P. S. Grant, S. C. Hogg, I. G. Palmer, University of Oxford, Oxford, United Kingdom
This work describes the microstructure and properties of a range of spraycast Al-(4-6)Mg-(1.2-1.6)Li-(0.3-0.4)Zr-(0-0.2)Sc alloys. Spraycasting provided refined microstructures, extended solid solubility and a reduction in the concentration of embrittling impurity elements such as H, Na and K. Billets of 20-25kg were produced with as-spraycast grain sizes in 5-15µm range using an Osprey spraycasting plant installed at Oxford University. Following hot isostatic pressing to close any porosity and to precipitate a fine, coherent Al3Zr or Al3(Zr,Sc) dispersoid population, forging of 1kg samples at 250 and 400°C led to a substantial refinement of the microstructure with grain sizes in the range 0.8 to 5µm. A large intra-granular orientation gradient with distance measured using Electron Backscattered Diffraction showed that at 250°C, partial dynamic recrystallisation by progressive lattice rotation led to a ‘necklace’ structure of very fine grains surrounding larger deformed grains. At 400°C, dynamic recrystallisation occurred by nucleation and growth of new grains at prior grain boundaries and triple points. The strength of as-forged alloys was 200-350MPa, with high ductilities of up to 30% that rendered the alloys amenable to post-forging cold work. A proof strength of 460MPa with 9.5% elongation was achieved in a non-heat-treatable Al-6Mg-1.3Li-0.4Zr alloy forged at 250°C and cold worked 20%, matching the best properties of similar mechanically alloyed AA5091, and exceeding the properties of AA7010-T74. The as-forged alloys showed excellent thermal stability up to ~0.9Tm, with no abnormal grain growth after 7 days at 500°C and grain size stagnation in the 4-6µm range due to Zener pinning. Strain rate sensitivity testing revealed an m value of ~0.45 at 400 and 500°C and strain rates of 0.001-0.05s-1 and indicated strongly the potential for superplasticity.
Summary: Third generation Al-Li alloys are now being specified for use in large civil airliners, with Al-Cu-Li based alloys such as AA2099 and AA2196 specified for use in the Airbus A380, A350 and
A340-600E, and Boeing 787 aircraft. Whereas these heat-treatable alloys are made by direct chill (DC) casting, the spraycasting process is being investigated at Oxford University to develop new
lower density, non-heat-treatable Al alloys containing Li and Mg. These alloys avoid problems associated with quench distortion and cracking in complex cross-sections. This paper describes the
microstructure and properties of a range of spraycast and forged Al-(4-6)Mg-(1.2-1.6)Li-(0.3-0.4)Zr-(0-0.2)Sc alloys. From an initial as-spraycast equiaxed grain size of 5-10µm, average grain sizes of 0.8-5µm have been achieved by dynamic recrystallisation during moderate forging deformation. Tensile properties of a range of
this type of alloy have been shown to be as good as the best published values for a similar mechanically alloyed material AA5091. Because of the relatively high Zr and Sc concentrations
possible in the spraycast alloys, a higher volume fraction of fine, coherent dispersoids results and the alloys exhibit remarkable thermal stability, even at temperatures close to the
melting point. Finally, strain rate sensitivity testing has revealed the potential for superplasticity at 400 and 500°C and strain rates of up to 0.05s
-1.
