WO1991007513A2

WO1991007513A2 - Dual processing of aluminum base alloys

Info

Publication number: WO1991007513A2
Application number: PCT/US1990/003608
Authority: WO
Inventors: Santosh K. Das; Michael S. Zedalis; Paul S. Gilman
Original assignee: AlliedSignal Inc
Current assignee: Honeywell International Inc
Priority date: 1989-11-09
Filing date: 1990-06-26
Publication date: 1991-05-30
Anticipated expiration: 1992-05-09
Also published as: AU7162191A; WO1991007513A3

Abstract

An aluminum based alloy is produced from a charge containing a rapidly solidified aluminum alloy and a carbidiferous agent in an amount ranging from about 0.01 to 10 % by weight of the charge. The charge is ball milled energetically to uniformly mix the carbidiferous agent within the aluminum base alloy while maintaining the charge in a pulverulent state. Upon completion of the ball milling step, the charge is hot consolidated at suitable temperatures to react the carbidiferous agent with the aluminum base alloy, resulting in the formation of carbide and oxide particles, and to provide a powder compact having a formable, substantially void-free mass. The compact is especially suited for use in aerospace, automotive, electronic, strength critical components, and the like, which often encounter service temperatures approaching 500 °C.

Description

DUAL PROCESSING OF ALUMINUM BASE T-T.OVS

Background of the Invention 1. Field of the Invention

This invention relates to a process for improving the mechanical properties of metals, and more particularly to a process for stabilizing an aluminum base alloy by incorporation of oxides and carbides through mechanical alloying following rapid solidification thereof.

Z_* Description of the Prior Art

High strength aluminum base alloys, i.e., alloys containing greater than 50% by weight aluminum have been made by rapid solidification or mechanical alloying techniques. These alloys have useful mechanical characteristics at room temperature and elevated temperatures. A problem with rapidly solidified aluminum base alloys is their tendency to degrade at temperatures approching 500°C. Mechanically alloyed aluminum base alloys depend in part for strength on age hardened and/or work hardened internal structures. Such alloys also depend for strength on the formation, in-situ, of a fine dispersion of aluminum carbide (A1₄C₃) and aluminum oxide (A1₂0₃) by reaction of aluminum with the break-down products of a carbidiferous agent (e.g., stearic acid) used in the mechanical alloying process.

Although carbidiferous agents, said to be necessary for the mechanical alloying of aluminum base alloys, can become constituents in the final product (see, for example U.S. Patent 4,627,959) prior art teachings suggest that the resulting A1₄C₃ particles are not suitable for use at temperatures greater than 100°C. Specifically, it has been taught that upon exposure to temperatures above 100°C, age hardened structures and/or work hardened structure tend to soften. At higher temperatures the dispersion of AI4C₃ in the alloy is said to coarsen, thus lessening the contribution of carbide to the strength of the alloy. In consequence, aluminum base alloys of the prior art as produced by mechanical alloying are said to be generally unsuitable for use in the temperature range of 100°C to 500°C. For the above reasons, in use of a carbidiferous processing aid, it has been proposed (see U.S.P. 4,624,705) that strong carbide formers such as titanium be added to produce in the final alloy carbides more thermally stable than A1₄C3 at temperatures in excess of 100°C. Summary of the Invention

The present invention provides a process for producing a stabilized rapidly solidified aluminum base alloy suitable for use at temperatures approaching 500°C wherein a strong carbide former is not needed.

In addition, with the process of the invention, the ability to mechanically alloy the rapidly solidified material is not dependent on the presence of a carbidiferous agent. Consequently, the desired volume friction of resulting carbides and oxides can be engineered into the material without the restrictions heretofore required to control the mechanical alloying process. More specifically, there is provided a process for producing an aluminum base alloy comprising the steps of forming a charge containing, as ingredients, a rapidly solidified aluminum alloy and carbidiferous agent in an amount ranging from about 0.01 to 10 wt. percent and ball milling the charge energetically to mix the carbidiferous agent within the aluminum base alloys maintaining the charge in a pulverulent state. Upon completion of the ball milling step, the resultant powder is hot pressed or sintered using conventional powder metallurgical techniques, to react the aluminum base alloys with the carbidiferous agent resulting in the formation of carbides and oxides, and to form a powder compact having a mechanically formable, substantially void-free mass. The compressed and treated powder compact is then mechanically worked to further react the carbidiferous agent and the aluminum base alloys, and to increase its density and provide engineering shapes suitable for use in aerospace components such as stators, wing skins, missile fins, actuator casings, electronic housings, automotive components such as piston heads, piston liners, valve seats and stems, connecting rods, cam shafts, brake shoes and liners, tank tracks, torpedo housings, radar antennae, radar dishes, space structures, sabot casings, and the like. Brief Description of the Prayings

The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiment of the invention and the accompanying drawings in which: Figs. 1A and IB are transmission electron micrographs of a rapidly solidified aluminum based iron, vanadium and silicon alloy ribbon and a rapidly solidified aluminum based titanium containing alloy ribbon produced by melt spinning;

Figs. 2A and 2B are photomicrographs of an aluminum based iron, vanadium and silicon containing alloys and an aluminum based titanium containing alloy fabricated by conventional ingot casting; and Fig. 3 is a photomicrograph of a rapidly solidified aluminum based iron, vanadium and silicon containing alloy powder processed using a carbidiferous agent in accordance with the present invention.

