Iterative laser straightening of welded plates of varying thickness

Abstract

Laser forming uses a defocused laser beam which scans over the sample, generating high surface temperature and large thermal gradients, to produce bending and upsetting/shortening of the material. Laser forming is promising in a wide spectrum of industrial applications, for example flexible shaping of metallic components without mechanical contact, rapid prototyping, or creating complex 3D shapes and removing distortion. An iterative approach to laser forming has the advantage that unknown initial conditions can be accommodated, compared with single pass laser forming. In this study iterative laser forming (ILF) was extended to plates of varying thickness, and the first ILF of pillow and saddle shapes of varying thickness plates is reported. This forming is done incorporating a finite element (FE) model into the process, which clearly requires the plate thickness to be known. Furthermore, a method is proposed which would adjust laser parameters, based on surface temperature measurements, when the plate thickness is not known. An important factor for industrial application of ILF is the understanding of its effect on the material formed. Therefore, a systematic study for two common materials on mechanical properties, residual stress (RS) and process temperature of laser formed samples is conducted. Based on the through thickness temperature ranges identified, a method is proposed to determine the extent of affected material, independent of system parameters. For interpreting those results, an analogy between the microstructure in a laser formed heat affected zone and that observed in welding is proposed for the first time, which has proved extremely useful. Transverse residual strain measurements were used to further validate an analyticalnumerical model of laser forming, proposed previously. The through thickness transverse residual strain profile is predicted in a simplified way by the model, which shows its relation to characteristic line energy ranges and laser forming mechanisms. Finally, ILF of varying thickness plates is demonstrated for the removal of welding induced distortion of industrial specimens, specifically plates with welded stringers. Laser parameters for those forming trials were chosen, based on previous investigations. Finally, the choice of laser parameters is validated through a study of the effect of the iterative laser straightening process on the material properties.