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.