Experimental Work

Initial testing showed the promise of AA7020 within and SPF forming process, with good levels of linear elongation within the target five minutes cycle time. The addition of 1.6wt% nickel to the alloy V3C showed significant improvements in ductility up to 204% linear elongation and an improvement in strength suggesting this would be an even more promising alloy.

Owing to this improved formability scale up of production of the V3C alloy in the form of V3CN was conducted. The effect of this scale up was a further increase in material strength to 342 MPa, but a decrease in formability due to differences in the size distribution of Al3Ni particles within the alloy. Primarily differences in the solidification rate of the two materials and to a lesser extent the difference in homogenisation treatments led to this difference between the two alloys.

Testing at SUSA using truncated cone tools with sharp male radius geometries showed the ability of AA7020 to form parts with in excess of 400% equivalent strain, the material requiring a minimum of 100% equivalent strain before contacting a male radius to form successfully. In the same geometries the V3CN was seen to have an upper limit of 100% equivalent strain, but being able to form parts successfully with lower strains where the AA7020 failed. This was attributed to greater static recrystallization by PSN due to the Al3Ni particles present in the V3CN compared to the AA7020, the AA7020 then possibly undergoing dynamic recrystallization during the forming, hence requiring the minimum of 100% equivalent strain before contact with the tool.

PSN is likely occurring at a lower level in AA7020 compared to V3C and V3CN due to the MgZn2particles present, the larger amount of Al3Ni particles in the V3C and V3CN leading to greater levels of recrystallization due to this mechanism. The higher strain rates employed during testing mean that dislocation creep rather than GBS is responsible for material elongations. All alloys likely undergo PSN, dislocation creep and possibly some limited amounts of dynamic recrystallization, but due to the different chemistries these mechanisms act at different amounts at different stages of the process. To properly identify and quantify this difference in-situ techniques such as heated tensile stage confocal microscopy or EBSD will be required.

Testing using custom made tooling which combines a slight mechanical preforming in combination with gas bulge testing at higher strain rates whilst allowing for material flow reinforced testing at SUSA. This testing showed the ability of the V3CN to form parts again with an upper limit of around 100% equivalent strain. Whilst the AA7020 showed the need for at least 100% equivalent strain before contact. The testing also confirmed the need for a mixture of graphite and a lower lubricity lubricant in areas of contact with male radii.

Heated stage EBSD and FSD analysis of the three alloys established the recrystallization temperatures of the three alloys with AA7020 showing recrystallization occurring at 327°C. V3C was seen to recrystallize at 277°C and V3CN at 285°C, the Al3Ni particles leading to earlier recrystallization due to greater levels of PSN. The coarser particles in V3CN compared to the V3C slightly retarding this added mechanism. This negative impact of the coarser particles within the V3CN was confirmed with a fast ramp to 300°C and fast cooling to room temperature which lead to a nearly fully recrystallized structure for the V3C but only 33% recrystallization within V3CN. The AA7020 under the same

for recrystallization are acting within all three alloys however at varying levels at different points within the preheat and subsequent forming stages.

Bulk EBSD highlighted differences in grain size of the alloys with AA7020 having the

coarsest structure with an average 11μm grain size, the V3C 6μm and V3CN 8μm. This

showed the influence of the Al3Ni particles on recrystallization, the added PSN from these particles leading to a finer microstructure in comparison to AA7020. The AA7020 showed less grain growth after forming than V3C and V3CN and also a lack of grain elongation which was observed in both V3C and V3CN. This suggests that there was some limited dynamic recrystallization within AA7020 with none in either V3C or V3CN, or that the same mechanisms were present in all three alloys but due to the Al3Ni particles they acted at different levels at different stages of the process.

Failures within both V3C and V3CN were observed to be “tearing” type failures, compared to more localised necking and thinning within the AA7020. This again is due to the Al3Ni particles which act as areas of nucleation of cavitation as they are not affected by the temperature of the forming process and remain present within the matrix as coarse intermetallics throughout. This was strengthened by the levels of cavitation observed within the three alloys after failure, 1.5% in AA7020, 3.5% in V3C and 8.5% in V3CN.