[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.