The investment casting process can produce high quality, complex castings with great accuracy and controlled grain structure. However, many challenges face process designers hoping to achieve these results. Fortunately, FLOW-3D CAST v5.1 includes an Investment Casting Workspace which provides the necessary tools to study the wide range of process parameters in a virtual space and determine an optimal design before casting a single part.
In this blog, we’ll walk through the Investment Casting Workspace and show how easy it is to simulate a directionally-cooled investment casting using a Bridgman process. The casting we’ll be investigating is this multi-cavity casting on the right.
Shell Building Tool
An investment casting process begins with a wax representation of the part to be cast. The next step is to dip the wax part into a ceramic slurry mixture successively to build up a shell around the part. This is done until a sufficient thickness shell is achieved. FLOW-3D CAST’s shell building tool allows users to create water-tight shells of any thickness in a matter of minutes.
Using the shell building interface in the GUI, the first step is to select the geometry around which the shell should be created. Next, select Fit Mesh to create a computational mesh around the geometry to be shelled. The edge of the mesh where the pouring sprue is located would be moved into the part slightly so that the generated shell is open there. The only other required inputs are the shell thickness and the cell size which should be roughly half the shell thickness.
Calculating View Factors
A critical aspect of investment casting is the calculation of view factors between all surfaces in the simulation. Every surface that “sees” another surface requires a calculation of “how” each of the surfaces see each other. The orientation of each surface relative to others and the emissivity of each must be evaluated. For complex shapes, the surface is subdivided, or clustered, and the view factor between each cluster is computed.
Surface Clustering
In a Bridgman process, where the solidifying casting is being moved slowly through a selectively heated and cooled oven, the view factors are updated continuously throughout the simulation. This simulation result shows the surface clustering computed for the shell mold and the internal surfaces of the oven.
The simulation result shows the surface clustering computed for the shell mold and the internal surfaces of the oven. In a Bridgman process, where the solidifying casting is being moved slowly through a selectively heated and cooled oven, the view factors are updated continuously throughout the simulation.
Cluster Generation
A number of user-adjustable controls for cluster generation are available to minimize memory use and simulation runtime. For example, the cluster size could be set relatively large so that iterative simulations can be run quickly. As the design options are reduced, more refined details can be added to zero-in on the final design.
Here we see the solidifying casting has moved downward from the heated portion of the oven through a cooling ring so that the casting solidifies from the bottom to the top. This process allows for equiaxed grain structure to be formed.
This simulation shows how the temperature distribution in the solidifying casting on the left and the solid fraction on the right. The feeders at the top of each part provide liquid metal to the casting as it solidifies and shrinks.
Many process parameters can affect the outcome of an investment casting. With FLOW-3D CAST v5.1 in your design toolbox, the effect of these parameters, including the temperature profiles of the heated and cooled sections of the oven, the initial shell temperature, and the rate of motion of the solidifying casting through the oven, can be studied in-depth before casting a single part.
John Ditter
Principal CFD Engineer at Flow Science
