WO1992018658A1 - Ameliorations concernant les alliages d'aluminium - Google Patents

Ameliorations concernant les alliages d'aluminium Download PDF

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Publication number
WO1992018658A1
WO1992018658A1 PCT/GB1992/000658 GB9200658W WO9218658A1 WO 1992018658 A1 WO1992018658 A1 WO 1992018658A1 GB 9200658 W GB9200658 W GB 9200658W WO 9218658 A1 WO9218658 A1 WO 9218658A1
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WIPO (PCT)
Prior art keywords
sheet
alloy
ageing
temperature
aged
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Application number
PCT/GB1992/000658
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English (en)
Inventor
Kevin Michael Gatenby
Ian Graham Palmer
Original Assignee
Alcan International Limited
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Filing date
Publication date
Application filed by Alcan International Limited filed Critical Alcan International Limited
Publication of WO1992018658A1 publication Critical patent/WO1992018658A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon

Definitions

  • This invention relates to aluminium alloys containing lithium which are particularly suitable for aerospace construction and have been found to have improved damage tolerance in regard to their fatigue crack deviation behaviour.
  • fatigue crack deviation refers to the change in mean direction of a propagating fatigue crack over a distance of several and sometimes tens of millimetres as compared with the type of microscopic crack branching which occurs across grains with a mean length of only a few tenths of a millimetre.
  • a production method is described which has the purpose of producing sheet or strip material having improved cold rolling characteristics. Specifically, there is claimed a method of producing sheet or strip material of improved cold rolling characteristics preferably with improved damage tolerance which comprises the steps of:-
  • the other grain-controlling elements are selected from hafnium, niobium, scandium, cerium, chromium, titanium and vanadium, and wherein at least one of (i) manganese, (ii) zirconium and (iii) one of the said other grain controlling elements is present,
  • step (c) annealing the said intermediate shape at a temperature sufficiently high for the intermediate shape to be softened sufficiently to be subsequently rolled, and high enough for essentially no ⁇ ' precipitate to be formed, but not so high as to form any significant amount of C phase, and for a time sufficient to precipitate any soluble constituents therein to an extent sufficient to decrease significantly the extent of work hardening needed in step (d), (d) cold rolling the annealed intermediate shape to an extent sufficient to cause an essentially fully recrystallised grain structure to be formed therein during step (e) and to produce a sheet or strip of the desired thickness, and
  • the alloy is cast, preferably by the direct chill method, and then heated at a controlled rate to a temperature sufficient to relieve internal stresses caused by the cooling from melt of the molten alloy.
  • this is generally between 300 and 500°C, preferably between 300 and 400°C. During this heating, some precipitation of at least some of the constituents held in supersaturated solid solution may occur.
  • the stress- relieved billet is heated at a controlled rate such that the low melting point phases are substantially all dissolved without melting, and the billet homogenised by holding it at a temperature and for a time sufficient to dissolve substantially all of the soluble phases.
  • the billet may then be cooled to room temperature and scalped.
  • the homogenised billet is then reheated generally to between 535 and 545°C and hot rolled, optionally with re-heating at intermediate stages, and optionally with hot widening, i.e. cross-rolling at elevated temperature, to produce an intermediate shape suitable for annealing.
  • the hot rolled metal may be heated to about 450°C in order to allow alteration of the distribution of the second phase particles to occur.
  • the hot rolled material is then annealed in order to precipitate any soluble constituents therein in order to reduce the extent of work hardening during cold rolling.
  • this is generally performed at between about 270°C and 350°C, preferably between about 270° and 325°C, and more preferably about 300°C, depending on the precise composition of the alloy used.
  • the annealing temperature should be sufficiently high for the intermediate shape to be softened sufficiently to be subsequently rolled, and high enough for essentially no ⁇ ' precipitate to be formed, but not so high as to form any significant amount of C phase.
  • the annealed material is then cold rolled to its final thickness, optionally with inter-annealing usually between 270 and 350°C, such that sufficient cold work is imparted to the sheet or strip to cause a fine re-crystallised grain structure to be formed during solution treatment.
  • the cold-rolled sheet or strip is then rapidly heated to a suitable heat-treatment temperature, preferably in a salt bath, and rapidly cooled, preferably by water quench, in order to produce a solution-treated, fully recrystallised grain structure therein.
  • a suitable heat-treatment temperature preferably in a salt bath
  • water quench preferably by water quench
  • this heat treatment can be done in two steps, the first step at a lower temperature of from about 450°C to below about 530°C in order to bring about recrystallisation and then a second step at about 530°C followed by water quench to solution treat the sheet or strip.
  • the heating step can be carried out. using a continuous heat treatment furnace, an air-recirculating furnace or by induction heating, but a salt bath is preferred.
  • recrystallisation can be performed again starting again from step 4 or from step 5.
