effects
Cezard, Pierre; Bigot, Regis; Becker, Eric et al

in AIP Conference Proceeding - 10th ESAFORM Conference on Material Forming (2007, April 07)

Through different papers, authors shown that the influence of thermal exchanges was a first order parameter on the semi-solid steel behaviour, and certainly for every semi-solid metallic materials. These ... [more ▼]

Through different papers, authors shown that the influence of thermal exchanges was a first order parameter on the semi-solid steel behaviour, and certainly for every semi-solid metallic materials. These thermal exchanges hide other parameters effect like, for example, the strain rate influence. This paper tries to determine the influence of these two parameters by using a new extrusion device on a hydraulic press. This new tools conception annihilated the influence of the decrease of the punch speed before stopping and permitted to have a constant speed during the experiment. This work also deals with the homogeneous flow during thixoforming of steel and shows the importance to couple initial temperature of the slug with punch speed. This paper presents different conditions which permitted to have a homogeneous flow by keeping a low load. [less ▲]

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See detailThixoforging of 7075 aluminium alloys at high solid fraction
Vaneetveld, Grégory ULg; Rassili, Ahmed ULg; Lecomte-Beckers, Jacqueline ULg et al

in Solid State Phenomena (2006), 116-117

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.5 < f(s) < 1), which involves the processing of alloys in the semi-solid state. 7075 alloy has been used as a feedstock for ... [more ▼]

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.5 < f(s) < 1), which involves the processing of alloys in the semi-solid state. 7075 alloy has been used as a feedstock for thixoforming in order to investigate thixoformability of a high performance aluminium alloy at high solid fraction. The microstructure in semi-solid state consists of fine spheroidal solid grains surrounded by liquid. Thixo behaviour allows laminar flow that avoids macro-porosity and other casting defects and gives better mechanical properties; so we can reduce the weight of functional parts. Higher solid fraction of 7075 alloy is less sensitive to temperature, avoids metal splash at high speed, allows laminar flow at high speed and gives a good smooth surface without stick-slip phenomenon. The process is quicker with less energy and so the production rate increases and the tool has longer life. However, high solid fraction promotes freezing flow because of faster solidification and the lack of liquid phase. This freezing flow creates solid/liquid segregation and increases the forming force. [less ▲]

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See detailSimulation of thixoforming: comparison of different constitutive equations
Pierret, Jean-Christophe ULg; Rassili, Ahmed ULg; Cezard, Pierre

in Solid State Phenomena (2006), 116-117

The behaviour of the semi-solid material is complex to simulate and many approaches are proposed in the literature to represent it. In this work, different constitutive equations are studied: the ... [more ▼]

The behaviour of the semi-solid material is complex to simulate and many approaches are proposed in the literature to represent it. In this work, different constitutive equations are studied: the classical Northon-Hoff law, Joly and Merhabian's one [1] and an innovative micromechanical model using homogenisation technique [2]. These constitutive equations have been implemented in the FE code Forge2 (R) to simulate simple forming tests, such as compression and direct extrusion. The load-displacement curves using the three constitutive equations are compared in order to estimate their capabilities. [less ▲]

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See detailDetermination of solidification parameters used for the prediction of the thixoformability of several steel alloys
Lecomte-Beckers, Jacqueline ULg; Rassili, Ahmed ULg; Robelet, Marc et al

in Solid State Phenomena (2006), 116-117

This paper focuses on the liquid fraction curves of several steels and the correlation between liquid fraction, temperature and heating rate. The work has been performed along two main axes. First, the ... [more ▼]

This paper focuses on the liquid fraction curves of several steels and the correlation between liquid fraction, temperature and heating rate. The work has been performed along two main axes. First, the solid fraction versus temperature has been obtained experimentally by differential scanning calorimetry (DSC), limited to low heating rates. Then, a shift of the liquid fraction curves has been noticed at high industrial heating rates. The quantification of this effect could not be carried out by DSC and required the elaboration of another experimental device. [less ▲]

