eli5 – why is cast metal weaker than forged metal?
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Liquid metal has gas and other impurities dissolved into it. Solid iron can’t dissolve as much, so as the metal solidifies it concentrates the impurities towards the front forming a horribly weak mess at the last place to solidify, with gas pockets and non- metallics. Forging breaks these up, do any cracks are smaller. It’s the size of the biggest crack that determines the toughness of the metal.
Forging also helps the strength through making the crystal grains smaller. The grain boundaries aren’t weak, but to stretch metal, you need to move slices of atoms called ‘dislocations’. These are hard to push through grain boundaries, do small grains make for strong metals. In most steels, heating and cooling will make the grain size smaller too.
I am a metallurgist.
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