|Title of Invention||
MANUFACTURING PROCESS TO REALIZE LIGHTWEIGHT INCONEL-718 PANELS FOR METALLIC THERMAL PROTECTION SYSTEM
|Abstract||ABSTRACT "A method of manufacturing lightweight, honeycomb metallic thermal protection panels." This invention relates to lightweight honeycomb metallic thermal panels, which are reusable, heat resistant and are useful in making aerospace vehicle parts. Structures made from such panels are capable of with standing temperature conditions at re-entry of space vehicles. These panels are made from honeycomb structures made from thin corrugated films of super alloys like NiCr alloy Titanium Aluminize and the like which are laser welded to form honey comb structures of the desired thickness. They are then sandwiched between two face plates, which are treated to withstand oxidation.|
A method of manufacturing lightweight, honeycomb metallic thermal protection
This invention relates to a method of manufacturing lightweight honeycomb metallic thermal protection panels which are particularly useful for thermal protection systems for re-entry of space vehicles.
Background and prior art:
This invention is the result of a continuous effort to improve the lifetime and reusability of thermal protection system for re-entry application of space vehicles. Several thermal protection systems are being used to protect re-entry vehicles from aerodynamic heating such as ablative based, foam based, and C-C based materials. Foam based materials are used in the temperature range 1000 °C to 1500 °C. But the mechanical properties of foams vary across the material thickness, making the design of thermal protection system for mechanical loads difficult. This can be improved using metallic based thermal protection system but the density of the panel is of concern. The design philosophy of honeycomb structures, which reduces the density of panels, can be used for metallic thermal protection system. But the conventional method of manufacturing lightweight honeycomb structures, using adhesives limits the use of metallic lightweight structures in the operating range of 150 - 200 °C, but re-entry application requires lightweight structures to withstand temperatures beyond 1000 °C. Such problems can be addressed by adopting alternate joining techniques like welding. The honeycomb panels are made with thinner material, thereby making it difficult to use the conventional joining techniques like TIG, MIG, resistance spot welding, etc. These techniques lack in control of penetration and the heat affected
zones lead to disjoint, cracks etc. Welding technique suitable for thin foil joining which are simple in automation can be adopted for high temperature resistant alloys like Inconel-718, titanium aluminize, and other super alloys so that panels made there from can be used for high temperature protection systems. However, there is a greater manufacturing challenge in making such panels.
An innovative manufacturing technique combining corrugation making, laser welding, diffusion brazing, and functionally graded coating has been developed to process Inconel-718 honeycomb panels for thermal protection systems. The present manufacturing method overcomes the existing problems in producing lightweight panels for aerospace re-entry.
To overcome the manufacturing difficulties experienced in producing metallic thermal protection systems, alternate metal joining techniques are developed and applied in two different ways to obtain honeycomb core in the following patents:
US5609288A1: This patent is for making honeycomb core of stainless steel or Titanium alloys or Aluminum alloys. It follows a process where sheets are placed one over the other and laser welded and later stretched to obtain honeycomb structure. The drawback of this process is that as thickness of sheets is lowered to 100 microns or less, the limiting of depth of weld penetration by laser welding to top two sheets becomes difficult because of variation in laser power during welding. And also maintaining intimate contact between sheets will become difficult for sheets of thickness less than 100 microns resulting in lack of joining and hole making. Even light variation leads to joining of more than two sheets that results in more rejections. Such process is adoptable only for sheets of thickness more than 100 micron. Another drawback of this process is that when two sheets are lap-welded, light weld bead protrusions are expected. These protrusions will obstruct smooth contact with next
sheet when it is placed for welding. This will have major impact in case of thin foils where little air gap will result in disjoints.
W003024659A1: This patent follows a method of corrugation and holding corrugated sheets matching troughs and successive laser welding. The process is repeated to achieve honeycomb structure. The success of this process depends on the kind of fixture used for the desired thickness of sheet. Fixture should be such that it brings corrugated sheets in intimate contact. The process is demonstrated for 0.5mm aluminum sheet by making honeycomb core of 60x60x13mm with density of 0.57 g/cc and in magnesium with density of 0.6g/cc. The Sheets were corrugated and held using simple fixtures and then welded. Further addition of corrugations for welding, the used fixture cannot apply local load, as it is very much essential for welding of foils less than 0.5mm. This requires special fixtures wherein part of fixture enters into hexagon cell just below the top corrugation and when loaded it makes local contact between them.
It can be noticed that the patents of the prior art are for making only honeycomb core and not the panels and are restricted to higher sheet thickness (above 0.5mm) and are also not specific to metallic thermal protection system. But, the present invention goes still farther by evolving a method of realizing honeycomb structures with foils of thickness 80|jm, which is much less than 0.5mm. This structure has tremendous advantage over the existing honeycomb structures particularly in heat and load bearing potentials. This leads to reusability and enhanced efficiency of the protection panels produced.
Objects of the invention:
The primary object of the invention is to develop reusable thermal protection systems with long life and reusability.
Another object of the invention is to develop honeycomb panels with very thin foils of in the range of 50 to 100|im to achieve high strength to weight ratio for metallic thermal protection.
Summary of the invention:
The invention relates to the development of a process for manufacturing lightweight panels towards metallic thermal protection systems of reusable space vehicles with reusability up to 50. The manufacturing process is developed using corrugation, laser welding and diffusion brazing of faceplates. The process involves corrugating thin foil of 80 [im, and assembling two corrugations such that troughs match using a specially designed fixture to get intimate contact at all troughs and successive laser welding at all contact places. Diffusion brazing is done by holding mechanically lapped and chemically cleaned honeycomb core, nicrobraze foil and face sheets together at a temperature 1050 °C for three hour under vacuum with the application of load ranging between 10 to 20 MPa. Further, metallic honeycomb sandwich panel is coated with functionally graded material of yttrium stabilized zirconia & NiCrAlY alloy to protect it from corrosion & oxidation at temperature higher than 900 °C. Finally, coated panel is tested for re-entry heatflux using radiative heating.
