US3667107A

US3667107A - Filler for roll-weld structures

Info

Publication number: US3667107A
Application number: US764064A
Authority: US
Inventors: Raymond H Anderson Jr; Richard A Rawe; Bennett V Whiteson
Original assignee: McDonnell Douglas Corp
Current assignee: McDonnell Douglas Corp
Priority date: 1968-10-01
Filing date: 1968-10-01
Publication date: 1972-06-06
Anticipated expiration: 1989-06-06

Abstract

A roll-welding process for fabricating beryllium roll-welded panels where the filler material is an austenitic manganese or Hadfield steel which can be chemically or mechanically removed from the structure that has been diffusion bonded by the process.

Description

United States Patent Anderson, Jr. et al.

[ 1 June 6,1972

[54] FILLER FOR ROLL-WELD STRUCTURES [72] Inventors: Raymond B. Anderson, Jr., Santa Ana; Richard A. Rawe; Bennett V. Whiteson, both of Granada Hills, all of Calif.

[73] Assignee: McDonnell Douglas Corporation [22] Filed: Oct. 1, 1968 [2I Appl. No.: 764,064

52 U.S.CI ..29/423,29 472.1,29 4723, 29 493 51 1nt.Cl ..B23p 17/00 58 FieldofSearch ..29/423,471.1,472.1,472.3, 29/493 [56] References Cited UNITED STATES PATENTS 3,044,160 7/1962 Jafi'ee ..29/423 UX 3,345,735 10/1967 Nicholls ..29/47l.l X 3,380,146 4/1968 Babel et a1. ..29/423 3,419,951 l/1969 Carlson ..29/423 X 3,427,706 2/1969 Jafiee ..29/47 1.1 3,453,717 7/1969 Pfafi'enberger et a1. ..29/423 Primary ExaminerJohn F. Campbell Assistant Examiner-Richard Bernard Lazarus Attorney-Walter .1. Jason, Donald L. Royer and Robert 0. Richardson [57] ABSTRACT A roll-welding process for fabricating beryllium roll-welded panels where the filler material is an austenitic manganese or Hadfield steel which can be chemically or mechanically removed from the structure that has been diffusion bonded by the process.

2 Claims, 3 Drawing Figures FILLER FOR ROLL-WELD STRUCTURES BACKGROUND OF THE INVENTION The roll-weld process is a method of combining difficult to weld materials such as beryllium or titanium into an autogeneous weld in the solid state by the application of heat and pressure during a hot rolling operation. The'parts to be welded are positioned in an abutting relationship within a surrounding frame of another material, also of a different composition, so as to provide for a pack having substantially no void spaces. Cover sheets are welded to the opposite sides of the frame and air evacuated from the pack. After the pack is subjected to a suitable temperature and pressure during rolling to unite the parts, the cover sheets, frame, and filler material are removed. This roll-weld process is more fully disclosed in U. S. Pat. No. 3 ,044, 160 entitled Method of Producing Ribbed Metal Sandwich Structures," by R. I. Jaffee, and issued July I7, 1962.

Problems in filler material selection have been encountered when metals such as titanium and beryllium have been used as parent metals in the roll-weld process. It was found that the use of mild steel filler materials containing up to 0. l 5 percent carbon resulted in contamination of the titanium surface. When the composition of the surface of steel filler is less than 0.15 percent carbon, such as a decarburized steel, a strong bond between the filler and titanium parent metal is developed during rolling, with resultant contamination of the parent metal surface. This led to the discovery that the amount of carbon at the surface of the steel directly affected the degree of ion contamination and subsequently the degree of bond between the filler and parent metal. It was also discovered that carbon contents greater than 0.40 percent at the tiller surface were necessary to obtain ease of mechanical removal of the steel filler from the titanium components. Disclosure of the carbon barrier concept of steel filler preparation is contained in U. S. Pat. No. 3,380,l46 forCarbon Barrier, issued Apr. 30, 1968 to Henry W. Babel, et al.

The fabrication of roll-weld panels from other low density materials such as beryllium will result in composite structures with exceptionally high strength-to-weight ratio. The inherent brittleness of beryllium resulting from its crystallographic structure places additional requirements on the selection of both filler and surrounding pack materials which are not necessary with the more ductile parent metals. In particular, filler materials must be selected to insure that levels of stress placed in the beryllium components during processing are not excessive, causing rupture of the bonds and cracking of the beryllium. Such problems could arise if the filler material undergoes an allotropic transformation during processing, resulting in expansion during cooling, does not possess a coefficient of thermal expansion compatible with beryllium, and does not possess deformation characteristics similar to those of beryllium. In addition, the filler material must not contaminate the parent metal surface during roll-welding. Failure of the filler material to possess these properties would result in unacceptable roll-weld panels and sever-l limits the application of the process to beryllium.

