Method for grain refinement of high strength aluminum casting alloys

Abstract

An improved aluminum base alloy having improved hot crack resistance when solidified into cast products, the alloy comprised of 4 to 5.5 wt. % Cu, max. 0.5 wt. % Mn, max. 0.55 wt. % Mg, max. 0.2 wt. % Si, up to 0.5 wt. % Fe, optionally 1.13 to 1.7 wt. % Ni, 0.005 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. Ser. No. 09/393,503, filed Sep. 10, 1999.

BACKGROUND OF THE INVENTION

[0002] This invention relates to improved aluminum base alloys having improved hot crack resistance when solidified into cast products.

[0003] It is well known that pure aluminum is soft. Thus, in order to produce high strength castings from aluminum, significant amounts of other elements must be added. These chemical additions strengthen the metal considerably, but have the problem that they tend to form low melting point eutectics. The practical consequence of this, from the foundryman's point of view, is that high strength casting alloys have a wide freezing range.

[0004] Relatively pure aluminum alloys (greater than about 99 wt. % Al) freeze over a temperature interval of 5-10° C., or less. High strength casting alloys, on the other hand, usually contain less than 95 wt. % Al and freeze over a temperature interval of 50-100° C., or more.

[0005] During solidification of high strength casting alloys, there is a ‘mushy’ mixture of solid and liquid metal present in the mold as it cools through this wide freezing range. There is thermal contraction of solid during this cooling and solidification process, and the shrinkage of the solid has the problem that it often results in the formation of hot cracks (hot cracks are also called hot tears). Hot cracking of high strength casting alloys is a serious problem, and has prevented significant commercial use of many alloys, in spite of their excellent properties.

[0006] There are few examples of grain refining practices proposed for casting alloys in the prior art. Sigworth and Guzowski (U.S. Pat. Nos. 5,055,256 and 5,180,447; and related foreign patents) discovered that an alloy containing a boride of “mixed” composition; (Al, Ti)B2, gave the best results. They proposed a master alloy having a nominal composition of 2.5 wt. % Ti and 2.5% B for best grain refinement in casting alloys. This method of grain refinement did not produce smaller grain sizes, however. It only produced equivalent grain sizes at reduced cost. As such, this method of refinement does not represent a solution to the hot cracking problem in high strength casting alloys.

[0007] Aimberg, Halvorsen and Tondel (EP 0553533) have proposed a Si—B alloy refiner for use in casting alloys. W. C. Setzer and co-workers (U.S. Pat. No. 5,230,754) have proposed an Al—Sr—B master alloy, to simultaneously grain refine and to modify the eutectic in Al—Si alloys. However, these methods do not produce the desired smaller grain sizes.

[0008] D. Apelian and J-J. A. Cheng have proposed an Al—Ti—Si master alloy (U.S. Pat. No. 4,902,475), but this alloy does not appear to be suitable for grain refinement of high strength casting alloys.

[0009] In addition to the patents mentioned above, U.S. Pat. Nos. 3,634,075; 3,676,111; 3,785,807; 3,857,705; 3,933,476; 4,298,408; 4,612,073; 4,748,001; and 4,812,290 disclose different master alloy compositions and methods of manufacture.

[0010] Other nucleating particles may be used and include several commercial master alloys for grain refining based on the Al—Ti—C system. These master alloys introduce microscopic TiC particles as nucleating agents into the melt. The TiC particles are disclosed in U.S. Pat. Nos. 4,710,348; 4,748,001; 4,873,054; and 5,100,488. Nucleating particles, such as sulfides, phosphides or nitrides (e.g., U.S. Pat. No. 5,100,488) may also be used.

[0011] It will be seen that there is still a great need for an improved aluminum alloy and method of grain refinement of high strength, aluminum-based casting alloys which permits use of high strength alloys without the attendant problem of hot cracking.

SUMMARY OF THE INVENTION

[0012] It is an object of this invention to provide an improved high strength aluminum alloy substantially free from hot cracking.

[0013] It is another object of this invention to produce a smaller grain size in cast parts made from high strength, aluminum-based casting alloys.

[0014] Yet, it is another object of this invention to reduce or eliminate the problem with hot cracking associated with solidification of these same casting alloys.

