Yannick Le Maoult

Thermal Modeling in Composite Transmission Laser Welding Process: Light Scattering and Absorption Phenomena Coupling

In previous studies [1, , we have presented a detailed formulation of a macroscopic analytical model of the optical propagation of laser beams in the case of unidirectional thermoplastic composites materials. This analytical model presented a first step which concerns the estimation of the laser beam intensity at the welding interface. It describes the laser light path in scattering transparent composites (first component) by introducing light scattering ratio and scattering standard deviation. The absorption was assumed to be negligible in regard to the scattering effect. In this current paper, in order to describe completely the laser welding process in composite materials, we introduce the absorption phenomenon in the model, in the absorbing material (second component), in order to determine the radiative heat source generated at the welding interface. Finally, we will be able to perform a three dimensional temperature field calculation using a commercial FEM software. In laser welding process, the temperature distribution inside the irradiated materials is essential in order to optimize the process. Experimental measurements will be performed in order to valid the analytical model.

Direct Examinations of Oxide Scales upon Cooling: A New Way to Analyse Oxide Scale Spallation

The spallation behaviour of thick alumina scale grown at 1300 degreesC on PM2000 alloy was investigated by recording the events occurring upon cooling from 1300 degreesC down to ambient temperature with CCD cameras. The evolutions of the spalled surface fraction with time and temperature as well as the trajectories of spalled particles were observed and analysed. These observations and analysis were used to determine the critical temperatures drops and incubation times for the onset of spallation as well as to calculate the kinetic energies of spalled oxide particles which is a part of the energy balance describing the interfacial decohesion, the fracture and finally the spallation of oxide scales. In addition to the usual spallation phenomenon resulting in the ejection of small but measurable oxide spalls, localized and sudden outbursts of very fine oxide powders were also observed, showing that oxide scale spallation is likely a phenomenon more complex than usually considered.

Advances in the Field of New Smart Thermal Barrier Coatings

This paper deals with the development of a new synthesis technique for functional materials such as Yttria Stabilized Zirconia (YSZ) in the field of thermal barrier coatings. Currently, Thermal Barrier Coatings (TBCs) are manufactured by dry route technologies (EB-PVD or plasma spray) but such methods are directional and often require costly investments and complex operations. For these applications, the sol-gel route, a non directional method, is developed, to process, by suitable chemical modifications, nanocrystalline materials with a controlled morphology. The main advantage of this method is to decrease the crystallization temperature, much lower than the conventional processes, allowing the synthesis of reactive substituted zirconia powders with nanometric particles size. In this study, several suitable architectures for thermal barrier coatings have been achieved in order to show that this process is appropriate for repairing damaged TBC compared to conventional processes. The next step is to investigate spallation mechanisms and overall TBC durability by cyclic oxidation. Preliminary results are promising and research will be develop further to optimize both processing and cyclic oxidation behavior.

In Situ Thermal Gradient Controlled Investigation of Spallation -Experimental Design and Preliminary Results

Based on a coupled numerical and experimental approach, the design and implementation of an in situ thermal-gradient-controlled cyclic oxidation test, dedicated to the investigation of the TGO and TBC spallation, is presented. The influence of the specimen through thickness thermal gradient as well as the benefits of the video real-time monitoring of the cooling phase of an oxidation test is discussed in the case of the spallation of an alumina scale grown on FeCrAl alloys.

Simulation of IR Heating for Composite Stamping

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.

In Situ and Ex Situ Investigation of the Spallation of Thermally Grown Oxides

A review of various experimental techniques, simply implemented or specifically developed to investigate the spallation of thermally grown oxides subject to thermal shocks and cycles, is proposed. This includes thermogravimetric analysis, acoustic emission, interfacial indentation and a newly developed technique based on the real time in situ monitoring of spallation events using CCD cameras. Benefits and drawbacks of each method are discussed. It is shown that the CCD monitoring technique shows enhanced mass resolution and, in addition to the global information derived from the spallation kinetics, reveals straightforwardly the local correlation between spall and substrate micro structures.

Innovative Superplastic Forming Based on In Situ Infra-Red Sheet Heating

This paper describes the research work that was performed in order to propose an innovative and low cost process route for superplastic forming of aluminum and titanium alloy sheets. The driving idea was to heat only the metallic sheet using heating elements included in the pressure chamber. Several heating configuration have been tested and equipment designs investigated. Based on experimental results and numerical thermal analysis, it was found that halogen heating lamps with a high reflective thermal insulation was the best for the upper pressure chamber, whereas low effusivity Refractory Castables materials seem optimal as low cost forming die. Energy consumption evaluation shows more than 80% energy saving in nominal titanium alloy forming. A pilot forming equipment was developed and first aluminum sheet forming trials give interesting results.

Buckling and spalling failure of alumina grown by oxidation on ODS FeCrAl alloys

Abstract In situ investigation of thermal shocks imposed to alumina grown on FeCrAl alloy at high temperature shows that interfacial buckling is the preferential route to spallation. Various parameters, either intrinsic or extrinsic to the substrate/oxide system, are analysed in terms of variation in interfacial fracture energies and propensity of alloy grains to wrinkle during high temperature exposure depending on their crystal orientation.