Investigation On The Fretting Wear And Fretting Fatigue Mechanism Of Ti-6Al-4V Strengthened By Wet Peening Treatment

In aeronautics, Ti-6Al;-4V alloy offers wide industrial applications in the blade/disk attachments in gas turbine engines owing to its high specific strength and corrosion resistance. However, the lower hardness and fatigue resistance of Ti-6A1-4V alloy are the key factors in these applications. Especially in the blade/disk dovetail joint part, the fretting-fatigue-related failure is one of the most insidious, difficult, and costly problems. Under vibratory loading condition, the localized fretting wear at the blade/disk dovetail joint results in premature initiation and subsequent growth of fatigue cracks, leading to shorter fatigue life of a component or failure at a stress much lower than the fatigue strength of the Ti-6Al-4V alloy. Such damage has been assumed to have caused many unanticipated disk and blade failures in turbine engines and thus has gained considerable attention over the years.In the present study, wet peening (WP) treatment which is a more economic and environmental impact technique was employed to form a modified surface layer of Ti-6A1-4V alloy. The samples treated with and without WP were subjected to fretting wear, fatigue and fretting fatigue tests and the results were compared. Based on the WP strengthening mechanism and the tests results, the influence of WP treatment to the fretting-fatigue-related failures were systematicly investigated. The main points studied were:(1) Ti-6A1-4V alloy was peened by WP method to 0.25 mmN Almen target intensity. After treament, a modified surface layer was formed on the surface layer of the alloy. The variations of the residual stress, nano-hardness and micro structure of the modified layer in function of depth were studied using X-ray diffraction analysis, nano-indentation analysis, scanning electron microscopy and transmission electron microscopy observations. Both the compressive residual stress and hardness decrease with depth, and the termination depth are 160 and 80?m, respectively. The strengthened layer is quantitatively analyzed by an equation which is revised and used in Ti-6A1-4V alloy. According to the microstructure observation, the WP grain refinement mechanism was discussed. Based on the grain refinement mechanism, A model to explain the formation of compress residual stress was proposed. That is, the compressive residual stress is an unrecoverable oppression among grains while the unequal plastic deformation occurs.(2) The fatigue life of Ti-6A1-4V alloy which suffered wet peening treatment was obviously increased. Compared with the fracture of un-peened sample, the fatigue crack initial site was transferred from surface to the interior region beneath the surface (about 1 mm depth). The co-action of grain refinement strengthening and dislocation strengthening dominated the transferred fatigue crack initial site. The compressive residual stress effectively delayed the crack propagation.(3) Wet peening was taken on Ti-6A1-4V titanium alloy as a surface strengthening treatment, and then study on the effect of fretting wear behavior. The results showed that wet peening has little influence on fretting wear behavior of Ti-6A1-4V titanium alloy. However, wet peening can inhibit crack propagation which arised in accumulation of plastic deformation zone effectively when partial slip occured. By calculating can find that the improving of local yield strength is the key reason for avoiding local fatigue damage.(4) A fretting fixture which is fixed on the fatigue test machine was designed and developed to investigate the effects of WP treatment on the fretting fatigue behavior and crack propagation in Ti-6Al-4V alloy. Results of the fretting fatigue tests show that the fretting damage could efficiently reduce the fatigue life of both the un-peened and wet peened samples. Moreover, wet peening could effectively increase the fretting fatigue resistance. Investigation on crack propagation mechanism revealed that the increased resistance is due to the synergistic effect of the enhanced hardness and the induced compressive residual stress in the wet peening modified layer. The enhanced hardness could resist the local fatigue in the fretting cracking stage and the compressive residual stress could retard crack growth.