Patent Application
20070102071
This invention relates to 1) an aluminum alloy for casting operations, such as sand, investment or permanent mold casting operations, 2) a heat treatment process for aluminum alloys and 3) the application of the newly invented aluminum alloy casting alloy for cast products for racing, aerospace and military (land, sea and air) applications.
Only a few aluminum alloy casting alloys have attractive properties for racing, aerospace and military applications. These aluminum alloy casting alloys are commonly designated 354, C355, A356, A357, A201 and A206. However, none of these aluminum alloy casting alloys have the desirable combination of high strength, high ductility, high toughness, good resistance to stress corrosion cracking, good weldability and good castability (i.e. good resistance to hot tearing and good fluidity). Hot tearing is a catastrophic event that occurs during the casting process and renders the cast product unusable. Hot tearing occurs when the metal contraction due to solidification produces tensile stresses higher than the strength of the casting. “Spongy” areas, which have high levels of porosity, due to poor feeding will have low strength and hot spots caused by the combination of thick and thin sections will have high tensile contraction stresses that promote hot tearing.
Aluminum alloy casting alloys 354, C355, A356 and A357 have good castability but do not have good mechanical properties. Aluminum alloy casting alloys A201 and A206 have good mechanical properties but do not have good castability, good resistance to stress corrosion cracking or good weldability. Both A201 and A206 alloy have poor fluidity and poor resistance to hot tearing during casting.
The chemical compositions and mechanical properties of aluminum alloy casting alloys are found in ASTM B26/B26M-99, “Standard Specification for Aluminum-Alloy Sand Castings”, Table 1, and ASTM B686-99, “Standard Specification for Aluminum Alloys Castings, High-Strength”, Table 2 and Table 3. A comparison of the castability/fluidity, corrosion resistance and weldability of aluminum alloy casting alloys is shown in Table 4.
*a single value denotes the maximum amount permitted
*www.sfsa.org/tutorials
The ASTM does not list A206 alloy as an aluminum alloy sand casting or high-strength aluminum alloy.
There are numerous aluminum alloy wrought alloys that have attractive properties for aerospace and military applications but products from these aluminum alloys must be machined from plate or billet, which is extremely time consuming and costly, or must be forged into useful shapes. Some of these aluminum alloy wrought alloys are 2024, 5083, 6061, 7075, 2219 and 2519.
The chemical compositions and mechanical properties of aluminum alloy wrought alloys are found in ASTM B209/B209M-95, “Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate” and are summarized in Table 5 and Table 6.
*a single value denotes the maximum amount permitted
**from MIL-DTL-46192C(MR)
*the properties of wrought alloys are a function of thickness
**from MIL-DTL-46192C(MR)
***long transverse direction
****longitudinal direction
The ASTM does not list 2519 alloy as an aluminum alloy wrought alloy. The chemical composition and mechanical property data for 2519 alloy was taken from MIL-DTL-46192C(MR).
Aluminum alloy wrought alloy 2519 is currently the premier aluminum alloy wrought alloy because of its excellent tensile strength and ballistic qualities. However, aluminum alloy wrought alloy 2519 requires “stretching” to achieve these properties (see e.g. U.S. Pat. No. 4,610,733, entitled “High Strength Weldable Aluminum Base Alloy Product and Method of Making Same” and issued Sep. 9, 1986 to Sanders, Jr., et al.). Because “stretching” is a cold working process, the benefit of stretching is lost if the product is welded or heat treated after “stretching”. Further, products that are cast to shape cannot be “stretched”.
U.S. Pat. No. 2,706,680 (entitled “Aluminum Base Alloy” and issued Apr. 19, 1955 to Criner) describes aluminum base alloys that are adapted for service at elevated temperatures, particularly such as required in certain parts of jet engines. This patent discloses a magnesium-free aluminum base alloy containing copper as the chief added component and small amounts of manganese, vanadium and zirconium which displays a combination of strength and resistance to fatigue and creep at high temperatures.
