ARC SPRAYING OF RAPIDLY SOLIDIFIED ALUMINUM BASE ALLOYS

Abstract: A rapidly solidified aluminum base alloy is formed into a wire and arc sprayed onto a substrate to provide a uniform and contiguous coating. Deposition and retention of the alloy onto the substrate are effected in a single process step. The coating exhibits improved mechanical and physical properties including excellent corrosion and oxidation resistance and improved elevated temperature strength and thermal stability.

Description: ARC SPRAYING OF RAPIDLY SOLIDIFIED ALUMINUM BASE ALLOYS BasfcgjQund of the Ioyentifln 1. Field of the Invention This invention relates to a process for improving the properties of materials, and more particularly to a process for producing a metallic coating from a rapidly solidified metal. 2. Description of the Prior Art Spray metallizing consists of heating a metal to a molten or semi-molten condition by passing it through a high temperature heat source, and depositing it in a finely divided form on a substrate. The molten or semi-molten particles flatten out on impacting the substrate and adhere to its surface. Subsequently deposited particles also flatten out, and adhere to those previously deposited, thus the structure of sprayed deposits is lamellar. The sprayed metal deposits resemble the derivative wire or powder chemically, but their physical properties, especially their microstructure, are quite different from those of the original wrought metal. Cohesion is achieved through mechanical and metallurgical bonding. After being sprayed, certain materials can be fused to form a dense and uniform coating that is metallurgically bonded to the substrate. Fused coatings usually are required for protecting the substrate material during service at high temperatures, in abrasive and corrosive environments, or for developing a surface of uniformly high hardness. For example, sprayed aluminum coatings on steel require heating to above 482 β C to etallurgically bond the coating to the steel. Typically, the material may be subsequently heated at 732°C to 1093 β C to provide a dense, uniform coating metallurgically bonded to the base metal. Problems may arise due to the spray metallizing of aluminum coatings and subsequent diffusing of the aluminum spray coating from the formation of coarse aluminum-iron intermetallics dispersed within the deposited particles and at the coating/substrate interface. These intermetallics are very brittle and can degrade the mechanical properties of the component, for example, by forming a brittle layer between the components. Also, because the sprayed aluminum coating requires a thermal diffusing treatment, conditions may exist wherein the substrate material may not be properly heat treated. Problems may be encountered with welded aluminum coated steel parts. The alloying of the aluminum and iron can create a loss of ductility and lowering of corrosion resistance in the weld and heat-affected zone. Finally, because of the mismatch in the coefficient of thermal expansion between the sprayed aluminum coating and the substrate, the coating may degrade and spall off during high temperature exposure. fτ * H1T«irτ of th Indention The present invention provides an economical and efficient process for arc spraying aluminum base alloys in which no subsequent thermal treatment is required. Advantageously, such properties as high temperature strength and stability, corrosion and oxidation resistance and compatibility with the substrate, of an aluminum spray metallized coating, are improved in accordance with the invention by arc spraying a rapidly solidified, high temperature aluminum alloy onto a designated substrate. This procedure, referred to hereinafter as arc spraying, results in the formation of a high temperature spray metallized coating. Subsequent thermal treatments, such as heating the coating to above the solidus of the alloy, heretofore required to adhere the coating to the substrate are virtually eliminated. Deposition and retention of a rapidly solidified alloy onto a substrate are effected in a single process step. The coated substrate exhibits improved * 5 ambient and elevated temperature mechanical an physical properties due to the microstructure of the *- rapidly solidified coating. Briefly stated, the invention provides a process for producing a rapidly solidified aluminum 10 base alloy coating, comprising the steps of: (a) forming a rapidly solidified aluminum base alloy into a wire; and (b) arc spraying said wire onto a substrate. Wire having a diameter suitable for arc 15 spraying may be fabricated directly by a friction actuated process or by conventional wire drawing techniques, and sprayed onto a substrate using arc spraying techniques to form a nearly fully dense spray metallized coating. Moreover, the attractive 20 properties of the rapidly solidified wire are retained. This process may be repeated such that the subsequent spraying is done on top of the sprayed coating. The sprayed metal coatings may then be finished by typical metal finishing operations such 25 as machining, grinding, burnishing and polishing provided that the precautions usually followed for sprayed metallized coatings are followed. Also components having the spray metallized coatings can withstand moderate forming operations such as 30 drawing, spinning, brake and roll forming, and embossing. The arc sprayed coatings are suitable for use * in components requiring corrosion, oxidation and elevated temperature protection for use as aerospace 35 components such as turbine blades, turbine vanes and fasteners; automotive components such as exhaust pipes, intake valves and cylinder barrels; and industrial components such as heat exchangers, fasteners for chemical piping and boilers, reactor tubes, and heat treating equipment. Applications such as molds appointed for subsequent casting may arise that specifically utilize the higher temperature capability, i.e. hardness, of the rapidly solidified coating. Alternatively, the arc sprayed layers can be used for