Method for Diffusion Welding Under Isostatic Pressure

The invention relates to a method for permanently securing structural components to each other by a diffusion welding process. The invention is used, for example, for interconnecting components of an injection head of a rocket combustion chamber.

European Patent Publication (EU-OS) No. 0,155,490 discloses a method for interconnecting of work piece parts made of nickel based super alloys. In the known method, two parts of different nickel super alloys are connected to each other to form a work piece by filling a gap between the parts with a powder having a similar or the same composition as the parts. The powder forms a layer having a thickness of about 1 to 2 mm. The permanent connection of the parts to form a monolithic work piece, is accomplished by a hot pressing operation resulting in a diffusion joint. In such a diffusion joining process the diffusion is enhanced by the deformation caused by the high pressure in the connection zone. Such improvement of the diffusion is due to the intimate kneading of the material by the force components extending in the contact zone of the two parts to be joined. However, this known method is not usable for structural components which are not suitable for the pressure application even by means of pressure application devices or by means of press fits or shrink fits.

European Patent Publication No. 90,762 discloses ways to apply the required diffusion pressure even under difficult conditions by encasing the components to be joined. Under many circumstances such encasing is not possible.

In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination:

to provide a method for diffusion joining structural components even under difficult conditions, thereby avoiding the drawbacks of the prior art, especially where the structural components cannot transmit the pressure required for the diffusion;

to optimally interconnect complicated or difficult structural components or parts with each other by a diffusion welding using isostatic pressure;

to avoid complicated, trouble-prone jigs for the joining of such complicated parts or components; and

to provide a substantial saving compared to other prior art welding methods, for example, the electron beam welding.

According to the invention structural components are joined to each other by diffusion welding involving the combination of the following steps. A metallurgical layer is applied to part or all of at least one of the joint forming surfaces of the parts to be connected with each other. The metallurgical layer has a melting point below the temperature required for the hot isostatic pressing. This temperature must be below the temperature at which the metallurgical layer would melt onto the welding joint forming surface or surfaces. When the two components are engaged with each other a joining gap is formed in which the metallurgical layer is located for sealing the joining gap. The metallurgical layer may be provided in several different ways prior to the assembly of two parts or after the assembly of these parts. The welding is performed by subjecting the assembled parts with said layer therebetween to a hot isostatic pressing process for accomplishing a material diffusion of said metallurgical layer through the joint forming surfaces.

The invention is basically a two-phase operation. In the first phase the metallurgical layer is applied to at least part of at least one of the surfaces which will form the welding joint, or to both surfaces. In this first phase and when the parts are assembled the joining gap is closed and sealed by the metallurgical layer. The second phase involves the diffusion welding in a hot isostatic press.

In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 is a partial sectional view through an injection head of a rocket propulsion plant, whereby the injection head has one pair of welding joint forming surfaces; and

FIG. 2 is a partial cross-section through an injection head similiar to that of FIG. 1, however having two pairs of joint forming surfaces resulting from the insertion of a ring into a ring groove.

The nature and shape of the two embodiments of an injection head of a rocket propulsion plant shown in FIGS. 1 and 2 do not permit a pressure transmission through the parts, for example, by way of a shrinking stress or with the aid of respective jigs permitting the pressure application to the joint or joining surfaces. However, according to the invention even work pieces of complicated shape, as shown in the example embodiments, may be joined permanently and even at substantial savings. Comparing the invention to electron beam welding, the savings are about 50%. Further, trouble-prone jig structures are avoided altogether according to the invention.

In connection with the hot isostatic pressing, it is necessary that the pressure is applied from all sides evenly in order to avoid the shifting of the joint forming surfaces relative to each other. Such a shifting could prevent the welding joint formation since it may interfere with a proper diffusion at the joint forming surfaces.

