Electric heating – Metal heating – For deposition welding
Reexamination Certificate
2000-04-14
2002-05-07
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
For deposition welding
C029S889100
Reexamination Certificate
active
06384365
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to a plasma sintering process for repair and fabrication of combustion turbine components which will provide improved results over repair techniques such as welding or wide gap brazing, for applications such as repair of cracked turbine blades or vane segment platforms; and which will allow manufacture of complex components for which casting processes are not cost effective.
2. Background Information
Utilities and other power producers have been adding capacity or replacing existing steam generators with smaller, more efficient combustion turbines. The higher efficiencies of the combustion turbines have been realized via higher operating temperatures which typically require use of gamma prime strengthened, nickel base superalloys for components such as blades and vanes. Due to the high replacement costs of these hot section components, development of new and improved repair processes is essential. Also, the advanced turbine systems typically require use of directionally solidified and single crystal hot section components. However as the design complexities and component size increase, the ability to cast single piece components in a cost effective manner decreases. In many cases, advance blade and vane design will require complex cooling geometries for which conventional casting processes are not possible.
In the past, gas tungsten arc welding has been a main primary repair process. However, the gamma prime strengthening mechanism used in manufacturing these superalloys results in hot cracking (during welding) or and/or strain age cracking (during post weld heat treatment) of the heat affected zones or of the weld metal itself. In order to alleviate this situation, repairs have been made using lower strength, solid solution strengthened filler alloys such as IN 625 (sold commercially by INCO International) for repair. Because of the poorer mechanical properties of the weld metal and high probability of microfissures in the heat affected zones, repairs have been limited to non-critically stressed regions of components. Advanced welding procedures utilizing high strength filler metals continue to result in inferior creep and fatigue properties.
In U.S. Pat. No. 5,554,837, (Goodwater et al.), laser welding a powder alloy feed, after preheating the weld area and an adjacent region to a ductile temperature, was used during manufacture and repair of jet engine components that operate at temperatures of up to 1093° C. (2000° F.). The problem solved there was, that as a result of such high temperature demands the components usually are manufactured from superalloys containing a gamma-prime phase and materials commonly known as the MCrAlY family of alloys. One particular problem with the gamma-prime precipitation hardenable alloys is the inability to weld or clad these alloys with like or similar alloys without encountering cracking and high production rejects. Because of the welding temperatures and stresses involved, these alloys encounter shrinkage, stress cracking and the like. Due to the difficulties in welding these specific superalloys, there is a need for a process by which gamma-prime precipitation hardened alloys can be joined consistently, without cracking, with similar or parent metal alloys.
Many years ago, sintering of discrete bodies by effecting a spark discharge between the bodies was disclosed by Inoue, in U.S. Pat. Nos. 3,250,892 and 3,241,956. This process involved disposing a mass of discrete electrically fusible particles, preferably consisting predominantly of conductive metallic bodies between a pair of electrodes which sustain a spark discharge. Since this mass tends to shrink as sintering proceeds, means were provided to maintain the electrodes in contact with the mass. To this end, the electrode means could be spring or gravity loaded to maintain the contact and, if desired, a mechanical pressure up to 100 kg./cm
2
was provided, when required. The spark discharge could be terminated upon the particles being welded together, at least preliminarily, while passage of the electric current could be continued without development of the spark to weld the particles further by resistance heating. A variety of articles were made using this process: nickel and cobalt discs, bodies of copper with carbon or lead and bodies of cadmium oxide.
Intermetallic compounds, such as Fe
3
Al have been mechanically alloyed (“MA”) and then disks 10-20 mm in diameter were prepared using plasma activated sintering (“PAS”), as described in “Mechanical Alloying Processing and Rapid Plasma Activated Sintering Consolidation of Nanocrystalline Iron-Aluminides”,
Materials Science and Engineering,
A207 (1996) pp. 153-158, by M. A. Venkataswamy et al. The PAS consolidation provided high density values in a very short time, and a very fine grain structure was maintained. PAS has also been utilized to consolidate difficult-to-sinter powders to provide AlN, Nb
3
Al and superconducting (Bi
1.7
, Pb
0.3
)—Sr
2
Ca
2.1
Cu
3.1
O
x
ceramics with absence of significant grain growth during densification, as described in “Plasma-Activated Sintering (PAS): A New Consolidation Method For Difficult-to-Sinter Materials”,
Powder Metallurgy in Aerospace, Defense and Demanding Applications—Proceedings of the
3
rd
International Conference,
published by the Metal Powder Industries Federation, (1993) pp. 309-315, by J. Hensley et al. They mention that common methods of consolidating ceramic and metallic powders include sintering, hot pressing, or hot isostatic pressing, which processes typically require long exposure (one to several hours) at high temperature, leading to grain coarsening of the microstructure and the formation of grain boundary impurities. The plasma-activated sintering (PAS) process significantly reduces the sintering times of many materials. In the PAS process, a spark plasma sintering process, the commercial powders are poured into carbon molds without additives, binders, or pre-pressing. Uniaxial pressure is applied to the powder and an external power source provides a pulsed current to activate the surface of the particles. The power supply is then switched to resistance heating for densification. To measure the sintering temperature, a thermocouple is inserted into the carbon mold and a linear gauge measures the shrinkage. They mention that the first commercial use of PAS was to manufacture Fe—Nd—Co—B magnets around 1990.
M. Tokita, in “Mechanism of Spark Plasma Sintering”,
Proceedings of the International Symposium on Microwave, Plasma and Thermochemical Processing of Advanced Materials,
published by the Joining and Welding Research Institute, Osaka University (1997) pp. 69-76, describes SPS as being based on a high temperature plasma (spark plasma) momentarily generated in the gaps between powder materials by electrical discharge at the beginning of pulse energizing. The large current pulse energizing method generates: (1) spark plasma, (2) spark impact pressure, (3) Joule heating, and (4) an electrical field diffusion effect. The SPS process is regarded as a rapid sintering method, using self-heating action from inside the powder, similar to self-propagating high temperature synthesis (SHS) and microwave sintering. That article states that SPS systems offer many advantages over conventional systems using hot press (HP) sintering, hot isostatic pressing (HIP) or atmospheric furnaces, including ease of operation and accurate control of sintering energy as well as high sintering speed, high reproducibility, safety and reliability. The SPS process is expected to find increased use in the fabrication of functionally graded materials (FGMs), intermetallic compounds, fiber reinforced ceramics (FRC), metal matrix composites (MMC) and nanocrystalline materials, which are difficult to sinter by conventional sintering methods. Tokita explains that there was little literature on research into this process until the latter half of the 1970s. The second generation was developed from the middle of the 1980s t
Seth Brij B.
Swartzbeck Gary W.
Wagner Greg P.
Dunn Tom
Pittman Zidia
Siemens Westinghouse Power Corporation
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