| Torsional fretting can be defined as a small amplitude relative motion which was induced by a reciprocating torsion in an oscillatory vibratory environment. As one of the four basic running modes of fretting, it occurred in a great deal actual operating conditions. So, the torsional fretting wear was a very important failure mode that cannot be ignored. The damage behavior and evolution of debris between the contact interfaces of torsional fretting have been not very clear up to the present. The in-depth studies on running behavior and wear mechanisms of torsional fretting deeply are not only helpful to reveal the principles of torsional fretting wear, but also are significant to engineering application and academic research.In this paper, the torsional fretting tests of TA2 and TC4 alloy flats against ZrO2 ceramic ball were carried out on a new torsional fretting tester in dry condition. After the tests, the studies on the running behavior and wear mechanisms of torsional fretting wear conducted systemically by using optical microscope, scanning electron microscopy(SEM), electron energy dispersive X-ray spectroscopy(EDX), profile-meter and so on. The obtained results were listed as follows:(1) Under the test parameters, the friction torque vs angular displacement amplitude (T-θ) curves of TA2 and TC4 alloys presented three types:i.e.near beeline, elliptic and parallelogram loops.(2) There are only three running regimes for torsional fretting of TA2 and TC4 alloys: i.e. partial slip regime, mixed fretting regime and slip regime. With the increase of angular displacement amplitude, the fretting regime shifted from the partial slip regime to the mixed fretting regime and slip regime accordingly. As the normal load increased, the width of the partial slip regime increased, and the mixed fretting regime and slip regime shifted to the higher angular displacement amplitude. Under a same condition, the TA2 and TC4 maybe run in different fretting regimes. It indicated that the change of the torsional fretting running regimes was induced by the difference of the mechanical properties of the materials.(3) For TA2 and TC4 alloys, the friction torques increased gradually with the increase of the angular displacement amplitudes and normal loads. and the values of friction torques of TA2 were higher than that of TC4. In the partial slip regime, the curves of friction torque were composed of running-in, absent and atable stages. However, in the mixed fretting and slip regimes, when the angular displacement amplitude reached a higher level, the removing of debris was relatively easy to induce the stable stage of friction torques postponing a longer time.(4) In the partial slip regime, the micro-slip and slight wear occurred in the narrow ring area of contact edge, the damage mechanisms were main abrasive wear and slight oxidative wear. In the mixed fretting regime, the damage was enhanced, the damage mechanisms were abrasive wear, oxidative wear and delamination. In the slip regime, the wear occurred at the whole contact zone with severer damage, and the wear scars appeared morphologies of ploughing and detachment. As similar to the mixed fretting regime, the wear mechanisms were main abrasive wear, oxidative wear and delamination.(5) For TA2 alloy, it presented a stronger plastic flow due to its lower hardness and better plasticity; and for TC4 alloy, the phenomenon of delamination was severer owing to its high hardness and anti-wear property. The wear scar profiles indicated that the debris accumulated at the center area of the scars both in the mixed fretting and slip regimes for TA2 and TC4 alloys. Under the same test conditions, the accumulation of TA2 alloy was severer due to the difficulty of debris removing, which was related to its better plasticity. During the friction processes of TA2 and TC4 alloys against ZrO2 ball, the transfer of Zr element from the ZrO2 ball to the surface of titanium alloys can be observed under higher angular displacement amplitudes. |
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