Research On Performance And Size Effect Of Laser Shock Hydraulic Flexible Micro-forming

Laser shock hydraulic flexible micro-forming is a new type of forming process,its principle is to use single pulse laser-induced shock wave pressure to impact the liquid medium to form metal foil.The forming performance and size effect of this new process present new characteristics,which need to be further explored.This dissertation uses a combination of theory,experiment and numerical simulation to systematically study the forming performance and size effect of laser shock hydraulic flexible micro-forming.The main content and results are as follows:First,the interaction between laser and matter and its force effect model,microplastic size effect model,impedance mismatch model,and the calculation method of material strain rate at high strain rates are discussed,which lays a theoretical foundation for subsequent experiments and numerical simulation research.Secondly,an experimental platform was built to obtain T2 copper foils with different grain sizes and foil thicknesses through annealing process,and under different laser power densities,laser shock hydraulic flexible micro-forming experiments were carried out to explore the forming performance and grain size effect.(1)In terms of forming performance,it found that: Increasing the laser power density,the maximum bulging depth,the surface roughness,and the unevenness of the thickness reduction increased,and the elastic recovery rate was reduced;the process greatly improved the thinning at the rounded corners,the largest thinning is in the bottom transition area,which is due to the pressure equalization of liquid and the flow supplement of material;the area with greatest hardness is the bottom of foil because of the coupling effect of high strain rate and plastic deformation on the increase of dislocation density.(2)In terms of grain size effect,it found that: when the ratio of foil thickness to grain size N(t/d)> 1,the value of N decreases,the maximum bulging height increases,the surface roughness is not significant,the thinning of foil thickness intensifies,and the deep elastic recovery rate decreases;when N<1,the maximum bulging height decreases,the roughness increases significantly,the regularity of thinning decreases,and the deep elastic recovery rate greatly increases;N<4.3,The larger the value of N,the higher the hardness,which is in line with the conventional Hall-Petch relationship.Because dislocation movement is the main mechanism;when N>4.3,the larger the value of N,the lower the hardness,which conforms to the inverse Hall-Petch relationship.Because twin deformation is the main mechanism.Finally,the Hypermesh/LS-DYNA software is used to carry out numerical simulation research on the forming process using the fluid-structure coupling method.(1)Research on the forming performance found that the simulation of the bulging height of the workpiece is highly consistent with the experimental results;the liquid medium will generate four shock waves,and the pressure of the last two shock waves will be dissipated due to the existence of friction in the liquid.The second shock wave pressure acts on the surface of the workpiece,which obviously inhibits the rebound of the workpiece,and the rebound of the workpiece displacement is very small;the liquid medium prolongs the action time of the shock wave,and the forming quality of the foil is high;the forming occurs fbined with the dimple morphology in the SEM image,indicating that the ductile fracture occurred here;the rubber layer and the liquid greatly homogenirst from the entrance of the fillet,Instead of the center position where the pulse laser energy is the largest;the velocity vector of the xy plane in the center edge area is the largest,comized the shock wave pressure,It is helpful to improve the uniformity of forming.(2)Based on the principle of similarity,the mold system with different scale factors was simulated,and the influence of the geometric size effect on the forming effect was explored.The research found that: a large scale factor mold system has a large number of crystal grains,a large coordination effect between the crystal grains,less geometrically necessary dislocations(GND)inside the crystal grains,local softening,large bulging depth,low elastic recovery rate,and forming high precision;a large scale factor mold system has a small equivalent plastic strain,which to a certain extent reflects the improvement of the workpiece forming ability in the high strain rate laser shock hydraulic flexible micro-forming process;a large scale factor mold system,on the contrary,the instantaneous maximum liquid pressure is small,which requires more laser energy when forming difficult-to-form materials with high yield strength.This dissertation explored the forming performance and size effect of laser shock hydraulic flexible micro-forming,laying a theoretical and technological foundation for the application of this process.