WO2021003528A1 - Alliages d'aluminium - Google Patents

Alliages d'aluminium Download PDF

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Publication number
WO2021003528A1
WO2021003528A1 PCT/AU2020/050708 AU2020050708W WO2021003528A1 WO 2021003528 A1 WO2021003528 A1 WO 2021003528A1 AU 2020050708 W AU2020050708 W AU 2020050708W WO 2021003528 A1 WO2021003528 A1 WO 2021003528A1
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heat
hours
alloy
temperature
process according
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PCT/AU2020/050708
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Thomas Henri DORIN
Alireza VAHID
Santu RANA
Sunil Gupta
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Deakin University
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Priority claimed from AU2019902444A external-priority patent/AU2019902444A0/en
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Publication of WO2021003528A1 publication Critical patent/WO2021003528A1/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

Definitions

  • the present invention relates generally to aluminium based alloys.
  • the invention particularly relates to high strength heat-treated aluminium alloys where the properties have been optimized by controlling the composition and heat-treatment parameters.
  • the invention further relates to processes for optimising the characteristics of the aluminium alloy through control of the manufacturing process.
  • the invention particularly relates to process where the duration of heat treatment steps and temperature are controlled to allow for precipitation of particular dispersoids and precipitates to be formed.
  • Aluminium alloys have generally been developed as they demonstrate excellent mechanical and physical properties that make them useful for many applications.
  • One particular advantage of aluminium alloys is that they are relatively low density, and as such they are useful for lightweight applications yet still demonstrating significant strength characteristics.
  • the aircraft and automotive industries utilize aluminium alloys extensively to take advantage of the light weight and strength characteristics and many alloys have been specifically developed for those purposes.
  • Alloys may be categorized into 2 principle classifications, namely cast alloys and wrought alloys.
  • Cast aluminium alloys generally have lower tensile strength than those of wrought alloys and generally have relatively high levels of silicon which contribute to good casting characteristics.
  • Wrought aluminium alloys vary in the alloying material used to provide different characteristics to the alloy.
  • the alloys are generally classified by a numbering system which has been developed by the Aluminium Alloy Association of America. This numbering system may indicate the main alloying constituents of the alloy which leads to an indication of its properties.
  • the wrought alloy numbering system includes various series from the 1xxx series through to the 8xxx series.
  • the 2xxx series for example have higher levels of copper and are precipitation hardened but have been susceptible to cracking. They have been increasingly replaced in the aircraft industry in particular by the 7xxx series which include magnesium and greater levels of zinc.
  • the 8xxxx series for example is inclusive of aluminium-lithium alloys.
  • Wrought aluminium alloys are generally used in shaping processes, and may be shaped for example through rolling, forging, extrusion, pressing or stamping.
  • There are two principle groups namely non-heat treated alloys and heat-treated alloys.
  • the initial strength for non-heat treated alloys is achieved through a hardening process of the alloying elements in solid solution such as manganese, silicon and magnesium.
  • Alloys such as the 1xxx, 4xxx and 5xxx series are examples of non-heat treated aluminium alloys.
  • Heat-treatable alloys on the other hand gain initial strength through the hardening effect of the alloying elements, for example, copper, silicon, magnesium and zinc.
  • the solubility of these elements in solid aluminium depends on the temperature, so it is possible to harden the alloys from this group by heat treatment.
  • the heated composition is allowed to cool and is aged which contributes to the hardening process through the formation of metastable precipitates.
  • the process may also be called precipitation hardening or age hardening.
  • Precipitation hardening achieves strengthening by precipitation of fine particles of a super saturated solid solution.
  • titanium forms a solidified grain structure that is finer than may be observed with other grain structures which may lead to improved fabricating capabilities.
  • Scandium is another element that has been used to strengthen aluminium alloys without increasing the density. Lithium may be included to decrease density, and maintain or improve strength.
  • the aluminium alloys may be developed to vary in relation to its tensile strength, density, ductility, formability, weldability and corrosion resistance amongst other properties.
