FE Simulation of the Hole Expansion Experimental Test of High Strength Aluminium Alloys

Edge failure is one of the major problems associated with forming operations of metal sheets. During the recent years, some Aluminium Alloys, as AA7075, are gaining attention and interest in academia due to their increasing utilization in manufacturing processes, especially in the automotive. As a result, there is an interest in developing numerical techniques able to predict the edge failure in forming simulations and consequently optimize the forming operations in the automotive industry for vehicle light-weighting, in agreement with the most recent regulations. The stretch-flangeability of AA7075-T6 were predicted with FE hole expansion simulations involving different boundary conditions, as temperature and punch shape variations. The standard conical punch as well as a custom flat punch were used to investigate how punch shape affect the formability when the hole is expanded out-of-plane (conical punch) and in-plane (flat punch). The effect of temperature was investigated considering simulations at room and warm conditions (25 and 200 Celsius degrees respectively). The primary objective of this thesis is then, basing on the available experimental data, to build a Finite Element Model (FEM) able to characterize the induced strain and stress within the metal blank during the various test condition and predict the failure of the blank. The FEM is also useful to verify how the material tensile properties affect the edge stretchability of the hole. Comparing the results with other aluminium alloys analysed in previous literature, it was found that the AA7075-T6 shows a relative high edge sensitivity in the hole expansion simulations with the conical punch (HER = 28% at room temperature, HER = 35% at warm temperature), with a predicted fracture in pure uniaxial stress on the edge for both the room and the warm condition; on the contrary, the expansion with the flat punch shows the most critical point some milimiters away from the hole edge , in a zone that present a stress state between pure uniaxial and plane deformation (HER = 19% at room temperature, HER = 24% at warm temperature). Relationship between hole expansion results and tensile-mechanical propoerties were developed for both conditions.