Production of ultra-fine grain structure in as-cast aluminium alloys

Abstract: A method of controlling grain size in as-cast aluminum alloy having the steps of a) providing a molten aluminum alloy including an alloying element selected from the group consisting of Ti, Sc, Zr, V, Hf, Nb and Y; b) adding a grain refiner to the molten aluminum alloy to form a melt; and c) solidifying the melt to form an ingot. The grain refiner includes Ti and B or C, and is added to the melt in an amount to yield the concentration in the melt of B or C from the grain refiner of about 0.003-0.010 wt. %. Grains in the as-cast aluminum alloy are about 200 microns or less in size.

Description: 1. Field of the Invention This invention relates to production of as-cast aluminum alloys with ultra-fine grain structure. More particularly, it relates to methods of adding a grain refiner to a molten aluminum alloy at levels which reduce the grain size to less than about 200 microns. 2. Prior Art The size and shape of grains in as-cast aluminum alloy impacts properties of wrought or cast products. Casting with large grains, particularly dendritic grains, is highly prone to cracking during casting and reduce ductility, fracture toughness and fatigue properties. Reduction of the size as well as the form of the grains may be accomplished by mechanical or electromagnetic stirring to break up the grains. Grain size may also be controlled metallurgically by adding a grain refiner to the aluminum alloy melt. A typical grain refiner used for aluminum alloys is cither an Al-Ti-B alloy or an Al-Ti-C alloy in the form of a rod or waffle. A grnin refiner consists of numerous fine boride or carbide particles in an aluminum matrix. When these grain refiners are added to the aluminum alloy melt, the boride or carbide particles are dispersed into the melt and serve as nucleating sites for grains during solidification. Commercially available grain refiners include alloys containing about 3-5 wt, % Ti and about 0.15-1 wt. % B or C and the balance Al. According to this practice, a residual amount of Ti is present in or is added to the aluminum melt (e.g. less than 0.015wt. %), and a controlled amount of the grain refiner is added thereto which increases the total Ti concentration in the final melt by about 0.001-0.003 wt. %. In this manner, the amount of B or C added to the melt via the grain refiner is about 0.0001-0.001 wt. %. The addition of grain refiners at these conventional levels can control the size of dendritic grains to be about 250-1000 microns. For certain cast or wrought aluminum products, such a grain structure is sufficiently fine and cracking or other mechanical problems are not experienced. However, a need remains for as cast aluminum alloys with an ultra-fine grain structure, i.e. about 200 microns in size or less. It has been found that ultra-fine grain size may be achieved by supersaturating a molten alloy with dispersoid-forming elements such as Zr, Mn, Cr, V, Ti, Sc and Hf as disclosed in U.S. Patent No. 6,004,506. That process requires the addition of a specialized, pre-alloyed ribbon of material containing the dispersoid-forming elements into a pool of molten metal formed during ingot casting. Accordingly, a need remains for a method of producing aluminum alloys with ultra-fine grain structure using readily available additives. The document US-A-4 377 425 discloses a method of producing fine grain aluminium comprising the steps of : a) providing a molten aluminium alloy from the AA 1000 and AA 5000 series; b) adding Ca so that the melt comprises 0.0005-0.05 wt% Ca; c) adding a Al-Ti-B grain refiner so that B is added from 0.0001 to 0.02 wt.%; d) solidifying the melt to form a D.C. cast ingot wherein the grain size of the central portion of the ingot inward of surface coarse cell zone is lower than 150 microns. This need is met by the method of the present invention. In accordance with one aspect of the present invention, there is provided a method ofproducing fine grain aluminium comprising the steps of: (a) providing a molten aluminum alloy which is a 1XXX, 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX series alloy,adding to said molten alloy; an alloying element selected from the group consisting of Sc, Hf, Nb and Y; (b) adding a grain refiner to the molten aluminum alloy containing said alloying element to form a melt, wherein the grain refiner comprises (i) Ti and (ii) B or C, such that the concentration in the melt of B or C from the grain refiner is 0.003-0.010 wt.%; and (c) solidifying the melt to form an ingot wherein grains in the ingot are 200 microns or less in size. In accordance with a further aspect of the present invention, there is provided a method of producing fine grain aluminum having the steps of providing a molten aluminum alloy selected from 1XXX, 2XXX, 3XXX, 5XXX, 6XXX, 7XXX and 8XXX alloys, adding, 0.015-0.030 wt.% Ti to the molten alloy and adding a grain refiner to the molten alloy containing the said titanium to form a melt, wherein the grain refiner includes Ti and B or C, and solidifying the melt, wherein the grain refiner is added in an amount such that the concentration in the melt of B or C from the grain refiner is 0.003-0.010 wt.% and the grains in the solidified melt are 100 microns or less in size. element is Sc at a concentration of about 0.030-0.10 wt. % in the molten aluminum alloy. The alloy is preferably an alloy of the 2XXX or 7XXX AA series. Particularly preferred wrought alloys are 7055 and 7050 alloys. Other features of the present invention will be further described in the following related description of the preferred embodiment which is to be considered together with the accompanying drawings wherein: Fig. 1 is a photomicrograph of 7055 alloy produced according to the present invention; and Fig. 2 is photomicrograph of 7055 alloy produced according to the prior art. For purposes of the description hereinafter, it is to be understood that the invention may assume a number of alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting. The present invention includes a method of controlling the grain size in oast aluminum alloys to about 200 microns or less. According to the inventive method, an alloying element is added to a molten aluminum alloy. Preferred added alloying elements are Ti, Sc, Hf, Nb and Y, more preferably Ti and Sc when Ti is added, the concentration of the alloying element Ti in the molten aluminum alloy is about 0.015-0.030 wt. %. When the alloying element is Sc, a preferred concentration of residual Sc in the molten aluminum alloy is about 0.030-0.1 wt, %. When the alloying element is Hf, Nb or Y, the preferred concentration thereof is on the order of the preferred concentrations of Ti and Sc. A grain refiner is added to the molten aluminum alloy containing the residual alloying element to form a melt. Commercially available grain refiners are alloys consisting of Ti and B or C with the balance aluminum. Typical concentrations in the grain refiner are about 3-5 wt. % Ti and about 0.15-1 wt. % B or C. The final concentration of B or C in the melt from the grain refiner is about 0.003-0.010 wt. %. Suitable commercially available grain refiners have compositions such as 3 wt. % Ti, 1 wt. % B and balance Al (referred to as Al-3%Ti-1%B), Al-5%Ti-1%B, Al-3%Ti-0.2%B, Al-5%Ti-0.2%B, Al-3%Ti-0.15%C, or Al-3%Ti-0.3%C. These grain refiners typically are provided in the form of a rod or waffle. The ratio of B or C to Ti in the grain refiner is normally fixed; hence, the amount of grain refiner added to the melt controls the final amount of Ti present in the melt. For example, in order to achieve a concentration of B of about 0.003-0.010 wt. % in 100 pounds of a melt containing 0.02 wt. % Ti, about 0.003-0.010 pounds of B are needed in the melt. When a grain refiner consisting of Al-3%Ti-1%B is used, about 0.3-1 pound of grain refiner are added to the melt. This results in an additional about 0.009-0.030 wt. % Ti added to the melt from the grain refiner for a total concentration of Ti in the melt of about 0.029-0.050 wt. %. Conventional grain refining practice dictates using less than 0.001 wt. % B in the melt which is 3 to 10 times less grain refiner than is added according to the present invention. Likewise, the incremental concentration of Ti from the grain refiner for conventional practice is 3 to 10 times less than the amount of Ti added to the melt by the grain refiner according to the present invention. The present invention may be used to control grain size in wrought and cast alloys. Suitable alloys include Aluminum Association (AA) wrought alloys of the 1XXX, 2XXX, 3XXX, 5XXX, 6XXX, 7XXX and 8XXX series. Alloys of the AA 2XXX and 7XXX series are particularly suited to treatment according to the present invention. As-cast aluminum alloy produced according to the present invention has globular grains which are about 200 microns or less in size, typically about 80 microns in size. In contrast, conventional grain refining practice of adding a grain refiner of Al-3%Ti-1%B such that the concentration of B in the melt is 0.001 wt. % produces dendritic grains sized about 1000 microns. These large dendritic grains interlock with each other and render the cast alloy rigid and prone to cracking, whereas the small, globular grains formed by the method of the present invention reduce crack initiation during casting and improve formability during deformation. Although the invention has been described generally above, the particular example gives additional illustration of the product and process steps typical of the present invention. Example 50 Pounds (lbs) of aluminum alloy 7055 was melted, and 0.0154 lb of 97% Ti powder compact was added to the molten alloy to achieve a concentration of about 0.03 wt, % Ti. A grain refiner of Al-3%Ti-1%B (0.335 lbs) was added to the melt at 1300° F. The melt was stirred for 1 minute after the addition of the grain refiner. The final concentration ofB in the melt was about 0.0066 wt. %, and the final concentration of Ti in the melt was 0.05 wt. % (0.03 wt. % residual Ti, plus 0.02 wt. % Ti added from the grain refiner.) The melt was cast into an ingot. The microstructure of the ingot is shown in Fig. 1. The bright areas of globular grains are clearly seen and are less than about 100 microns in size. 50 Pounds of aluminum alloy 7055 was melted. A grain refiner of Al-3%Ti-1%B was added to the melt at 1300° F according to conventional commercial practice. The melt was stirred for 1 minute after the addition of the grain refiner. The melt was cast into an ingot. The microstructure of the ingot is shown in Fig. 2. The bright arcas of dendritic grains are clearly seen and are up to about 1000 microns in size. It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Such modilications are to be considered as inoluded within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims.