  • the selection of alloying elements is generally selected in order to maximize the particular property of the aluminium alloy that is desired, together with the consideration of ease of manufacture, cost and durability.
  • the ratios between various properties are also a key factor as, for example, the ratio between tensile strength and density influences strength to weight capabilities.
  • US patent 7060139 to Senkov et. al. aims to develop an aluminium alloy having improved strength and ductility. It discusses the benefit of the addition of scandium and high levels of zinc which the Inventors have found can improve the strength, particularly in alloys that are to be used at cryogenic temperatures, that is temperatures around -190 oC.
  • US patent 5137606 looks at aluminium alloys that include lithium as one of the alloying minerals.
  • the alloys of this disclosure demonstrate a decrease in density and an improvement in tensile and yield strengths.
  • the alloys referred to in this patent include relatively low levels of zinc and higher levels of copper which are typical levels used in the 2xxx series of aluminium alloys. The presence of copper and zinc in those alloys at levels to achieve the desired characteristics of a 2xxx series of aluminium alloys can interfere with fracture toughness of the alloy.
  • the present invention has been developed in order to overcome or at least alleviate one or more of the difficulties associated with known practices and the prior art.
  • the present invention has been further developed to provide a method for optimizing the characteristics for heat-treated aluminium alloys.
  • the present invention is particularly applicable to aluminium alloys that have been strengthened by heat treatment.
  • the present invention resides in both a heat-treated aluminium alloy and a process for manufacturing that aluminium alloy.
  • the present invention resides in a heat-treated aluminium based alloy that is high strength and has low density characteristics that makes it suitable for use in areas where high strength but light-weight materials are needed.
  • the present invention resides in a heat-treated aluminium alloy comprising: i. up to 1 wt.% lithium;
  • the source of the aluminium may be from any suitable source. This may include fresh aluminium or recycled aluminium.
  • a suitable source of recycled aluminium may for example be recycled 7xxx series aluminium, such as that which may be available from decommissioned aircraft.
  • a preferred aluminium source would be a recycled 7050 or 7075 series aluminium which has the base elements of the alloy of the present application except for scandium, zirconium and lithium.
  • a combination of recycled and fresh aluminium may also be used.
  • the 7xxx series aluminium alloys have greater levels of zinc in comparison with the 2xxx series of aluminium alloys.
  • the 2xxx series have greater levels of copper. Both alloys have been used extensively in the aeronautical and automotive industries.
  • the alloys of the present application have greater levels of zinc and are therefore preferably aimed at recycling of 7xxx series alloys if recycled alloys are to be used, but if the final composition of elements is adjusted, the composition and process may be applied to the recycling of a range of aluminium alloys.
  • the present invention resides in a process for the manufacture of the heat-treated aluminium alloy of the present invention. Since solubility of the elements, or precipitates formed from the elements used to form the aluminium alloy depend on the temperature, it is possible to harden the alloys from this group by heat treatment which is also known as precipitation hardening during an aging process after extrusion.
  • Precipitation hardening heat treatment processes generally involve the following stages:
  • Casting The process of the present application generally includes a casting step where the elements are heated and melted at a temperature of at least 720 oC in an induction furnace. This composition may be poured into a cylindrical mould or direct chill cast into cylindrical billets cooled and then subjected to a number of heat-treatment steps together with a forming step, a solution treatment step and aging step.
  • homogenisation Accordingly, in a further embodiment of the invention, there is provided an alloy wherein the alloy is homogenised during a heat-treatment process where Al 3 Sc and Al 3 Zr dispersoids are formed.
  • a multi-step homogenisation treatment is preferred as it leads to the formation of an AI 3 Sc/AI 3 Zr core-shell dispersoids.
  • the developed alloy is primarily for extruded products but other forming processes can be used such as rolling and forging.
  • the billet may be preheated to a given temperature and then extruded at a suitable strain rate and container temperature so that the temperature does not exceed the solidus temperature when exiting the extrusion press.
  • the extruded product may be press-quenched via air/water spray or by exiting into a coolant bath.
