超声冲击表面改性工艺的影响因素

Processing Factors of Ultrasonic Impact Surface Modification

超声冲击表面改性技术采用机械方式,施加频率数万赫兹的正弦振动波,通过静荷载、步进速度、车床转速、冲击头尺寸、振幅等参数组合进行超声冲击,从而在金属表面诱导产生晶粒细化,最终获得纳米晶层。机械性能随静荷载和单位面积冲击次数的增加有明显改善,但过大的静荷载对表面平整度有损伤。超声冲击下表面塑性变形后的微裂纹易成为表面裂纹源且会加速表面裂纹扩展。较大尺寸8mm直径的冲击头处理后,表面纳米晶和残余应力场共同作用改变了裂纹萌生机制,弯曲疲劳下裂纹大量呈现内部萌生,不再都表现为表面起裂。振幅、润滑、温度等也会影响表面处理效果。这些因素对材料性能的影响不一致,需要针对不同材料不同工况进行优化组合,以达到最好的处理效果。

The technology of ultrasonic impact surface modification adopts the mechanical method.It applies a sine vibration wave with a frequency of tens of thousands of Hertz on the surface of metals.The ultrasonic strike carries out through the combination of static load,stepping speed,rotating speed,the size of ball-tip,the amplitude and so on.It helps to induce the grain refinement of the surface,and the nanocrystalline surface layers can be achieved ultimately.The mechanical properties can be improved with the increase of the static load and the strike number per unit.However,the excessive static load is harmful to the surface smoothness.The microcracks can be observed due to the surface plastic deformation.They are easy to be the source of surface cracks,and they accelerate the propagation of surface cracks.After the treatment with an 8mm diameter impact ball tip,the surface nanocrystals and the field of residual stress change the mechanism of fatigue crack initiation,mass of inner cracks appear in the rotating-bending fatigue instead of surface crack initiation.The amplitude,lubrication and temperature also affect the surface treatment.The influences of these main factors on the properties are not consistent.To achieve the best treatment effect,it is necessary to optimize the combination of them for different materials and different working conditions.