Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Laser surface treatment of engineering alloys

It has been proved that laser surface treatment can be successfully applied to real world steam turbine components. It has been demonstrated that high quality coatings, in particular, the finely dispersed carbide phase, which has been tested successfully under drop impingement, can be obtained also when treating real world components. The remelted bonding plane between coating and base material promises a very high adhesive strength in comparison with brazed coatings. The risk of interfacial corrosion between brazed coatings and the steel substrate is avoided. The treatment of the first original turbine blades and their investigation has demonstrated that the distortion can be accepted in case of laser treatment after the machining of the blades.

The preheating of the blades and other turbine components is a suitable measure to reduce the residual stresses, which have shown to be the cause for cracks during a subsequent stress relief heat treatment. The formation of martensite must be prevented carefully.

With respect to titanium alloy blades it has been established that laser surface treatment produces coatings with satisfactory erosion resistance (erosion rate is less than 10 times that of the titanium-6 aluminium-4 vanadium substrate). Compared to traditional cobalt base alloys (stellites) wear is identical in terms of weight loss and slightly higher in terms of eroded volume.

This resistance to wear is satisfactory and the following is observed:
little or no residual stresses in the deposit and substrate;
insensitivity to stress corrosion;
satisfactory resistance to fatigue, although in comparison with the base material significant reduction of the endurance limit is observed.

From a laser processing point of view, relating to processing complex components of both steel and titanium alloy, it is recommended to use a 5-axis workstation in order to control the beam spot, powder and gas parameters sufficiently and thereby obtain a homogeneous coating. Further, the programming of the laser cell is complicated, because precise positioning of the sharp blade edges is required. Small deviations caused by thermal distortion or an unprecise clamping may lead to an interruption of the process.

A method for significantly improving the corrosion/oxidation properties of gas turbine blades has been established involving 2 separate activities. The first one concerns the development of diffusion aluminide coatings, as well as hafnium modified aluminide coatings. This technique has been shown to be easily applied independently of the component shape, producing uniform and high quality coatings.

The second activity, following the aluminizing process, involves excimer laser irradiation of the preplaced aluminium coatings. A computerized 4-axes robotic system was established to permit the treatment of both surfaces of the blade by taking into account the characteristic features of their shapes.

The technique of reverse machining using a continuous wave carbon dioxide (CO2) laser has been proposed and demonstrated for the build up of the abradable seal at the outer end of the turbine blade. Conditions have been established for the building of a narrow ridge approximately 5 mm high which can be machined to dimensions appropriate to a fin.

The capabilities of the different nondestructive testing (NDT) methods in relation to laser surface treatments have been investigated and some of these techniques have been found to be realistic quality monitoring tools. With the use of conventional NDT equipment, various methods (ultrasonic, eddy current and acoustic emission inspections) have shown possibilities as (nearly) online systems where defects will be detected as they as formed or shortly after and recorded for further evaluation.

Exploitation includes: the development processing procedure will be put in production in 1993 by GEC Alsthom on a large steam turbine generator set for last stage titanium blades. Surface treatment of steel components will comprise turbine blade edges, slide plates and valave seats.


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