Experimental Study On Laser Shock Clinching Of Copper Foil And Perforated Stainless Steel Sheet

Laser shock clinching?LSC?is a cross-application of laser shock forming?LSF?and joining technology.In a typical LSC process,laser-induced explosive plasma deforms the sheet/foil into specific interlock structure,joining two or more layers together.LSC technology shows a promising application prospect in automotive,aircraft and other fields.This paper presents an experimental study on LSC of copper foil and perforated stainless steel sheet.A multi-parameter adjustable LSC mould system was developed and the influence mechanism of technological factors in forming process was discussed.The deformation evolution,metallurgical microstructure,failure modes and failure mechanism were discussed based on the analysis of geometric features,plastic flow of material and microstructural characteristics.The LSC process windows of copper foil and perforated stainless steel sheet were established.The mechanical properties of LSC specimen were tested.The main contents and conclusions are as follows:?1?The experimental system mainly consisted of a Nd:YAG laser system,a digital displacement stage,the LSC mould system,quartz glass as confinement layer and black paraffin as ablative layer.?2?Factors,such as laser beam,material properties and feature sizes of LSC mould,were discussed.The bulge height increases with increasing pulse energy.Top fracture,partial blanking and complete blanking are the common fracture mode.The usage of holder board maks it easier to update the ablative layer in LSC process,and improves some issues,such as the control of laser spot,the coaxiality of laser beam and the enegy distribution of laser spot.Feature sizes of LSC mould have a quantitative relation:diameter of holes in spacer D₂>diameter of bottom support D₃>diameter of holes in holder D₁?diameter of holes in stainless board D0.?3?In the first stage of LSC,the foil bulges into lump shape.The axial and radial deformation mainly arises from the thickness decrease of material,and the thickness declines from hole edge to the axis gradually.Materials around the hole can flow into the forming cavity,which can make up for the thickness decrease.The foil has the smallest thickness nearby the top of lump,where top fracture may occur.The foil may also fracture at the edge of hole because of foil thickness decrease and shearing pressure from plasma.In the second stage of LSC,the foil bulges into boss shape.The axial deformation gradually change into radial deformation because of the constraint from bottom support.The diameter of boss surface increases with the accumulation of pulses ending with a cylindrical shape.Materials around the hole can flow into the forming cavity,but the flow resistance becomes stronger.The thickness decreases obviously at hole edge and the fracture here becomes easier.The radical deformation depends on the material flow on boss surface,and the material flows more intensely nearby the boss edge.In the third stage of LSC,the bottom of cylindrical shape bulges radically.The diameter of the bottom becomes lager than that of the hole,forming an interlock structure.?4?After the 400? recrystallization annealing of copper foil,the microstructure is improved,the residual stress and work hardening are eliminated,the anisotropy is reduced and the mechanical properties are optimized.Region with fine grain tends to deform uniformly,and region with large grain is likely to fracture when the material flow is intense.Grain elongation,rotation and crushing may occur along with the plastic deformation.?5?There are two primary reasons for foil fracture:overmuch ablation and constant plastic deformation.Fracture in plastic deformation commonly take place at the hole edge and the bottom of clinch.The fracture at the bottom includes circumferential fracture nearby interlock and central bottom fracture.The defects of LSC includes hole edge fracture and partial clinching.?6?The process windows were established-According to the pump voltage versus cavity height windows and the pump voltage versus pulse number windows,the prime parameters for LSC could be determined.?7?Shear test and axial peel test were conducted.The clinching joint had good shear resistance but poor peel resistance.In the shear test,the clinching joint tended to have shear failure of foil when interlock was Iarge,and deformed separation was prone to occur when interlock was small.In the peel test,the interlock region was more likely to rotate under axial load.