MXPA01008002A - Weld repair of directionally solidified articles - Google Patents

Abstract

A directionally solidified nickel-based superalloy article (40) has a defect (58) therein extending parallel to the solidification direction (54). The article (40) is repaired by removing any foreign matter present in the defect (58), and then heating the article (40) to a repair temperature of from about 60 to about 98 percent of the solidus temperature of the base material in a chamber containing a protective gas that inhibits oxidation of the base material. The defect (58) is filled with a filler metal while maintaining the article (40) at the repair temperature. The filling is accomplished by providing a source of the filler metal of substantially the same composition as the base material of the directionally solidified article (40), and melting the filler metal into the defect (58) progressively while moving the source of the filler metal relative to the article (40) in a direction parallel to the solidification direction (54). Optionally, additional artificial heat extraction is accomplished in a heat-flow direction that is within about 45 degrees of the solidification direction (54), as the filler metal solidifies within the defect (58). The article (40) may thereafter be heat treated.( AF ) French Abstract:Un article en superalliage (40)àbase de nickel,àsolidification directionnelle présente un défaut (58) s'étendant parallèlementàla direction de solidification (54). L'article (40) est réparéparélimination de la matièreétrangère présente dans le défaut (58), puis par chauffage de l'article (40)àune température de réparation comprise entre environ 60àenviron 98 pourcents de la température dite 2 Solidus de la matière de base, dans une chambre contenant un gaz protecteur qui inhibe l'oxydation de la matière de base. Le défaut (58) est combléavec un métal d'apport tandis que l'article (40) est maintenuàune température de réparation. On effectue le remplissage en fournissant une source de métal d'apport de composition sensiblement identiqueàcelle de la matière de base de l'articleàsolidification directionnelle (40), et en faisant fondre progressivement le métal d'apport dans le défaut (58) tout en déplaçant la source du métal d'apport par rapportàl'article (40) dans une direction parallèleàla direction de solidification (54). Une extraction de chaleur artificielle additionnelle estéventuellement effectuée dans une direction de flux de chaleur se situant environà45 degrés au maximum par rapportàla direction de solidification (54), tandis que le métal d'apport se solidifie dans le défaut (58). L'article (40) peut ensuiteêtre traitéthermiquement.

Description

WELDING REPAIR PE ARTICLES SOLIDIFIED DIRECTIONALLY FIELD OF THE INVENTION This invention relates to the repair by welding of articles having an oriented grain structure produced by the direction solidification and more particularly with the repair by welding of superalloys with solidified nickel base.

BACKGROUND OF THE INVENTION Metallic articles can be made with a solidified grain structure in directional form to improve their mechanical properties at elevated temperatures. In the directional solidification, the molten metal in a mold that defines the shape of the article is cooled unidirectionally from one end of the mold. The metal solidifies first at the end of which the heat is removed and then along the length of the mold as the temperature drops below its solidification temperature. The resulting structure has a number of grains that extend along the length of the mold parallel to the direction of heat flow. Also, grain boundaries are parallel to the direction of heat flow. Typically, the grains exhibit a grain structure oriented in accordance with the fastest growing crystallographic direction or a shifted orientation introduced at the first solidified end. The grain orientation is selected to achieve good high temperature properties. In service, the article made by directional solidification is placed in such a way that the highest mechanical load is applied parallel to the direction of heat flow during solidification. The orientation of the grain structure parallel to the direction of heat flow places the greatest strength of the material in this direction. Additionally, the orientation of grain boundaries parallel to the direction of heat flow reduces the incidence of plastic runoff from grain boundaries. Directional solidification is used to manufacture molded articles of nickel-based superalloys to be used in the hottest portions of gas turbine overhead engines. When the article solidifies directionally, there may be molding defects, both of common types in the molding processes and also of the unique types of directional solidification. These defects often manifest as cracks, particularly intergranular cracks, which extend parallel to the direction of solidification. Other kinds of defects may occur during solidification and also during service. Directionally solidified items are relatively expensive to produce. Therefore, it is desirable to repair these defects produced during molding or service, if repair is feasible.

