Pulsed welding, which is somewhat like a series of overlapping spot welds:
http://www.lincolnelectric.com/en-us/su ... etail.aspx
As for the bumper, Weldtec, it looks amazing! The tube fitting and welding look top notch.
I have a question and comments. I was wondering if having the big angled flat plate under the front had any effect on the front end at highway speeds? I ask because I have been looking at something like your front bumper to provide a layer of protection for the front end components for 4x4 but it does look like it might act as a bit of a wing at highway speeds. And yes, I know lots of vehicles have them, including the off-road racers but that doesn't rule out that it might have an effect on lightening the front end at speed.
I don't think a tyre carrier would look amiss on the rear at all and personally I think the rear would look a little more 'finished' if the ends of the bottom tube extended up and around to join the wrap around top tube.