1. Liquid shrinkage is the contraction of the liquid before solidification begins. It is not an important design consideration.

2. Liquid-to-solid shrinkage is the shrinkage of the metal mass as it transforms from the liquid’s disconnected atoms and molecules into the structured building blocks of solid metal. The amount of solidification shrinkage varies greatly from alloy to alloy. Shrinkage can vary from low to high shrinkage volumes.

Alloys are further classified based on their solidification type: directional, eutectic-type and equiaxed. The type of solidification shrinkage in a casting is just as important as the amount of shrinkage. Specific types of geometry can be chosen to control internal integrity when solidification amount or types are a problem.

Figs. 1-3 illustrate what is implied by the three solidification shrinkage types. In each case, a simple plate casting is shown with attached risering (a “riser” is a reservoir of liquid metal attached to a casting section to feed solidification shrinkage). Cross sections of the plate and riser(s) show conceptually how solidification takes place; metallurgical reality is similar, but microscopic.

Fig. 1

Fig. 1 shows solidification on and perpendicular to the casting surfaces, known as “progressive” solidification. At the same time, solidification moves at a faster rate from the ends of the section(s) toward the source of feed metal (risers)—this is known as directional solidification. Directional solidification moves faster from the ends of the sections because of the greater amount of surface area through which the solidifying metal can lose its heat. The objective is for directional solidification to beat out progressive solidification before it can “close the door” to the source of the feed metal. As shown, directionally solidifying alloys require extensive risering and tapering, but they also have the capability for excellent internal soundness when solidification patterns are designed properly.

Fig. 2 illustrates the eutectic-type alloy, the most forgiving of the three. Such alloys typically have less solidification shrinkage volume. Risers are much smaller, and in special cases can be eliminated by strategically placed gates. The key feature with these alloys is the extended time that the metal feed avenue stays open. The plate solidifies more uniformly all over and all at once, similar to eutectic solidification. Eutectic-type alloys are less sensitive to shrinkage problems from abrupt geometry changes.

Fig. 2

Alloys that exhibit equiaxed solidification respond the most dramatically to differences in geometry (Fig. 3). Shrinkage in these alloys tends to be widely distributed as micropores, typically along the center plane of a casting section. The reason is that solidification occurs not only progressively from casting surfaces inward and directionally from high surface area extremities toward lower surface area sections, but also equiaxially via “islands” in the middle of the liquid. These islands of solidification interrupt the liquid pathway of directional solidification. Gradually, the pathways freeze off, leaving micropores of shrinkage around and behind the islands that grew in the middle of the pathway. Larger risers, thicker sections and tapering (shown at center of Fig. 3) are counterproductive, causing micropores to coalesce into larger pores across more of the casting cross section. As illustrated at the bottom of Fig. 5, microporosity is kept small and confined to a narrow mid-plane in the casting section by more “thermally neutral” geometry with smaller, further-spaced risers.

As illustrated in Figs. 1-3, there is a significant bilateral and reciprocal relationship between solidification shrinkage and geometry. Most simply, eutectic-type solidification is tolerant of a wide variety of geometries; the least reciprocity is required. Most complex, equiaxed solidification requires the most engineering foresight in the choice of geometry and may require supplemental heat transfer techniques in the mold process. In the middle lies directional solidification; while capable of the worst shrinkage cavities, it is the most capable of very high internal integrity when the geometry is properly designed. Well-planned geometry in a directionally solidifying alloy can eliminate not only shrinkage but the need for any supplemental heat transfer techniques in the mold.

Fig. 3

In fact, the real mechanism behind the bilateral and reciprocal relationship between solidification shrinkage and geometry is heat transfer. All three modes of heat transfer, radiation, conduction and convection are involved in solidification of castings, and all three depend on geometry for transfer efficiency. Convection and conduction are very important in casting solidification, and transfer rates are highly affected by geometry.

3. Patternmaker’s Contraction is the contraction that occurs after the metal has completely solidified and is cooling to ambient temperature. This contraction changes the dimensions of the casting from those of liquid in the mold to those dictated by the alloy’s rate of contraction. So, as the solid casting shrinks away from the mold walls, it assumes final dimensions that must be predicted by the pattern- or diemaker. This variability of contraction is another important casting design consideration, and it is critical to dimensional accuracy. Tooling design and construction must compensate for it.

Achieving dimensions that are just like the blueprint requires the foundry’s patternmaker and/or diemaker to be included. The unpredictable nature of patternmaker’s contraction makes tooling adjustments inevitable. For example, a highly recommended practice for critical dimensions and tolerances is to build the patterns/dies/coreboxes with extra material on critical surfaces so that the dimensions can be fine-tuned by removing small amounts of tooling stock after capability castings have been made and measured.