With a measure the defect can be repaired by a welding process, in which the defect is filled with a filling material and solidified, or by other filling procedures. These techniques are known for equidimensional items. However, when applied in directionally solidified articles, the result is an inadequate repair that has a non-homogeneous microstructure and whose mechanical properties are unacceptably low. The repaired article may tend to have less ductility than the defect-free article. There is a need for a measure to improve the repair of directionally solidified metallic articles. The present invention satisfies this need and also provides certain related advantages.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a method for repairing directionally solidified articles. This measure produces a chemically homogeneous structure and a grain structure that is oriented similarly to the rest of the article. The result is that the repaired item has the same properties or very similar to those of the defect-free portion of the item. Thus, the repaired item can be used in service without a major reduction in its properties compared to a defect-free item. A method for repairing a directionally solidified article comprises the steps of providing a directionally solidified article comprising a base material with a solidus temperature and having a repair region with a grain structure of elongated kernels essentially parallel to the solidification direction. The repair region includes a defect that is elongated parallel to the solidification direction. The article is heated to a repair temperature of about 60 to 98 percent of the solidus temperature, preferably about 60 to 80 percent of the solidus temperature, of the base material in a chamber containing a protective gas that inhibits the oxidation of the base material. The defect is filled with a filler metal while maintaining the item at its repair temperature. The filling step includes the steps of providing a source of a filler metal of essentially the same composition as the base material of the directionally solidified article and melting the filler metal within the defect progressively, while the source is moved to the metal of filling in relation to the article in a direction parallel to the direction of solidification, so that the filler metal solidifies within the defect. Optionally, the heat can be extracted artificially from the article in a heat flow direction that is within about 45 degrees of the solidification direction. Preferably, the article is made of a nickel-based superalloy. The defect in the article is typically a crack that extends parallel to the solidification direction, and more typically is an intergranular crack. The invention is also operative to repair other types of defects. Before heating, it is preferred to remove any foreign matter present in the defect. The removal of foreign matter is usually achieved by grinding base material around the defect, which creates a cavity that must be filled with the filler metal, and chemically clean the repair region containing the defect. The present measurement produces a repaired region in which the original defect is filled with the same material as the base metal of the article. Heating the article to a high temperature during the filling of the defect reduces the incidence of incompatibility between the filler metal and the base metal, and also reduces the probability of failure of the base metal due to the low ductility of the base metal in the intermediate temperature intervals. The grain structure of the repaired region that originally contained the defect is similar to the rest of the article. The grain structure of the repaired region has grains of the base metal composition oriented parallel to the original direction of heat flow and grain boundaries are also parallel to the direction of heat flow. The grain size may be different, but the oriented grain structure of the repaired region does not result in any damage to the properties as can be seen if the grain structure in the repaired region is equidimensional or had grain boundaries perpendicular to the grain. original direction of heat flow. In this way, the present measure allows defects in directionally solidified articles used in service or mold to be repaired with very little or practically no reduction in the properties of the articles. Other features and advantages of the present invention will become apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. Nevertheless, the scope of the invention is not limited to the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block flow diagram of a measure for practicing the invention; Figure 2 is a perspective view of an article having a defect; Figure 3 is a plan view of the article under repair; Figure 4 is a sectional view of the article of Figure 3, taken along line 4-4; Figure 5 is a plan view of the article repaired with another measure, which is not within the scope of the invention; Figure 6 is a sectional view of the article of Figure 5, taken along the line 6-6; Figure 7 is a perspective view of an internal or external duct covering board.