Study On Self-piercing Riveting Of Friction-heated Magnesium Alloy Sheets

Due to the need of environmental protection and energy saving, lightweight has become the direction of motor vehicle development. Magnesium not only has lightweight, but also has many excellent properties, so it is considered as the most potential green materials for exploitation and application. It is possible to improve the security of motor vehicle, achieve the purpose of environmental protection and save energy by using magnesium alloys to produce the components of motor vehicle. Since magnesium alloys have high electrical conductivity, thermal conductivity and easy to form oxide layer while welding, the conventional welding technology is not suitable for joining magnesium alloys. Self-piercing riveting (SPR) technology, which has widely been used by automobile manufacturers, is usually used to connect metal sheets that have good ductility. However, it can not be directly used to join magnesium alloys because of their poor toughness at room temperature. Preheating can improve the toughness of magnesium alloys, so it is possible to obtain good SPR effect for preheated magnesium alloys. At present, there have been some preheating methods for magnesium alloys. But they cannot yet be applied in practical production. Therefore, the author of this paper presented a new method of friction-heating magnesium alloy sheets, and combined the method with SPR technology to join AZ31 magnesium alloy sheets. The author studied systematically the SPR effect, the shear and fatigue properties of the SPR joints of the frication-heated AZ31 magnesium sheet. The main studying contents and the results are summarized as follows.1. The feasibility of preheating AZ31 magnesium sheets using friction-heating method and the effects of friction speed, the diameter of the grinding head, the grinding pressure and the mold temperature on the variation of the magnesium sheets temperature were investigated. The results showed that the friction-heating method was feasible, and it could rapidly increase the temperature of the sheets to the required temperature. Under a certain condition, increasing friction speed could increase the rising rate of the sheet temperature. When the friction speed was 3600r/min, the rising rate of the sheet temperature was the fastest, and the damage of the magnesium sheets was also slighter. With the increasing of the diameter of grinding head, the rising rate of the sheets temperature increased, while the depth of wear track on the surface of the magnesium sheets firstly decreased and then increased. The higher the pressure of the grinding head was, the faster the rising rate of the sheets temperature, while the damage to the sheets surface also increased. Compared with the sheets fixture with room temperature, the higher the temperature of the sheets fixture was, the faster the rising rate of the sheets temperature during friction-heating, and the temperature of the friction-heated sheets decreased more slowly after stopping heating.2. The effects of friction-heating temperature on the appearances of the SPR joints of AZ31 magnesium sheets, the morphology of the cross-sections of the joints and the mechanical interlock between the rivets and riveted sheets were investigated. The results showed that when the SPR of the magnesium sheets was carried out at room temperature, the cracking of the materials at the riveting site on the sheets would occur. And a lot of cracks would present on the riveting cap and the cross-section of the SPR joints. Friction-heating could effectively reduce the embrittlement tendency of the magnesium sheets. With the increasing of the friction-heating temperature of the sheets, the crack number on the joints decreases. When the sheets temperature rose to 180℃-200℃, the cracks on the joints virtually disappeared. Moreover, the mechanical interlock between rivets and riveted sheets increased with the increasing of the sheets temperature to some extent.3. The shear strength and fatigue performance of the SPR joints of the friction-heated magnesium sheets were investigated. The results showed that the shear strength of the SPR joints of the friction-heated magnesium sheets was significantly improved when compared with the SPR joints fabricated at room temperature, and the highest strength could be obtained when the magnesium sheets were friction-heated to 180℃. But when the temperature of the magnesium sheets was continuously increased to 200℃, the shear strength of the joints would slightly decrease. The fatigue performances of the SPR joints fabricated using the magnesium sheets preheated to 150℃,180℃and 200℃respectively are comparable, but significantly higher than that of the SPR joints of the unheated sheets. When compared with the SPR joints of the magnesium sheets preheated to 180℃by an electro thermal plate, the shear strength of the SPR joints of the friction-heated magnesium sheets at the same temperature is higher. These results suggest that the damage caused by friction-heating on the surfaces of the magnesium sheets has a little affect on the shear strength and the fatigue performance of the SPR joints. It further proved that it was feasible to preheat magnesium sheets using the friction-heating method for SPR.