GB2061791A

GB2061791A - Making a noise attenuation panel

Info

Publication number: GB2061791A
Application number: GB8027290A
Authority: GB
Original assignee: Rohr Industries Inc
Current assignee: Rohr Inc
Priority date: 1979-10-29
Filing date: 1980-08-21
Publication date: 1981-05-20
Also published as: DE3035722C2; FR2477965A1; GB2061791B; DE3035722A1; FR2477965B1

Abstract

Method of manufacturing broad band noise attenuation structure and the structure resulting therefrom. The panel comprises a central cellular core (10) between an imperforate sheet (16) and perforate facing sheet (14) which is overlaid with a stainless steel wire fabric layer (20). The components are either liquid interface diffusion, or braze bonded together depending on their materials of construction. The resulting structure is usable at temperatures in the range of 1600 DEG F. <IMAGE>

Description

SPECIFICATION Method of manufacture of honeycomb noise attenuation structure for high temperature application This invention relates to a process for producing improved high temperature noise attenuation panels and more particularly to sandwich attenuation panels utilized in a severe environment including elevated temperatures as high as 1 600 F. An example of its use is sound attenuation within the exhaust area of a modern jet aircraft engine.

In the design and manufacturing of sound attenuation sandwich panels which additionally provide structural integrity in their severe environment, it has been common practice to provide attenuation sandwich panels wherein honeycomb core material is sandwiched between a perforated and an imperforate sheet of thin facing material. Panels of this type of construction, although satisfactory for attenuating some specific sound frequencies, are found to be inadequate over a broad range of frequencies customarily encountered within and around modern jet engines. It has also been found that the perforations, when placed adjacent to high speed gas and airflow areas within aircraft engines, creates some turbulence to those high speed flows, reducing engine efficiency.It has been further discovered that these panels are generally constructed by adhesive bonding which limits their use to low temperature areas to prevent degradation or failure when in use. Although a co-pending application which is assigned to the assignee of the instant invention reduces and substantially eliminates the turbulence and the narrow frequency range problems for attenuation sandwich in and around modern aircraft jet engines, this new concept attenuation sandwich panel cannot be used in elevated temperatures due to its adhesive bonded construction.

There has not been an entirely satisfactory attenuation material with structural integrity, capability with standing severe high temperature environment conditions such as those encountered in exhaust areas of modern jet aircraft engines until the emergence of the instant invention.

It is the primary object of this invention to provide a manufacturing process and the resulting structure therefrom which provides an improved attenuation sandwich material that can be utilized in a high temperature environment.

It is the further object of this invention to provide an attenuation sandwich structure that has the strength required to be utilized as aircraft supporting structure.

A still further object of this invention is to provide an attenuation sandwich material employing the Helmholtz resonant cavity sound attenuation principle.

A still further object of this invention is to provide a manufacturing method that provides a predetermined flow resistance between the outer surface adjacent the sound to be attenuated and the resonant cells of the core.

The invention comprises a method for the production of high temperature noise attenuation panels in which the components comprise a central honeycomb cellulose core between an imperrforate sheet and a perforated sheet with the outer surface of the perforate sheet having a wire cloth overlay comprising plating the outer surface of the imperforate sheet and prepared surface of the perforate plate with layers or silver, copper and nickel; or wetting the surface of the perforated sheet and core and applying a powdered braze alloy thereto; applying a predetermined pressure to the panel, placing the panel in a vacuum furnace, evacuating the furnace to meet the coating metals to cause them to flow to bond the components without closing the perforations, and allowing the panel to cool to ambient temperature.

The invention will be described with reference to the accompanying drawings:~ Figure 1 is the perspective partially cutaway view of a completed attenuation sandwich panel constructed by process of the instant invention.

Figure 2 is an enlarged fragmentary vertical section of the attenuation sandwich panel shown in Fig. 1.

