CLADDING OF THE HIGH-STRENGTH SUPERPLASTIC ALUMINIUM ALLOY.
Aluminum AA7XXX series alloys are widely used in high performance structural aerospace and transportation due to their low density and high strength. Some AA7XXX alloys exhibit superplasticity (SP) though at very low strain rates.The main problem of AA7XXX alloys is their low corrosion resistance at T6 state and thus cladding technology can be used for corrosion protection of the sheets. Traditional cladding by pure Al or Al-1%Zn decreases elongation by approximately 2 times and non superplastic cladding layer is destroyed during superplastic forming (SPF) after small strains. The cladding material should thus exhibit SP under the same deformation conditions as base alloy, good corrosion resistance and high plasticity to consolidate layers by rolling process, therefore alloys of Al-Mg and Al-Mg-Si meets the requirement.The Al-Mg alloys of AA5XXX have excellent corrosion resistance and ability to superplastic deformation (SPD). Alloying of Al-Mg alloys by Mn, Ti, Cr, Zr and Sc allow achieving a fine grain structure and SP due to the Zener pinning (ZP). Effect Zr and Sc provide both low temperature and high strain rate of SP. High density of fine nanoscale precipitates of Al3Zr, Al3Sc, Al3(Zr, Sc) phases has been observed by different scientists, Duan et al.(2016). Such type of precipitates provides a non-recrystallized grain structure before commencing SPD. The Al-Mg-Si alloys of the AA6XXX exhibit good corrosion resistance but low strain rate of SPD. The Al-Mg-Si alloys are moderate strength alloys and develop high resistance to corrosion equal or approaching resistance of Al-Mg and Al-Mn alloys.This study is focused on finding corrosion resistant cladding material for high-strength SP aluminium alloys, SP behavior, mechanical propertiesand corrosion resistance analyses of cladding sheets. Al-3Mg-0.3Zr-0.1Ti, Al-8.2Mg-4.7Si, Al-5Zn-5Ca and Al-3.8Mg-2.2Si-0.1Zr alloys are investigated as candidates for cladding materials. The Al-Mg alloy with Zr addition contains only fine particles for ZP of the grain boundaries. Two eutectic alloys contain a high amount of coarse eutectic Mg2Si or Al3CaZn particles to refine grains by particle stimulated nucleation effect. The Al-3.8Mg-2.2Si-0.1Zr alloy contains both coarse eutectic Mg2Si and fine Al3Zr particles to realize both grain refinement effects by PSN and ZP.