  • the quenched sheet or strip is then if desired stretched and/or planished and then under aged, for example at about 150°C for 24 hours, to produce the finished product. Natural ageing may be possible for certain alloys depending on the particular combination of toughness and strength that is desired.
  • a method of producing a damage tolerant sheet of lithium-containing aluminium alloy having reduced susceptibility to fatigue crack deviation comprising providing a sheet of the said alloy with an unaged structure, ageing the sheet at a temperature and for a time near to, at, or over the peak aged condition for that alloy based upon its proof stress properties sufficient to increase precipitation of at least one planar slip blocking phase, heating the sheet to an elevated temperature higher than its ageing temperature and holding the sheet at that temperature for a time sufficient to facilitate significant dissolution of the ⁇ ' phase precipitates therein without significant dissolution of any planar slip blocking phase, and then cooling the sheet.
  • the heating of an aged alloy to a temperature higher than its ageing temperature for a relatively short time is generally referred to as a reversion treatment.
  • a reversion treatment The heating of an aged alloy to a temperature higher than its ageing temperature for a relatively short time.
  • planar slip is known to be encouraged by the presence of ⁇ ' precipitate and is discouraged by the presence of precipitates such as S' phase and/or other phases in the structure. It has been found that standard ageing, i.e. under ageing, treatments result in the formation of high levels of ⁇ ' precipitate before an adequate amount of the at least one planar slip blocking phase, usually S', has been formed.
  • the high levels of ⁇ ' formed by such conventional ageing are undesirable because, in addition to encouraging planar slip, this phase increases the strength and decreases the fracture toughness of the resulting alloy.
  • ⁇ ' formed by such conventional ageing
  • the reversion treatment of the present invention is therefore designed to produce a controlled amount of & while retaining the S' or similar phase in the structure. Two such similar phases are the T 1 phase and the ⁇ ' phase.
  • the present invention utilizes at least near peak under ageing, which generally means ageing to a proof strength of at least about 85% of peak aged proof strength.
  • the optimum ageing treatment will depend upon the composition of the particular alloy used.
  • the present invention is particularly applicable to control the fatigue behaviour of alloys at relatively high values of ⁇ K above the threshold region, e.g. between 10 and 50 MPa ⁇ m, especially between 15 and 35 MPa ym.
  • the present invention can be applied to any lithium-containing aluminium alloy which exhibits a ⁇ ' phase and at least one planar slip blocking phase such as S' or T ⁇
  • examples of such alloys are those containing lithium 1.7- 2.8%, copper 1.0-3.0% and magnesium 0-1.9%, in addition to other components, such as the alloys designated as AA's 8090, 8091, 2090 and 2091.
  • the reversion temperature is from 200 to the ⁇ ' solvus temperature which is about 300°C, the optimum reversion conditions depending on the composition of the particular alloy used and on its ageing treatment.
  • the reversion temperature is from 210 to 270°C and for 8090 more preferably from 220 to 250°C and most preferably about 230°C.
  • the holding time is generally from 10 seconds to 8 minutes preferably about 4 minutes.
  • the sheet is aged at between about 170 and 190°C for at least 50 hours at the lower temperature, or at least 12 hours at the higher temperature prior to the reversion treatment of the present invention, and usually the sheet is stretched prior to ageing.
  • the alloy has a recrystallised structure, and the sheet is rapidly heated to its reversion temperature and is rapidly cooled thereafter.
  • the reversion treatment is carried out after cooling from the first ageing temperature to some intermediate temperature e.g. most conveniently, although not necessarily, room temperature, optionally holding for some time at the intermediate temperature. Alternatively it could be done without cooling directly following the first ageing treatment.
  • the alloy's susceptibility to fatigue crack deviation is significantly reduced by the described reversion treatment, it has been found that for some alloys a better balance of its macroscopic properties, including tensile strength, can be obtained by subjecting the treated alloy to a further ageing step at an elevated temperature, but not so high as to allow significant precipitation to occur at the grain boundaries. Preferably this further and final ageing step is carried out at or below the first ageing temperature.
  • Such secondary ageing treatment can improve the long term stability of the mechanical properties of the alloy.
  • the method of the present invention can be applied to any sheet of a lithium-containing aluminium alloy, such as described in EP-B-0088511, EP-B-0124286, EP-A-0157711 and EP-A-0210112, it is preferred that the initial sheet be produced by a process as described in the aforementioned PCT specification. It will be appreciated that the method of the present invention is well suited to the use of a continuous heat treatment furnace, although batch treatment can be used for example in an air furnace or in salt baths.
  • a 1.6 mm sheet was prepared in the T3 condition from a cast alloy number E140, the composition of which is set out below in Table 1. It is known that this material in its standard damage tolerant temper, viz. aged for 24 hours at 150°C, is susceptible to fatigue crack deviation.
  • Tensile data was obtained at 170°C and 210°C and two peak aged conditions selected (82h at 170°C and 4h at 210°C).
