Additive manufacturing (AM), the process o f joining materials to make objects from three-dimensional (3D) model data, usually layer by layer, is distinctly a different form and has many advan tages over traditional manufacturing processes. Commonly known as "3D printing," AM provides a cost-effective and time-efficient way to produce low-volume, customized products with compli cated geometries and advanced material properties and function ality. As a result o f the 2013 National Science Foundation (NSF) Workshop on Frontiers of Additive Manufacturing Research and Education, this paper summarizes AM's current state, future potential, gaps and needs, as well as recommendations for technology and research, university-industry collaboration and technology transfer, and education and training.

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A three-dimensional laser-keyhole welding model is developed, featuring the self-consistent evolution of the liquid/vapor (L/V) interface together with full simulation of fluid flow and heat transfer. Important interfacial phenomena, such as free surface evolution, evaporation, kinetic Knudsen layer, homogeneous boiling, and multiple reflections, are considered and applied to the model. The level set approach is adopted to incorporate the L/V interface boundary conditions in the Navier-Stokes equation and energy equation. Both thermocapillary force and recoil pressure, which are the major driving forces for the melt flow, are incorporated in the formulation. For melting and solidification processes at the solid/liquid (S/L) interface, the mixture continuum model has been employed. The article consists of two parts. This article (Part I) presents the model formulation and discusses the effects of evaporation, free surface evolution, and multiple reflections on a steady molten pool to demonstrate the relevance of these interfacial phenomena. The results of the full keyhole simulation and the experimental verification will be provided in the companion article (Part II).