Relationship between Material Strength and Friction Coefficients for Aluminum Alloy Bolt

Multi-material structures are demanded to reduce weight of vehicles. We have to reduce the weight of not only structural material but also joining elements to achieve multi-material structures. Some aluminum alloy bolts have begun to be used in the automotive fields recently. In our previous study, we investigated the tightening characteristics of Aluminum alloy A5056 bolt. The results showed that friction coefficients of thread surfaces and bearing surfaces are obviously different in comparison with those of steel bolt. The tightening strength, especially the proof clamp force, is very important to determine the target clamp force of bolted joint. However the proof clamp force of bolt is different from the proof tensile strength of bolt because the proof clamp force significantly depends on the friction coefficient of thread surfaces. Therefore we can easily know the proof clamp force for each bolt material if we can estimate the friction coefficients from the tensile strength. In this study, we have investigated a relationship between the tensile strengths of aluminum alloy bolts and these friction coefficient of contacting thread surfaces to easily estimate the proof clamp force of bolt using the tensile strengths. We have revealed the relationships of four different aluminum alloy bolts, A5056, A6056, A6061 and A2024. The results showed that there is a certain correlation between the friction coefficients of thread surfaces and tensile strengths for each bolt material if the bolted joints were lubricated by machine oil ISO VG46. However the correlation between the friction coefficient and the tensile strength was not high if the bolted joints were not lubricated.