Defect formation and mitigation during laser powder bed fusion of copper

Additive manufacturing (AM) is a group of processing technologies which has the potential to revolutionize manufacturing by allowing easy manufacturing of complex shapes and small series. One AM-method which is of high interest for processing of metals is laser powder bed fusion (LPBF), where a powerful laser is used to melt powder in a layer-by-layer fashion into a consolidated part.Many defects can form during LPBF processing, but copper, copper alloys, and similar metals are particularly prone to the formation of pores.The reason why these metals tend to have these defects is the high reflectivity decreasing the effective energy input, and the high thermal conductivity transporting the absorbed energy away. Several ways of decreasing the pore formation tendency are known: increasing the power, increasing the absorptivity, and alloying to decrease thermal conductivity.The relative effectiveness of these strategies are however not known, making it difficult to choose the correct strategy for the different industrial applications of these metals.In this thesis a simple analytical model for predicting the occurrence of these defects is proposed, allowing a lowest possible processing power given the material properties to be estimated. The model agrees well with experiments, and can be used to predict the transition from so called balling mode processing to conduction mode processing, and for qualitative analysis of different processing strategies. Two strategies of decreasing the porosity are proposed and tested: coating of the powder with an absorbing layer and processing with a green laser, which is mor e strongly absorbed by copper than the conventional IR-lasers.The copper powder was coated with thin layers of tin and nickel using a simple and cheap immersion deposition method, and it is shown that the amount of porosity is decreased more than can be expected of the alloying alone, showing both that the method is working and that laser interaction with the solid is important for the heat balance of the system.The interaction with the solid is shown through computational fluid dynamics simulations to arise from periodic fluctuations of the molten metal which cause laser light to be reflected forward, pre-heating the powder bed.The preheating is larger the higher the solid absorptivity is, explaining the porosity decrease when using coated powders, and shows that processing using a green laser increase the laser coupling more than the absorptivity of the liquid would.Single line tracks made from fine 5 µm pure copper with a green laser source show that the desired conduction mode melting can be achieved at a power below 70 W, an order of magnitude lower than what is needed for processing of the conventionally used 45 µm powder with an IR-laser.

High-temperature wettability and thermal conductivity of aluminium nitride reinforced metal matrix composites

Masahiro Kida

Aluminum nitride (AlN) has been applied to various applications, including electronic substrates and packaging, semiconductor production components, wide band gap semiconductor, light-emitting diodes (LED) and acoustic wave devices; at the same time, it is also attractive as a reinforcement in Metal Matrix Composites (MMCs) for thermal management applications with tailored thermal properties. In this thesis, two main subjects are investigated in terms of both the composite processing and its thermal property, namely (i) the high-temperature wettability between AlN and liquid metals and (ii) the thermal conductivity behavior of AlN reinforced metal matrix composites and the associated question of the interface thermal conductance at the AlN/metal interface. High-temperature wettability of AlN particle preforms with pure Al, Cu, Sn and Pb, was investigated by means of drainage curve measurements during pressure infiltration. The measured drainage curves show good agreement with the semi-empirical model in soil science proposed by Brooks and Corey. The total work of immersion, Wi, can be directly calculated by integration of the drainage curves, in turn giving the apparent contact angle during infiltration at high temperature. For Cu, Sn and Pb, the apparent contact angles deduced by integration of experimental drainage curves are in the range 120-140° and agree well with corresponding sessile drop data from the literature. Data for AlN and α-Al2O3 are furthermore similar for both methods of measurement, which results from the presence of a native oxide layer on the AlN particles, which is not eliminated from the particle surface in these systems. For the wettability between AlN/Pb and α-Al2O3/Pb systems, a slight difference is observed and this behavior is attributed to the transformation of the native oxide layer with temperature and the presence of dissolved oxygen in the melt. On the contrary, the wettability of AlN/Al system gives higher value than the sessile drop data measured in vacuum, corresponding rather to the value measured under He-H2 gas at 1373K. This indicates that in the present study there is no elimination of the native oxide layer on the AlN by the formation of Al2O gas, which normally occurs in high vacuum. Moreover, the results of the present technique gradually deviate to the sessile drop data as the initial contact angle gets close or below 90° especially in the α-Al2O3/Al system, suggesting that there is a minimum value, around 105-110°, below which the present infiltration-based contact-angle measurement technique may lose validity. Thermal conductivity behavior of AlN reinforced metal matrix composites produced by pressure infiltration technique has been characterized at both room and elevated temperature in two different microstructural systems, namely (i) particle reinforced system (PRCs) and (ii) partially sintered particle system, i.e., an interpenetrating composite in which the AlN is also a continuous phase (IPCs). The phase contrast of thermal conductivity between AlN (Κd) and matrix metals (Κm) is varied within a range of 0.4

