Research On Deformation Behavior Of Ultrafine-grained Pure Copper In Laser Shock Texturing

Friction and wear between parts of mechanical system are unavoidable in a mechanical system.It is believed that nearly 1/3 of the global energy have been consumed in various types of friction and wear.In addition,more than 80%of the mechanical failure derives from the friction consumption.Therefore,it is urgent to find an appropriate method to achieve the reduction of friction and wear in order to save energy and improve the working efficiency of mechanical equipment.The fabrication of the microarray structure on the surface of the friction pair can effectively improve the tribological properties of the surface,and it becomes one of the important ways to improve the wear resistance of the surface.Laser shock texturing technology is a new micro-structure processing technology developed in recent years.Because of the advantages of laser beam,such as good controllability,high energy intensity and flexibility,the process has unique technical advantages.Moreover,ultrafine-grained material is a kind of engineering materials with great prospective of application owing to its special microstructure and excellent comprehensive mechanical properties.However,due to the high strength and deformation resistance of ultrafine-grained material,it is difficult to fabricate the array microstructure on the surface of ultrafine-grained material.Considering the above problems,micro-dimples were fabricated on the surface of ultrafine-grained pure copper by using laser shock wave.The deformation behavior of ultrafine-grained pure copper under laser shock was investigated through both finite element analysis and experimental approaches.The main research work and conclusions of this paper are as follows(1)An equal-channel angular pressing(ECAP)die was designed and fabricated,and multi-pass ECAP process was carried out to prepare ultrafine-grained pure copper material.The structure and properties of multi-pass extrusion samples were characterized,and then the microstructure evolution and microhardness changes of pure copper were analyzed under different extrusion passes.According to the examinations,the pure copper with an average grain size less than 1μm and uniform distribution was successfully prepared.(2)Quasi-static compression and split Hopkinson pressure bar tests were carried out for both coarse-grained and ultrafine-grained pure copper to obtain the material parameters in Johnson-Cook constitutive equation,which provided the material mechanical property data for the subsequent finite element analysis of laser shock texturing.According to the measurements,it is found that the mechanical properties of ultrafine-grained pure copper are significantly higher than those of coarse-grained pure copper.However,the strain hardening effect of ultrafine-grained pure copper is weaker than that of coarse-grained pure copper.The flow stress of coarse-grained and ultrafine-grained pure copper increases with the increase of strain rate,whereas the coarse-grained pure copper is more sensitive to the change of strain rate.(3)The finite element model of laser shock texturing was established,and the key issues of model establishment were concerned.The plastic deformation process,the evolution of stress and strain field,the propagation behavior of stress wave and the adiabatic temperature rise of ultrafine-grained pure copper under laser shock wave were studied by using the validated finite element model.The results indicate that the micro-dimple depth obtained by laser shock of ultrafine-grained pure copper increases firstly,and then the rebound deformation occurs after reaching the peak value.The micro-dimple depth decreases and finally reaches a stable value.A "crater" is formed at the edge of the micro-dimple,and the height of the crater increases until it reaches saturation.The temperature rise caused by the compression of the material during the propagation of the shock wave and the plastic deformation of the material at a high strain rate are both small,and thus it do not affect the deformation behavior of the material in laser shock texturing.(4)The effect of number of laser pulses,laser energy and laser spot diameter on the forming quality of micro-dimples was investigated through a series of experiments for both coarse-grained and ultrafine-grained pure copper.The surface morphology and microhardness characteristics of micro-dimples of two kinds of materials were analyzed.The results show that the average depth of dimple bottom surface increases with the increase of number of laser pulses and laser energy.As the laser spot diameter becomes smaller,the micro-dimple depth of the two materials increases firstly.When the diameter decreases to a certain value,the average depth of micro-dimple of coarse-grained pure copper remains unchanged due to the plasma shielding effect,while the average depth of micro-dimple of ultrafine-grained copper decreases slightly.The microhardness of coarse-grained pure copper decreases firstly,and then tends to be stable with the increase of dimple depth,while the microhardness distribution of ultrafine-grained pure copper is generally unchanged.