[0015] Still, it is another object of this invention to produce high strength casting alloys having a better distribution of gas porosity, smaller diameter gas pores, a lessor amount of porosity, and higher fatigue strength.

[0016] And still, it is another object of this invention to produce improved grain refinement of high strength, aluminum-based casting alloys at reduced cost.

[0017] These and other objects will become apparent from a reading of the specifications, examples and claims appended hereto.

[0018] In accordance with these objects there is provided an improved aluminum base alloy having improved hot crack resistance when solidified into cast products, the alloy comprised of 4 to 5.5 wt. % Cu, max. 0.5 wt. % Mn, max. 0.55 wt. % Mg, max. 0.2 wt. % Si, up to 0.5 wt. % Fe, optionally 1.13 to 1.7 wt. % Ni, 0.005 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities.

BRIEF DESCRIPTION OF THE DRAWING

[0019] FIG. 1 illustrates a scale drawing of the casting used to evaluate the new grain refining practice and locations where cracks were observed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] It will be useful to consider some examples of alloys at this point. In the United States it is customary commercial practice to refer to alloy grades established by the Aluminum Association (900 19th. Street, Washington, D.C. 20006). These alloy grades are detailed in the “Registration Record of Aluminum Association (AA) Alloy Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot” and by reference thereto are incorporated herein by reference as if specifically set forth.

[0021] It will be useful to explain in more detail the nomenclature system adapted by the Aluminum Association, and to also define technical terms used herein.

[0022] The term “ingot” as used herein is meant to include semi-finished castings intended for further processing in the foundry and may include billet or slab or other solidified aluminum. This further processing may include bringing the ingot into the molten state, subjecting the resulting molten metal to various refining operations (such as degassing), and making small amounts of chemical additions (such as grain refiners) to the melt. The prepared molten alloy is then poured into a shaped mold, wherein it freezes. When it is fully solidified, the now solid alloy is removed from the mold to provide a casting. The term “casting” as used herein is meant to include a net shaped casting; or since the casting often receives subsequent processing steps (such as machining, polishing, or sometimes forging), a near net-shape casting.

[0023] It should be noted that reference to AA alloy 206 includes two separate alloys: 206.0 and 206.2. The term 206.0 refers to the alloy in the form of a casting. The term 206.2 refers to the name of the same alloy in the form of ingot.

[0024] For AA alloy 206, the AA chemical composition limits are the same for both, except the maximum allowable iron content in the casting (206.0) is 0.15%, whereas the maximum iron allowed in the ingot (206.2) is lower, 0.10%. This difference in iron content is common in most of the AA chemical composition limits. This results from the use of iron tools (ladles, skimmers, and so on) when handling the molten metal, and it is inevitable that a certain amount of this iron dissolves into the liquid aluminum and thereby is incorporated in the casting.

[0025] The suffix “0” in the alloy name (as in 206.0) always refers to a casting. The suffix “1” or “2” (both are used for historical reasons) always refers to ingot.

[0026] There is also an “A” version of 206 alloy (A206.0 and A206.2) which is similar to 206 except that lower quantities of undesirable impurities (Si, Fe, and Ni) are called for.

[0027] The term “high strength casting alloy” refers to an alloy which contains more than about 5% total alloying elements therein, and consequently, less than about 95% aluminum. A high strength casting will normally have a yield strength greater than about 30,000 pounds per square inch (psi) in the fully heat treated (aged) condition; or more than about 20,000 psi in castings which do not receive artificial aging, or heat treatment. The meaning of the term ‘high strength casting alloy’ is further elucidated by considering the following two examples.

