Study On Damage Behaviors Of Torsional Fretting In Early Stage Of Ti6A14V Alloy

Torsional fretting can be defined as a relative angular motion which was induced by reciprocating torsion in an oscillatory vibratory environment. As one of basic fretting modes, it occurred in many actual working conditions, such as ball valves, enarthrosis of artificial implant, ball and socket joints in, rotational fasteners and so on. Thus, the torsional fretting wear is a very important failure mode that cannot be ignored. The damage behaviors and evolution of debris in the early stage between the contact surface of torsional fretting have been not very clear up to the present. Therefore, the study on the torsional fretting running and damage behaviors of the Ti6A14V alloy and its varied counter-pairs has an important theoretical and practical significance. The torsional fretting wear tests of Ti6A14V alloy plates against different counter-balls (Ti6Al4V alloy and Si3N4 ceramics) were carried out on a new torsional wear tester under the angular displacement amplitude (θ) from 1°to 15°, the normal load (Fn) of 50 N and the number of cycles (N) form 1 to 500. Based on the analyses of the frictional kinetics of torsional fretting, the wear scars were analyzed by profilometer, scanning electron microscope (SEM) and 3D-profiler. The main conclusions are summarized as follows:1. The running characteristics of torsional fretting of Ti6A14V alloy can be described by the friction torque-angular displacement amplitude (T～θ) curves. During the test processes, the T～θcurves for the two counter-pairs only presented two types:i.e. elliptic (the motion of the contact interfaces was coordinated by the elasto-relative plastic deformation) and parallelogram (the contact interfaces are in the state of gross slip) loops. The friction torque of the Ti6Al4V/Si3N4 counter-pair was higher than that of the Ti6Al4V/Ti6Al4V counter-pair in the all tests due to the difference of material property. It was the reason that the contact stiffness of the Ti6Al4V/Ti6Al4V counter-pair was lower, which induced the increasing of the contact area and the reducing of contact stress.2. The results indicated that there were three running regimes for the torsional fretting. Whenθ=1°, fretting ran in the partial slip regime (PSR); Whenθ=2.5°andθ=5°, fretting ran in the mixed fretting regime (MFR); Whenθ=15°, fretting entered into the slip regime (SR). The friction torque curves of the PSR and MFR went up persistently during the test processes, and not stabilized until 100 cycles. However, the friction torque curves of the SR appeared 4 stages:i.e. ascent, peak value, descend and steady stages.3. For the Ti6Al4V/Ti6Al4V counter-pair, in the PSR, there were a few detachment and slight plastic deformation in the contact zone. In the MFR, the contact zone occurred strong plastic deformation and particle detachment. With the increase of the angular displacement amplitude, the fatigue wear was enhanced, the sizes of the detached particles increased obviously, and some debris was removed to the outer of the contact zone. The analyses of the cross-section of the wear scar indicated that the white layer played a very important role during the process of particle detachment, and it also was the main reason induced the material detachment in shape of block. Some tilted cracks can be observed in the MFR. In the SR, severer damage and strong plastic flow can be found, and the wear mechanisms mainly were delamination, oxidative wear and abrasive wear.4. When the Ti6Al4V alloy plate against the Si3N4 ball, the damage of the contact zone was slight in the PSR. There was only a few smaller detached pits scattered in the contact zone. In the MFR, some detached pits appeared in shape of annularity near the center of the scars. With the increase of the angular displacement amplitude, the plastic deformation of the contact zone was enhanced, and the sizes of the detached particles increased. It was found from the cross-section that the lateral cracks were easy to form paralleled to the interface of the plastic deformation zone and the white layer, but no titled crack was observed. In the SR, obvious plastic deformation generated, the blocks of detached material were ground repeatedly to form the third-body layer to carry the loads.5. It was found that the damage of the Ti6Al4V/Ti6Al4V counter-pair was serever than that of the Ti6Al4V/Si3N4 counter-pair. It was the reason that the debris formation process of the Ti6Al4V/Si3N4 counter-pair companied with the hydration, which increased the adhesion of the debris to form the debris layer (the third-body layer). The debris layer took part in the carrying of the load, and played an action of solid lubricant. In addition, the wear mechanisms have not been changed with the variation of the counter-pairs.