Solidification Of Pure Metals In Casting And Growth Mechanisms

In general, the bulk of pure metals are produced with sand casting methods. Sand casting is not an easy process in terms of proper cooling and solidification of cast metal. The cooling and solidification of pure metals have a very specific mechanism itself. Here, we explain the solidification mechanism of pure metals and their solidification mechanisms in sand casting molds.

How The Solidification Of Metals Takes Place?

Fundamentals of Modern Manufacturing: Materials, Processes and Systems, Seventh Edition

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Just like other elements in nature, pure metals have a constant melting temperature. This melting temperature of metals is also equal to the freezing temperature. But if we take a look at the solidification inside a sand casting, it has a quite original solidification mechanism.

When the liquid metal pouring inside a sand mold, it has a liquid form and the temperature of this liquid metal is higher than the melting temperature to prevent the solidification inside gatings. When the mold cavity inside the sand casting is filled with liquid metal, the solidification process starts.

Temperature-time curve of pure metals(Src: Fundamentals Of Modern Manufacturing).

The solidification of liquid metal can be expressed with the above diagram showing the temperature and time relation of solidifying pure metals. As you from the diagram, pouring temperature is the highest temperature and the beginning of the casting. When the liquid pure metal fills the mold cavity, cooling will begin. When the freezing temperature is reached after a time passed, the solidification of metal starts from the outer surface of the mold.

From the interface of mold wall and liquid metal, the solidification of liquid metal starts. After the formation of a thin solid film, this film thickens with the passing time and gives up the ‘latent heat of fusion’ of metal through the mold walls.

What Is ‘Dendritic Growth’ In Metal Casting?

Sections of pure metal near the mold wall will have fine grains in structure because of the rapid cooling of these sections. These grains are randomly oriented and they grow with the passing time.

Dendritic growth of metals inside molds(Src: Fundamentals Of Modern Manufacturing).

When the solidification goes into the inner sections of the mold, these grains will turn into spikes that are oriented inside the mold. Up to the full solidification of pure metal, these spike grains will turn into long branches that are oriented inside the mold. These long branches of molecular structures unite in the innermost section of the mold cavity. This molecular structure is illustrated above. This solidification phenomenon of metals is called dendritic growth.

The time that is passed between the starting of freezing and the ending of it, called ‘local solidification time.’ In this time section, solidification of pure metal takes place but the temperature is constant. When the pure metal is completely solidified, solid cooling starts up to room temperature.

‘Total solidification time’ refers to the time between pouring and complete cooling of pure metal in sand casting operations.

This can be the general expression of solidification of pure metals inside sand casting molds. Metal parts and molds are designed according to these solidification and grain growth mechanisms.


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