Experimental Study On Size Effect Of Laser Shock Micro-bulging/Micro-punching Process For Copper Foil

The widespread application of micro-parts promotes the development of micro-manufacturing technologies,among which laser shock micro-forming has got extensive attention for its high productivity and forming accuracy.The principle of laser shock micro-forming is to utilize a high-power laser pulse to generate shock wave pressure and subsequently make the metal target deform plastically at high strain rates to obtain different micro-structured parts.With the miniaturization of the part,the feature dimension reduces from macro-scale to micro-scale and mechanical properties of the material are different from that in macro-scale due to so-called size effects.As a result,the forming performance is changed and the forming process as well as the quality of formed parts are influenced.Hence,it is of great importance to investigate the influence of size effects on micro-forming.In this paper,laser shock direct and indirect micro-bulging as well as laser shock direct and indirect micro-punching experiments are carried out,and then size effect are investigated and the effect of laser power density as well as the application of rubber are studied.The main contents are as follows:Laser shock direct micro-bulging experiments are conducted using annealed T2 copper with thickness of 50?m.Maximum bulging height,thickness distribution,micro hardness,grain size and surface morphology are measured to evaluate forming quality.The effects of grain size and laser power density are investigated.Results show that with the increase of grain size,the maximum bulging height increases while thickness and micro hardness of the foil decrease.Meanwhile,the hardness distribution tends to be uneven and the grain refinement is enhanced,while the surface still tends to be rough.Moreover,with the increase of laser power density,the maximum bulging height and micro hardness is increased,while foil thickness is decreased and the distributions of thickness and micro hardness are both nonuniform.Grain size is slightly refined and the foil surface tends to be slightly rough.However,the laser power should be limited to avoid fracture of the bulged part.The same indexes are applied in laser shock indirect micro-bulging experiments to evaluate forming quality of the parts.The effects of grain size,characteristic dimension,existence of rubber and laser power density are studied using T2 copper with thickness of 80?m and 100?m.The results show that both grain size and laser power density have the same influence on the quality of formed parts as in direct laser shock micro-bulging.Moreover,with the increase of foil thickness,the maximum bulging height and the reduction rate of maximum thickness are decreased while micro hardness is increased.Comparing laser shockdirect and indirect micro-bulging,it can be seen that the existence of rubber enhances the maximum bulging height and forming stability and improves the uniformity of foil thickness.Laser shock direct micro-punching experiments are carried out to investigate the effects of grain size and laser power density on the quality of punched holes using annealed T2 copper with thickness of 30?m.the characteristics of shape accuracy,dimension accuracy and fracture surface morphology of punched holes are measured to examine forming quality.It is observed that with the increase of grain size,the shape accuracy and dimension accuracy decrease and the fracture surface quality descends.Meanwhile,with the increase in laser power density,the quality of punched holes with smaller grain size improves while no improvement is observed when grain size is relatively large.The fracture surface quality is not significantly improved and in some circumstances even worsens.Laser shock indirect micro-punching experiments are conducted using annealed T2 copper with thickness of 20?m,30?m,40?m and 50?m to investigate the effects of grain size,characteristic dimension,process dimension,rubber and laser power density on the quality of punched holes.The indexes are the same with those in direct laser shock punching experiments mentioned above.The results show that grain size and laser power density have the same influence on the quality of punched holes as in laser shock direct micro-punching.In spite of different grain sizes,the shape accuracy of the punched holes are all enhanced and the roundness varies from 20?m to 55?m.However,when the diameter of the die is below 1.6mm,the shape accuracy decreases with the increase of foil thickness;when the diameter of the lower die is above 1.6mm,the foil thickness has no influence on the shape accuracy.It is also noticed that when the diameter of the die exceeds 1.0 mm,the punched holes with thickness of 30?m have the best dimension accuracy while the variation of biger die diameter has no significant influence on dimension accuracy.Comparing laser shock direct and indirect micro-punching,it can be observed that the application of rubber can improve the shape accuracy and dimension accuracy of the punched holes.