[0001] The present subject matter described herein, relates to a laser shock peening process for inducing compressive residual stress in gas turbine bucket root. More particularly, the present invention relates to introducing compressive residual stress in Inconel 738 frame 5 stage 3 gas turbine bucket root.
BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention.
[0003] In general Gas Turbine (GT) components are operated at high speeds and high temperatures. At this rated speeds the last stage frame 5 and Frame 6 buckets on gas turbine shaft undergo high vibrations. Due to which there is a chance of failure in these GT blades which may cause catastrophic damage to casing and surroundings. In order to extend the life of the GT bucket, widely conventional shot peening is being used, which bombards the sample surface with multiple spherical shots to generate compressive residual stress. However, shot peening induces compressive residual stress to a depth of 300 microns and increases surface roughness.
[0004] Laser shock peening (LSP) is an advanced surface treatment process in which laser energy is used to introduce compressive residual stresses in the metals. Depth of compressive residual stresses induced using LSP is much higher than the normal shot peening which helps to increases the life of component further. One more advantage with LSP is that it doesn’t alter the surface finish of parent metal.

PRIOR ART IN THIS FIELD
[0005] Conventional shot peening is being used for improving fatigue life properties of the various components which are operated at high speeds by inducing compressive residual stress. Shot peening induces compressive residual stress to a certain depth only and alters surface finish. Due to this limitation, after certain cycles of operation, components are prone to fatigue failure at root portion. It results in shutdown of power station which results in huge revenue loss.
[0006] W.r.t US patent reference No: US5591009A, laser shock peening is being used on gas turbine engine blades particularly on leading edge. W.r.t US patent reference No: US6551064B1, Laser shock peening is being done on intermetallic parts of gas turbine engine. W.r.t. US patent No: US5675892A, Laser shock peening is being used for gas turbine engine vane repair after gap brazing. In addition to above laser peening is being used for other components made of material like stainless steel, titanium alloys, aluminum alloys, etc. Apart from above processes no attempts were made on gas turbine root to improve surface properties by laser peening method. And there is no procedure to carryout laser peening process on gas turbine root. This has resulted in this invention.

OBJECTS OF THE DISCLOSURE
[0007] Some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed herein below.
[0008] The principal object of the present subject matter is to provide a laser shock peening process to induce deeper compressive residual stress in gas turbine root.
[0009] Another object of the present subject matter is to provide a gas turbine components operating at high speeds and high temperatures are prone to fatigue failure or stress corrosion cracking
[0010] Another object of the present subject matter is to provide a shot peening is being used to improve fatigue life of the gas turbine bucket by inducing compressive residual stress in gas turbine bucket root
[0011] Another object of the present subject matter is to provide a laser peening can induce compressive residual stress much deeper in comparison with shot peening
[0012] These and other objects and advantages will become more apparent when reference is made to the following description and accompanying drawings.
SUMMARY
[0013] This summary is provided to introduce concepts related to a laser shock peening process for inducing compressive residual stress in gas turbine bucket root.
[0014] In this invention LSP samples were prepared using wire cutting method from GT Buckets. Sample material was IN738 and dimensions were 25mm X 25 mm X 6mm. These samples were stress relived at 6000C for 10hrs followed by slow cooling under inert atmosphere using tubular furnace. Robot programming was made using KUKA robot to inline laser spot with sample. Laser peening trials were carried by varying different parameters like energy, frequency, water flow, etc. These samples were characterized for surface roughness and residual stress measurement. GT bucket holding fixture was designed and fabricated. Robot programming was made to inline spot with GT Bucket root. Laser peening was carried out on GT bucket root at optimized laser peening parameters.
[0015] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The illustrated embodiments of the subject matter will be understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
[0017] Fig. 1a and 1b illustrate holding fixture (a) CAD model (b) holding GT bucket, in accordance with an embodiment of the present subject matter;
[0018] Fig. 2 illustrates residual stress measurement, in accordance with an embodiment of the present subject matter;
[0019] Fig. 3 illustrates a laser peening process for inducing compressive residual stress in gas turbine bucket root
[0020] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
[0021] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure as defined by the appended claims.
[0022] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0023] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a",” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0024] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0025] In addition, the descriptions of "first", "second", “third”, and the like in the present invention are used for the purpose of description only, and are not to be construed as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Thus, features defining "first" and "second" may include at least one of the features, either explicitly or implicitly.
[0026] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0027] Sample preparation
[0028] In this invention LSP samples were prepared using wire cutting method from GT Buckets. Sample material was IN738 and dimensions were 25mm X 25 mm X 6mm. These samples surface were smoothened by using emery paper with 150-1000 grit size followed by cloth polishing with 3µ diamond paste. Subsequently these samples were stress relived under at 6000C for 10hrs under inert atmosphere followed by slow cooling in tubular furnace. After cooling, sample surface was covered with ablative layer (black tape of 100 microns thick) which creates localized plasma by vaporizing the ablative layer.
[0029] Laser peening procedure
[0030] Sample was mounted in holding fixture to a KUKA robot. A laminar water curtain was maintained at the spot position which helps to travel shock wave towards the sample surface. Subsequently robot programming was made to inline laser spot with the sample surface. Laser peening was carried out by using Ekspla diode laser system and parameters are as shown in Table1.