More specifically the aluminum alloy includes from 5 to 13% copper, 0.15 to 1.7% manganese, 0.05 to 0.20% vanadium, 0.05 to 0.30% zirconium, with an iron impurity not exceeding 0.75% and a silicon impurity not exceeded 0.40%. The disclosed alloy contains no more than about 0.02% magnesium, hence it is referred to as being “magnesium-free”. To obtain a finer grain size or enhance minor characteristics of the alloy it is disclosed to be desirable to add 0.01 to 0.25% of one or more of the following elements: cobalt, nickel, molybdenum, tungsten, chromium, titanium, boron, tantalum and niobium. The thermal treatment disclosed to enhance the alloy properties consists of heating to a temperature between 960 and 1000° F. for a period of 2 to 24 hours followed by quenching, preferably in water at 70 to 160° F. The quenched alloys are then reheated to 350 to 450° F. for a period of 1 to 50 hours. Mechanical properties are disclosed for elevated temperatures (400 and 600° F.).
U.S. Pat. No. 2,784,126 (entitled “Aluminum Base Alloy” and issued Mar. 5, 1957 to Criner) is similar to U.S. Pat. No. 2,706,680 (discussed above) and discloses an aluminum base alloy that is adapted for service at elevated temperatures. In this patent, the disclosed chemistry of the alloy consists of from 5 to 13% copper, 0.15 to 1.7% manganese, 0.05 to 0.20% vanadium, 0.05 to 0.30% zirconium and the addition of 0.05 to 0.70% magnesium. In this patent, the disclosed addition of magnesium is claimed to improve the strength and resistance to creep and fatigue at high temperatures. In this patent, samples were cast in the form of ingots and forged to 1′ square bars. The bars were given a solution heat treatment of 2 hours at 990- 1000° F., quenched in cold water and precipitation hardened by heating them for 12 hours at 375° F. The disclosed room temperature properties of an alloy of composition 5.98 wt % Cu, 0.11 wt % Fe, 0.07 wt % Si, 0.21 wt % Mn, 0.10 wt % V and 0.23 wt % Zr are an average ultimate tensile strength of 61,600 psi, an average 0.2% offset yield strength of 43,000 psi and an average elongation of 17%. The disclosed room temperature properties of an alloy of composition 6.09 wt % Cu, 0.15 wt % Fe, 0.11 wt % Si, 0.32 wt % Mn, 0.18 wt % V, 0.20 wt % Zr and 0.25 wt % Mg are an average ultimate tensile strength of 71,100 psi, an average 0.2% offset yield strength of 55,700 psi and an average elongation of 13%. In this patent, it is presumed that forging was required to increase the density of the disclosed aluminum alloy. This patent does not disclose information on the properties of castings made from the disclosed alloy.
One of the reasons that castings typically have inferior properties compared to wrought products is porosity. However, a low cost process known as hot isostatic pressing (HIP) is available. Use of the HIP process with a proper process cycle can produce significant improvements in mechanical properties of castings with respect to porosity. The industry standard practice is to (1) produce a casting, (2) HIP the casting to eliminate detrimental porosity and then (3) heat treat the casting to develop the appropriate mechanical properties.
An aluminum alloy casting alloy and heat treatment process that produces a cast product with properties equivalent to the aluminum alloy wrought alloys is desired to reduce material usage, energy usage and machining time and expense.
In accordance with principles of the present invention, a casting is made from an aluminum alloy containing, in weight percent:
The aluminum alloy casting is solution heat treated, then hot isostatically pressed, then solution heat treated again. This process produces a cast product having a multitude of second phase particles, and in particular a cast product in which an interdendritic network of second phase particles is eliminated.
More specifically, the aluminum alloy casting is solution heat treated at 950-960° F. (510-516° C.) for 2-4 hours followed by 980-1005° F. (527-541° C.) for 16-120 hours followed by quenching in water, hot isostatically pressed (HIP) at 950-975° F. (510-524° C.) and 15,000±500 psi (103±3.4 MPa) for 2 to 3 hours, heat treated at 950-960° F. (510-516° C.) for 2-4 hours followed by 980-1005° F. (527-541° C.) for 16-120 hours followed by quenching in water and aged. The casting may be either naturally aged at room temperature or artificially aged at 310-390° F. (154-199° C.) for 1 to 96 hours.
The resulting aluminum alloy product in the naturally aged condition T4 has a minimum ultimate tensile strength of 57,100 psi (394 MPa), a minimum 0.2% offset yield strength of 38,500 psi (265 MPa), a minimum elongation of 6.2%, a minimum unnotched impact strength of 88 joules/cm2 and passes the ASTM G47 test for resistance to stress corrosion cracking at an applied stress of 30,000 psi (207 MPa).
