Patent Publication Number: US-2013248518-A1

Title: Brazing method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part to U.S. patent application Ser. No. 13/428,006, filed on Mar. 23, 2012, which is fully incorporated herein by reference and made a part hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention described herein relates generally to brazing. More specifically, the invention relates to a method of brazing. 
     Armature stator bars in large generators are usually liquid cooled and contain a combination of individually insulated conductors, comprising both solid and hollow strands. The hollow strands are used to transmit liquid coolant, as well as electric current through the length of the armature stator bar. A header, including generator connection rings, must be fluid tight and capable of conducting electric current. This assembly is affixed to each end of each armature stator bar. All plumbing should be impervious to hydrogen leaks from the generator atmosphere, pressurized hydrogen, into the circulating cooling fluid inside the plumbing. 
     The headers and connection rings presently in service in generators serve as electrical connections between the stator bars or phase rings of the armature circuit and are also the sealed enclosure for transferring the liquid coolant to and from the stator bars. Conventional headers and connection rings are normally brazed to both the hollow and solid strands at each end of the stator bars. Thus, the liquid coolant is in direct contact with the brazed joints, which can result in liquid coolant leaks due to braze joint corrosion or braze joint imperfections. One known method of reducing leaks is to use phosphorus—free BAg alloys. However, the phosphorus—free BAg alloy family typically cannot be brazed in air without a flux or a vacuum or reducing atmosphere. Process issues with use of a flux or reducing atmosphere present major challenges to plumbing assembly or repair. Even with the known methods, ensuing leaks of liquid coolant can damage the armature insulation and result in costly maintenance outages. In addition, on-site repair of generator components is extremely difficult, time consuming and costly. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an aspect of the present invention, a brazing method for a dynamoelectric machine includes the steps of providing a first dynamoelectric machine part and a second dynamoelectric machine part, at least a first portion of the first dynamoelectric machine part is configured to fit inside a second portion of the second dynamoelectric machine part. Preplacing a non-self-fluxing braze alloy on one or more of the first portion and the second portion. Thermally treating at least one of the first portion and the second portion, to create a temperature differential and size differential between the first portion and the second portion. Inserting the first portion into the second portion, and heating at least one of the first portion and the second portion to melt the non-self-fluxing braze alloy. The first portion is joined to the second portion by brazing without the use of a flux, vacuum or inert atmosphere. 
     In another aspect of the present invention, a brazing method for a dynamoelectric machine is provided. The dynamoelectric machine includes a first dynamoelectric machine part and a second dynamoelectric machine part. At least a first portion of the first dynamoelectric machine part is configured to fit inside a second portion of the second dynamoelectric machine part. The method includes the steps of preplacing a non-self-fluxing braze alloy on one or more of the first portion and the second portion. Thermally treating at least one of the first portion and the second portion, to create a temperature differential between the first portion and the second portion. Inserting the first portion into the second portion, and heating at least one of the first portion and the second portion to melt the non-self-fluxing braze alloy. The first portion is joined to the second portion by brazing in an ambient air environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a simplified perspective view of two parts before they are brazed together, according to an aspect of the present invention; 
         FIG. 2  illustrates a simplified perspective view of two parts after they are brazed together, according to an aspect of the present invention; and 
         FIG. 3  illustrates a flowchart of a brazing method, according to an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more specific aspects/embodiments of the present invention will be described below. In an effort to provide a concise description of these aspects/embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design method, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with machine-related, system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters and/or environmental conditions are not exclusive of other parameters/conditions of the disclosed embodiments. Additionally, it should be understood that references to “one embodiment”, “one aspect” or “an embodiment” or “an aspect” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments or aspects that also incorporate the recited features. 
     A dynamoelectric machine is defined as a machine that converts mechanical energy to electrical energy or vice-versa, including but not limited to generators and motors. However, it is to be understood that the present invention could also be applied to turbomachines and general brazing methods as well. 
     A brazing method is herein described for joining two parts in a reliable manner without requiring the use of a flux, vacuum, inert gas or reducing atmosphere. Brazing is generally defined as a joining process wherein coalescence is produced by heating to a suitable temperature above about 800° F. and by using a suitable brazing alloy, having a melting point below that of the materials to be joined. Brazing in an ambient air environment greatly expands potential uses for the brazing method, as well as, reduces costs and shortening service/construction times, thereby returning the machine to service much more promptly. An ambient air environment is defined as an environment comprising a substantially colorless, odorless, tasteless, gaseous mixture of mainly nitrogen (approximately 78 percent) and oxygen (approximately 21 percent) with lesser amounts of argon, carbon dioxide, hydrogen, neon, helium, and other gases. Ambient air is a mixture with varying amounts of moisture and particulate matter, enveloping the earth and may also be referred to as the atmosphere. Ambient air may also be the air that surrounds the average person when the person is located outside or within a building or enclosure, or the air in the immediate surroundings of something. 
