Tensile Properties Affecting the Formability of Aluminum-Alloy Sheet at Elevated Temperatures

The tensile properties thought to influence formability have been determined for 15 aluminum-alloy sheet materials tested at elevated temperatures. Complete load-extension diagrams have been obtained, and the effects of temperature, strain rate, and exposure time have been noted. An appreciable increase in the elongation over a zero gage length is found at elevated temperatures. This suggests that forming operations in which this property is a criterion may be facilitated at such temperatures. A decrease in the limit of uniform elongation is noted at elevated temperatures, suggesting that operations in which this property is a criterion may be less successful. A fiftyfold change in strain rate has an appreciable effect on properties at the higher temperatures, an increase in strain rate appearing to be equivalent to a decrease in temperature. In the case of materials subject to precipitation hardening, the time at temperature appears to be a minor factor for times ranging from 5 to 20 min. and temperatures up to at least 450 °F. In certain cases unusual load-extension diagrams were noted. I N T R O D U C T I O N In the aircraft industry aluminum-alloy sheet is often subjected to plastic forming, generally at room temperature. It has been found that some of the recent highstrength materials exhibit poor forming characteristics under these conditions and it has been suggested that formability may be improved at elevated temperatures. Previous investigations 2> 3 dealing with the elevated temperature properties of aluminum alloys have not included some of the recent materials and have not been concerned with formability. Consequently, an investigation was undertaken to evaluate over a considerable range of temperatures the tensile properties related to formability. The program comprising a portion of the ''restricted'' Project NRC-548, was financed by the Office of Production Research and Development of the W.P.B. and was supervised by the War Metallurgy Committee. This paper, which presents some of the results, has been released for publication by the latter agencies. TENSILE DATA, FLOW CURVES AND FORMABILITY The load deformation data derived in a tensile test are of interest in connection with formability since they Received January 8, 1946. * Research Engineer. t Engineer. + Associate Professor of Mechanical Engineering. describe the strain hardening characteristics of the material and the conditions for necking and fracture. It is often desirable, however, to express the data in terms of a true stress-true strain relation (flow curve) in preference to the more familiar but less fundamental engineering stress-strain diagram. This is possible on the assumption that the flow curve approximates a straight line between the points of maximum load and fracture and that the incidence of maximum load corresponds to the beginning of necking (the limit of uniform elongation). True stress and true strain may then be expressed as:** a = 5(1 + £ ) , * = log, (1 + E) up to and including the limit of uniform elongation, and subsequently as a = s (Ao/A)t <p '= log, (AQ/A) where a = true stress (load divided by the corresponding momentary cross-sectional area)tt s ^ engineering stress (load divided by the original cross-sectional area) E — engineering strain = (Z — k)/l = true strain = dA/A = / dl/l up to the limit of uniform elongation Ao dA/A beyond the limit of uniform elonga-L A\ tion • A§ = original cross-sectional area A — momentary cross-sectional area Zo = original gage length I — momentary gage length From this point of view, two of the most significant features of the load-extension diagram are the points of maximum load and fracture, and associated with these points are the following desirable data: ** These relations and other aspects of the flow curve are discussed in references 4 and 5. t t Here we neglect the undoubted presence of a nonuniform stress distribution through the cross section of a tensile specimen in the necked region. 