Key Engineering Materials Vols. 554-557

Abstract: Heat transfer coefficients are playing an important role in forming of metastable stainless steel sheets. Metastable austenitic stainless steels are highly influenced by heating of forming tools due to generation of latent heat during forming process. Strain-induced martensite formation and hence the TRIP-effect is directly coupled with the temperature development within forming tools as well as the temperature induced by heat controlled tools. Measurements of heat development in serial deep drawing processes are showing the need for an accurate determination of heat transfer coefficients considering actual process conditions. Heat transfer coefficients were determined with a simple and easy applicable measurement device for tool materials AMPCO 25 and cold work tool steel EN 1.2379 in combination with aluminum, austenitic EN 1.4301 and ferritic EN 1.4016 stainless steel grades. Special attention was paid to production-related individual influences such as surface conditions, lubrication and deep drawing film. Experiments were accomplished between 1-15 N/mm² showing high influence of intermediate media on heat transfer between forming tool and part and serve as boundary conditions for fully thermo-mechanical coupled forming simulations. A strong influence of deep drawing film, lubrication and surface pressure on heat exchange could be determined and this basic knowledge is seen as mandatory for dimensioning of heat controlled metal forming tools. Finally the experimental determined results are discussed and compared to common heat transfer models and similar experiments from literature.

Abstract: 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.

Abstract: Epoxy resins have several applications in the aerospace and automobile industry. Because of their good adhesive properties, superior mechanical, chemical and thermal properties, and resistance to fatigue and micro cracking, they produce high performance composites. In the technology presented here, the composite is cured in an IR oven which includes halogen lamps. The liquid resin infusion (LRI) process is used to manufacture the composite, whereby liquid resin is infused through a fiber reinforcement previously laid up in a one-sided mold. These epoxy resins release an exothermic heat flux during the curing process, which can possibly cause an excessive temperature in the thickness. Consequently, for the production of high performance composites, it is necessary to know the thermal behavior of the composite during curing. Therefore, IR interactions with the graphite/epoxy system were modeled as a surface radiation transport. In our work, we have studied IR interactions with the composite, which is placed in an IR oven. Using an IR spectrometer Bruker Vertex 70 (1-27 μm), we measured radiative properties and determined the fraction of IR rays absorbed by the composite. Since it is necessary to optimize the manufacturing time and costs and to determine the performance of these composites, the purpose of this study is to model the IR curing of a composite part (carbon fiber reinforced epoxy matrix) in the infrared oven. The work consists in two parts. In the first part, a FE thermal model based on radiosity method was developed, for the prediction of the infrared incident heat flux on the top surface of the composite during the curing process. This model was validated using a reference solution based on ray tracing algorithms developed in Matlab® (In-lab software called Rayheat based on ray tracing algorithms is used to compute the radiative heat flux that impacts the composite). Through the FE thermal model, an optimization study on the percentage power of each infrared heater is performed in order to optimize the incident IR heat flux uniformity on the composite. This optimization is performed using the Matlab® optimization algorithms based on Sequential Quadratic Programming method. In a second part, the optimized parameters set is used in a three-dimensional numerical model which is developed in the finite element commercial software Comsol Multiphysics ™, where the heat balance equation is coupled with the cure kinetic model of the resin. This numerical model allows calculation of the temperature distribution in the composite during curing, which is a key parameter that affects its mechanical properties. In this model, we can predict also the evolution of the degree of cure as function of time. Experimental measurements were used to validate simulations of the whole infrared composite curing process. Keywords: Curing composite, infrared oven, Radiation, Optimization, Epoxy resin, Carbon fibers.

