Transformation process of Al—Cu—Li alloy sheets
11667994 · 2023-06-06
Assignee
Inventors
Cpc classification
C22F1/057
CHEMISTRY; METALLURGY
International classification
Abstract
The invention concerns a process to manufacture a flat-rolled product, notably for the aeronautic industry containing aluminum alloy, in which, notably a flattening and/or stretching is performed with a cumulated deformation of at least 0.5% and less than 3%, and a short heat-treatment is performed in which the sheet reaches a temperature between 130° C. and 170° C. for a period of 0.1 to 13 hours. The invention notably makes it possible to simplify the forming process of fuselage skins and to improve the balance between static mechanical strength properties and damage tolerance properties.
Claims
1. A process for manufacturing a flat-rolled product comprising an aluminum alloy for the aeronautic industry, said process comprising a) preparing a molten metal bath comprising aluminum, said molten metal bath comprising from 2.1% to 3.9% Cu by weight, from 0.7% to 2.0% Li by weight, from 0.1% to 1.0% Mg by weight, from 0% to 0.6% Ag by weight, from 0% to 1% Zn by weight, at most 0.20% Fe+Si by weight, at least one element selected from the group consisting of Zr, Mn, Cr, Sc, Hf and Ti, the quantity of said element, if chosen, being from 0.05% to 0.18% by weight for Zr, from 0.1% to 0.6% by weight for Mn, from 0.05% to 0.3% by weight for Cr, from 0.02% to 0.2% by weight for Sc, from 0.05% to 0.5% by weight for Hf and from 0.01% to 0.15% by weight for Ti, other elements at most 0.05% by weight each and 0.15% by weight in total, remainder aluminum; b) casting a rolling ingot from said molten metal bath; c) optionally, homogenizing said rolling ingot; d) hot rolling said rolling ingot obtained after step c) if step c) is performed or after step b) if step c) is not performed, and optionally cold rolling, into a sheet; e) solution heating and quenching said sheet; f) flattening and/or stretching said sheet obtained after step e) with a cumulated deformation of at least 0.5% and not more than 3%; g) performing short heat-treatment in which said sheet obtained after step f) reaches a temperature ranging from 130° C. to 170° C. for 0.1 to 13 hours, wherein the short heat treatment is carried out to obtain an equivalent time at 150° C. from 0.5 hour to 6 hours, wherein equivalent time t at 150° C. is defined by formula:
2. The process according to claim 1, wherein g) performing short heat-treatment in which said sheet obtained after step f) reaches a temperature ranging from 150° C. to 160° C. for 1 to 5 hours.
3. The process according to claim 1, wherein the sheet obtained after step i) is manufactured into an aircraft structural element.
4. The process according to claim 1, wherein said sheet obtained after step d) has a thickness of from 0.5 mm to 15 mm.
5. The process according to claim 1, wherein, at step f), controlled stretching is performed with permanent deformation from 0.5% to 1.5%.
6. The process according to claim 1, wherein said cold working is performed by one or more forming processes comprising drawing, stretch-forming, stamping, spinning and/or bending.
7. The process according to claim 1, wherein the molten metal bath comprises 3% to 3.5% of copper by weight.
8. The process according to claim 1, wherein the molten metal bath comprises 0.85% to 1.2% of lithium by weight.
9. The process according to claim 1, wherein the molten metal bath comprises 0.2% to 0.6% of magnesium by weight.
10. The process according to claim 1, wherein the molten metal bath comprises 0.1% to 0.5% of silver by weight and/or less than 0.4% of zinc by weight.
11. The process according to claim 1, wherein silver is present in an amount from 0.15% to 0.4% by weight and/or zinc is present in an amount of less than 0.2% by weight.
12. The process according to claim 1, wherein the molten metal bath comprises from 0.08% to 0.15% of zirconium by weight, from 0.01% to 0.10% of titanium by weight, and in which Mn, Cr, Sc and Hf are present in total in an amount that is at most 0.05% by weight.
