Patent Grant
7028881
This invention relates generally to the field of welding, and, more particularly, to a method for welding objects having a restricted access region behind a region to be welded.
Weld backings are used to temporarily provide support for weld material deposited in a region being welded, to prevent, for example, a weld root from sagging below the welded region. An uncontrolled weld root may result in varying weld thickness and sharp notches in the weld requiring further machining that may be difficult and time consuming to perform. Furthermore, complex weld root profiles may be difficult to obtain without the use of a weld backing. Typically, a mechanical support is used for holding a heat resistant material against a backside of a region to be welded. Once the weld has cooled sufficiently, the backing can be removed, leaving a smooth weld root. Typically, ceramics, fiberglass tape, solid metal, and weld metal deposited using a tungsten gas process are used for weld backings. However, irregularly shaped objects having restricted access to the backside of regions to be welded pose problems when attempting to provide weld backings in a conventional manner. In particular, objects having relatively complex structures defining irregular cavities with restricted access have been difficult to weld while maintaining a desired weld root profile.
The invention will be more apparent from the following description in view of the drawings that show:
The inventors of the present invention have innovatively realized that welding of objects having backside access that is restricted by a structure defining a cavity may be accomplished by at least partially filling the cavity proximate a region to be welded with a fugitive backing material. For purposes of this invention, the term fugitive backing material means a type of material that is provided at one stage of fabrication, a state of material is changed, a fabrication step is performed, the state of the material is changed a second time, and the material is subsequently removed by a process during a later stage of fabrication. Generally, a method of welding such objects includes filling at least a portion of the cavity directly below the region with a fugitive backing material, welding the region, and then removing the fugitive backing material from the cavity.
In one form, the fugitive backing material 16 may include a fusible material, such as metal powder or a brazing compound. Accordingly, a portion of the fusible material 16 facing the region 14 to be welded may become part of a weld 24 after welding of the region 14 is complete. In another aspect, the fugitive backing material 16 may include a refractory material, such as fused silica, alumina, or quartz, that does not become a part of the weld 24 during welding and, therefore, may be completely removed from the cavity after welding.
A powdered material may be used for the fugitive backing material 16. For example, the object 10 may be positioned so that the region 14 to be welded opens in an upward direction. The cavity 12 may then be filled with the powdered material and the region 14 welded, such as by laser or electron beam welding. If necessary, a plug may be provided to block the opening 22 to prevent the powdered material from exiting the opening 22 under the force of gravity. In an aspect of the invention, the powdered material may be compacted in the cavity 12. The inventors have experimentally determined that a welding process such as gas tungsten arc or plasma arc welding may require the use of a binder mixed with the powdered backing material to provide a fugitive backing material 16 that is less likely to be displaced than a powdered material when welding. For example, a binder such as sodium silicate or hydrolyzed ethyl silicate may be added to the powdered material to achieve a desired viscosity, so that the mixed material remains in place after being packed in the cavity 12. Accordingly, the mixed material may be mixed to a desired viscosity suitable for packing into the cavity 12 without extending into the region 14 to be welded regardless of the orientation of the object with respect to gravity. The mixed material may be prevented from extending into the region 14 to be welded during packing, such as by a plug conforming to the dimension of the weld region. After the plug is removed, the mixed material may retain its packed configuration without extruding into the region 14. In an aspect of the invention, a liquid form of the binder may be mixed with the powdered material to create a relatively higher viscosity mixture that may be poured into the cavity and allowed to harden into a solid state.
After the cavity 12 has been filled with the fugitive backing material 16, the region 14 may be welded so that a weld root 26 of a weld 24 assumes the profile of a surface 20 of the fugitive backing material 16 exposed to the region 14 to be welded. After welding of the region 14, the fugitive backing material may then be removed through the opening 22, for example, by a chemical leaching process as would be appreciated by a skilled artisan. The leaching process may include injecting, into the filled cavity 12, a chemical capable of reducing the viscosity of the fugitive backing material 16 so that the fugitive backing material 16 may be made to flow out of the cavity 12 through the opening 22. In the case of a powdered material, the material may be simply removed by orienting the object so that the force of gravity causes the material to flow out through the opening 22. After removal of the fugitive backing material 16, the cavity may be flushed with, for example, steam, water, or compressed air to ensure that fugitive backing material 16 is thoroughly removed. In addition, if a material, such as fused silica, is used as a fugitive backing material 16, a chemical etching process, as known in the art, may be used to remove any remaining material from the cavity 12.
The above process may be further explained in terms of states of the fugitive backing material during the steps of the process. For example, an installation state of the fugitive backing material may include states such as a loose granular state, a liquid state, or a paste state, so that insertion of the fugitive backing material into the cavity of is easy to accomplish. A rigid state, for example, existing after transformation from the installation state, may include a gravity held state, a compacted state, a hardened state, or a cured state sufficiently rigid for supporting weld formation. A removable state may include the resulting state of the fugitive backing material after a process of, for example, melting, sublimation, dissolving, or releasing pressure or a resistance to gravity, so that the fugitive backing material is made easy to remove.
In another aspect of the invention, the fugitive backing material 16 may comprise multiple layers of material. For example, the object 10 may be positioned so that the opening 22 opens in an upward direction (that is, inverted from the view shown in
In another aspect, a fugitive, pre-formed weld backing may be placed in a first portion of a cavity directly under a region to be welded, and a second portion of the cavity directly under the pre-formed weld backing may be filled with a fugitive filler.
While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
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| Number | Date | Country | |
|---|---|---|---|
| 20050061858 A1 | Mar 2005 | US |
