Brahim Bourouga

Les aspects théoriques régissant l'instrumentation d'un capteur thermique pariétal à faible inertie

The theoretical aspects of the instrumentation of a weak inertia parietal thermal sensor. From a simple semi-infinite model, we present theoretical developments allowing the optimisation of the instrumentation of parietal thermal sensors. Generally equipped with two thin thermocouples, these sensors must present a weak thermal inertia in order to characterise thermal boundary conditions in fast unsteady regime. Performances of such sensors are described by three characteristics: response time, measurement sensitivity and accuracy. For a given material, the constraint on the response time determines diameter and position of the second thermocouple. In a second time, the position of the first thermocouple results from a good compromise between measurement sensitivity and accuracy. This compromise depends itself on the sensor response time, particularly at the beginning of the studied thermal phenomena.

Influence de la vitesse et de la charge sur la conductance thermique de transport entre les bagues d'un roulement à rouleaux

The heat transfer between the rings of a bearing are governed by two modes of transport related respectively to the two periodical movements of the rolling element. First is related to the orbital movement of the roller around the shaft: that is the orbital transport mode. The second mode is related to the rotational movement of the roller around its own axis in the double contact zone: it is the mode of transport by simple rotation. We model this heat transfer between the rings (solid convection) by a Transport Thermal Conductance (TTC) related to the raceway of external ring. The reference temperature is the one of the internal ring. The study of the influence rotational speed on the TTC emphasised that for the weak loads, the heat transfer by orbital transport increases quickly in the range 100–400 rpm. Under the influence of the centrifugal force, orbital transport becomes sufficiently significant to restrict the cooling of the external ring in double contact zone. This increase is braked when the speed corresponding to a permanent contact between rollers and the external ring is reached (430 rpm approximately). Beyond this speed, both transport modes are stimulated by the same mechanism: the reduction in thermal constriction resistances present on both sides of the rolling contact by the increase in speed. At low speed, the influence of the load is felt only in the double contact zone. For loads larger than 1000 N and speeds higher than 500 rpm, the two modes of transport become strongly coupled and the TTC is then monotonous increasing according to the two parameters.

Modèle predictif de résistance thermique de contact dynamique adapté au cas de l’interface pièce–outil de forgeage

Resume We propose a model of dynamic thermal contact resistance. This model is intended to be implanted in numerical codes of thermomechanics with as first application: the workpiece–die interface during a hot forging process. All the interfacial parameters are temporal functions. The real rate of contact is represented by the ratio local normal stress on flow stress of the workpiece corresponding to the contact temperature. The principle of the model suggested consists to connect the real rate of contact to the density of contact spot and to the average interstitial thickness. These parameters are functions which are established from a surface analysis processing of the workpiece and die before the forging operation. Calculated at each step of time, the real rate of contact gives access thus to the other parameters and thus to the instantaneous thermal resistance of contact. Le modele de resistance thermique de contact (RTC) dynamique propose est destine a etre implante dans des codes de calcul de thermomecanique avec comme premiere application: l’interface piece–outil de forgeage. Tous les parametres d’interfaces sont des fonctions temporelles. Le taux reel de contact est represente par le rapport contrainte normale locale sur contrainte d’ecoulement de la piece correspondant a la temperature de contact locale. La premiere est calculee par le code et la seconde est une donnee rheologique. Le principe du modele propose consiste a relier le taux reel de contact a la densite de points de contact et a l’epaisseur interstitielle moyenne par des fonctions que l’on etablit a partir d’un traitement des releves topographiques des surfaces de la piece et de l’outil avant l’operation de forgeage. Calcule a chaque pas de temps, le taux reel de contact permet d’acceder ainsi aux autres parametres et donc d’estimer la RTC instantanee.

Le contact thermique pièce–outil lors d'une opération de forgeage à chaud: validation de l'hypothèse de résistance thermique de contact et influence de la loi de comportement de la pièce

Abstract The heat transfer at the workpiece–die interface is controlled by thermal interface resistance. In the cases of dry contact or mixed lubrication, one speaks about thermal contact resistance (TCR). During hot forging, the workpiece–die contact is not static since the normal stress at the interface monotonously increases as the forging force. In this paper, the assumption of TCR in the case of the workpiece–die interface during the hot forging process is validated experimentally. The experiment shows that the TCR depends only on the instantaneous strain, which makes it an intrinsic parameter related to the instantaneous structure of the interface and not to the forging time. The significant role of the constitutive law of the workpiece on the evolution of the TCR is also shown.