13. The process according to claim 1, wherein, from 0 to 50 days after short heat-treatment, the sheet further comprises a combination of at least one property selected from the group consisting of: (i) R.sub.m(L) of at least 340 MPa and (ii) R.sub.m(LT) of at least 320 MPa, and at least one property selected from the group consisting of (1) R.sub.m/R.sub.p0.2 (L) at least 1.40 and (2) R.sub.m/R.sub.p0.2 (LT) at least 1.45.
14. The process according to claim 1, wherein the sheet obtained after step i) comprises a tensile yield strength R.sub.p0.2(L) at least equal to tensile yield strength by a process not comprising said short heat-treatment of step g), and a toughness K.sub.R greater by at least 5% than a toughness obtained by a process not comprising said short heat-treatment of step g).
15. A process for manufacturing a flat-rolled product comprising an aluminum alloy for the aeronautic industry, said process comprising a) preparing a molten metal bath with a composition of a AA2198 alloy; b) casting a rolling ingot from said molten metal bath; c) optionally, homogenizing said rolling ingot; d) hot rolling said rolling ingot obtained after step c) if step c) is performed or after step b) if step c) is not performed, and optionally cold rolling, into a sheet; e) solution heating and quenching said sheet; f) flattening and/or stretching said sheet obtained after step e) with a cumulated deformation of at least 0.5% and not more than 3%; g) performing short heat-treatment in which said sheet obtained after step f) reaches a temperature ranging from 130° C. to 170° C. for 0.1 to 13 hours, wherein the short heat treatment is carried out to obtain an equivalent time at 150° C. from 0.5 hour to 6 hours, wherein equivalent time t at 150° C. is defined by formula:
16. The process according to claim 15, wherein the sheet obtained after step i) is manufactured into an aircraft structural element.
17. A process for manufacturing a flat-rolled product comprising an aluminum alloy for the aeronautic industry, said process comprising a) preparing a molten metal bath with a composition of a AA2198 alloy; b) casting a rolling ingot from said molten metal bath; c) optionally, homogenizing said rolling ingot; d) hot rolling said rolling ingot obtained after step c) if step c) is performed or after step b) if step c) is not performed, and optionally cold rolling, into a sheet; e) solution heating and quenching said sheet; f) flattening and/or stretching said sheet obtained after step e) with a cumulated deformation of at least 0.5% and not more than 3%; g) performing short heat-treatment in which said sheet obtained after step f) reaches a temperature ranging from 130° C. to 170° C. for 0.1 to 13 hours, wherein the short heat treatment is carried out to obtain an equivalent time at 150° C. from 0.5 hour to 6 hours, wherein equivalent time t at 150° C. is defined by formula:
18. The process according to claim 17, wherein the sheet obtained after step i) is manufactured into an aircraft structural element.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
(3) Unless otherwise stated, all the indications concerning the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed as a percentage by weight is multiplied by 1.4. Alloys are designated in conformity with the rules of The Aluminium Association, known to those skilled in the art. The definitions of the metallurgical tempers are indicated in European standard EN 515.
(4) The static mechanical properties under stretching, in other words the ultimate tensile strength R.sub.m, the conventional yield strength at 0.2% offset (Rp.sub.0.2) and elongation at break A %, are determined by a tensile test according to standard EN ISO 6892-1, and sampling and test direction being defined by standard EN 485-1.
(5) Plane stress fracture toughness was determined from a curve of the effective stress intensity factor as a function of the crack extension, known as R-curve, is determined according to standard ASTM E 561. The critical stress intensity factor K.sub.C, in other words the intensity factor which makes the crack unstable, is calculated from R-curve. The stress intensity factor K.sub.CO is also calculated by allotting the initial crack length at the beginning of the monotonic load, at critical load. These two values are calculated for a test-specimen of the required shape. K.sub.app represents factor K.sub.CO corresponding to the test specimen that was used to carry out the test of R-curve. K.sub.eff represents factor K.sub.C corresponding to the test specimen which was used to carry out the R-curve test. Δa.sub.eff(max represents the crack extension of the last valid point of the R-curve.