Abstract: Composite stamping is a two steps process that includes an infrared heating oven in order to melt the composite sheets before forming. This study deals with the numerical simulation of the heating step of the process. The numerical model has been validated using three woven glass and carbon / PA6.6 composites provided by Solvay Rhodia. This type of simulation consists in solving the heat equation with a radiative flux that characterizes the interaction of the material with the IR heating. The model thus considers the IR properties of the material (emission and reflexion). Considering a homogeneous composite, the optical and thermal properties of sheets have been first measured. The material’s emissivity has been measured using a FTIR spectrometer from the reflective and transmitive spectra, by using the Kirchoff law and considering a Lambertian material. Three complementary measurement techniques were used to determine the thermal properties of the composites. Differential Scanning Calorimetry (DSC) measurements have been performed to identify the heat capacity of the composites. On another hand, a hot disc system (measurements performed at the LTN, France) has been used in transient conditions to determine the heat capacity and the thermal conductivity of the composites is all three directions. Finally, the in-plane thermal matrix of conductivity has also been measured by thermography by using an inverse method. The simulation of composites heating has been performed with Comsol MultiphysicsTM and the simulation procedure was validated by comparison with experimental results. The simulated IR oven is composed of 9 IR emittors provided by Toshiba Lighting Company that emit mainly in short IR wavelength (0.75-2µm). The emission properties of the tungsten filament were implemented in order to simulate the IR heating. Free convective heat transfer was also taken into account in the oven. In order to validate the model, an experimental set-up was instrumented with a calibrated IR pyrometer that measured the back side of the heated composite sheets. The experimental results confirm a low thermal gradient through composite thickness, in particular for carbon-reinforced composite. This result is consistent with the low Biot number of the composites. Moreover, experimental and simulated temperatures are in good agreement with an error lower than 15% in the entire heating stage from room to melt temperature.

Abstract: Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tubes production because of the high heat transfer rates provided by the boiling of the subcooled water jet. As far as metallurgical phase transformations, residual stresses and deformations in the workpiece are concerned, the temperature drop during cooling must be perfectly controlled thanks to a fully understanding of the heat transfer mechanisms. In a previous study [1] the effect of surface to jet velocity ratio on heat transfer has been characterized and it has been shown that this parameter has a significant influence on shoulder of flux collapse. In order to understand the effect of more industrial quench process parameters, an innovative experimental quenching device has been designed and built. It allowed us to make heat transfer measurements at the surface of a hot nickel cylinder impinged by a subcooled water jet, according to several angles of impact and three jet directions against gravity. The results clearly highlight an effect of these two parameters on the heat transfer mechanisms at the surface of the tube. These results allow a better understanding of the origins of temperature heterogeneities inside the tube during the quench.

Abstract: A numerical model for a multiphysics problem is presented. It includes the movement of subdomains, which are embedded in a global air domain. The description of the movement is based on a discrete level set representation of the moving boundaries. It is based on the original geometry of the moving tools, such that the mesh quality is not reduced in subsequent time steps.

Abstract: Car manufacturers use many different mechanical or thermo-mechanical processes to assemble thin metal sheets. Each of these processes may induce –more or less localizedundesirable deformations of the assembled sheets. The numerical simulation of these deformations via the use of predictive thermo-mechanical finite element analyses may help and reduce development time and cost. To validate such complex non-linear numerical simulations by comparison of the numerical results with experimental evidence, it is necessary to develop well-controlled and highly instrumented tests. Welding-brazing is one the various assembly processes involving a heat source used by car manufacturers to assemble thin –typically 0.7 to 1mm thick- metal sheets. Predictive heat transfer simulations of this process require accurate modelling of both the heat source and the boundary conditions imposed onto the assembled sheets –including the contact between the sheets. Using ABAQUS general-purpose finite element code for this study, the welding-brazing heat source is modelled as the combination of a 3D volumetric source corresponding to the welding-brazing joint, and a 2D surface source corresponding to the heat flux emitted by the laser beam onto the sheets in the vicinity of the joint. Besides, the contact between the sheets is modelled by a thermal resistance depending on the pressure between the sheets when they are in contact, or the distance between them when they are apart. Identification of heat source and contact models is derived from the results of weldingbrazing tests instrumented with thermocouples and infrared cameras. Micrographic observations in the vicinity of the welding-brazing joint are used to (partially) validate the numerical predictions of the Heat Affected Zone (HAZ). After validation of this heat transfer numerical analysis, the thermo-mechanical analysis of the process will be validated by comparison of the predicted displacement fields with the results of 3D Digital Image Correlation (stereo-correlation) measurements made during the welding-brazing tests.

