Investigation Of Self-piercing Riveting Of AZ31 Mg Alloy

This paper is a part of GM Joint Funding Project "Joining of Dissimilar Materials with Crash-Toughened Adhesives" carried out by Zhengzhou University and General Motors Corporation, which aims to explore the possibility of applying magnesium alloy in the automobile industry. One of the key tasks is to investigate the self-piercing riveting and rivet-bonding process between AZ31 magnesium alloys and between AZ31 magnesium alloy and DP600 dual phase steel. In order to get ideal riveted and rivet-bonded joints, method of preheating magnesium alloy before riveting was adopted to improve the ductility of AZ31 alloys. Effect of overlap distance and preheating temperature on the shear strength and failure mode of riveted and rivet-bonded joints was investigated; Effect of size, geometry of rivets and dies on the riveted and rivet-bonded joints was investigated. Rivet-bonding process was investigated and the joining results of bonding, riveting and rivet-bonding were compared. Effect of hot and humid environment on the riveted and rivet-bonded joints was investigated. The results show that:Pre-heating magnesium alloys between 180℃and 200℃before riveting can improve its ductility effectively and get desired riveted and rivet-bonded joints. Pre-heating can not only improve the ductility of magnesium alloys, is also beneficial to increase the interlock distance between sheets and rivets, thereby enhancing the shear strength of the joint. The shear strength of riveted joint is decided by interlock distance and cracking emerged on the joint. The optimum preheating temperature and riveting speed is 180℃and 110mm/sec.Too short overlap distance will decrease shear strength of the joint. The shear strength of rivet-bonded joints decreases more seriously than riveted joints when decreasing the overlap distance. The proper overlap distance of joint is 25mm.In order to get desired joint, it is necessary to adopt the rivets with 2-4mm longer length than the total thickness of the sheets being riveted and lower hardness (no more than 450Hv), and dies with a higher profile height and profile diameter of 9mm or 10mm. For the joining between magnesium alloy and steel sheet, if the 0.75mm thick steel sheet was served as lower sheet, the interlock distance of the joint is very small and the lower sheet is easily pierced. So, the steel sheets should be served as the upper sheet.The shear strength of riveted Mg joints is close to that of the riveted Al joints and the shear strength of rivet-bonded joints is close to that of the spot welded Steel joints. The shear strength of riveted joints is mainly determined by mechanical interlock force. It can be simulated using a simplified lever model and expressing as:The presence of the adhesive affected the head height, interlock distance of joints and the remaining thickness of lower sheet because of its lubrication action. So, it is necessary to properly adjust the riveting velocity for the rivet-bonded joints to achieve the desired riveting result comparing to the riveted joints. Rivet-bonding process not only gives joints ideal strength, but also overcome the unpredictability shortcomings of the joints.Environmental exposure increase the shear strength of the riveted joints slightly and decrease the rivet-bonded joints. The rivet-bonded AZ31 joints with phosphate pretreatment have higher shear strength than the rivet-bonded joints with abrasion pretreatment.