Dynamic distortion measurements during laser forming of Ti-6Al-4V and their comparison with a finite element model

Laser forming is, potentially, an attractive flexible manufacturing technique for the controlled forming of aerospace alloys. Laser forming experiments using a continuous-wave CO2 laser were performed on coupons of material 80 mm x 80 mm in area and 2 mm thick, with sequential passes of the laser beam, at a surface scanning rate of 20mm/s with 90s of convective cooling between passes. A novel surface profilometer that was specifically developed to operate under the conditions of high vibration and stray light typically found in laser machining applications recorded transient surface shape changes during individual laser passes at frame rates of 4 and 0.2Hz. A finite element model was developed using ABAQUS for the laser forming of linear bends in free Ti-6Al-4V sheets, with sequentially coupled thermal and elastic-plastic analysis incorporating temperature-dependent material properties. Transient heat source scanning was implemented to simulate the experiment. Good agreement was found between the experimental three-dimensional shape data and those predicted by the transient model. In particular, the formation of an unwanted 'camber' distortion perpendicular to the desired main bend was correctly predicted; its magnitude and temporal evolution throughout the three laser passes, and during the periods of convective cooling, agreed well with the experimental data. The model and the shape measurement technique will enable the future predictive controlled laser forming of more complex three-dimensional shapes.