  • Solution treatment The alloy may be heated in one or more further heat- treatment steps to a temperature to dissolve the alloying element into a super saturated solid solution.
  • the dissolved solid solution is held at a temperature which may vary from 1 hour to 24 hours to complete the dissolving.
  • One or more heat-treatment steps may be applied with comparable varying duration.
  • the temperature and the duration for which each heat-treatment step is held should be such as to not prevent excessive growth of the grains and of the dispersoids.
  • Quenching Solution treatment is normally followed by quenching generally with water, although a water and air mixture or sometimes just air may be used. This leads to a super saturation of a solid solution at room temperature.
  • the quenching phase is not necessary and results can be achieved without a quenching step, but quenching or cooling can lead to improving the hardness of the alloy and is generally employed.
  • Stretching depending on the nature of the extruded profile, stretching might be conducted after quenching in order to straighten the extrusion. This step also provides work hardening and might generate dislocations that can help nucleating the strengthening precipitates.
  • Aging Following the solution treatment and quenching, an aging process takes place. Desired precipitates form during the aging process. Aging may be either natural aging at essentially room temperature, or artificial aging where the temperature of the composition is elevated, or a combination of both.
  • the aging process in the present application is controlled such that the present application includes heating the solid composition to a desired temperature to achieve precipitation of desired precipitates.
  • a combination of the following precipitates, or corresponding precursors, is expected to form, AI 2 CuU, Al 3 Li, Al 2 Cu , Al 2 CuMg and Mg 2 Zn. These precipitates might be enhanced by preferred nucleation on the Al 3 Sc/Al 3 Zr core-shell dispersoid.
  • the aging step is optimised to provide a distribution of fine precipitates that provide most of the strengthening.
  • the invention resides in a process for producing a heat-treated aluminium alloy, said process including the steps of: i) melting the elements of the preferred composition as outlined above and cast into billets.
  • the elevated temperature for the first heat-treatment steps are preferably maintained for a period of greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 20.5 hours.
  • the elevated temperature for the second heat-treatment steps are preferably maintained for a period of greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 23.5 hours.
  • the process preferably includes a number of heat-treatment steps.
  • the composition is subject to a third heat treatment step wherein the temperature is raised to between 440oC to 490oC , preferably from 445oC to 480oC to assist homogenisation and removal of any iron and silicon impurities.
  • the elevated temperature of the third heat-treatment step is maintained for a period of greater than 6 hours, preferably from 6 to 25 hours, and more preferably 10 to 18.5 hours at the elevated temperature.
  • the process may include an extrusion step where the composition is allowed to cool to room temperature and the billets are extruded at a temperature of from 430oC to 455oC. The exit temperature should n ot exceed 490 oC.
  • the extruded billets are preferably subjected to a fourth heat-treatment step (solution treatment) wherein the temperature is then raised to between 460oC to 490oC, preferably between 470oC to 480oC to dissolve the m agnesium, manganese, zinc, copper and any lithium.
  • solution treatment solution treatment
  • the elevated temperature of the fourth heat-treatment step is maintained for a period of greater than 6 hours, preferably from 6 to 25 hours, and more preferably from 13.5 to 22 hours.
  • the composition is then allowed to cool following the further heat-treatment step and undergo an aging process.
  • the aging process preferably includes a natural aging step where the composition is left to age for a period of from 3 to 7 days, preferably 3 to 5 days. This is then followed by an artificial aging step where the temperature is raised to between 110 oC and 150 oC for a period of from 3 to 30 hours.
  • the combination of both the natural and artificial aging steps allows for the formation of metastable precipitates, and corresponding precursors, such as Al 2 CuLi, Al 3 Li, Al 2 Cu, and/or Al 2 CuMg and Mg2Zn precipitates.
  • the process may be carried out without the natural or artificial aging steps and may for example include only artificial aging.
