Mechanical Properties and Microstructure of Argon Atomized Aluminum-Lithium Powder Metallurgy Alloys
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1.0 I N T R O D U C T I O N
THE addition of lithium to aluminum increases the elastic modulus and decreases the density? These changes, which are very desirable for structural applications, are obtained at the expense of reduced toughness and ductility, increased material cost, and production problems relating to loss of lithium during melting and ingot cracking during casting. 2 One method of avoiding the ingot cracking problems is to start with a hot pressed powder billet instead of an ingot. This technique initially appeared to have the potential of alleviating the low toughness of aluminumlithium alloys by allowing the use of rapidly cooled powder having a very fine grain size and a very fine constituent particle size, although work by Webster 3 on rapidly cooled A1 2024 powders produced by melt spinning and containing up to 2.5 pct Li indicated that the embrittlement of AI 2024 by lithium was similar for both powder source and ingot source material. The work reported here is similar to the earlier aluminumlithium program, except that larger scale powdermaking equipment was used and additional alloys were tested. A summary of the mechanical properties obtained by earlier investigators is given in Table I. The alloys in Table I are made by ingot metallurgy except where noted.
2.0 M A T E R I A L S The materials used in this investigation were produced by argon gas atomization at Kawecki-Berylco Industries (KBI). The powder-producing equipment which was formerly used for making A1-Be powder
D. WEBSTER is Staff Engineer, Lockheed Missiles and Space Co., Sunnyvale, CA 94086, G. WALD is Senior Research and Development Engineer, Lockheed California Co., Burbank, CA, and W. S. CREMENS is Materials Scientist, Lockheed Georgia Co., Marietta, GA 30060. Manuscript submitted January 11, 1980. METALLURGICAL TRANSACTIONS A
(Lockalloy) produces powder in approximately 40-1b batches. The target chemical compositions of the alloys are given in Table II. The Zr, Ti, and B are grain-refining elements and the Be addition is designed to reduce lithium loss during melting and atomization by forming an oxide skin over the molten metal. The function of Cd in A1-Cu alloys is to increase the strength by promoting a more uniform precipitation reaction of 0'. The actual compositions achieved are given in Table III. The lithium values obtained are 10 to 33 pct lower than the target compositions given in Table II. Lithium is normally lost during the melting of aluminum-lithium alloys even in vacuum or under a protective atmosphere and it is apparent that additional lithium must be added to counteract the losses. In the case of A1-Li powder manufacture, lithium will be lost in both alloy melting and the powder formation.
3. E X P E R I M E N T A L T E C H N I Q U E The A1-4.5-Li alloy powder was vacuum hot pressed into 5 cm billets at Lockheed Missiles and Space Company (LMSC), while the other alloys in 14-cm cans were processed according to the procedures shown in Table IV. Tensile testing on the A1-4.5-Li material was conducted on fla
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