Patent Application
20080078477
Illustrated in
It is well known to use laser shock peening to counter possible fatigue failure of portions of an article. The airfoil 34, for instance, is subject to a significant tensile stress field due to centrifugal forces generated by the blade 8 rotating during engine operation. The airfoil 34 is also subject to vibrations generated during engine operation and nicks 52 and tears operate as high cycle fatigue stress risers producing additional stress concentrations around them. Typically, laser shock peening surfaces 54 on one or both sides of the article such as the blade 8 are laser shock peened producing laser shock peened patches or laser shock peening surfaces 54 and pre-stressed regions 56 having deep compressive residual stresses imparted by a laser shock peening (LSP) method extending into the article from the laser shock peened surfaces 55.
The laser shock peened surfaces 55 may extend all the way along the leading edge LE from the airfoil base 28 to the airfoil tip 38 and may also be along the trailing edge TE or along the airfoil tip 38. The laser shock peened surfaces 55 may also extend over the entire airfoil 34 on the pressure and suction sides 46 and 48, respectively. The leading edge LE, the trailing edge TE, and the airfoil tip 38 are all sections of the airfoil that may be very thin and subject to delamination due to the laser shock peening. Thin gas turbine engine airfoils for which laser shock peening may be used includes those found in stator vanes and rotor blades of fans, compressors and turbines in the engine. These are examples of thin articles or articles having thin sections which may be laser shock peened and be subject to delamination due to laser shock peening.
In order to avoid or reduce delamination, a variable surface fluence laser shock peening method for laser shock peening a thin article with varying thickness T was developed which varies a surface fluence f of the laser beam 2 over the laser shock peening surface 54 as a function F of the thickness T of the article beneath a laser shock peened spot 58 formed by a laser beam 2 on the laser shock peening surface 54. There are many ways to vary the surface fluence f of the laser beam 2. The strength of the beam 2 may be increased or decreased and the laser shock peened spot 58, the area the laser beam forms on the laser shock peening surface 54, is held fixed. Alternatively, the area of the laser shock peened spot 58 may be increased or decreased and the strength of the laser beam 2 is held fixed or constant.
An example of an article with varying thickness is a compressor blade airfoil as illustrated in
The airfoil is thinnest at the leading and trailing edges LE and TE and gradually becomes thicker in leading and trailing edge sections 51 and 53 away from the leading and trailing edges LE and TE as illustrated further in
The method provides lower surface fluence or laser energy at the leading edge which is the thinnest portion of the airfoil and higher surface fluences or laser energies aft of the leading edge. The exemplary method illustrated herein employs a change in the energy of the laser beam 2 to produce changes in the surface fluence in the laser shock peening process. Illustrated schematically in
The airfoil 34 illustrated in
The exemplary incremental varying of the surface fluences illustrated in
Chart 2 below illustrates individually adjusted or varied laser energies, as opposed to incrementally adjusted or varied laser energies, that could be used to laser shock peen at the first through ninth positions P1-P9 respectively in each of the four rows in the laser shock peened leading edge section 51. Individually adjusted or varied surface fluences or laser energies more closely maintains the volumetric fluence factor at about 1500 J/cm3 in the laser shock peened leading edge section 51. More than the first through ninth positions P1-P9 are laser shock peened as indicated in
Chart 2 Local laser Energy, based on fixed spot diameter and constant Volumetric Fluence (1500 J/cm3)
The laser energies in Chart 2 are exemplary values and illustrate a range of individually adjusted or varied laser energies required to maintain a constant Volumetric Fluence (1500 J/cm3). A more detailed illustration of the individually varied laser energies is illustrated for the airfoil 34 in
Illustrated in
A clear confining medium 68 to cover the laser shock peening surface 54 is provided by a curtain of clear fluid such as water 21 supplied by a water nozzle 20 at the end of a water supply tube 19. The curtain of flowing water 21 is particular to the exemplary embodiment illustrated herein, however, other types of confining mediums may be used. The laser shock peening system 10 illustrated herein includes a laser beam apparatus including a generator 31 having an oscillator 33 and a pre-amplifier 47 and a beam splitter 43 which feeds the pre-amplified laser beam into two beam optical transmission circuits 100 each having a first and second amplifier 39 and 41, respectively, and optics 35 which include optical elements that transmit and focus the laser beam 2 on the laser shock peening surface 54. A laser controller 24 is used to modulate and fire the laser beam apparatus to fire the laser beam 2 on the bare laser shock peening surface 54 in a controlled manner. The CNC controller 128 usually is used to control the operation of the laser controller 24 particularly as to when to fire the laser beams 2.