  • the artificial aging process may be carried out as two separate steps where the composition is heated to a temperature of 110 °C to 150 oC in a first artificial aging step, followed by cooling and then raising the temperature to 120 oC to 180 oC, more preferably 140 oC to 160 oC in a second artificial aging step to further assist the precipitation of metastable precipitates.
  • Two steps can become favourable depending on the thermodynamic and kinetics of formation of the desired precipitates but generally only one artificial aging step may be used.
  • Precipitation hardening or aging is the strengthening part of precipitation of fine particles of a second phase from a super saturated solid solution.
  • Aging may however be carried out either exclusively artificially, that is by heating to a temperature to assist the aging process, or naturally at room temperature which can take from several days to even several weeks. It is however preferred to have both a natural and artificial aging step.
  • Quenching also forms part of the process in achieving the desired properties for the alloy, and a quenching step is preferably included between one or more of the heat treatment steps, and the extrusion and aging phases, but preferably between each of the heat-treatment steps and phases. Quenching is not preferred in the heat treatment step preceding extrusion.
  • the present application relates to a heat-treated aluminium alloy composition and a process for producing that alloy.
  • Aluminium alloys maybe classified in 8 main series depending upon the main alloying elements.
  • Typical alloying elements for an aluminium alloy include copper, magnesium, zinc, zirconium and scandium, each of which can provide various characteristics to the aluminium alloy.
  • aluminium alloys Most of the mechanical properties of aluminium alloys are dictated by the presence of nanometer sized precipitates.
  • the aluminium alloys contain a number of refining elements which help nucleate grains during casting and allow the aluminium to form dispersoids.
  • the dispersoids that may be formed are dependent upon the main alloying elements used.
  • composition of the present application has been developed by the control of the formation of preferably three main precipitates and dispersoids providing the desired characteristics for the alloy.
  • the applicants have found that the desired characteristics have been achieved with the formation of desired dispersoids through control of the homogenisation process and the precipitation of desired precipitates during the aging treatment.
  • one of those precipitates is inclusive of lithium however an alloy having the desired strength and properties may be achieved without lithium being present.
  • the applicants have found however that the inclusion of lithium provides for an alloy with improved strength and a decrease in the density of the alloy allowing for improved yield strength to weight ratio.
  • the 7 xxx series of aluminium alloys have been developed as strong and light-weight materials. They acquire their high strength through the formation of nanometer-size particles, known as precipitates and are generally rich in zinc and magnesium. The strength contribution from precipitates is strongly influenced by the heat-treatment process applied in developing the alloy.
  • the process of producing such alloys is a heat treatment process where the precipitates are formed during heat- treatment sequences. They acquire strength through an age-hardening process.
  • the present application has been developed such that various precipitates are formed by control and optimization of the heat treatment process.
  • the composition of the present application has been developed by optimizing the precipitates that are formed though the heat treatment process.
  • an AI 3 Sc/AI 3 Zr core-shell dispersoid is formed through the combination of Al 3 Sc and Al 3 Z particles.
  • the scandium is able to provide strength to the aluminium alloy. This is enhanced with the added combination of zirconium in the spherical dispersoids formed.
  • the AI 3 Sc/AI 3 Zr spherical dispersoids form with core-shell morphology with a scandium enriched core and a zirconium enriched shell.
  • the scandium controls the nucleation process while zirconium creates a strong shield against coarsening which leads to high dispersoid density and great thermal stability of the dispersoids.
  • a further precipitate is an AI 2 CuU or Al 3 Li particle which is preferably formed in order to incorporate lithium into the alloy. With the inclusion of this precipitate, it has been found the strength of the alloy is increased and the density is decreased which significantly improves the strength to weight ratio. Alternatively an Al 2 Cu and/or Al 2 CuMg particle may be formed without the presence of lithium. [0051 ] A further precipitate is an Mg 2 Zn particle which the applicants have found is the biggest contributor to the strength of the alloy.
  • a preferred method is to use the Bayesian optimisation technique which together with ThermoCalc enables the inventor to optimize the processing conditions including the selection of the elements and temperature and duration of each heat-treatment step to achieve a high strength alloy having the desired characteristics.
