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3D printed metal is now as strong as machined metal, assuming an identical alloy. The process has been pretty well perfected.

The strength loss comes from the fact that not all alloys are 3d-printing friendly, so you often have to compromise and you end up with a less than ideal alloy for your application.

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Sure, but I mean what's the technical reason a material isn't it 3D printing friendly? Are we talking grain structure here? Is it something that can be at least partly mitigated by some post-printing heat treatments, like tempering?

Some alloys don’t like to be melted. If an alloy has a large solidification range, certain areas can partially solidify without the liquid part keeping up to fill in the gaps so to speak. This leads to solidification cracking / hot tearing. This is a simplification and only one possible cause, but there are literal books written about this kind of thing (I like Solidification by Dantzig and Rappaz). This is also why you see things like friction stir welding for rocket bodies. No melting means no solidification means no solidification issues.

Eh. A lot of materials get their strength from being worked, which 3d printing doesn't do at all.

Try machining some printed 316 stainless! It’s basically pre work hardened from the thermal stresses!

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