The laser beam shock induced deep compressive residual stresses in the compressive pre-stressed regions 56 are generally about 50-150 KPSI (Kilo Pounds per Square Inch) and extend to a depth of about 20-50 mils into the airfoil 34. The laser beam shock induced deep compressive residual stresses are produced by repetitively firing a high energy laser beam 2 that is defocused±a few mils with respect to the laser shock peening surface 54. The laser beam 2 typically has a peak power density on the order of magnitude of a gigawatt/cm2 and is fired with a curtain of flowing water 21 or other fluid that is flowed over the laser shock peening surface 54 or some other clear confining medium. The laser shock peened surfaces 55 may be bare or as illustrated herein may be coated with an ablative coating 59 such as paint or adhesive tape to form coated surfaces as disclosed in U.S. Pat. Nos. 5,674,329 and 5,674,328. The coating 59 provides an ablative medium over which the clear containment medium is placed, such as a fluid curtain such as a curtain of flowing water 21. During laser shock peening, the blade 8 is moved while the stationary laser beams 2 are fired through curtains of flowing water 21, dispensed by water nozzles 20, on the laser shock peened surfaces 55. The laser shock peening process is typically used to form overlapping laser shock peened circular spots 58 on laser shock peened surfaces 55.
The coating or bare metal surface 14 is ablated generating plasma which results in shock waves on the surface of the material. These shock waves are redirected towards the laser shock peening surface 54 by the clear liquid confining medium 68, illustrated herein as the curtain of flowing water 21, or confining layer to generate travelling shock waves (pressure waves) in the material below the laser shock peening surface 54. The amplitude and quantity of these shockwave determine the depth and intensity of compressive stresses. The shockwaves and the laser beam shock induced deep compressive residual stresses may cause delamination in the thin leading and trailing edge regions 51 and 53. The exemplary variable surface fluence laser shock peening method illustrated herein simultaneously laser shock peens opposite sides of the article illustrated by the pressure and suction sides 46 and 48. This method is also referred to as dual sided laser shock peening. Other embodiments of the variable surface fluence laser shock peening method can be used to laser shock peen just one side of an airfoil or other part or article.
In order to reduce or prevent the delamination, the airfoil 34 which generally represents an article with a varying thickness T is laser shock peened along the laser shock peening surface 54 using a laser beam 2. The exemplary embodiment of the variable surface fluence laser shock peening method, illustrated in
The exemplary embodiment of the variable surface fluence laser shock peening method illustrated herein uses the CNC controller 128 to send instructions to the laser controller 24 to modulate the energy of the laser beams 2 to vary the surface fluence. The surface fluence f is equal to the thickness T multiplied by a volumetric fluence factor VF and the volumetric fluence factor is held constant over the laser shock peening surface 54. The volumetric fluence factor VF is in a range of about 1200 J/cm3 to about 1800 J/cm3 and a particularly useful value of the volumetric fluence factor is about 1500 J/cm3 for thin gas turbine airfoils made of a Titanium alloy. A device external to the laser generating apparatus described above may also be used to change the energy of the laser beam 2. One such device is an optical attenuator.
The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. While there have been described herein, what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.
