Characterization of Flow Drill Screwdriving Process Parameters on Joint Quality

A state of the art proprietary method for aluminum-to-aluminum joining in the automotive industry is Resistance Spot Welding. However, with spot welding (1) structural performance of the joint may be degraded through heat-affected zones created by the high temperature thermal joining process, (2) achieving the double-sided access necessary for the spot welding electrodes may limit design flexibility, and (3) variability with welds leads to production inconsistencies. Self-piercing rivets have been used before; however they require different rivet/die combinations depending on the material being joined, which adds to process complexity. In recent years the introductions of screw products that combine the technologies of friction drilling and thread forming have entered the market. These types of screw products do not have these access limitations as through-part connections are formed by one-sided access using a thermo-mechanical flow screwdriving process with minimal heat. The friction drilling, thread forming process, hereto referred to as FDS, is an automated continuous process that allows multi-material joining by utilizing a screw as both the tool and the fastener. The process uses the friction caused by the rotating screw to pierce and extrude the material. Threads are then created in this formed extrusion which allows the fastener to be screwdriven into the parts. A final torquing then securely clamps together the sheets of material. This study explores the quality design space as represented by resultant joint geometry as a function of the critical process parameters of fastener force and drilling speed. Feasible design space regions are explored to determine how process parameters affect joint geometry, and strength testing performed to validate the findings