Beryllium and its alloys represent a relatively new structural material for use in aerospace industries. The material has long been used as reflectors or moderators in nuclear reactors but, more recently, serious consideration has been given to its structural applications. An important feature of beryllium is that it is approximately 35 percent lighter than aluminum and possesses a specific heat approximately two times that of aluminum. The materials elastic modulus is higher than that of any other structural material and persists at temperatures where other light metals are no longer operative. In combination, the properties of relatively high modulus and density characterize a material with unusual potential. The material also possesses useful mechanical properties to temperatures above 800 F as experienced by high performance space, aerial and certain other vehicles.

Fabrication, handling, and joining of beryllium products is difficult. Forming must be carried out at temperatures above l,200 F except for beryllium alloys containing extremely low interstital content. Joints in beryllium cannot be satisfactorily prepared by ordinary welding techniques. Grain growth further embrittles the joint. Adhesive bonding has been successfully used where the joints are not excessively loaded or do not operate at high temperatures. Mechanical joints have been considered to be most applicable to beryllium although they are difficult to fabricate, lack strength, and are inefi'icient compared with normal welded joints. Handling and processing of beryllium must be carried out in special facilities due to the toxicity of the BeO dust and vapors. On the other hand, joints in beryllium produced by roll-welding possess strengths comparable to the parent metal as well as resulting in a joint of higher efficiency.

A basic part of the roll-weld process is the use of filler material to support the parent metal components during rolling. A picture frame yoke and cover sheet arrangement is used to provide a hermetically sealed envelop around the panel. After rolling, the yokes, cover sheets, and filler material are either chemically or mechanically removed. In roll-welding of titanium panels, steel filler bars containing greater than 0.40 percent carbon are normally specified to minimize iron contamination of the titanium and to provide for mechanical removal of the filler bars. It was discovered, however, that the use of carbon steel filler material in the roll-welding of beryllium panels was unsatisfactory because of the brittleness of the parent metal and metallurgical changes which occur in the steel during processing. These allotropic transformations are obvious from a study of the iron-carbon phase diagram. Steel will transform upon heating from the body centered cubic crystal structure to the face centered cubic structure with an accompanying contraction in the lattice. On cooling to about 1,200 P, this face centered cubic structure undergoes the reverse transformation to the body centered cubic structure with a resultant expansion of the lattice structure. This expansion of the filler material after the panel has been rolled to final dimensions results in high stress levels in the beryllium components. Cracking of the beryllium is the end result. The

use of steel filler with other materials such as titanium containing a lower modulus of elasticity than beryllium does not produce cracking of the parent metal components.

SUMMARY OF THE INVENTION Elimination of allotropic transformations and their adverse effects on beryllium in the roll weld process can be accomplished by selection of a filler material which does not undergo such changes during processing temperatures, contains similar deformation characteristics as beryllium, will not contaminate the beryllium surfaces, and can be readily removed by conventional methods. The discovery was made that the use of austenitic manganese steel, sometimes referred to as Hadfields Steel, with beryllium in the roll weld process eliminates cracking of the beryllium components, results in good beryllium-beryllium bonds, and provides a filler material with deformation characteristics similar to beryllium and one which can be readily removed after processing. The austenitic manganese steel normally contains 1.2 percent carbon and 12 to 15 percent manganese as the essential elements. The material is similar to other austenitic steels in that it remains face centered cubic in structure through all processing temperatures encountered in beryllium roll-welding. The coefficient of thermal expansion is quite similar to that of austenitic stainless steel and compatible with that of beryllium. Its deformation characteristics are similar to those of beryllium at rollweld temperatures. No evidence of contamination of the beryllium surface has been discovered with the use of austenitic manganese steel as filler material.

Removal of the filler material from the beryllium roll-weld panel must be accomplished by chemical leaching because of the brittleness exhibited by beryllium. Mechanical removal is extremely hazardous with beryllium, resulting in high impact and vibrational loads. Under such conditions, cracking of the beryllium components during. removal would be inevitable. Chemical leaching of the austenitic manganese steel filler can be accomplished readily with the use of a solution of nitric acid in concentrations of 50 percent or greater. The beryllium panel including the filler material can be submerged in the solution and left until the filler material has been leached out. It was discovered that more diluteacid concentrations resulted in an attack on the beryllium during filler removal.

