Study on the Effect of Milling Surface Plastic Deformation on Fatigue Performance of 20Cr and TC17 Specimens

In addition to the micro-topography of machined surfaces, plastic deformation is an important factor affecting the machined surface integrity and the fatigue performance of specimens. For 20Cr and TC17 materials, the effect law of milling surface plastic deformation on high- and low-cycle fatigue performance of specimens was studied. Experimental results show that the effect law of milling surface plastic deformation on high- and low-cycle fatigue performance of specimens is different. When the high-cycle fatigue life is about 2 × 105 cycles, severe surface plastic deformation can increase the maximum stress suffered by TC17 specimens from 1060 MPa to 1080 MPa; when the low-cycle fatigue life is about 6 × 104 cycles, severe surface plastic deformation can reduce the maximum stress suffered by 20Cr specimens from 680 MPa to 660 MPa. Therefore, severe surface plastic deformation can improve the high-cycle fatigue performance and, on the contrary, it has a negative effect on the low-cycle fatigue performance. Combined with the analysis of the surface integrity index and the fatigue fracture topography of specimens, it is concluded that the machined surface plastic deformation significantly reduces the plastic deformation capacity of surface layer material and enhances the stress concentration phenomenon caused by the surface micro-topography, so that when a specimen is subjected to a large load, it is very easy to make microcracks appear on the machined surface and form the “over-plastic deformation” phenomenon, resulting in a sharp decrease in the low-cycle fatigue performance of specimens. This suggests that the plastic deformation degree of machined surfaces needs to be optimized depending on the magnitude of the working load and the micro-topography.