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 is a block flow diagram of a preferred measure for practicing the invention. A directionally solidified article is provided, which is indicated with the number 20. Figure 2 illustrates such article 40 directionally solidified, in this case, a turbine blade for a gas turbine engine. The turbine blade 40 includes an aerodynamic surface 42 against which the flow of hot exhaust gas is directed when the turbine blade is in service in a gas turbine engine. The blade 40 of the turbine is mounted on a turbine disc (not shown) by a dovetail 44 extending downwardly from the aerodynamic surface 42 and engages a groove in the turbine disc. A platform 46 extends longitudinally outwardly from the area where the aerodynamic surface 42 joins the dovetail 44. A number of internal passages may extend through the interior of the aerodynamic surface 42, ending at the openings 48 in the airfoil. surface of the aerodynamic surface 42. All or some portions of article 40 can be covered with a protective coating. The present invention can be operated with turbine blades, the preferred application, and also with other directionally solidified articles. Examples of other articles used in gas turbine engines include turbine blades, mixing nozzles, external and internal transition duct coating boards, and transition duct shrouds. Figure 7 illustrates a typical internal and external duct coating board. The turbine blade 20 is made of a metal alloy base material that is preferably, but not necessarily, a nickel-based superalloy. The preferred alloy is nickel base, which means that it has more nickel in weight percentage than any other element. The preferred nickel base alloy is a superalloy, and is reinforced by the precipitation of gamma prime particles (typically, Ni3 (AI, Ti)) in the gamma matrix. The alloy has a solidus temperature, which is the temperature at which when heating the solid material with the composition of the alloy, the liquid phase first appears. The invention is also operative with other alloys. Examples of preferred nickel-based superalloys operative with the present measure include Rene 80H, which has a nominal composition, in weight percent, of about 9.5 percent cobalt, about 14 percent chromium, about 4 percent molybdenum about 4 percent tungsten, about 3 percent aluminum, about 5 percent titanium, about 0.75 percent hafnium, about 0.2 carbon, and about 0.015 percent boron, nickel balance and impurities; Rene 108, which has a nominal composition, in percent by weight, about 9.4 cobalt, about 8.2 chrome, about 0.5 percent molybdenum, about 9.5 tungsten, about 3.2 tantalum, about 5.6 percent aluminum, about 0.7 titanium percent, approximately 1.5 hafnium, approximately 0.1 carbon, approximately 0.015 boron, nickel balance and impurities; Rene 150, which has a nominal composition, in percent by weight, of about 12 percent cobalt, about 5 percent chromium, about 1 percent molybdenum, about 5 percent tungsten, about 2.2 percent vanadium, about 6 percent tantalum, about 5.5 percent aluminum, about 3 percent iron, nickel balance and impurities; Rene 142 has a nominal composition, in percent by weight, of about 12 percent cobalt, about 6.8 percent chromium, about 1.5 percent molybdenum, about 4.9 percent of tungsten, about 6.4 percent of tantalum, about 6.2 percent of aluminum, approximately 2.8 iron, approximately 1.5 percent hafnium, approximately 0.1 percent carbon, approximately 0.015 percent boron, nickel balance and impurities; and Mar-M247 having a nominal composition, in percent by weight, of approximately 10.3 cobalt, approximately 8.4 percent chromium, approximately 0.75 percent molybdenum, approximately 9.9 percent tungsten, approximately 3.1 percent tantalum, approximately 5.5 percent aluminum, approximately 1 percent titanium, approximately 1.5 percent hafnium, approximately 0.2 percent carbon, nickel balance and impurities. Also, the present invention is operative with other alloys. Article 40 is initially prepared by directional solidification. The result of the directional solidification processing, which process is well known within the art, is a plurality of grain boundaries 50, extended along the length through the article 40, separating the grains 52. The grains 52 are oriented with a preferred crystallographic direction, as in [001] in the case of superalloys with a majority of nickel, parallel to the solidification direction 54. The grain boundaries 50 are also parallel to the solidification direction 54. These grain boundaries 50 are visible on the surface of article 40 and penetrate its interior. The crystallographic directions of the grains 52 and grain boundaries 50 need not be exactly parallel to the solidification direction 54, but are within approximately 15 degrees of the solidification direction. The article 40 may also include some grains and grain boundaries that are not parallel to the solidification direction 54, particularly in irregular areas such as the platform 46. The article 40 has a repair region 56, where the grains 52 are essentially elongated parallel to the solidification direction 54 in the manner described. The repair region 56 includes a defect 58. The most common type of defect 58, will be described in more detail herein, is a crack extending inward from the surface and extending parallel to the solidification direction 54 . Typically, defect 58 is granular, that is, at grain boundary 54 between two adjacent grains 52. The intergranular crack can be formed during the directional solidification molding process, in which the article 40 is molded or can be formed during service. The invention is also operative to repair other types of defects. Examples of these defects include those that are generally formed during molding, such as inclusions, unfilled regions and porosity, and defects that are more directly associated with directional solidification processes, such as spots and cores of unsuitable grains. Service defects include, for example, impact cases, corrosion and hot streaks. Typically, defect 58 has various forms of foreign matter embedded in defect region 56, such as along the sides of the intergranular crack. Foreign matter may include, for example, oxides, particles of impurities and their like. Where the foreign matter is present, it is preferred to remove it from the region 56 of the defect 58, before the subsequent steps of the repair process, number 22. In case foreign matter is not removed, it is likely to interfere with the filling the defect and also leaving the defect repaired weak. Preferably, removal of foreign matter is carried out by grinding the base metal of article 40 away from the sides of defect 58, typically at least twice the width as the original width of defect 58. region 56 which contains the defect can also be chemically cleaned, by using an acid to remove a layer on the surface of region 56. The result of removal of foreign matter is usually a larger volume of material to be filled than the size of the defect original, but for present purposes the volume will be determined by the defect. The article is heated to a repair temperature, number 24. The subsequent filling operation must be carried out at a high repair temperature. The filling operation should not be carried out at room temperature or at a temperature lower than that specified later as the repair temperature, or the repair will not be successful. The repair temperature is about 60 to 98 percent of the solidus, wall temperature of about 60 to 80 percent of the solidus temperature, of the base material of the alloy that forms the article 40. In case the temperature be less, the subsequent filling operation will not be successful and / or the final repaired item will not have the proper properties. The repair temperature can not be higher, since the article will run the risk of melting.

During the subsequent filling operation, the temperature of the region 56 containing the defect 58 may be locally higher by the filling metal to be melted in the defect 58. The "repair temperature" specified above refers to the temperature of the mass base material of article 40 near defect 58 produced by a general heating of the article, but not within defect 58. Generally, article 40 is heated in a welding chamber, by any operable measure. The welding chamber surrounds the article with a protective gas, which inhibits the oxidation of article 40 during the filling process. The preferred protective gas is argon at a slightly above atmospheric pressure, to prevent runoff into the chamber. The protective gas may allow a small amount of oxidation, but the amount of oxidation is greatly reduced compared to the amount that would occur in the absence of the protective gas. The welding chamber is preferably a glove box operating at a light positive pressure [typically about 0.453 kg to 0.907 kg per square cm (1 to 2 pounds per square inch) over the atmospheric pressure] of the inert gas processed as described to achieve greater purity. The inside of the glove box is preheated before the start of welding to expel oxygen, moisture and other residual gases. In the preferred case of welding, under a high purity inert argon gas, care must be taken to keep the oxygen content of the inert gas to less than 1 part per million (ppm), since the residual oxygen strongly oxidizes the elements of the alloy in the superalloy with nickel base, to reduce the quality of the welding. For the same reason, the moisture content of the inert gas is kept very low, with the moisture condensation point of the inert gas at less than about 26 ° C (-80 ° F). The oxygen and moisture contents can be maintained at these low levels by passing the inert gas through a nickel train gas purifier operated at 871 ° C (1600 ° F) before being introduced into the welding chamber, and continuously monitor the oxygen content of the inert gas. Defect 58 (modified by the removal of foreign matter) is filled with a filler metal while article 40 is generally maintained at repair temperature, number 26. In the filling process, a source of a metal is provided of filler, number 28. The filler metal is essentially of the same composition as the metal-based material from which article 40 is generally formed. However, minimal variations in composition can be accepted. The purpose of the filling process is to fill the defect with a metal of approximately the same composition as the metal base material, and also to achieve a directionally oriented grain structure. The filler metal can be in any operable form. Preferred forms include rod and welding powder. Figures 3 and 4 illustrate a repair region 56 in greater detail. In Figure 3, the original defect is indicated by the number 58 'and the defect after removal of foreign matter (step 22) is indicated by the number 58. A source 59 of filler metal is melted into defect 58, in Progressively while moving the source 59 of the filler metal relative to the article 40 in a direction parallel to the solidification direction 54. In the illustration of Figure 3, the source of the filler metal begins at the first end 60 of the defect 58, and moves gradually parallel to the solidification direction 54 toward a second end 62. This relative movement can be achieved by maintaining the stationary article 40 and moving the source 59, keeping