A Note on forged aluminum vs forged steel.
Generally, steel forgings are more expensive than castings by a good margin, and depending on the shape of the part can be the same cost as billet. Forging steel is hard on the dies and requires a big press, and a good size furnace, the cost of forging compared to the cost of whittling away everything that doesn't look like your part with a saw, profile mill, or end mill and the tooling that will get used up is equal to forging for some given production figure. So, below that number, it is best to go with machining from billet.
Generally, aluminum forgings cheaper in the long run than billet, and aluminum castings have very different mechanical properties from billet, extrusion or forged aluminum, so are a class all to themselves. Aluminum forgings do not require as much energy, so a smaller press can be used and the dies last much longer. The result is the break even number for forging is much lower, which is why they show up more often than billet aluminum parts, unless the production run is very very small.
As far as engine parts, I would figure forged pistons would be much cheaper than billet pistons, given the amount of material that would have to be removed, and thus be the preferred method of manufacture.
And, to address that age old, often fought over argument - forging vs [casting/billet]:
Firstly, it depends how you define strength. If you strictly mean ultimate or yield strength (or even stiffness, aka modulus of elasticity) under a single loading, then all materials of equal hardness and alloy (forged vs. machined from solid, vs machined from cast, with the exception of aluminum which has different properties if cast) will behave effectively the same, and have the same stiffness, yield strength, and ultimate strength. (And density, forging will not change the density of the material. That is a myth) This is because these are strictly a function of the strength of the bonds between atoms in the crystal lattice, and the ability of dislocations to propagate through the lattice (at yield). Dislocation propagation is a function of the dislocations already in the material, and the frequency and size of various impurity precipitates (alloying elements) within the lattice. It is not a function of grain structure.
However, if you are talking about fatigue strength under cyclic loading (such as found in a con-rod), then this is an issue of crack propagation, not crystalline structure or bond strength. Crack propagation is a messy field that I don't understand well, but in general I understand that they most easily propagate along grain boundaries. Hence, a forging which orients grain boundaries perpendicular to the load should demonstrate improved resistance to crack propagation (read: improved fatigue strength) as compared with a material which has isotopic grain structure (such as a casting). But you don't get something for nothing. Forgings show improvement these improvements in the longitudinal direction, but also show a decrease in the short transverse direction.
Now, billet steel or aluminum is rolled or extruded which will give it an elongated grain structure like a forging, so the properties not uniform in the longitudinal and transverse directions. How you layout the part on the billet matters.
