Study On Laser Shock Flat Hole Clinching With Double Junction Of Copper Foil And Perforated Stainless Steel Sheet

The development of modern manufacturing has put forward higher requirements for the miniaturization and refinement of industrial devices,especially in terms of material connection technology.In addition to meeting the basic requirements of stable connection,it is also hoped that it has the characteristics of high efficiency,high precision and economy.The laser shock flat hole clinching with double junction is a combination of double junction clinching,flat hole clinching and laser shock clinching.It has the advantages of the three and can be quickly completed in the micro-scale with high connection strength,high surface smoothness,high forming accuracy,high material applicability,and high energy utilization efficiency.We studied the laser shock flat hole clinching with double junction of copper foil and stainless-steel sheet with cone-shaped through holes.The bulging test confirmed the feasibility of forming double junction under the designed system;the comparison of the results of different forming methods clarified the double laser shock forming method of low-energy pre-shock+high-energy shock,and analyzed the thinning effects during the forming process.Under different material thickness and forming height the process window with shock energy,was established,and the optimum shock energy was also determined;Based on the three joint shapes resulted from the different ratio of forming height/material thickness,and analyzed the material flow law according to its cross-sectional microstructure;observed the clinching insufficiency and damage during the forming process,and proposed corresponding solutions;performd tensile shear tests on the well-formed clinched parts,analyzed the causes of connection broken and the influencing factors of clinching strength from the load-displacement curve and the broken mode.(1)Construction of test system.Designed the test device,selected high-strength quartz glass as the confinement layer,black paint as the ablative layer,used SGR series Nd:YAG pulsed laser for processing operations,and the center of the focused beam is aligned with the center of the double junction.The designed junction diameter is 0.8mm,the distance between junctions is 1.8mm,the thickness of the copper foil is 20-100μm,and the thickness of the stainless steel sheet is 50-200μm.(2)Feasibility study of double point forming under single spot shock..The results of the double bulging test show that under the single spot shock,the material around the two tapered holes plastically flows into the hole,and the forming area is symmetrical,but the material is still thinner,so it is feasible to simultaneously form double junction under a single spot.Studied the combination of multiple process parameters of laser energy and number of shocks,the results showed that the two-shot forming method of low-energy pre-shock+high-energy shock is the optimal forming method with the highest energy efficiency.(3)Formulate the clinching process window.Taking different material thickness-forming height combinations as the research object,taking pre-shock energy and secondary shock energy as test variables,the clinching process window corresponding to various process combinations are drawn according to the experimental results.Based on the principle of highest energy efficiency,the optimal clinching energy for different material thickness-forming height combinations is determined.(4)Analysis of clinching plastic forming and microstructure.From morphology of the central section of the clinched part,it is found that the clinched structure is an interlocking structure formed by the plastic deformation of the copper foil in the tapered hole and embedded in the bottom corner of the hole.According to the microstructure of the clinching area,material flow characteristics,and the different ratio of forming height/material thickness,,the clinching structure is divided into Extrusion type,Entermediate type and Bulging type.Under laser shock,the crystal grains will elongate,turn,break and refine.(5)Analysis of clinching defects.The main cause of defects in the clinching process is excessive or insufficient laser shock energy.When the shock energy is too low,the material connection will not be achieved or the connector load is weak.At this time,the pre-shock or secondly shock energy should be appropriately increased according to the relative thickness of the hole entrance and the bottom material of the buckle;too large energy will leads to the destruction of the hole entrance or the material at the bottom of the buckle,which reduces the load-bearing capacity of the connector.Insufficient material flow outside the hole leads to excessive thinning of the material at the entrance,so the pre-shock or secondly shock energy should be appropriately reduced.After the copper foil touches the bottom,the frictional force hinders the radial flow of the material and easily leads to the fracture of the bottom of the buckle.The difference between the two shock energy should be reduced to reduce the amount of secondary deformation..(6)Analysis of the mechanical properties of clinching.Tensile-shear test is carried out on clinched parts.The load-displacement curve showed that the clinching broke may occur in the elastic deformation stage,or occur in the plastic deformation stage,and manifested as shear broke and detach broke.Factors such as the number of junctions,material thickness,and forming height affect the clinching strength by affecting the bearing area of the buckle,the side thickness and undercut of the buckle.