Forging and casting are fundamentally different processes. In forging, a solid item of metal is repeatedly struck by the anvil(s) of a forge in order to produce a near-nett shape i.e. closer to the finished dimensions. An advantage of forged components is that strength levels can be higher than for cast components, as the microstructure is refined during the reduction process. As the grain structure also tends to follow the shape of the component, this provides favourable mechanical properties across section changes. Therefore, forging is typically used for shafts.
Casting is when a molten metal is poured into a mould, which when solidified can achieve quite intricate shapes with little waste of metal and a reduced need for machining. However, casting requires the creation of a pattern, from which the mould is fabricated, which can add significant cost and time. The design of the mould and runner system that takes the molten metal can be challenging, to allow for shrinkage of the metal as it cools. The mechanical properties of a casting are generally much lower than a forged bar, depending upon the alloy. Casting is most typically used to produce medium- to large-runs of the same component, justifying the cost of the pattern and mould-making process.