The principal advantage in the use of the present invention is the ability to fabricate a beryllium composite structure wherein removal of the filler material from the beryllium rollweld-structure is done without damage to the parent metal. Satisfactory removal will result in a beryllium structure in which the parent metal components have been strongly rollwelded together with no contamination or cracking of the parent metal components resulting from filler material interaction or incompatibility.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view showing the relationship of the various elements comprising a rollweld pack used in the practice of the present invention;

FIG. 2 is an enlarged sectional view of a beryllium panel before the filler material has been removed; and I FIG. 3 is a sectional view of an alternate form of beryllium panel, also without the filler material being removed.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS Referring now to the exploded view of the pack assembly for producing roll-weld truss core sandwich panels, as shown in FIG. I, there is shown a bottom cover plate 10, top cover 12, and yoke 14 which when assembled comprises the pack within which a composite structure may be placed for the rollwelding process. Yoke 14 has a thickness to separate the cover plates and 12 equal to the thickness of the composite structure placed within the central opening 18 therein. The composite structure in this illustrative embodiment consists of a beryllium top cover sheet 20, a beryllium bottom cover sheet 22, and beryllium ribs 24 of such length as to abut the inner surfaces of these cover sheets. In forming a truss core, these ribs are spaced apart and are inclined approximately 70. In order to fabricate a truss core sandwich panel by the roll-welding process, all of the void spaces within the yoke 14 must be filled. Accordingly, steel filler bars 26 are so shaped to fit between the ribs 24. The enlarged view in FIG. 2 shows this more clearly. After machining and prior to assembly, all components are chemically cleaned. To ensure proper fitting in the cavity, the beryllium ribs were chemically etched. A solution of 95cc nitric acid (70-71 percent reagent grade), 95cc water, and 23 grams of ammonium fluoride is a typical chemical milling solution for this purpose. The faying edges of the ribs and the inner surfaces of the

skin sheets

20, 22 were grit blasted and then dipped in a solution of 10 percent nitric acid 70-71 percent reagent grade), 90 percent water, rinsed in warm water, dipped in a 10 percent sulphuric acid (95.5 to 96.5 percent reagent grade) 90 percent water solution, again rinsed in warm water and finally rinsed twice in ethyl alcohol.

The steel components were generally degreased either in acetone or trichloroethylene, rinsed in alcohol and annealed in a vacuum for a half hour at l,600-l,700 F. A coating of high grade aluminum oxide applied to the stainless

steel cover plates

10 and 12 facilitates separating of the cover plates from the

skin sheets

20, 22 after the bonding process has been performed. After assembly, the cover plates l0, l2 and yoke 14 were welded in an argon-helium atmosphere. After welding, the pack was evacuated at room temperature to approximately 1 micron or less. The pack was then heated to approximately l,525 F and subjected to a rolling pressure where its thickness was reduced by about 10 percent for each pass. A series of passes from 4 to 8 is considered typical in reducing the thickness as desired. After the rolling operation has been completed, the pack was allowed to slow cool by placing it in a furnace operating at 1,200 l ,300 F and then shutting off the furnace. After the rolling and cooling, the weldment was cut and the stainless steel covers were easily removed. The balance of the yoke was carefully removed from the panel with a cutoff wheel. Removal of the steel filler bars 26 from the roll bonded structures is best done by leaching in nitric acid. The higher the acid concentration, the higher the rate of removal of the tiller bars. Also it was observed that a weaker solution in some instances does attack the beryllium. Accordingly, a chemical leaching solution of from 50 to 75 percent nitric acid is preferred.

While the beryllium panel shownin FIGS. 1 and 2 was a truss core sandwich panel, other configurations are possible and in some instances are preferable. The configuration shown in FIG. 3 consists of a bottom cover sheet 30 having vertical rib sections 32 with segmented reinforcing strips-34 providing a T-shaped reinforcement to the bottom cover sheet 30. Filler material 36 fills the spaces between the rib sections 32 and reinforcing segments 34 in the same manner as that shown in FIGS. 1 and 2.

We claim:

1. The method of roll-welding of beryllium parts comprising the steps of:

a. assembling the parts to be welded with sections abutting in a metal yoke,

b. filling the metal yoke with filler material of an austenitic manganese steel with a nominal composition of approximately 1.2 percent carbon and l2-l 3 percent manganese,

c. covering the parts and filler material with metal cover plates,

d. withdrawing the air therefrom,

e. heating and rolling the metal yoke and its contents until an autogenous weld is made between the abutting surfaces of the beryllium components, and

f. removing the filler material, yoke and cover plates from the beryllium parts.

2. In the method set forth in claim 1, the improvement of:

employing a chemical leaching solution of 50-75 percent nitric acid to remove the austenitic manganese steel filler material from the beryllium components thereby eliminating the isolated attack on the beryllium components associated with the use of more dilute acid solutions.

Claims

2. In the method set forth in claim 1, the improvement of: employing a chemical leaching solution of 50-75 percent nitric acid to remove the austenitic manganese steel filler material from the beryllium components thereby eliminating the isolated attack on the beryllium components associated with the use of more dilute acid solutions.