Patent Application
20120048912
The following disclosure relates generally to gas turbine engines and, more particularly, to embodiments of a method for depositing MCrAlY coatings on gas turbine engine components.
As the operating temperature of a gas turbine engine (“GTE”) increases, so too does the engine's efficiency. The maximum operating temperature of a gas turbine engine is, however, limited by the ability of hot section components (e.g., combustor liners, turbine seals, turbine blades, nozzle guide vanes, duct members, and the like) to withstand direct exposure to high temperature gas flow without excessive structural degradation due to hot corrosion, oxidation, thermal fatigue, and erosion. Extensive engineering efforts have resulted in various advances in cooling techniques, superalloy materials, and coating systems (e.g., thermal barrier and environmental barrier coatings), which have collectively increased the operational temperature limits of modern GTEs by several hundred degrees Fahrenheit within the past few decades.
It is, of course, desirable to reduce cost in both the fabrication and the repair of hot section components. While many newly-manufactured hot section components are applied with environmental barrier coatings (e.g., dual layer platinum-modified aluminide/MCrAlY coatings), the production and the application of such protective coatings can add considerable cost to the hot section component fabrication. For example, while platinum-modified aluminide coatings provide excellent oxidation resistance and good corrosion resistance, such coatings are especially costly to produce due, in large part, to their requisite platinum content. In addition, application of a platinum-modified aluminide coating to a selected hot section component typically entails the performance of multiple time consuming steps; e.g., plating, diffusion heat treatment, aluminizing processes such as pack or above pack aluminizing, chemical vapor deposition, and heat treatment steps. By comparison, MCrAlY coatings have lower raw material costs and provide good oxidation and excellent corrosion resistance. However, conventional application techniques utilized to deposit MCrAlY coatings (e.g., low pressure plasma spraying and electron beam physical vapor deposition processes) are also undesirably complex and costly to perform. There thus exists an ongoing need to provide embodiments of a lower cost process for forming an MCrAlY coating on a gas turbine engine component. Ideally, embodiments of such a low cost process would produce a metallurgically sound coating providing oxidation and corrosion protection properties equivalent to or surpassing those of conventionally-deposited MCrAlY coatings.
It is further desirable to reduce costs in the repair of hot section components that have, for example, cracked or eroded as a result of prolonged exposure to hot combustive gas. Several innovative processes have been developed to repair damaged areas of hot section components through crack healing and restoration of eroded surfaces to original dimensions and contours. However, conventional repair techniques typically cannot provide improved corrosion and oxidation resistance to the restored area of the component beyond that provided by the component's parent material. Thus, after reinstallation and subsequent usage of the refurbished component, the repaired area may again erode, crack, or otherwise suffer structural damage in the presence of combustive gas flow and further repair may become necessary. There thus further exists an ongoing need to provide embodiments of a method for repairing a damaged (e.g., eroded or cracked) area of a hot section component (e.g., a turbine airfoil) utilizing materials that provide improved oxidation and corrosion protection. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and this Background.
Embodiments of a method for forming an MCrAlY coating on a gas turbine engine component are provided. In one embodiment, the method includes the step of preparing an MCrAlY slurry containing an MCrAlY powder, a low melting point powder, a binder, and a dilutant. After application over the gas turbine engine component, the MCrAlY slurry is heated to a predetermined temperature that exceeds the melting point of the low melting point powder to form an MCrAlY coating on the gas turbine engine component. In a first exemplary implementation of the foregoing method, the MCrAlY powder includes about 8-15 wt. % aluminum; about 15-25 wt. % chromium; about 15-22 wt. % cobalt; about 0-3 wt. % zirconium; about 0.1-1 wt. % yttrium; about 0-5 wt. % of each of hafnium, rhenium, ruthenium, silicon, and tantalum; and the balance nickel. In a second exemplary implementation, the MCrAlY powder includes about 7.5-8.5 wt. % aluminum, about 20-22 wt. % chromium, about 38-40 wt. % cobalt, about 0.2-0.60 wt. % yttrium, and the balance nickel. In a third exemplary implementation, the MCrAlY powder includes about 11.5-13.5 wt. % aluminum; about 18-20 wt. % chromium; about 20-22 wt. % cobalt; about 0.15-0.5 wt. % hafnium; about 0.2-0.6 wt. % of each of silicon and yttrium; and the balance nickel.
Methods for forming an MCrAlY coating on a structurally-damaged area of a gas turbine engine component to repair the structurally-damaged area are further provided. In one embodiment, the method includes the step of preparing an MCrAlY slurry containing an MCrAlY powder, a braze powder, a binder, and a dilutant. After application over the structurally-damaged area of the gas turbine engine component, the MCrAlY slurry is heated to a predetermined temperature surpassing the melting point of the braze powder to form an MCrAlY coating on the structurally-damaged area of the gas turbine engine component.
At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and:
The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description.
It is noted that, in embodiments wherein the below-described method is utilized to repair a structurally damaged gas turbine engine component, the method will typically include one or more steps similar to the steps of the JetFix® process developed by Honeywell International, Inc., headquartered in Morristown, N.J. However, embodiments of the exemplary method described herein differ from the conventionally-performed JetFix® process in several manners, including in the production and application of an MCrAlY-based slurry. Thus, in a general sense, embodiments of the below-described method provide an improved JetFix® process that can be performed at reduced cost to restore structurally-damaged GTE components utilizing a slurry-deposited MCrAlY material having improved corrosion and oxidation resistive properties, as described above.
With reference to
After selection of the GTE component (STEP 12,
In addition to its named components (i.e., chromium, aluminum, yttrium, and “M,” wherein M represents nickel, cobalt, or a combination thereof), the MCrAlY powder can, and typically will, include lesser amounts of one or more additional metallic or non-metallic constituents, which may be added in powder form to a master alloy during processing to optimize desired metallurgical properties, such as oxidation and corrosion resistance, of the resulting MCrAlY coating. In a preferred group of embodiments, an MCrAlYX powder is utilized wherein X comprises one or more of the following elements: hathium, rhenium, ruthenium, platinum, palladium, silicon, tantalum, titanium, lanthanum, cerium, and zirconium. TABLES 1-3 below provide exemplary compositions of an MCrAlYX powder well-suited usage in implementations of exemplary method 10 utilized to repair damaged GTE components, as well as in implementations of method 10 utilized to form environmentally-protective overlay coatings on newly-manufactured or pre-existing, non-damaged GTE components. The values set-forth in TABLES 1-3 below are approximations of the maximum and minimum weight percentages of each component included within the MCrAlYX powder.
The composition of the low mp powder will typically be determined, at least in part, by whether the MCrAlY slurry is intended to form an environmental coating on an undamaged GTE component or, instead, to repair a structurally-degraded area of a service-run GTE component. In embodiments wherein the MCrAlY slurry is intended to form an environmental barrier coating on an undamaged GTE component, the low mp powder is preferably an aluminum-containing powder and, more preferably, an aluminum powder or an aluminum-silicon powder. An aluminum-silicon powder having a preferred composition is set-forth in TABLE 4 below. The values set-forth in TABLE 4 below are approximations of the maximum and minimum weight percentages of each component included within the aluminum-silicon powder.
In embodiments wherein the MCrAlY slurry is utilized to repair structural damage of an engine-run GTE component, the low mp powder preferably comprises a braze alloy, such as a nickel- or cobalt-based braze alloy. Three braze alloys well-suited for usage in embodiments wherein the MCrAlY slurry is utilized for repair purposes are set-forth in TABLES 5-7 below. As previously indicated, the values set-forth in TABLES 5-7 below are approximations of the maximum and minimum weight percentages of each component included within the braze powder.
Next, during STEP 12 of exemplary method 10 (
Continuing with exemplary method 10 (
Next, during STEP 18 (
After application of the MCrAlY slurry (STEP 18,
In a first exemplary embodiment wherein method 10 is performed to repair a GTE component having structural damage, the following steps may be performed during STEP 20 (
In a second exemplary embodiment wherein method 10 is performed to form an environmentally-protective overlay coating on a new or otherwise undamaged GTE component, the following steps may be performed during STEP 20 (
At this juncture in exemplary method 10, one or more machining steps are optionally performed (STEP 22,
The foregoing has thus provided multiple exemplary embodiments of a method for forming a unique slurry-deposited MCrAlY coating over gas turbine engine components. In certain embodiments, the above-described method is especially useful in the formation of environmentally-protective overlay coatings over newly-manufactured or otherwise undamaged GTE components. In other embodiments, the above-described method is especially useful in repair of GTE components having structural damage (e.g., cracking or material loss). In either case, the MCrAlY slurry is applied utilizing a relatively straightforward and low cost application technique, such as brushing, dipping, or spraying; and is heat treated to form a highly dense, adhesive MCrAlY coating having exceptional corrosion and oxidation resistance over a gas turbine engine component.
Corrosion Testing Example
Corrosion testing was performed on an embodiment of the MCrAlY slurry coating formed over a substrate fabricated from MM247, a nickel-based superalloy commonly service-run for turbine engine components such as blades and vanes. For comparison purposes, two uncoated alloy superalloy specimens were also tested from MM247 and HS188, a cobalt based superalloy with good corrosion resistance that is commonly used in turbine engine components such as combustions cans and transition ducts.
Button samples approximately 25.4 mm in diameter by 3.2 mm in thickness were machined from MM247 and HS188. Some of the MM247 samples were coated with a slurry-deposited environmental overlay coating of the type described above. The surfaces of all of the samples were sanded and wet blasted using 240 mesh silica grit. The surfaces of the samples were then ultrasonically cleaned in toluene. An aqueous solution of sodium sulfate (NaSO4) and magnesium sulfate (MgSO4) in a 60:40 ratio, by weight, was applied to one face of the button samples so as to leave approximately 5 mg of salts on the surface after drying. The samples were then place in a low temperature oven (about 40° C. to 90° C.) until the salt solution was dry.
Five samples of each test condition (MCrAlY braze-coated MM247, bare MM247, and bare HS188) were placed in a furnace chamber maintained at 900° C. Periodically, the samples were removed from the furnace, ultrasonically cleaned in toluene, dried, and then weighed in order to determine the weight change as a function of the number of hours exposed to 900° C. After weighing, the salt solution was reapplied to all of the samples as described above, and then returned to the furnace for continued testing.
While at least one exemplary embodiment has been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended Claims.
