Method of producing a shaped Al alloy panel for aerospace applications

Abstract

A method of producing a shaped aluminum alloy panel, preferably for aerospace or automotive applications, from 5000-series alloy sheet. The method includes: providing a sheet made of 5000-series alloy having a thickness of about 0.05 to 10 mm and a length in the longest dimension of at least 800 mm; and stretch forming the sheet at a forming temperature between 100 C. and 25 C., to obtain a shaped aluminum alloy panel. A shaped article formed by the above method is also provided.

Claims

1. A method of producing a shaped aluminium alloy panel from 5000-series aluminium alloy sheet, the method comprising: providing a sheet made of 5000-series alloy having a thickness of about 0.05 to 10 mm and a length in a longest dimension of at least 800 mm; and stretch forming the sheet at a target forming temperature between 90 C. and 25 C., to obtain the shaped aluminium alloy panel, wherein the shaped aluminium alloy part is not showing any Portevin-Le-Chatelier (PLC) lines and has a tensile strength in L-T direction of above 350 MPa, and an elongation above 7%.

2. The method according to claim 1, wherein the target forming temperature is below a value T.sub.crit characterized by the formula
T.sub.crit[ C.]=log.sub.10({acute over ()}[s.sup.1])18.8+13.8 C. wherein {acute over ()} is the strain rate during forming.

3. The method according to claim 1, wherein the stretch forming is performed at a strain rate between 0.1 and 10.sup.4 s.sup.1.

4. The method according to claim 3, wherein the sheet is made of a Sc-containing aluminium alloy having Sc in a range, in weight percent, of 0.05% to 1%.

5. The method according to claim 1, wherein the strain rate is above 110.sup.3 s.sup.1.

6. The method according to claim 1, wherein the sheet is stretched, at least in some positions, by a total strain of 1 to 8%.

7. The method according to claim 1, wherein the target forming temperature is between 90 C. and 40 C.

8. The method according to claim 1, wherein the temperature during forming is held constant to within 10 C. of the target forming temperature, during the stretch forming.

9. The method according to claim 1, wherein the sheet is cooled down prior to the stretch forming by use of dry ice and no further cooling is done during the stretch forming.

10. The method according to claim 9, where the sheet is cooled down by immersion in or spraying with the dry ice.

11. The method according to claim 1, wherein the sheet made of 5000-series alloy has been produced by casting an ingot; hot rolling; cold rolling; annealing.

12. The method according to claim 1, comprising a step of annealing the shaped aluminium alloy panel at a temperature of 250-350 C., or of inter-annealing the aluminium alloy panel between two stretch forming steps at a temperature of 250-350 C.

13. The method according to claim 1, wherein the aluminium alloy panel is for aerospace or automotive applications.

14. The method according to claim 1, wherein the strain rate is above 210.sup.3 s.sup.1.

15. The method according to claim 1, wherein the sheet is stretched, at least in some positions, by a total strain between 3% and 8%.

16. The method according to claim 1, wherein the sheet is stretched, at least in some positions, by a total strain between 3.5% and 6.5%.

17. The method according to claim 1, wherein the target forming temperature is between 90 C. and 50 C.

18. The method according to claim 1, wherein the temperature during forming is held constant to within 15 C. of the target forming temperature, during the stretch forming.

19. The method according to claim 1, wherein the sheet made of 5000-series alloy has been produced by casting an ingot; hot rolling; cold rolling; annealing for 1-2 hours at 270-280 C.

20. The method of claim 1, wherein the sheet is made of a 5000-series alloy having a thickness of about 0.05 to 10 mm and a length in the longest dimension of at least 800 mm.

21. The method of claim 1, wherein the sheet is made of a 5000-series alloy having a thickness of about 0.6 to 6 mm, and a length in the longest dimension of at least 800 mm.

22. The method according to claim 1, wherein the sheet is made of a Sc-containing aluminium alloy having Sc in a range, in weight percent, of 0.05% to 1%.

23. The method according to claim 22, wherein the sheet is made from Sc-containing aluminium alloy comprising, in weight %, 3.0-6.0% Mg, 0.05-0.5% Sc, 0.05-0.25% Zr, optionally up to 2% Zn, balance is made by Fe, Si, regular impurities and aluminium.

24. The method according to claim 22, wherein the sheet is made from Sc-containing aluminium alloy comprising, in weight %, 3.8-5.3% Mg, 0.10-0.15% Zr, optionally up to 2% Zn, balance is made by Fe, Si, regular impurities and aluminium.

