*3.2. Optimized Mold Design*

The optimization problem is to obtain high quality casting nearly free from defects especially porosity. The objective function is to minimize porosity in castings (test specimens in this case) so that they have a mechanical performance similar to their sound counterparts. The design variables are elements of gating and runner system (Shape and size of sprue, choking, runner geometry and shape, addition of exothermic sleeves, etc.) and the constraints are cast and mold material, melting and pouring temperatures. The new mold design includes changes such as removal of choke from the sprue, choke at the beginning of runner bar, and use of exothermic sleeves. The new casting layout, as shown in Figure 7 is divided into 1,989,414 elements. The simulation settings are kept the same as already discussed earlier. Once again, simulation results indicated temperature gradient in specimens as shown in Figure 8a, and the risers are found to solidify at the end as shown in Figure 8b. It took 16 min for complete solidification using this mold design.

Residual stresses in specimens are shown in Figure 8c which are reduced from ~50 MPa to ~30 MPa. Once again, the X-ray view of the software, as shown in Figure 9, enabled analysis of the simulated porosity, microporosity, and total porosity in specimens. It can be observed that nearly the same porosity is predicted in all specimens as shown in Figure 9a. The connections between runner and specimens are found to be pore-free in the new mold design. Some microporosity is observed, which is uniformly distributed in the specimens as shown in Figure 9b. Total porosity is found to be identical and reduced in all specimens as compared to the total porosity shown in Figure 6c.

**Figure 7.** Optimized mold design for casting fatigue specimens.

**Figure 8.** (**a**) Temperature profile within the mold at 50% solidification, (**b**) Percentage fraction solid at 50% solidification, and (**c**) Residual stresses in specimens at ejection.

**Figure 9.** X-ray views of (**a**) porosity, (**b**) microporosity and (**c**) total porosity in simulated cast specimens using optimized mold design.