the source 59 stationary and moving the article 40, or any combination of these movements. The source 59 of the filler metal is gradually melted by any appropriate heating technique, number 30. In the case of an illustrated welding rod source, the source 59 can be melted by an electric arc strike between the source 59 and the article 40, by forming an arc between an electrode and the article and supplying the filler metal within the arch, by laser fusion, by a separate heat source, or by any other source of operable heat. In the case of a powder source 59, the heating can be achieved by a plasma spray or by another operable heat source. The molten filler metal forms a reservoir that fills the volume of the defect 58. At the same time, the heat, number 32, of the article 40 is withdrawn in a direction of heat flow 64 or in multiple directions of heat flow. The direction 64 of the heat flow is desirably parallel to the solidification direction 54 and opposite to the relative movement of the source 59 relative to the article 40. That is, in case the source 59 moves from the first end 60 towards the second end 62, the direction of heat flow is from the first end 60 in a direction away from the second end 62. The direction 64 of the heat flow, while preferably parallel to the solidification direction 54, may deviate from the solidification direction 54 as much as about 45 degrees. However, the greater the deviation of the direction 64 from the heat flow of the solidification direction 54, the less desirable the final structure of the repaired defect will be. The heat removal causes the filler metal to progressively solidify as a filler metal deposit 65 within the defect volume 58, from the first end 60 to the second end 62. The directional removal of the heat in the direction 64 of the flow of heat occurs naturally as a result of the relative movement of the source 59 of the filler metal. Optionally, the extraction of heat can be aided and accelerated artificially with the use of antifreeze as the flow of a cooling gas that hits the surface of article 40 or a block of freezing, placed as the heat extraction is accelerated as length of the direction 64 of heat flow. Figures 3 and 4 illustrate the grain structure that results from this repair process by controlled welding. The filler metal has an oriented grain structure, which comprises grains 66 within the repaired defect extended from the first end 60 to the second end 62. The preferred orientation of the grains 66 is parallel or substantially parallel to the solidification direction 54 , and therefore parallel to the preferred orientation within the grains 52 of the base metal of article 40. The grain boundaries 68 of the grains 66 within the repaired defect 58 are parallel or substantially parallel to the solidification direction 54 and thus to the limits of the 50 grain of the base metal. This repaired structure is essentially homogeneous in composition, since the filler metal that fills the defects 58 is essentially the same composition as the metal base material of the article 40. The grains, including the grains 52 of the article 40 of base and the grains 66 of the repaired region 56 are more oriented with their preferred directions parallel to the solidification direction 54. The grain boundaries, including both the grain boundaries 50 of the base material and the boundaries 68 of the grains of the repaired region 56, are more oriented parallel to the direction of solidification 54. The result of this homogeneity of composition and Uniformity of orientation is that the repaired item has properties that are the same or almost the same as an article that has no defects or repaired regions.

After the filling step 26, the article 40 can optionally be heat treated, number 34. When the heat treatment is used, it is usually selected to provide the optimum structure for the properties desired in the final article. In accordance with this, the selected heat treatment is associated with the specific alloy that has been repaired. For example, the article can be a treated or stale solution, and / or free of tension, in accordance with the specific procedures of the alloy. This structure is superior to that in its composition of the filler metal is essentially different from that of the base metal and where the direction of the heat flow is not controlled so that the flow of heat is mainly along the direction 54 of solidification. As shown in Figures 5 and 6, where the direction 70 of the heat flow is perpendicular to the solidification direction 54, the grains of the repaired defect are also generally perpendicular to the solidification direction 54. The result is that, in case the composition of the filler metal is different from that of the base metal, there is a lack of homogeneity in strength, corrosion and other properties. Because the direction of heat flow is perpendicular to the direction of solidification, the grains in the repaired defect are oriented so that the grain boundaries are perpendicular to the solidification direction 54 and at least to the main load axis of the article in service. This orientation can lead to premature failure. An equidimensional grain structure in the repaired defect will also suffer this problem. Although a particular embodiment of the invention has been described in detail, for the purpose of illustration, some modifications and improvements can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited except by the appended claims.