The resulting aluminum alloy product in the artificially aged condition T6 has a minimum ultimate tensile strength of 66,100 psi (458 MPa), a minimum 0.2% offset yield strength of 47,800 psi (330 MPa) and a minimum elongation of 3.1%.
The resulting aluminum alloy product in the artificially aged condition T7 has a minimum ultimate tensile strength of 48,600 psi (335 MPa), a minimum 0.2% offset yield strength of 43,900 psi (303 MPa), a minimum elongation of 1.0%, a minimum unnotched impact strength of 28 joules/cm2 and passes the ASTM G47 test for resistance to stress corrosion cracking at an applied stress of 40,000 psi (276 MPa).
The resulting aluminum alloy product in the artificially aged condition T61 has an average ultimate tensile strength of 69,660 psi (480 MPa), an average 0.2% offset yield strength of 59,390 psi (409 MPa), an average elongation of 6.3% and an average unnotched impact strength of 41 joules/cm2 and has similar ballistic performance to aluminum alloy wrought alloy 2519 in the T87 condition.
The aluminum alloy casting alloy of the present invention is desirable because it is weldable and retains the desired properties when heat treated after welding.
In the drawing:
Aluminum alloy casting alloys A201 and A206 were purchased as ingots from a supplier. The aluminum alloy casting alloy in the illustrated embodiment was produced using A206 ingot with addition of aluminum-copper, aluminum-manganese, aluminum-chromium, aluminum-vanadium, aluminum-zirconium master alloys and pure magnesium. Commercially available aluminum-titanium-boron and aluminum-titanium-carbon grain refiners were used. The chemical compositions of a plurality of aluminum alloy casting alloys produced are shown in Table 7. In Table 7, the columns represent a weight percentage of the indicated element and each row represents one mixture of the constituent elements, termed a heat and designated by a letter A through W.
Castability. The castability of the aluminum alloy casting alloys of the illustrated embodiment is determined by qualitatively comparing the fluidity and hot tearing tendency of A201 and A206 alloys to that of the illustrated embodiment. A complex seat frame casting that has thick and thin sections is poured from each alloy at various temperatures in chemically bonded sand molds that contain aluminum chills. Pictures of such seat frame castings are shown in
For the seat frame casting, good castings could not be produced in aluminum alloy casting alloy A206. Good castings could be produced in aluminum alloy casting alloy A201 when poured in the temperature range 1350-1360° F. Good castings could be produced in the aluminum alloy casting alloy of this embodiment (Heats A, B, D & E) when poured in the temperature range 1330-1380° F., a wider temperature range than with aluminum alloy casting alloy A201. Good seat frame castings could not be poured from Heat C. Thus, an aluminum alloy casting alloy should have a minimum copper content of about 5.20 wt % to produce good fluidity and good resistance to hot tearing. Seat frame castings were not poured from Heats F-W.
An example of an aluminum alloy casting alloy A206 casting with hot tears is shown in
Heat Treatment and Mechanical Properties. The aluminum alloy casting alloy of the illustrated embodiment is processed using a pre-HIP solution heat treatment. That is, instead of applying a HIP process to the cast product, that product is first heat treated. The mechanical properties of an aluminum alloy cast product are determined by soundness, chemistry and microstructure. Soundness is a measure of porosity, which is determined by the feeding characteristics of the aluminum alloy cast alloy. Soundness can be improved by hot isostatic pressing (HIP) the cast product. The chemistry of the aluminum alloy cast alloy ultimately determines what microstructural phases can be produced. The size, quantity and distribution of the microstructural phases and porosity determine the mechanical properties. The size, quantity and distribution of the microstructural phases are determined by heat treatment.
The aluminum alloy casting alloys of the illustrated embodiment produced less porosity than A201 but more porosity than A206 alloy. For the seat frame casting (
The aluminum alloy casting alloy of the illustrated embodiment produced better mechanical properties compared to A201 alloy. Table 8 compares the mechanical properties of samples cut from seat frame castings produced from aluminum alloy casting alloy A201 and aluminum alloy casting alloy of heat A of the illustrated embodiment.
