Relationship between Hydrogen Content and Low Ductility in Aluminum- Lithium Alloys

Abstract

Abstract : An investigation was conducted to determine the relationship between hydrogen content and ductility in Al-Li alloys. A series of Al-3.4Mg-1.9Li-0.3Mn alloys was prepared in the laboratory by either arc melting or induction melting so as to produce a range of hydrogen contents. The lowest hydrogen contents were produced in alloys induction melted and cast in a rapidly flowing dry-argon atmosphere. Hydrogen is retained much more readily in Al-Mg-Li alloys than in conventional aluminum alloys, and hydrogen contents ranging from 14 to 45 ppm were measured in seven alloys that were processed to 0.040 inch sheet. Hydrogen was found to have an appreciable effect on properties; lower hydrogen contents resulted in significantly higher ductility (tensile elongation) and toughness (notched tensile strength/yield strength) and slightly lower yield strength. Hydrogen pickup occurred during processing, and the hydrogen contents of some of the alloys in sheet form were over twice as high as those measured in the ingots from which they were produced. Contamination did not appear to be limited to a surface scale; it extended some distance below the surface. Carrying out all thermal treatments in an argon atmosphere, including heating for forming and rolling, appeared to minimize contamination. Prolonged vacuum annealing was ineffective in removing hydrogen from Al-Mg-Li alloy sheet, but did not cause some loss of Mg and/or Li. Efforts to determine whether hydrogen was present in the alloys in solid solution or as a hydride phase by optical metallographic examination were unsuccessful.