Casting Defect Prediction - Air Entrainment
A front gear housing, 380 die cast
alloy. Results are colored by
percent of entrained air.
Image courtesy of Albany-Chicago Co.
A support arm cast in A380. Result
is colored by air entrainment. The
major concern with the part was
filling the deep and thin ribs.
Image courtesy of Littler Diecast Co.
The air entrainment model in FLOW-3D Cast is used to estimate the amount of entrained air occurring in metal casting systems, such as gravity-poured casting processes. It is based on simple physical mechanisms, which means it can also be used to estimate the amount of entrained air occurring in metal casting systems, such as gravity-poured casting processes.
Recent additions of even more physical details in the model allow entrained air, which is assumed to be in the form of bubbles, to be modeled as rising in the surrounding liquid because of buoyancy forces and even leaving the liquid if it reaches a free surface.
Sprue and Runner Systems
Gravity poured castings typically use sprue and runner systems to direct poured metal into a mold. During the initial stages of pouring there can be a significant amount of splashing in these systems while they are being filled with metal. If air is entrained during this process it may be carried into the mold and result in unacceptable defects in the final part.
A simple arrangement for casting a rectangular plate can be used to illustrate this type of application. The plate has dimensions of 20 by 10 by 1.64cm. A 22.2cm high sprue and 25cm long runner is connected to the plate. Aluminum enters the sprue with a pressure head of 5cm.
In Figure 1, the amount of air entrainment in the metal is recorded using a passive scalar concentration, which did not affect the liquid dynamics. Figure 2 shows the same casting with the entrained air allowed to rise and leave the liquid metal as it reaches the free surface.
|Figure 1: Air volume fraction (max 3%) with no effect on metal density||Figure 2: Air volume fraction (max 5%) including air drift and air escape at surface|