Although laser welding can produce relatively high quality and low distortion structures, distortion remains a problem when welding high accuracy 3D components for aerospace applications. Low Stress No Distortion (LSND) techniques have been demonstrated to eliminate buckling distortion with arc welding. The present paper describes the extension of this technique to laser and friction stir applications.
The present LSND system utilises a transient thermal tensioning technique to introduce equal and opposite forces to those which the solidifying and cooling weld metal normally experiences, and which lead to plastic deformation during the welding cycle. Thermal tensioning is here achieved by spraying a cryogenic liquid, delivered from a nozzle which moves along with the laser, to the solidifying weld metal at the rear of the laser spot. The theory of operation of the system, as well as the critical parameters for an optimised system will be described.
Demonstration of the elimination of buckling distortion when direct diode laser conduction welding 1.6mm 304L sheet, both for bead on plate and butt welds, will be shown. Here sample distortion amounting to 20mm from flat can be completely eliminated with LSND. The effect of LSND on the residual stresses in friction stir welding will also be described.