The invention avoids these problems by providing, prior to any diffusion welding step, in the gap 13 between the joint forming surfaces 12a, 12b, 12c, and 12d of the parts 10, 11 to be joined, a metallurgical gap layer which initially seals the gap. This metallurgical layer is selected from a group of gap materials which have a normal melting point, under atmospheric or reduced pressure, below the given diffusion temperature needed for the hot isostatic pressing and which cannot be melted onto these surfaces by said given diffusion temperature under the isotatic pressure. In other words, these materials also have a higher melting point, under the isostatic pressure, above said diffusion temperature. Thus, these materials can melt if heated above the normal melting point under atmospheric or reduced pressure, but they cannot melt at the diffusion temperature prevailing at the same time when the isostatic pressure is applied and when the diffusion takes place. Gold, nickel phosphorus-alloys or nickel based solders have been found to satisfy these conditions and hence are suitable for the present purpose. These gap materials may be used in powder form, in wire form, or they may be deposited electrogalvanically or electrochemically or by sputtering or vapor deposition. The metallurgical gap layer so formed may cover part or the entire area of the joint forming surface or surfaces 12a, 12b, 12c, and 12 d. The gap layer may be applied, prior to or after assembling the parts to be joined, to one or both of these surfaces. It is, however, essential that the application of the gap layer is so localized that the gap 13 is sealed by the metallurgical gap layer when the parts to be joined are assembled. Regardless whether the metallurgical gap layer has been applied prior to or after the parts 10, 11 have been assembled as shown in FIG. 1, any hollow spaces 14a that may result outside the seal in the gap 13 are vented to the atmosphere through a venting bore 14.

Where the material forming the metallurgical gap layer is applied into the gap 13 after the assembly, it is best to perform this phase in a vacuum having a reduced pressure within the range of 10.sup.-3 to 10.sup.-5 bar, and at a temperature within the range of 900.degree. C. to 1250.degree. C. Under these conditions a gravity enhanced capillary action seals the gap 13.

The temperature application prior to the hot isostatic pressing is such that the layer 15 provides a seal of the gap 13 while simultaneously the melting-on is prevented, for example, by an out diffusion of individual components of the layer or by a diffusion with the material of the parts 10, 11 to be diffusion welded. Such out-diffusion or diffusion raises the melting point of the layer so that the undesired melting-on during the subsequent hot isostatic pressing is prevented.

After the gap layer is formed and the parts 10, 11 have been assembled the actual hot isostatic pressing is performed to form the welded joint by a further material diffusion through the joint forming surfaces 12a to 12d. Thus, the present method permits the construction of work pieces having a complicated shape in which a pressure may be applied directly through the structural components which otherwise would not permit the application of a direct pressure by means of a jig or the like which applies the required pressure to the surfaces to be joined.

In the embodiment of FIG. 2 two structural parts 20 and 21 are to be joined for constructing an injection head of a rocket combustion chamber, whereby a so-called plug-in connection is involved. The dome forming part 20 has a rim 20' with conical side walls 22a. The part 21 has a respectively shaped conical groove 25 so that upon insertion of the rim 20' into the groove 25 gaps 23 are formed. The gaps 23 are again filled to an extent sufficient to achieve the mentioned sealing with the above described metallurgical layer, whereby any of the mentioned steps and possibly other steps may be used to form the gap layer. Any hollow spaces 24a formed outside the gaps 23 proper, are vented to the atmosphere by a vent channel 24. The gap layers in the gaps 23 are formed to such a thickness in the range of 2 .mu.m to 100 .mu.m, that the layer material will diffuse into the material of the parts 20, 21 during the subsequent hot isostatic pressing. A formation of the layer on the surfaces 22a and/or on the surface 25 by electrochemical deposition, has been found to be quite satisfactory. The gap width is generally in the range of 0.0 to 0.1 mm.

The material for forming the diffusion gap layer does not form any brittle phases with the material of the parts 10, 20, and 11, 21. It has been found that the formation of the solid solution during the diffusion of the layer material into the material of the structural components 10, 11 and 20, 21 results in an increase of the strength of the formed diffusion welded zones as compared to conventionally fused joints. Further, whether the metallurgical layer is applied before or after the assembly of the parts to be joined, will determine the manner or method of applying the material to form said metallurgical layer. In both possibilities it is important that the normal melting point at atmospheric pressure is sufficiently increased above the hot pressing temperature at the hot pressing isostatic pressure so that diffusion and not melting takes place during the hot isostatic pressing. Thus, it is possible that even the normal melting point could be above the hot pressing temperature. The hot pressing temperature and the isostatic pressure will depend on the materials used. However, hot pressing temperatures in the range of 980.degree. C. to 1200.degree. C., and isostatic gage pressures within the range of 1000 to 1500 bar have been found to be suitable.

Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims.