Study On Ti6Al4V Low Temperature Thermal Field Composite Ultrasonic Impact Surface Strengthening And Cavitation Erosion Resistance

Ti6Al4V is widely used in ultrasonic chemistry as radiation head because of its high specific strength.Long-term service in fluid environment makes its surface suffer serious cavitation,which leads to premature failure of Ti6Al4 V radiation head due to surface damage.Single surface hardening technology can only improve the surface properties of Ti6Al4 V,which is difficult to meet the rigorous working requirements.Therefore,thermal ultrasonic impact technology is used to strengthen the surface properties of Ti6Al4 V in the paper,and the plasticity of Ti6Al4 V is strengthened by the softening effect of temperature field to promote the hardening effect.The surface microstructure,microhardness and residual stress after hardening were characterized,and the cavitation resistance in deionized water and under neutral pressure was studied by ultrasonic vibration.The main research work of this paper is as follows:(1)The thermal ultrasonic impact technology strengthening plan is maked,combining with the service temperature,the stress-relieving orientation temperature of Ti6Al4 V and the temperature rise of ultrasonic shock,the temperature thermal ultrasonic impact technology plan is set,and the thermal ultrasonic impact technology experimental platform and the ultrasonic vibration cavitation erosion platform are constructed.Ultrasonic vibration cavitation test adopts direct cavitation erosion method,and combines the actual working conditions of ultrasonic chemistry to select reasonable cavitation erosion experimental parameters.The sample is designed for direct cavitation erosion and a fixture used in the composite strengthening process.(2)The effects of different process parameters of thermal ultrasonic impact on surface microstructure,micro-hardness,residual stress and surface roughness of Ti6Al4 V were studied,and the influence mechanism of temperature and pre-pressure on surface plastic deformation layer,microhardness,residual stress and surface roughness of Ti6Al4 V was analyzed.The results show that the surface plastic deformation layer gradually increases with the increase of temperature and pre-pressure;XRD diffraction analysis shows that proper temperature is beneficial to promote grain refinement of the surface layer;the surface micro-hardness increases first and then decreases with the increase of temperature,reaching a maximum of 898HV;compared with thermal hardening,the residual stress induced by ultrasonic impact at room temperature is larger,which is-855MPa;The surface roughness increases with increasing pre-pressure and temperature.(3)The cavitation erosion resistance of the material strengthened by thermal ultrasonic impact is studied.The cavitation resistance of the surface of the composite strengthened sample is increased by 28%?53%.The optimum process parameter for composite hardening is 65N-150?.Cavitation erosion mechanism of material surface before and after reinforcement was analyzed.Cavitation erosion mechanism of material changed from substrate's ductile fracture failure to reinforced fatigue failure and brittle fracture.Fine surface grains and high residual compressive stress effectively hindered crack propagation.Change of surface cavitation erosion performance of composite reinforcement material under pressure was studied.Cavitation erosion strength under pressure increased significantly,after the plastic deformation layer is completely corroded in 50 min,the cavitation erosion mechanism of the material surface is the same as that of the substrate.