(6) “Structural element” of a mechanical construction here refers to a mechanical part for which the static and/or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural analysis is usually prescribed or performed. These are typically elements for which its failure is likely to endanger the safety of said construction, its users or others. For an aircraft, these structural elements include the parts which make up the fuselage (such as the fuselage skin, stringers, bulkheads, and circumferential frames), the wings (such as the top or bottom wing skin, stringers or stiffeners, ribs and spars) and the tail unit, made up of horizontal and vertical stabilizers, as well as floor beams, seat tracks and doors.
(7) According to the invention, after rolling into sheet form, solution heat-treatment, quench and flattening and/or stretching, at least a short heat-treatment is performed with a duration and temperature such that the sheet reaches a temperature between 130° C. and 170° C. and preferably between 150° C. and 160° C. for a period of 0.1 to 13 hours, preferably 0.5 to 9 hours and preferably still from 1 to 5 hours. Typically, following this short heat-treatment, the yield strength R.sub.p0.2 decreases significantly, i.e. by at least 20 MPa or more, while the elongation A % increases, i.e. is multiplied by a factor of at least 1.1, or by even at least 1.2 or even 1.3 in relation to the temper obtained without short heat-treatment, typically T3 or T4. The short heat treatment is not an artificial aging for obtaining a T8 temper but a special heat treatment that provides a non-standardized temper particularly suitable for forming. In fact, a sheet in T8 temper has a yield strength greater than that of a T3 or T4 temper, whereas after the short heat treatment according to the invention the yield strength is on the contrary lower than that of a T3 or T4 temper. Advantageously, the short heat-treatment is carried out to obtain an equivalent time at 150° C. from 0.5 h to 6 h and preferably from 1 h to 4 h and preferentially from 1 h to 3 h, the equivalent time t.sub.i at 150° C. is defined by the formula:
(8)
where T (in Kelvin) is the instantaneous treatment temperature of the metal, which changes with time t (in hours), and T.sub.ref is the reference temperature set at 423 K. t.sub.i is expressed in hours. The constant Q/R=16,400 K is derived from the activation energy of the diffusion of Cu, for which the value Q=136,100 J/mol was used.
(9) Surprisingly, the present inventors noted that the mechanical properties obtained following short heat-treatment are stable over time, which allows the sheets to be used in a temper obtained from short heat-treatment instead of a sheet with on O-temper or a W-temper for the forming process.
(10) The present inventors were surprised to note that the short heat treatment not only simplified the manufacturing process of the products by doing away with the forming process on temper O or W, but in addition, the balance between mechanical resistance and damage tolerance is at least identical or even improved owing to the process of the invention, in an aged temper compared to a process not including short heat treatment. In particular for an additional cold working of at least 5% after the short heat treatment, the balance between static mechanical strength and toughness is improved relative to the prior art.
(11) The advantage of the process according to the invention is obtained for products having a copper content between 2.1% and 3.9% by weight. In an advantageous embodiment of the invention, the copper content is at least 2.8% or 3% by weight. A maximum copper content of 3.5% or 3.7% by weight is preferred.
(12) The lithium content lies between 0.7% or 0.8% and 2.0% by weight. Advantageously, the lithium content is at least 0.85% by weight. A maximum lithium content of 1.6% or even 1.2% by weight is preferred.
(13) The magnesium content lies between 0.1% and 1.0% by weight. Preferably, the magnesium content is at least 0.2% or even 0.25% by weight. In one embodiment of the invention, the maximum magnesium content is 0.6% by weight.
(14) The silver content lies between 0% and 0.6% by weight. In an advantageous embodiment of the invention, the silver content is between 0.1% and 0.5% by weight and preferably between 0.15% and 0.4% by weight. The addition of silver helps to improve the balance of the mechanical properties of the products obtained by the process according to the invention.
(15) The zinc content lies between 0% and 1% by weight. Zinc is generally an undesirable impurity, in particular owing to its contribution to the density of the alloy. However, in certain cases, zinc can be used alone or in combination with silver. Preferably, the zinc content is lower than 0.40% by weight, preferably 0.2% by weight. In one embodiment of the invention, the zinc content is less than 0.04% by weight.
