Forging is stronger than casting

What happens to grain structure when metal is deformed rather than poured.

Forging produces a stronger part than casting of the same metal because forging squeezes and works solid metal into shape, physically compacting and aligning its internal grain structure along the direction of the load applied, while casting simply lets molten metal solidify however it happens to cool, leaving its grain structure randomly oriented and, in a die casting, riddled with the small trapped voids that fast filling leaves behind.

How grains form in a poured part and a pressed one

A metal's strength depends heavily on how its internal grains, the microscopic crystals the whole piece is actually made from, are arranged and how soundly they are bonded to their neighbours. A casting's grains form wherever the molten metal happens to solidify, growing outward from wherever cooling begins with no preferred direction. The metal that freezes last, usually in the thickest sections, is also where shrinkage leaves small cavities and where impurities gather, so a casting's weakest spots are set by how it cooled rather than by where it will be loaded.

Forging takes solid metal below its melting point and deforms it under enormous compressive force. That force changes the outer shape and also drags the internal grains along with the deformation, stretching and aligning them in the direction the metal was pushed and pressing shut any small internal voids the starting billet carried. The result is a grain flow that follows the contour of the finished part, much as the grain of a wooden branch follows its bend, and grain running along a load path resists that load, and especially repeated loading, considerably better than grain arranged with no relationship to it.

Hammered clay against slumped clay

Hammering a lump of modelling clay repeatedly with the flat of a hand shows the same principle in a material soft enough to feel the difference. The hammered clay has been compacted and worked in the direction of every blow, pressed denser and flatter exactly where the hand struck it. The same lump left to slump under gravity spreads with no internal organisation at all. Both might end up looking roughly similar once flattened, but only one has had its internal structure worked by a deliberate force, the same distinction that separates a forged part's aligned grain from a cast part's grain, which simply froze wherever it ended up. Cut the hammered lump open and the layers lie flat, parallel to the table, pressed that way by every blow, and a forged part sectioned and etched shows the same thing in metal, with its grain lines sweeping around corners and along the length of each arm instead of stopping at them.

Same alloy, different material

Because the difference comes from how the grain structure was formed, two parts melted from the exact same batch of metal can end up with very different real strength once one has been worked and the other poured. The gap shows most clearly in fatigue, the slow growth of a crack under repeated loading, since a crack starts most easily at a void or a boundary lying across the load, and a forging has fewer of both. A crankshaft forged so its grain wraps around each crank throw puts the grain along the path the bending load follows, while one machined from a straight bar cuts straight through that grain at every change of section. This is why a critical, highly loaded component such as a crankshaft, a connecting rod or an aircraft undercarriage leg is usually forged even when casting the same shape would be cheaper, because in a part that cannot be allowed to fail, the strength matters more than the extra cost.

It also changes how a designer should read a specification. Matching the alloy alone does not guarantee two parts perform equally, because a forged and a cast version of the identical alloy behave, mechanically, close to two different materials. Substituting a casting for a part that was designed and tested as a forging, purely because the alloy call-out matches, is a mistake that can go completely unnoticed until the part is loaded the way the original forging was.

Shapes a forging die cannot reach

Forging has limits of its own. The metal has to be pushed and squeezed into shape while solid, which becomes progressively harder and more expensive as geometry grows more intricate, and deep internal cavities, thin walls or complex branching shapes are beyond what a die can press solid metal into however much force is applied. Molten metal, by contrast, flows into any cavity a mould can be shaped to hold, so a part that needs an intricate internal shape more than raw strength along a simple load path is very often a casting whatever the strength penalty.

Some components combine the two, made first as a rough blank and then forged or worked at the specific highly loaded features that need aligned grain, accepting the cost of two processes only where each is doing a job the other cannot. Paying twice only makes sense once neither process alone could deliver both the shape and the strength the part needs.