25. The method according to claim 1, wherein the sheet is made from an aluminium alloy of the AA5024-series.

26. The method according to claim 1, wherein the target forming temperature is between 80 C. and 40 C.

27. The method according to claim 26, wherein the target forming temperature is between 70 C. and 40 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram summarising the tests made at different strain rates and temperatures, indicating the appearance of PLC lines or no PLC lines.

(2) FIG. 2 is a diagram of tensile strength and yield strength of various samples stretched at different temperatures.

(3) FIG. 3 is a diagram of elongation of different samples stretched to a total strain of 6% at different temperatures.

(4) FIG. 4 is a diagram illustrating the effect of total strain on strength.

(5) FIG. 5 is a diagram of elongation against total strain.

(6) FIG. 6 is a diagram of unit propagation energy against total strain.

(7) FIG. 7 is a diagram of strength against strain rate.

(8) FIG. 8 is a diagram of elongation against strain rate.

(9) FIG. 9 is a diagram of unit propagation energy against a strain rate.

(10) FIG. 10 is a diagram of various properties, compared for samples stretched at low strain and strain rate vs. high strain and strain rate.

(11) FIG. 11 are photographs of 5xxx sheet stretched at 50 C. (left) and 150 C. (right) tested for corrosion resistance according to ASTM G-66.

(12) FIG. 1 summarises a number of experiments which have been carried out to find out the critical temperature, i.e. the maximum temperature below 0 C. at which 5000-series alloy sheet can be stretched without PLC lines appearing. The circular data points indicate sample with no PLC lines, square data point represent samples with PLC lines. Surprisingly, one has found a relationship between the strain rate and the temperature, which can be summarised by the formula:
T.sub.crit[ C.]=log.sub.10({acute over ()}[s.sup.1])18.8+13.8 C.

(13) The critical temperature is drawn in FIG. 1 as a line separating samples with no PLC lines from those which showed PLC lines. Surprisingly, the higher the strain rate, the higher the stretching temperature can be. Thus, at the temperature range above about 100 C. and below the critical temperature, homogeneous flow occurs during stretching. Experiments show that the dislocation movement at these temperatures is rather homogeneous, because the solute atoms cannot catch up with the moving dislocations to pin them, caused by the low diffusivity of the solute Mg atoms at the low temperatures. The experiments of FIG. 1 were carried out with an AlMgSc alloy having the following composition: Mg 4.5%, Sc 0.27%, Zr 0.10%, impurities <0.05% each and <0.15% in total, remainder aluminium.

EXAMPLES

(14) Alloys were cast, processed to sheet products and stretched at various temperatures and at various strain rates and total strains to investigate the advantages of the present invention. In particular, an alloy containing 4.5% Mg, 0.26% Sc, 0.10% Zr, impurities <0.05% each and <0.15% in total, remainder aluminium, was cast to ingots having a diameter of 262 mm and 1400 mm length. From these ingots, rolling blocks were machined with a gauge of 80 mm. The rolling blocks were hot-rolled to an intermediate gauge of 8 mm, cold rolled to a thickness of 4 mm, annealed for 1 hour at 275 C., cold rolled to 1.6 mm, and annealed for two hours at 325 C. From these cold rolled sheet, panels were machined which were subjected to a cryogenic stretching operation at various temperatures, strain rates and total strains, as indicated in the below tables 1 and 2.

(15) Tensile properties were tested according to DIN EN-10.002. In tables 1 and 2, Rp stands for the yield strengths, Rm for the ultimate tensile strength, and A stands for elongation. TS stands for tear strength and was measured in L-T and T-L direction according to ASTM-B871-96. UPE stands for unit propagation energy and was also measured according to ASTM-B871-96. It is a measure for the propagation of cracks, while TS is indicative of the amount of crack formation.

(16) TABLE-US-00001 TABLE 1 Summary of tear strength TS, UPE and TS/Rp for 8 samples of the same sheet, but stretched at different temperatures, strain rates and total strain. Forming Sam- tem- Strain TS/ ple perature Rate Strain TS UPE Rp ID [ C.] [s1] [%] L-T T-L L-T T-L L-T 1 50 1.3E03 6 583 560 101 122 1.62 2 50 9.3E04 6 546 571 88 140 1.53 3 50 1.0E03 4 554 580 126 159 1.68 4 50 2.0E04 4 539 561 129 126 1.58 5 40 2.3E03 6 576 577 96 119 1.58 6 40 1.9E04 4 573 577 136 137 1.70 7 20 2.6E04 6 537 557 149 79 1.46 8 20 2.6E04 4 547 549 112 172 1.58

(17) Table 1: Summary of tear strength TS, UPE and TS/Rp for 8 samples of the same sheet, but stretched at different temperatures, strain rates and total strain.