Corrugated sheets are made from heat resistant NiCr alloy that is commercially known as Inconel-718. This alloy is a precipitation hardenable alloy of Ni & Cr, which contains significant amount of iron, Niobium, Molybdenum in addition to lesser amounts of other metals. Other super alloys like Titanium Aluminide and Ni Aluminide may also be used in producing the panels.
This invention relates to a method of manufacturing lightweight honeycombs metallic thermal protection panels which comprises the steps of forming a honey comb core structure by corrugating thin foils of high temperature resistant Ni Cr alloy,
Titanium Aluminide, nickel aluminate and the like being made by welding two such corrugates such that the troughs of corrugates matches to create contact between the under surface of the top corrugate and the top surface of the lower corrugate, placing and welding a third corrugate over said welded structure and repeating the steps to obtain a honeycomb of desired thickness and thereafter sandwiching said honeycomb structure between two face plates.
The invention is described herein after with NiCr alloy known as Inconel-718 but is not exclusive to that. The foils may be laser welder and the face plates may be diffusion bonded and then coated with functionally graded coatings of yttrium stabilized Zirconia and NiCrAlY alloy. The honeycomb structure may be made from foils of Inconel-718 alloy in the range of 50 to 80|j,m though foils of lesser thickness may also be used.
Detailed Description of the invention:
The fabrication of typical honeycomb core structure begins with the formation of corrugating Inconel-718 thin foil of 80|im using press tools and then assembling of two corrugated sheets on laser welding fixture such that the troughs of corrugations matches. This creates local intimate contacts between undersurfaces of top corrugation and top surfaces of lower corrugation and then programming CNC to join all the contact surfaces using laser energy. Laser welding parameters in the following range were used for 0.08mm thick Inconel-718 foils: Laser type: pulsed Nd-YAG, Power: 10 to 15W, Pulse energy: 2 to 3J, Welding speed: Im to 2m/min, DOF: -1 to -5 mm. The welded structure of two corrugations is taken and placed such that portion of fixture will enter inside the cell. The next corrugated sheet is placed over previous welded structures such that undersurface of top corrugation and top surface of lower welded sheets match their troughs and laser welded as earlier. The process is repeated to obtain required size of the honeycomb core. In this invention for welding of
Inconel-718 foil of Sodium thickness, laser-welding parameters are optimized to obtain consistent depth of penetration. Then all the faces of welded structure were trimmed, and sliced to desired thickness using wire-Electro Discharge Machining process. Before joining face sheets with core using diffusion brazing, all the elements were mechanically lapped and cleaned using acetone. Diffusion brazing of face sheets with core was carried out under vacuum with the use of nicrobraze foil and the application of load ranging between 10 to 20 MPa at a temperature 1050°C for three hours. The process is demonstrated for 80|jm thick foils of Inconel-718 by realizing 220x220x13mm honeycomb core with density of 0.4 g/cc. Also this process can be adopted for Solid or less thick foils to get advantage of weight reduction. In order to protect the metallic honeycomb sandwich panel from oxidation and corrosion at temperature more than 900 °C, one face of panel is coated with functionally graded material having nine layers with composition variation of 10% of yttria stabilized zirconia (YSZ) & NiCrAlY alloy using plasma spray process under atmospheric condition. Finally, functionally graded coating panel is tested for re-entry heatflux with peak power density of lOW/cm , 20 W/cm and 30 W/cm^ under radiative heating.
This invention includes panels manufactured by the method disclosed herein before and structural articles made from such panels.
Obvious alterations and modifications knowing to persons skilled in the art are not excluded from the scope and ambit of the appended claims.
1. A method of manufacturing lightweight honeycomb meteoric thermal protection panels comprising the steps of forming a honeycomb core structure by corrugating thin foils of high temperature resistant NiCr alloy Titanium Aluminide, Nickel Aluminide and the like super alloys, said honeycomb structure being made by welding two such corrugates in such a way that the troughs of corrugates match to create contact between the under surface of this top corrugate and the top surface of the lower corrugate placing and welding a third corrugate over said welded structure and repeating the steps to obtain a honeycomb of desired thickness and thereafter sandwiching said honeycomb structure between too face plates.
2. The method as claimed in claim 1 wherein said thin foils are of 50 to 80 thickness.
3. The method as claimed in claims 1 or 2, wherein said face pates are provided with at least one layer of coating of yttria stabilized zirconia and NiCrAlY alloy.
4. The method as claimed in claims 1 to 3 wherein said honey comb structure is sandwiched between face plates by diffusion bonding and the honey comb structure is produced by laser welding the corrugated foils.
5. The method as claimed in claim 4, wherein said diffusion bonding is carried out by brazing the face plates with the honeycomb under vacuum and under the application of load ranging from 10 to 20MPa at a temperature range of atleast 1050°C for about 3 hours.
6. A lightweight honeycomb metallic thermal protection panel manufactured by the method as claimed in any of the preceding claims.
7. Structured articles made from lightweight honeycomb metallic thermal panels as claimed in claim 6.
|Indian Patent Application Number||2983/CHE/2007|
|PG Journal Number||30/2012|
|Date of Filing||13-Dec-2007|
|Name of Patentee||INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ISRO HEADQUARTERS, DEPARTMENT OF SPACE, ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE 560094|
|PCT International Classification Number||E04C2,B31D3|
|PCT International Application Number||N/A|
|PCT International Filing date|