repairing coatings as well as engineering shapes made directly from the rapidly solidified materials. Specifically, the coating can be applied to a substrate to repair a surface defect thereof. The arc sprayed layers can also be used to make the preforms for various composite materials wherein the substrate consists of continuous or woven fibers, bundles, whiskers or particulate made from a hard or semi-hard material such as refractory carbides, oxides or nitrides. Also, the rapidly solidified alloys may be combined with a reinforcing phase to form a composite a described in U.S. Patent Application Serial No. 242,989, filed September 12, 1988, which application is incorporated herein by reference thereto, prior to being formed into a wire. Brief Description of the Drawings 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: Fig. 1 is a scanning electron photomicrograph of the surface of a wire arc sprayed coating composed of rapidly solidified aluminum based iron, vanadium and silicon containing alloy uniformly deposited on planar flow cast aluminum based iron, vanadium and silicon containing ribbon fabricated by the present invention; Fig. * 2 is an optical light photomicrograph of a cross section of a wire arc sprayed coating composed of rapidly solidified aluminum based iron, vanadium and silicon containing alloy deposited onto planar flow cast aluminum based iron, vanadium and silicon containing ribbon fabricated by the present invention; and Fig. 3 is a transmission electron photomicrograph of a wire arc sprayed coating composed of rapidly solidified aluminum based iron, vanadium and silicon containing alloy fabricated by the present invention. Description of the Pre erred KmWlliaffntff 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 bal Fe a Si b X c wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 1.5-8.5 at %, "b" ranges from 0.25-5.5 at %, "c" ranges from 0.05-4.25 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 1.5-8.5 at %, vanadium ranges from about 0.25-4.25 at %, and silicon ranges from about 0.5-5.5 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 bal Fe a Si b X c wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 1.5-7.5 at %, "b" ranges from 0.75-9.5 at %, "c" ranges from 0.25-4.5 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 suitable for use in the process of the invention has a composition consisting essentially of the formula Al bal Fe a Si b X c wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, Ce, Ni, Zr, Hf, Ti, Sc, "a" ranges from 1.5-8.5 at %, H b H ranges from 0.25-7.0 at %, and "c" ranges from 0.05 to 4.25 at %, the balance being aluminum plus incidental impurities. 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 the 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. A low density aluminum-lithium base, rapidly solidified alloy suitable for use in the present process has a composition consisting essentially of the formula Al bal Zr a Li b Mg c T d , wherein T is at least one element selected from the group consisting of Cu, Si, Sc, Ti, B, Hf, Cr, Mn, Fe, Co and Ni, "a" ranges from 0.05-0.75 at %, "b" ranges from 9.0-17.75 at %, "c" ranges from 0.45-8.5 at % and "d" ranges from about 0.05-13 at %, the balance being aluminum plus incidental impurities. Those skilled in the art will also appreciate that other dispersion strengthened, rapidly solidified or mechanically alloyed alloys or composites may be appointed for use as the coating material in the process of the present invention. One mechanically alloyed material that is suitable is that disclosed in the aforementioned application Serial No. 242,987, filed September 12, 1988. Specifically, the powder can be composed of rapidly solidified alloy combined with the particles of a re nforcing material present in an amount ranging from about 0.1 to 50 percent by volume, the powder having been ball milled to enfold metal matrix material around each of the particles. The metal alloy quenching techniques used to fabricate these alloys generally comprise the step of cooling a melt of the desired composition at a rate of at least about 10 5o C/sec. Generally, a particular composition is selected, powders or granules of the requisite elements in the desired portions are melted and homogenized, and the molten alloy is rapidly quenched on a chill surface, such as a rapidly moving metal surface, 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 microstructure and chemical composition. The 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 matrix, the rapidly solidified aluminum alloy particulate has properties that make it amenable to direct friction actuated extrusion into wire, as well as numerous powder metallurgy techniques used to fabricate such powders, including vacuum hot degassing and compacting the rapidly solidified powder into near fully dense billets at temperatures where the majority of the adsorbed gases are driven from the powder surfaces and that decomposition of any dispersed phases does not occur. The billets may thereafter be compacted to full density in a blind died extrusion press, forged, or directly extruded into various shapes including profiled extrusions and wire. The substrate may be water or gas cooled, or may be heated directly or indirectly during the processing. The optimum substrate temperature is dependent on the rapidly solidified alloy and the dispersed phases which must be formed during solidification. The rapidly solidified alloy in the form of a wire is arc sprayed to form a coating. The arc spraying step comprises the steps of (i) striking an arc between two strands of said wire to melt the tips thereof; and (ii) atomizing said melt in said arc by impinging a high pressure inert gas thereagainst. Arc spraying involves initially striking an arc between two strands of a conductive metal wire and essentially atomizing any molten metal which forms in the arc by impinging a high pressure inert gas onto the molten wire tips. Since arc spraying is a consumable process, wire is continually fed and