  • This technique reduces the need for trial and experimentation based on the multitude of options that are available to the inventors. Some trial and experimentation is still necessary but appropriate utilisation of the Bayesian optimisation technique has the benefit of reducing substantially the trial and experimentation requirements.
  • the first step in the process is to select the alloying elements which will be used to form the alloy. This will vary depending upon the base aluminium element to be used, for example, selection of the elements to be added is dependent upon the base aluminium product.
  • the process of this application may use a recycled aluminium product as the base aluminium such as a recycled aluminium alloy.
  • the 7xxx series aluminium alloys have appropriate zinc and magnesium levels for use in the alloy of this application. Fresh aluminium may also be used either on its own, or together with recycled aluminium alloy. Recycled 2xxx series aluminium which is rich in copper as an alloying element may also be used.
  • the applicants aim to achieve a modified 7xxx series aluminium alloy.
  • the elements however are selected based upon the properties of the final product to be achieved.
  • the inventors in the present application aim to produce a high strength aluminium alloy with reduced density so as to produce a strong lightweight product that maybe suitable for such industries as the aeronautical or automotive industries.
  • the preferred properties that are to be achieved include a yield strength of between 600 to 900 MPa, preferably between 630 to 800 MPa for the final extruded product at room temperature.
  • the alloy also has total elongation ranges from greater than 2% at room temperature, preferably from 2% to 10% at room temperature, and more preferably from 4% to 10% at room temperature.
  • the composition of the alloy includes lithium as it has been found that lithium may contribute to reducing the density of the alloy product. It is however possible that lithium is not included yet still achieve the desired characteristics of the final aluminium alloy product.
  • a heat- treated aluminium alloy having the desired properties may be achieved with the selection of elements in the desired proportions.
  • the invention resides in a heat-treated aluminium alloy comprising the following elements: i. up to 1 wt.% lithium;
  • the aluminium which forms the base of the alloy, may be sourced from a recycled aluminium alloy, preferably a 7xxx series aluminium alloy, and/or fresh aluminium and/or additional alloying elements in the form of Master alloys.
  • the heat-treated aluminium alloy preferably includes lithium as it reduces density and improves the strength of the alloy.
  • alloys that include lithium may be difficult to cast. Therefore, in a number of applications, such as for example fasteners, the alloy may not include any lithium, but may include up to 1 wt.% lithium.
  • the alloy may include from 0.03 to 0.5 wt.% lithium, or alternatively from 0,05 to 0.44 wt.% lithium, or alternatively from 0.05 to 0.3 wt.% lithium.
  • the alloy also includes zirconium and scandium which form nanometer sized particles that remain stable at high processing temperatures.
  • the zirconium and scandium particles also add significant strength to the alloy with limited impact on ductility.
  • the particles may also act as recrystallisation inhibitors. Surface recrystallisation may be an issue with high strength wrought aluminium products and the zirconium and scandium particles may assist in mitigating this issue.
  • the alloy includes from 0.05 to 0.4 wt.% zirconium, preferably from 0.1 to 0.3 wt.% zirconium, or alternatively 0.1 to 0.29 wt.%, or alternatively 0.25 to 0.29 wt.% zirconium.
  • the alloy also includes 0.02 to 0.3 wt.% scandium, preferably 0.03 to 0.2 wt.% scandium, or alternatively 0.05 to 0.15 wt.%, or alternatively 0.11 to 0.15 wt.% scandium.
  • the invention preferably resides in a heat-treated aluminium alloy comprising the following elements: i) 0.03 to 0.6 wt. % lithium, preferably 0.05 to 0.3 wt % lithium
  • the invention resides in a method for producing a heat-treated aluminium alloy by carefully controlling both the temperature and duration of each heat-treatment step depending on the element selected for the alloy.
  • the elements, including any recycled aluminium product are melted at a temperature preferably of 720oC or above so as to I iquefy and homogenise the elements. This may be conducted in an induction furnace optionally under a flux of argon. Stirring takes place utilising a magnetic field to assist homogenisation. The homogenised liquid is then poured into moulds to form billets and allowed to cool back to room temperature.