Referring now to the Figures in detail. The process of this invention provides an attenuation sandwich panel 8. The constituent elements of the sandwich panel including a honeycomb core 10, having a plurality of endwise direction cells 12, outer facing sheets 14 and 16 and wire cloth material 20, such as a Dutch Twill woven stainless steel material. In one preferred process, the materials for the core and facing sheets are titanium alloy, 321 Stainless Steel, PH hardening stainless steel and the like. Other materials having stable temperature characteristics in the range of 1 600' F. that can be Liquid Interface Diffusion (LID) bonded or brazed may be used equally as well to practice this invention. The facing sheet 14 is perforated with a plurality of small perforations 18, their size, for example, could range from 0.05 to 0.25 inches.

The perforations 18 provide a range of from 12% to 35% open area to the facing sheet 14. The perforations 18 may be punched, drilled, or chem milled through the sheet 14.

Chem milling is preferred as the cross-sectional area of the perforation can be predetermined and both surfaces of the sheet 14 remain smooth and do not require deburring, grinding, filing, etc., prior to use. The perforations may be spaced at 0.25 inch intervals, and for example, in staggered rows. Various other spacing intervals and patterns may be used to successfully practice the invention.

Sheet 16 is imperforate and forms one closed surface of each cell Helmholtz resonator cavity. The wire cloth material is preferably constructed of inconel, nickel, monel, stainless steel alloys or other materials having like characteristics.

The first step of the process is to cut to size the materials to be joined having in mind the desired size requirement of the desired attenuation panel or for economics of manufacture.

Various methods well-known in the metal working art may be employed to size the components, such as, but not limited to, shears, friction saws, scissors or the like.

The cut to size components are then cleaned so as to be free of any surfactants that would affect their bonding. Any suitable pre-plating cleaning method known in the art may be utilized.

After cleaning, rinse in a deionized water, then dip in a mild acid and a final rinse in deionized water and drying.

In the first method of manufacture using the LID process, after cleaning, the imperforate sheet 10 is masked on one side with any suitable maskant, such as, but not limited to, microshield. The opposite side of the imperforate face sheet 10 is dust blasted by any suitable means known in the art to prepare the surface for plating. Imperforate sheet 10 is then plated first with a layer of silver, (Ag), copper (Cu), and nickel (Ni) in that order resulting in an increase of weight in the range of from eight to ten grams per square foot evenly distributed on one surface of the imperforate sheet 10. The various plating layers are substantially equal in weight per square foot (2-2/3 to 3-1/3 grams per square foot each). The plating is preferably by electroplating which is weil-lenown in the metal-plating art.

The perforate sheet 14 is dust blasted on both surfaces and electroplated in the same manner as the imperforate sheet 10. Each side of the perforated sheet is sequentially plated from the surface out with Ag, Cu, Ni in that order with the coating weight being in the range of 2-2/3 to 3-1/3 grams per square foot resulting in an increase of weight of the perforate sheet 14 from 16 to 20 grams per square foot.

The components are now ready to be bonded. The components are positioned as shown in the figures. The honeycomb core 12 is positioned between the imperforate sheet 10, plated side against the core, and the perforated sheet 14. The woven material 20 is placed on the outside of the perforate sheet 14. The stacked components are secured in place by means of holding straps 22 which are made from stainless steel foil or the like.

The straps 22 are tack welded to each of the four vertical edges of the component. Any other convenient method may be used to hold the various components in position for bonding.

The stacked and secured components are then placed on a reference surface which may be constructed of graphite, ceramic, alloyed steel or the like. Pressure is then applied from the outer free surface of the components towards the reference surface. A weight may be employed to provide uniform pressure from the outer surface of the woven material 20 toward the reference or delta alpha tooling may be utilized. Delta alpha tooling is tooling constructed from material having a smaller coefficient of expansion than the stacked components when heated. This type of tooling is well-known in the bonding art and, therefore, needs no detailed explanation.

The assembly including the stacked and secured components, the reference surface and the pressure applying means is then placed in a vacuum furnace. The furnace then evacuated to approximately 1 X 10-5 torr.