457 458 J O U R N A L O F T H E A E R O N A U T I C A L S C I E N C E S — S E P T E M B E R , 1 9 4 6 (1) Ult imate tensile stress (defining the limit of uniform elongation). (2) Elongation at maximum load (the limit of uniform elongation). (3). Reduction in area a t the fracture. (4) Load or stress a t fracture. In the present investigation complete load-extension diagrams were obtained for a large number of materials and conditions, and the da ta listed above under points 1, 2, and 3 were derived and tabulated. Considerable difficulty was encountered, however, in the precise determination of the load at fracture—a mat ter t ha t will be discussed presently. I t is not the purpose of this paper to prescribe methods of applying tensile da ta in the prediction of limits of formability. In certain types of forming operations tensile da ta are directly applicable, whereas in other operations the method of application is far from simple and such factors as combined stresses and criteria for rupture and necking may be involved. CLASSIFICATION OF M A T E R I A L S The materials investigated may be grouped as follows according to the anticipated effects of elevated temperature exposure on subsequent room temperature properties: Group 1A.—Nonprecipitation hardenable alloys in the annealed condition. These materials may be formed a t any temperature up to the temperatures of grain growth or hot shortness without significant effect on the final properties. Group IB,—Precipitation hardenable alloys in the annealed condition. These materials respond in a manner similar to t h a t characterizing Group 1A. After forming, however, they may be heatt reated to obtain marked improvement over the original properties. Group 2A.—Precipitation hardenable alloys previously subjected to solution heat t rea tment folio wedby aging a t room temperature . These materials are subject to additional precipitation a t elevated temperatures dependent upon the temperature and t ime of exposure. Brief exposures a t temperatures below approximately 500°F. may be expected to increase the room temperature yield stress. Overaging attended' by a loss of strength may be encountered as a result of longer exposures and higher temperatures. Group 2B.—Precipitation hardenable alloys previously subjected to solution heat t rea tment followed by aging a t elevated temperatures. Additional exposure a t elevated temperatures may be expected to induce little additional strengthening. As in the case of the 2A materials, however, overaging may result a t the higher temperatures. SCOPE OF THE INVESTIGATION The following materials were studied. All were obtained from the producers' commercial stock. Unless TABLE 1 NOMINAL COMPOSITIONS OF THE ALLOYS T E S T E D * 2S Aluminum and normal impurities. No alloying additions 3S 1.2 per cent manganese 52S 2.5 per cent magnesium, 0.25 per cent chromium 24S 4.5 per cent copper, 0.6 per cent manganese, 1.5 per cent magnesium 61S 0.25 per cent copper, 0.6 per cent silicon, 1.0 per cent magnesium, 0.25 per cent chromium R301 4.5 per cent copper, 1.0 per cent silicon, 0.8 per cent manganese, 0.4 per cent magnesium XB75S No information has been released on the precise composition. The principal hardening elements are zinc and magnesium with smaller additions of copper and other elements * In each case the remainder consists of aluminum and normal impurities. TABLE 2 Standard Heat-Treatments Used in Production of the Age Hardened Materials 24S-T 24S alloy solution heat-treated at 910-930 °F. followed by quenching in cold water and aging at room temperature 24S-RT 24S-T strain hardened by, rolling to a reduction of about 5.5% 61S-W 61S alloy solution heat-treated at 960-980 °F. followed by quenching in cold water and aging at room temperature 24S-T84 24S-T stretched 3V2-4 per cent and aged 6 hours at 375 °F. or 9 hours at 365 °F. 24S-T86 24S-RT aged 5V2 hours at 375 °F. or 8 1 / , hours at 365 °F. 61S-T 61S-W aged 8 hours at 350 °F. or 18 hours at 320 °F. XB75S-T XB75S alloy solution heat-treated at 860-930°F. followed by quenching in cold water and aging 24 hours at 250 °F. R301-T R301 alloy solution heat-treated at 930-950 °F. followed by quenching in cold water and aging 6 hours at 350 °F. or 18 hours at 320 °F. otherwise indicated, all were in the bare (unclad) condition. Group 1A 2S-0 3S-0 52S-0 Group IB 24S-0 61S-0 XB75S-0 Alclad* R301-0 (clad) Group 2A 24S-T 24S-RT 61S-W Group 2B . 24S-T-84 24S-T-86 61S-T XB75S-T* R301-T (clad) The nominal chemical compositions of the alloys and the s tandard heatt reatments for the precipitation hardened materials are listed in Tables 1 and 2. In each case elevated temperature tensile tests were made on 0.064-in. sheet to determine: ' (1) The complete load extension diagram. (2 ) . Ult imate tensile stress. (3) Limit of uniform elongation. (4) Local elongation at the fracture (based on reduction in area). (5) Elongation in 2 in. (6) Effect of gage length on elongation J * The alloy formerly designated XB75S by the Aluminum Company of America is now known as 75S. t The extensive data relating per cent elongation to gage length have been omitted from this paper. They are included in reference 5. F O R M A B I L I T Y O F A L U M I N U M . A L L O Y S H E E T ' 459 CROSSHEAD SPEED , 3 in. /minute 200 . X B 7 5 S 0 6 5 8