Comment by jackmott42
14 hours ago
3d printing metal has some strength downsides, I'm not sure what it is for the raptor engines, but I've heard other space companies claim ~5% less strength that traditional methods for aluminum structures, but that can be worth it in cases where you are able to make shapes that wouldn't be possible with traditional methods, or if you save enough money by printing it. Rocket engines often can benefit from intricate internal channels and shapes that you can 3d print as once piece with no way to do it via subtractive manufacturing.
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
What's the current theory for why this is?
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.
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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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Don't get good crystal/grain structure from 3d printing.
One of the things that 3-D printing allows is a fine honeycomb structure. This comes with a somewhat bigger strength reduction, but a dramatic weight reduction. Less materials is cheaper.
Also subtractive manufacturing requires paying to melt away a lot of metal. 3D printing saves.
I don't know all of the ways in which SpaceX is using