Molten droplet solidification and substrate remelting in microcasting - Part I: numerical modeling and experimental verification

This paper presents a numerical model of a molten metal droplet impinging, solidifying and bonding to a solid substrate. The physical and numerical model includes dissimilar materials, multi-dimensional axisymmetric heat transfer, tracking of solid/liquid interfaces during remelting and solidification, and coupled treatment of the continuous droplet/substrate region. The numerical model solves for the evolution of the temperature distribution in the droplet and substrate, predicts the position of the remelting and solidification fronts, and accounts for convective motion. The effect of the convection induced by the droplet spreading is modeled through a time-dependent effective thermal conductivity. High-speed filming of the molten droplet impinging and spreading on the substrate is performed to obtain the required parameters to determine this time dependent effective conductivity. The accuracy of the model is investigated with experimental techniques. This research is directly related to the development of microcasting Shape Deposition Manufacturing (SDM) which is a process for automatically fabricating complex multi-material objects by sequentially depositing material layers. Microcasting is a molten metal droplet deposition process in SDM, which is able to create fully dense metal layers with controlled microstructure. Important issues in the production of high quality objects manufactured with microcasting SDM are: attainment of interlayer metallurgical bonding through substrate remelting, control of both substrate and droplet cooling rates, and minimization of residual thermal stresses. To validate experimentally the numerical modeling approach, predicted cooling rates are compared with thermocouple measurements and substrate remelting depths are verified through optical metallographic techniques.