Abstract: A temperature sensor with a thermocouple placed at ~0.5 mm from roll surface is used in hot rolling conditions to evaluate by inverse calculation heat transfers in the roll bite. Simulation analysis in industrial hot rolling conditions with short contact lengths (e.g. short contact times) and high rolling speeds (7 m./sec.) show that the temperature sensor + inverse analysis with a high acquisition frequency (> 1000 Hz) is capable to predict with a good accuracy (5 to 10% error) the roll bite peak of temperature as well as the roll surface temperature evolution all around the roll rotation. However as heat flux is more sensitive to noise measurement, the peak of heat flux in the bite is strongly under-estimated (40% error) by the inverse calculation and thus only an average roll bite heat flux could be expected from the sensor (these simulation results will be verified with an industrial trial that is being prepared). Rolling tests on a pilot mill with low rolling speeds (from 0.3 to 1.5 m./sec.) and strip reductions varying from 10 to 40% have been performed with the temperature sensor. Analysis of the tests by inverse calculation show that at low speed (<0.5 m="" sec="" and="" large="" contact="" lengths="" reduction:="" 30="" to="" 40="" the="" roll="" bite="" peak="" of="" heat="" flux="" reconstructed="" by="" inverse="" calculation="" is="" correct="" at="" higher="" speeds="" 1="" 5="" smaller="" reduction="" :="" 10-20="" reconstruction="" incorrect:="" in="" under-estimated="" though="" its="" average="" value="" analysis="" reveals="" also="" that="" transfer="" coefficient="" htc="" sub="">roll-bite (characterizing heat transfers between roll and strip in the bite) is not uniform along the roll bite but is proportional to the local rolling pressure. Finally, based on the above results, simulations with a roll thermal fatigue degradation model in industrial hot rolling conditions show that the non uniform roll bite Heat Transfer Coefficient HTC_roll-bite may have in certain rolling conditions a stronger influence on roll thermal fatigue degradation than the equivalent (e.g. same average) HTC_roll-bite taken uniform along the bite. Consequently, to be realistic the roll thermal fatigue degradation model has to incorporate this non uniform HTC_roll-bite.

Abstract: The Young modulus and tensile strength of flax fibre reinforced polypropylene were determined and compared with the micromechanical models usually used in the case of short glass fibre reinforced composites. The fibre length and fibre diameter distributions of the injected reinforced of 2, 4, 8 and 22vol% compound were determined and used to the models in order to evaluate the expected properties of the composites. The mechanical properties were interpreted on the base of real fibre content, fibre orientation, fibre length and diameter distributions and morphology of the composites. The Kelly-Tyson’s model of the tensile strength prediction has been modified to take in consideration the fibre property variability due to the large distribution of fibre shape ratio induced by the process. Finally matrix modulus has been adjusted to take into account the change of crystallinity with fibre content.

Abstract: The homogeneity of the wall thickness (distribution) is considered to be the most critical parameter in the quality assessment of a thermoformed product. Numerous previous studies have characterized the thickness distribution by means of manual discrete tactile measurements. Such approach is slow, operator dependent and only gives results on specific points of the final product, resulting in complicated judgements on the causes of the thinning of the polymer sheet. This work presents a methodology to use digital image correlation (DIC) for on-line, full field wall thickness measurements of thick gauge thermoformed parts during and after thermoforming. Such technique offers the following advantages. Firstly, it provides the user with thickness results over a complete area instead of a discrete measuring point. Secondly, it allows on-line measurements so that a better insight can be obtained in the deformation mechanisms during the forming process. Finally, a correlation is made between each undeformed point in the base image and the same point in the deformed images during thermoforming, resulting in a full-field strain image where intermediate sheet thinning can be calculated. This makes it easier to determine a causal relation between thermoforming parameters and final thickness distribution of the product.