EPFL2008

Numerical Modeling of Annular Laminar Film Condensation in Circular and Non-Circular Micro-Channels under Normal and Micro-Gravity

Stefano Nebuloni

A theoretical and numerical model to predict film condensation heat transfer in mini, micro and ultra micro-channels of different internal shapes is presented in this thesis. The model is based on a finite volume formulation of the Navier-Stokes and energy equations and it includes the contributions of the unsteady terms, surface tension, axial shear stresses, gravitational forces and wall thermal conduction. Notably, interphase mass transfer and near-to-wall effects (disjoining pressure) are also included. This model has been validated versus various benchmark cases and versus published experimental results from three different laboratories, predicting micro-channel heat transfer data with an average error of 20 % or better. The conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid and the heat conduction in the channel wall has been studied and analyzed. The work has focused on the effects of three external wall boundary conditions: a uniform wall temperature, a non uniform wall heat flux and single-phase convective cooling. The thermal axial and peripheral conduction occurring in the wall of the channel can affect the behavior of the condensate film, not only because it redistributes the heat, but also because the annular laminar film condensation process is dependent on the local saturation to wall temperature difference. When moving from mini to micro and ultra-micro channels, the results shows that the axial conduction effects can become very important in the prediction of the wall temperature profile and they can not be ignored. Under these conditions, the overall performances of the heat exchanger become dependent not only on the fluid properties and the operative conditions but also on the geometry and wall material. Results obtained for steady state conditions are presented for circular, elliptical and flattened shape cross sections for R-134a and ammonia, for hydraulic diameters between 10 µm and 3 mm. Microscale condensation finds applications in heat pipes and compact heat exchangers for electronic equipment or spacecraft thermal control, in automotive condensers, in residential air conditioning and in refrigeration applications: the influence of the steady or unsteady gravitational field and of the inertia forces on the flow field and consequently on the heat transfer performances is investigated allowing the model to be applied as a design and optimization tool for enhanced heat exchangers.

EPFL2010

Heat transfer monitoring between quenched high-temperature superconducting coated conductors and liquid nitrogen

Daniele Colangelo, Bertrand Dutoit

High-temperature superconducting coated conductors (HTS-CCs) are good candidates for resistive superconducting fault current limiter (RSFCL) applications. However, the high current density they can carry and their low thermal diffusivity expose them to the risk of thermal instability. In order to find the best compromise between stability and cost, it is important to study the heat transfer between HTS-CCs and the liquid nitrogen (LN2) bath. This paper presents an experimental method to monitor in real-time the temperature of a quenched HTS-CC during a current pulse. The current and the associated voltage are measured, giving a precise knowledge of the amount of energy dissipated in the tape. These values are compared with an adiabatic numerical thermal model which takes into account heat capacity temperature dependence of the stabilizer and substrate. The result is a precise estimation of the heat transfer to the liquid nitrogen bath at each time step. Measurements were taken on a bare tape and have been repeated using increasing Kapton® insulation layers. The different heat exchange regimes can be clearly identified. This experimental method enables us to characterize the recooling process after a quench. Finally, suggestions are done to reduce the temperature increase of the tape, at a rated current and given limitation time, using different thermal insulation thicknesses.