(16) The alloy also contains at least one element which may contribute to the control of grain size chosen from Zr, Mn, Cr, Sc, Hf and Ti, the quantity of the element, if it is chosen, being from 0.05% to 0.18% by weight for Zr , 0.1% to 0.6% by weight for Mn, 0.05% to 0.3% by weight for Cr, 0.02% to 0.2% by weight for Sc, 0.05% to 0.5% by weight for Hf and 0.01% to 0.15% by weight for Ti. Preferably, it is chosen to add between 0.08% and 0.15% by weight of zirconium and between 0.01% and 0.10% by weight of titanium and to limit the Mn, Cr, Sc and Hf content to 0.05% by weight maximum, as these elements can have a detrimental effect, particularly on density and are added only to further help obtain a primarily non-recrystallized structure, if necessary.
(17) In an advantageous embodiment of the invention, the zirconium content is at least 0.11% by weight.
(18) In another advantageous embodiment of the invention, the manganese content is between 0.2% and 0.4% by weight and the zirconium content is less than 0.04% by weight.
(19) The sum of the iron content and the silicon content is at the most 0.20% by weight. Preferably, the iron and silicon contents are each at the most 0.08% by weight. In an advantageous embodiment of the invention the iron and silicon contents are at the most 0.06% and 0.04% by weight respectively. A controlled and limited iron and silicon content helps to improve the balance between mechanical strength and damage tolerance.
(20) The other elements have a content of at most 0.05% by weight each and 0.15% by weight in total, this concerns inevitable impurities, the remainder is aluminum.
(21) The manufacturing process according to the invention includes the stages of preparing, casting, rolling, solution heat-treatment, quenching, flattening and/or stretching and short heat-treatment.
(22) In the first stage, a molten metal bath is prepared in order to obtain an aluminum alloy composed according to the invention.
(23) The molten metal bath is then cast in the form of a rolling ingot.
(24) The rolling ingot can optionally be homogenized in order to reach a temperature ranging between 450° C. and 550° C. and preferably between 480° C. and 530° C. for a length of time ranging between 5 hours and 60 hours. The homogenization treatment can be carried out in one or more stages.
(25) The rolling ingot is then hot rolled, and optionally cold rolled, into a sheet. Advantageously, said sheet is between 0.5 mm and 15 mm thick and preferably between 1 mm and 8 mm thick.
(26) The product so obtained is then solution treated, typically by heat treatment making it possible to reach a temperature ranging between 490° C. and 530° C. for 15 min. to 8 hours, then quenched typically with water at room temperature or preferably with cold water.
(27) Said sheet then undergoes flattening and/or stretching with a cumulated deformation of at least 0.5% and less than 3%. When flattening is carried out, the deformation obtained during the flattening operation is not always known precisely although it is estimated at approximately 0.5%. When it is carried out, controlled stretching is performed with permanent deformation between 0.5% and 2.5% and preferably ranging from 0.5% to 1.5%. The combination between controlled stretching with preferred permanent deformation and a short heat-treatment allows optimal results to be expected in terms of formability and mechanical properties, notably when additional forming and aging are carried out.
(28) The product then undergoes a short heat treatment, already described.
(29) The sheet obtained by a process according to the invention preferably has, between 0 and 50 days and preferably between 0 and 200 days after short heat-treatment, a combination of at least one property chosen among R.sub.p0.2(L) of at least 220 MPa and preferably of at least 250 MPa, R.sub.p0.2(LT) of at least 200 MPa and preferably at least 230 MPa, R.sub.m(L) of at least 340 MPa and preferably at least 380 MPa, R.sub.m(LT) of at least 320 MPa and preferably at least 360 MPa with a property chosen among A % (L) at least 14% and preferably at least 15%, A % (LT) at least 24% and preferably at least 26%, R.sub.m/R.sub.p0.2 (L) at least 1.40 and preferably at least 1.45, R.sub.m/R.sub.p0.2 (LT) at least 1.45 and preferably at least 1.50.
