Patent Publication Number: US-2019185979-A1

Title: Annealing Process

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. patent application Ser. No. 14/910,269, filed on Feb. 5, 2016, which, in turn, is a 371 continuation of International Application No. PCT/US14/052046, filed Aug. 21, 2014, which claims the benefit of U.S. Provisional Application No. 61/868,230, filed Aug. 21, 2013, the entire disclosures of which are hereby incorporated by reference as if set forth fully herein. 
    
    
     STATEMENT OF GOVERNMENTAL INTEREST 
     This invention was made with government support under Contract No. N000141210505 awarded by the Office of Naval Research. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to the field of thermal treatment of aluminum alloy. In particular, the present invention is directed to an annealing process for treating aluminum alloy to increase corrosion resistance. 
     2. Description of the Related Technology 
     Due to their light weight and good corrosion resistance properties as compared to steels, aluminum alloys have the potential of replacing steels in a wide range of applications, such as in aircraft, ships and boats, trucks, cars and other vehicles. For example, aluminum alloys of the AA5XXX series, which have magnesium as a major alloying element, have been broadly used in marine applications. AA5XXX series aluminum alloys may be sensitized to enhance resistance to corrosion, as a result of which magnesium atoms in Al—Mg alloys may precipitate out and form a β-phase (Al 3 Mg 2 ) structure along the grain boundaries, especially in alloys with Mg levels above ˜3 wt. %. 
     The β-phase structure is anodic to the matrix material, leading to formation of small openings along the grain boundary network. These small openings within the matrix material may initiate a process called stress corrosion cracking (SCC). SCC is a type of cracking induced by the combined influence of tensile stress and a corrosive environment. SCC can lead to unexpected sudden failure of normally ductile metals when they are subjected to a tensile stress, due to the ability of SCC to cause gaps to grow faster within the aluminum alloy than otherwise expected. 
     To increase strength and/or corrosion resistance, aluminum alloys are traditionally cast into an ingot of approximately 12 to 28 inches in thickness. The ingot is then scalped and preheated, after which it may be hot rolled to about 0.125 inch, cold rolled to about 0.020 to 0.060 inch, and subjected to further heat treatment, such as batch annealing or solution heat treatment. One concern with such traditional approaches is the intermetallic particles present in the as-cast aluminum alloy ingots, which is a function of the alloy composition and the solidification rate in the casting process. The intermetallic particles can participate in a fracture initiation and propagation and, as a result, may limit the formability or design tolerance of the aluminum alloys. The intermetallic particles also act as void nucleation sites during sheet forming processes, such as stretching and, therefore, may contribute to fracture initiation within the aluminum alloy matrix. 
     The aluminum alloy may also be strengthened by using conventional mechanical methods such as solid-solution hardening, precipitation hardening, grain size refinement, dispersion, work hardening, and fiber reinforcement. Several heating based processes have been developed attempting to improve various properties of aluminum alloys, especially their corrosion resistance. 
     Kamegawa et al. (“Grain Refinements of Al—Mg Alloy by Hydrogen Heat-Treatments,”  Materials Transactions , vol. 46, pages 2449-2453, 2005) discloses a heat treatment method for an aluminum alloy with 7.8 wt. % Mg. This hydrogen heat-treatment method uses the so-called Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) process. The aluminum alloy undergoes homogenization treatment at 450° C. for 24 hours; the alloy is then quenched in water; oxide is removed from surface of the alloy plates by using SiO 2  paper; followed by ultrasonic washing in acetone. The produced alloy plate is then cut and ground into coarse powder with a diameter of less than 100 mm in an Ar-filled glove box. The powders are subjected to the heat treatment at 250-450° C. under a hydrogen pressure of 7.5 MPa, and then dehydrogenation in the same temperature range for 0.5-4 hours by using a rotary oil-pump. 
     Kramer et al. (“Locally Reversing Sensitization in 5XXX Aluminum Plate,”  Journal of Materials Engineering and Performance , vol. 21, pages 1025-1029, 2012) discloses a method for heat treatment of aluminum alloy in the AA5XXX series, involving a short exposure to a specific elevated temperature. The method first uses a sensitization treatment of the aluminum alloy at 150° C. for 24 hours, followed by heat treatment at a temperature from 200 to 340° C. for 24 hours. This method is capable of reversing the adverse effects of sensitization and restoring corrosion resistance. 
     Conserva and Leoni (“Effect of thermal and thermo-mechanical processing on the properties of Al—Mg alloys,”  Metallurgical Transactions A , vol. 6, pages 189-195, 1975) discloses thermal and thermo-mechanical processing of Al—Mg alloys for improving tensile strength and stress corrosion resistance of the alloys. One method comprises hot rolling at a temperature from 380 to 420° C. to reduce the thickness of the alloy from 10 mm to 4 mm; annealing at 400° C. for 2 hours followed by air quenching, cold rolling to a thickness of 2 mm; and heterogenization treatment at 225° C. Another disclosed method comprises warm rolling at a temperature from 250 to 280° C. to reduce the thickness of the alloy from 10 mm to 3 mm; cold rolling to a thickness of 2 mm; and heterogenization treatment at 225° C. 
     US 2012/0119407 discloses a method for enhancing both stress corrosion cracking and intergranular corrosion of Al—Mg alloy sheet or plate product. The method includes the steps of: (a) continuously casting an Al—Mg alloy comprising from about 6 wt. % to about 10 wt. % Mg; (b) hot rolling the Al—Mg alloy to a thickness of less than 6.35 mm; (c) annealing the Al—Mg alloy via a furnace, wherein the annealing step comprises: (i) heating the Al—Mg alloy at elevated temperature and for a time sufficient to achieve an O temper; and (ii) cooling the Al—Mg alloy, wherein, after the cooling step, the Al—Mg alloy comprises a plurality of grains, and wherein the Al—Mg alloy is substantially free of a continuous film of β-phase structure at the grain boundaries after the Al—Mg alloy has been age sensitized. 
     To improve corrosion resistance of the aluminum alloy in the AA5XXX series, the present invention employs an annealing treatment of the aluminum alloy. The treatment process is capable of minimizing or preventing β-phase structure formation in the treated aluminum alloy during future sensitization. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides an annealing process comprising the steps of heating an aluminum alloy to a first temperature of from about 350° C. to about 450° C. at a rate of temperature increase from about 0.1° C./s to about 0.5° C./s; and cooling the annealed aluminum alloy to a temperature below about 50° C. 
     In another aspect, the annealing process further comprises a sensitizing step prior to the annealing step, wherein the sensitizing step is performed by heating the aluminum alloy to a temperature of from about 200° C. to about 350° C. at a rate of temperature increase from about 0.1° C./s to about 0.5° C./s; and cooling the annealed aluminum alloy to a temperature below about 50° C. 
     In yet another aspect, the sensitizing step comprises heating the aluminum alloy to a second temperature of from about 200° C. to about 350° C. at a rate of temperature increase from about 0.1° C./s to about 0.5° C./s; maintaining the aluminum alloy at the second temperature for a time period of from about 1 minute to about 10 minutes; and cooling the aluminum alloy to a temperature below about 50° C. 
     In yet another aspect, the present invention provides an aluminum alloy of the AA5XXX series treated by the annealing process of the present invention. 
     These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a flow chart representing an annealing process for treatment of an aluminum alloy of the AA5XXX series according to one embodiment of the present invention. 
         FIG. 2  shows a flow chart representing an annealing process for treatment of an aluminum alloy of the AA5XXX series according to another embodiment of the present invention. 
         FIG. 3  shows a flow chart representing an annealing process for treatment of an aluminum alloy of the AA5XXX series according to yet another embodiment of the present invention. 
         FIG. 4  is a TEM (transmission electron microscope) image of an aluminum alloy after sensitization, as explained in Example 1. 
         FIG. 5  is a TEM of the aluminum alloy of  FIG. 4  after the annealing of the present invention, as explained in Example 1. 
         FIG. 6  is a TEM of the aluminum alloy of  FIG. 5  after an additional sensitization, as explained in Example 1. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     For illustrative purposes, the principles of the present disclosure are described by referencing various exemplary embodiments. Although certain embodiments are specifically described herein, one of ordinary skill in the art will readily recognize that the same principles are equally applicable to, and can be employed in other systems and methods. Before explaining the disclosed embodiments of the present disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of any particular embodiment shown. Additionally, the terminology used herein is for the purpose of description and not of limitation. Furthermore, although certain methods are described with reference to steps that are presented herein in a certain order, in many instances, these steps may be performed in any order as may be appreciated by one skilled in the art; the novel method is therefore not limited to the particular arrangement of steps disclosed herein. 
     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Furthermore, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. The terms “comprising”, “including”, “having” and “constructed from” can also be used interchangeably. 
     The present invention provides a process for annealing of aluminum alloys for enhancing corrosion resistance. Aluminum alloys suitable for the process of the present invention include aluminum alloys of the AA5XXX series in the Aluminum Association Register. The AA5XXX series aluminum alloys have magnesium as the major alloying element. 
     In some embodiments, the process of the present invention may be used for annealing stainless steel for enhancing its strength and corrosion resistance. 
     The AA5XXX series of alloys are considered non-heat treatable because they cannot generally be appreciably strengthened by solution heat treatment. Instead, the AA5XXX series aluminum alloys are usually strengthened by solid-solution formation of second-phase microstructural constituents, dispersoid precipitates and/or strain hardening. In addition the AA5XXX series is readily weldable, and for these reasons these alloys may be used in a wide variety of applications such as shipbuilding, transportation, pressure vessels, bridges and buildings. These alloys are often welded with filler alloys, which are selected after consideration of the magnesium content of the base material, and the application and service conditions of the welded aluminum alloy. 
     Examples of suitable aluminum alloy include the aluminum alloy AA5456-H116, which is a wrought aluminum alloy comprising (in weight percentage) about 5.5% Mg, 1.0% Mn, 0.2% Cr, with the balance being aluminum. This alloy is currently used in marine applications because it is considered to be resistant to seawater corrosion. The temper designation “H116” indicates a soft temper alloy that has been strain hardened without thermal treatment. 
     Among the AA5XXX series aluminum alloys, some are more suitable for the present invention. These alloys generally have higher amounts of magnesium, such as at least about 3.0 wt. % Mg, or even at least about 4.0 wt. % Mg, or at least about 5.0 wt. % Mg, or even at least about 6.0 wt. % Mg. In one embodiment, the aluminum alloy products include not greater than about 10 wt. % Mg, such as not greater than about 9.5 wt. % Mg, or not greater than about 9.0 wt. % Mg, or not greater than about 8.5 wt. % Mg. 
     Other elements may be included in these aluminum alloys in non-incidental amounts. For example, the aluminum alloy may include up to about 0.8 wt. % copper, up to about 1.2 wt. % manganese, up to about 0.5 wt. % chrome, up to about 1.0 wt. % zinc, and up to about 0.3 wt. % zirconium, up to about 0.23 wt. % scandium, to name a few. When the aluminum alloy products are produced via slab casting, the aluminum alloy generally includes non-incidental amounts of beryllium, such as at least about 0.0003 wt. % beryllium. The aluminum alloy may include small amounts of incidental elements and impurities. For example, trace amounts of iron and silicon may be included in the aluminum alloy. Iron may be included in the aluminum alloy in an amount of up to about 0.15 wt. %. Silicon may be included in the aluminum alloy in an amount that will allow for the precipitation of Mg 2 Si phase during solidification. The actual amount of Si required for this purpose will depend on the Fe content of the metal and cooling rate applied in solidification. In other embodiments, silicon may be included in the aluminum alloy as an alloying ingredient. 
     In one aspect, the present invention provides an annealing process for dissolving, or preventing the formation of, β-phase in an aluminum alloy of the AA5XXX series, therefore increasing the corrosion resistance of the treated aluminum alloy. Referring to  FIG. 1 , the annealing process may comprise the steps of heating an aluminum alloy of the AA5XXX series to a temperature of from about 350° C. to about 450° C. at a rate of temperature increase from about 0.1° C./s to about 0.5° C./s; and cooling down the annealed aluminum alloy to a temperature below about 50° C., or at about 25° C. Typically, the heating is initiated at about room temperature or ambient conditions but could also be initiated at any temperature up to about 100° C. The annealing process of the present invention can be used to desensitize the alloy and/or dissolve β-phase in the alloy, as well as preventing growth of β-phase in the treated aluminum alloy. The annealing process can dramatically reduce formation of β-phase, in aluminum alloys, particularly along the grain boundary, and thereby increase the corrosion resistance of these aluminum alloys. 
     In some embodiments, the annealing step comprises heating the aluminum alloy to a temperature of from about 350° C. to about 450° C., or from about 370° C. to about 430° C., or from about 380° C. to about 420° C., or from about 390° C. to about 410° C. 
     The heating of the aluminum alloy is conducted slowly to allow sufficient time for the dissolution of β-phase in the aluminum alloy. The rate of temperature increase for the annealing step is from about 0.1° C./s to about 0.5° C./s, or from about 0.15° C./s to about 0.45° C./s, or from about 0.2° C./s to about 0.4° C./s, or from about 0.25° C./s to about 0.35° C./s, or from about 0.28° C./s to about 0.32° C./s. 
     In an exemplary embodiment, the annealing step may be performed using a Gatan™ heating holder. In other some embodiments, the annealing step may be performed in a heat treating furnace. The heat treating furnace must be capable of accurately controlling the furnace temperature and temperature variation must be limited to within a range of about ±5° C. Overheating should be avoided, especially not exceeding the initial eutectic melting temperatures of the aluminum alloy. Though not apparent at early stages of overheating, a deterioration of mechanical properties of the aluminum alloy may result from overheating the aluminum alloy. 
     After the annealing step raised the temperature of aluminum alloy to the desired temperature, the aluminum alloy is then cooled to a temperature below about 50° C., or at about 25° C. The cooling step should be performed such that the temperature of the annealed aluminum alloy is reduced quickly to avoid an appreciable time where the temperature is from about 200° C. to about 300° C. in order to minimize the risk of β-phase formation. The final temperature of the annealed aluminum alloy after the cooling step should be below about 50° C. 
     The cooling step may be air cooling or quenching. Although the mode of cooling is not critical, the annealed aluminum alloy is preferably cooled rapidly to a temperature at which the diffusion rate of the elements in the aluminum alloy matrix is not appreciable, and formation of precipitates, particularly on the grain boundaries of the aluminum alloy, is thereby prevented. In some embodiments, the cooling rate may be from about 10° C. to about 30° C. per minute, or from about 15° C. to about 25° C. per minute, or from about 18° C. to about 22° C. per minute, until the temperature of the aluminum alloy is reduced to below a desired temperature. 
     In one embodiment, the annealed aluminum alloy is quenched. Quenching can help to keep dissolved constituents in solution after cooling the annealed alloy to a temperature below about 50° C., or to about 25° C. The speed of quenching is important as excessive delay in transferring the aluminum alloys to a quenching medium may adversely affect the properties of the aluminum alloy. The quenching medium may be, for example, water or oil at a temperature below about 50° C., or to about 25° C. Other rapid cooling methods known to skilled persons may also be used for the present invention. 
     Referring to  FIG. 2 , in some embodiments, prior to the annealing step, the aluminum alloy is treated in a sensitization step. The sensitization step comprises heating the aluminum alloy to a temperature from about 200° C. to about 350° C., or from about 250° C. to about 325° C., or from about 270° C. to about 310° C. The sensitization step may be used to form β-phase at the grain boundary of the alloy. 
     In certain embodiments, the temperature is gradually ramped up in the sensitization step. For example, the rate of temperature increase in the sensitization step may be from about 0.1° C./s to about 0.5° C./s, or from about 0.15° C./s to about 0.45° C./s, or from about 0.2° C./s to about 0.4° C./s, or from about 0.25° C./s to about 0.35° C./s, or from about 0.28° C./s to about 0.32° C./s. 
     The sensitization step may also involve a period of maintaining the aluminum alloy at or near the maximum sensitization temperature for a time period of from about 1 minute to about 10 minutes, or from about 2 minutes to about 8 minutes, or from about 3 minutes to about 7 minutes, or from about 4 minutes to about 6 minutes. 
     Subsequently, the aluminum alloy is cooled to a temperature below about 50° C. by any suitable means as discussed above in relation to the cooling carried out subsequent to the annealing step. This cooling step also should be performed such that the temperature of the sensitized aluminum alloy is reduced quickly to a desired temperature, which is below about 50° C., or to about 25° C. 
     As a result of the sensitization step, β-phase (Al 3 Mg 2 ) precipitates at the grain boundaries in a semi-continuous fashion. The β-phase is anodic to the grain interiors and thus structures formed from aluminum alloys with a sensitized microstructure possessing β-phase are susceptible to intergranular corrosion, exfoliation, and stress corrosion cracking when exposed to stress and corrosive media such as seawater. 
     The sensitized aluminum alloy may then be annealed and cooled as described above in relation to the annealing step ( FIG. 2 ). The annealing step reduces or eliminates the β-phase from the aluminum alloy, and makes the aluminum alloy less likely to develop β-phase in the future, even when exposed to heat or corrosive conditions. 
