Classifications

• • 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
  • C22F1/04—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

Definitions

• the 7XXX wrought Al-Zn-based alloys are commonly used in structural applications demanding high specific strength. Compared to wrought alloys, castings decrease the fabrication cost and associated logistics lead time, because castings enable near-net-shape products.
• the known 7XXX alloys are susceptible to hot tearing during solidification and therefore not optimal for casting. The hot tearing is caused by a relatively high thermal expansion coefficient and significant volumetric difference between liquid and solid.
• Senkov et al. U.S. Patent 7,060,139 (incorporated by reference herein) disclose a high-strength aluminum alloy with a nominal composition of Al - 6.0-12.0 Zn - 2.0-3.5 Mg - 0.1-0.5 Sc - 0.05-0.20 Zr - 0.5-3.0 Cu - 0.10-0.45 Mg - 0.08-0.35 Fe - 0.07-0.20 Si, in wt%.
• the alloy by Senkov et al. shows high tensile strength while maintaining high elongation in ambient temperatures and cryogenic temperatures. The freezing range of the alloy by Senkov et al.
• the present invention comprises high-strength aluminum casting alloys that are resistant to hot tearing.
• the yield strength of the casting alloys ranges from about 410 MPa to about 540 MPa, at room temperature.
• the invented alloys are Al-Zn-based and comprise the major alloying elements Sc, Zr, Mg, and Cu.
• the amounts of Sc and Zr are optimized to produce primary Ll 2-phase particles which refine the grain size and improve the hot-tearing resistance as well as fatigue resistance and toughness.
• the amounts of Zn, Mg, and Cu are optimized for high resistance to hot-tearing and high strength.
• the amounts of Fe, Mn, and Si are kept low and at a minimum because these elements have a detrimental effect on strength and hot-tearing resistance.
• the solvus temperature of the Ll 2 phase must be above the solvus temperature of the fee phase.
• the solvus temperatures can be computed with thermodynamic database and calculation packages such as Thermo- Calc ® software version N offered by Thermo-Calc Software. Alternatively, in the composition space of the alloys, the solvus temperatures can be approximated by the following equations:
• the amount of Zr is kept below about 0.3 wt% to minimize the formation OfAl 3 Zr which has a DO 23 crystal structure.
• DO 23 particles quickly grow too large [Hyde, K. 2001. The Addition of Scandium to Aerospace Casting Alloys. Ph.D. diss., University of Manchester (incorporated herewith)], and are not very effective for refining the fee grain size.
• small Al 3 (Sc, Zr) particles with an Ll 2 crystal structure are employed instead to inoculate small fee grains during melt cooling.
• the alloys of the invention use as much Zr as possible, about 0.25 ⁇ 0.05 wt%. However, where cost is not a limiting factor, as little as 0.15 wt% Zr can be used in combination with a larger amount of Sc.
• the amounts of Sc and Zr in the casting alloys are optimized for cooling rates up to about 100 0 C per second.
• the Zl 2 -Al 3 (Sc, Zr) particle size distribution depends on the melt cooling rate. Casting into a sand mold results in a cooling rate of about 0.5 0 C per second. Higher cooling rates are accessible through direct-chill casting where the billet is cooled, for example, with water during solidification. Cooling rates above about 100 0 C per second are accessible through casting methods such as the Continuous Rheoconversion Process (CRP).
• CRP Continuous Rheoconversion Process
• Solidification parameters such as the freezing range, the solidus temperature, and the eutectic phase fraction can be computed with thermodynamic database and calculation packages such as Thermo-Calc software.
• Thermo-Calc software To compute solidification parameters of complex alloy systems with Thermo-Calc software, the Gibbs free energy of relevant phases must be assessed following the CALPHAD (CALculation of PHAse Diagrams) approach.
• One such relevant phase is the metastable ⁇ ' phase, because the 7XXX wrought alloys employ ⁇ ' phase precipitates for strengthening.
• the mean radius of ⁇ ' precipitate should be less than about 5 nm.
• the ⁇ ' phase precipitation kinetics can be simulated with PrecipiCalc ® software version 0.9.2 offered by QuesTek Innovations LLC after assessing the thermodynamic description.
• the predicted particle size distribution can be used as input to a mechanistic model of the yield strength, which comprises contributions from precipitation strengthening, grain-size strengthening, solid-solution strengthening, and dislocation strengthening.
• the amounts of Zn, Mg, and Cu of the alloys are chosen to optimize the solidification parameters at various yield strength levels.
• the amounts of Fe, Mn, and Si are kept as low as possible because these elements otherwise form large insoluble constituent particles OfAIi 3 Fe 4 , Al 7 Cu 2 Fe, Mg 2 Si, and Al 6 Mn which negatively affect the toughness, fatigue, and SCC resistance.
• the amount of Fe is preferably kept below about 0.0075 wt%, Mn below about 0.2 wt%, and Si below about 0.03 wt%.
• the homogenization or solution treatment temperature should be below the final solidification temperature, preferably with a safety margin of about 10 to 30 0 C.
• the calculated final solidification temperature is about 493°C.
• the homogenization and solution treatment should be at about 460 to 480 0 C. The time of such treatments should be long enough to eliminate the majority of as-cast segregation.
• Figures IA and IB respectively are graphs depicting the simulated primary Zl 2 particle radius and simulated grain size as a function of the alloy Sc and Zr;
• Figures 2A, 2B, and 2C respectively are graphs depicting strength and solidification parameter contours as a function of Zn, Mg, and Cu content wherein the following legends are utilized:
• Figure 3 is a time-temperature diagram illustrating the processing steps for processing an embodiment of the alloy of the invention.
• Figure 4 is a homogenization simulation of the examples of the invention.
• FIG. 5 is a micrograph of alloy A of the invention.
• the micrograph is typical of the examples of the invention.
• a melt was prepared comprising Al - 6.3 Zn - 3.2 Mg - 1.1 Cu - 0.52 Sc - 0.20
• the exemplary alloy preferably includes a variance in the constituents in the range of plus or minus ten percent of the mean value.
• the alloy was cast through the CRP reactor into a sand-casting mold at measured cooling rates of 50 ⁇ 100°C/second. As shown in Figure 3, the optimum processing condition was to apply hot isostatic pressing, homogenize and solutionize at 460 0 C for 2 hours and 480 0 C for 1 hour, quench with water, hold at room temperature for 24 hours, and age at 120 ⁇ 10°C for 20 hours.
• the ambient yield strength in this condition was 521 ⁇ 12 MPa.
• the grain diameter was about 50 ⁇ m, or an ASTM (American Society for Testing and Materials) grain size number of about 5.7.
• the calculated freezing range is 136°C, solidus temperature 493°C, and the eutectic phase fraction formed at late stages of solidification is 10%.
• a melt was prepared comprising Al - 5.3 Zn - 3.0 Mg - 1.1 Cu - 0.55 Sc - 0.25
• the exemplary alloy preferably includes a variance in the constituents in the range of plus or minus ten percent of the mean value.
• the alloy was cast through the CRP reactor into a sand-casting mold at a measured cooling rate of 100°C/second. As shown in Figure 3, the optimum processing condition was to apply hot isostatic pressing, homogenize and solutionize at 460 0 C for 2 hours and 480 0 C for 1 hour, quench with water, hold at room temperature for 24 hours, and age at 120 ⁇ 10°C for 20 hours.
• the ambient yield strength in this condition was 482 ⁇ 6 MPa.
• the grain diameter was about 54 ⁇ m, or an ASTM grain size number of about 5.5.
• the calculated freezing range is 139°C, solidus temperature 494°C, and the eutectic phase fraction formed at late stages of solidification is 9%.
• a rectangular panel of alloy B was cast successfully without hot tearing in accord and otherwise generally with the protocol of alloy A.
• a melt was prepared comprising Al - 4.5 Zn - 2.3 Mg - 0.62 Cu - 0.42 Sc - 0.25
• the exemplary alloy preferably includes a variance in the constituents in the range of plus or minus ten percent of the mean value.
• the alloy was cast through the CRP reactor into a sand-casting mold. As shown in Figure 3, the optimum processing condition was to apply hot isostatic pressing, homogenize and solutionize at 460 0 C for 2 hours and 480 0 C for 1 hour, quench with water, hold at room temperature for 24 hours, and age at 120 ⁇ 10°C for 15 hours.
• the calculated ambient yield strength in this condition is 410 ⁇ 40 MPa.
• the calculated grain diameter is about 50 ⁇ m or an ASTM grain size number of about 5.7.
• the calculated freezing range is 145°C, solidus temperature 494°C, and the eutectic phase fraction formed at late stages of solidification is 6%.
• Two panels were successfully cast from one heat of alloy C without hot tearing and otherwise generally in accord with the protocol used for alloy A.
• Table 1 summarizes the compositions of the examples set forth above and sets forth the general range of the constituents for the practice of the invention in weight percent:
• Table 2 summarizes the information with respect to the microstructural elements of the examples set forth above and considered relevant to the range of the constituents in the practice of the invention. [40] TABLE 2

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• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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• 2009
  • 2009-01-16 CN CN2009801026580A patent/CN101952467A/zh active Pending
  • 2009-01-16 US US12/863,148 patent/US20110044843A1/en not_active Abandoned
  • 2009-01-16 JP JP2010543275A patent/JP2011510174A/ja active Pending
  • 2009-01-16 RU RU2010133971/02A patent/RU2010133971A/ru unknown
  • 2009-01-16 WO PCT/US2009/031251 patent/WO2009126347A2/en active Application Filing
  • 2009-01-16 EP EP09730142A patent/EP2252716A2/en not_active Withdrawn

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