Influence of tightening procedures and lubrication conditions on titanium screw joints for lightweight applications

In recent years, the need for lighter vehicles led to the widespread of lightweight alloys, such as titanium, also in the field of threaded fasteners. Unluckily, the replacement of steel bolts with titanium ones, usually suggested because of their favourable ratio between strength and weight, is not quite straightforward. The coefficient of friction, a key parameter in bolted joints design, changes drastically when switching from steel to titanium. Some results concerning the frictional behaviour of bolted joints involving titanium screws are here presented: friction and torque coefficients were calculated, according to ISO 16047, for joints made up of a hexagon socket head screw made of titanium alloy (Ti–6Al–4V), a bush made of aluminium alloy (EN AW 7075 T6) and a steel nut (ISO 4032). Data were collected by performing several tightening tests on ad-hoc designed specimens, which allowed the evaluation of the different tribological behaviour of the same joint under three different conditions of lubrication (dry, Teflon® added oil, and ceramic paste). 20 repeated tightenings (re-tightenings) have been analysed in order to simulate some maintenance operations. The DOE method was applied to manage the tests, while the results were analysed by the ANOVA and P-value methods. Out of the two lubricants examined, the ceramic paste showed the best results in terms of friction coefficients constancy throughout the re-tightening operations, as well as the best protection of the thread and underhead surfaces against wear.