Casting is a common metal solidification process which utilized the melting and re-solidification of a metal or alloy within a mold to produce a final desired product. Casting is often used to create complex shapes that would be complicated or expensive to manufacture using other methods. When customers ask us what the solidification process involves, the concept is a frequent source of confusion. Here is a step-by-step explanation of the solidification of metals and alloys in castings.
The casting process starts by heating a metal alloy in a crucible until it melts. When a metal is heated above its freezing point, it becomes liquid. This is also known as its melting point. The melting point of metal depends upon the type of metal or alloy being used. For instance, our zinc alloys melt around 900 degrees F, whereas some of the bronze alloys we pour melt above 2000 degrees F (hotter than lava). In addition, pure metals melt at the same constant temperature. Conversely, metal alloys will melt within a range of temperatures depending on the composition of the materials. In its molten state, a metal contains a high amount of energy. The alloy is heated above its melting point to allow for enough time for the metal to cool during the pouring process.
When we pour zinc alloys or aluminum alloys, they must be degassed prior to pouring. This is accomplished by inserting a graphite lance into the melt. The lance spins very fast and argon is injected through the lance dispersing it through the melted alloy. The argon moves dissolved hydrogen and other contaminants to the surface of the melt. This contamination is then removed from the crucible prior to pouring.
Many alloys require modification prior to pouring. These modifications increase metal flowability, improve grain structure, remove contaminants, etc. Some of the materials we use to accomplish this are titanium-boron, copper-phosphorous, strontium, manganese, etc.
After degassing and/or modification, the metal is tested to ensure it is approximately 50 degrees (Fahrenheit) above its desired pouring temperature. This allows enough time for the metal to cool during the pouring process. The crucible is transported via an overhead rail to the pouring lines. The liquid metal is then poured into a sand mold. Inside the hollow cavity of the sand mold is the shape of the desired end product. Sometimes this cavity is only one part and sometimes it is several individual parts. It is imperative to keep the lip of the crucible as close to the sand mold as possible to reduce the velocity the metal enters the mold cavity.
Once the molten liquid has been poured into the mold it cools rapidly. When the temperature of the liquid metal changes below the melting point of that particular metal or alloy, the solidification process begins. This usually takes less than a few minutes.
As the temperature drops further, the molten metal loses energy and crystals begin to form. This process starts near the mold walls where it cools first. These crystals eventually become grains within the final structure. If the metal solidifies slowly, the grains are longer. If it cools quickly, the grains are visibly shorter. The crystals (or dendrites) continue to form and harden until the entire melt is solidified. During the solidification process, the metal is shrinking. It is important to feed this shrinking to ensure the castings are free of voids and shrink defects. This is accomplished by the use of risers.
Once hardened, the cooled metal is removed or broken from the sand mold to complete the solidification process. This finished piece is also called a casting or castings. The casting(s) are then trimmed, finished and polished based on the specifications of its final application.
Although different methods employ different equipment and techniques, all forms of casting observe the metal solidification process. It takes an experienced and skilled foundry to control this process and avoid casting defects that can result from improper handling of this process.
Interested in learning more about casting services like sand casting, precision casting, aluminum casting, and more? Contact Patriot Foundry to learn more about our diverse non-ferrous foundry services.