Patent Application
20120279066
The increasing use of integrally bladed rotor hardware in large high-performance gas turbine engines is driven by the demand for improvements in performance and efficiency. In conventional rotors, rotating airfoils are retained by dovetail slots broached into the rim of a disc. In an integrally bladed rotor, the airfoils and disc form one continuous piece of metal. The weight and fuel savings afforded by integrally bladed rotors result from their ability to retain rotating airfoils with less disc mass than would be required in a conventionally designed rotor. Furthermore, the reduced disc mass of an integrally bladed rotor disc permits weight reduction in other components which react upon or obtain a reaction from the rotors, i.e. shafts, hubs, and bearings.
In the past, a major disadvantage associated with the use of integrally bladed rotors in large gas turbine engines has been the lack of a reliable method for repairing integrally bladed rotor airfoils that have been damaged beyond blendable limits. Because the airfoils are integral with the disc, damage to airfoils beyond blendable limits requires a removal of the entire rotor from service and replacement with a new integrally bladed rotor, at significant expense.
Other concerns associated with integrally bladed rotors relate to the fabrication method employed to manufacture them. They can be machined out of a single large forging; however, this approach is not desirable. A large forging (e.g. large billet) has lower property capability, and it can be very expensive due to high buy to fly ratio. Also, the part may be at risk of scrap out due to machining errors during manufacture. Another approach for manufacturing integrally bladed rotors is to attach separately forged airfoils to a rotor by a friction welding process.
A titanium alloy having a nominal composition in weight percent of Ti-6Al-2Sn-4Zr-6Mo (referred to as Ti-6246) is a desirable alloy for integrally bladed rotors due to its high toughness, tensile and fatigue strength. However, the fusion weldability of Ti-6246 is limited by the nature of the weld zone microstructure which may form brittle orthorhombic martensite under rapid cooling from the fusion weld. As such, the original equipment manufacturer (OEM) friction weld must be post-weld heat treated to stabilize the microstructure and relieve stresses. Secondly, the integrally bladed rotor must be able to undergo subsequent in service weld repairs due to foreign object damage. While weld properties can be restored with full solution plus age heat treatment after one weld repair, it is impractical to perform full solution heat treatment after weld repairs due to potential high risk of airfoil distortion and surface contamination, especially for non-OEM welds.
The invention is a method to weld or repair damaged Ti-6246 alloy airfoils in integrally bladed rotors. Damaged regions of the airfoil are built up with repair metal by fusion welding. Following welding or repair, the airfoil is given a stress relief heat treatment of about 1300° F. for 1 to 4 hours. Optional laser shock peening introduces surface compressive residual stress in the airfoil for additional mechanical integrity. Ti-6242 alloy filler metal in one embodiment advantageously minimizes undesirable weld microstructure.
A schematic cutaway view of Ti-6246 alloy integrally bladed rotor (IBR) 20 is shown in
Prior to depositing repair metal in the damaged site (Step 4), the site is cleaned by those methods known to those in the art. Material may be removed around the damage sites, such as cracks and foreign contaminants, to allow for easier metal deposition.
Repair metal may then be deposited in the damaged site until the repaired region exceeds the initial dimensions of airfoil 20. (Step 4). Damaged sections may also be cut away and replaced by new sections. Repair may be performed by many methods known in the art. A preferred embodiment is repair by fusion welding. Preferred embodiments are gas tungsten arc welding (GTAW), laser beam welding, plasma arc welding and electron beam welding.
Titanium alloy candidates for integrally bladed rotor (IBR) or bladed disc (BLISK) applications for compressor stages behind the fan include, but are not limited to, in weight percent, Ti-6Al-2Sn-4Zr-6Mo (Ti-6246), Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and Ti-6Al-4V (Ti-6-4). Ti-6246 alloy exhibits improved elevated temperature properties as compared to Ti-6242 and Ti-6-4 alloys and is a leading candidate. Someone skilled in the art of weld repair of Ti-6242 and Ti-6-4 alloys would find difficulties in the weld repair of Ti-6246 alloy. In particular, enhanced crack growth behavior leading to reduced mechanical properties in the weld.
