Evaluating the Performance of Current Self-Pierce Rivet Technology for the Joining of High Strength Steel and Aluminum Alloys

The performance of a current self-pierce rivet (SPR) alloy, 10B37 boron steel, at commercial processing conditions has been evaluated in two case studies; the joining of USIBOR 1500 high strength steel to heat-treated 6000 series aluminum and the utilization of standard rivet and die combinations to join multiple different sheet metal stack ups. The mechanical properties of the rivet and related wire (raw material) were also assessed over the commercial automotive hardness range to draw correlations between rivet performance and material properties. Analysis of the rivet failures from attempts to join USIBOR 1500 to 6000 series aluminum indicated rivets with hardness values below 550 HV exhibited buckling of the rivet legs and rivets with hardness values above 550 HV developed fractures at the tail of the rivet. The fractures were consistent with those observed when attempting to join an alternate sheet metal stack up with a standard rivet and die combination. These fractures most likely form as a result of a hoop stress that develops at the rivet tail during flaring into the bottom substrate. It is inferred that embrittlement at prior austenite grain boundaries contributes to the formation of these fractures in rivets with hardness values in excess of 550 HV, causing limited performance in tension and the development of fractures along the circumference of the rivet after insertion. In order to provide satisfactory joining of these two different joining scenarios, an alternate alloy must be employed for use as a self-pierce rivet which has higher ductility than the current alloy at the strength levels required to prevent buckling of the rivet legs.