3.1 Surface Roughness, Microhardness and Residual Stress
The surface roughness distribution of different specimens is shown in Figure 12. It can be seen that the surface roughness of Ti-6Al-4V is significantly improved after USRP. The surface roughness (Ra) of AsR specimen is 0.217 μm. After USRP, it reduces to 0.143 μm. However, compared with the AsR specimen, the surface roughness increases after SP, which is 1.279 μm.
It also can be seen from Figure 12 that the surface asperities of the specimen under different surface treatments are not the same. For the AsR specimen, the highest point of the asperity reaches 0.9 μm. The distance between asperities on the left side of 1.5 mm is relatively close while on the right side is larger and the distance can be up to 0.3 mm in width. The whole two-dimensional contour map is small and sharp. The gap between two adjacent asperities is likely to be a potential microcrack. The larger the gap, the more likely it is to become a crack. However, the distance between the two asperities of the specimen after SP is relatively large and the maximum point can reach 0.4 mm. Throughout the overall trend in Figure 12b, almost every two asperities have different sizes of spacing and even have obvious characteristics similar to pits. The whole two-dimensional contour map is wide and high. The more the gap between the asperities, the wider the gap spacing, which means that the material is more likely to initiate microcracks on the surface and more likely to be damaged under the same working conditions. The height of the asperities of the USRP specimen is mostly about 0.4 μm and the height tends to be horizontal. The asperities are relatively tightly connected and do not have obvious gap. The whole two-dimensional contour map is dense and sharp. By comparison, the number of asperities of specimen after SP has increased significantly, while it greatly reduced after USRP.
The microhardness distribution of different specimens is shown in Figure 13. The microhardness of USRP and SP specimen are obviously greater than AsR specimen at the same depth. USRP specimen reaches the maximum value of 374 HV at the depth of 150 μm. SP specimen reaches the maximum value of 368 HV at the depth of 80 μm. It can also be seen from Figure 13 that the microhardness of USRP specimen is greater than that of SP specimen. The microhardness of SP specimen is basically the same as the as received specimen at the depth of 450 μm, while the USRP specimen at the depth of 550 μm. Therefore, USRP has a deeper influence on the hardness of specimen than SP. The main reason for the increase in microhardness is grain refinement [29]. At the same time, because the microhardness distribution results of the specimens can objectively reflect the distribution trend of the plastic strain of the material [30], the plastic strain layer thickness of the specimen after USRP is also deeper.
The residual stress distribution of different specimens is shown in Figure 14. The residual compressive stresses of specimens after USRP and SP are first increase and then decrease in the depth direction and all reach the maximum value on the subsurface. Wohlfahrt [31] used Hertzian contact theory to explain this phenomenon. He believed that it was mainly caused by the competition mechanism between Hertzian dynamic pressure and surface plastic deformation. When the subsurface strain is dominant in the competition with surface plastic deformation, the maximum residual stress is located in the subsurface layer. The residual stress of the specimen that strengthened by USRP reaches the maximum of − 550 MPa at the depth of 0.18 mm, while the SP-strengthened specimen reaches the maximum of − 380 MPa at the depth of 0.1 mm. In addition, the influence range of residual compressive stress field and residual compressive stress value at the same depth after USRP are greater than those after SP strengthening. The surface residual compressive stress of the AsR specimen is between 35 MPa and 72 MPa and the affected layer is very shallow, about 20 μm. Some scholars believed that a deep residual compressive stress layer is beneficial to prolong the propagation life of cracks, because it can effectively delay the propagation rate of cracks. In addition, the high residual compressive stress can well offset the external load on the surface, which has a positive effect on the resistance to crack initiation [32]. Therefore, USRP has better effect than SP on the improvement of fretting fatigue life of Ti-6Al-4V.
3.2 Wear Profile
The morphology of fretting areas on the surface of specimens after fretting fatigue experiment are shown in Figure 15. The locations of fracture failure are all located in the fretting area. Observing the fretting area, the specimen has a certain degree of elastic and plastic deformation. It can be seen from Figure 15a, b that there are large areas of slip marks in AsR specimens, mainly adhesion and a small amount of delamination. The SP specimen (Figure 15c, d) has obvious flaking of massive material at the fracture position, which may be caused by the initial collection of microcracks under the combined action of axial fatigue load and contact stress. In the fretting area, there are slight wear marks, delamination and a few micro-cracks and the surface also appears embrittlement. This is probably because a large amount of shots bombards the surface of the specimen during the SP process, the surface grain of the specimen is refined but not uniform. In contrast, only slight delamination and smaller wear pits exist on the USRP specimen (Figure 15e, f). This is because the surface and sub-surface grains are refined and relatively uniform, the wear resistance is improved. In addition, no cracks can be found in the wear area and only a few wear pits exist. It is possible that the specimen is in a global slip state during the fretting process. Similar to the results of Mohd [33], cracks initiate at the boundary between the adhesion zone and the sliding zone in the local sliding zone, while only accumulated debris is found in the overall sliding zone. It means fretting wear is dominant in the competition mechanism with fatigue during the fretting process. The wear rate is greater than the crack initiation rate so that the initiating microcracks are worn away.
