Turbine Repairs Made Possible
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Repairing worn gas turbine blades, instead of replacing them, helps to reduce costs |
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Laser powder build-up welding technique reduces the cost of repairing gas turbines
Nickel magazine, Mar. 02 -- High-efficiency gas turbines use materials that are expensive to
manufacture and replace. Last year Sulzer Elbar, a subsidiary of Sulzer Turbomachinery Services, introduced a
new automatic laser powder welding technology that can repair superalloy components and which can reproduce
the directionally solidified or monocrystalline structure of gas turbine hot section components.
This proprietary technology combines traditional welding and cladding techniques and permits
accurately-controllable input in small zones, limited distortion and good dimensional control of the welding
bead. Conventional cast components that previously had to be scrapped, for example, the superalloy
N07718, can now often be repaired. The technique is suitable for most nickel-based superalloys.
A component repair begins with the creation of a 3D digitized model of the part for a CNC-controlled workstation. With this model the CNC-controlled laser welder can precisely follow the geometry of the part, producing metal deposits of near-net shape.
The laser beam is focussed just above the substrate surface to create a weld pool in the area to be repaired. A cover argon gas stream with entrained superalloy powder particles (the specification of the powder depends on the base material) is directed into the molten pool, which is quickly cooled by the substrate.
The deposition rates can be as high as 90 grams per minute. The weld thickness can be from 0.5 to 1.0 millimetre (mm) thick; the weld width can be adjusted from as little as 0.5 to as much as 5.0 mm. Both thickness and width can be controlled to an accuracy of plus or minus 0.1 mm.
To date, Sulzer Elbar's primary area of mastery of laser powder welding is with build-up welding of different types of conventionally-cast superalloy components.
Laser powder welding can be used for directionally solidified (DS) and single crystal (SX) components, as long as the epitaxial growth of the microstructure is achieved during the solidification of the melt pool. As the laser powder welding takes place, planar front, cellular or dendritic solidification microstructures form in the solid/liquid interface. The formation depends on variables such as the alloy composition, the temperature gradient and the speed at which the welded area solidifies.
By matching the welding parameters with the component geometries and alloy compositions, DS or SX
microstructures can be reproduced. "The successful parameters are available for DS and SX, but you have a
very small temperature window in which to work. The company wants to expand that window and thus expand the
flexibility of the operation," says Sulzer Elbar.
Adapted from an article originally published in the 4/2001 issue of the Sulzer Technical
Review.
Photo: SULZER ELBAR
 Berend van Zanten Director of Sales Tel.: 31 77 473 8607 Fax: 31 77 472 2785 Mobile: 31 653 184 176 Email: Berend.vanZanten@Elbar.com Website: www.elbar.nl |
