Rosenhain Centenary Conference - 3. Materials development present and future 3.6 Development of Al-Zn-M g-Cu alloys for aircraft

Abstract
Historical development of the high strength Al—Zn—M g-Cu alloys during the last 50 years reflects the influence of progressive changes in engineering and functional requirements related to the evolution of aircraft size, design concepts and manufacturing methods with consequent changes in dimensions and configurations of structure components. If the only criterion of merit for such application were high strength uncomplicated by stress-concentrating geometric factors or environmental influences, compositions available at least 40 years ago would have been highly suitable, since strength of the early materials fully equalled that of the alloys in use today. These early alloys were, however, not adequately resistant to stress-corrosion cracking even in the form of thin-section products such as sheet, thin extrusions and tube prevalent in aircraft of the earlier portion of the period. Development of the chrouiium-containing alloy 7075—T6 in the 1940s provided a solution to this problem for the thin-section products. As experience accumulated in extending the use of 7075-T6 to larger and thicker components produced from plate, thick-section extrusions or forgings, transverse direction stress-corrosion cracking was encountered along with decreasing strength and an undesired degree of mechanical anisotropy, particularly ductility. Alloy 7079, which had more uniform mechanical properties, enjoyed a transient period of popularity but proved to be even more deficient'in resistance to stress-corrosion. The latter problem was eliminated with development of 7075-T73 involving use of a two-stage precipitation treatment with an accompanying significant strength penalty. Despite the strength trade-off, this material is extensively used in medium section thickness components of current large American aircraft and has an eminent success record.

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