(30) In an advantageous embodiment of the invention, after the short heat treatment, a sheet obtained by the method according to the invention has a ratio R.sub.m/R.sub.p0.2 in the LT direction of at least 1.52 or 1.53.
(31) Advantageously, between 0 and 50 days and most preferably between 0 and 200 days after the short heat treatment, the sheet obtained by the process according to the invention has a yield strength R.sub.p0.2 (L) of less than 290 MPa and preferably less than 280 MPa and R.sub.p0.2 (LT) of less than 270 MPa and preferably less than 260 MPa.
(32) Following short heat-treatment, the sheet is thus ready for additional cold working, notably a 3-dimensional forming operation. An advantage of the invention is that this additional cold working operation may reach values of 6% to 8% or even 10% locally or in a generalized manner. In order to attain sufficient mechanical properties at the completion of an artificial aging to a T8 temper, a minimum cumulated deformation of 2% between said additional deformation and the cumulated deformation by flattening and/or by controlled stretching performed before the short heat-treatment is advantageous. Preferably, the additional cold working is locally or in a generalized manner at least 1%, preferably at least 4% and preferably still at least 6%.
(33) Aging is performed in which said sheet reaches a temperature ranging between 130° C. and 170° C. and preferably between 150° C. and 160° C. for 5 to 100 hours and preferably from 10 to 70 hours. Aging may be performed in one or more stages.
(34) Advantageously, cold working is carried out by one or several forming processes such as drawing, stretch-forming, stamping, spinning or bending. In an advantageous embodiment of the invention, forming takes place in three dimensions to obtain a part of complex shape, preferably by stretch-forming.
(35) The product thus obtained through short heat-treatment can be formed as an O-temper product or a W-temper product. However, compared to an O-temper product, it has the advantage of no longer requiring solution heat-treatment or quenching to attain the final mechanical properties, as simple aging is sufficient. Compared to a W-temper product, it has the advantage of being stable, and does not, require a cold room and does not pose problems associated with deformation from this temper. The product also has the advantage of generally not generating unacceptable Lüders lines during forming. The short heat-treatment can thus be performed on the sheet manufacturer's premises and forming can take place on premises of the aeronautic structure manufacturer, directly on the product delivered.
(36) Surprisingly, the balance between the static mechanical properties and the damage tolerance properties obtained following aging is advantageous compared to that obtained by a similar treatment not comprising a short heat-treatment. The inventors noted in particular that the mechanical strength, particularly the tensile yield strength R.sub.p0.2 (L) is high and increases with the additional deformation but that contrary to their expectations, the toughness measured by the R curve (values of K.sub.R) does not decrease significantly, notably for a crack extension value of 60 mm when the additional deformation increases, even up to a generalized deformation of 8%. Advantageously, the product obtainable by the process, comprising the additional deformation and aging steps, has a tensile yield strength R.sub.p0.2(L) at least essentially equal to a toughness K.sub.R greater, preferably by at least 5%, than that obtained by a similar process not comprising a short heat-treatment. Typically, the tensile yield strength R.sub.p0.2 (L) is at least equal to 90% or preferably 95% of that obtained by a similar process not comprising a short heat-treatment.
(37) The method according to the invention allows to obtain in particular a AA2198 alloy sheet with a thickness of between 0.5 and 15 mm and preferably between 1 and 8 mm having, after artificial aging to a T8 temper, a combination of at least one static mechanical property selected from R.sub.p0.2 (L) of at least 500 MPa and preferably of at least 510 MPa and/or R.sub.p0.2 (LT) of at least 480 MPa and preferably at least 490 MPa, and at least one toughness property measured on CCT760 (2ao=253 mm) test specimens selected from K.sub.app in the T-L direction at least 160 MPa√{square root over (m)} and preferably of at least 170 MPa√{square root over (m)} and/or Keff in the T-L direction at least 200 MPa√{square root over (m)} and preferably of at least 220 MPa√{square root over (m)} and/or Δaeff(max) in the T-L direction of at least 40 mm and preferably at least 50 mm.
