Residual Stresses in Flow Drill Screwdriving of Aluminum Alloy Sheets

With an increase in fuel economy standards and the need for reducing emissions set for the automotive sector, has resulted in the increased demand for lightweight vehicles. It is well know that the single heaviest component of a passenger vehicle is the body structure, thus has the greatest potential for significantly reducing the vehicles mass. Therefore, transitioning from steel-based bodies to ones composed of lightweight materials, such as: aluminum, magnesium and advanced high strength steels are of great interest. However, with the introduction of these new materials comes with a new means of joining, where conventional methods do not work. Therefore, this work examines a novel joining technique, flow drill screwdriving which is a thermo-mechanical process for joining aluminum and dissimilar materials. The focus of this work is to examine the residual stress distribution in a joint, because mechanical behavior and joint quality are greatly affected by the residual stress. Neutron diffraction was used for the determination of the residual stress in two samples processed with low and high fastener force. The high penetration depth of neutron radiation allows for the determination of triaxial residual stress states inside the material without destruction of the sample. It was found that the stress field around the joint location is primarily in tension, which is problematic if external forces are applied near the joint. Therefore, additional stress measurements were conducted under applied load through a lap shear test. Two load levels were applied to determine the effects on stress concentrations around the proximity of the joint.