WO2018025275A1 - Procédé de vieillissement thermique à quatre étapes pour l'amélioration de la résistance à la fissuration influencée par l'environnement d'alliages d'aluminium de série 7xxx - Google Patents

Procédé de vieillissement thermique à quatre étapes pour l'amélioration de la résistance à la fissuration influencée par l'environnement d'alliages d'aluminium de série 7xxx Download PDF

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Publication number
WO2018025275A1
WO2018025275A1 PCT/IN2016/000280 IN2016000280W WO2018025275A1 WO 2018025275 A1 WO2018025275 A1 WO 2018025275A1 IN 2016000280 W IN2016000280 W IN 2016000280W WO 2018025275 A1 WO2018025275 A1 WO 2018025275A1
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Prior art keywords
alloys
samples
temperature
nacl
aluminium alloys
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PCT/IN2016/000280
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English (en)
Inventor
Ajay Krishnan
Srinivasan Raja Vngaranahalli
Surendra Muralidhar Vaidya
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Indian Institute Of Technology, Bombay
Godrej & Boyce Mfg. Co. Ltd.
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Application filed by Indian Institute Of Technology, Bombay, Godrej & Boyce Mfg. Co. Ltd. filed Critical Indian Institute Of Technology, Bombay
Priority to US15/563,393 priority Critical patent/US20190153578A1/en
Priority to EP16831924.2A priority patent/EP3294918B8/fr
Publication of WO2018025275A1 publication Critical patent/WO2018025275A1/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
    • C22F1/053—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 of alloys with zinc as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/10—Alloys based on aluminium with zinc as the next major constituent
    • 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
    • C22F1/002—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working by rapid cooling or quenching; cooling agents used therefor