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See detailSteel grades adapted to the thixoforging process : metallurgical structures and mechanical properties
Robelet, Marc; Rassili, Ahmed ULg; Fischer, Dirk

in Solid State Phenomena (2006), 116-117

Thixoforming of steel offers the advantages of casting technology in combination with high mechanical strength that can only be achieved by forging. The progress in establishing this technology in ... [more ▼]

Thixoforming of steel offers the advantages of casting technology in combination with high mechanical strength that can only be achieved by forging. The progress in establishing this technology in industry depends on the success in the development of suitable steel grades. Recent investigations dealt-with the development of steel grades that are especially adapted to the thixoforming process. For this, alloys were developed with a lower solidus temperature and a wider process temperature range compared to classic forging steels. In consequence, the inductive heating process is more tolerant to inaccuracies and for a given liquid fraction the process temperature window is easier to handle. It is desired to obtain great degrees of deformation at rather low forming forces as these parameters determine the size of the needed presses. This behaviour is affected by the present liquid fraction in the slug and the heat transfer between work piece and die. It was detected that variations of the forming force have a direct influence on the quality of the thixoformed parts. In order to make the thixoforming technology of steels competitive versus other forming technologies, the parts must show a favourable microstructure and thus, good in-use properties. In this paper various solutions are compared. The main results obtained in the optimization research, namely, the steel grades adapted to semi-solid forming, the resulting process parameters and the mechanical properties of thixoforming parts will be presented for two exemplary steel grades. By producing a real automotive part, thixoforging of steels with regard to the adapted materials and to the ongoing industrial implementation of this process is proved. [less ▲]

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See detailSeries production of thixoformed steel parts
Behrens, B. A.; Fischer, Dirk; Haller, B. et al

in Solid State Phenomena (2006), 116-117

In recent years several attempts were made to transfer the thixoforming technology of steel parts into industrial applicable processes. This paper gives an overview about the progress of a European ... [more ▼]

In recent years several attempts were made to transfer the thixoforming technology of steel parts into industrial applicable processes. This paper gives an overview about the progress of a European consortium that established a fully automated thixoforming process for the series production of automotive steel parts. Due to the multi-faceted nature of this technology, problems concerning the development of suitable steel grades and tool materials as well as the development and application of an inductive heating system, a handling unit and of a complex forming tool had to be solved. Besides the development of adapted steel grades and the inductive heating, the handling of the semi solid billets plays a special role because during the manipulation of the parts from the heating station into the tool a loss of heat is unavoidable. Furthermore, scaling of the parts must be prevented. By means of a fully automated process line existing constraints were reduced and the forming process is kept reproducible. Improved silicon nitride composites have been developed as a tool material, which show good mechanical properties in combination with an acceptable chemical stability at the occurring process temperatures as well as in contact with semi solid steel. Basing on the practical experience a comparison of the thixoforming technology to existing processes and an outlook for the ftiture are, given. [less ▲]

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See detailThixoforming of carbon steels: Inductive heating and process control
Rassili, Ahmed ULg; Robelet, Marc; Fischer, Dirk

in Solid State Phenomena (2006), 116-117

The development of steel grades for the production of thixoformed parts made a great progress but the process itself is still difficult to control. The most critical point is the inductive heating of the ... [more ▼]

The development of steel grades for the production of thixoformed parts made a great progress but the process itself is still difficult to control. The most critical point is the inductive heating of the billets to assuring a homogeneous distribution of the desired liquid fraction. One of the principle approaches is based on investigations of the thermo-physical properties of steel grades. These data are also used for numerical simulation of the inductive heating. From these results a mathematical model is derived and implemented in the heating chain for controlling the parameters that determines the billet temperature indirectly by computation and not by direct measuring of the temperature. Optimizing the coil geometry leads to an adapted magnetic field assuring a homogenous temperature distribution in the billet considering expected energy losses. We will also present new results of the liquid fraction within the billet as well as the electrical resistivity of the thixotropic steel at high heating rate corresponding to the industrial scale. Last but not least, some new parts made out of adapted steel will be shown. [less ▲]