  • the peak aged materials were then given a reversion treatment consisting of 1 min. at 270°C in a salt bath followed by a cold water quench.
  • the water quench was used in order to minimize both matrix and grain boundary precipitation which might occur during a slower cool. It was noted that no distortion occurred during the water quench.
  • the proof strength of the reverted material is slightly lower than the standard aged material and the tensile strength is significantly lower.
  • the NTS/YS values are about 8% higher for the reverted material and show that the toughness of the reverted material is higher than the standard aged material.
  • microstructures of the materials were examined in the TEM and the results are summarized below.
  • the matrix of this material contained uniform distributions of both fine ⁇ ' and coarse S' precipitate.
  • the grain boundaries showed narrow precipitate free zones (pfz's) and some precipitation of both I phase and phase.
  • the matrix of this material showed a very fine uniform precipitation of ⁇ ' resulting from the reversion treatment, plus some coarser undissolved ⁇ ', presumably resulting from incomplete dissolution at 270°C
  • the material therefore contained a bimodal distribution of ⁇ '.
  • the distribution of coarse S' precipitate was essentially unchanged from the distribution observed prior to reversion.
  • the grain boundaries showed essentially the same amount of precipitation as observed prior to reversion but the pfz's were much wider.
  • the material aged 4h at 210°C was not examined in detail but the reverted material showed the following features.
  • the matrix showed a bimodal distribution of ⁇ ' , as described above for the 82h 170°C + 1 min 270°C material, and a uniform distribution of S' also similar to that observed in the 170°C aged material. There was, however, a greater amount of the unidentified phase present.
  • the grain boundaries showed more precipitation than the 170°C aged material and wider pfz's.
  • the fracture toughness of the reverted material was then measured by generating R curves, as described in ASTM Standard E561-86, using 760 mm wide centre cracked panels in both L-T and T-L orientations.
  • the R curves obtained for the two orientations were almost identical, showing that the material was highly isotropic, much more so than for the standard age.
  • the R curves and hence toughness were equivalent to, or slightly better than, the established data for similar standard aged material.
  • the actual Kc numbers obtained were invalid due to net section yielding.
  • the fatigue crack growth rate was then measured using a 160 mm wide centre cracked panel, in the T-L orientation, with an R value (load ratio) of 0.385.
  • the fatigue crack ran straight; this confirmed the absence of crack deviation found in the 75 mm wide panels previously tested.
  • the fatigue crack growth rate was higher at low ⁇ K and lower at high ⁇ K compared with similar material given the standard age. The higher growth rate at low K was attributed to less planar slip. The lower growth rate at high ⁇ K was attributed to the higher toughness.
  • samples of the E140 material of Example 1 were subjected to either peak ageing (PA) for 82 hours at 170°C or near peak under ageing (UA) for 32 hours at 170°C, before being subjected to reversion treatment in a salt bath at either 270°C or 250°C for times between one and eight minutes, followed by water quenching.
  • PA peak ageing
  • U near peak under ageing
  • the transverse tensile and NTS/YS properties were then measured.
  • the double edged notched tensile samples had a gauge width of 12.7 mm and contained two 60° notches 2.12 mm deep with a root radius of 0.018 mm.
  • PA 1 293 361 9.8 1.32 0.5 321 407 8.7 1.23
  • PA 2 320 395 10.1 1.25 - - - - - PA: 4 320 390 8.3 1.22 - - - - - PA : 8 308 377 7.8 1.27 - - - - - -
  • the material was in the damage tolerant temper (24h/150°C) and the sample had a thickness of 3.2 mm. Previous work had shown that in this condition it was susceptible to fatigue crack deviation.
  • the material was additionally aged for 82 h at 170°C (as used for E140) and then reverted for 4 min. at 270°C in a salt bath followed by a cold water quench.
  • the 4 min. time was selected as a more commercially feasible time for this treatment than the 1 min. used previously.
  • Tensile properties were measured on the as-reverted material and the following data obtained:
  • the fatigue crack deviation test used in this and the other Examples was a standard centre cracked panel test in which there exists the possibility of two fatigue crack deviations per sample being formed, one from either end of the centre crack.
  • the results are presented in Table 6, and it will be noted that the reversion treatment of the present invention is effective in suppressing fatigue crack deviation where the reversion temperature was at least 230°C. Reversion at 210°C and 190°C resulted in some deviation, particularly for the lower temperature.
  • Panels 1 and 5 were subjected to a secondary ageing treatment with half of the specimens of Panel 5 being subjected to the same secondary ageing treatment as for Panel 1, i.e. 1 hour at 150°C, whilst the other half of the specimens were subjected to secondary ageing at 70oC for 672 hours.
  • the results of the centre cracked panel tests for these secondary aged specimens are set out in Table 6 and show that for the particular heat treatments used secondary ageing of the reverted material resulted in crack deviation starting to return.
  • Table 7 presents the effect of reversion temperature on the tensile properties of the alloy tested.
  • Table 9 sets out its mechanical properties under different ageing conditions and presents its transverse tensile proof stress as a percentage of that achieved by peak ageing the alloy.