  • a preferred cooling rate may be for example 0.1 -10oC per second. Cooling may be achieved through water quenching, or natural cooling with air, or a combination of both.
  • the cooled billets are then subjected to a first heat-treatment step where the temperature is raised preferably to about 250oC to 350oC and maintained for a period of about 6 hours or more.
  • the temperature range is between 275oC to 300oC and for a duration of greater than 6 hours, more preferably between 6 to 25 hours, and more preferably 8 to 20.5 hours.
  • Al 3 Sc particles are formed.
  • the Al 3 Sc particles will precipitate as small particles in a size of from 5 to 10 nm. These particles have a very fine distribution combined with excellent stability during conventional thermo-mechanical treatment steps and provide strength for the alloy.
  • the composition is allowed to cool following this period of Al 3 Sc precipitation to form billets. Again, the cooling may be carried out by quenching with water, or air-water mix or air alone. Quenching is not however essential after this step.
  • the temperature of the billets may then be raised in a second heat-treatment step to a temperature of preferably about 420° to 470oC, preferably 430oC to 450oC and maintained at that temperature for a duration of preferably greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 23.5 hours.
  • a temperature of preferably about 420° to 470oC, preferably 430oC to 450oC and maintained at that temperature for a duration of preferably greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 23.5 hours.
  • Al 3 Zr particles form and precipitate around the scandium particles to form Al 3 Sc/Al 3 Zr dispersoids.
  • the heavier zirconium particles surround the lighter scandium particles to form a core-shell structure of a size of approximately 5-25 nm. The benefit of this is that zirconium and scandium particles are very stable particularly at higher processing temperatures.
  • the Al 3 Zr shell makes the dispersoids extremely stable and prevents
  • the temperature of the billets is then raised to preferably around 440oC to 490oC, more preferably 4 45oC to 480oC and maintained preferably for a duration of greater than 6 hours, more preferably 6 to 25 hours, and more preferably 8 to 23.5 hours.
  • the raising of the temperature may act as a continuum rather than quenching the composition between each heat-treatment step.
  • the composition is inclusive of up to 0.25 wt.% manganese. At these temperatures in the third heat-treatment step, the manganese is able to trap impurities such as iron and silicon which assists in the ability to remove these impurities from the subsequently formed alloy.
  • the composition is then preferably allowed to slowly cool to room temperature without quenching. Slow cooling is preferable for this step to allow for a homogenised composition free from residual stress, that will be easier to extrude.
  • the solid composition then undergoes an extrusion phase where the billet is heated to a temperature of preferably around 430° to 455oC. At this temperature, the scandium and zirconium dispersoids are not subjected to change.
  • the composition may then be extruded at a rate preferably of between 0.5 to 2 mm per second, preferably about 1 mm per second to form profiles with the AI 3 Sc/AI 3 Zr dispersoids evenly spread. These extruded profiles are cooled down to room temperature and can be quenched at the exit of the extrusion press. However, quenching is not a requirement.
  • the profile may be stretched in order to straighten the extrusion. This may be conducted after quenching if quenching has taken place. Stretching provides work hardening and may generate dislocations that can assist in nucleating the strengthening precipitates.
  • the extruded profiles then undergo a fourth heat-treatment step where they are treated to a temperature of preferably around 460° to 490oC, more preferably 470 oC to 480 oC.
  • the duration of the four th heat-treatment step is greater than 6 hours, more preferably from 6 to 25 hours, and more preferably 8 to 22 hours.
  • magnesium, manganese, zinc, copper and any lithium elements will dissolve and undergo continued solutionisation.
  • the composition is then preferably allowed to cool rapidly back to room temperature through a water quenching process.
  • recrystallisation should be avoided during this step and the texture should remain fibrous.
  • the applicants have found that the presence of the Al 3 Sc/Al 3 Zr dispersoids helps to prevent the development of peripheral coarse grains at the profiles surfaces, which has the potential to increase the fracture strength, even more in thin walled extrusions.
