Aluminum-lithium powder metallurgy alloys with improved toughness
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I.
INTRODUCTION
THEinvestigation of A1-Li alloys made by powder metallurgy was originated at Lockheed Missiles & Space Company in 1976 using flakes of A12024 + 2.5 Li melt spun by R.E. Maringer of Battelle Columbus Laboratories. The work was undertaken to determine whether what appeared to be a fundamental brittleness problem in A1-Li alloys produced by ingot metallurgy could be overcome by rapid cooling from the molten state. Unfortunately, the results 1'2 showed very similar properties for both ingot route and powder route materials processed identically. This appeared to limit the use of AI-Li powder alloys to applications requiring lithium levels above that at which sound DC cast ingots could be produced. Depending on the size of the ingot and the secondary alloying elements, this would be in the range 2.5 to 2.8 pct Li. There may, however, be limited markets where the superior general corrosion resistance of A1-Li powder alloys would be a deciding factor at even lower lithium levels. The intent of the present work was to produce AI-Li powder alloys with properties superior to the less expensive ingot materials. The approach chosen was to produce a 'composite' powder consisting of an AI-Li powder mixed with 10 to 30 pct of unalloyed aluminum. The very high toughness of the unalloyed aluminum powder was expected to improve the toughness of alloys by acting to blunt a growing crack. A small loss of strength and an increase in density were expected as a result of the reduced average lithium level of the powder mixture. This technique (U.S. Patent 4,597,792 issued 1986) is applicable to both aluminum alloys and metal matrix composites. The mechanical properties and microstructure of nine A1-Li alloys made by this technique are described below.
II.
MATERIAL
The AI-Li powder alloys were made by Homogeneous Metals, Inc. using argon gas atomization. The A1-Li-CuMg-Zr (A18090) powders were -100 mesh (< 149/xm) ex-
D. WEBSTER, formerly with Kaiser Aluminum & Chemical Corporation, P.O. Box 877, Pleasanton, CA 94566, is a Consultant for Advanced Material Development at 20565 Verde Vista Lane, Saratoga, CA 95070. Manuscript submitted July 23, 1986. METALLURGICALTRANSACTIONS A
cept for one batch, alloy 96, which was made with + 100 mesh powder to determine the influence of powder particle size on properties. The unalloyed aluminum powder (99.6 pct A1, 0.11 pet Fe) was also - 1 0 0 mesh with 99.3 pet being between - 2 0 0 mesh (74/xm) and 10/xm. The compositions of the alloy powders are given in Table I, together with the compositions of the powder mixtures tested. Two batches of the A18090 alloy were obtained. The first batch, alloy 1611 (2.6 pct Li), was mixed with 10 pct aluminum to make alloy 1612. The second batch was diluted with 15, 20, 30, and 15 pet aluminum to make alloys 93, 94, 95, and 96 (+ 100 mesh - 2 0 mesh). Alloy 1614 was a higher lithium version of A18090 with 3.3 pct Li and this was mixed with 10 pct aluminum to produce alloy 1615.
III.
EXPERIMENTAL TECHNIQUE
The powder mixtures were made by blen
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