Alloy Cooling Rate

Cooling of a casting is a very complicated process. The most important part is the rate at which a casting cools down. Many properties of the casting are affected by this rate, which eventually determines the quality of the finished product. The cooling rate affects: -

• Microstructure of the casting.
• Quality of the casting.
• Properties of the casting.

Much depends on the type of casting and the underlying processes. Let’s understand this with an example. Sand-casting and slurry-mold processes lead to slower cooling because of large and thick walls. As a result the grain size of the metal increases and hence its strength decreases (because of a coarse microstructure). Sand casting and slurry-mold processes lead to: -

• Slower cooling.
• Increased grain size of the metal.
• Weak casting.

On the other hand, die-casting and metal-mold processes cool relatively quickly. As a result the grain size remains small and the casting gains in strength(because the microstructure is fine). Die-casting and metal-mold processes lead to: -

• Faster cooling.
• Small grain size of the metal.
• Strong casting.

There is another angle to the story also if we take alloy-segregation and gases (including waste metal) into account. Slower cooling allows elements of alloys to escape, which decreases strength. But it also allows more gases and waste metal to escape which leads to the strengthening of the casting. Slower cooling causes: -

• Escape of alloy elements, thus decreased strength.
• Escape of gases and waste metal, thus increased strength.

If we consider faster cooling, it does not allow alloys to escape but it also retains much of the gases and waste metal. One factor strengthens the casting and the other one weakens it. Faster cooling: -

• Holds the alloy elements, hence increased strength.
• Holds gases and waste elements, hence decreased strength.

Thus, we see that there are two opposing factors in competition. No matter which casting method we use, we can’t do away with those factors that lead to a weak casting. A judicious choice (based on experience) of a particular process for a particular type of casting is the solution to the puzzle.