Definitions

  • This invention relates to a four-step thermal aging method for improving environmentally assisted cracking resistance of 7xxx series aluminium alloys.
  • Aluminium alloys find extensive use across a wide range of applications from structures to household appliances. Low cost, light weight, versatility and recyclability are some of the main advantages of aluminium alloys. Various grades of aluminium alloys are available depending mainly on their end use and requirement. AA 7xxx or AA 7000 series aluminium alloys such as AA 7010, AA 7050, and AA 7085 are characterized by their high strength and high environmentally assisted cracking resistance and are extensively used for structural applications in aerospace and armor industries.
  • SCC Stress corrosion cracking
  • HE hydrogen embrittlement
  • EAC environmentally assisted cracking
  • the 7xxx series alloys are subjected to various thermal aging treatments in order to enhance their strength and EAC resistance.
  • T C Tsai T H Chuang, Atmospheric stress corrosion cracking of a superplastic 7475 aluminum alloy, Metall Mater Trans A 27 (1996) 2617-2627;
  • Solutionizing of the alloys is a pretreatment for thermal aging.
  • the precipitates that evolve will impede the dislocation movement in the material and impart strength to the alloys.
  • the precipitates that evolve in the grain boundary of the alloys impart EAC resistance to the alloys.
  • the grain boundary precipitates are mainly Mg (Zn,Al,Cu) 2 complexes which are highly anodic in nature. Since Mg is a very active anodic element, the grain boundary precipitates can undergo selective dissolution in the corrosive environments adversely affecting the grain boundary cohesive strength of the alloys. In the presence of stresses, such dissolution can lead to intergranular brittle fracture of the alloys.
  • the rate of intergranular stress corrosion cracking is higher if such anodic precipitates are located continuously at grain boundary.
  • the various thermal aging treatments known and reported in the prior art as stated above do not impart sufficient strength and EAC resistance to the alloys for high strength applications, especially in the aerospace and armor industries and it is still desirable to have aluminium alloys having improved strength and EAC resistance for such applications.
  • a four-step thermal aging method for improving environmentally assisted cracking resistance of 7xxx series aluminium alloys comprising solutionizing and aging 7xxx series aluminium alloys, wherein the aging is carried out by treating the alloys at a temperature of 120 to 130°C for 0.3 to 0.5h, water quenching the alloys to a temperature of 25 to 27°C and further treating the alloys at a temperature of 80 to 95°C for 100 to 120h and at a temperature of 120 to 130°C for 20 to 24h and at a temperature of 155 to 160°C for 1 to 5h and air cooling the alloys to room temperature, sequentially.
  • the four-stage thermal aging of the aluminium alloys employing temperature cycles sequentially as described above according to the invention assist in bringing about novel micro structural changes in the alloys so as to improve the EAC resistance of the alloys. It is quite evident from the experimental studies that follow that according to the method of invention more copper is diffused into the grain boundary of the alloys and the distance between the grain boundary precipitates is increased.
  • the copper enrichment and the discontinuously located grain boundary precipitates produce microstructures that significantly reduce the anodic dissolution of the grain boundary precipitates and increase the EAC resistance of the alloys in corrosive environments (especially in 3.5wt.% NaCl).
  • the aging cycles also aid to nucleate more precipitates at the grain matrix and increase the strength levels of the alloys.
  • the thermally aged aluminium alloys of the invention are ideal for high strength applications, especially in the aerospace and armor industries because of their excellent EAC resistance and higher strength. However, they can be used in any other applications also.
  • the experiments were conducted on dog-bone shaped tensile specimens of AA 7010, AA 7050 and AA 7085 prepared according to ASTM E8. Prior to thermal aging, the alloy samples were solutionzied at 460°C and water quenched in known manner. The thermal aging of the samples was carried out in a laboratory scale oil bath furnace fitted with temperature control of accuracy ⁇ 2°C. Slow strain rate tests (SSRT) were carried out at a strain rate of 10 "6 s "1 and 10 "7 s " 1 . Gauge length of the specimens was continuously exposed to 3.5 wt% NaCl that was freely exposed to air, till failure. Each experiment was at least triplicated. The electrical conductivity of the heat treated samples were measured using an EDDY current type conductivity meter.
  • EAC susceptibility of the samples was evaluated based on elongation and ultimate tensile strength (UTS) of the samples measured in air (considered as inert) and in corrosive environment (3.5 wt% NaCl freely exposed to air).
  • Fractured samples from SSRT were ultrasonicated in acetone immediately after the failure and were dried and examined under a scanning electron microscope (SEM) to determine the mode of fracture. Ductility (% elongation) of the samples was calculated by measuring the gauge length of the failed samples manually. Microstructures of the specimens were examined in a field emission transmission electron microscope (FEG TEM) operating at 200 kV.
  • FEG TEM field emission transmission electron microscope
  • Table 4 shows that the peak aged samples showed an elongation of about 7.5% in 3.5 wt% NaCl against about 13.7% in air.
  • the drop of elongation in NaCl was about 6.2%.
  • the over aged samples showed an elongation of about 12% in 3.5 wt% NaCl against about 13.6% in air.
  • the drop of elongation in NaCl was about 1.6% compared to air.
  • the samples treated according to the invention showed an elongation of about 14% in 3.5 wt% NaCl against about 15% in air.
  • the drop of elongation in NaCl was only about 1 % compared to air.
  • the peak aged samples reached a UTS (ultimate tensile strength) value of about 577 MPa in air compared to about 548 MPa in 3.5wt% NaCl.
  • the drop in strength in 3.5wt% NaCl was about 69 MPa.
  • the over aged samples reached a UTS value of about 539 MPa in air compared to about 530 MPa in 3.5wt% NaCl.
  • the drop in strength in 3.5wt% NaCl was about 9 MPa.
  • the samples according to the invention showed an increased UTS value upto about 580MPa in air and upto about 569 MPs in NaCl.
  • the drop of UTS value in NaCl was about 1 1 MPa
  • the increase in the elongation and UTS values of the samples treated according to the invention are significant and substantial in the overall structural strength and EAC resistance of the 7xxx series aluminium alloys in 3.5 wt% NaCl.
  • the increase or improvement in the elongation and UTS values are indicative of the changes in the microstructures of the samples treated according to the invention.
  • the graph in Fig 1 of the drawings is a representation of stress against % elongation of the alloys in 3.5 wt% NaCl.
  • the graph in Fig 1 supports the findings in Table 4.
  • Table 5 shows that the alloy samples treated according to the invention had a lower electrical conductivity as compared to OA samples but had improved EAC resistance as shown by Table 4 above.
  • Fractographs or fracture surface maps taken at the edges of the samples treated as per PA and OA temper conditions and according to the invention and failed in 3.5 wt% NaCl were as shown in Figs 2a, 2b and 2c of the drawings, respectively.
  • the peak aged samples of AA7010 in Fig 2a shows extensive intergranular cleavage fracture having brittle facets indicative of stress corrosion cracking (SCC) failure.
  • SCC stress corrosion cracking
  • Table 4 supports this finding in terms of drop in ductility (% elongation) .
  • the over aged samples in Fig 2b and the samples according to the invention in Fig 2c did not show any signs of intergranular SCC failure.
  • Example 2 AA 7050 samples in as-received condition were used for the study.
  • the samples as- received were already treated according to T7451 (OA temper condition).
  • a few of the samples were further subjected to thermal aging according to the invention under the same conditions as given in Table 3 of Example 1.
  • the SSRT results for the AA 7050 samples as-received and after the thermal treatment according to the invention in air and 3.5 wt% NaCl were as shown in the Table 6 below:
  • the as-received and over aged samples showed an elongation of about 13.92 % in 3.5 wt% NaCl against about 15.25 % elongation in air.
  • the as-received and over aged samples had a UTS value of about 523 MPa in air compared to about 51 1 MPa in 3.5 wt% NaCl.
  • the samples treated as per the invention showed about 16.36% elongation in air and about 15.18% elongation in 3.5 wt% NaCl.
  • the samples treated as per the invention showed a UTS value of about 600 MPa in air and about 597 MPa in NaCl.
  • the samples treated according to the invention exhibited significant improvement in the strength and EAC resistance as compared to the over aged samples even at a strain rate of 10 "7 /s.
  • TEM micrographs of the as-received and over aged samples and samples treated according to the invention were taken in a FEG TEM and were as shown in Figs 7a and 7b of the drawings respectively.
  • the as-received and over aged samples of Fig 7a show a discontinuous network of grain boundary precipitates, whereas the treated samples of Fig 7b show a relatively coarser and more discontinuous grain boundary precipitates.
  • a graphical representation of % Cu in the grain boundary precipitates of the samples taken through FEG TEM is shown in Fig 8 of the drawings.
  • the grain boundary precipitates of the over aged alloys and alloys treated as per the invention showed 3.4 and 5.3 wt% Cu respectively.
  • the microstructural changes in the grain boundary precipitates are understood to be the reasons or causative factors for the improved EAC resistance of the treated samples.
  • AA 7085 samples in as-received condition were used for the study.
  • the samples as- received were already thermally treated according to T7651 (OA temper condition).
  • a few of the samples were further treated according to the invention under the same conditions as given in Table 3 of Example 1.
  • the SSRT results for the AA 7085 samples in as-received and over aged condition and after the thermal treatment according to the invention in 3.5 wt% NaCl were as shown in Table 8 below.
  • the as-received and over aged samples showed an elongation of about 5.5 % at a strain rate of 10 "6 s "1 , whereas the elongation dropped to about 3.3 % at a strain rate of 10 ⁇ 7 s " 1 in 3.5 wt% NaCl.
  • the samples treated as per the invention showed high strain to failure at the strain rate of 10"V and even at the strain rate of 10 "7 s "1 in 3.5 wt.% NaCl indicating that the samples treated as per the invention had significantly increased the EAC resistance.
  • the samples treated as per the invention also had significantly higher strength (MPa) values.
  • the graphical representation of stress vs % elongation of the samples in 3.5 wt.% NaCl as shown in Fig 9 of the drawings supports the finding in Table 8.
  • TEM micrographs of the as-received and over aged samples and samples treated according to the invention were taken in a FEGTEM and the images were as shown in Figs 1 l a and 1 l b of the drawings respectively.
  • the as-received and over aged samples in Fig 1 l a show a discontinuous network of grain boundary precipitates, whereas the samples treated as per the invention in Fig l i b show a relatively coarser and more discontinuous grain boundary precipitates.
  • the microstructural changes in the grain boundary precipitates are understood to be the reasons or causative factors for the improved EAC resistance of the treated samples as per invention.
  • the method of the invention is unique, innovative and inventive.
  • the method of the invention has been found to be effective in 7xxx aluminium alloys having a wide range of quench sensitivity. Therefore, it is believed that the method of the invention is applicable to at least all the 7000 series aluminium alloys, if aluminium alloys in general.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Preventing Corrosion Or Incrustation Of Metals (AREA)
  • Investigating And Analyzing Materials By Characteristic Methods (AREA)