       FIG. 1  illustrates a simplified perspective view of two dynamoelectric machine parts needing to be joined together by brazing. The two parts may be pipes, conduit or any other element for transporting a gas or fluid, and may comprise portions of a generator cooling circuit. A first portion  112  of a first dynamoelectric machine part  110  is configured to fit inside a second portion  122  of a second dynamoelectric machine part  120 . In one example, the first dynamoelectric machine part  110  is a copper pipe having a first outer diameter (e.g., 1 inch). The second dynamoelectric machine part  120 , in this example, may also be a copper pipe or copper fitting, and the second portion  122  may have an inner diameter about the same or slightly greater than the outer diameter of the first dynamoelectric machine part  110  or first portion  112 . For example, if the first part/portion has a 1 inch outer diameter, then the inner diameter of the second portion  122  might be about 1.004 inches to about 1.008 inches, resulting in a clearance gap of about 0.002 inches to about 0.004 inches, respectively. These dimensions are only exemplary and any suitable part dimensions and clearance gap may be used with the method of the present invention. 
     The two parts  110 ,  120  can be joined by brazing and both portions of the joint should be precleaned. However, an important feature of the present invention is that both parts can be brazed without requiring the use of a flux, a vacuum, an inert gas or reducing atmosphere. The method herein described has resulted in substantially improved results that were unexpected, because typical practice has required the use of a flux or a reducing atmosphere. The terms vacuum, inert gas and reducing atmosphere are viewed as generally equivalent in the sense that they are all used to prevent contamination of the parts during brazing. For example, when copper is heated to elevated brazing temperatures, contamination, such as, oxidation and scaling can occur quickly and this contamination interferes with or prevents a successfully brazed joint. The braze alloy is prevented from satisfactorily adhering to the parts by the contaminated or oxidized layers. In the past, the only method to avoid this was to use either self-fluxing braze alloy, a flux, a vacuum, an inert gas or a reducing atmosphere. It can be appreciated that it is extremely difficult, costly and time consuming to create a reducing atmosphere around a large scale generator at the point of use. It can also be appreciated that a joint that must remain water-tight through many years of service cannot be contaminated by a self-fluxing braze alloy containing phosphorus, which causes copper to corrode, or any sort of residual flux, which is also corrosive. 
     In order to obtain high-quality brazed joints, the parts must be closely fitted, and the base metals must be exceptionally clean and free of oxides. In most cases, joint clearances of about 0.002 inches to about 0.008 inches are recommended for the best capillary action and joint strength. However, in some brazing operations it may be desirable to have joint clearances above or below this range. Cleanliness of the brazing surfaces is also important, as any contamination can cause poor wetting (i.e., flow of the filler metal or braze alloy). Two methods for precleaning parts, prior to brazing, are chemical cleaning, and abrasive or mechanical cleaning In the case of mechanical cleaning, it may be desirable to maintain a predetermined surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry. 
     A phosphorus free braze alloy (or phosphorus free filler metal)  116  may be used in the brazing method, according to an aspect of the present invention. A non-self-fluxing braze alloy  116  may also be used. For example, a BAg-18 alloy is comprised of silver (Ag), copper (Cu) and tin (Sn), and has a solidus point of about 1,115° F. and a liquidus point of about 1,325° F. A BAg-24 alloy may also be used and is comprised of silver (Ag), copper (Cu), zinc (Zn) and nickel (Ni), and has a solidus point of about 1,220° F. and a liquidus point of about 1,305° F. It is to be understood that other non-phosphorous, phosphorus free, BAg alloys or non-self-fluxing brazing alloys may also be used as desired in the specific application, as long as they meet desired brazing and performance specifications. The phosphorous free braze alloy and non-self-fluxing braze alloy may be any suitable BAg alloy. 
     The braze alloy  116  may be preplaced on at least one of the parts to be joined. For example, the braze alloy can be preplaced on part  110  in the region (i.e., portion  122 ) of the joint. The second part  120  can be thermally treated by preheating to thermally expand the inner diameter of portion  122 . The thermal treatment creates a temperature differential (and resulting size differential) between the first portion and the second portion. For example, the second part  120  may be heated by induction heating to a temperature of about 400° F. to about 500° F. It would be desirable to prevent or reduce oxidation of the heated parts during the thermal treatment step, so lower temperatures are desired. Other heating methods (e.g., torch heating, furnace, carbon arc, resistance, etc.) and other temperature ranges above or below those listed may also be used as desired in the specific application. After the second part  120  is preheated and portion  122  has thermally expanded, the first part  110  can be inserted into the second part  120 . 
