MET 110 Course Modification - Aluminum Casting
Casting is the creating of objects by pouring a molten substance (most commonly metals) into a mold to form them into a certain shape. There are various types of materials used for casting as well as for the type of molds that are used. Several methods of casting are sand, die, investment, lost foam, and continuous casting. All these different types casting differ only in the type of cast or the process in which the material is desired to be made (specifically continuous casting). All the processes are almost identical in general steps. The minute details during individual steps of the processes are what distinguish each type. These will be discussed in more detail in a future section.
Prior to executing an aluminum casting, the students had a lecture focused on the history of metal casting (Bronze Age, Iron Age), discussion on the melting point of aluminum, microstructure of metal, and a safety lecture.
Figure 1 demonstrates the importance of thermal (heat) when casting a material. The thermal processing (how high of temperature we heat, how long we hold at a specific temperature, how fast we cool, etc.) can dictate properties of a material upon cooling.
Figure 1: Image from Li1, demonstrating the difference the temperature of the fluid, (a) and (b), has on the microstructure of upon cooling (c) and (d). In (a), the metal is hot enough (at high enough temperature) to be liquid and cools into longer grains that appear to be almost dendritic-like in pattern (c). In (b), the metal is not completely liquid (slurry means partially liquid and partially solid - similar to runny mud), and has some solid components that upon cooling show circular shape with more clearly defined grain boundaries.
Casting is used in industry in situations where other methods of fabrication are not possible or economical. For instance, if a metal is not be easy to weld or blacksmith due to low ductility and malleability, it will not be simple or efficient to try to force the metal to change shape through those methods. However, if you melt the metal, you can simply pour it into a mold of the desired shape to create the part that is desired. So, oftentimes, casting replaces other methods when casting is more efficient or is at least possible.
In general, with all casting, you will need to gather a few things in order to complete the process. These include any molds and/or materials to fill or alter molds as well the molten metal and a device to make/keep the metal molten. You will also need a pair of tongs to hold the container with the molten metal as you remove it from the device that keeps the metal warm and pour it into the mold. Also, since you are working with metal at high temperature, standard work clothes, safety glasses, and gloves are very important for safety.
Figure 2: A MET 110 student successfully executing an aluminum metal casting pour.
Figure 3: On the left hand side are three molds, a cross, a bucket, and a heart. On the right hand side is a tin can of filled with water beads.
Figure 4: The result of molten aluminum cast into a bucket of water beads.
When beginning the casting process, it is important to make sure you have enough molten metal to make your object. One of the casting process by melting your metals in the furnace so that they can be poured. Assuming you have your molds already premade, you can then set them up near your molten metal (so you don’t have to walk far to pour). When you are ready to begin the actual process of casting, you must take the tongs, grab the container with your molten metal and begin pouring it into the mold, Figure 2. Care must be taken to not pour too quickly or slowly as this will result in the final object being filled with air bubbles or could cause part of the metal to solidify before other parts, making the piece weak. After you have poured sufficient metal into the container, you then should wait for it to cool unless your process requires you to quench it quickly in water or oil, Figure 3. After the metal has cooled enough, the cast can be removed from the mold, Figure 4.
1Li, G.; Lu, H.; Hu, X.; Lin, F.; Li, X.; Zhu, Q. Current Progress in Rheoforming of Wrought Aluminum Alloys: A Review. Metals 2020, 10, 238. https://doi.org/10.3390/met10020238
Note this section on aluminum casting was written with the help of the Fall 2021 MET 110 students.