Comment by fbd_0100

16 hours ago

It has mild strength downsides, but very severe fatigue and damage tolerance downsides. Knowing this, it makes sense that 3d printing tech would make headway in space industry but not (yet) in aviation

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fbd_0100

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naasking  16 hours ago

What's the current theory for why this is?

  • fbd_0100  15 hours ago

    crack growth in metals is driven by microscopic flaws that cause high-intensity but very localized stress concentrations. Over time even low stress levels cause these flaws to grow to the point where they start causing strength problems. Even in traditional aluminum machined parts, increased surface roughness can have a large impact on fatigue life.

    3D printed parts are chock-full of these microscopic flaws, porosity, and have horrible surface roughness (most parts you see in production are post-machined to improve the finish). Additionally, the repeated heating-cooling of the layers as they are deposited builds up residual stress in the part. All just due to the nature of how they are manufactured.

    • naasking  15 hours ago

      Is there a known source of internal flaws/porosity in an otherwise solid part? Presumably laser melting produces a puddle which shouldn't allow for internal pores, as long as it isn't printed too fast (or solidifies too fast, which is why I think most chambers are internally heated to near melting temp).

      Re: surface roughness, I can understand that the powder grain size creates a sort of minimal structure size, and can in principle be the start of a crack if a surface grain gets knocked loose. Is that the sort of thing you mean? I can see that for any internal or external surfaces, and a rocket engine combustion is certainly applying a lot of heat and pressure on surface grains. Can this be alleviated by smaller grain sizes, or is there some limit there?

      Re: repeated heating/cooling and internal stresses, this strikes me as just requiring standard post-printing stages like tempering to alleviate internal stresses.

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  • tredre3  15 hours ago

    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.

    • naasking  15 hours ago

      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?

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