Experimental Study On Preparation Of Copper Alloy Forming Layer By The Hybrid Process Of Electro-Spark Deposition And Ultrasonic Rolling

The failure of metal parts often results from surface damage such as wear,corrosion and scratch.Therefore,the repair of surface damage of metal parts is of great significance to the saving of resources and the sustainable development of economy.Copper alloy has great limitations in the repair of its parts because of its good thermal conductivity,easy oxidation at high temperature and low laser absorption.Electro-spark deposition technology(ESD)was often used to repair the surface of parts,because of the characteristics of small heat input to the base metal,metallurgical bonding between the forming layer and the substrate,and easy to operate.In this work,firstly,ESD was used to prepare the surface forming layer of copper alloy.The effects of main process parameters on the forming layer and its properties were studied,and the surface,cross-section morphology and microstructure of the forming layer were characterized and analyzed.Then,the ultrasonic rolling(Ultrasonic surface rolling,USR)was used to strengthen the forming layer,so that the wear resistance of the forming layer exceeded the substrate.The strengthening effect and mechanism of USR on the forming layer were analyzed,and the influence of process parameters on the strengthening effect of USR was explored.The main research contents and conclusions are as follows:(1)QAl9-4 aluminum bronze was selected as the research object.The effects of ESD process parameters(deposition voltage,deposition capacitance,specific deposition time and discharge frequency)on different indexes(deposition efficiency,density and surface micro-hardness)were studied,and the microstructure of the forming layer was characterized and analyzed.The influence of process parameters on different indexes is different or even opposite,so it should be optimized according to the requirements of different indexes in engineering application.The grains of the forming layer are columnar crystals growing perpendicular to the surface of the substrate.the elements are uniformly distributed,and the phases are Cu₃Al-based metal compounds.(2)USR was used to further improved the performance of the forming layer.The strengthening effect of USR on the forming layer was evaluated and its strengthening mechanism was analyzed.USR can reduce the surface roughness,refine the grains,heal the pore defects,improve the micro-hardness and surface nano-hardness,enhance the resistance to static creep,eliminate the residual tensile stress and introduce the residual compressive stress.The severe plastic deformation caused by USR is the main reason for the improvement of the surface properties of the forming layer.USR makes dislocations multiply and accumulate,resulting in the formation of new grain boundaries in the grains,which refines the grains in the forming layer.The effect of dislocation strengthening and grain boundary strengthening can improve the wear resistance of the formed layer,which shows that the average friction coefficient and wear amount was reduced,resulting in the wear resistance of the formed layer better than that of the substrate after USR treatment.QAl9-4 aluminum bronze could be repaired by the hybrid process of ESD and USR.(3)The effects of USR process parameters(static pressure,rolling times,rolling speed and feed)on the surface properties(micro-hardness,residual stress and roughness)of the formed layer were investigated by single factor test.The process parameters affect the surface property of the forming layer by affecting the plastic deformation degree of material,the degree of grains refinement and the uniformity of USR.The increase of static pressure,the increase of rolling times and the decrease of rolling speed and feed quantity will increase the degree of plastic deformation of the material,so as to improve the surface performance of the forming layer.However,too severe plastic deformation will cause the destruction of the forming layer,which will reduce the performance of the forming layer.