Claims: A method of producing fine grain aluminum comprising the steps of (a) providing a molten aluminum alloy which is a 1XXX, 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX series alloy,adding to said molten alloy an alloying element selected from the group consisting of Sc, Hf, Nb and Y; (b) adding a grain refiner to the molten aluminum alloy containing said alloying element to form a melt, wherein the grain refiner comprises (i) Ti and (ii) B or C, such that the concentration in the melt of B or C from the grain refiner is 0.003-0.010 wt.%; and (c) solidifying the melt to form an ingot wherein grains in the ingot are 200 microns or less in size. The method of claim 1, wherein the alloying element is Sc and the concentration of Sc in the molten aluminum alloy is 0.030-0.10 wt.% The method of either of the preceding claims, wherein the alloy is a 2XXX or 7XXX series alloy. The method of claim 3, wherein the alloy is a 7055 or 7050 alloy. The method of any of claims 1-4, wherein grains in the ingot or casting are 100 microns or less in size. A method of producing fine grain aluminum having the steps of providing a molten aluminum alloy selected from 1XXX, 2XXX, 3XXX, 5XXX, 6XXX, 7XXX and 8XXX alloys, adding 0.015-0.030 wt.% Ti to the molten alloy and adding a grain refiner to the molten alloy containing the said titanium to form a melt, wherein the grain refiner includes Ti and B or C, and solidifying the melt, wherein the grain refiner is added in an amount such that the concentration in the melt of B or C from the grain refiner is 0.003-0.010 wt.% and the grains in the solidified melt are 100 microns or less in size.

Inventors: CHU MEN GLENN, CHU, MEN GLENN
Applicants: ALCOA INC, ALCOA INC.
Application year: 2001
Main classification: C22C1
IPC codes: C22C1, C22C21