(38) Thus, the products obtainable by the process according to the invention are particularly advantageous.
(39) The use of a product obtainable by the process according to the invention comprising the steps of short heat-treatment, cold working and aging for the manufacture of an aircraft structural element, notably fuselage skin, is particularly advantageous.
Example 1
(40) A rolling ingot made of AA2198 alloy was homogenized then hot-rolled to a thickness of 4 mm. The sheets obtained in this manner were solution heat treated for 30 minutes at 505° C., then water quenched.
(41) The sheets were then elongated in a controlled manner. The controlled stretching was carried out with permanent elongation of 2.2%.
(42) The sheets were then subjected to short heat-treatment of 2 hours at 150° C.
(43) The mechanical properties were measured prior to the short heat-treatment and between two and sixty-five days after the treatment. The results are given in Table 1. It is noted that the temper obtained after short heat-treatment is remarkably stable over time.
(44) TABLE-US-00001 TABLE 1 Rm R.sub.p0.2 E % Rm R.sub.p0.2 E % (L) (L) (L) (LT) (LT) (LT) Before short heat- 438 323 13 404 287 23 treatment Duration after short heat- treatment (days) 2 396 270 16.8 370 244 27.1 8 396 269 15.3 372 247 28.0 15 398 273 14.5 374 248 27.2 43 397 270 14.9 375 248 27.5 65 398 271 15.0 373 250 27.2 104 398 273 14.3 373 250 26.9 203 401 277 16.1 375 253 26.9 239 402 278 16.7 376 255 27.7
Example 2
(45) A rolling ingot made of AA2198 alloy was homogenized then hot-rolled to a thickness of 4 mm. The sheets obtained in this manner were solution heat treated for 30 minutes at 505° C., then water quenched.
(46) The sheets were then flattened and stretched in a controlled manner. The controlled stretching was carried out with permanent elongation of 1%.
(47) The sheets were then subjected to short heat-treatment of 2 hours at 150° C.
(48) The sheets thus obtained then undergo additional cold working by controlled stretching with permanent elongation of 2.5%, 4% or 8%. After deformation, the sheets showed no unacceptable Lüders lines.
(49) The sheets were subjected to an aging treatment at 155° C. for 12 hours to obtain a T8 temper.
(50) For reference a sheet was, directly after quench, stretched 2% and aged 14h at 155° C. to a T8 temper, without intermediate short heat treatment.
(51) The static mechanical properties were characterized following the aging treatment and are presented in table 2 below: samples #1, #2 and #3 are according to the invention and sample #4 is a reference.
(52) TABLE-US-00002 TABLE 2 Static mechanical properties (MPa) Additional cold work after short heat- Rm R.sub.p0.2 E % Rm R.sub.p0.2 E % Sample treatment (L) (L) (L) (LT) (LT) (LT) #1 2.5% 511 474 11.0 499 464 11.0 #2 4% 526 499 10.4 513 485 10.4 #3 8% 541 518 9.7 516 491 9.7 #4 No short heat 497 454 10.2 486 440 12.7 treatment
(53) The R curves were measured in the T-L direction according to standard E561-05 on the CCT760 test samples, which had a length of 760 mm L. The initial crack length was 2ao=253 mm. The R curves obtained are presented in
(54) Plane stress fracture toughness results are provided in Table 3. It is noted in particular that even for a further deformation of 8%, the values of K.sub.app and K.sub.eff are high. Thus the decrease of K.sub.app in the T-L direction is low, less than 5%, between 2.5% and 8% stretch.
(55) TABLE-US-00003 TABLE 3 Additional cold K.sub.app K.sub.eff work after short (MPa√{square root over (m)}) (MPa√{square root over (m)}) Sample heat-treatment T-L T-L Δa.sub.eff max (mm) # 1 2.5%.sup. 182 262 79 # 2 4% 177 265 97 # 3 8% 174 238 68 # 4 No short heat 190 274 60 treatment
(56) It is noted that even after additional deformation of 8%, the R-curve remains quite satisfactory: the curve is sufficiently long, in excess of 60 mm, and the values of K.sub.R are near those obtained with lesser deformation (
Example 3
(57) In this example the conditions of time and temperature of the short heat treatment were studied. A rolling ingot made of alloy AA2198 was homogenized and then hot rolled to 4 mm thickness. The sheets obtained in this manner were solution heat treated for 30 minutes at 505° C., then water quenched.
