STRONG WELDING ALLOYS BASED ON COBALTQ-CHROME-PALADIUM fk BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION 5 This invention relates to brazing metals composed of cobalt-chromium-palladium-based alloys containing transition metals such as cobalt , nickel, tungsten, molybdenum and certain metalloids, and more particularly to alloys of multiple components that
contain cobalt, chromium, palladium, nickel, tungsten, molybdenum, boron and silicon, which are especially useful for welding metals at high temperatures to produce very solid welds, highly resistant to oxidation at high temperatures and corrosion. The alloys of this
invention have a composition represented by the formula: CraNibWcPddSieBfCoba; A (plus incidental impurities), where the subscripts "a", "b", "c", "d", "e" and "f" are presented in atomic percentages and "a" is within the range of 15 to approximately
22, "b" is between about 0 and about 20, "c" is within the range of about 1 to about 5, "d" is within the range of about 1 to about 10, "d" is found within the range of approximately 5 a
approximately 12 and "f" is in the range 5 to approximately 12 and "bal" is the remaining amount to reach a total of 100%. 2. DESCRIPTION OF THE PREVIOUS TECHNIQUE Brazing is a procedure for joining metal parts, often of different composition, among them. Typically, brazing is achieved by interposing a filler metal having a melting point lower than the melting point of the parts to be joined to form an assembly. The assembly is then heated to a temperature sufficient to melt the weld filler metal. Upon cooling, a strong bond is formed, preferably of high oxidation resistance, high temperature resistance and high resistance to corrosion. Some kinds of products produced by brazing processes are used as critical parts of turbines for the production of energy, for example as aircraft engines in the aeronautical industry and in stationary generation plants to generate electric power. Particularly turbine parts for producing energy, for example turbine seals, first stage turbine nozzle guide fins, and turbine blades are subjected, in operation, to highly oxidative high temperature environments. Thus, the welded parts used in these applications must be able to withstand such difficult operating conditions in order to achieve a high energy efficiency that is directly related to the operating temperature. Another important application of brazing technology is the fabrication of light-weight and high-temperature-resistant honeycomb structures for wing and other body parts of supersonic aircraft and reusable space shuttles. In these applications, the base metals to be bonded are mainly nickel and cobalt-based super alloys and iron-based alloys containing a high chromium content. Such iron-chromium-based superalloys and alloys have complex compositions comprising alloys of some or all of the elements in a group of transition elements such as cobalt, nickel, chromium, iron and certain refractory elements. In addition, all these alloys also typically contain aluminum, titanium and sometimes, additions of yttria to improve their resistance to high temperatures and high oxidation. This last characteristic is achieved due to the intrinsic formation of a surface protection film of alumina oxide / titania on such metal base parts. Of particular importance to all parts subjected to a high temperature service environment is their resistance to oxidation while maintaining the mechanical integrity of the parts. The resistance to oxidation of these base metals is due to the existence of the aforementioned dense alumina / titania protection film on the surface of the part. Unfortunately, brazing by the use of filler metals containing active metalloid elements such as boron and silicon causes a partial or even complete dissolution of these protective oxide films in the welded areas. As a result, the welded interfaces act as conduits for the penetration of oxygen which causes a catastrophic oxidation of the part. Therefore, during the brazing of materials it is of essential importance to preserve the integrity of the brazing interfaces even if these oxide films can not be preserved in the initial state. Previously, some amorphous brazing filler metals consisting of cobalt-nickel-chromium-based alloys have been developed which exhibit sufficient strength and good resistance to corrosion at elevated temperatures. Such alloys have been disclosed, for example, in U.S. Patent Nos: 4,260,666, 4,515,868, 4,515,869, 4,515,870 and 4,801,072. The alloys disclosed in these patents, however, present drawbacks that make them unsuitable for brazing products that require a long service life at high temperatures and in highly oxidizing and corrosive environments. For example, the alloys disclosed in US Patent Nos: 4, 260, 666,
