Patent Application
20080054048
Referring to
The turbine module also includes a set of circumferentially distributed outer airseals 18 each having hooks 20 for mounting the airseal on a case, not shown. The airseal is made of a temperature tolerant substrate alloy. Suitable alloys include nickel and cobalt base alloys and superalloys. An example of one specific suitable alloy is a nickel base superalloy having the composition disclosed in U.S. Pat. No. 4,719,080. This and other alloys typically used to make outer airseals are nontransparent to microwave radiation. The airseal also includes a coating 22 applied to the substrate to impart additional thermal tolerance and/or to protect the substrate from oxidation and corrosion and abrasion. A circular opening 26 with a shoulder 28 penetrates selected outer airseals. When the turbine module is assembled, the tips 30 of the blades are spaced from the outer airseal by a clearance gap G, shown highly exaggerated in
A subassembly 32 comprising a window frame 34 and a cylindrical window 36 occupies the opening 26 in the outer airseal. The window frame is made of the nickel base alloy described above and has a circular hole 40 for receiving the window. The window is made of a microwave transparent material such as a ceramic. Ceramics having suitable microwave transparency include alumina, quartz, silicon nitride, zirconia toughened alumina and sapphire. Other suitable microwave transparent materials include materials, whether ceramic or not, having a dielectric constant of no more than about 11.5, although materials having a higher dielectric constant may be satisfactory if the electronics package and algorithms in the clearance sensing system is configured to compensate for the diminished microwave transparency associated with the higher dielectric constant. One suitable nonceramic material having acceptable microwave transparency is diamond. The window frame 34 is brazed to the outer airseal 18. The ceramic window 36 is brazed to the host nickel base window frame 34 as described below.
The turbine module also includes a waveguide 42 having a circular external cross section. The waveguide 42 is a component of a microwave clearance sensing system that uses microwave radiation and processing circuitry to determine the clearance gap G. When the turbine is assembled, the window 36 is axially and circumferentially aligned with the waveguide so that the clearance sensing system can detect the proximity of the blade tips 30.
Referring additionally to
A braze alloy 48, which may be in the form of a paste, tape, powder or other suitable form is introduced between the coated, plated window and the window frame. One suitable braze alloy is American Welding Society AWS BNi-9 having a composition by weight of about 15% chromium, 3.6% boron, remainder nickel. A second suitable braze alloy has a composition described in U.S. Pat. No. 5,437,737, the contents of which are incorporated herein by reference, specifically the composition identified at “Table II Second Particulate Component” and whose composition by weight is reproduced below:
A third suitable braze alloy is a blend of a first constituent whose composition is that of the above described second suitable braze alloy and a second constituent having a composition described in U.S. Pat. No. 5,437,737, specifically the composition identified at “Table II First Particulate Component” and whose composition by weight is reproduced below:
Preferably, the first constituent makes up between about 40% and 95% of the third braze alloy and the second constituent makes up essentially the balance of the third braze alloy, i.e. between about 60% and 5% of the blend.
The subassembly 32 is then exposed to a thermal cycle to create a braze joint 49 between the TiW coated, nickel plated ceramic window and the nickel alloy window frame. The thermal cycle is carried out in a vacuum of 0.0005 Torr or lower with a maximum leak rate of 15 microns per hour. A representative thermal cycle is described below, however it will be appreciated that the optimum cycle parameters depend on factors that include the composition of the braze alloy:
As a result of the thermal cycle, the braze joint 49 forms to join the window to the window frame. As a result, the individual layers 44, 46, 48 as well as an optional nickel rich zone 54 (discussed below) are no longer clearly differentiated as they are in
As already noted, the optimum thermal cycle depends on factors including the composition of the braze alloy. The above described cycle, which includes a brazing temperature of about 2100 deg. F., a diffusion temperature of about 1975 deg. F. and a diffusion time of about 4 hours is believed to be satisfactory when using the third suitable braze alloy with the first constituent making up about 75% to 95% of the blend and the second constituent making up the remainder of the blend. However if the amount of the first constituent is decreased toward 40% of the blend and the amount of the second constituent is increased toward 60% of the blend, a brazing temperature of about 2200 deg. F., a diffusion temperature of about 2100 to 2200 deg. F. and a diffusion time of about 5-10 hours may be more satisfactory. In general, the method described herein is believed to be satisfactory for high temperature brazing in which the brazing temperature, which is the peak temperature of the cycle, is at least about 2050 deg. F. The resulting braze joint is therefore suitable for service in high temperature environments such as the turbine module of a gas turbine engine.
The subassembly 32, which comprises a ceramic component (the microwave transparent window) joined to a nonceramic component (the nickel alloy window frame) is then installed in the opening 26 in the nonceramic (nickel alloy) outer airseal 28 and joined thereto by any suitable method, for example by brazing as indicated at 50.
The above described method may also include an optional step of preparing the nonceramic component (the nickel alloy window frame 34) prior to brazing to render the nonceramic component more compatible for being brazed to the TiW coated, metal plated (nickel plated) ceramic component. The preparatory step is preferably selected from a group of processes consisting of nickel plating and fluoride ion cleaning. The preparatory step is applied to the perimeter of the hole 40 in the window frame and results in a nickel rich zone 54 with a scarcity of elements that could, if present in higher concentrations, promote undesirable oxide formation during brazing.
In the above described method, and as seen in
Although this disclosure refers to specific embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the subject matter set forth in the accompanying claims.
