Thermal Stress Model for FLOW-3D
The new thermal stress model incorporated into FLOW-3D allows you to simulate a larger range of casting processes! During solidification of molten metal, stresses form due to frustrated shrinkage at mold walls and shape irregularities, and due to nonuniform cooling. The new thermal stress model predicts the formation of this stress.
Figure 1: Temperature distribution. The color
scale ranges from 430K (blue) to 564K (red)
Figure 2: Magnitude of displacement. The color
scale ranges from 0mm (blue) to 1.7mm (red)
Figure 3: Pressure (negative of mean isotropic stress). The color
scale ranges from -1.9×106Pa (blue) to 1.52×105Pa (red)
Figure 4: Von Mises Stress. The color scale r
anges from 1.35×104Pa (blue) to 1.43×106Pa (red)
The thermal stress model is built upon the elastic stress model, which was newly available in Version 8.2. What is added is the expansion and contraction of the material due to changes in temperature. During a simulation, regions containing liquid (i.e., where the material temperature is greater than its solidus temperature) are not affected by the model. Once the temperature drops below the solidus temperature, the model is active and the elastic stress is computed incrementally each time step. Regions with rapid cooling (e.g. in narrow channels or near walls) will shrink fastest. In regions that remain in a liquid state, the elastic stress is zero. Furthermore, a yield-stress limit can be defined to specify a limit to the stress at which point the material is allowed to relax. All elastic material properties can be a function of temperature.
A sample result is shown in the Figures below. The part is a casting of an alternator housing composed of Al 201, an Aluminum-Copper alloy. The simulation begins with a full mold at a temperature of 825K, in the mushy zone between the liquidus and solidus temperatures. The surrounding mold is cooled at a constant temperature of 300K. Over a period of 15 seconds, the casting is rapidly cooled below the solidus temperature of the alloy throughout. All of the plots show a 3-D surface contour, as well as a 2-D slice through the mold 2.35cm from the top.
Figure 1 shows that the temperature does not drop uniformly; rather, for example, thick regions cool more slowly. The inlet sprue has the highest temperature at this point. Figure 2 shows the magnitude of the displacement from the onset of solidification; here the greatest displacement occurs in the thickest
part of the casting because a given amount of shrinkage over a large region results in more motion than over a small region. The plot of pressure (Figure 3) indicates regions that are under isotropic tension (blue) or compression (red). What are of particular importance here are the thin blue regions
visible on the 2-D plot; these regions are most at risk of cracking due to the large tension the material is undergoing. The 3-D surface plot appears mottled because the model predicts the separation of the metal from the mold where it is allowed to pull away from the mold walls, which
relieves the stress in the vicinity. Therefore, the red color indicates areas under low tension (perhaps even compression), while blue-green indicates areas where the material is frustrated and cannot separate from the mold. Figure 4 shows the Von Mises stress; this is a scalar invariant of the elastic stress tensor where large values (yellow, orange or red in color) indicate regions where the material is undergoing significant stretching, squeezing and/or shearing. For this example, this is seen in some of the narrow parts of the casting, as well as adjacent to the mold where significant shearing occurs as the material tries to slide over the mold surface.
FLOW-3D now offers the ability to easily model the filling, solidification and the development of thermal stresses all in one package! Furthermore, such simulations can be performed as one continuously integrated computation. Please visit our casting applications section to see more examples of this new and powerful simulation tool.