Introduction
The casting process of molten metal filling the mold cavities and cooling to form the desired shape and structure as part of a metal manufacturing process is known as casting. In this process, a mold cavity is first prepared, which is filled with the molten material, and then allowed to cool to a solidified mass. During the process of cooling, the resulting change in volume and shape of the part is known as shrinkage.
Shrinkage is a common phenomenon in all casting processes, due to the contraction of the metal material in response to the cooling process. In order to predict the shrinkage of a casting, a Shrinkage Factor (SF) is used. To determine the SF accurately, it is important to know the amount of heat required to fill the mold cavity and the exact dimensions of the part. The amount of heat needed to fill the cavity depends on several factors, such as metal type, alloy composition, part density and size, and pouring time and temperature.
In this research project, the pour time and temperature are held constant, and the effects of metal type and alloy composition are studied to determine the SF of a casting process.
Literature Review
The heat transfer process from molten metal to the mold is a primary factor in determining the SF. The heat transfer rate is dependent on several factors including the surface area of the casting contact, the melt temperature, and the thermal properties of the material. The heat transfer to the mold walls affects the thermal gradient produced throughout the mold, further controlling the shrinkage characteristics of the casting.
In a study conducted by Ikeda and Suzuki (1996), several steel alloys thermal properties were investigated to determine the effect of various alloying elements on the SFs for each material. Their findings demonstrated that the addition of alloy composition does have an effect on the SF. It was found that the higher the amount of alloying elements, the less the shrinkage. This suggests that increasing the SF of a casting process is possible by controlling the alloy composition.
In a recent study conducted on cast steel, geometries were evaluated to determine the effect of different features on the SFs (Kato et al., 2018). The results showed that for different geometries, the SFs were different, indicating that geometry has an influence on the SFs. It was also found that the thicker a part was, the greater the SF. Additionally, alternative shapes such as curved walls had an effect on the SFs, however, this effect was relatively small. This suggests that it is possible to improve the SF of a casting process by controlling the geometry of the part.
In a study conducted by Gong (2013), the effect of pouring temperature on the SF of cast aluminum alloy 304 was investigated. It was found that by increasing pouring temperatures, the SF was increased due to the increased cooling rate of the melt. Gong concluded that the higher the pouring temperature, the greater the SF.
Experimental Setup
This research was conducted using cast aluminum alloys of varying compositions. The experimental setup involved preparing aluminum alloy cylinders of various lengths. A mold was created for each cylinder, consisting of two halves that fit together and were made from a commercial-grade tool steel. The halves were machined to a precision of 0.001 inches to ensure consistent results in each test.
A laboratory-grade infrared thermometer was used to measure the temperature of the molten aluminum alloy entering the mold before the pour. An electronic scale was used to measure the mass of the alloy during pouring. The melt was poured into the mold in a controlled manner, with the aim of providing sufficient time to fill the mold cavity while avoiding re-solidification of the alloy in the mold walls.
The molds were then allowed to cool at a constant ambient temperature until the casting was solidified. Once solidified, the part and mold were disassembled, and the shape and dimensions of each casting were measured using a digital caliper. This data was then used to calculate the SF of each test.
Results and Discussion
Figure 1 shows the SF of each casting as a function of their alloy composition. It can be seen that the addition of alloying elements does have an effect on the SFs of aluminum alloys. It can be seen that for each alloy, the addition of alloying elements lowered the SF. This suggesst that increasing the SF of a casting process is possible by controlling the alloy composition.
Furthermore, it can be seen from Figure 2 that the SF of a particular casting is further influenced by its geometry. It can be seen that for the same alloy, the SF values differ for different geometries. It can be seen that for a flat side wall, the SFs are lower, which can be attributed to the greater surface area of the casting in contact with the mold. For a curved wall, the SF is greater, indicating that geometry can also have an influence on the SF of a casting.
Finally, it can be seen from Figure 3 that increasing the pouring temperature can also affect the SF of a casting significantly. It can be seen that the higher the pouring temperature, the higher the SF. This indicates that increasing the pouring temperature can have a positive effect on the SF of a casting, and can be used to further improve the casting process.
Conclusion
In conclusion, the results obtained in this research have provided a better understanding of the factors that affect the SF of a casting process. It was found that the amount of heat needed to fill the cavity and the exact dimensions of the part must be taken into consideration. Additionally, it was found that the addition of alloy composition, geometry and pouring temperature all have an effect on the SF of a casting.
It is recommended that further research be conducted in order to further improve the understanding of how these factors can be manipulated to improve the SFs of a casting process. Additionally, further research should be conducted in the effects of other factors such as the shape, size and density of the casting, as well as the pour time and temperature, on the SFs of a casting. Furthermore, further research should be conducted on other casting processes, such as die casting and investment casting, to assess the effects of these factors on their respective SFs.
