NONEQUILIBRIUM MODELING OF HEAT TRANSFER IN A GAS-SATURATED POWDER LAYER SUBJECT TO A SHORT-PULSED HEAT SOURCE Yuwen Zhang Department of Mechanical and Aerospace Engineering, University of Missouri—Columbia, Columbia, Missouri, USA Heat transfer in a gas-saturated powder layer subject to a short-pulsed volumetric heat source is modeled using a two-temperature model. The energy equations for the powder par- ticles and gas in the powder layer are coupled by an interphase coupling coefficient. The effect of the heat source pulse width, particle diameter, and the heat source fluence on the powder and gas temperatures are investigated. The results show that the nonequilibrium is significant when the heat source pulse width is less than 1ls, and therefore heat transfer during nanosecond laser–powder interaction should be modeled using a nonequilibrium model. For the case with long pulse width, on the other hand, the equilibrium model is valid. 1. INTRODUCTION Solid freeform fabrication (SFF) is a rapid prototyping=manufacturing tech- nology in which three-dimensional parts can be built from computer-aided design (CAD) data in a layer-by-layer fashion [1]. SFF technologies are powered by fabri- cation of three-dimensional objects from (1) powders (e.g., selective laser sintering, SLS), (2) gases (e.g., selective area laser deposition, SALD), or (3) a combination of both (e.g., selective area laser deposition vapor infiltration, SALDVI), by a direc- ted laser beam (generally CO 2 or yttrium-aluminum garnet, YAG). The SALD= SALDVI processes that are based on laser chemical vapor deposition (LCVD) [2] technique are excellent for fabricating ceramics and refractory metal parts that are either impossible or very difficult to manufacture by any other method, such as SLS. While the SALD technique uses precursors to create free-standing parts directly or to join together simple shapes to create parts with higher complexity, SALDVI uses gas precursors and powder particles to build three-dimensional parts (see Figure 1). The mechanism to bound powder particles together in a SALDVI process is LCVD of solid material on the powder particle surface by decomposition of gas precursors. During the SALDVI process, a thin (100–250 mm thick) powder layer is laser-scanned to form the two-dimensional slice on an underlying solid piece, which consists of a series of stacked two-dimensional slices. After laser scanning, a fresh powder layer is spread and the scanning process is repeated. Loose powder Received 15 July 2005; accepted 17 December 2005. Address correspondence to Yuwen Zhang, Department of Mechanical and Aerospace Engineering, University of Missouri—Columbia, Columbia, MO 65211, USA. E-mail: zhangyu@missouri.edu 509 Numerical Heat Transfer, Part A, 50: 509–524, 2006 Copyright # Taylor & Francis Group, LLC ISSN: 1040-7782 print=1521-0634 online DOI: 10.1080/10407780600619956