     Referring to  FIG. 3 , in some embodiments, the present invention may be performed in more than one cycle. In some embodiments, an aluminum alloy undergoes two cycles of the process: (i) first cycle: a sensitization step, cooling the sensitized aluminum alloy, then annealing step, and cooling the annealed aluminum alloy to a temperature below about 50° C., followed by (ii) a second cycle: a second sensitization step, cooling the sensitized aluminum alloy down to a temperature below about 50° C., a second annealing step, and cooling the annealed aluminum alloy down to a temperature below about 50° C. In some embodiments, the process may comprise three or more cycles. 
     The aluminum alloys treated by the annealing process of the present invention are substantially free of β-phase, even after exposure to further sensitization, heat and/or corrosive elements which will improve corrosion resistance, especially seawater corrosion resistance. The aluminum alloy treated by the process of the present invention is particularly suitable for naval vessels where seawater corrosion is of a significant concern since the formation of anodic β-phase at the grain boundary has been found to accelerate seawater corrosion of aluminum alloys. Thus, use of this process on an industrial scale to produce highly corrosion resistant aluminum alloy for naval vessels could reduce maintenance on naval vessels. 
     Another area where the present invention may be valuable is refurbishing the sensitized parts on an aircraft, ship, or boat. Taking the naval vessels as an example, the sensitized portion or part of the vessels, which may be susceptible to sea water corrosion, may be treated with the process of the present invention to refurbish the portion or part. Such treatment will remove substantially all the β-phase in the sensitized portion or part, therefore restore its resistance to corrosion. This may reduce the need for part replacement. For example, parts may be sent to a furnace for treatment with the annealing process of the present invention. 
     In some embodiments, the annealing process of the present invention may be used by onboard personnel of a naval vessel to treat a sensitized portion or part of the vessel to reduce or remove β-phase that may have formed in the portion or part. This is especially advantageous since a ship superstructure may not be easily removed and furnace treated. Another advantage is that the removal of β-phase in the sensitized plate may be done in a speedy fashion, even at sea, in order to reduce or prevent further damage that may be caused by the β-phase. 
     EXAMPLE 
     The following example is illustrative, but not limiting, of the methods and compositions of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field are within the scope of the disclosure. 
     Example 1 
     A bulk plate of 5456-H116 aluminum alloy (Al-5.5Mg-1.0Mn-0.2Cr) was used for this study. The bulk plate (received plate) was cut into 2.5×2.5 cm blocks and a 300 micron slice was taken from the transverse plane. The 300 micron slice was then polished down to 50 microns, punched out as a 3 mm TEM disk, and Ion milled to achieve final electron transparent thickness. 
     The annealing process was carried out via a Gatan heating holder. The sample was first sensitized by heating to 300° C. at a rate of temperature increase of 0.3° C./s and held at the temperature for 5 minutes. After the sensitization step, the sample was then cooled back to room temperature. The sample was heated to 400° C. at a rate of temperature increase of 0.3° C./s to dissolve the β-phase structure grown in the sensitization step. Once the 400° C. temperature was achieved the sample was cooled down to room temperature (about 25° C.). 
     To investigate the effects of the annealing step on the sample, the sensitization step at 300° C. at a rate of temperature increase of 0.3° C./s was repeated again to try to re-grow β phase in the same area. 
     The same area of the sample at different stages of this annealing process was observed in a JEOL 2100 LaB 6  TEM.  FIG. 4  is a TEM image of the sample after the initial sensitization step at 300° C. Along the grain boundaries, β-phase structures (precipitates of Al 3 Mg 2 ) may be seen in the TEM image (indicated by arrows). In addition, a few β-phase structures may also be seen within the actual grain. 
       FIG. 5  is a TEM image of the sample after the annealing step and cooling step, in the same area as in  FIG. 4 . It can be seen that the β-phase structures have been substantially completely dissolved. The grain boundaries were devoid of precipitates and the precipitates in the grain have also dissolved.  FIG. 6  is a TEM image of the sample after repeating the sensitization step, in the same area as in  FIG. 4 . The grain boundaries and within the grain in this area were seen with no β-phase structures, which indicates that the exposing to the same elevated temperature fails to regrow the β-phase structures in the alloy treated with the annealing step. The annealing process of the present invention can successfully remove/dissolve the β-phase structures induced by sensitization of aluminum alloys and further prevent growth of the β-phase structures in the treated aluminum alloys even after exposure to further sensitization temperatures. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.