Hot isostatic pressing (HIP'ing) is a well known, commercial process for reducing the porosity in castings. HIP'ing is typically performed before any other heat treatment. In the illustrated embodiment, however, solution heat treatment performed before HIP'ing produces improved mechanical properties, particularly improved resistance to stress corrosion cracking. However, solution heat treatment before HIP'ing is not a requirement to produce satisfactory properties using the aluminum alloy casting alloy of the illustrated embodiment.
Sections were cut from castings produced from the aluminum alloy casting alloy of the illustrated embodiment and heat treated in various ways to quantitatively determine the effect of HIP'ing and heat treatment cycle on mechanical properties.
The heat treatment cycles were: (1) a long pre-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) 96 hours followed by quenching in water, followed by HIP'ing at 950-975° F. (510-524° C.), 15,000+/−500 psi (103+/−3.4 MPa) for 2 to 3 hours, followed by a post-HIP solution heat treatment (see below) and age; (2) a short pre-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. for 16-20 hours followed by quenching in water, followed by HIP'ing at 950-975° F. (510-524° C.), 15,000+/−500 psi (103+/−3.4 MPa) for 2 to 3 hours, followed by a post-HIP solution heat treatment (see below) and age; and (3) HIP'ing at 950-975° F. (510-524° C.), 15,000+/−500 psi (103+/−3.4 MPa) for 2 to 3 hours, followed by a post-HIP solution heat treatment (see below) and age (i.e. no pre-HIP solution heat treatment).
The T4 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.) and then naturally aging at room temperature for a minimum of seven days before testing. The T6 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.), naturally aging at room temperature for 8-24 hours and then artificially aging at 325° F. (163° C.) for 24 hours. Material heat treated to the T6 condition exhibited the best combination of strength and ductility: HIP'ing increased the tensile ductility by 60 to 101%. The T61 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.), naturally aging at room temperature for 8-24 hours and then artificially aging at 325° F. (163° C.) for 36 hours. The T7 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.), naturally aging at room temperature for 8-24 hours and then artificially aging at 390° F. (199° C.) for 24 hours.
The mechanical properties of samples cut from seat frame castings produced from the aluminum alloy casting alloy of the illustrated embodiment and heat treated in various ways are listed in Table 9.
Compared to aluminum alloy casting alloy A206, the aluminum alloy cast alloy of the illustrated embodiment has improved yield (design) strength in all heat treatment conditions. Also, the tensile ductility increased by 167% when subjected to the short solution heat treatment, followed by HIP'ing, followed by the T6 heat treatment. For comparison, sections from seat frame castings of each alloy were heat treated using identical processing conditions (e.g. solution heat treated, HIP'ed or not HIP'ed, then T6) and the results are listed in Table 10.
To more fully determine the effect of chemistry, sections were cut from Y-block castings produced from different formulations of the aluminum alloy casting alloy of the illustrated embodiment, and heat treated using the same conditions. The heat treatment cycle was a long pre-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 96-120 hours followed by quenching in water, HIP'ing at 950-975° F. (510-524° C.), 15,000+/−500 psi (103+/−3.4 MPa) for 2 to 3 hours, followed by a post-HIP solution heat treatment and age.
The T4 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.) and then naturally aging at room temperature for a minimum of seven days before testing.
The T6 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.), naturally aging at room temperature for 8-24 hours and then artificially aging at 325° F. (163° C) for 24 hours.
The T7 condition was produced by post-HIP solution heat treatment at 950-960° F. (510-516° C.) for 2-4 hours followed by 990-995° F. (532-535° C.) for 16-20 hours followed by quenching in warm water at 120-180° F. (49-82° C.), naturally aging at room temperature for 8-24 hours and then artificially aging at 390° F (199° C) for 24 hours.
For an aluminum alloy casting alloy to have good mechanical properties, copper content should be limited to about 6.25 wt %, chromium content should be limited to about 0.20 wt % and magnesium content should be limited to about 0.50 wt %. The addition of silver was shown to significantly increase yield strength. The resulting data shows that ductility (e.g. tensile elongation) decreases as copper content increases, as chromium content increases and as magnesium content increases. Increasing manganese content or vanadium content was shown to decrease yield strength and increase ductility. Increasing zirconium content was shown to have an inconsistent effect on mechanical properties. The mechanical properties of samples cut from castings produced from the aluminum alloy casting alloy of the illustrated embodiment and heat treated in similar ways are displayed in Table 11.