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See detailSimulation of thixoforming: comparison of different constitutive equations
Pierret, Jean-Christophe ULg; Rassili, Ahmed ULg; Cezard, Pierre

in Solid State Phenomena (2006), 116-117

The behaviour of the semi-solid material is complex to simulate and many approaches are proposed in the literature to represent it. In this work, different constitutive equations are studied: the ... [more ▼]

The behaviour of the semi-solid material is complex to simulate and many approaches are proposed in the literature to represent it. In this work, different constitutive equations are studied: the classical Northon-Hoff law, Joly and Merhabian's one [1] and an innovative micromechanical model using homogenisation technique [2]. These constitutive equations have been implemented in the FE code Forge2 (R) to simulate simple forming tests, such as compression and direct extrusion. The load-displacement curves using the three constitutive equations are compared in order to estimate their capabilities. [less ▲]

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See detailStudy of liquid fraction evolution of semi-solid steels for thixoforming
Lecomte-Beckers, Jacqueline ULg; Rassili, Ahmed ULg; Carton, Marc ULg et al

in Proceedings of the 8th ESAFORM Conference on Material Forming (2005)

Major challenge for semi-solid processing includes broadening the range of alloys that can be successfully thixoformed and developing alloys specifically for thixoforming. One important parameter is ... [more ▼]

Major challenge for semi-solid processing includes broadening the range of alloys that can be successfully thixoformed and developing alloys specifically for thixoforming. One important parameter is appropriate solidus-liquidus interval. The wider the solidification interval, the wider the processing window. This study is related to the experimental determination of this critical parameter on eight different steel compositions. This parameter was obtained using Differential Scanning Calorimetry. This technique allows to obtain the solid fraction versus temperature. [less ▲]

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See detailStudy of the liquid fraction and thermophysical properties of semi-solid steels and application to the simulation of inductive heating for thixoforming
Lecomte-Beckers, Jacqueline ULg; Rassili, Ahmed ULg; Carton, Marc ULg et al

in Bananic, Dorel (Ed.) Advanced Methods in Material Forming (2005)

The thixoforming of steels is so complex that it requires more investigations regarding both the materials and the technical tools dedicated to the elaboration of the process. In this paper we will show ... [more ▼]

The thixoforming of steels is so complex that it requires more investigations regarding both the materials and the technical tools dedicated to the elaboration of the process. In this paper we will show the experimental determination of appropriate solidus-liquidus interval on eight different steel compositions. This critical parameter was obtained using Differential Scanning Calorimetry. The paper also presents the results of thermophysical property determination. These parameters are important for the inductive heating phase of a semi-solid forming (SSF) process. Thanks to the simulations of the inductive heating process, the other main results consist on the developments of the heating techniques that are suitable for the achieving of the sine qua none condition to the semi-solid process, which is the uniform temperature distribution in the reheated billet. [less ▲]

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See detailCharacterisation of thermophysical properties of semi-solid steels for thixoforming
Lecomte-Beckers, Jacqueline ULg; Rassili, Ahmed ULg; Carton, Marc ULg et al

in Proceedings of the 8th International Conference on Semi-Solid Processing of Alloys and Composites (S2P) (2004)

Major challenge for semi-solid processing includes broadening the range of alloys that can be successfully thixoformed and developing alloys specifically for thixoforming. One important parameter is ... [more ▼]

Major challenge for semi-solid processing includes broadening the range of alloys that can be successfully thixoformed and developing alloys specifically for thixoforming. One important parameter is appropriate solidus-liquidus interval. The wider the solidification interval, the wider the processing window. This study is related to the experimental determination of this critical parameter on eight different steel compositions. This parameter was obtained using Differential Scanning Calorimetry. This technique allows to obtain the solid fraction versus temperature. The paper also presents the results of thermophysical properties determination such as thermal diffusivity, heat capacity and thermal conductivity. These properties were measured from room temperature to semi-solid state in one particular steel. The thermal diffusivity was measured using the Laser Flash method and the heat capacity using a DSC calorimeter. The thermal conductivity was obtained by calculation knowing the thermal dilatation measured with a dilatometer. All these measurements were performed for temperatures up to the liquidus. These parameters are difficult to measure but they are important to determine for the conductive heating phase of a semi-solid forming (SSM) process. [less ▲]

