Simple Solutions That Work! Issue 6

Contact: DAVID C. SCHMIDT [email protected] 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 along with formation of macro- and micro- porosity defects. Flow Modeling: Flow modeling is an integral part of the simulation process. Flow modeling allows flow-related defects, such as misrun and oxide 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. Flow simulation is accomplished through the use of Computational Fluid Dynamics (CFD), a technique that solves the equations of fluid flow for mold 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. Filling simulation lets the foundry engineer visualise the flow of the liquid metal from the pouring point, through the gating system and into the mold cavity during the entire filling process. This allows the design of the gating system to be verified. If the gating is not functioning as intended (for example, there is unequal metal flow through various gates), the design can be modified and a new design can be re-tested. In addition, the fluid flow calculations are coupled with thermal calculations so that the heat transfer from the liquid during filling and the resulting temperature distribution within the liquid metal can be viewed. This allows the prediction of areas within the casting where premature solidification may be occurring during filling, leading to defects such as misruns and folds. Accurate calculation of the temperature distribution of the liquid metal in the full mold results in highest accuracy of the subsequent modeling of cooling and solidification of the metal, Figure 3. where excessive turbulence is likely, leading to formation of oxides as the turbulence entrains oxygen- containing gas into the metal stream. Of course, the criticality of this effect does depend somewhat on the affinity of the particular alloy for oxygen (the tendency of the alloy to form oxides), so this is somewhat more important in alloys such as aluminium, which has a great tendency to oxidize as opposed to, say, carbon steel, which has a relatively lower affinity for oxygen. Almost all alloys, however, do have some tendency to form oxides and using flow simulation to design gating systems which minimize velocity and turbulence of the metal can be quite helpful in reducing flow-related defects in castings. Figure 3 Plot of temperature distribution during mold filling. Another aspect of filling simulation which is quite useful in improvement of casting quality is prediction of the velocity of the liquid metal during filling, Figure 4. Areas of higher velocity tend to be areas Figure 4 Plot of velocity distribution during mold filling. 49