The crack growth behavior in Ti-6246 alloy weld metal is determined by grain boundary morphology and microstructure. Schematic sketches of fusion zones 40 and 50 and microstructures 42 and 52 behind the fusion zones during welding are shown in
Another contributor to weld property in Ti-6246 alloy is the formation of a brittle orthorhombic martensite phase in the weld fusion zone microstructure. Orthorhombic martensite forms due to excessive rapid cooling rate in the weld zone immediately after welding. In Ti-6242 and Ti-6-4 alloys, the transformation can be suppressed by a slower cooling rate resulting in a more ductile alpha plus beta phase microstructure. In Ti-6246 alloy, the martensitic transformation occurs even at slower cooling rates and is difficult to suppress. The brittle martensite phase significantly increases the susceptibility of fracture in the weld metal. Furthermore, the brittle martensitic microstructure is not significantly altered by conventional weld stress relief anneals in the vicinity of 1100° F.
A number of strategies have been identified for use either individually or in combination to improve weld property of Ti-6246 alloy IBR repair. These are first, alter the thermal dynamics of the weld process by changing the weld parameters and/or joint geometry to control the weld cooling rate. Changing the composition of the weld metal to a compatible Ti alloy having a significantly lower or no propensity to form deleterious phases such as orthorhombic martensite in the weld metal is another strategy. As mentioned above, an inventive embodiment comprises using Ti-6242 alloy filler metal when welding Ti-6246 alloy to minimize centerline weld fracture. In addition to the above, commercially pure Ti is an alternative titanium welding filler metal. Another but not limiting example is to use post-weld thermal processing to alter the formation of deleterious phases in Ti-6246 alloy welds.
A post-weld heat treatment of about 1300° F. can eliminate the brittle orthorhombic martensite phase in Ti-6246 alloy.
A recommended stress relief anneal of airfoil 20 following deposition of repair metal may be heating the airfoil to about 1275° F. to about 1325° F. for about 1 to about 4 hours in an inert atmosphere to prevent alpha case formation. When Ti-6246 alloy is heated above 1000° F. in the presence of oxygen for an extended period of time, an embrittled zone of oxygen enriched alpha phase forms at the surface that is called “alpha case” in the art. The formation of alpha case on a titanium alloy turbine blade causes the blade to be highly susceptible to fatigue failure and deleterious impact damage by foreign objects, and needs to be avoided or significantly curtailed. For this reason, titanium alloys susceptible to alpha case formation are preferably heat treated in inert atmospheres. Following the post-weld heat treatment, the airfoil may be cooled at a rate of from about 40° F. to about 100° F. per minute. (Step 8)
During the stress relief heat treatment, it may be important that adjacent airfoils or hub sections are not thermally affected. In particular, the root area of airfoil 20 may be preferably maintained at temperatures less than 800° F. This may be accomplished by surrounding the repaired airfoil with a fixture containing localized heat sources that heat only the airfoil under consideration.
Following stress relief heat treatment (steps 6 and 8), repaired airfoil 26R is machined to predetermined dimensions and blended surface configurations. (Step 10)
To further enhance the mechanical integrity of repaired airfoil 26R, airfoil 26R is preferably subjected to laser shock peening to introduce residual surface compressive stresses. Laser shock peening is described in commonly owned U.S. Pat. No. 6,238,187, which is incorporated herein in its entirety as reference. In laser shock peening, a high intensity laser beam impinges on airfoil 20 and injects a compressive shock wave into the part. The stress level in the shock wave exceeds the yield strength of the part resulting in a plastically deformed surface and sub-surface region containing compressive residual stresses much like ordinary shock peening but deeper in extent to airfoil 26R. During laser shock peening, the laser moves over the surface creating a series of overlapping laser shock peened spots. The spots are normally circular but other shaped spots such as elliptical, square, triangular, etc. can be used. The depth of the compressive stress zone is controlled by the pulse intensity, i.e. the power of the laser.
Following laser shock peening (Step 12) repaired airfoil 26R is returned to service (Step 14).
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