     Alternatively, the first part  110  and/or first portion  112  can be thermally treated by cooling to contract or shrink the first part  110  and/or first portion  112 . The thermal treatment creates a temperature differential (and resulting size differential) between the first portion and the second portion. For example, the first portion  112  may be immersed in a nitrogen bath. The nitrogen bath may comprise liquid and/or solid nitrogen, or mixtures thereof. In this aspect, the nitrogen bath shrinks or contracts the first portion, while also providing a benefit to the method of reducing oxide formation or contamination. At cooler temperatures, oxide formation on metals (e.g., copper) is greatly reduced (or effectively eliminated) compared to a method where the metal is heated. In other aspects of the present invention, a combined cooling of the first portion and pre-heating of the second portion may be employed to create a compressive fit-up between the two parts. 
     The two parts form a compressive type fit-up and the thermal treatment (resulting in relative expansion and/or contraction) allows for easier insertion of the first part  120  (portion  112 ) into the second part  120  (portion  122 ). The small clearances also permit the avoidance of using a flux, vacuum, inert gas or reducing atmosphere. After the brazing operation and upon cooling of the parts, a high quality and long lasting mechanical bond is formed between the two parts. 
     To braze, a heating step can be performed on both parts, and this can be performed by induction heating or other suitable heating method (e.g., torch heating, furnace, carbon arc, resistance, etc.). The two parts  110 ,  120  (and/or portions  112  and  122 ) may be heated to about 1,300° F. to about 1,500° F., or any other suitable temperature range as required by the specific materials. The heating cycle melts the braze alloy and the braze alloy distributes along the joint through capillary action. The braze alloy bonds to both parts and forms a seal preventing any undesired leaks in the joint. As stated previously, conventional wisdom and practice always required a flux or reducing atmosphere when brazing at such elevated temperatures. However, the combination of the compressive fit-up and phosphorous free or non-self-fluxing braze alloys (such a BAg alloys) enables the brazing method to be performed in air without any corrosive flux or phosphorus, and without the need for a vacuum, inert, or reducing atmosphere. 
       FIG. 2  illustrates a simplified perspective view of both parts after a brazing method, according to an aspect of the present invention. The first part  110  has been inserted into the second part  120  and both parts are joined or brazed together by joint  230 . 
       FIG. 3  is a flow chart of a brazing method  300  according to an aspect of the present invention. The brazing method  300  includes the steps of providing (step  310 ) a first dynamoelectric machine part and a second dynamoelectric machine part, where at least a first portion of the first dynamoelectric machine part is configured to fit inside a second portion of the second dynamoelectric machine part, precleaning (step  320 ) the first portion of the first part and the second portion of the second part, and preplacing a self-fluxing braze alloy (step  330 ) on one or more of the first portion and the second portion, thermally treating (step  340 ) at least one of the first portion and the second portion, to create a temperature differential (and size differential) between the first portion and second portion. Step  340  may include one or more of cooling the first portion and/or pre-heating the second portion. For example, the first portion could be cooled by immersing the first portion in a nitrogen bath. As another example, the second portion could be heated by induction heating. The method also includes the steps of inserting (step  350 ) the first portion into the second portion, and heating (step  360 ) at least one of the first portion and the second portion to melt the braze alloy. Method  300  joins the first portion to the second portion by brazing without the need for a flux and/or reducing atmosphere. 
     Method  300  may also include a step of providing an inert gas purge inside the second portion or an inert atmosphere in the area surrounding the joint and/or any heated or pre-heated elements. The braze alloy may be a BAg-18 alloy or a BAg-24 alloy. The thermal treatment and/or heating steps may be performed by induction heating, and one or both parts, or portions thereof may be heated to about 400° F. to about 500° F. The heating step  360  may also be performed by induction heating, and both parts should be heated to about 1,300° F. to about 1,500° F. As a further advantage provided by the present invention, a fluxing step and/or a fluxing step before the precleaning step may be avoided. Further, a cleaning step after the heating step may also be avoided so that one does not have to perform a cleaning step after the heating step. However, a cleaning step could be performed if desired. 
     Method  300  may also include the steps of providing the first part  110  made of copper and the second part  120  made of copper. Copper is to be understood as any predominantly copper alloy including but not limited to tough-pitch copper, oxygen-free copper, and silver-bearing copper. A thermal expansion or contraction during the thermal treatment step of the first portion or second portion provides a compression fitting (or fit-up) between the first portion  112  and the second portion  122  upon the first portion  112  and second portion  122  reaching a temperature equilibrium. 
     As stated previously, an important feature of the present invention is that copper can be brazed using BAg alloys without requiring the use of a flux, a vacuum, an inert gas or reducing atmosphere. In the past, conventional wisdom always relied on a flux or reducing atmosphere when brazing with BAg alloys, as satisfactory brazed joints were not obtained without the use of a flux or reducing atmosphere. In contrast, the method herein described has resulted in substantially improved results that were unexpected, because satisfactory brazed joints can be obtained when brazing BAg alloys and copper in air, without the use of a flux and/or reducing atmosphere. The new method will greatly reduce the cost of brazing, the time required to braze and increase the locations where brazing may be performed. As one example only, brazing can now be completed at a customer site or point of use, rather than only in a specialized factory. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.