Development of Al–Mg–Sc Thin Foils for Fiber – Reinforced Metal Laminates

Fiber metal laminates (FML) are hybrid composite materials built up from interlacing layers of thin metal foils and fiber reinforced adhesives. The need to reduce weight, increase damage tolerance and corrosion resistance was the motivation that led to the development of these new class of materials. Basically, FML possess better mechanical characteristics when compared to both aluminum alloys and composites individually. Most metal foils in use are Al-Cu based and are prone to corrosion due to the high percentage of Cu; a drawback often overcome by subjecting the foils to sophisticated and partially hazardous surface pre-treatment prior to bonding in laminates. On the other hand, it is well known that Al-Mg foils possess excellent corrosion resistance and high ductility properties; however, strength properties are inferior. Contemporary research has revealed that incorporating small quantities of transition metals such as Scandium (Sc) and Zirconium (Zr) could improve the mechanical properties of Al-Mg. Equipped such knowledge, researchers have been able to fabricate alloys such as the 1570C alloy. Regardless, further research is still mandatory, particularly for consideration of the cost of Sc whose market is yet to stabilize.

Furthermore, despite the long-term research of the Al–Mg–Sc alloys, manufacturing of thin sheets (or foils) with a thickness of less than 1 mm is still an issue. The typical defects are buckling and a thickness tolerance. To address this, a team of researcher from the Austrian Institute of Technology: Dr. Evgeniya Kabliman, Dr. Salar Bozorgi and Dr. Johannes Kronsteiner, in collaboration with Engineer Alexey Nikolas at the SMW Engineering Ltd (now at 2AM, SIA) in Russia has studied a process for the production of Al–Mg–Sc thin sheets with different controlled thickness. Their work is currently published in the research journal, Advanced Engineering Materials.

For this purpose, the research team designed a special alloy based on the 1570C alloy. The special alloy has reduced Mg content and a different rare earth element. Consequently, a special rolling technology for production of thin metal foils with a maximal thickness of 0.5 mm at a semi-industrial scale was also developed. The microstructure of the samples was subsequently characterized by using the metallography, EBSD, as well as SEM/EDX techniques. Additionally, in order to eliminate the rolling defects, the researchers subjected the thin sheets to hot pressing. Lastly, numerical simulations of asymmetric hot rolling by using Finite Element Method were performed.

The authors reported that the mechanical properties of produced Al–Mg–Sc foils with a thickness of 0.5 mm achieved 377 and 419 MPa for yield and ultimate strength, respectively, with the elongation at break about 11% (in the longitudinal direction). Remarkably, the estimated average corrosion rate of the Al–Mg–Sc sheet material with a thickness of 2.7 mm was observed to be not more than 0.006 mm/year.

In summary, the study by Dr. Evgeniya Kabliman and her colleagues presented an in-depth investigation on the manufacturing process of the Al–Mg–Sc thin foils with the thickness of 0.5 mm, in a lab and semi-industrial scale. Generally, by applying the semi-continuous casting, the alloy was produced and thereafter rolled into thin foils with a thickness of max. 0.5 mm by using the duo rolling mill. Overall, the rate of fatigue crack growth was found to be lower than for a conventional AA2024-T3 sheet material as well as the material density (2.65g cm^-3).

The manufacturing process was studied mainly in the lab scale, however, the produced Al–Mg–Sc alloy (AFML1) might be considered as a perspective material for aeronautical applications, in particular for fiber metal laminates. The produced laminates showed similar mechanical properties when compared to GLARE 3-3/2 by the reduced weight and better corrosion resistance of metallic layers made of the AFML1 alloy.

The work was conducted within the project “AeroFML – Development of innovative Al-Mg-Sc Fibre Metal Laminates for aeronautical applications” ID # 128 funded by the DLR Project Management, the Austrian Research Promotion Agency (FFG) and the Foundation for Assistance to Small Innovative Enterprises (FASIE).