Key Engineering Materials Vols. 504-506

Abstract: Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tube production because of the high heat transfer 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 controlled thanks to a full understanding of the heat transfer mechanisms. One of the main characteristic using jet impingement is that the transition boiling regime may exist for very high superheat and thus the Leidenfrost temperature is higher than in pool boiling; consequently, boiling curves generally have a particular shape in the transition boiling regime which is usually called “shoulder of flux”. In this study, an innovative experimental quenching device has been used for analyzing the effect of the wall velocity of the surface to be cooled on the boiling curves (i.e. heat transfer) and we especially point out that the “shoulder of flux” (i.e. transition boiling regime) is strongly dependent on the surface to jet velocity ratio (r*). We found that a very small increase of the wall velocity has a high influence on shoulder of flux collapse.

Abstract: Plate and strip hot rolling lines are equipped with water cooling systems used to control the deformed material temperature. This system has a great importance in the case of thermal - mechanical deformation of steel which is focused on formation a proper microstructure and mechanical properties. The desired rate of cooling is achieved by water spray or laminar cooling applied to the hot surface of a strip. The water flow rate and pressure can be changed in a wide range and it will result in a very different heat transfer from the cooled material to the cooling water. The suitable cooling rate and the deformed material temperature can be determined based on numerical simulations. In this case thermal boundary conditions have to be specified on the cooled surface. The determination of the heat transfer coefficient distribution in the area of the water spray nozzle would improve numerical simulations significantly. In the paper an attempt is made to determine the heat transfer coefficient distribution on the hot plate surface cooled by the water spray nozzle. In the inverse method direct axially symmetrical and three dimensional solutions to the plate temperature field have been implemented. The computation time and the achieved accuracy have been compared for five cases. The studied cases differed in the maximum value of the heat transfer coefficient in nozzle spray axis and its distribution in the cooling time.

Abstract: The heat transfers are investigated between a phenolic pin sliding against a steel disc. Inverse heat conduction methods are used to identify the temperature fields and the heat fluxes at the contact surface for each part. Experimentals tests are performed on the High Speed Tribometer designed to reproduce mechanical and thermal conditions met in forming and braking applications at reduced scale. It allows to measure temperatures inside rotating part in seven locations with K-type thermocouples sensors connected to a telemetry system. Their positions have been chosen regarding a sensibility analysis performed in expected thermal conditions. The heat distribution on the disc surface, the heat repartition parameter α between the pin and the disc and the sliding thermal conductance h_c are discussed regarding the friction conditions during the tribological tests performed. In this tribological system, the temperature and the pressure influences on hc are highlighted. The originality of this work is the possibility to calculate two dimensional transient heat fluxes for sliding speed until 15m.s^-1 thanks to several thermocouple measurements.

Abstract: Selective laser melting (SLM) first developed for rapid prototyping (RP) is now used for rapid manufacturing of parts with inner complex shapes that cannot be made by more conventional routes. For example, production of injection moulds with cooling channels is of special interest. In this paper, a numerical model of SLM process was investigated to simulate the genesis of residual stresses. The proposed numerical modelling is based upon a double meshing with a multi step birth and death technique of manufactured part. The influence of the mesh size is analysed with element as small as the powder layer thickness for 2D and 3D geometries of simple parts. Comparisons between plane stress, plane strain, and 3D analysis are presented in order to propose a simplified approach.

Abstract: This paper presents a new technique to model the effect of intermediate induction heat treatment (IIHT) on pre-strained aluminium sheets, predominantly AA5182. IIHT is a heat treatment technique carried out between two conventional cold forming steps, which eventually lead to enhanced formability of aluminium alloys. The aim of IIHT is to alleviate the strain hardening of the material which is introduced in the first cold forming step and there by reducing the yield limit and increasing the hardening modulus for subsequent forming steps. As a result, a remarkable increase in formability can be achieved in the subsequent forming steps at room temperature. The scientific aspect of the IIHT process is demonstrated by defined pre-strained tensile test specimens at different object temperatures to establish a process window. To accurately model the effect of IHTT in simulations, it is necessary for the material model to consider the plastic recovery that the material undergoes during heat treatment. To this effect, material model Mat133 (Barlat_YLD2000) in LS-Dyna has been enhanced to account for the effect of intermediate heat treatment. The numerical simulation is carried out in four steps namely pre-forming, springback simulation to account for residual stresses, thermo-mechanical coupled simulation for heat treatment, and final forming with enhanced material model. To validate this model, experiments have been carried out on a simple cross-die deep drawn cup and compared with simulation results.

Abstract: 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.

Abstract: The main purpose of this study is to cure a 3D geometry composite part (carbon fiber reinforced epoxy matrix) using an infrared oven. The work consists of two parts. In the first part, a FE thermal model 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®. 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 and dynamically linked with the FE software COMSOL Multiphysics®. In a second part, the optimized parameters set is used in a model developed for the thermo-kinetic simulations of the composite IR curing process and the predictions of the degree of cure and temperature distribution in the composite part during the curing process.

Abstract: In this study, the effective thermal conductivity tensor of carbon/epoxy laminates was investigated experimentally in the three states of a typical LCM-process: dry-reinforcement, raw and cured composite. Samples were made of twill-weave carbon fabric impregnated with epoxy resin. The transverse thermal conductivity was determined using a classical estimation algorithm, whereas a special testing apparatus was designed to estimate in-plane conductivity for different temperatures and different states of the composite. Experimental results were then compared to modified Charles & Wilson and Maxwell models. The comparison showed clearly that these models can be used to accurately and efficiently predict the effective thermal conductivities of woven-reinforced composites.

Abstract: Injection Stretch Blow mouldng is a two step processing that was designed and optimized mainly using unfilled PET resins. This study focuses on stretch blow moulding of a PET filled with a few percent of sub micronic mineral fillers. Based on DSC, DMA, tensile tests as well as blowing on prototype machine main effects of fillers are analysed. It is demonstrated that fillers increases crystallization kinetics resulting in a reduction of the processing range. Difference in strain hardening induced by fillers makes it necessary to adjust blowing temperature. However main effect occurs during heating phase. Temperature within the perform is much less homogeneous than in PET making thermal gradient totally different if heating protocole is kept unchanged. Once heating is controlled to reach to equivalent thermal gradients as for PET blowing is possible and rather equivalent to that of pure PET.

Abstract: Plugs are a common feature of most deep-draw thermoforming processes and are used to ensure that the wall thickness distribution in the final product is controlled and balanced. Through contact with a moving mechanical plug, the heated sheet is locally captured and protected from excessive deformation and thinning. Previous work has clearly demonstrated that slip plays a critical role during this process and that its magnitude is determined by frictional properties that are strongly dependent on temperature. Work to discover the appropriate friction relationships has been very limited to date and this has greatly hampered the progress towards effective thermoforming process simulations. In this paper the magnitude of slip that occurs during the plugging stage of the thermoforming process was experimentally investigated. Preform shapes were created by pushing a specially designed plug into a heated sheet and then freezing it at the end of the plug displacement. A variety of processing parameters such as the plug and sheet materials, the temperature and plug displacement were evaluated. The results show that large variations in slip occur when different combinations of plug and sheet materials are employed and these are most affected by the contact temperature. A finite element based simulation of the plugging process is currently being constructed and it will be used to investigate different friction relationships and compare their performance with the experimental behaviour.