Abstract

Cette invention concerne un procédé de vieillissement thermique à quatre étapes conçu pour améliorer la résistance à la fissuration influencée par l'environnement d'alliages d'aluminium de série 7xxx. Le vieillissement thermique est effectué par traitement des alliages à une température de 120 à 130 °C pendant 0,3 à 0,5 h, trempe à l'eau des alliages à une température de 25 à 27 °C et traitement ultérieur des alliages à une température de 80 à 95 °C pendant 100 à 120 h, et à une température de 120 à 130 °C pendant 20 à 24 h, et à une température de 155 à 160 °C pendant 1 à 5 h, et refroidissement à l'air des alliages à la température ambiante, séquentiellement.
PCT/IN2016/000280 2016-08-04 2016-11-30 Procédé de vieillissement thermique à quatre étapes pour l'amélioration de la résistance à la fissuration influencée par l'environnement d'alliages d'aluminium de série 7xxx WO2018025275A1 (fr)

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US15/563,393 US20190153578A1 (en) 2016-08-04 2016-11-30 Four-step thermal aging method for improving environmentally assisted cracking resistance of 7xxx series aluminium alloys
EP16831924.2A EP3294918B8 (fr) 2016-08-04 2016-11-30 Procédé de vieillissement thermique à quatre étapes pour l'amélioration de la résistance à la fissuration influencée par l'environnement d'alliages d'aluminium de série 7xxx

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Cited By (1)

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WO2020007437A1 (fr) * 2018-07-02 2020-01-09 Otto Fuchs - Kommanditgesellschaft Alliage d'aluminium et produit d'alliage d'aluminium survieilli obtenu à partir d'un tel alliage

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2020007437A1 (fr) * 2018-07-02 2020-01-09 Otto Fuchs - Kommanditgesellschaft Alliage d'aluminium et produit d'alliage d'aluminium survieilli obtenu à partir d'un tel alliage
CN112218963A (zh) * 2018-07-02 2021-01-12 奥托福克斯两合公司 铝合金以及由这种合金制造的过老化的铝合金产品
US11970756B2 (en) 2018-07-02 2024-04-30 Otto Fuchs Kommanditgesellschaft Aluminum alloy and overaged aluminum alloy product of such alloy

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US20190153578A1 (en) 2019-05-23
EP3294918B8 (fr) 2019-02-27
EP3294918B1 (fr) 2018-11-28

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