The main issue with welding in space is the lack of convection based cooling, which means the welds take longer to cool through conduction, which can result in a larger HAZ. Increasing the mass and heat capacity of the adjacent material greatly reduces this.
The lack of an atmosphere and contaminants makes space a near ideal welding environment.
Similar results can be reliably recreated on earth. Ultrasonic welding rubs two pieces of metal together until the oxide breaks apart leaving pure metal to fuse. Explosive welding creates a plasma that strips off the oxide layer, and then propels the metals into each other. This method has the benefit of bonding dissimilar metals, and usually produces bonds that are as strong as the weakest metal.