Adapted Laser-Flash Method for Estimating Thermal Diffusion Properties of a Porous Material Surrounded by Compact Layers

Composite structures obtained by superimposing layers of different materials find application in various technological fields. The number of layers and the characteristics of the constituent materials are chosen based on different needs, e.g. for thermal protection of metal components or to preserve high mechanical strength by reducing the overall weight of the structure. For reasons related to production methods, some layers are not available separately from others and the study of their thermal properties cannot be achieved using methods conceived for homogeneous samples. For the evaluation of the properties of these non-free-standing layers, some modifications of the traditional laser-flash method are necessary, especially regarding measurements post-processing. In the present work, the measurements on three-layer samples were carried out using the classic laser-flash technique, but the analysis of the thermal response of the sample was carried out with a significantly different approach compared to that proposed by Parker and for which the method introduced by Cowan allows heat losses to be considered. The non-normalized thermal response that is measured on the rear surface of the sample was analyzed with a 1D heat diffusion analytical solution, considering both the heat losses on the front and rear surfaces of the sample and the finite pulse effect. The tested samples are three layers of the same material (Ti6Al4V) but with a different structure. In fact, the two external layers are compact, while the intermediate one is made up of sintered powder of various porosities, which is deposited through additive manufacturing. Since the thermal properties of the internal porous layer are of great interest (in particular its apparent thermal conductivity), the article describes the procedure adopted to estimate them and discusses their identifiability. Furthermore, using the thermal conductivity data available in the literature, a first validation of the adapted laser-flash method is also provided.