Electric heating – Metal heating – By arc
Reexamination Certificate
1999-07-13
2001-12-04
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121640, C219S121830, C219S121840, C219S121850, C219S121860
Reexamination Certificate
active
06326585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to improved apparatus and method for repairing a metallic portion of an article using a metal powder and a laser beam, and specifically to repairing a blisk assembly such as are found in gas turbine engines.
2. Description of the Prior Art
An aircraft gas turbine engine or jet engine draws in and compresses air with an axial flow compressor, mixes the compressed air with fuel, burns the mixture and expels the combustion product through an axial flow turbine that powers the compressor. The compressor includes a disk with blades projecting from its periphery. The disk turns rapidly on a shaft, and the curved blades draw in and compress air. In the past, the blades and the disk were structures that were manufactured separately and assembled together. More recently, the blades have been manufactured integral with the disk forming a single integral structure known as a blisk.
Blisk components manufactured by all known methods of manufacture (milling, electrochemical machining) are subject to foreign object damage (F.O.D.) of the airfoils. This can occur during the original manufacturing process or during field service of the blisk. In either case, repair of the damaged region is often desirable due to the high replacement cost of the components. To date, the only repair method demonstrated on an engine test is mechanical removal of the damaged material and essentially leaving the blisk as-is. While this repair method is suitable for minor airfoil edge damage, it is not acceptable for more significantly damaged areas due to imbalance problems and potential mechanical property degradation. Severe airfoil damage, as experienced by the XTC46 Core Driven fan Stage (CDFS), requires a complete replacement of airfoil material from the midspan shroud outward.
Techniques have been developed for forming a compressor blade integral with a substrate in new manufacture. One technique deposits layers of filler material on a substrate through the interaction of a laser beam. A CNC program is used to control the shape of the deposit, and a contoured buildup of suitable height is achievable through multiple layer buildups, with each layer having the desired geometry. Apparatus for performing controlled laser deposition of material on a substrate is described in U.S. Pat. No. 4,730,093, and this technique is described in U.S. Pat. No. 5,038,014, both of which U.S. Patents are incorporated herein by reference. While the technology described in these patents has been used to successfully repair minor damage to titanium (Ti 6-4) materials by applying the same feed or replacement material as the damaged base materials, no large scale repairs have been accomplished, and no repairs have been performed by depositing a feed or replacement material that is different than the substrate base material.
One of the problems experienced in attempts to manufacture and repair items using laser welding techniques such as described above include low integrity welds. These low integrity welds have been due to oxygen and nitrogen pickup due to loss of the argon cover over the molten weld metal, referred to as the weld puddle. Contamination due to oxygen and nitrogen causes a brittle weld repair, which is an unacceptable result for an airfoil repair. Impact due to foreign object ingestion by a gas turbine engine could result in a failure of the repaired area, that could lead to separation of the portion of the blade above the repaired area.
Drawbacks with the system set forth in these patents include operator fatigue. Even though the system is computer controlled, an operator must visually observe the application of the powder and must override the system if there is any deviation form the computer controlled program. Furthermore, as noted in the prior art patents, the systems are sensitive and failure to override in a timely fashion can lead to an unacceptable region of repair. Also, the systems of the previous patents relies upon an argon feed to assist in carrying the powder from the reservoir to the article and to shield the pool of molten metal, and upon a short interaction time of the powder in the molten pool created by the laser. However, problems with this technique are frequent loss of the protective argon gas over the molten metal, as set forth above. Spattering of molten metal onto adjacent blades and the undesirable heating of adjacent blades from laser energy reflections from the weld pool due to the close arrangement of blades around the blisk pose additional problems.
SUMMARY OF THE INVENTION
In accordance with the present invention, it is desirable to provide a laser twist weld (LTW) repair of compressor blisk airfoils. These airfoils are of a complex configuration, having a curvicular shape. The airfoils can be characterized as a plurality of substantially parallel cross sections of predetermined thickness extending from the base of each airfoil outward toward the tip of the airfoil. The airfoils are repaired by depositing at least one bead weld filler material on the underlying substrate corresponding to the predetermined thickness of the cross section. Typically, it is necessary to deposit two or three parallel beads of predetermined thickness in order to obtain the desired cross section. LTW can also be used to provide a replacement of contoured blisk airfoils. Following suitable post-weld heat treatment (PWHT), the blisk airfoils are machined to the desired geometry and inspected for assurance of weld integrity. Compressor blisk airfoil replacement may use dissimilar materials for the substrate portion and the airfoil portion (e.g., Ti-17 replacement material for Ti-6242 blisk base metal), or it may utilize as replacement material the same composition as the damaged material. The advantage of the bi-alloy repair process is that it allows both the disk and airfoil materials to be optimized independently, and can also result in improved weldability of the deposited material, which may not be achievable when using replacement material that is the same as the damaged material.
A fixture, a local dry box, provides a convenient means for enclosing a portion of the substrate being welded, usually several airfoils at a time, to allow a clean dry argon atmosphere to be maintained over the weld pool and in the region adjacent to the welding. The fixture also encloses at least a tip of the weld nozzle. This results in deposits having minimal contamination, which is essential when welding titanium materials. The need for gas shielding in the vicinity of the weld does not limit the process to the use of compact dry box; a larger argon filled enclosure that covers the entire part could be used and may reduce weld repair cycle time.
A laser weld feed nozzle is used for blisk repair to accomplish material deposition. The nozzle includes a powder feed system having two copper feed lines connected at one end to a supply of powder material and inert gas and at the other end to a powder “outer” feed nozzle which in turn is connected to an “inner” nozzle, as opposed to a single feed line separate from laser nozzle. This “coaxial” powder feed system differs considerably from the “single point” powder feed system described in U.S. Pat. No. 5,038,014, which patent is incorporated herein by reference. This design allows for multidirectional processing and more uniform deposition compared to prior art designs. Attached to the nozzle or provided as an integral part of the nozzle is a laser that provides the energy for heating the filler metal and the substrate coaxially with the powder feed or filler material. Also included as a part of the laser weld feed nozzle is a copper chill positioned at the nozzle tip at the end opposite the laser.
A means for viewing the substrate being welded is provided by a video system that is also used to monitor the amount of material deposited and to determine the upward movement of the nozzle assembly required after each layer to maintain the proper nozzle to part standoff. Although the means for monitoring the stando
Aleshin Stephen
Brunck Michael J.
English Christopher L.
Elve M. Alexandra
General Electric Company
Hess Andrew C.
Narciso David L.
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