Experiment And Numerical Simulation Research On Microscale Laser Dynamic Flexible Forming

Micro-scale laser dynamic forming (?LDF) is a novel high velocity micro-forming technique. As laser shock wave induced high strain rate forming process, ?LDF can not only solve the technological problems in conventional micro-forming, but also improve the forming limits. Thus ?LDF has become research hot spot in micro-forming field.Based on the analysis of laser dynamic forming both at home and abroad, combining rubber pad assisted micro-forming,a novel microscale laser dynamic flexible forming is presented.The main contents are as following:The mechanism of rubber-induced smoothing effect on confined laser shock wave is proposed. Plasticine is used to perform the smoothing effect experiments due to its excellent material flow ability. The influence of rubber on the uniformity of laser shock wave pressure is studied by measuring the surface micro topography of the deformed plasticine. The research results show the possibility of smoothing laser shock wave pressure using rubber. The comparative experiments with and without rubber were performed to study the effect of rubber on forming capability. The experimental results show that, the shock wave energy attenuates during the propagation through the rubber layer, so the rubber can avoid the premature fracture. Although the initial shock wave pressure is reduced, rubber can improve the forming capability of ?LDF.The experimental systems for micro-forming was developed, and the effects of laser energy and rubber thickness on the deformation were investigated. The samples are observed by optical microscope, and fracture morphologies are characterized by scanning electron microscope. The samples have good forming quality when laser energy is low. And the severe oxidation and melting phenomena were detected along the sheared edge when laser energy is too high, which indicates that the large plastic strain is generated before fracture. When the number of laser shock loads reaches certain threshold value, micro-defects were observed on the mold surface. Laser shock target experiments are performed to investigate the surface degradation evolution process,which provides guidance for the life cycle prediction of micro mold.Based on the observed different microstructure features before and after fracture using TEM, the possible microstructural evolution processes include the following four typical states: (1) Initial shock loading stage: High-density dislocations are generated; (2)Bending stage: dynamic-recrystallizations-induced nanometer grains are formed under shock compression deformation; (3) Stretching stage: In order to accommodate the high strain in this stretching stage at the nano-scale, grain boundary sliding(GBS) becomes the dominant deformation mechanism. However, the voids easily nucleated under the tension stress; (4) Fracture stage: as the stretching deformation continues, Heat-induced grain growth phenomena may degrade the material flowing ability, the local high tensile stress make the voids grow up and coalesce to form cracks. The the adiabatic shear failure mechanism can be used to guide the design of the micro-forming process.The effect of the collision between the mold and workpiece on the surface state and forming precision of workpiece are investigated. According to the rebound behavior during mold forming, a kind of modified laser dynamic flexible forming is presented. A low density plasticene is inserted between the rubber and the thin metal sheets. The plasticene layer can improve the stiffness of workpiece ,and absorb the redundant shock wave, so the rebound behavior is reduced. Finally the forming precision is improved.The surface nano-hardness and elastic modulus of two typical workpieces (with and without fracture) were measured using nanoindentation system. The measurement results show that the surface nano-hardness and elastic modulus are improved after forming, particularly at corner radius area of workpiece without fracture.Three types of dynamic responses of the samples are observed in numerical simulation, including bulge forming, tensile tearing and shear rupture. These three types of dynamic responses are consistent with the experimental results. Since the temperature field and stress state during micro-forming are very difficult to measure by experiment,the simulation can be used to predict the corresponding results. Numerical simulations considering micro-defect on mold surface were performed to investigate the effect of micro-defect on workpiece. Numerical results show that the replicating behavior on rear surface cause the inhomogeneous material flow. Collision behavior in mold forming is investigated,and rebound deformation law is revealed.In conclusion,This paper systematically studies the characteristics of laser shock rubber loading. The deformation and fracture laws are summarized through free forming and mold forming using experimental and numerical simulation methods. It lays the foundation for the engineering application of laser dynamic flexible forming process.