[0028] Alloy A356 is an alloy which finds extensive use in the production of high quality aerospace and automotive castings. It is also used for a wide variety of commercial castings. The alloy is easily cast, and through heat treatment can be brought to a wide variety of strength levels. A356 alloy contains 6.5 to 7.5 wt. % Si and 0.25 to 0.45 wt. % Mg, plus other normally occurring impurity elements at concentrations less than 0.2% each. The typical mechanical properties expected in permanent mold castings of this alloy (as published by the American Foundrymen's Society in a book entitled Aluminum Casting Technology, 2nd. Ed.) when heat treated to the T6 (strongest) condition are shown below: 1 Typical Mechanical Properties for A356.0 Alloy Temper Yield Strength (psi) Ultimate Strength (psi) Elongation (%) T6 30,000 41,000 12.0

[0029] Another important alloy is A206.0, which contains 4.2-5.0 wt. % Cu, 0.2-0.35 wt. % Mn, 0.15-0.35 wt. % Mg and 0.15-0.30 wt. % Ti plus normally occurring impurity elements. Typical mechanical properties of permanent mold castings in this alloy are: 2 Typical Mechanical Properties for A206.0 Alloy Temper Yield Strength (psi) Ultimate Strength (psi) Elongation (%) T7 50,000 63,000 11.7

[0030] The AA 206 alloy casting is significantly stronger. This means that castings from this alloy could be made lighter for the same load bearing properties. In the case of automotive applications, this would mean a lighter, faster, and more fuel-efficient automobile. But the AA 206 alloy is rarely used, while 356 alloy is commonly used because the freezing range of 356 alloy is about 50 degrees, and it is relatively immune to hot cracking. The freezing range of 206 alloy is about 120 degrees, and it is well known to be susceptible to hot cracking problems.

[0031] High strength casting alloys have the problem that they are more difficult to grain refine than pure aluminum or wrought alloys. Thus, the usual procedure has been to employ larger additions of titanium, and this procedure has often been codified into the Aluminum Association chemical composition limits. It will be seen that in the case of A206 alloy, a minimum Ti concentration of 0.15% is specified, and a maximum of 0.30% is allowed.

[0032] The situation is the same for a number of other high strength casting alloys. In the AA 200 series of alloys (which contain Al and 3.5-9 wt. % Cu) alloys 201, A201, B201, 203, 204, and 206 all have a specified minimum Ti content of 0.15%. Alloys 242 and 243 have a minimum Ti specified of 0.07% and 0.06% respectively. It will be noted that minimum Ti levels are also specified for AA alloys A355, B356, C356, A357, B357, C357, D357, 358, 393, 516, 535, B535, 712, 771 and 772 alloys, the composition of these alloys included herein by reference as if specifically set forth.

[0033] Even in alloys where no minimum Ti content is specified, the maximum allowable is quite high—generally 0.20 or 0.25 wt. % Ti—and the usual practice is to use fairly large amounts of Ti in the alloy.

