Mechanical properties and microstructure of Al-Li-Cu-Mg-Zr die forgings
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I.
INTRODUCTION
THEaerospace industry is presently in the initial stages of replacing existing high strength and high toughness aluminum alloys, such as A1 7075 and A1 2024, by low density A1-Li alloys. This will result in an 8 to 12 pct reduction in weight, 1 providing the A1-Li alloys can be produced with certain minimum mechanical properties. The properties that are used in aircraft design, e.g., strength, toughness, stress corrosion resistance, and fatigue crack growth, should be equivalent to those of existing alloys to avoid the need for structural redesign. At the present time, there are concerns regarding the short transverse toughness and stress corrosion resistance of A1-Li alloys, 2,3 particularly when properties equivalent to A1 7075-T73 are required. The present program was designed to determine whether A1 8090 forgings could be substituted for parts which presently use A1 7075-T73.
II.
MATERIAL
The composition of the AI 8090 used in this work is shown in Table I. The bulk hydrogen content was determined by Leco Corporation using a technique which melts the alloy under a stream of nitrogen gas and determines the hydrogen content from the change in thermal conductivity. Bulk hydrogen and surface hydrogen contents were determined, but only the bulk hydrogen is reported here. The bulk hydrogen content of 0.3 ppm is considerably higher than the values obtained on conventional aerospace quality aluminum alloys by the same technique (0.03 to 0.08 ppm), but is amongst the lowest values obtained for A1-Li alloys such as A1 8090 and A1 2090 which have been found to be in the range 0.3 to 0.8 ppm. 4 The sodium and potassium contents were determined by flame atomic emission.
DONALD WEBSTER, formerly Senior Staff Research Engineer, Kaiser Aluminum & Chemical Corporation, Pleasanton, CA 94566, is a Metallurgical Consultant, 20565 Verde Vista Lane, Saratoga, CA 95070. RICHARD KIRKBRIDE is Quality Control Representative, Boeing Company, Glendale, CA 91203. Manuscript submitted November 5, 1985.
METALLURGICALTRANSACTIONS A
III.
EXPERIMENTAL PROCEDURE
Four small die forgings were made at Kaiser Aluminum & Chemical Corporation's Oxnard Forging plant. The forgings were made from sections of a 6.4 cm-diameter extrusion, machined from a 14.6 cm-thick rolling block of A1-Li-Cu-Mg-Zr. The rolling block was turned to 12.7 cm diameter, homogenized for 4 hours at 790 K, air cooled, reheated to 744 K, and extruded to 6.4 cm diameter. Pieces of this extrusion, 10.2 cm long, were used for each forging. The forging conditions are given in Table II. The forgings were solution treated for 1 hour at 790 K, cold water quenched, aged 2 days at room temperature, followed by 48 hours at 444 K. Portions of the original extrusion stock were heat-treated identically so that the longitudinal tensile properties could be measured for comparison with the forging properties. Test specimens were cut from the forgings as shown in Figures 1 and 2. The original extrusion direction is shown in Figure 1 and the forging direction is shown in F
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