Investigation of plastic deformation mechanism in elastic preloaded laser forming of sheet metal

Elastic preloaded laser forming (EPLF) is a promising plastic-forming technology for sheet metals. Compared with pure laser forming (PLF), the forming effect of the sheet can be significantly enhanced due to the introduced elastic preload. However, the fundamental plastic deformation mechanism of the EPLF process, which is crucial for process design and optimization, remains unclear. In this paper, a basic model of EPLF is investigated to reveal the mechanism. Based on thermal elastic-plastic mechanics theory, the variations of both stress and strain through the forming process are analyzed systematically to describe the deformation process, during which the plastic deformation mechanism is proposed. It is demonstrated that with laser scanning, the original force and moment equilibrium of the sheet created by preload is broken. The local high-temperature regions inside the sheet are plastically deformed by load exerted from low-temperature regions to build new force and moment equilibrium, which brings about plastic deformation. A finite element (FE) simulation is also conducted, which is in good accordance with experiments. The simulation results verify the reliability of the proposed mechanism and are used to make a more detailed analysis.