ANALYSIS OF THE TRIBOLOGICAL BEHAVIOUR OF TITANIUM FASTENERS UNDER DIFFERENT TIGHTENING STRATEGIES

The present work aims at investigating the tribological behaviour of Ti6Al4V, M6 screws, applied to cast aluminium underhead and threads, when subjected to different tightening strategies. All tightenings have been performed in the dry condition, after degreasing of the cast parts by means of a commercial spray degreaser. Namely, a standard steady tightening operation at 20 rpm has been compared to a mixed high speed – standard speed strategy (200 rpm – 20 rpm), observing the tribological output in terms of underhead and thread friction coefficients at first tightening. All measurements have been performed by means of a tightening test bench that allows sampling the separate frictional contributions at the underhead and at the thread. For each specimen, force – torque diagrams have been plotted and observation of the underhead surfaces have been performed (before and after first tightening). The underhead and thread friction coefficients have been sampled at both 25% and 100% of the target tightening torque. The main outcomes of the study are: (i) the frictional behaviour of the Ti6Al4V screw, mating to aluminium substrates is nearly the same for both tightening strategies, in terms of average total coefficient of friction; (ii) in the case of mixed high – low speed strategy, a slightly better behaviour in terms of stick slip occurrence has been observed at first tightening, (iii) both processes allow achieving friction coefficients in the range suggested by the relevant standards.