Numerical Modelling of Deformation Prediction for Laser-Assisted Laser Peen Forming

Author:  Mingsheng Luo, Yongxiang Hu
Source:  ICSP-13
Doc ID:  2017136
Year of Publication:  2017
Abstract:  
Introduction: Laser peen forming (LPF) is a forming technique using laser-induced plasma to impact the workpiece spot-by-spot and forming the metal sheet into a specified curvature. It is a complex physics process including laser ablation, shock wave propagation, material yielding and deformation accumulation. A high-intensity short-pulsed laser irradiates on the surface of the workpiece, as a result, high pressure plasma generates on the surface of the workpiece in the confinement of water. In the impact of high pressure plasma, the shock pressure generates and spreads to the depth direction. The amplitude of the shock wave arrives at 1-10 GPa and the time duration is around 100 nanoseconds, as a consequence LPF is a high-dynamic response process due to the laser shock process. When the shock wave exceeds the Von Mises stress of materials, the workpiece in the shock zone yields and generates plastic strain. After laser scanning over the workpiece surface, the deformation is accumulated with the expanding of the plastic zone, resulting in the workpiece bending into a small curvature convex shape. In order to overcome the difficulty of improving the pulse energy, laser-assisted laser peen forming(LALPF) was proposed by Hu et al. [1] to improve the bending capability. LALPF adapted CW laser heating to increase the deformation. Laser heating plays an assisted role in softening the material and altering the stress state of material during LALPF. Simutationly, the back surface of the workpiece is heating by continuous wave (CW) laser which is coaxial with the pulsed laser. The material absorbs the CW laser energy resulting in high temperature field in the local area. The local high temperature decreases the yield strength, inducing thermal stress even plastic thermal strain owing to the nonuniform expansion of material. Thus, lager and deeper plastic strain is generated in the laser shock process and the deformation of the workpiece increase compared with the LPF in room temperature. The coupled thermo-mechanical mechanism in LALPF cannot be observed through experiment directly, therefore it is necessary and effective to use finite element method (FEM) to predict the bending of the workpiece in LALPF. However, this hybrid process of the continuous laser heating and noncontinuous of laser shock peening results in high computation cost because of the unmatch of the time scale in fully coupled thermo-mechancal numerical model. The time scale of single laser shock in LALPF is nanosecond in the discontinuous laser shock, while the time scale of the heating process is second in the continuous laser heating process, which results in huge computation during the simulation of fully coupled method. Therefore, the fully coupled method is not suitable for multi-time scale simulation of LALPF. To reduce computation costs, the sequentially coupled method simulates the two process dividedly: the temperature of laser heating is first simulated and then treated as the initial condition of following model. This method is used in many laser assist heating process, such as laser-assisted single point incremental forming [2], laser-assisted micro-milling [3]. Unlike these processes, LALPF is the characterized by extremely high shock pressure and short shock time, and the laser heating not only soften the material strength by increasing the temperature, but also may have an influence on the stress wave propagation due to superimposed stresses, which affects the plastic strain and finial deformation in the workpiece after laser shock. Furthermore, different from the common laser heating process, the water in LALPF worked as confined layer relatively moving on the surface of the workpiece during laser heating makes sequentially coupled FEM is more difficult to build.