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  • Chemical & Material Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Engineering & Computer Science (AREA)
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  • Metallurgy (AREA)
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Abstract

L'invention concerne des alliages d'aluminium contenant du lithium qui sont particulièrement utiles dans la construction aérospatiale et se sont avérés comme présentant une résistance améliorée à l'endommagement compte tenu de leur comportement en déviation de fissures dues à la fatigue. Pour obtenir une feuille d'alliage d'aluminium contenant du lithium qui résiste à l'endommagement et présente une susceptibilité réduite en ce qui concerne la déviation des fissures dues à la fatigue, on confère à une feuille dudit alliage une structure non vieillie, on viellit la feuille à une température et pendant le temps nécessaires pour atteindre la condition de viellissement de pointe dudit alliage, ou une condition approchant ou dépassant celle-ci, déterminée à partir de ses caractéristiques de limite apparente d'élasticité, ladite condition étant suffisante pour augmenter la précipitation d'au moins une phase de blocage de glissement des plans; on chauffe la feuille à une température élevée, supérieure à celle de sa température de vieillissement et on maintient la feuille à ladite température pendant un temps suffisamment long pour faciliter la dissolution généralisée des précipitats de la phase δ' sans dissolution importante d'une éventuelle phase de blocage de glissement des plans, après quoi on refroidit la feuille. De préférence, l'alliage présente une composition correspondant à celle spécifiée pour l'un des alliages déposés sous les numéros 8090, 8091, 2090 et 2091. La durée préférée du post-viellissement (réversion) pour le 8090 est de 4 minutes à 230 °C.
PCT/GB1992/000658 1991-04-12 1992-04-10 Ameliorations concernant les alliages d'aluminium WO1992018658A1 (fr)