3.3 Fracture Analysis
Through the fretting fatigue experiment, the crack initiation and fracture failure of all specimens are located on the side of the lower edge of the fretting contact area. The wear marks on the surface of the fretting contact area is a typical fretting fatigue failure. The macroscopic view of the fracture surface after fretting fatigue failure are shown in Figure 16. The fatigue crack originates close to the contact surface of the specimen. Because the normal load is applied through the micro-motion pad and the contact form between the micro-motion pad and the specimen is “cylindrical-planar” contact. According to Hertz contact theory, there is serious stress concentration in the contact area. Thus, the main crack source must be formed in the contact area. Regardless of whether there are defects in the crack source area, fatigue cracks are initiated from the maximum stress concentration area because the initiation of cracks must undergo repeated slippage processes to form [34]. Moreover, the form of crack propagation is radial expansion with the main crack source as the center, similar to "ripple" diffusion. Eventually, the fracture failure occurred in the transient area. In addition, there are wear pits in the crack source area of the AsR (Figure 16a, b) specimen. This is due to the low hardness and poor wear resistance, the titanium alloy is easy to be affected by fretting wear during the fretting process [35]. While after SP process, the surface hardness of the specimen is improved, and the subsurface also has a certain degree of nano gradient structure, which can effectively resist fretting wear. However, due to the surface embrittlement and the increase of roughness, the wear resistance will be weakened to a certain extent. As a result, the traces of wear damage can be found in the crack source area. However, the surface hardness of the specimen after USRP is improved, and the surface roughness is greatly reduced. Therefore, the specimen after USRP can better resist fretting damage.
3.4 Fretting Fatigue Life
Residual stress and surface roughness are considered the most critical factors affecting fretting fatigue life. The residual stress introduced by surface and sub-surface layers of the specimen after surface strengthening is compressive stress, which can effectively reduce the average stress caused by the applied load and reduce the stress ratio, thereby preventing the initiation of fatigue cracks and reducing the crack growth rate. Therefore, residual stress is often considered to be the average stress. The effect of surface roughness on fatigue is usually attributed to the stress concentration effect. Lower roughness helps the specimen avoid surface stress concentration during the fretting fatigue experiment and prevent crack initiation that caused by surface defects, especially for low-cycle fatigue experiment under high stress levels. This is mainly because the residual stress is prone to relaxation under high stress levels [32]. For the fretting fatigue damage with the coupling effect of fretting wear and fatigue damage, the importance of surface hardness cannot be ignored, especially in the overall sliding state. According to the Archard wear equation, the wear volume is proportional to the surface hardness of the material. Therefore, the higher the surface hardness is, the stronger the wear resistance is and the longer the fretting fatigue life is.
The fretting fatigue life curves are shown in Figure 17. The fretting fatigue life under three different test conditions are all decrease with the increase of stress level. For the dispersion of fretting fatigue lifetime, this is mainly because the dispersion of fatigue data itself is relatively large, coupled with the effect of wear. Moreover, the lower the stress level, the more obvious the strengthening effect. Among two strengthening methods, USRP has a more significant effect on the improvement of fretting fatigue life, especially under the stress level of 600 MPa. Some studies have shown that in the early fretting fatigue crack propagation stage, compressive residual stress can significantly decrease tensile stress at the crack tip [36]. Similar to clamping stress, compressive residual stress can also close up fretting fatigue cracks in the early stages of fretting fatigue crack propagation [18]. At the same time, the surface roughness and surface microhardness have also been greatly improved, which can well improve the wear resistance and stress concentration of the specimen. The coupling effect of the above three parameters greatly increases the fretting fatigue life. In the stress range of 600–650 MPa, SP does not significantly improve the fatigue life of Ti-6Al-4V. The research on the effect of SP on the fatigue strength of cast iron structure shows that higher residual compressive stress and nanocrystalline structure help to improve the fatigue strength, but the high roughness caused by SP will greatly weaken the above effect [37]. Generally speaking, parts with larger friction coefficients are more susceptible to fretting damage, especially titanium alloys that are more sensitive to fretting wear. In addition, some researchers [38,39,40] proved that SP will change the surface morphology and increase the roughness of the material while introducing residual stress. The increase of roughness will cause some unfavorable results like surface stress concentration, microcracks, etc. In the study of the effect of different SP treatments on the fretting fatigue life of 7075-t7351 aluminum alloy, it is found that laser SP can increase the fatigue life of 7075-t7351 aluminum alloy at the early and late stages of crack propagation by 7 and 3 times respectively. However, SP can only increase by 2 to 3 times. The main reason is that the SP process can easily cause embrittlement and roughening of the material surface. In addition, in the process of SP, the surface of the material will form a region which is easy to promote the rapid propagation of cracks. These will greatly weaken the beneficial effect of residual compressive stress. The surface brittleness and roughness caused by SP can also aggravate the fretting wear of the material, which makes the surface of the material easy to fall off, delamination and even appear pits or microcracks. Moreover, under the action of high strain amplitude, the residual compressive stress will relax, which weakens the ability of residual compressive stress to resist crack initiation.