  • the profiles then undergo an aging process.
  • the aging process may be a natural aging process or an artificial aging process or a combination of both.
  • the aging process itself assists in strengthening the aluminium alloy and allows for formation of desired precipitates with appropriate control of the aging process. Maximum strength may be achieved when the precipitates reach a high number density and volume fraction and remain small and semi-coherent.
  • the semi-coherent particles will assist in preventing the dislocation movement and hence strengthen the alloy. Highest strengthening is obtained when the precipitates are small and closely spaced.
  • the process of the present application preferably includes a natural aging phase where the composition is allowed to age for a period of greater than 3 days, preferably from 3 to 7 days, more preferably 3 to 5 days.
  • the process then preferably includes an artificial aging phase where the temperature of the composition is raised in a fifth heat-treatment step.
  • the temperature of the artificial aging step is raised to a temperature of for example, 110oC to 150 oC, more p referably 120oC to 140 oC.
  • the composition may be allowed to age for greater than 3 hours, preferably for 3 to 15 hours, up to a day. Higher temperatures through this aging process may decrease the strength of the alloys and the temperature should be selected to provide a compromise between high strength and the shorter aging time.
  • the artificial aging process may be conducted instantaneously following quenching from the previous heat-treatment step.
  • the following precipitates, or precursors may form, Al 2 CuLi, Al 3 Li or Al 2 Cu and/or Al 2 CuMg in the absence of lithium, together with Mg 2 Zn.
  • These precipitates may nucleate on the zirconium / scandium core-shell dispersoids resulting in smaller and higher number density of precipitates.
  • a preferred natural aging time is for greater than 3 days, preferably from 3 to 7 days or longer, and more preferably 3 to 5 days, while the artificial aging step may be from greater than 3 hours, more preferably from 3 to 30 hours and more preferably 8 to 21 hours.
  • the composition may be allowed to cool following the artificial aging step, and then additionally subjected to a second artificial aging step or a sixth heat-treatment step where the temperature is again raised to about 120 oC to 180 oC, more preferably 140oC to 160 oC having comparable duration conditions to the first artificial aging step. This sixth step may assist in further Al 2 CuLi, Al 3 Li, Al 2 Cu, Al 2 CuMg and/or Mg 2 Zn precipitation.
  • the composition is preferably cooled or quenched between each heat- treatment step and the extrusion and aging phases, although it is preferred to allow the composition to naturally cool prior to the extrusion step.
  • the cooling or quenching may be achieved with the application of water, air or a combination of water and air. Similar results may however be achieved by a continuum in elevating the temperature conditions, or not allowing complete cooling back to room temperature, but the applicants have found that if the composition is allowed to cool to room temperature between each heat-treatment step and/or the extrusion and aging phases, that greater stability is achieved with the formation of the dispersoids and particles leading to greater stability of the alloy.
  • a preferred method for assisting in the calculation of the preferred compositional elements, the desired temperature and duration of each of the heat- treatment phases, the rate of cooling and aging profile may be calculated using Bayesian Optimisation together with ThermoCalc.
  • This method allows for calculation of the desired characteristics based on assessing anticipated characteristics of the desired alloy For example, utilising this method allowed for calculations to be performed to maximize the different heat-treatment and hardening phases of the preferred AI 3 Sc/AI 3 Zr, AI 2 CuLi, Al 3 Li or Al 2 Cu and/or Al 2 CuMg and Mg 2 Zn precipitates with careful control of the temperature and duration during the heat-treatment phases and the extrusion and hardening phases.
  • the alloying elements were a combination of some of the following elements namely Zn, Mg, Cu, Cr, Ti, Sc, Zr, Mn and Li together with four or five heat-treatment steps having varying temperatures and duration.
  • the balance of each alloy was aluminium, some of which included a recycled 7075 aluminium alloy.