[0034] Other aluminum alloys suitable for cast products included within the purview of this invention are set forth in the following table. 3 TABLE Alloy Compositions in Weight Percent Alloy Si Fe Cu Mn Mg Cr Ni Zn Sn Ti L201.0(1) 0.20 0.15 4.0-5.2 0.20-0.60 0.15-0.6  — — — — 0.01-0.12 L201.2(1) 0.20 0.10 4.0-5.2 0.20-0.60 0.15-0.6  — — — — 0.01-0.10 LA201.0(1) 0.05 0.10 4.0-5.0 0.20-0.40 0.15-0.35 — — — — 0.01-0.12 LA201.1(1) 0.05 0.07 4.0-5.0 0.20-0.40 0.15-0.35 — — — — 0.01-0.10 LB201.0(2) 0.05 0.05 4.5-5.0 0.20-0.50 0.25-0.35 — — — — 0.01-0.12 L203.0(3) 0.30 0.50 4.5-5.5 0.20-0.30 0.10 — 1.3-1.7 0.10 — 0.01-0.12 L203.2(3) 0.20 0.35 4.8-5.2 0.20-0.30 0.10 — 1.3-1.7 0.10 — 0.01-0.10 L204.0 0.35 0.40 4.2-5.2 0.10 0.15-0.35 — 0.05 0.10 0.05 0.01-0.12 L204.2 0.15 0.10-0.20 4.2-4.9 0.05 0.15-0.35 — 0.03 0.05 0.05 0.01-0.10 L206.0 0.20 0.20 4.2-5.0 0.20-0.50 0.15-0.35 — 0.05 0.10 0.05 0.01-0.12 L206.2 0.10 0.10 4.2-5.0 0.20-0.50 0.15-0.35 — 0.03 0.05 0.05 0.01-0.10 LA206.0 0.05 0.10 4.2-5.0 0.20-0.50 0.15-0.35 — 0.05 0.10 0.05 0.01-0.12 LA206.2 0.05 0.07 4.2-5.0 0.20-0.50 0.15-0.35 — 0.03 0.05 0.05 0.01-0.10 LA242.0 0.6  0.8  3.7-4.5 0.10 1.2-1.7 0.15-0.25 1.8-2.3 0.10 — 0.01-0.06 LA242.1 0.6  0.6  3.7-4.5 0.10 1.3-1.7 0.15-0.25 1.8-2.3 0.10 — 0.01-0.07 LA242.2 0.6  0.35 3.7-4.5 0.10 1.2-1.7 0.15-0.25 1.8-2.3 0.10 — 0.01-0.07 L243.0(4) 0.35 0.40 3.5-4.5 0.15-0.45 1.8-2.3 0.20-0.40 1.9-2.3 0.05 — 0.01-0.06 L243.1(4) 0.35 0.30 3.5-4.5 0.15-0.45 1.9-2.3 0.20-0.40 1.9-2.3 0.05 — 0.01-0.06 LA355.0 4.5-5.5 0.09 1.0-1.5 0.05 0.45-0.6  — — 0.05 — 0.01-0.03 LA355.2 4.5-5.5 0.06 1.0-1.5 0.03 0.45-0.6  — — 0.03 — 0.01-0.03 LA357.0(5) 6.5-7.5 0.20 0.20 0.10 0.40-0.7  — — 0.10 — 0.01-0.03 LA357.2(5) 6.5-7.5 0.12 0.10 0.05 0.45-0.7  — — 0.05 — 0.01-0.03 LB357.0 6.5-7.5 0.09 0.05 0.05 0.40-0.6  — — 0.05 — 0.01-0.03 LB357.2 6.5-7.5 0.06 0.03 0.03 0.45-0.6  — — 0.03 — 0.01-0.03 LC357.0(5) 6.5-7.5 0.09 0.05 0.05 0.45-0.7  — — 0.05 — 0.01-0.03 LC357.2(5) 6.5-7.5 0.06 0.03 0.03 0.50-0.7  — — 0.03 — 0.01-0.03 LD357.0(5) 6.5-7.5 0.20 — 0.10 0.55-0.6  — — 0.05 — 0.01-0.09 LA358.0(6) 7.6-8.6 0.30 1.0-1.5 0.05 0.45-0.6  — — 0.05 — 0.01-0.09 LA358.2(7) 7.6-8.6 0.20 1.0-1.5 0.03 0.45-0.6  — — 0.03 — 0.01-0.09 L516.0(8) 0.30-1.5  0.35-1.0  0.30 0.15-0.40 2.5-4.5 — 0.25-0.40 0.20 0.10 0.01-0.09 L516.1(8) 0.30-1.5  0.35-0.7  0.30 0.15-0.40 2.6-4.5 — 0.25-0.40 0.20 0.10 0.01-0.09 L535.0(9) 0.15 0.15 0.05 0.10-0.25 6.2-7.5 — — — — 0.01-0.10 L535.2(10) 0.10 0.10 0.05 0.10-0.25 6.6-7.5 — — — — 0.01-0.10 LB535.0 0.15 0.15 0.10 0.05 6.5-7.5 — — — — 0.01-0.10 LB535.2 0.10 0.12 0.05 0.05 6.6-7.5 — — — — 0.01-0.10 L712.0 0.30 0.50 0.25 0.10 0.50-0.65 0.40-0.6  — 5.0-6.5 — 0.01-0.10 L712.2 0.15 0.40 0.25 0.10 0.50-0.65 0.40-0.6  — 5.0-6.5 — 0.01-0.10 L771.0 0.15 0.15 0.10 0.10 0.8-1.0 0.06-0.20 — 6.5-7.5 — 0.01-0.10 L771.2 0.10 0.10 0.10 0.10 0.85-1.0  0.06-0.20 — 6.5-7.5 — 0.01-0.10 L772.0 0.15 0.15 0.10 0.10 0.6-0.8 0.06-0.20 — 6.0-7.0 — 0.01-0.10 L772.2 0.10 0.10 0.10 0.10 0.65-0.8  0.06-0.20 — 6.0-7.0 — 0.01-0.10 Notes: Single numbers refer to maximum amounts. The alloys can include other elements in minor amounts, such as Ag, Sb, Co, Zn, Zr, V, Be and B, for example. (1)Ag is present in the range of 0.4 to 1 wt. %. (2)Ag is present in the range of 0.5 to 1 wt. %. (3)This alloy contains 0.1 to 0.4 wt. % Sb, 0.1 to 0.4 wt. % Co, and 0.1 to 0.4 wt. % Zr, with Ti + Zr less than 0.5 wt. %. (4)V is present in the range of 0.06 to 0.20 wt. %. (5)Be is present in the range of 0.04 to 0.7 wt. %. (6)Be is present in the range of 0.1 to 0.3 wt. % (7)Be is present in the range of 0.15 to 0.3 wt. %. (8)Pb may be present up to 0.1 wt. %. (9)Be is present in the range of 0.003 to 0.007 wt. %, and B is less than 0.005 wt. %. (10)Be is present in the range of 0.003 to 0.007 wt. %, and B is less than 0.002 wt. %.

