Here are two video guides by youtube user Scorch on using a Dremel based DIY friction welder. Great work Scorch!
Here is a video on how to make a Buckyball from 3d printed hexagons:
... and video on how to repair 3d printed objects:
Friction welding works with both PLA and ABS.
While good people of Hackaday think this is a new 3d printing technique it is actually used for a quite some time now.
here you can see it in action making and repairing stuff:
... and here you can see a guide on how to make it work and make a small DIY plastic welder:
And if you want to use 3d printed plastic rivets for friction riveting things together:
If you want to go the ultrasonic way or DIY heat plastic welding in the "old school style":
And here is how a friction welding seam looks like (from Scorch's video):