Development of a hot gas formable AA7XXX alloy & hybrid SPF/warm forming technology for light-weight automotive structures in niche vehicle manufacturing

This work looked to develop a new hybrid superplastic forming process in combination with a high strength aluminium alloy. A combination of physical testing, elevated temperature tensile testing, cone gas bulge and custom designed tool gas bulge tests were used to assess material formabilities to define the optimum forming conditions and establish an industrially suitable forming process. Testing established the suitability of a forming cycle within the industrial five minute target time, with the alloys undergoing recrystallization during preheat stages, forming at higher than typical strain rates and achieving full solutionization during the forming cycle. A two stage aging treatment (90°C for 8 hours and 130°C for 18 hours) achieved a high strength T5 temper meeting the industrial target of 300 MPa. AA7020 alloy was capable of achieving strains in excess of 400% in the 5 minute forming cycle under optimal forming conditions provided the alloy had experienced a minimum 100% strain before contact with male tooling. After aging treatment the alloy was seen to achieve a yield strength of 305 MPa, slightly in excess of the industrial target. A 1.6wt% nickel addition to AA7020 labelled alloy V3C initially showed increased performance in terms of formability and strength. The scale up of this alloy in the form V3CN showed decreased performance when compared to the V3C but was capable of strains in the region of 120% in the five minute forming cycle, a significant reduction compared to AA7020 but with no minimum strain requirement before contact with male radii. After aging treatment the alloy achieved a yield strength of 342 MPa, suggesting use as a higher strength alternative to AA7020 in applications with less extreme geometries