Prediction Of Fracture Behavior Of Lead Sheath In Forming Process Based On Microscopic Damage Mechanics

The role of lead sleeve in bullet warhead is to prevent the high-speed rotation of steel core in bullet during shooting,and directly rub with bullet shell,resulting in the destruction and failure of bullet during shooting.At the same time,lead sleeve can effectively improve the lethality of bullet.But pure lead has soft texture and low strength,so it is easy to crack in the forming process of lead sleeve.In order to improve the forming quality of lead sleeve,the failure of lead sleeve in forming process was analyzed and predicted by means of GTN micro-damage model,and the forming process and die scheme were optimized.Friction is an important topic in material forming.The friction between the forming die and the formed workpiece will increase the resistance of material flow and cause damage to the surface of the die and workpiece.At the same time,friction and wear will reduce the accuracy of the die and thus reduce the life of the die.The application of effective lubricant on plastic deformation interface has always been an effective means to improve the friction and wear of the interface.However,for the forming process of lead sleeve,if lubricant is used,a closed pit will be formed in the lead sleeve,resulting in uneven wall thickness difference of the lead sleeve.At the same time,drying process should be added after forming.Therefore,this paper proposes to use self-lubricating coating to modify the surface of the die,so as to reduce the use of lubricant,improve the wear resistance of the die,and then improve the life of the die.The contents and conclusions of this paper are as follows:(1)The parameters of GTN damage model of lead were obtained by scanning electron microscopy(SEM)metallographic analysis and response surface finite element inverse method.The parameters of lead’s GTN damage model are coupled to unidirectional tension simulation.And the results of load-displacement simulation and the variation of void volume fraction with strain of key elements are obtained.The results are compared with the load-displacement curve measured by experiments and the void volume fraction obtained by SEM statistics under several strains.The main conclusions are as follows: The simulation results are well matched with the experimental results.It is proved that the parameters of lead’s GTN damage model obtained by SEM metallographic analysis and response surface finite element inverse method can characterize the damage behavior of lead during deformation.(2)Coupled the lead GTN damage model to the simulation process of lead sleeve forming,predicted the location of lead sleeve damage and forming defects.And the influence of process parameters(friction coefficient and fillet radius of deep drawing die)on thinning and deep drawing is determined by single factor test,in order to obtain the best forming process.On the basis of the optimum process,the drawing die is optimized to improve the forming quality of lead sleeve.The main conclusions are as follows: in the process of deep drawing,the lubrication condition of the bottom die is good.But the punch is not suitable for lubrication.The radius of the fillet of the bottom die should not be too large or too small,and the proper radius of the fillet of the bottom die can improve the forming quality.In the process of optimizing the forming bottom die,two passes of deep drawing are carried out on one die,the first drawing coefficient is smaller,and the second drawing coefficient is larger.Through the optimized scheme,there are no cracks and other phenomena after deep drawing,and the forming quality of lead sleeve has been improved successfully.(3)In this paper,magnetron sputtering technology is used to sputter self-lubricating coatings(DLC and Cr N coatings)on T10 A die steel with different surface roughness.The comprehensive mechanical properties of the two coatings and their bonding properties with die steel are measured.The friction and wear properties of the two coatings are compared,which provides a new idea for surface modification of die steel.The main conclusions are as follows: From the test results,both coatings have the characteristics of improving the lubricity of dies.Among them,DLC coatings have the lowest friction coefficient and the best self-lubrication.However,compared with Cr N coatings,DLC coatings have higher internal stress,lower elastic modulus,toughness and bonding performance with the die steel.The bonding performance of coatings on rough interface is higher than that on smooth interface.The main reason for the bonding performance is that the real surface area of the rough interface is large,and the contact area between the coating and the die steel is large.Moreover,in the rough interface,the coating is liable to couple and rivet with the substrate.