Influence of Hydrogen on the Ductility of 7091 and 7090 PM Aluminum Alloys

Abstract

Two high-strength aluminum alloys, X7091 and X7090, were produced by powder metallurgy (PM) processing, which included gas (air or helium) atomization, cold compaction, hot vacuum degassing, hot compaction, and extrusion. The products were then solution heat treated, water quenched, stress relieved, and aged as follows: Alloy X7091 was aged for 24 h at 120°C plus 4 h at 163°C and Alloy X7090 for 24 h at 120°C plus 1 h at 163°C. The mechanical properties in the longitudinal and transverse directions and the hydrogen (H2) content were then measured. The author observed that the H2 content of the different products varies from 1 ppm to more than 10 ppm as a function of the quality of the degassing process and that, though the mechanical properties (yield strength, ultimate tensile strength, and elongation) are not affected in the longitudinal direction, the elongation in the transverse direction can decrease from 10% for an alloy with low H2 content to 0% for an alloy with high H2 content. It is, however, difficult to correlate the hydrogen content with ductility: a high-H2 alloy can have good ductility when highly oxidized. This phenomenon can be explained by a beneficial hydrogen getter effect by the oxide. It seems, then, that the best way to characterize the soundness of the PM aluminum alloys is by their hydrogen and oxygen contents.