McrAlY Cladding Layers and Method for Making Same

The present invention relates to laser cladding and more particularly to MCrAlY coatings formed by laser surface fusion. MCrAlY coatings, where Mis a base metal of iron, cobalt, nickel or a combination thereof, are generally known in the art. MCrAlY coatings were developed for high temperature oxidation and corrosion protective applications. An ability to form a uniform protective oxide layer which retards further oxidation of the underlying metal made the coating particularly suited to applications in the aerospace industry. Due principally to the alloy's content of active elements such as yittrium and aluminum, preparation of MCrAlY coatings have been confined to high vacuum, typically prepared by physical vapor deposition (alternately referred to herein as P.V.D.) such as with an electron beam. A teaching of one such application may be found in U.S. Pat. No. Re. 27,920. Aside from the evident constraints on the workpiece size and production time, which eliminates physical vapor deposition for many applications and makes it cost prohibitive in others, P.V.D. formed MCrAlY layers require controlled high temperature (about 1700.degree. F.) heat treatment during and/or post deposition. Several such processes require an additional step of glass peening the MCrAlY layer.

A relatively new technology in metallurgical coatings has evolved with the application of high powered lasers to the industry. Generally, a workpiece to be coated is irradiated with an intense laser beam, melting a coating material onto a workpiece surface. The technique may be used to alloy the coating material with the underlying metal such as taught by U.S. Pat. No. 4,212,900, to provide a surface layer having unique properties on a common base substrate such as taught in U.S. Pat. No. 4,218,494 and Defensive Publication No. T 967,009, or utilizing the high energy density to alter surface properties of a material as taught in U.S. Pat. No. 4,122,240. A somewhat modified application uses the laser energy to melt particles of the coating material during their transit from a feedstock source to the coating workpiece. This is generally referred to in the art as "flame spraying," examples of which may be found in U.S. Pat. Nos. 3,310,423 and 4,269,868.

The use of controlled duration high intensity/power density laser irradiation provides an advantage of controlled heating of the coating material and the coating workpiece. That is, in cladding applications such as the present invention, laser cladding permits rapid and selective heating of the coating material and portion of the contiguous substrate surface while maintaining the bulk or body of the coated workpiece at a relatively reduced temperature. The technique consequently permits rapid cooling after irradiation since the bulk of the coated item is not necessarily heated to the melting point of the alloy coating.

The present invention is directed to a high temperature oxidation resistant cladding of MCrAlY formed by laser fusion of the coating layer to the underlying metal substrate. In the formula MCrAlY, M represents a base metal comprising Iron, Nickel, Cobalt or a combination thereof. Laser irradiation of controlled duration and power density is utilized to fuse the MCrAlY coating layer to the substrate forming a metalurgical bond between the substrate and the coating layer. The workpiece is maintained in an inert atmosphere during laser irradiation to protect the hot process area from oxidation. The process demonstrates a novel advantage in producing relatively small and uniform dispersoids of the oxides of the alloy components required for forming strongly adherent, oxidation resistant films, permitting a lowered aluminum content. The process further eliminates the conventional constraints imposed by physical vapor disposition including preparation under high vacuum and high temperature heat treatments.

The singular drawing is a side view of an apparatus used in the practice of the present invention.

The present invention is directed to a high temperature corrosion resistant MCrAlY cladding having novel metalurgical characteristics and a method for producing same. To illustrate the novel characteristics of the method used in producing the MCrAlY cladding, FIG. 1 describes one embodiment of an apparatus used in fabricating the MCrAlY coatings. A workpiece 10 having a coating surface 11 is generally metallic and preferably a relatively low carbon content iron base alloy. The terminology "relatively low carbon content" as used herein, and generally recognized in the art, refers to an alloy having a carbon content less than about 0.2%.

In the illustrated embodiment, the workpiece 10 is placed or secured on conveyor means 12 which provides lateral movement of the workpiece, illustrated by arrow 14. Due to the collimated nature of laser light, utilizing the conveyor means 12 permits control of the laser beam dwell time upon any specific area of the workpiece within the effective area of the laser beam by means of controlling the speed in which the workpiece is moved into and from the beam.

The coating surface 11 is uniformly covered with a layer of coating material 16 comprising MCrAlY. In the term MCrAlY, M is a base metal comprising iron, cobalt, nickel or a combination thereof. In one embodiment layer 16 comprises MCrAlY powders uniformly spread over the coating surface. Examples of such MCrAlY powders include iron based alloys having about 15% to 30% Cr, 4% to 10% Al, 0.1% to 1.5% Y; cobalt based alloys having about: 15% to 30% Cr, 4% to 10% Al, 0.1% to 1.5% Y; nickel based alloys having about 15% to 30% Cr, 4% to 10% Al, 0.1% to 1.5% Y; and alloys of Fe, Ni, and Co base containing about 60% to 70% of the base elements and the aforedescribed alloying elements. Selection of the powder granulation and the thickness of layer 16 is primarily dependent upon the particular alloy and the desired thickness of the cladding layer. Uniformity of the powder layer 16 can be achieved by any of a number of techniques known to one skilled in the art such as using a template or doctor blade spreading device and automatic powder feeder for example.

The workpiece having layer 16 is transported by conveyor means into the beam path 18 of laser 20. An inert environment, illustrated in the drawing by dashed line 22 is used to envelope at least that portion of the workpiece being irradiated by the laser beam 18. In one embodiment, the inert environment is maintained by blanketing at least the irradiated area with Ar gas.