The aluminum alloy casting alloy of the illustrated embodiment has good stress corrosion cracking properties that are enhanced when solution heat treated before HIP'ing. Aluminum alloy cast alloys and wrought alloys that contain copper typically have unacceptable stress corrosion cracking properties in the T6 condition but often have acceptable stress corrosion cracking properties in the T4 or T7 conditions. Samples from two heats (Heat B, Heat D) were given a variety of heat treatments and then subjected to the standard stress corrosion cracking test, ASTM G47-98 (2004). The heat subjected to long solution heat treatment prior to HIP'ing (Heat D) exhibited significantly improved resistance to stress corrosion cracking (almost produced “acceptable” results) in the T6 condition. The stress corrosion cracking performance, as determined by ASTM G47-98 (2004), for the two different heat treatment processes and aluminum alloy casting alloys of the illustrated embodiment are displayed in Table 12.
*failed on the last day of the test
The aluminum alloy casting alloy of the illustrated embodiment is weldable. Aluminum alloy casting alloys are not normally welded so little or no published data exists for comparison purposes. However, aluminum alloy wrought alloys are often welded and the aluminum alloy casting alloy of the illustrated embodiment compared favorably to published data for material tested in the heat treated then welded condition. The welder had no prior experience welding the aluminum alloy casting alloy of the illustrated embodiment and very little experience with aluminum alloy 2319 welding wire. The aluminum alloy casting alloy of the illustrated embodiment can be heat treated after welding to develop improved properties. Further, solution treatment after welding but prior to HIP'ing results in significantly improved yield strength. Solution treatment after welding but prior to HIP'ing resulted in an 85% increase in yield strength in the T4 condition and a 27% increase in yield strength in the T6 condition. A comparison of the tensile properties after welding of aluminum alloy wrought alloy 2219, aluminum alloy wrought alloy 2519 (from published data) and the aluminum alloy casting alloy of the illustrated embodiment is displayed in Table 13 and the tensile properties of the aluminum alloy casting alloy of the illustrated embodiment after welding followed by heat treatment is displayed in Table 14.
*from U.S. Pat. No. 4,610,733
The aluminum alloy casting alloy of the illustrated embodiment in the T61 condition has ballistic properties similar to aluminum alloy wrought alloy 2519 in the T87 condition. Samples of the aluminum alloy casting alloy of the illustrated embodiment in the T61 condition and aluminum alloy wrought alloy 2519 in the T87 condition were machined to the same size and dimensions and shot at with 0.223 caliber standard rounds at a distance of approximately 50 meters (150 feet). Single plates, 0.5″ thick, of the aluminum alloy casting alloy of the illustrated embodiment in the T61 condition and the aluminum alloy wrought alloy 2519 in the T87 condition were completely penetrated, as illustrated in
Double plates of the aluminum alloy casting alloy of the illustrated embodiment in the T61 condition and the aluminum alloy wrought alloy 2519 in the T87 condition were not penetrated after the same ballistic testing described above, as illustrated in
Articles Made From the Aluminum Alloy Casting Alloy of the Illustrated Embodiment
The aluminum alloy casting alloy of the present illustrated embodiment is ideally suited for articles requiring high strength, high toughness, resistance to penetration by ballistic objects, resistance to stress corrosion cracking and light in weight. These articles are commonly used in racing, aerospace and military (land, sea and air) vehicles. Specifically, a seat frame for a military vehicle, (
Preferred Embodiments
The composition for an aluminum alloy casting alloy according to principles of the present invention, in weight percent, is as follows:
wherein the aluminum alloy casting alloy is grain refined using a 0.04-2.00 weight % addition of aluminum-5 weight % titanium-1 weight % boron and a 0.07-2.00 weight % addition of aluminum-3 weight % titanium-0.15 weight % carbon containing grain refiner.
An optimum composition for the aluminum alloy casting alloy, in weight percent, is as follows:
wherein the aluminum alloy casting alloy is grain refined using a 0.04-0.08 weight % addition of aluminum-5 weight % titanium-1 weight % boron and a 0.07-0.10 weight % addition of aluminum-3 weight % titanium-0.15 weight % carbon containing grain refiner.
A heat treatment for an aluminum alloy casting alloy according to principles of the present invention is as follows:
The optimum heat treatment for the aluminum alloy casting alloy is as follows:
