A novel approach to hot die-less clinching process for high strength AA7075-T6 sheets

High strength AA7075-T6 aluminum sheets were joined by hot die-less clinching by locally heating the clinching region with an electrical resistance heating method. This method applied a large amount of current in the range 7.5–15 kA over a time period of 2–3 s to enhance the local ductility of AA7075-T6 sheets. A modified die-less clinching tool was developed to carry current from the blank holder to the anvil through a pair of AA7075-T6 sheets to be clinched. The above range of applied current and time duration led to an increase in sheet metal temperature in the range 175–260 ℃ leading to material softening and a consequent reduction in the clinch forming force as well as improved material flow. The clinched joints produced with constant forming force of 60 kN resulted in an increase in lap shear joint strength up to 41% with an applied current of 15 kA. Microstructure examination of the clinched region for a range of electrical resistance heating conditions revealed sound joints with previously reported forming and force locking mechanisms as well as recently identified elevated temperature material locking mechanism in the literature. The geometric interlock resulted in nonlinear increase in force with displacement in the lap shear test as well as instantaneous drop in the force due to failure of material locking.