(58) The sheets were then flattened and stretched in a controlled manner. The controlled stretching was carried out with permanent elongation of 1%.
(59) The plates were naturally aged to reach stable T3 temper.
(60) The plates were then subjected to a short heat treatment at 145° C., 150° C. or 155° C. The equivalent time at 150° C. was calculated by taking into account a temperature rise rate of 20° C./h. The static mechanical properties of the sheets were characterized after short heat treatment in the TL direction.
(61) The results are presented in Table 4 below and shown graphically in
(62) TABLE-US-00004 TABLE 4 Short heat Short heat treatment Equivalent treatment time temperature time t.sub.i at Rp.sub.0,2 TL Rm TL Rm/Rp.sub.0,2 (h) (° C.) 150° C. (MPa) (MPa) E TL (%) (TL) 0 0 0 288.0 407.3 22.6 1.41 2.5 145 1.90 245.7 371.7 29.1 1.51 5 145 3.47 251.3 373.7 27.6 1.49 7 145 4.73 264.3 378.7 27.7 1.43 10 145 6.62 283.3 386.3 25.9 1.36 0.5 150 1.02 240.3 369.3 25.9 1.54 1 150 1.52 237.3 366.0 26.1 1.54 2 150 2.52 240.3 369.3 27.6 1.54 3 150 3.52 246.7 369.3 28.1 1.50 4 150 4.52 253.0 373.3 26.3 1.48 5 150 5.52 259.3 376.7 27.9 1.45 6 150 6.52 264.7 375.7 26.5 1.42 0.5 155 1.63 235.0 364.0 28.1 1.55 1 155 2.41 238.3 367.7 26.4 1.54 2 155 3.98 246.7 369.3 29.2 1.50 3 155 5.55 262.0 380.7 24.8 1.45 4 155 7.12 275.3 382.3 25.5 1.39 5 155 8.70 295.3 392.0 25.1 1.33
Example 4
(63) In this comparative example, the effect of strain rate on toughness in a process not involving short heat treatment was studied. A rolling ingot alloy AA2198 was homogenized and then hot rolled to 3.2 mm thickness. The sheets obtained in this manner were solution heat treated for 30 minutes at 505° C., then water quenched.
(64) The sheets were then flattened and stretched in a controlled manner. The controlled stretching was carried out with permanent elongation of 3% or 5%.
(65) The plates were then subjected aged 14 h at 155° C. to reach a T8 temper.
(66) Mechanical properties were characterized after aging and are presented in Table 5 below.
(67) TABLE-US-00005 TABLE 5 R.sub.p0,2 R.sub.p0,2 E % Sample Strech Rm (L) (L) E % (L) Rm (LT) (LT) (LT) #5 - 3% 3% 525 486 11.1 499 459 14.1 #6 - 5% 5% 545 519 10.4 518 487 14.0
(68) R-curves were measured according to standard E561-05 test on CCT760 test samples, which had a width of 760 mm, in the direction of T-L and L-T directions. The initial crack length was 2ao=253 mm.
(69) Toughness results obtained are presented in Table 6. It is noted in particular that the decrease in K.sub.app in the T-L direction is significant, about 9%, between 3% and 5% stretch.
(70) TABLE-US-00006 TABLE 6 T-L L-T Thickness K.sub.app K.sub.eff Δa.sub.eff max K.sub.app K.sub.eff Δa.sub.eff max Sample [mm] (MPa√m) (MPa√m) (mm) (MPa√m) (MPa√m) (mm) #5 - 3% 3.2 mm 151 178 61 124 152 115 #6 - 5% 3.2 mm 138 174 67 119 142 55