the arc and metal source are maintained. The rapidly solidified alloy must be provided as a wire that can range in size from 0.05 cm to 0.25 cm in diameter and more preferably from about 0.1 cm to 0.18 cm in diameter, the optimum wire diameter depending on the alloy composition, the voltage across the wires and the feed sizes physically allowed by the arc spraying apparatus. The wire suitable in diameter for arc spraying may be fabricated directly by a friction actuated process or by conventional wire drawing techniques. Arc spraying may be performed for varying lengths of time depending on the thickness of the sprayed preform required. The attractive microstructure and mechanical and physical properties of the rapidly solidified wire are retained. This process may be repeated such that subsequent spraying is done on top of the sprayed coating, and multi-layered coatings may be fabricated. The sprayed coatings require no diffusion treatment as the arc sprayed material retains the attractive microstructure and mechanical and physical properties of the rapidly solidified wire. KT MfT.R γ Wire composed of a rapidly solidified alloy having a diameter of 0.16 cm and the composition aluminum balance, 4.06 at % iron, 0.70 at % vanadium, 1.51 at % silicon (hereinafter designated alloy A). was wire arc sprayed onto a planar flow cast, two inch wide ribbon composed of alloy A wrapped upon a mandrel approximately 30 cm in diameter. Wire arc spraying was performed for approximately 10 minutes at 34 volts, 100 amps and 50 psi (0.34 MPa) atomizing gas pressure to achieve a deposited layer approximately 0.015 cm thick. Fig. 1 is a scanning electron photomicrograph of the surface of wire arc sprayed coating composed of rapidly solidified aluminum based iron, vanadium and silicon containing alloy matrix deposited on planar flow cast aluminum based iron, vanadium and silicon containing ribbon. Individual areas or splats corresponding to solidified droplets of sprayed molten alloy were observed. The coating was uniform and contiguous. Fig. 2 is a light photomicrograph of a cross section of a wire arc sprayed preform composed of rapidly solidified aluminum based iron, vanadium and silicon containing alloy matrix deposited on planar flow cast aluminum based iron, vanadium and silicon containing ribbon. Some porosity was observed, however, discrete primary intermetallic compound particles were not seen in the matrix alloy A microstructure indicating that solidification of the arc sprayed wire occurred at a rate rapid enough to suppress the formation of coarse primary dispersoid particles. ■hΛAMPTiK TT Transmission electron microscopy (TEM) was performed on wire arc sprayed coating to further examine the microstructure of the deposited layer. Samples were prepared by mechanically grinding off the planar flow cast alloy A substrate and thinning the sample to approximately 25 micrometers in thickness. TEM foils were prepared by conventional electro-polishing techniques in an electrolyte consisting of 80 percent by volume methanol and 20 percent by volume nitric acid. Polished TEM foils were examined in a Philips EM 400T electron microscope. A transmission electron photomicrograph of a wire arc sprayed coating composed of rapidly solidified aluminum based iron, vanadium and silicon containing alloy fabricated by the present invention is shown in Fig. 3. The microstructure of the deposited layer was observed to be composed of fine 50-100 nm diameter Al 13 (Fe,V) 3 Si dispersoids uniformly distributed in an aluminum solid solution matrix. This microstructure is very similar to that typically observed in the planar flow cast, rapidly solidified alloy A ribbon as well as in components consolidated from rapidly solidified powder particles using powder metallurgical techniques. Having thus described the invention in rather full detail, it will be understood that such detail need, not be strictly adhered to but that further 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 a rapidly solidified aluminum spray metallized coating, 5 comprising the steps of: (a) forming a rapidly solidified aluminum base alloy into a wire; and (b) arc spraying said wire onto a substrate. 2. A process as recited in claim 1, wherein JLO said rapidly solidified alloy has a substantially uniform structure. 3. A process as recited in claim 2, wherein said rapidly solidified aluminum base alloy is prepared by a process comprising the steps of forming ^5 a melt of the aluminum base alloy and quenching the melt on a moving chill surface at a rate of at least 10 5o C/sec. 4. A process is recited in claim 2, wherein said rapidly solidified aluminum base alloy is on prepared by a process comprising the step of quenching a melt of said alloy on an impinging gas or liquid at a rate of at least about 10 * ^°C/sec to form a powder. 5. A process as recited in claim 1, wherein 2 said arc spraying step comprises the steps of (i) striking an arc between two strands of said wire to melt the tips thereof; and (ii) atomizing said melt in said arc by impinging a high pressure inert gas thereagainst. - 0 6. A process as recited in claim 5, wherein said wire has a diameter ranging from 0.05 cm to 0.25 cm in diameter. 7. A spray metallized coating formed from arc spraying a rapidly solidified aluminum alloy onto a __ substrate. 8. A spray metallized coating as recited in claim 7, wherein said coating exhibits, in combination, excellent corrosion and oxidation resistance and improved elevated temperature strength and thermal stability. 9. A spray metallized coating as recited in claim 7, wherein said coating is applied to a substrate to repair a surface defect thereof. 10. A spray metallized coating as recited in claim 9, wherein said substrate is a rapidly solidified aluminum alloy.

Inventors: DAS SANTOSH K, GILMAN PAUL S, ZEDALIS MICHAEL S, DAS, SANTOSH, K., GILMAN, PAUL, S., ZEDALIS, MICHAEL, S.
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL INC.
Application year: 1991
Main classification: C73C4
IPC codes: B22F9, C23C4