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Metal Casting Technologies : March 2010
42 www.metals.rala.com.au BacktoBASICS Introduction Flow modeling is an integral part of the casting simulation process, particularly in castings where rates of heat transfer are high during mould filling. This includes thin-section castings and those processes where mould materials have higher thermal conductivity, such as the permanent mould, or gravity die process. Flow modeling allows flow-related defects such as miss-run, cold lap and oxide (dross) formation due to excessive velocity to be predicted and reduced or eliminated through design changes prior to production of the casting. Flow modeling can be used for the evaluation of gating design to ensure the desired delivery of metal in the casting cavity. In addition, flow modeling provides a more accurate initial temperature field for modeling the subsequent cooling and solidification of the casting along with the gates and risers so that correct feeding of the casting can be obtained. The design process Typically, the design process begins with receipt of part specifications from the customer. Traditionally this involved a paper drawing, however, nowadays most part geometry is contained within a 3D CAD file as exemplified by the pump body shown in Figure 1, this facilitates the use of computer design and simulation. One of the first tasks of the foundry engineer is to decide on a basic process design for the casting, i.e., in what orientation the part will be cast, how it is to be gated, how it is to be fed (risered) during solidification and how many castings are to be produced within a single mould or tree. Computer simulation can be immediately helpful even at this early stage of design. Many foundry engineers have adopted the practice of running a "naked" simulation of the part as received from the customer, completely surrounded by mould material and without gates or risers. This can often be accomplished in just a few minutes with the right software, and allows the part to be viewed from a thermal standpoint, i.e., showing the progression of "natural" solidification and the location of thermal centers within the casting as shown in Figure 1. In many cases this analysis will determine the orientation of the casting within the mould; contact points for risers (feeders) become obvious, and the best orientation of the casting in order to accommodate those contact points can be decided immediately. It is helpful if the simulation software has built into it design rules for feeding and gating the casting (such as a Gating Design Wizard and a Riser Design Wizard) so that location, number and size of risers (feeders) and suggested size and shape of gating components can be calculated more or less automatically so as to establish an initial rigging design for the casting (see Figure 2). Once the initial design is developed, it is necessary to prove and fine-tune the design by running complete flow and solidification analyses. This is due to the fact that design rules are general in nature and cannot take into account all of the dynamics that will occur within a complex casting/ gating system. This means that it is necessary for the foundry to construct a 3D model of the casting with the complete proposed rigging system for computer modeling. Facility with 3D computer design is becoming a commonplace requirement in the engineering of casting processes. Typically the simulation process occurs in two phases: Simulation of the flow of the liquid metal as it enters and fills the mold cavity, and simulation of the subsequent cooling and solidification of the metal alloy along with formation of porosity defects. Flow simulation is accomplished through the use of Computation Fluid Dynamics (CFD), which means a computer software program that solves the equations of fluid flow for a given situation such as mould filling. The basic equations governing the flow of a liquid are the Navier-Stokes equations; these relate the flow of liquid to the principle of conservation of momentum as well as movement in reaction to body forces on the liquid (such as gravity, pressure and friction). A full and complete solution of these equations is not yet within the realm of computer science, and some theoreticians believe that a complete fluid flow solution is so complex that it may never be feasible. This means that all such programs must make some simplifying assumptions in order to be practical. A truly useful solution will make the correct simplifications in such a way that Using flow simulation to cure casting production problems J F Meredith Casting Solutions Pty Ltd Lawrence E Smiley and David C Schmidt Finite Solutions Inc F
Media Kit 2010