[0035] Fe and Si levels in these alloys such as L204 and L206 type alloys do not need to be less than 0.015 wt. % for each of Fe and Si.

[0036] The following examples are further illustrative of the invention.

EXAMPLE 1

[0037] A series of melts of Al-4.5 wt. % Cu alloy were prepared, and small additions of titanium briquette were added to the melts to produce various dissolved Ti levels. This alloy, 4.5 wt. % Cu, remainder aluminum, is similar to a number of the AA 2000 series casting alloys, which were discussed herein. The melt was allowed to sit for two hours, so that all of the Ti added went into solution, and so that it would no longer produce grain refinement. During this time, the melt was held at a temperature of 730° to 750° C., which is sufficient to put all of the added Ti in solution.

[0038] A constant addition of a grain nucleating agent comprised of boron was made by adding a quantity of commercial Al-3Ti-1B (3 wt. % Ti, 1 wt. % B, remainder aluminum) master alloy to the melts. The addition made was equivalent to an increase of 0.002 wt. % B, or 0.006 wt. % Ti in the melt.

[0039] Grain size samples were then taken by using a hockey puck test. In this test, a steel ring was placed on top of a polished refractory block, and molten metal was poured inside the ring. The bottom surface was etched by placing briefly in acid, and the grain size was determined with a low powered binocular microscope, by using the line intercept method described in ASTM E112. The resulting grain size, as measured by the average intercept distance, is given below: 4 Test No. Alloy Composition Grain Size (microns) 1 Al-4.5Cu-0.18 wt. % Ti 158 2 Al-4.5Cu-0.05 wt. % Ti 127 3 Al-4.5Cu-0.025 wt. % Ti 107 4 Al-4.5Cu-0.005 wt. % Ti  93

[0040] Only in the the first test was the amount of titanium sufficiently high (0.18%) to meet the chemical composition limits required by the Aluminum Association for 206 alloy, and for other similar AA 200 series alloys. However, this test produced the largest grain size. Reducing the dissolved Ti level significantly improved (reduced) the grain size. That is, the lower Ti levels resulted in significantly lower grain sizes.

[0041] This result is contrary to the teaching of the art. It is the usual commercial practice to add Ti, in relatively large quantities, in the form of various master alloys. From the above results, it is apparent that the Ti content should be reduced, and minimized as far as possible, not increased as in the current practice.

EXAMPLE 2

[0042] A permanent mold casting was selected to evaluate the new grain refining practice The casting to be used in these trials was a design subject to hot cracking. The part selected was the support bracket shown in FIG. 1. This casting has two legs, each supported with a thin flange of metal on the outside of the leg. The casting is 11 inches wide (from left to right in FIG. 1), 5.2 inches high (from top to bottom in FIG. 1), and 1.5 inches thick (not shown in FIG. 1). The allows indicate the four corner locations where cracks are observed in the castings, when subjected to a die penetrant test.