(18) TABLE-US-00002 TABLE 2 Tensile values for 8 different samples of sheet stretched at various temperatures, strain rates and total strains. Tem- Sam- pera- Strain ple ture Rate Strain Rp Rm Ag A PLC ID [ C.] [s1] [%] [MPa] [MPa] [%] [%] Lines 1 50 1.3E03 6 359 400 8.0 9.2 No 2 50 9.3E04 6 357 400 8.1 9.2 No 3 50 1.0E03 4 330 383 11.8 12.6 No 4 50 2.0E04 4 342 393 9.2 10.5 No 5 40 2.3E03 6 365 403 6.8 7.0 No 6 40 1.9E04 4 337 390 9.1 9.6 No 7 20 2.6E04 6 369 410 8.2 8.8 YES 8 20 2.6E04 4 347 397 9.9 10.7 YES Base 0 293 374 11.7 13.0 No

(19) Table 2: Tensile values for 8 different samples of sheet stretched at various temperatures, strain rates and total strains.

(20) FIG. 2-11 shall be discussed in the following to illustrate some important properties of the sheet stretched according to the invention. According to FIG. 2, a significant amount of work hardening occurs by stretching to a total strain of 6%, resulting in an increase of ultimate tensile strength from about 375 MPa of the unstretched reference to above 390 MPa for forming temperatures of 40 or 50 C. Yield strength increases from about 290 to above 350 MPa. Although the best results are achieved at about room temperature, this technique does not form an alternative, due to the clear appearance of PLC lines at these temperatures. It is furthermore evident from FIG. 2 that the work hardening effect is considerably higher at cryogenic temperatures than at temperatures above 100 C., thus cryo-stretching yields considerably better results in this regard.

(21) FIG. 3 shows values for the elongation after stretching by 6%, which appears to be fairly constant for temperatures between 50 C. and 100 C. This is of great advantage, since it demonstrates that the temperature need not be constant during stretch forming, but may vary by for example 20 C., as long as the critical temperature for cryo-stretching is not overstepped.

(22) Thus, one can summarise that the tensile properties yield strength, ultimate tensile strengths and elongation have very low temperature dependency, thus, there will be low inhomogeneous deformation when stretch forming is performed at inhomogeneous or varying temperature. Furthermore, the strain hardening increases with decreasing stretch forming temperature.

(23) The effect of total strain on various properties will be discussed with reference to FIGS. 4-6. According to FIG. 4, an increase of the total strain from 4% to 6% results in an 8% increase in Rm and a 5% increase in Rp. This difference is quite small, which is also very good, allowing the technique to be applied for commercial panels which are not stretched by the same amount at every position. According to the invention, the variation of tensile properties across the formed panel will nevertheless be small.

(24) FIG. 7-9 demonstrate the effect of strain rate on various properties. As evident from FIG. 7, the effect on strength is generally very low. Elongation seems to decrease with increasing strain rate, whereas unit propagation energy appears to be relatively unaffected by the strain rate. Thus, there appears to be no obstacle to using a high strain rate, in order to achieve a relatively high critical temperature according to FIG. 1, and which also has the advantage of a high throughput of formed panels.

(25) FIG. 10 gives a summary of various properties, comparing a low strain (4%) and low strain rate with high strain (6%) and high strain rate at a temperature of 50 C. The diagram clearly shows that all properties remain relatively constant, which is a good indication for a homogeneous distribution of properties over a formed panel which is stretched by different amounts in different positions.

(26) The invention has the additional advantage that cryo-stretching does not sensitize the material, therefore there will be no loss of corrosion resistance, see Table 3 and FIG. 11 in which the exfoliation and pitting corrosion for cryo-streched 5xxx sheet according to ASTM G-66 is compared with that of sheet stretched at +150 C. to prevent PLC lines. In Table 3, PA and PB stand for slight pitting and moderate pitting respectively, PN stands for no pitting, and EA stands for slight exfoliation. Because there is no recovery of the deformed microstructure, the strength values are retained. The strain hardening increases with decreasing stretch temperature.

(27) TABLE-US-00003 TABLE 3 Stretch Degree of Temperature Degree of Exfoliation Pitting/Pit-Blistering 50 C. EA PN +150 C. EA PB

(28) Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described.