Freezing Of Metal Alloys In Sand Casting Molds; Segregations, Dendritic Growth
In general, 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. Cooling and solidification of pure metals has very specific mechanism itself. In here, we explain this solidification mechanism of pure metals and solidification mechanisms of them in sand casting molds.
Just like other elements in nature, pure metals have a constant melting temperature. This melting temperature of metals are also equal with the freezing temperature. But if we take a look at the solidification of inside a sand casting, it has a quite original solidification mechanism.
When the liquid metal pouring inside a sand mold, it has 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, solidification process starts.
The solidification of liquid metal can be expressed with above diagram shows 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, solidification of metal starts from the outer surfaced of mold.
From the interface of mold wall and liquid metal, solidification of liquid metal starts. After the formation of a thin solid film, this film thickens with the passing time and giving up the ‘latent heat of fusion’ of metal through the mold walls.
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.
When the solidification goes into the inner sections of 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 mold cavity. This molecular structure is illustrated above. This solidification phenomenon of metals called as dendritic growth.
The time that is passed between the starting of freezing and ending of it, called as ‘local solidification time.’ In this time section, solidification of pure metal takes place but the temperature is constant. When the pure metal completely solidified, solid cooling starts up to the room temperature.
‘Total solidification time’ refers 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 this solidification and grain growth mechanisms.
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