Low Stress No Distortion ( LSND) Welding of Aerospace Components

Welding related distortion is a problem in many industries resulting in a reduction in performance, compromise in design and expensive remedial action. For aerospace components, distortion can influence gas and air flow across a welded surface, creating hot spots and premature failure.

Distortion occurs due to forces produced by differential contraction as material solidifies and cools from the non-uniform heat produced by the welding process. Fabrications in thin material, as used in the aerospace sector, are more prone to distortion than thicker fabrications because there is less stiffness against out of plane bending moments and lower resistance to buckling. In thin sheet, compressive forces are produced adjacent to the weld. If the compressive forces are too high they can cause the sheet to buckle.  The aim of low stress no distortion (LSND) welding is to reduce the plastic strains, by changing the temperature distribution in the component and thereby reduce the tendency for buckling.

 LSND  technique may involve additional heating, cooling and mechanical constraint to reduce the build up of stresses and buckling distortions in a welded component.  The techniques describe in this paper  for gas tungsten arc welding (GTAW) use selective cooling of the weld bead and surrounding area just behind the weld, to change the temperature distribution in the component. Cooling techniques using cryogenic (liquid) carbon dioxide and atomised water are described. FE modelling has been used to gain a better understanding of the process and to optimise cooling parameters.

 The LSND technique has been demonstrated for a range of materials including stainless steel, titanium and nickel alloys, varying in thickness between 1 and 4 mm. This paper describes the techniques that have been applied and the results obtained.