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Comment by jackmott42 | Hacker News Reader

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.

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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?

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.

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!

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.

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.

I can't speak to the proximal cause of the roughness and porosity, but if you've ever held a raw printed aluminum part in your hand it is immediately apparent. That said there are processes to deal with porosity like Hot Isostatic Pressure (HIP) treatment that basically crushes all the voids with immense pressure. This does come at the cost of dimensional accuracy though (HIP will compress the part somewhat).

Similarly, annealing a 3D-print to relieve residual stress does work, but it also will cause warping as those stresses are relieved. Again, sacrificing dimensional accuracy. Frontier AM companies have ways to compensate for all of these effects but it's a trial and error process for each part essentially.

At this point you're now stacking multiple processes on each other just to try to get to near-billet properties. Calibration Trials > Print > HIP > Anneal > Machine. The cost adds up quickly. It can be justified especially in non-fatigue-critical applications but it's no free lunch

I'm not sure if it make sense but... is it possible to put the printer inside a vacuum chamber so there is no air to fill the internal bubbles?

Cooling would be an interesting problem. Atmosphere takes away the heat pretty readily.

You could have a cooling plate the work is on. But the higher temperature difference is apt to cause warping.

Vacuum filled pores would also be an issue.

I expect them to collapse due to surface tension or something.

[Slightly related https://en.wikipedia.org/wiki/Cavitation The problem is that the bubbles are filled with vapor instead of CO2 like in sparkling water.]

SLS printers lay a layer height worth of powdered metal and fuse it with a laser engraver. Thee bed lowers one layer and the process is repeated. They don't bring materials used like inconel to full melting temperature, only do what it takes for the metal sand to clump together. That's one source of pores.

I assume you can just anneal or print then recast in sand or whatever, maybe even lostwax with Al as wax, but the point is that porosity in a print itself is inevitable with current powder based tech.

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

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