Description of the Preferred Embodiments The aluminum base, rapidly solidified alloy appointed for use in the process of the present invention has a composition consisting essentially of the formula Al^_g^Fe_gSi_j-X_j, wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 2.0 to 7.5 at %, "b" ranges from 0.5 to 3.0 at %, "c^H ranges from 0.05 to 3.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X]:Si ranges from about 2.0:1 to 5.0:1. Examples of the alloy include aluminum-iron-vanadium-silicon compositions wherein the iron ranges from about 2.0-7.5 at %, vanadium ranges from about 0.05-3.5 at %, and silicon ranges from about 0.5-3.0 at %.

Another aluminum base, rapidly solidified alloy suitable for use in the process of the invention has a composition consisting essentially of the formula Al_balFe_aSi_bX_c wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, , Nb, Ta, "a" ranges from 1.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c" ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X] :Si ranges from about 2.01:1 to 1.0:1. Still another aluminum base, rapidly solidified alloy that is suitable for use in the process of the invention has a composition range consisting essentially of about 2-15 at % from a group consisting of zirconium, hafnium, titanium, vanadium, niobium, tantalum, erbium, about 0-5 at % calcium, about 0-5 at % germanium, about 0-2 at % boron, the balance being aluminum plus incidental impurities. Rapid solidification of those alloys is accomplished in numerous ways, including planar flow or jet casting methods, melt extraction, splat quenching, atomization techniques and plasma spray methods. These metal alloy quenching techniques generally comprise the step of cooling a melt of the desired composition at a rate of at least about 10^°C/sec. Generally, a particular composition is selected, powders or granules of the requisite elements in the desired portions are melted an homogenized, and the molten alloy is rapidly quenched on a chill surface, such as a rapidly moving metal substrate, an impinging gas or liquid.

When processed by these rapid solidification methods the aluminum alloy is manifest as a ribbon, powder or splat of substantially uniform structure. This substantially uniformly structured ribbon, powder or splat may then be pulverized to a particulate for further processing. By following this processing route to manufacture the aluminum base alloy, the resultant microstructure is significantly refined and homogeneous. Such microstructural improvements typically result in improved ambient and elevated temperature strength, fracture toughness and ductility when compared to alloys of similar composition fabricated by conventional ingot casting or other techniques wherein the molten metal cools at relatively slow rates. The aluminum base alloy must be provided as a particulate that can range in size from 0.64 cm in diameter down to less than 0.0025 cm in diameter.

As used herein, the term "carbidiferous agent" means carbon based material including compounds and mixtures such as stearic acid, methanol, oxalic acid, etc. as well as carbonitrides and carbides containing free carbon. The term "energetic ball milling" in the context of the present specification and claims means milling at prescribed conditions where the energy intensity level is such that the carbidiferous agent is optimately kneaded into the aluminum matrix. As used herein, the phrase "prescribed conditions" means conditions such that the ball mill is operated to physically deform, fracture, cold weld and re-fracture the matrix metal alloy powder so as to distribute the carbidiferous agent therewithin. The phrase "optimately kneaded", as used herein, means that carbidiferous agent is distributed more uniformly than the distribution produced by simple mixing or blending, and approaches a substantially homogeneous distribution of carbidiferous agent within the matrix. Energetic ball mills include vibratory mills, rotary ball mills and stirred attritor mills.

After the ball milling step is completed, the resultant powder is compacted alone or mixed with additional matrix material, under conditions to promote the decomposition of the carbidiferous agent, and formation of carbides and oxides. Consequently, the resultant composite compact is vacuum hot pressed or otherwise treated under conditions such that the carbidiferous agent decomposes and reacts with the aluminum matrix, and that no significant melting of the matrix occurs. Generally, the consolidation step is carried out at a temperature ranging from about 400°C to 600°C, and preferably from about 450°C to 550°C, the temperature being below the solidus temperature of the metal matrix. The Al-Fe-V-Si alloy composite containing a carbidiferous agent may be canless vacuum hot pressed at a temperature ranging from 435°C to 500°C and more preferably from 450°C to 475°C, followed by forging or extrusion. Those skilled in the art will appreciate that other time/temperature combinations can be used and that other variations in pressing and sintering can be employed. For example, instead of canless vacuum hot pressing the powder can be placed in metal cans, such as aluminum cans having a diameter as large as 30 cm or more, hot degassed in the can, sealed therein under vacuum, and thereafter reheated within the can and compacted to full density, the compacting step being conducted, for example, in a blind died extrusion press. In general, any technique applicable to the art of powder metallurgy which does not involve liquefying (melting) or partially liquefying the matrix metal can be used. Representative of such techniques are explosive compaction, cold isostatic pressing, hot isostatic pressing and direct powder extrusion. The resultant billet can then be worked into structural shapes by forging, rolling, extrusion, drawing and similar metal working operations.