Laser Energy Spot size Frequency Overlap Period
0.5J – 3J 2mm X 2mm 5Hz 20% 10ns pulsed
Table 1: Laser peening parameters.
[0031] Fig. 1a and 1b illustrate holding fixture (a) CAD model (b) holding GT bucket according to some example embodiment of the invention. In an embodiment, holding fixture for GT bucket and a laser peening comprising a suitable fixture was designed and subsequently fabricated in order to hold the GT bucket and fixing on to the robot tools. A CAD model sketch as designed in NX software is shown in figure 1(a) and fabricated fixture is shown in figure 1(b). Subsequently robot (KUKA Robot) programming was made for continuous laser peening on the blade root. Subsequently the robot programming and laser peening was done at the root of the blade to improve the fatigue properties of the GT Bucket. Fluorescent Dye Penetration (FDP) test was carried out before and after the laser peening and no surface cracks was observed. The benefit of the laser peening process is that the process takes nearly 10 minutes time to complete the whole bucket whereas the shot peening process takes about 30 minutes time for complete shot peening of the bucket.
[0032] Surface roughness Measurement
[0033] Surface roughness was measured using MarSurf M300 instrument and values are shown in the Table 2. From Table 2 it can be observed that change in surface roughness of laser peened samples is very low in comparison with shot peened samples.

