The block-shaped casting is sound. The more progressive the freezing process and the higher the local cooling rate, the smaller the volume fraction of shrinkage porosity that will form and the smaller will be the individual pores. The higher the cooling rate, the more effectively the alloy responds to solution heat treatment. Reducing the volume fraction and the size of shrinkage microporosity and improving the effectiveness of heat treatment will both improve the strength and ductility of the cast alloy. The use of chills and tapering of the cross section of the casting in the direction of feeding can be used to make the freezing pattern more progressive and, thus, improve the efficiency of risers in feeding shrinkage.
Chills also increase the local cooling rate.
Chills and tapered cross sections can be used in combination to improve the strength and ductility of castings. Experimental data that can be used to design the rigging of a casting are presented in Figure 10. A series of A356 plates were cast in sand molds, with a chill at one end of the plate and a riser at the other end of the plate. After casting the plates were cut into coupons, the coupons were given the T6 precipitation hardening heat treatment, and the heat treated coupons were machined into test bars. The closer the location of a coupon in the cast plate is to a chilled end, the higher the tensile strength and the higher the elongation to failure.