Experimental Approach for Thermal Contact Resistance Estimation at the Glass / Metal Interface

An experimental device is designed and developed in order to estimate thermal conditions at the Glass / Metal contact interface. The device is made of two parts: the upper part contains the tool (piston) made of bronze and a heating device to raise the temperature of the piston to 700° C. The lower part is composed of a lead crucible and a glass sample. The assembly is provided with a heating system, an induction furnace of 6 kW for heating the glass up to 950° C. Both parts are put in contact through a mechanical system consisting of a pneumatic cylinder sliding on a column and a pump providing the required pressure in the enclosure. The developed experimental procedure has permitted the estimation of the Thermal Contact Resistance TCR using a developed measurement principle based on an inverse technique. The semi-transparent character of the glass has been taken into account by an additional radiative heat flux and an equivalent thermal conductivity. After the set-up tests, reproducibility experiments for a specific contact pressure have been carried out. Results shows a good repeatability of the registered and estimated parameters such as the piston surface temperature, heat flux density and TCR. The maximum dispersion of the estimated TCR doesn’t exceed 6%.Copyright

Thermal Study of Hot Stamping with Heated and Cooled Tooling to Obtain Tailored Properties

To produce parts with tailored properties, i.e. parts with high strength in some areas and high ductility on some other areas, one of the most popular method, called the tailored tempering process, is to heat up locally the tools. In the hot areas, the blank follows a different thermal path leading to different microstructure evolutions and thus different final mechanical properties. In this paper, a tool is designed to have a side heated up to 500°C and a water cooled side. The hot side is heated up thanks to heated cartridges. A PID regulation is used to control the temperature of the hot side (from 200°C to 500°C) while the cold side is maintained at a low temperature using a thermostated water circulation. A uniform temperature on the working surface is successfully reached on both sides. Instrumentation by thermocouples is designed to be able to fully characterize the heat transfer: solving 2D heat conduction problems, the temperature fields in the tools and the thermal contact resistances at the blank/tool interfaces are estimated. Hardness measurements are also performed on the blank: the possibility to confer a distribution of mechanical properties is highlighted.

Estimation of the Heat Transfer Conditions in a die Radius during Hot Stamping

Established process in the automotive industry, the hot stamping process consists in heating a blank until complete austenitization in a furnace before transferring it to a press where it is formed at high temperature before being quenched by contact with the cold tools. During the forming step the hot blank slides on the die radius. Locally, the contact pressure can reach very high values. Due to this contact, heat transfer between the hot blank and the die can be significant. Using an omega die instrumented with eight thermocouples localized in the die radius, a 2D inverse method is used to estimate the heat flux that crosses the Blank/Die interface and the temperature field in the die radius and on the die surface. Four thermocouples are located in the blank thickness and a FE analysis is performed to estimate their positions as function of the time. The temperature in the thickness of the blank is considered as uniform according to Biot number value. This assumption is checked afterward. Thus, it is possible to estimate the sliding thermal contact resistance between the blank and the die as a function of time in front of each thermocouple of the blank. The estimation of the temperature field in the die can be useful for investigating the fatigue that occurs in the die. On the other hand, the knowledge of the interface condition in the die radius can present a high interest for improving the numerical simulations of this process.

Latent Heat Estimation of the Martensite Transformation Through Inverse Methods During the Hot Stamping Process

In the hot stamping process, the temperature evolution drives the metallurgy, as well as the metallurgical transformation influences the temperature evolution through the exothermic nature of the austenite to martensite transformation. This heat release has already been highlighted by previous experimental work. This heat release leads to a source term in the heat equation in the blank. This source term must be quantified in order to accurately predict blank temperature evolution. Moreover, the end of the heat release corresponds to the end of the metallurgical transformation. It allows the determination of the minimum quenching time, relevant information for industry to minimize the process time.This paper presents a method to quantify the heat released by the metallurgical transformation during the hot stamping of Usibor 1500P® ArcelorMittal steel, solving inverse conduction problems. It allows the determination of this heat release as a function of temperature or time. Then, integrating it, the latent heat of the transformation can be estimated. This can be done for different contact pressures between tool and blank. Finally, it can be linked to the martensite proportion to estimate it as a function of time or temperature and determine the Koistinen-Marburger model parameters. These results should improve the accuracy of numerical simulations of the hot stamping process.Copyright

Metallurgical Model Fitted to Experimental Data Using a Genetic Algorithm

The hot stamping process is an established process in the automotive industry to satisfy challenges concerning security aspects and lightweight construction. Now, innovative processes have arisen which consist in heating locally the tools and thus adjust local final mechanical properties of the parts. To simulate accurately this so called tailored tempering process, a coupling between thermal and metallurgical phenomena must be considered as the metallurgical transformations lead to a source term in the heat equation and the thermal evolution drives the transformation. To improve the model, a genetic algorithm optimizes the metallurgical model parameters to fit both the CCT and TTT diagrams, taking in account the cooling rate dependence. This method for creating a metallurgical data file, that is directly usable by the industrial software and that fits the TTT diagram and the final constituent proportions of the different constituents, is presented. This method, tested on hypothetical experimental data, is then validated and results are presented. Moreover, the principle of this work can be adapted to various softwares that industries use.