EXAHELE- . Ten kilogram batches of aluminum alloys of the compositions aluminum-balance, 4.06 at % iron, 0.70 at % vanadium, 1.51 at % silicon (hereinafter designated Alloy A), and aluminum-balance, 4.7 at % titanium were produced by planar flow casting. Transmission electron photo-micrographs of the rapidly solidified ribbon are shown in Figs. 1A and IB, respectively. The aluminum - iron - vanadium - silicon base alloy microstructure is composed of a microcellular network of aluminum intermetallic compound particles, Al₁₃(Fe,v)₃Si, uniformly distributed in the aluminum solid solution network. The aluminum - titanium base alloy microstructure is composed of titanium-rich cell boundaries, within which is a uniform distribution of fine aluminum intermetallic compound particles, Al₃Ti. For comparison, light photomicrographs of these two alloys made by conventional ingot casting are shown in Figs. 2A and 2B respectively. The dispersed phases present in these alloys are observed to be much coarser and less uniformly distributed than the dispersed phases formed in planar flow cast alloys.

EXAMP E II A five gram sample of -40 mesh (U.S. standard sieve) powder of Alloy A was added to 0.10 grams of Nopcowax®, i.e., stearic acid. The sample was processed by pouring the powders into a Spex Industries hardened steel vial (Model #8001) containing 31 grinding balls. Each of the balls had a diameter of about 0.365 cm and was composed of Alloy SAE 52100 steel. The filled vials were then sealed and placed into a Spex Industries 8000 mixer mill. The powder batch containing the aforementioned ingredients was then processed for 240 min. The processing procedure described above provides a rapidly solidified aluminum base alloy that exhibits a substantially uniform dispersion of the carbidiferous agent. A photomicrograph of said ball milled particles is shown in Fig. 3. Having thus described the invention in rather full detail, it will be appreciated that such detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims.

Claims

We claim:

1. A process for producing an aluminum base alloy comprising the steps of: (a) forming a charge containing, as ingredients, a rapidly solidified aluminum base alloy and a carbidiferous agent in an amount ranging from about 0.01 to 10 % by wt;

(b) ball milling the charge energetically to mix the carbidiferous agent within the aluminum base alloy while maintaining the charge in a pulverulent state; and

(c) consolidating said charge to react the rapidly solidified aluminum base alloy with the carbidiferous agent resulting in the formation of carbides and oxides, and to provide a mechanically formable, substantially void-free mass.

2. A process as recited in claim 1, wherein said rapidly solidified aluminum based alloy has a substantially uniform structure.

3. A process as recited in claim 2, wherein said rapidly solidified aluminum based alloy is prepared by a process comprising the steps of forming a melt of the aluminum based alloy and quenching the melt on a moving chill surface at a rate of at least about 10^5oC/sec.

4. A process as recited in claim 3, wherein said ball milling step is continued until said carbidiferous agent is enveloped by said aluminum base alloy.

5. A process a recited in claim 4, wherein said consolidation step is carried out at a temperature ranging from about 400°C to 600°C, said temperature being below the solidus temperature of said metal matrix.

6. A process as recited in claim 3, wherein said rapidly solidified aluminum based alloy has a composition consisting essentially of the formula Al_balFe_aSi_bX_c wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, w, Nb, Ta, "a" ranges from 2.0 to 7.5 at %, "b" ranges from 0.5 to 3.0 at %, "c" ranges from 0.05 to 3.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X] :Si ranges from about 2.0:1 to 5.0:1.

7. A process as recited in claim 6, wherein said rapidly solidified aluminum based alloy is selected from the group consisting of the elements Al-Fe-V-Si, wherein the iron ranges from about 2.0-7.5 at %, vanadium ranges from about 0.05-3.5 at %, and silicon ranges from about 0.5-3.0 at %.

8. A process as recited in claim 3, wherein said rapidly solidified aluminum based alloy has a composition consisting essentially of the formula ^A--bal^Fea^s--b^xc wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 2.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c" ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X] :Si ranges from about 2.01:1 to 1.0:1.

9. A process as recited in claim 3, wherein said rapidly solidified aluminum based alloy has a composition consisting essentially of about 2-15 at % from a group consisting of zirconium, hafnium, titanium, vanadium, niobium, tantalum, erbium, about 0-5 at % calcium, about 0-5 at % germanium, about 0-2 at % boron, the balance being aluminum plus incidental impurities.

10. An alloy having at least 50 % matrix material formed from a rapidly solidified aluminum based alloy, said matrix material having substantially uniformly distributed therein carbides and oxides formed by reaction of a carbidiferous agent with said matrix material.