Parameters Unpeened Shot peened Laser peened at 2J energy
Surface roughness
(Ra in µm) 0.162 1.705 0.166
Table 2: Surface roughness measurement.
[0034] Fig. 2 illustrates residual stress measurement according to some example embodiment of the invention. In an embodiment, the residual stress is measured by Empyrean X-Ray Diffraction System available at SCT department using cradle sample stage, omega stress, incident mask 2mm, anti-scatter slit 10 and nickel Beta filter. The residual stress measurement was done at 139.10 2theta angle. Electro Polishing was done after every measurement of residual stress for depth measurement. Electro polishing helps to remove the metal layer by layer to check the depth of penetration of residual stress. In this IN738 metal around 50 microns layer was removed and residual stress measurement was carried out up to a depth of thickness 800 microns. Residual stress was also measured on shot peened sample (IN738). Test results are of laser peening and shot peening is shown in figure 2.
[0035] From figure 2 it can be observed that sample laser peened at 2J energy is having compressive residual stress to a depth of 700 microns and beyond 700 microns stresses are tensile in nature. In comparison to laser peened samples, shot peened sample (using standard parameters) depth of penetration was lower and it was up to a depth of 300 microns. Laser peened at 0.5J energy showed the depth of penetration to a level of 350 microns. Hence laser peening done at 2J energy is preferred due to high depth of compressive residual stress which are beneficial for the improvement of fatigue life of GT bucket.
[0036] Fig. 3 illustrates a laser peening process for inducing compressive residual stress in gas turbine bucket root according to some example embodiment of the invention. In an embodiment, the process comprising carrying out a laser peening on Inconel 738 metal at 2J energy, spot size of 2mm X 2mm, at 5 Hz frequency and 20% overlap. Providing a surface preparation and a stress relieving of samples at 6000C for 10hrs under inert atmosphere. Applying an ablative layer on sample surface and maintaining a laminar water flow at spot position. Mounting of sample to a robot and a robot programming to an inline laser spot with sample surface. Adjusting the laser peening parameters to desire values in a computer controlled laser peening system and the laser peening on sample surface. Inducing compressive residual stress to a depth of 700 micros. Changing in surface roughens is very minimal in the range of 2%-3%. Completing the laser peening on a GT bucket root within 10 minutes’ lesser time than the conventional shot peening.
[0037] Conclusions:
[0038] Based on above results conclusions as follows.
• The laser peening of IN738 metal is beneficial.
• In this project laser preening at 2J energy, with 2 mm spot diameter and 5 Hz frequency has shown better results when compared to shot peening using standard parameters.
• Laser peening induces compressive residual stress to a depth of 700 µm whereas shot peening induces to a depth of 300 microns.
• Average surface roughness of unpeened sample is 0.162 microns, laser peened sample is 0.166 microns and shot peened sample is 1.705 microns. Thus laser peened sample has a better surface finish compared to others.
• Laser peening take nearly 10 minutes time to complete the whole bucket whereas the shot peening process takes about 30 minutes time thus there is time saving.
[0039] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances, where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
[0040] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims:We claim:
1. A laser peening process for inducing compressive residual stress in gas turbine bucket root, the process comprising:
carrying out a laser peening on Inconel 738 metal at 2J energy, spot size of 2mm X 2mm, at 5 Hz frequency and 20% overlap;
providing a surface preparation and a stress relieving of samples at 6000C for 10hrs under inert atmosphere;
applying an ablative layer on sample surface and maintaining a laminar water flow at spot position;
mounting of sample to a robot and a robot programming to an inline laser spot with sample surface; and
adjusting the laser peening parameters to desire values in a computer controlled laser peening system and the laser peening on sample surface.

2. The process as claimed in claim 1, wherein inducing compressive residual stress to a depth of 700 micros.

3. The process as claimed in claim 1, wherein changing in surface roughens is very minimal in the range of 2%-3%.

4. The process as claimed in claim 1, wherein completing the laser peening on a GT bucket root within 10 minutes’ lesser time than the conventional shot peening.

Dated this 5th day of September, 2020

We claim:
1. A laser peening process for inducing compressive residual stress in gas turbine bucket root, the process comprising:
carrying out a laser peening on Inconel 738 metal at 2J energy, spot size of 2mm X 2mm, at 5 Hz frequency and 20% overlap;
providing a surface preparation and a stress relieving of samples at 6000C for 10hrs under inert atmosphere;
applying an ablative layer on sample surface and maintaining a laminar water flow at spot position;
mounting of sample to a robot and a robot programming to an inline laser spot with sample surface; and
adjusting the laser peening parameters to desire values in a computer controlled laser peening system and the laser peening on sample surface.

2. The process as claimed in claim 1, wherein inducing compressive residual stress to a depth of 700 micros.

3. The process as claimed in claim 1, wherein changing in surface roughens is very minimal in the range of 2%-3%.

4. The process as claimed in claim 1, wherein completing the laser peening on a GT bucket root within 10 minutes’ lesser time than the conventional shot peening.