The temperature within the furnace is then elevated to a temperature of approximately 1 725' F and held at this temperature. When the temperature within the furnace reached approximately 1 635' F the plated materials will form an eutectic melt wherein this melt will close the spacings between the honeycomb core and the face sheets and may also form a bond between the wire cloth modes and the perforated face sheet. The temperature within the furnace is maintained at approximately 1 725' F for from 10 to 90 minutes to allow solid state diffusion of the component after the melt condition ceases.This solid state diffusion bonding creates grain growth between the components resulting in a strong unitized structure with the perforations through the perforated face sheet and the wire cloth material remaining open. After maintaining the stacked components at the elevated temperature for the required time, the furnace is then allowed to cool to ambient temperature and the now LID bonded structure is ready to be utilized.

In the second method of manufacturing using the braze process, after sizing and cleaning a nickel (Ni) base braze alloy is applied to one surface of the perforated sheet 14. The amount of braze alloy applied is determined by the size of the mesh of the wire cloth 20. The smaller the mesh size, the less braze alloy used. The principle criteria is to keep the amount of braze alloy applied below that amount which would result in blockage of the mesh in the finished structure.

The amount of brazed alloy applied varies from about 6 to 10 grams per square foot evenly distributed, the amount depending on the mesh size, for example, mesh opening size of 23 Rayl, would require an amount of braze alloy in the range from 6 to 7 grams per square foot.

The preferred method of brazed alloy appli cation is by spray wetting the skin surface with an acrylic binder, such as nicrobraz 600 cement manufactured by Wall Calmonoy, or the like, then while the binder is still wet applying the braze alloy in powder form evenly over the binder. The powdered braze alloy used may have a mesh from - 140 to + 270 and approximately 8 grams per square foot is used. The same process is then repeated to coat the inside walls of the core with approximately 120 grams per square foot.

The brazed alloy is sprayed on the binder coating of the core inside walls evenly by spray apparatus well-known in this art to insure uniformity. An example of one such spray apparatus is fully described in the United States Patent No. 3,656,224.

The various components are now ready to be bonded. The components are positioned or stacked as shown in the various figs., the honeycomb core 12 is positioned between the imperforate sheet 16 and the clean surface of the perforate sheet 14. The woven material is placed on the brazed alloy deposited surface of the perforated sheet 14. The stacked components are then secured in place by means of holding straps 22 which may be made from stainless steel foil or the like. These straps 22 are tack welded to each of the four vertical edges (two shown in Fig. 1) of the stacked components.

These stacked and secured together components are then placed upon a reference surface constructed of graphite, ceramic, alloy steel, or the like. A positive pressure is then applied from the outer free surface of the stacked components or toward the reference surface with the reference surface in a fixed position. A dead weight may be utilized that provides uniform pressure or Delta Alpha tooling as hereinbefore discussed.

The assembly including the reference surface is then placed in a vacuum furnace. The furnace is then evacuated to approximately 1 X 10-5 torr. The interior temperature of the furnace is then elevated to the melting point of the braze alloy. When melt occurs, the now liquidous braze alloy flows along the walls of the honeycomb core, face sheet and porous fibrous material by capillary action, leaving the perforations 18 and the pores of the porous fibrous material 14 open.

The assembly is then cooled and the braze alloy then solidifies, forming a unitized structure that can now be used for sound attenuation in an elevated temperature environment.

Although throughout the various Figs., flat components are shown for ease of explanation and disclosure, curved sandwich panels could also be formed and bonded together by the method of this invention. The surved panel components are formed prior to the cleaning step with the desired curvature required of the finished bonded sandwich structure.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it should be understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended

Claims

claims. CLAIMS

1. A method for the production of high temperature noise attenuation panels in which the components comprise a central honeycomb cellulose core between an imperforate sheet and a perforated sheet with the outer surface of the perforate sheet having a wire cloth overlay comprising plating the outer surface of the imperforate sheet and prepared surface of the perforate plate with layers or silver, copper and nickel, or wetting the surface of the perforated sheet and core and applying a powdered braze alloy thereto to provide interface diffusion bonding; applying a predetermined pressure to the panel, placing the panel in a vacuum furnace, evacuating the furnace to melt the coating metals to cause them to flow to bond the components without closing the perforations, and allowing the panel to cool to ambient temperature.

