Comment by azalemeth | Hacker News Reader

Comment by azalemeth | Hacker News Reader

If you're reading this article, you may wish to know that arguably a "counterpart" to heavy press forging is explosive forming [1] in which a chemical high explosive is used to force a template material against a template. The overpressure generated by the explosive can be equivalent or maybe even greater than heavy press forgings (a 50,000 US short ton force press exerts ≈500 MN force; peak overpressure close to detonating TNT or PETN explosives can be MPa or higher [2] so depending on the geometry of the part they may be comparable) and it has the added "fun" fact that complex cylindrical or spherical shapes can be made very easily and accurately.

The only people I know who have worked with this have used it to make superconducting magnets, explosively forming either titanium or high grades of nonmagnetic stainless (A4, which has µr ≈ 1) without causing marsenite formation due to machining. This includes a major international MRI scanner manufacturer, for one relatively niche product. It's like the "extreme" version of metal spinning [3] – forcing a rotating chunk of metal against a rotationally symmetric mandrel.

[1] https://en.wikipedia.org/wiki/Explosive_forming [2] https://www.researchgate.net/figure/Variation-of-peak-over-p... [3] https://en.wikipedia.org/wiki/Metal_spinning

Explosive forming has been used to make aluminum boat hulls.[1] It's a useful way to form very large sheet metal parts.

[1] https://www.youtube.com/watch?v=CbS6rS0seuk

I used to work as a design engineer for a pressure vessel manufacturer - we used explosion bonding all the time to bond more expensive corrosion resistant layers to carbon steel backing parts.

Example: In a shell and tube heat exchanger, the tubes might have some really reactive stuff in it so you might make the tube side out of indium , titanium, nickel, or even an expensive stainless steel like S32205/S31803. The shell side might just have river water for cooling, and can just be painted and have a sacrificial anode somewhere inside.

The bulkhead where all the tubes penetrate (the "tubesheet") might be 6' (180cm) in diameter and 4-8 inches (10-20cm) thick - an extraordinarily expensive hunk of material (or possibly not even available in the thickness needed) when made 100% of the more exotic materials; easily in the 6 figure range.

Sometimes this problem is solved by having a welder coat the entire surface with weld metal that's good enough to withstand the corrosion characteristics of the process stream, but with larger parts this can take _days_; with some metallurgies (e.g. brass) it's not even possible.

Instead, the practice was to explosion bond a "thin" layer (1/4" (6mm) or so) of the expensive stuff to a more standard carbon steel forging. The tubes are usually very thin walled and welded/brazed to the cladding.

What's cool is the interface layer between the two metals looks like when two liquids meet with swirls and whorls of the two materials interleaving, but frozen solid.

That sounds amazingly fun -- thank you for sharing, and for including si units! Do you have any pictures of the whorls? (I'm imagining something like damask steel)

See figures 3 and 11 in this[0] paper - sometimes this is visible with the naked eye, though only up close with your eye practically touching the part

[0]: https://www.mdpi.com/2075-4701/10/7/969

There are some popular videos making spheres using explosive hydroforming, which is quite fun, and much lower tech than explosively forming magnets to avoid the formation of marsenite (sp?).

I heard it from my brother who visited one such company, where they use explosive forming to shape metal parts used oil refinery; if I remember correctly for the corrugation on the metal that separate the hot and cold liquid in an heat exchanger, with the idea behind using explosive forging was that it allowed them to shape a big sheet in one go.

*martensite

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