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Application Number Priority Date Filing Date Title
GB919107875A GB9107875D0 (en) 1991-04-12 1991-04-12 Improvements in or relating to aluminium alloys
GB9107875.8 1991-04-12

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2002070770A1 (fr) * 2001-03-08 2002-09-12 Commonwealth Scientific And Industrial Research Organisation Traitement thermique d'alliages d'aluminium durcissables par vieillissement a l'aide d'une precipitation secondaire
US7025839B2 (en) 1999-12-23 2006-04-11 Commonwealth Scientific And Industrial Research Organization Heat treatment of age-hardenable aluminum alloys
EP3153601A1 (fr) * 2015-10-06 2017-04-12 BAE Systems PLC Production d'un objet métallique

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0273837A1 (fr) * 1986-12-02 1988-07-06 Pechiney Rhenalu Méthode de traitement thermique des alliages à base d'Al et contenant du Li et produit ainsi obtenu
US4808248A (en) * 1986-10-10 1989-02-28 Northrop Corporation Process for thermal aging of aluminum alloy plate
EP0394155A1 (fr) * 1989-04-21 1990-10-24 Pechiney Rhenalu Alliage Al-Li-Cu-Mg à bonne déformabilité à froid et bonne résistance aux dommages
EP0412204A1 (fr) * 1987-12-14 1991-02-13 Aluminum Company Of America Procédé de vieillissement en deux étapes d'un alliage d'aluminium et pièce d'usinage

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4808248A (en) * 1986-10-10 1989-02-28 Northrop Corporation Process for thermal aging of aluminum alloy plate
EP0273837A1 (fr) * 1986-12-02 1988-07-06 Pechiney Rhenalu Méthode de traitement thermique des alliages à base d'Al et contenant du Li et produit ainsi obtenu
EP0412204A1 (fr) * 1987-12-14 1991-02-13 Aluminum Company Of America Procédé de vieillissement en deux étapes d'un alliage d'aluminium et pièce d'usinage
EP0394155A1 (fr) * 1989-04-21 1990-10-24 Pechiney Rhenalu Alliage Al-Li-Cu-Mg à bonne déformabilité à froid et bonne résistance aux dommages

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
ACTA METALLURGICA, PERGAMON JOURNALS LTD vol. 35, no. 6, 1987, OXFORD, GB pages 1213 - 1219; A.K. VASUDEVAN ET AL: 'OVERVIEW NO. 58: GRAIN BOUNDARY FRACTURE IN PRECIPITATION HARDENED ALUMINIUM ALLOYS' *
TREATISE ON METERIALS SCIENCE AND TECHNOLOGY, ACADEMIC PRESS vol. 31, no. 15, 1989, NEW YORK, US pages 445 - 462; A.K. VASUDEVAN ET AL: 'FRACTURE AND FATIGUE CHARACTERISTICS IN ALUMINIUM ALLOYS' cited in the application *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7025839B2 (en) 1999-12-23 2006-04-11 Commonwealth Scientific And Industrial Research Organization Heat treatment of age-hardenable aluminum alloys
WO2002070770A1 (fr) * 2001-03-08 2002-09-12 Commonwealth Scientific And Industrial Research Organisation Traitement thermique d'alliages d'aluminium durcissables par vieillissement a l'aide d'une precipitation secondaire
AU2002233063B2 (en) * 2001-03-08 2006-03-09 Commonwealth Scientific And Industrial Research Organisation Heat treatment of age-hardenable aluminium alloys utilising secondary precipitation
US7037391B2 (en) 2001-03-08 2006-05-02 Commonwealth Scientific And Industrial Research Organization Heat treatment of age hardenable aluminium alloys utilizing secondary precipitation
EP3153601A1 (fr) * 2015-10-06 2017-04-12 BAE Systems PLC Production d'un objet métallique

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