1. A method for the production of high temperature noise attenuation panels in which the components comprise a central honeycomb cellulose core between an imperforate sheet and a perforated sheet with the outer surface of the perforate sheet having a wire cloth overlay comprising plating the outer surface of the imperforate sheet and prepared surface of the perforate plate with layers or silver, copper and nickel, or wetting the surface of the perforated sheet and core and applying a powdered braze alloy thereto; applying a predetermined pressure to the panel, placing the panel in a vacuum furnace, evacuating the furnace to meet the coating metals to cause them to flow to bond the components without closing the perforations, and allowing the panel to cool to ambient temperature.

2. Method for manufacturing honeycomb noise attenuation structure for high temperature application, said structure comprising a central honeycomb cellular core sandwiched between an imperforate sheet and a perforated sheet with the outer surface of the perforate sheet having a wire cloth overlay, as in claim 1 comprising the steps of:: (a) sizing the structural components; (b) cleaning the structural components; (c) masking one surface of the imperforate sheet; (d) preparing the surface opposite the masked surface of the imperforated sheet and the central core and wire cloth surfaces of the perforate sheet for metal plating; (e) plating the prepared surface of the im- perforate sheet and prepared surfaces of the perforate sheet sequentially with layers silver, copper and nickel; (f) stacking the structural components in assembly order; (g) securing the stacked structural components together with securing means; (h) positioning the stacked and secured together structural components on a reference surface; (i) applying a positive pressure between the stacked components;; (j) placing the secured together structural components and reference surface in a vacuum furnace; (k) evacuating the furnace; (I) elevating the evacuated furnace temperature to cause a eutectic melt of the plating materials and solid state diffusion bonding of the adjacent stacked structural components; and (m) returning the stacked and now bonded together structural components to ambient temperature.

3. The method of manufacturing as set for in claim 1, comprising an additional step of forming the surface contour of the structural components prior to step (b).

4. The method of manufacturing as set forth in claim 2 wherein the securing together of the stacked structural components comprises attaching straps between the stacked structural components and removing said straps after step (m).

5. Noise attenuation structure for use in environments having temperature ranges from 0 to 1 600' F manufactured according to claim 1 or claim

2.

6. Noise attenuation structures for use in environment having temperature ranges from 0 to 1 600' F manufactured according to claim

3.

7. Method for manufacturing honeycomb noise attenuation structure for high temperature application, said structure comprising a central honeycomb cellular core sandwiched between an imperforate and a perforate sheet with the outer surface of the perforate sheet having a wire cloth overlay, as in claim 1 said method comprising the steps of:: (a) sizing the structural components; (b) cleaning the structural components; (c) wetting one surface of the perforated sheet and central core cell walls with a binder; (d) applying a selected amount of a powered braze alloy evenly over the binder coated surfaces; (e) stacking the structural components in assembly order; (f) securing together the stacked components; (g) positioning the secured stacked components on a reference surface; (h) applying a positive pressure between the stacked structural components; (i) placing the secured together structural components and the reference surface in a vacuum furnace; (j) evacuating the vacuum furnace; (k) elevating the internal temperature of the evacuated furnace to a level causing the braze alloy to melt and flow; and (I) returning the stacked components to ambient temperature.

8. The method of manufacturing as set for in claim 2, comprising an additional step of forming the surface contour of the structural components prior to step (b).

9. The method of manufacturing as set for in claim 2, wherein the securing together of the stacked structural components comprises attaching straps between the stacked components and removing said straps after step (1).

10. Noise attenuation structure for use in environments having temperature ranges from 0 to 1 600' F manufactured by the method of claim 1.

11. Noise attenuation structure for use in environments having temperature ranges from 0 to 1 600' F manufactured by the method of claims 2 or 3.

12. A noise attenuation panel when produced by the method of Claim 1 substantially as described with reference to the accompanying drawings.

CLAIMS (12Feb1981)