Freezing Of Metal Alloys In Sand Casting Molds; Segregations, Dendritic Growth

Metal alloy systems have very profound effect in our society and most of the metal alloy systems are very important as engineering material. First shapes of metal alloys are generally given with casting methods, especially sand casting methods. But, alloy systems have very specific solidification characteristics. Because of these solidifcation caracteristics in sand molds, there are some issues and parameters must be considered in the phase of mold design of alloy systems. In here, we explain the solidification chracteristics of metal alloys in sand molds.

How Metal Alloys Solidify In Sand Molds?

Phase diagrams of copper-nickel alloys(SRC: Fundamentals of modern manufacturing).

If you take a look at the temperature-composition and temperature-time curves of copper-nickel alloy systems, they are quite different from pure metal curves.

As you see, there are ‘liquidus’ and ‘solidus’ points. The temperatures of these liquidus and solidus points are different. And also these liquidus and solidus temperatures changes with the changing composition of metal alloy. You can see this change on the curve sets of copper-nickel alloy at rightside. Solidus and liquidus temperatures are decreses with the increasing copper percenatage in copper-nickel alloy. If you pick a place between these liquidus and solidus curves, you will obtain a phase between liquid and solid.

Above the liquidus curve, the all alloy system will be liquid. And below the solidus curve, all the alloy system will be solid. If you take a ‘tie line’ in a specific percentage of copper, you can obtain a exact temperature-time curve of this composition of alloy system. For example at above, 50% of copper alloy tie line is taken.

As you see in left curve, the pouring temperature is the highest temperature. When the alloy system is pouring into the gating system of sand casting system, temperature begins to decrease. When the temperature reaches to the ‘freezing point’, sectional solidification of alloy systems starts. This freezing point is also called liquidus point. But, unlike the pure metal systems, the temperature is not constant in the solidification. Up to the completion of freezing, temperature decreases gradually.

When the solidification completes, temperature is at the ‘solidus’ point. These solidus and liquidus temperatures, correspond to the edges of tie line that is taken from the temperature-composition curve.

How Is The Dendritic Growth In Metal Alloys?

Illustration of segragation in metal alloy solidification(SRC: Fundamentals of modern manufacturing).

The dendritic growht in metal alloys is also quite different from pure metals. The dendritic growth starts with the formation of finer grains near the mold walls, upon rapid cooling. And these grains turns into the spikes of grains and these spikes turns into the grain branches oriented inside the mold.

But these grain branches are rich with the metal of which melting temperature is lower. These dendrits surrounds the liquid other alloy compound that has higher melting temperature. This section of solidification is called as mushy zone. With the advancing of solid dendrites, this mushy zone turns into solid phase at last. But in microscale, alloy compounds are not regularly spread through the mold. This phenomenon is called as segragation of different compounds in microscale.

Dendritic growth finishes with the depletion of the compound that has lower melting temperature. So, remaining other phase forms finer grains at innermost of sand mold. This situation is also called bulk segregation.

As you understand that, this is a very different mechanism of solidification. Mold designs and gating systems, and part that made with alloy systems must be designed by considering this issue.

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