As we all know, forged wheels are stronger than cast wheels, but what’s the reason of stronger?

Let’s see:

The forging process creates parts with a finer, more advantageous grain structure and less porosity (voids within the metal. This results in parts with higher tensile strength, better toughness, and better fatigue resistance. Part of the improvement is the fact that working semi-solid metal tends to orient the crystal lattice grain along the part surface while casting leaves chaotic grain and machining leaves the grain unchanged from the original plate or ingot that was produced.

Forging works because it creates a more overlapped, interlocking structure. Individual crystals are far stronger than the boundary bonds between crystals. Flattening and aligning the mass of crystals within a metal piece makes a material that is acting somewhat like wood which is far stronger resisting forces in the grain direction than perpendicular to the grain. The highest service stresses in most parts occur at the surfaces and the stresses are oriented paralleled the surface. Strengthening the surface greatly increases the overall applied force required to start a fracture.