Tightening characteristics of nonferrous bolts and usefulness of magnesium alloy bolts

In this study, tightening tests for four types of commercial bolts made of nonferrous materials were conducted to investigate torque coefficients that show the tightening accuracy. These four types of bolts were made of AZ31 and AZX912 magnesium alloy, A5056 aluminum alloy and TB340 pure titanium respectively. Materials for the clamped parts corresponded to the bolts: AZ31 magnesium alloy, A5056 aluminum alloy and TB340 pure titanium. These clamped parts were tightened by bolts and nuts of the same material except when using AZX912 bolts, in which case AZ31 nuts and AZ31 clamped parts were used. Tightening tests using A5056 clamped parts with magnesium alloy bolts were also conducted to verify the usefulness of the magnesium alloy bolt in the tightening of aluminum parts. Tensile tests for all bolts were conducted to investigate the difference between their tensile strength and tightening strength. The results showed that if an A5056 clamped part was tightened with an A5056 bolt, the scatter band of its torque coefficient was the widest. However the scatter band of the torque coefficient in the experiment using an A5056 clamped part tightened with an AZ31 bolt or AZX912 bolt was reduced. The torque coefficient for the TB340 bolt was the twice as much as in other cases. The results also showed that the tightening strength of the magnesium alloy bolts was greater than that of the aluminum alloy bolts although the tensile strength was lower. These results indicate that there may be an increase in the use of magnesium alloy bolts.