The Tightening Characteristics of Magnesium Alloy Bolts

In recent years, due to the rapid progress of lightweight technology, the demand for lightweight mechanical and electronic parts has been increasing. To indicate an appropriate fastening guideline of magnesium alloy or aluminum alloy components is very important for safe use of lightweight components. In this study, tensile tests and tightening tests for magnesium alloy bolts and aluminum alloy bolts have been conducted to investigate the effectiveness of magnesium alloy bolts. The magnesium alloy bolts used in the tests were made of AZ31 and AZX912 (9Al-Zn-2Ca) magnesium alloy, and the aluminum alloy bolts were made of A5056 aluminum alloy. Test results showed that the tensile strength of the aluminum alloy bolts was higher than that of the magnesium alloy bolts. However the results of the tightening tests indicated that the tightening strength of the magnesium alloy bolts was greater than that of the aluminum alloy bolts. The reason the results were different in each test comes down to the magnitude of twisting torque. When a bolt is tightened to a specific clamping force, the bolt receives not only tensile force but also twisting torque. In general, the friction coefficient of aluminum alloy is higher than that of magnesium alloy. Hence an equivalent stress on the aluminum alloy bolts was larger than the same stress on the magnesium alloy bolts because the tightening torque generated when the aluminum alloy bolt was tightened, became larger than in the case of the magnesium alloy bolt. Accordingly the ultimate clamping force of an aluminum alloy bolt turned out to be less than that of a magnesium alloy bolt.