Claims (11)

1. CLAIMS 1. A method for repairing a directionally solidified article, which comprises the steps of: providing a directionally solidified article which comprises a base material having a solidus temperature and having a repair region, with a grain structure of elongated grains essentially parallel to the solidification direction, the repair region includes a defect that is elongated parallel to the solidification direction; heating the article to a repair temperature of about 60 to 98 percent of the solidus temperature of the base material in an atmosphere of a protective gas that inhibits the oxidation of the base material; and filling the defect with a filler metal while maintaining the article at the repair temperature, the filling step includes the steps of: providing a source of a filler metal of essentially the same composition as the base material of the solidified article directionally; and melting the filling metal within the defect progressively, while moving the source of the filler metal relative to the article in a direction parallel to the direction of solidification.
2. 2. The method according to claim 1, wherein the step of providing includes the step of providing a nickel-based superalloy.
3. 3. The method according to claim 1, wherein the step of providing includes the step of: providing a nickel-based superalloy having a composition in percent by weight, selected from the group consisting of Rene 80H, having a nominal composition , in percent by weight, of about 9.5 cobalt, about 14 percent chromium, about 4 percent molybdenum, about 4 percent tungsten, about 3 percent aluminum, about 5 percent titanium, about 0.75 percent percent of hafnium, approximately 0.2 percent carbon, and approximately 0.015 percent boron, nickel balance and impurities; Rene 108 having a nominal composition, in weight percent of about 9.4 percent cobalt, about 8.2 percent chromium, about 0.5 percent molybdenum, about 9.5 percent tungsten, about 3.2 tantalum, about 5.6 percent of aluminum, approximately 0.7 of titanium, approximately 1.5 percent of hafnium, approximately 0.1 percent of carbon, approximately 0.015 percent of boron, nickel balance and impurities; Rene 150 having a nominal composition, in percent by weight, of about 12 percent cobalt, about 5 percent chromium, about 1 percent molybdenum, about 5 percent tungsten, about 2.2 percent vanadium, about 6 percent tantalum, approximately 5.5 percent aluminum, approximately 3 percent iron, nickel balance and impurities; Rene 142 having a nominal composition, in percent weight of approximately 12 percent cobalt, approximately 6.8 percent chromium, approximately 1.5 percent molybdenum, approximately 4.9 percent tungsten, approximately 6.4 tantalum, approximately 6.2 percent aluminum, approximately 2.8 percent iron, approximately 1.5 percent hafnium, approximately 0.1 percent carbon, approximately 0.015 boron, nickel balance and impurities; and Mar-M247 having a nominal composition, in weight percent of approximately 10.3 percent cobalt, approximately 8.4 percent chromium, approximately 0.75 percent molybdenum, approximately 9.9 percent tungsten, approximately 3.1 percent tantalum, about 5.5 percent aluminum, about 1 percent titanium, about 1.5 percent hafnium, about 0.2 percent carbon, nickel balance and impurities.
4. 4. The method according to any of claims 1 to 3, wherein the defect is a crack extending parallel to the direction of solidification.
5. 5. The method according to any one of claims 1 to 4, including the additional step, before the heating step of: removing any foreign matter present in the defect.
6. 6. The method according to claim 5, wherein the step of removing includes the step of removing foreign matter from the defect region by grinding the material around the defect.
7. 7. The method according to any one of claims 1 to 6, including the additional step, after the filling step of: heat treating the article with the defect repaired.
8. 8. The method according to any one of claims 1 to 7, including the additional step, carried out simultaneously with the melting step, of: artificially extracting the heat of the article in a heat flow direction, which is within about 45 degrees of the solidification direction, so that the filler metal solidifies within the defect.
9. 9. A directionally solidified repaired article, which comprises: an article comprising a base material having a solidus temperature and having a repair region that includes a defect that is elongated parallel to the solidification direction, the article has a structure of grain of elongated grains essentially parallel to the direction of solidification; and a filling material within the defect, the filling material has essentially the same composition as the base material of the article and has a grain structure with elongated grain boundaries that are essentially parallel to the direction of solidification.
10. 10. The article according to claim 9, wherein the base material is a nickel-based superalloy.
11. 11. The article according to claim 9 or claim 10, wherein the defect is a crack extended parallel to the direction of solidification.