^ k 4,515,868 and 4,801,072 contain the transitional and refractory elements and boron and silicon. Unfortunately, boron 5, due to its very small atomic radius, diffuses significantly outside the bonding area in alloys, particularly in alloys containing chromium, due to the tendency to form strong chromium borides. These borides are formed preferably in grain boundaries which results
^^ 10 in a fragile alloy and in excessive oxidation or even in a complete failure. At the same time, these alloys do not contain elements that protect the base metal against the diffusion of boron. As for the alloys of multiple components
disclosed in US Patent Nos. 4,515,869 and
4,515,870 also contain similar transition and refractory elements as well as boron and silicon but are based on nickel. Therefore, these multi-component alloys contain only a moderate amount (less
than 30% atomic) of cobalt and as a result are insufficient to protect the welded parts against an environment of high temperatures and high oxidation. For the above reasons, previously known alloys are not effective for use in products
soldiers to be used in high temperature, high oxidation, and high voltage environments that exist in turbine engines and supersonic aerospace structural applications. Accordingly, there remains a need in the art for improved solder fillers for welding super alloys or alloys based on iron-chromium at high temperatures that can withstand service in high temperature and high oxidation environments under high stress for a long time . Specifically, there is a need in the art for a solder filler metal that naturally forms a protective layer of high temperature resistant phases at the solder interface, protecting the base metal bases against excessive penetration of boron upon completion of the welding. In addition, it would be beneficial if this box kept the borro inside the union, thus preventing its excessive diffusion in base metal. Accordingly, it is an object of the present invention to provide said weld metal. It is a further object of the present invention to provide solder filler metals that first contain larger metallic elements compatible with base metals resistant to high temperatures; second, that they can humidify a surface covered with rust during the brazing operation; and third, which contain an element or several elements that migrate predominantly and form a layer of protection phase in the interface of the union. SUMMARY OF THE INVENTION The present invention provides an improved braze filler metal having melt characteristics at high temperatures and forming strong welds having high strength, and high oxidation resistance at very high service temperatures. Welding alloys particularly suitable for use as filler metal contain cobalt, chromium, palladium, nickel, tungsten, molybdenum, boron and silica which are especially useful for welding metals at high temperatures to produce high strength welds, highly resistant to oxidation and high temperatures and corrosion. The alloys have a composition represented by the formula: CraNibWcPddS ieBfCObal plus incidental impurities, where the subscripts "a", "b", "c", "d", "e", and "f" are atomic percentages and "a "is within the range of about 15 to about 22," b "is between about 0 and about 20," c "is within the range of about 1 to about 5," d "is within the range of about 1 and approximately 10, "e" is in the range of about 5 to about 12, and "f" is in the range of about 5 to about 12., and "bal" represents the rest to reach the rest of a total of 100%. The alloys of the present invention exhibit numerous beneficial properties not recognized or disclosed before. These alloys exhibit a high melting temperature within a range of about 1050 to about 1180 ° C. These alloys present virtually no significant diffusion problems associated with alloys containing boron since they contain only a low amount of boron and, more important, since they contain palladium. Palladium forms a predominant layer of an aluminum-palladium AlPd intermetallic phase resistant to oxidation with high melting temperature at the bonding interfaces thus preventing the penetration of boron, changing, favorably retreating the microstructure of the bond and protecting the joints against oxidation. Furthermore, despite the presence of palladium, the minimization of the boron concentration together with the maintenance of the silicon concentration at relatively low levels, the alloys of the present invention can be manufactured as a ductile product. Furthermore, increasing the concentration of palladium at coasts of the cobalt concentration allows to conserve the capacity of the alloy to be formed in the amorphous state and to remain ductile in the sheet