Identification of critical factors affecting shrinkage porosity in permanent mold casting using numerical simulations based on design of experiments

Vital process parameters affecting the size and location of a shrinkage pore defect in an aluminum alloy permanent mold casting of varying section thicknesses were sought. The aim was to either modify or eliminate the defect by manipulating those parameters so that the lower inlet manifold casting would pass a leak test. The study was carried out ‘off-line’ using numerical simulations of the casting process. This was because the just-in-time production schedule of the busy mass production automotive foundry involved did not allow for any interruptions to accommodate physical experimentation. In a relatively novel approach, the design of experiments (DOE) method was employed to limit the number of simulations required, realizing significant savings in both labor cost and the time spent to arrive at a solution. Mold coat thickness and mold temperature were identified as the vital two parameters from a field of five potential factors nominated by experienced foundry personnel. It was determined that a thicker mold coat and a higher mold temperature would modify temperature profiles in the casting in such a way as to move the shrinkage porosity away from the critical location to less critical regions in a dispersed form. This solution would assume greater importance in applications where there is only a limited supply of feed metal either due to geometric constraints or due to a conscious effort aimed at increasing material yield. As a consequence of incorporating the project recommendations, the foundry achieved a reduction of more than 13% in its annual scrap rate for the casting involved.