Patent Application
20160045982
This invention relates generally to the fields of metals joining and filler manufacture and, more particularly, to a process for building a tip cap on a turbine blade by additive welding.
Worn gas turbine blade tip caps can be replaced by grinding the old cap away and installing a new one on the blade tip. However, gas turbine blades are often made of a superalloy for heat tolerance, and installing a superalloy blade tip cap by a known process is difficult and time-consuming. In one process, the worn tip cap is removed, and a superalloy tip cap is welded to the blade tip in a hot box. An existing version of this process takes about 24 hours per blade, partly due to slow heating in the hot box, and gradual cooling required in order to minimize cracking of the superalloy material. In spite of precautions, this process often produces cracks that must be manually corrected. Similar issues arise when installing a tip cap on a newly manufactured blade when fixturing of a ceramic core prevents the casting of a closed blade tip during the primary casting process.
Superalloy materials are among the most difficult materials to weld due to their susceptibility to weld solidification cracking and strain age cracking. The term “superalloy” as used herein means a highly corrosion and oxidation resistant alloy with excellent mechanical strength and resistance to creep at high temperatures. Superalloys typically include high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g., IN 738, IN 792, IN 939), Rene alloys (e.g., Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g. CMSX-4) single crystal alloys.
United States patent application publication number US 2013/0298400 A1, also commonly owned with this application, describes a method of repairing a turbine blade tip which uses a specially shaped tip cap in order to minimize welding stresses during installation. Further improvements are desired.
The invention is explained in the following description in view of the drawings that show:
The present inventors created a hybrid welding/printing process that is faster and more dependable than prior methods for building a tip cap on a turbine blade for worn tip cap replacement or for original blade manufacture. The inventive process combines the concepts of welding a pre-formed blade tip with an additive manufacturing process. The inventors recognized that the welding of a full size blade tip cap can create a high level of stress in the component, and they minimize such stresses by initially welding only a thin base sheet of metal onto the blade tip. Subsequently, a full thickness of the tip cap is created by applying successive thin layers of metal over the base sheet with a powder deposition process using controlled laser heating. The result is the fabrication of a thick blade tip cap while avoiding the build-up of stresses to a level where cracking is a hindrance to productivity.
Using a powder feed apparatus and method as shown allows unattended operation over multiple layers. This cladding operation has been performed automatically in a Huffman® laser welder and cladding system in about 1 hour to clad six to eight layers on a Siemens SGT6-6000G row 2 turbine blade. The tip cap edges are then machined flush with the blade exterior surface, and the cladding may be machined to a final thickness. Any desired thickness of multi-layer cladding 34 may be applied over the tip cap base sheet 24, and in one embodiment the cladding 34 may be twice as thick as the base sheet 24, and in another embodiment may range from about 0.03-0.20 inches. The base sheet functions as scaffolding that supports the cladding. The first layer of cladding fuses to the base sheet and to the tip surface 22. Each successive layer of cladding fuses to a previous cladding layer. The cladding conforms to each surface as it builds, providing relatively low process stress and a more integrated blade tip with lower operational stress than in prior methods of welding a full-thickness tip cap onto the blade tip.
52—Prepare a turbine blade comprising an open tip, such as by grinding off a used tip to a smooth, planar surface;
54—Form a tip cap base sheet to cover the blade tip except for a margin;
56—Place the base sheet on the blade tip to cover the blade tip except for the margin;
58—Perform a root weld pass around a periphery of the base sheet using a more ductile alloy than that of the base sheet;
60—Perform laser cladding on the base sheet and tip surface, thus building and fusing a tip cap onto the blade tip, the tip cap formed of the base sheet and cladding.
The method described herein takes 3-5 hours to complete after the old tip cap is removed. In contrast, a prior method of tip cap replacement in a hot box takes about 24 hours. Argon consumption is much less with the new method, for example using a Huffman® laser welder and cladding system, compared to the prior hot box method. The new method provides more consistent and reliable results. Thus, the method herein is much faster, more reliable, and more efficient than prior methods.
While various 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 may be made without departing from the invention herein. For example, the hybrid process described herein may be used on components other than turbine blades and may be used for joining any two substrates with a cap where welding a full thickness of the cap would produce undesirable levels of stress in the base material. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