In a preferred embodiment, the inert environment also envelopes the laser beam path, avoiding absorption of the laser energy by an uncontrolled environment and ionized gases. The laser 20 is generally characterized as comprising a continuous wave (CW) relatively high power (multi kilowatts) laser. The laser is selected to provide strong absorptivity of the output wavelength of the laser by the coating layer 16. In a preferred embodiment, laser 20 comprises a 15 kilowatt CO.sub.2 laser to accomplish the melting and fusion of the cladding layer. The use of laser light energy is of further advantage in that upon melting of coating material layer 16, the layer in the molten state becomes highly reflective of the light energy. The reflected energy no longer serves to heat the layer, self-regulating the laser melting process.

In a preferred embodiment, an integrator lens 24 may be interposed between the laser 20 and th workpiece 10 to direct the laser beam energy uniformly over a specific area of the workpiece. Although not shown in the drawing, the lens may also be used to divert or angle the laser beam 18.

Under cover of the inert environment 22, the coating material 16 is irradiated by laser beam 18 for a preselected dwell time. The dwell time is a function of the power density of the laser beam and the composition of the alloy. For example, upon irradiating, the coating material layer is rapidly melted along with a small amount of the underlying substrate coating surface 11. Depending upon the dwell time, about 5% to 50% of the coating comprises material attributable to the melting of the substrate, preferably 10-40%.

Upon movement out of the laser beam 18, the molten alloy is solidified due principally to autoquenching. Conventional cooling capability may be added to the conveyor means or the inert environment to supplement the autoquenching. Using laser energy to melt the coating layer 16 results in a rapid rise rate of temperature which permits the selective heating of the coating material and the area immediately contiguous to the surface without heating the remainder of the workpiece to the high temperatures necessary for the cladding process.

The solidified MCrAlY alloy layer 17 is tenaciously adherent to the underlying substrate. Metallurgical studies reveal the aluminum composition of the MCrAlY to be less than about 6.0%, yet as evidenced in the examples detailed hereinafter the coatings exhibit excellent high temperature oxidation resistance. As presently understood, the rapid heating and cooling results in relatively small, uniform dispersants of the oxides of the alloy components, resulting in the evidenced metallurgical characteristics. The layer thickness dependent as described heretofore upon the thickness of the coating material, generally ranges from about 20 mils to about 80 mils. The coating area may be increased without necessitating larger area laser beams (or multiple beams) by multiple pass processing where a small portion of the previously irradiated layer is re-melted.

To assist one skilled in the art, the following examples detail specific embodiments of the present invention.

A type 304 stainless steel substrate, approximately one-quarter inch in thickness was uniformly coated with a powder of FeCrAlY alloy which contained approximately 24.5 percent chromium, 4.0 percent aluminum and 0.5 percent yittrium, by weight, and the balance iron. The powder form having a particulate of about -325 mesh size was evenly spread across the substrate using a template to a thickness of about 0.035 inch. the workpiece was placed onto a conveyor system having a controlled scan speed of about 12 inches per minute. A commercially available CO.sub.2 laser was adjusted to provide a power of about 6.5 kilowatts to the surface of the workpiece. An optical integrater lens was used to provide a square beam which at the workpiece surface has an area of about 0.5 inch by 0.5 inch. The conveyor belt was set to provide a scan speed of 12 inches per minutes, which under the conditions specified results in a power density of 4 kilowatts per square centimeter and a dwell time of about 2.5 seconds. Argon was used to envelope the laser beam and the workpiece in the area proximate to the irradiation zpone. Visual observation of the process confirmed virtually instant melting of the alloy powder and solidification upon exiting the irradiation zone.

After cooling, the coating was subjected to oxidation studies which comprised exposing the coated samples in air at temperatures ranging from about 1000.degree. to 1200.degree. C. The uncoated stainless steel substrate rapidly oxidized under these conditions while the segment of the surface coated with the alloy evidenced virtually no oxidation attack. The oxidation study was continued until virtually all of the stainless steel substrate was oxidized leaving only the coating layer.

A powder of CoCrAlY alloy containing 29.5% chromium, 6.0% aluminum and 0.8% yittrium, by weight, the balance being cobalt, was uniformly spread onto the surface of a type 304 stainless steel substrate having a thickness of about one-quarter inch. The powder alloy having a particulate size ranging from about -80 mesh to about +270 mesh was uniformly spread across the surface using a template to a thickness of about 0.035 inch. The laser and conveyor apparatus described in Example I was utilized. The scan speed was set at 14 inches per minute, which for a laser beam delivered through the optical integrator to form a square beam of 0.5 inch by 0.5 inch, resulted in an area power density of 4.0 kilowatts per centimeter square and a dwell time of about 2.1 seconds. Visual observance again confirmed rapid melting of the powders resulting in a coating ranging in thickness from about 0.05 to about 0.06 inches. The coating was subjected to oxidation studies in air between 1000.degree. and 1200.degree. C. in which the CoCrAlY coating evidenced corrosion resistance substantially similar to that described in Example I.

A cladding of FeCrAlY alloy produced in the manner substantially identical to that of Example I was subjected to metallurgical testing. An analysis of the composition of the FeCrAlY coating indicated an average aluminum content of about 2.4 weight percent. The Energy Dispersive X-ray (EDX) analysis in a scanning electron microscope also identified the constituents of the substrate confirming the partial melting of the substrate surface during the irradiation process.

The sample was then subjected to an oxidation study virtually identical to that described in Example I. After 1536 hours at a temperature of about 1000.degree. C., the stainless steel substrate was virtually totally lost to oxidation while the cladding layer of FeCrAlY remained substantially intact.

A CoCrAlY coating substantially similar to that set forth in Example II was subjected to EDX analysis. The EDX analysis revealed that the layer contained an average of about 4.2 weight percent aluminum. The sample was then subjected to oxidation studies and after exposure to air at an elevated temperature of about 1000.degree. C. for about 1536 hours, the stainless steel substrate was substantially consumed by oxidation, whereas the cladding layer exhibited no significant oxidation.