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See detailCrystal-field effects on the thermal conductivity of localized spin metallic compounds
Rassili, Ahmed ULg; Durczewski, K.; Ausloos, Marcel ULg

in Physical Review. B : Condensed Matter (1998), 58(9), 5665-5671

The influence of the crystal-electric-field (CEF) splitting on the thermal conductivity is calculated on the basis of a two-level system model applicable to intermetallic magnetic compounds. The localized ... [more ▼]

The influence of the crystal-electric-field (CEF) splitting on the thermal conductivity is calculated on the basis of a two-level system model applicable to intermetallic magnetic compounds. The localized spin scattering contribution kappa(s), in a manner similar to the total electronic thermal conductivity kappa(e), shows a larger increase at low and intermediate temperatures as compared to the case iii which-no crystal-electric-field splitting is taken into account. The influence of some theoretical parameters is also discussed. It is shown that the CEF effect enhances the effect of the magnetic scattering potential, and impurity contributions screen such an enhancement at temperatures below the Debye temperature. Other scattering contributions, e.g., electron-phonon and electron impurities, are also taken into account in our calculation. The theory is in quantitative agreement with data on RA1(2) systems taken as test cases, and leads to values of the level splitting in the 50 K range. [less ▲]

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See detailTheory of the thermal conductivity of metallic localized spin compounds in a magnetic field (vol 163, pg 153, 1996)
Rassili, Ahmed ULg; Ausloos, Marcel ULg

in Journal of Magnetism & Magnetic Materials (1997), 170(1-2), 241-241

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See detailTheory of the thermal conductivity of metallic localized spin compounds in a magnetic field
Rassili, Ahmed ULg; Ausloos, Marcel ULg

in Journal of Magnetism & Magnetic Materials (1996), 163(1-2), 153-163

We calculated the influence of a de magnetic field on the thermal conductivity of localized spin metallic systems as a function of temperature. A variational method was used. The influence of the joint ... [more ▼]

We calculated the influence of a de magnetic field on the thermal conductivity of localized spin metallic systems as a function of temperature. A variational method was used. The influence of the joint impurity and phonon scattering processes together with the localized spin influence is shown. Remarkable features are found at low temperature and near the critical transition. An increase of conductivity with field near T-C is discussed. It is also argued that a coherent effect due to the field is the cause of the thermal conductivity enhancement at low temperature. The magneto-thermal conductivity is shown at different temperatures. [less ▲]

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See detailTHEORY OF THE THERMAL-CONDUCTIVITY OF RE AL-2 COMPOUNDS
Rassili, Ahmed ULg; Ausloos, Marcel ULg

in Journal of Magnetism & Magnetic Materials (1995), 147(3), 341-345

On the basis of the variational method for transport coefficients we have calculated the thermal conductivity of RE Al-2-like systems as a function of temperature. The low-temperature limit obeys ... [more ▼]

On the basis of the variational method for transport coefficients we have calculated the thermal conductivity of RE Al-2-like systems as a function of temperature. The low-temperature limit obeys Matthiessen's rule 1/kappa = 1/kappa(ph) + 1/kappa(s) + 1/kappa(imp) where the three components are the contributions arising from e(-)-phonon, e(-)-localized spin and e(-)-impurity interactions, respectively. The temperature dependence of each term has been calculated, as well as the superposition of all three. The influence of the physical parameters (spin value, Debye temperature, cut-off wave vector, Fermi energy, etc.) is shown. The theory is in good agreement with reported data for GdAl2. [less ▲]

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