[0043] Two alloys were prepared. One was a conventional AA 206 alloy, which had 0.20-0.24 wt. % Ti after grain refining additions were made. A total of 45 castings were poured with the conventional AA 206 alloy. The second melt had a much lower Ti content, between 0.06 and 0.09 wt. % Ti after grain refiner additions. A total of 54 castings were poured from this new alloy. This alloy is called L206 below; the ‘L’ designating a low Ti content.

[0044] Aside from the difference in Ti content, the two alloys were nearly the same composition. An average of all chemical analyses, taken from sections cut from the casting, are tabulated below. All other casting parameters, such as pouring temperature and dissolved gas content, were maintained the same as far as possible. 5 wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % Alloy Cr Cu Fe Mg Mn Ni Si Ti V Zn 206 0.001 4.32 0.12 0.23 0.39 0.002 0.061 0.239 0.011 0.005 L206 0.001 4.40 0.12 0.18 0.27 0.002 0.061 0.075 0.008 0.002

[0045] A grain refiner addition was made to the furnace by adding a quantity of Al-10T-1B master alloy. Castings were poured. Then additional grain refiner was placed in metal transfer ladle, in the form of pieces of cut rod. Al-5Ti-1B and Al-1.7Ti-1.4B rod were both used to add nucleating particles. Additional castings were poured at the higher boron addition levels.

[0046] In some castings the foot at the lower left hand side (below arrow 4 in FIG. 1) was cut off and subjected to metallographic examination. The piece was ground and polished, and etched with Keller's reagent. The grains were examined under a microscope with polarized light, and the average intercept distance (AID) was measured. The results of the measurements are shown below: 6 wt. % Grain Size Addition wt. % B Alloy Ti (microns) Made Added L206 0.062 59 10 Ti-1 B  0.006 L206 0.084 56 5 Ti-1 B 0.02  L206 0.066 68 1.7 Ti-1.4 B 0.026 206 0.224 120  10 Ti-1 B  0.006 206 0.209 118  5 Ti-1 B 0.02  206 0.241 99 1.7 Ti-1.4 B 0.026

[0047] Two important facts may be drawn from this result. Firstly, in all cases the grain size in the new L206 allows is significantly smaller than in the conventional alloy. And secondly, the method of adding nucleating particles does not seem to be important.

[0048] All casting were examined for cracks by using the dye penetrant test. The results of this inspection are shown below: 7 Casting Location Alloy Number of Cracks 206  3-1 3 206  3-2 3 206  3-3 1, 3, 4 206  7-1 2 206  7-2 2, 3, 4 206  7-3 1, 2, 3, 4 206 10-2 2, 3 206 10-3 1, 2 206 12-2 3 206 12-3 2 L206 5L-2 3

[0049] This is very significant result. Ten of the 206 alloy castings (22% of the 45 castings poured) exhibited a total of 19 cracks. Only one of the L206 castings (5L-2, 1.9% of the 54 castings poured) was cracked, and only a single crack was observed. Thus, the occurrence of hot cracks in L206 alloy castings was reduced by a factor of ten or twenty times, which is a marked improvement.

[0050] In a number of castings a tensile sample was cut from one of the legs of the casting. These samples were pulled until fracture, yielding the following test results: 8 Yield Strength Ultimate Strength Elongation Alloy (psi) (psi) (%) 206 34,700 45,900  9.2 L206 35,200 49,700 11.8

[0051] It can be seen that the new alloy also exhibits better mechanical properties in the final casting.

[0052] It can be seen from the above examples that maintaining the Ti content in the ingot at a level below about 0.1 wt. % produces the desired smaller grain size, and significantly reduced hot cracking. Further, it is preferred to maintain the Ti content below a maximum of 0.06 wt. %. And a still smaller maximum Ti content of 0.03 wt. % will produce the smallest grains. The titanium can range from about 0.005 to 0.1 wt. %, with typical amounts of titanium being in the range of 0.01 to about 0.06 wt. %.

[0053] In the above example the nucleating particles were microscopic borides added in the form of commercial Al—Ti—B master alloys. These master alloys are well known commercially.