  • Example 1 Base 7075 alloy
  • Example 2 Mode 7075 alloy
  • Figures 2, 3, 4 and 5 Tables 2, 2a 3a and 3b
  • Examples 1 and 2 did not include the first heat-treatment phase as do the other Examples, as there is no scandium to precipitate.
  • Example 1 utilises a recycled base 7075 alloy, while Example 2 utilises a modified 7075 alloy where the zinc content has been increased and the copper content decreased based on the base 7075 alloy.
  • the yield strength, ultimate tensile strength and total elongation were measured and shown in Figures 14 to 16.
  • the base 7075 aluminium had weaker yield strength and Ultimate Tensile strength but greater total elongation.
  • the temperature in zone 1 varied from between 275oC to 330oC for a duration of from 6 to 23 hours.
  • Al 3 Sc particles are formed and precipitate.
  • the composition was allowed to cool to room temperature between heat-treatment phases.
  • phase 2 the temperature varied between 430 oC to 450 oC for a duration of from 6 to 23.5 hours.
  • Al 3 Zr particles precipitate and surround the Al 3 Sc particles to form Al 3 Sc/Al 3 Zr dispersoids.
  • the composition is then again allowed to cool to room temperature.
  • phase 3 the composition is again heated to a temperature of between 460oC to 480oC for a duration of from 6 to 18 hours . At these temperatures, all the elements are dissolved without changing the Al 3 Sc/Al 3 Zr dispersoid structures while allowing for continued homogenisation. The composition is slowly allowed to cool back to room temperature. [0097] The composition then underwent an extrusion step where the extrusion rate was 1 mm per second to form profiles where each of the elements is substantially homogenised. The profiles were allowed to cool back to room temperature.
  • the composition then underwent a further heat-treatment step 4 at a temperature of between 470oC and 480oC. During thi s heat-treatment step, all the elements were dissolved and homogenised, apart from the dispersoids and intermetallic forming elements. The composition was then allowed to rapidly cool back to room temperature via a water quenching step.
  • the composition then undergoes an artificial aging step at step 5 where the temperature is raised between 120oC and 140oC for a duration of from 6 to 19 hours.
  • the following precipitates or precursors may form, Al 2 CuLi or Al 3 Li and Mg 2 Zn and may nucleate on the edge of the already existing Al 3 Sc/Al 3 Zr core-shell dispersoids and thicken in both directions embedding partially in the dispersoids.
  • Example 3 Similar temperature conditions and duration conditions for the five heat- treatment stages including both a natural and artificial aging step as for Example 3 were used. A further second artificial aging step zone 6 under similar temperature and duration conditions was conducted in Example 5 and a graph of the duration and temperature variation is shown in Figure 13, in order to complete the Al 2 CuLi, Al 3 Li and the Mg 2 Zn precipitation for the test with T-alloy 2.

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Abstract

L'invention concerne un alliage à base d'aluminium traité thermiquement comprenant les éléments suivants : i. jusqu'à 1 % en poids de lithium ; ii. 0,02 à 0,3 % en poids de scandium ; iii. 0,4 à 3 % en poids de cuivre ; iv. 1,5 à 5 % en poids de magnésium ; v. 6 à 12 % en poids de zinc ; vi. 0,05 à 0,4 % en poids de zirconium ; vii. jusqu'à 0,25 % en poids de manganèse ; viii. jusqu'à 0,25 % en poids de chrome ; ix. jusqu'à 0,2 % en poids de titane ; x. jusqu'à 1 % en poids de fer ; xi. jusqu'à 1 % en poids de silicium ; xii. un reste d'aluminium.
PCT/AU2020/050708 2019-07-10 2020-07-06 Alliages d'aluminium WO2021003528A1 (fr)

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CN115961194A (zh) * 2022-04-25 2023-04-14 江苏大学 锶锆钛铒四元复合微合金化的790MPa超高强度高塑性耐晶间腐蚀铝合金及制备方法

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CN115401293A (zh) * 2022-09-14 2022-11-29 中南大学 一种可mig焊接异种铝合金的铝镁硅挤压板的应用

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