Rectification of distortion
If the measures listed above are not effective, remedial measures to rectify the distortion will be necessary. These may be based upon those used for steel but great care needs to be exercised if such techniques are used. The most effective methods are those that use some form of mechanical working or stretching as these will not significantly affect the mechanical properties of the base materials. Longitudinal bow in welded beams should preferably be done cold by pressing, and buckled plate may be pressed flat.
As a last resort, local spot or line heating may be used to heat-shrink items that have been distorted by the welding of, for instance, stiffeners. Some examples of how these techniques can be applied are illustrated in Fig. 5.24. The high thermal conductivity of aluminium means that local heating with an oxy-gas torch is not very effective. If this technique is to be used then electric induction heating is the most effective method of introducing sufficient heat into the component.
If heat must be used then this should not exceed 400 °C for the non-heat treatable alloys. Remember, though, that temperatures over 250 °C will produce full softening in the alloy if it is in the work-hardened condition. The age-hardened heat-treated alloys should not be heated to more than 150 °C as this will cause softening due to overageing of the precipitates.
Whenever these techniques are used then reference must be made to the design engineer to ensure that the potential loss of strength is taken into account.