[0054] Nucleating particles or agents such as TiAl3, TiC or TiB2, for example, can be used to initiate nucleation to provide small grains in the aluminum alloys of the invention. Examples of master alloys which provide nucleating agents include A15Ti1B, Al3Ti1B, Al2.5Ti2.5b, Al1.5Ti1.4B, Al3Ti0.1C and Al6Ti. While the invention has been demonstrated using nucleating particles containing Ti, it will be understood that other elements also form stable aluminides, borides or carbides. Thus, elements such as Nb, Sc, Ta, V, Y and Zr can be used to provide suitable grain refining compounds. The alloy ranges provided herein include all the numbers within the range as if specifically set forth.

[0055] It can readily be seen that the alloys of the invention will find commercial use in a number of products where high strength and light weight are required. Some examples of aircraft, missile and other aerospace applications include: structural casting members, gear and pump housings, landing gear components, generator housings, aircraft fittings, supercharger housings, and compressors. Light weight is also important for fuel economy in automotive applications. Examples of vehicular members or near net shape cast products for transportation applications include: cylinder heads, pistons, gear and air conditioning housings, spring hangers, superchargers, support brackets, front steering knuckles, subframes and cross-members, differential carriers, transmission and belt tensioner brackets, and pedestal rocker aims.

[0056] Typically, cooling or solidification times for castings made in accordance with this invention can range from about 10 to 300 seconds, in order to obtain small grain size and improved hot tearing resistance. Grain sizes obtainable for cast products can range from 10 to 125 microns, preferably 20 to 100 microns, and typically 30 to 80 microns. In permanent mold castings the grains will be smaller, and in sand castings the grain size tends to be larger, because of slower cooling rates.

[0057] While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass other embodiments which fall within the spirit of the invention.

Claims

1. An improved aluminum base alloy casting having improved hot cracking resistance in the as-cast condition, the alloy comprised of 4 to less than 5 wt. % Cu, 0.15 to 0.55 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0.015 to 0.2 wt. % Si, 0.015 to 0.5 wt. % Fe, 0.01 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities, cast products formed from the alloy having a grain size of less than 125 microns.

2. A near net shape cast product formed from an aluminum base alloy, the cast product having increased resistance to hot cracking in the as-cast condition, the alloy comprised of 4 to less than 5 wt. % Cu, 0.15 to 0.55 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0.015 to 0.2 wt. % Si, 0.015 to 0.5 wt. % Fe, 0.01 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities, the cast product having a grain size of less than 125 &mgr;m.

3. A method of casting an aluminum base alloy to provide a cast product having improved hot crack resistance in the as-cast condition and a grain size of less than 125 microns, the method comprising:

(a) providing an aluminum base alloy comprised of 4 to less than 5 wt. % Cu, 0.15 to 0.55 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0.015 to 0.2 wt. % Si, 0.015 to 0.5 wt. % Fe, the balance comprised of aluminum, incidental elements and impurities;

(b) maintaining Ti in the range of 0.005 to 0.12 wt. % to improve the resistance of said alloy to hot cracking;

(c) adding a nucleating agent selected from the group consisting of carbides, aluminides and borides; and

(d) solidifying said alloy to provide a cast product substantially free of hot cracks.

4. An improved vehicular or aerospace casting having increased resistance to hot cracking, the member formed from an aluminum alloy comprised of 4 to less than 5 wt. % Cu, 0.15 to 0.55 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0. 015 to 0.2 wt. % Si, 0.015 to 0.5 wt. % Fe, 0.01 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities, the casting having a grain size of less than 125 microns.

5. A near net shape cast vehicular casting formed from an aluminum base alloy, the vehicular casting having increased resistance to hot cracking, the alloy comprised of 4 to 5.5 wt. % Cu, 0.15 to 0.55 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0.015 to 0.2 wt. % Si, 0.015 to 0.5 wt. % Fe, 0.01 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities, the casting having a grain size of less than 125 &mgr;m.

6. A near net shape cast aircraft casting formed from an aluminum base alloy, the aircraft casting having increased resistance to hot cracking, the alloy comprised of 4 to 5.5 wt. % Cu, 0.15 to 0.55 wt. % Mg, 0.2 to 0.5 wt. % Mn, 0.015 to 0.2 wt. % Si, 0.015 to 0.5 wt. % Fe, 0.01 to 0.12 wt. % Ti, the balance comprised of aluminum, incidental elements and impurities, the casting having a grain size of less than 125 &mgr;m.