form. The welded base metal parts are protected against oxidation and their resistance to high temperatures is of a high level because the alloys of the present invention exhibit excellent protection against harmful excessive diffusion of boron into the base metal due to the formation of a beneficial layer of AlPd phase in the binding interfaces. In the same way, since the solid phase at high temperatures of AlPd is first formed at the interface between solid base metal / liquid filler metal, the erosion of the thin base metal is substantially limited due to a protective interaction between the Liquid filler and solid base metals. The strong welds produced using such filler metals have a substantially uniform microstructure and possess a high resistance to high temperatures. In addition, the invention offers brazing filler metals in the form of homogeneous ductile sheets composed of metastable materials preferably having at least 80% amorphous structure. Further, in accordance with the present invention, there is provided an improved process for joining superalloys and / or honeycomb structures based on iron or chromium, said process comprising the steps of: interposing a filler metal of the composition described above between portions of Use a base mix to form an assembly, heat the assembly to a temperature of about 25 to 50 ° C above the liquidus temperature of the braze filler metal and hold at this temperature for a sufficient time to form a strong and resistant bond to oxidation. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully understood and additional advantages will be apparent with reference to the following detailed description of the preferred embodiments of the invention and the accompanying drawing in which: Figure 1 is an SEM micrograph of an iron-based union -chrome-aluminum made using a filler metal manufactured in accordance with the prior art as presented in the US Patent No. 4,260,666, the micrograph shows the presence of a substantial amount of chromium borides (in the black arrows) precipitated in the body of the base metal part. These borides are predominantly segregated in planes parallel to the direction of base metal rotation. Figure 2 is an ESM micrograph of an iron-chromium-aluminum binding made using a filler metal containing 3% by weight of palladium and which was manufactured in accordance with the present invention, the micrograph represents a dense layer of intermetallic phase of AlPd formed in the interface of union (in empty arrows) and protecting the base metal against the penetration of boron and the formation of harmful chromium borides. The base metal has a single phase, substantially uniform microstructure, with a very limited amount of precipitated chromium borides. Figure 3 is an ESM micrograph of an iron-chromium-aluminum-based junction effected using a filler metal containing 5% by weight of palladium and which was manufactured in accordance with the present invention, the micrograph showing the same basic characteristics beneficial to the binding microstructure as illustrated in Figure 2 but containing a substantially greater amount of the AlPd phase (in the empty arrows). This nstrates that the AlPd phase formation is in fact related to the amount of palladium in the filler metal alloy of the present invention. Figure 4 is a x-ray diffraction pattern taken from sheet sample number 4 showing a diffuse halo Characteristic of the amorphous state. DETAILED DESCRIPTION OF THE INVENTION In any brazing process, the brazing material must have a sufficiently high melting point to provide strength to meet the service requirements of the metal parts to be welded together. In addition, the material must be compatible, both chemically and metallurgically, with the base metal parts welded. Ideally, the welding material should be in sheet form that contains 100%
^ k of metal in such a way that complex shapes can be stamped from there in such a way that strong welding of complex structures can be easily effected. In addition, brazing sheets must be homogeneous and ductile. That is, they must not contain binders or other materials that could otherwise form voids or contaminating residues during the welding process
^ & 10 strong, and to demonstrate sufficient flexibility such that the blade can be bent to a round radius of a size up to about 10 times the thickness of the blade without fracture. In accordance with the present invention, there are provided cobalt / nickel-chromium based multicomponent alloys having a liquidus temperature of at least about 1090 ° C, and which are particularly suitable for brazing alloy-based alloys. iron-chromium-aluminum. 20 Welding filler metals have a composition represented by the formula: CraNibWcPddSieBfC? Bai More incidental impurities, where the subscripts "a", "b",
"c", "d", "e" and "f" are in atomic percentage and "a" is
is within the range of about 15 to about 22, "b" is within a range of about 0 to about 20, "c" is within the range of about 1 to about 5, "d" is within of the range between about 1 and about 10, "e" is within the range between about 5 and about 12, and "f" is within a range between about 5 and about 12 and "bal" represents the rest to reach a total of 100%. Boron and silicon are added to increase the capacity of the alloys to resist in the amorphous form and to lower the melting point of the alloys. The additions of boron and silicon also provide wetting capacity and ensure a metallurgical bond between the filler metal and the base. Individually, boron is present in the amount of from about 5 to about 12, and more preferably from about 6.5 to about 8.0 atomic%, to improve the strength in the amorphous state and the interaction between base and filler metals. The silicon content is within a range of about 5 to about 12 and more preferably within a range of about 5.0 to about 10.5 atomic%. In these amounts, it is believed that the silicon operates to induce the formation of the amorphous structure and contribute to the weldability of the filler metal. The total content of boron and sackcloth can not be so great as to cause excessive erosion of the base metal during the alloy or to cause excessive formation of fragile intermetallic compound phases separated in the weld. Preferably, the total boron and silicon content of the filler metal is within a range of about 14.5 to about 20 atomic%, and more preferably within a range of about 14.5 to about 17 atomic%. Of critical importance is palladium in an amount of 1 atomic% to about 10 atomic% and more preferably from 1.5 to 7 atomic% to form a high temperature intermetallic palladium-aluminum phase protection layer at the interface between the weld and the base metal. In accordance with binary phase diagrams of aluminum-palladium constitution, both elements form an intermetallic component of AlPd of very high melting point (TfUs? Ón = 1645 ° C) strong and resistant to oxidation that exists in a range of narrow composition . See "Binary Alloy Phase Diagnosis" (Diagnosis of Alloy Binary Phases), Ed. T. Misoalski, ASM 1990, pages 139-191. This compound, in accordance with the standard thermodynamic data, has a high enthalpy value of formation compared to what is obtained from any other potential intermetallic phase or other types of phases of cobalt, chromium, tungsten, iron and silicon can form with aluminum, the high value of Formation enthalpy is a basic measurement of the intensity of a chemical reaction between elements involved and an indication of the chemical stability and mechanical strength of its substance (s), of resulting product (s), is say AlPd in this particular case (see table 1). It was discovered in this invention that during the welding operation, the binary AlPd intermetallic phase segregates first at the interface between the base metals of the multiple and fill controls. It is believed that the formation of a protective phase layer occurs as a result. In addition, preferably, tungsten is comprised between about 1 and about 5 atomic%, and more preferably in an amount ranging from about 3.0 to about 3.5 atomic%. The function of tungsten in the alloys of the present invention is to preserve the high melting temperature of the alloy while increasing the overall strength of passive film formation in strong welds. Cobalt and chromium, the main elemental components, form the basis of the composition of the alloys of the present invention and are especially suitable for providing resistance to oxidation at high temperatures of the strong welds formed. The presence of nickel in alloys of the present invention greatly improves its resistance to certain non-oxygenating corrosive media. Nickel also provides other desired properties, for example amorphous character, ductility and the like. The alloys of the present invention can be produced in various forms, such as, for example, sheets, tapes, and wires by the application of several well-known techniques. The alloys of the present invention may also be produced in the form of metastable powders, homogeneous, ductile sheets or wires by casting alloys of the composition described above using fast solidification techniques. Methods commonly used to make alloys in powder form include gas or water atomization or mechanical pulverization. The most preferred method used for the manufacture of alloys of the present invention in sheets, tapes or wires is rapid solidification. The alloys of the present invention have numerous advantages that have not been recognized or disclosed in advance. These alloys have a high melting temperature and do not present significant diffusion problems, generally associated with alloys with a high boron content, since the concentration of boron is kept at a minimum level while the presence of palladium predominantly prevents the presence of boron inside the brazing. At the same time, the concentration of boron, together with a strong concentration of silicon, allows the production of a product of ductile and coarse tape by rapid solidification technology. Furthermore, since it contains a combined concentration of boron and silicon at sufficient levels and maintains a chromium concentration of approximately 21 atomic%, the alloy capacity to be formed in the amorphous state and to remain ductile in tape form retains the ability. Finally, the alloys of the present invention do not substantially erode the base metal, thereby retaining the integrity of thin fin portions that are employed in honeycomb or plate structures. The alloys of the present invention can be produced in the form of homogeneous, ductile sheets or wires by melting alloys of the composition described above using fast solidification techniques. More specifically, the homogeneous brazing filler metals of the present invention can be manufactured by a rapid solidification process comprising the formation of a melt of the composition, and the rapid cooling of the solution in a rotating rapid cooling rule at a speed of approximately 105 degrees C per second. Said process is disclosed in the North American Patent No. 4,142,571. Under these conditions of rapid cooling, a ductile, homogeneous, metastable product is obtained. The metastable material can be amorphous, in this case there is no long-range order in accordance with what is evidenced by x-ray diffraction patterns that show a diffuse aura, similar to what is observed in the case of inorganic oxidation glasses ( figure 4). Preferably, the microstructure of the alloys of the present invention contained at least 50% amorphous phase to achieve superior ductility, and more preferably is at least about 90% amorphous. Metastable products can also be a solution of constituent elements. In the case of the alloys of the present invention, such solid, metastable solution phases are not customarily produced in conventional processing techniques employed in the manufacture of crystalline alloys. Accordingly, the casting processes described above are employed. These metastable products can be quickly solidified sheets or spots that are also ductile. The sheets produced by the fast solidification process described herein have a thickness comprised between about 13 and about 100 microns, usually between about 13 and about 76 microns in thickness and up to 200 mm in width or more. Because they are homogeneous products (ie, of substantially uniform composition in all directions), the strong welds produced therefrom are fairly uniform and substantially free of voids. Within the wide range of the compositions of the present invention, a preferred embodiment has been found to have a composition from about 18 atomic% to about 22 atomic% chromium, from 12 to about 17 atomic% nickel, from about 1.5 to about 7 atomic palladium from about 3.0 to about 3.5 atomic% tungsten, from about 5 to about 10.5 atomic% silicon, and from about 6.5 to about 8.0 atomic% boron, the remainder being essentially cobalt and incidental impurities. The alloys within this most preferred embodiment have a range of melting temperatures of between about 1010 ° C and about 1180 ° C, and more preferably, within a range of about 1015 ° C to about 1160 ° C. Specific to these alloys include the ability to form strong welds at elevated temperatures and provide a braze that can be used in an environment of high temperatures with high level of oxidation and corrosion without significant degradation of mechanical properties. The alloys produced in accordance with the present invention are especially suitable for the brazing of turbine parts and structures of aircraft and spacecraft employed in the aeronautical industries and power plants. The following examples are provided to allow a more thorough understanding of the invention. The specific techniques, specific conditions, specific materials, specific proportions, and reported data presented to illustrate the principles and practice of the invention are examples and should not be considered as limits of the scope of the present invention. Example 1 Tapes of about 2.54 to about 200 mm (about 0.1 to about 8 inches) wide and about 13 to about 76 microns (about 0.0005 to about 0.003 inches) thick are formed by the continuous deposition of a melt each of the compositions presented in table 2 below, by overpressure of argon in a rapidly rotating copper cooling sheet (surface velocity from about 915 meters / minute (3000 feet / minute) to about 1830 meters / minute (6000 feet / minute)). Metastable homogenous tapes are produced that
• have a substantially glassy structure. The liquidus and solidus temperatures of the tapes described in Table 2 are determined by the Differential Thermal Analysis (DTA) technique. Individual samples are heated side by side with an inert reference material at a uniform speed, and the temperature difference between them is measured as a function of temperature. A thermogram (a graph of the change in thermal energy vs. temperature) is produced from which the beginning of the melting and the end of the melting is determined, which are respectively known as solidus and liquidus temperatures. The values are reported in table 2 below. 15 TABLE 2 Nominal alloy composition (percentage by weight) and melting characteristics No. of Alloy Composition (Designation of 20 Laboratory) Co C Crr N Nii P Pdd W B YES
1 (MBF-100) Bal 2 211..55 - - 1.2 11.65 3.05 21. 0 4.50 2.40 1.60
2 (# 5) Bal 2 222..1177 - 1 1..5588 1.27 7.26 12.4 25 2 200..55 3 3..0000 4.15 1.40 6.20 3 (IPF-2050) Bal 21.33 - 1.49 1.29 7.81 10.34 21.0 3.00 4.50 1.60 5.50
4 (IPF-2051) Bal 20.55 13.65 1.50 1.31 7.9 8.37 20.0 15.00 3.00 4.50 1.60 4.40 5 (ipf-2052) Bal 20.72 13.76 2.53 1.32 7.97 8.44 20.00 15.00 5.00 4.50 1.60 4.40 Alloy No. Melting characteristics (Laboratory designation) Solidus Liquidus 1 (MBF-100) 1130 1160 2 (# 5) 1056 1136 3 (IPF-2050) 1056 1131 4 (IPF-2051) 1068 1156 5 (ipf-2052) 1018 1152 Example 2 Samples for metallographic tests are sized and manufactured as "sandwich" type samples. Each sample consists of two strips of 125 μm thick PM2000 alloy, which have a composition of 20% by weight of Cr, 5.5% of Al, 0.5% of Ti, 0.45% of Y203, and the rest is iron, and a single 25 μm thick sheet of one of the samples presented in table 2. The sheets include the number 1 sample manufactured in accordance with the prior art and nominal composition samples of numbers 4 and 5 manufactured in accordance with the present invention. The unique brazing sheet is pre-positioned between two PM2000 strips. The width of the strips of PM2000 and of all brazing filler metal alloys is approximately 10 mm. In the case of these brazing alloys, the tapes act as spacers. The alloy is carried out in a vacuum oven evacuated to a pressure equal to or less than 1.33 x 10"2 Pa (10" d Torr). The alloy is carried out at a temperature of 1195 ° C for 15 minutes. When cooling in the furnace, segments are cut from welded samples, assembled in the form of plastic, and polished in standard equipment to achieve the preparation of metallographic samples for metallographic observations with scanning electron microscopy. The microstructure of the joints is observed using analytical methods SEM / EDAX and Auger. The typical binding microstructure prepared using an alloy with a nominal composition of Sample 1 that is manufactured using the prior art is shown in Figure 1. The micrograph shows the presence of a substantial amount of chromium borides precipitated in the body of the base metal part. These borides (in the black arrows) segregate predominantly in planes, which are parallel to the winding direction of the base metal alloy. Figure 2 is a SEM micrograph of a sample prepared using a filler metal sheet with a Sample No. 4 of nominal composition containing 3% by weight of palladium and manufactured in accordance with the present invention. The micrograph shows a dense layer of intermetallic phase of AlPd (in empty arrows) that is formed at the bonding interface and that protects the base metal, through the penetration of boron and the formation of harmful chromium borides. The base metal has a substantially uniform, single-phase microstructure with a very limited amount of precipitated chromium borides. Figure 3 is a micrograph of a joint made using a filler metal sheet with a sample number 5 of nominal composition containing 5% by weight of palladium and manufactured in accordance with the present invention, the micrograph shows the same beneficial basic characteristics of the binding microstructure which is what is shown in Figure 2, but contains a substantially greater amount of AlPd phase (in empty arrows). This demonstrates that the formation of the AlPd phase is in fact related to the amount of palladium in the filler metal alloy of the present invention. Having thus described the present invention in detail, it will be understood that such details do not have to be followed strictly but that various changes or modifications may be presented to one skilled in the art, all these changes and modifications are within the scope of the invention of in accordance with that defined in the appended claims.