Method of applying braze materials to a substrate

Details Method of applying braze materials to a substrate Method of applying braze materials to a substrate

2001-02-22

2003-06-10

Dunn, Tom (Department: 1725)

Metal fusion bonding

Process

Repairing, restoring, or renewing product for reuse

C228S260000, C427S142000, C427S456000, C029S889100

Reexamination Certificate

active

06575349

ABSTRACT:

This invention relates to a method of applying braze material to a substrate. More particularly, this invention relates to a repair deposit on a substrate.
As is known, various types of turbomachinery components, such as steam and gas turbine blades and vanes that are used in environments which are detrimental to the components. For example, the environments in which these components operate cause dimensional reduction of the wall thicknesses of the components by erosion, high temperature, oxidation and hot corrosion or a combination of these factors. In addition, cracks may occur from time-to-time due to thermal fatigue. Over time, these cracks may grow to significant length and width.
Because of the high cost of producing these turbomachinery components, expensive repair methods have been economically justified. Typically, high temperature brazing is used extensively for the repair of these components. For example, compressors may be subject to a high temperature brazing such as described in ASME 97-GT-372.
High temperature brazing materials based on nickel (Ni), cobalt (Co) or a ferrous material (Fe) together with melting point depressing elements, such as silicon (Si) and boron (B) have been used for some time for brazing honeycomb seals and repairing small cracks. For wide cracks and wall thickness restoration, wide gap brazes have been developed, for example as described in Turbomachinery, September 1986. These brazes consist partly of a braze material as described above combined with a super alloy metal powder that does not melt during a brazing run. This allows the brazing of a wide gap and the restoration of a wall thickness to be accomplished.
Wide gap brazes also improve the properties, such as ductility and oxidation resistance, of the high temperature brazing material as compared to a conventional braze product.
The newer repair technologies employ an increasing use of a base material by partly alloying a super alloy with a melting point depressing element, such as silicon or boron, for example, as described in ASME 00-GT-500.
Generally, before a high temperature brazing material may be applied to a substrate, the substrate must be carefully cleaned. Cleaning may be accomplished mechanically, by grit cleaning or by blending with a belt abrasive combined with various degreasing methods. The substrate may also be cleaned in a chemical bath at ambient and at elevated temperatures such as described in Turbomachinery, January 1996. Special cleaning processes have also been developed at high temperatures in vacuum, hydrogen and a fluoride/hydrogen environment.
In the past, high temperature brazing materials have been applied with a binder, in a so-called “slurry”. However, such a binder usually consists of elements, such as oxygen or carbon, which may adversely influence the quality of the brazing.
Plasticized powdered metal alloy tapes have also been used in order to make the application of the brazing material easier. However, the use of such tapes has not improved the quality of the brazing.
In other techniques, a pre-sintered alloy tape has been used in order to reduce the need for a binder and/or an adhesive and thereby produce a better braze. If applied with a resistance welding technique, a pre-sintered alloy needs no binder or adhesive. This allows an improvement in the quality of the final braze significantly but commercial preforms are only available in plate form and need to be cut by laser or high-pressure water. Also, for some applications, a three dimensional preform is required. This makes the application of this technique, if possible at all, extremely expensive.
Except for sintering, air plasma sprays and even vacuum plasma brazes have been used to make preforms. Although no binder or adhesive needs to be used with either method, air plasma introduces porosity and oxidation products into the repair deposit. Spraying high temperature brazing materials by vacuum and air plasma also create another problem called “element migration”. Migration is caused when certain elements, such as boron and carbon, are vaporized out of the mixture. Migration may be compensated by adding more of those elements in the initial high temperature brazing material. However, migration prediction is unreliable thereby making the content of the critical elements inconsistent.
Generally, a heat treatment step is used to complete a brazing operation. Normally, the heat treatment step is performed under vacuum or other protective atmosphere. During a typical braze heat treatment, the binder/adhesive which is used with the brazing materials is degraded and become gaseous. After this, the temperature of the component being repaired is stabilized just below the solidus temperature of the braze. The furnace in which the heat treatment is being performed is then quickly brought to the actual brazing temperature for a short time and then slowly lowered. A diffusion heat treatment may also be included after the braze heat treatment.
After heat treatment, inspection of the braze quality takes place and the process repeated if required.
As is known, while a coating is applied to a base material in order to improve the resistance of the base material to the environment, braze material is applied for the purpose of restoring the shape and material properties, including the mechanical properties of the base material. It is important to restore the resistance to stresses and/or restore critical dimensions lost by a reduction in wall thickness or other defects.
U.S. Pat. No. 6,136,453 describes a thermal barrier coating system for applying a bond coat to a substrate. As described, the bond coat may be disposed on a substrate employing thermal spray processes, such as vacuum plasma spray (VPS or LPPS), air plasma spray (APS) and hyper-velocity oxy-fuel (HVOF) spray processes. As further described, the structure and roughness of the bond coat surface are dependent on the spray process. Bond coats deposited by HVOF are sensitive to particle size distributions. Dense and oxide-free bond coats can be deposited by HVOF using very lean conditions and finer particles. Rough bond coats deposited by HVOF using coarser powders require a low temperature which results in unmelted powders. Therefore, the coating is porous and less dense.
U.S. Pat. No. 5,735,448 describes a method of repairing surface and near surface defects in super alloy articles such as gas turbine engine components. As stated therein, brazing techniques have been employed to repair defective areas. However, difficulties encountered with this technique and variations thereof include the inability to completely remove contamination in the cracks and inability to completely fill narrow cracks with braze material. In order to overcome the problem, the use of a specific repair coating is described.
U.S. Pat. No. 5,151,308 describes a high density spray coating. In particular, use is made of accessory apparatus attachable to the nozzle of a supersonic velocity thermal spray gun which uses oxy fuel (propylene) products of combustion. The use of an inert gas shield confined within a metal shroud attachment which extends coaxially from the outer end of a thermal spray gun nozzle is said to reduce total volume fractions of porosity and oxide from a normal range of 3% to 50% to a level of less than 2%.
U.S. Pat. No. 5,915,743 describes a metal spray tool repair system which employs a mask in a thermal spraying process.
Accordingly, it is an object of this invention to reduce the need for brazing material or melting point depressing elements and thereby improve the properties of a high temperature braze material significantly.
It is another object of the invention to eliminate the use of binders or adhesives in a high temperature brazing material.
It is another object of the invention to reduce oxidation products from a repair deposit of high temperature brazing material.
It is another object of the invention to eliminate carbon deposits from a repair made using high temperature brazing material.
It is another object of the invention to eliminate or r

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