Research On Microscale Laser Dynamic Flexible Forming Assisted By Laser Pre-shocking

Microscale laser dynamic flexible forming(?LDFF)combines the advantages of high strain rate forming and microscale flexible forming technology to improve the production efficiency and forming quality of the microparts.The ?LDFF is especially suitable for the highstrength materials with complex features and has broad prospects in actual production.However,the mechanical properties and flow stress of the workpiece affect the forming quality because of the existence of size effect.Especially when the thickness of the workpiece is equivalent to the grain size,the forming accuracy and surface quality of the microparts cannot be guaranteed.It restricts the further development of microscale laser dynamic flexible forming technology.This paper proposes to introduce laser pre-shocking(LPS)into ?LDFF.Based on the related theories of laser shock forming and microscale flexible forming,a series of researches were conducted.The main research work and results are as follows:(1)The LPS experiments of T2 copper foils with different grain sizes were conducted to analyze the influence of laser pulse energy on the surface modification.The experimental results show that LPS has a positive influence on the grain refinement and the improvement of the surface quality.Moreover,the trend of first decreasing and then increasing on the surface roughness with the increase of laser pulse energy indicates that LPS can improve the surface quality of copper foils.In addition,different degrees of rebound phenomenon can be observed.The rebound caused by excessive laser energy cannot be ignored,and it also provides guidance for the parameter selection of subsequent pretreatment processes.(2)The bulging mold was prepared,and the corresponding microscale laser dynamic flexible bulging(?LDFB)experiment were constructed.And the influence of related process parameters(laser pulse energy,mold feature size,grain size and nano-hardness)on the forming quality of bulging samples was investigated from the aspects of forming depth,surface roughness,and forming uniformity.The surface roughening and increase of the grain size lead to the reduction of the forming quality of the bulging samples.The introduction of LPS is beneficial to the improvement of the forming quality.This is because LPS refines the internal crystal grains of the workpiece and suppresses the surface roughening in plastic deformation.Especially for coarse-grained samples,the grain size of the copper foil with LPS was reduced by 21.0%,and the surface roughness of the bulging sample with LPS was reduced by 13.7%.The improvement of the forming quality of tbulging samples with LPS is reflected in the improvement of the thickness distribution and forming symmetry,and the occurrence of local necking is suppressed.For workpieces with fixed feature sizes,there is a suitable mold feature size interval to ensure better forming quality.The increase of the mold feature size causes the decrease of forming uniformity,while the decrease of the mold feature size leads to the deterioration of the surface quality.The introduction of LPS increases the window of ?LDFB.(3)SEM was used to observe the microstructure of the fractured bulging samples,focusing on the analysis of local necking and premature fracture caused by surface roughening.For fine-grained parts,local necking tends to occur in the bulging central zone,which is located at the most severe surface roughening region.The local necking position of the coarse-grained parts is often unpredictable,and it usually occurs in the bulging transition zone,where the grain size distribution is uneven.This is because the difference in grain orientation and surface roughening cause local stress and strain concentration.Local necking is the precursor of cracks,and the location of the cracks mostly coincides with the location of the local necking.But for coarse-grained parts,cracks may even appear at the mold entrance zone,because the uneven microstructure and surface roughening cause local stress and strain concentration and premature fracture may occur.The direction of crack propagation is random but generally along the grain boundary,accompanied by strong grain boundary slip marks.The sharp thickness thinning at the edge of the crack is a sign of crack propagation.(4)The punching mold was prepared,and the corresponding microscale laser dynamic flexible punching(?LDFP)experiments were constructed.The influence of related process parameters(laser pulse energy,grain size)on the forming quality of punched samples was investigated from the aspects of section morphology,dimensional accuracy and shape accuracy.There is no fracture zone on the fracture surface of the punched samples,but only three areas of rollover,shearing zone and burr.The shearing zone of the punched samples with LPS is obviously enlarged and the surface quality is improved.LPS generally improves the shape accuracy and dimensional accuracy of punched samples,especially for coarse-grained samples.By suppressing surface roughening and refining the grains in the thickness direction,the stability of the ?LDFP is improved.