[sfink]

I wonder if cold welding could be useful in space, though. Make the two surfaces totally flat, clean them in vacuum, and then press them together to weld them.

Ok, "clean them in vacuum" is kind of a "now draw the rest of the owl" type of thing. But I wonder what's possible when you don't have an atmosphere to mess up the surface. Could you scrape off the oxide layer of aluminum, for example, and get it smooth enough to cold weld without worrying about it re-oxidizing because there's no air?

[yetihehe]

> Make the two surfaces totally flat

This is the hard part. They need to be flat to less than atom size over whole area, at least several square centimeters. We can't do that yet economically. If they are not so flat, your weld will be pretty weak.

BUT if you could make them flat enough and then wiggle them ultrasonically so that those almost flat surfaces rub the rest of bumps, that would probably not require a lot of heat and energy to make a pretty good connection.

[userbinator]

Surfaces that aren't that flat, already have an interesting weld-like behaviour, although they can still be separated relatively easily:

https://en.wikipedia.org/wiki/Gauge_block#Wringing

[pjc50]

Ultrasonic stir welding is already a thing: https://technology.nasa.gov/patent/TOP8-95

[namibj]

Ultrasonic welding is done commercial for cable harnesses to butt weld copper.

[mlindner]

In order to make a surface properly flat you need to have the cleave surface line up with crystal boundaries, but crystals are randomly oriented in three dimensions meaning the surface can't be truly flat.

And that's even assuming the solid you're working with is crystalline of the sort that can do this. Many materials are alloys meaning that cold welding would be further difficult.