A computational model for defect prediction in shape castings based on the interaction of free surface flow, heat transf
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INTRODUCTION
THE modeling of solidification processes using computational techniques has engaged the attention of the “numerical” research community for over 40 years. This attention has been merited because of the significance of the industries using casting processes and their potential to produce components of very high integrity and controllable structures. By high integrity castings we mean all external geometric features are within a specified tolerance, there is no external evidence of defects, components meet all NDT specifications, and cut samples do not exhibit microporosity above specifications. The problem with solidification-based processing from a modeling perspective is that it involves the interactions among a range of phenomena as follows: (1) heat transfer with solidification (and melting) phase change, (2) free surface and buoyancy driven Navier–Stokes flows, (3) electromagnetic fields, and (4) the development of residual stress and deformation of the solidified component and the mould. This is in the context of three-dimensional complex geometries. Because of the physical complexity, the computational modeling of casting processes has provided one of the most enduring challenges to the research community. Moreover, the prime interest of those involved in process design for manufacture and performance is not in the continuum physics per se but in avoiding the defects that arise as a consequence of the interactions of such phenomena. The objectives of this article are to describe an approach to the modeling of macrodefects in casting processes. Because S. BOUNDS, Research Fellow, G. MORAN, Research Fellow, K. PERICLEOUS, Professor, M. CROSS, Professor, and T.N. CROFT, Senior Research Fellow, are with the Centre for Numerical Modelling and Process Analysis, University of Greenwich, London, SE10 9LS United Kingdom. Manuscript submitted April 20, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
such defects arise as a result of interactions among a range of continuum phenomena, before moving on to describe the defect prediction models, it is useful to consider the computational modeling challenges that must be addressed to provide the appropriate context for this approach. At the outset of this work, it is important to distinguish between the class of defects addressed. There has been a sustained research effort in the last decade to characterize the factors that control microstructure and microporosity, as shown in the MCWASP conference proceedings series.[1] The length scales of these phenomena are micrometers compared to component dimensions of millimeter-centimetermeter. Nevertheless, micrporosity, etc. in castings can have a profound effect upon their mechanical and fatigue properties.[2] However, in this work, we are concerned with macrodefects, which are essentially visible to the naked eye. II. PROCESS MODELING OF THE CONTINUUM PHENOMENA INVOLVED IN SOLIDIFICATION-BASED MANUFACTURING PROCESSES—PROGRESS AND CONSTRAINTS The research efforts in this area have been led by the following three, in
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