Abstract:
A laser shock peening method and device for a bottom surface of a tenon groove of an aircraft blade. During the laser shock peening process, according to geometric characteristics of the bottom surface of a tenon groove, a circular facula of a laser beam is changed into a strip-shaped facula, at the same time as a flow-guiding injection device and a water pumping device are respectively arranged at two end surfaces of the bottom surface of the tenon groove to ensure the stability of a water confinement layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/CN2013/085743, filed Oct. 23, 2013, designating the United States of America and published as International Patent Publication WO 2015/027555 A1 on Mar. 5, 2015, which claims the benefit under Article 8 of the Patent Cooperation Treaty and under 35 U.S.C. §119(e) to Chinese Patent Application Serial No. 201310384555.6, filed Aug. 29, 2013, the disclosure of each of which is hereby incorporated herein in its entirety by this reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to the laser processing field, in particular, to a laser shock peening technique for the bottom surface of a tenon groove of an aircraft blade, which can improve the reliability of connection between the aircraft blade and a blade disc and prolong the service life of the aircraft blade. 
       BACKGROUND 
       [0003]    Engine is the power source and also heat of an aircraft. Blades are special parts that are in a large quantity and complex in shape, have high precision criteria, and are difficult to process. They are always the key items in engine manufacturing plants, and are reputed as the “heart of heart.” Aircraft blades are connected to tenon grooves in the blade disc by means of the tenons on the bottom of the blades; therefore, the bottom surface of the tenon groove of the tenons of an aircraft blade is a typical stress-concentrated area. When the engine operates, fatigue cracks may occur and develop on the bottom surface of the tenon groove, resulting in blade looseness, vibration, or unbalance, and thereby causing severely compromised engine safety and reliability, reducing service life, and increasing maintenance cost. Hence, it is of crucial importance to carry out surface peening for the bottom surface of the tenon groove to reduce abrasion and inhibit occurrence and development of fatigue cracks. 
         [0004]    Laser shock peening (LSP), which is also referred to as laser shot blasting, is a new material surface strengthening technique. LSP utilizes the mechanical effect of shock wave induced by strong laser to process the material, and has the features of high pressure, high energy, super-rapid and super-high strain rate, etc. Residual compressive stress layer formed by LSP can effectively eliminate stress concentration in the material and inhibit crack occurrence and development, and thereby significantly improve the fatigue life, corrosion resistance, and wear resistance of metal parts. It has been proven through many studies that LSP is an effective means for extending crack occurrence time, reducing crack development speed, and improving material life. 
         [0005]    However, the bottom of the tenon groove of the tenon of the aircraft blade is very narrow, usually only several millimeters. During a LSP process, the laser beam irradiates the side wall of the bottom of the tenon groove and produces plasma therein first, creating a “plasma shielding” effect, which hinders the laser beam from irradiating to the bottom surface of the tenon groove and affecting the effect of LSP. In addition, owing to the strong wall-attachment effect of water flow, it is difficult to form a uniform and stable water confinement layer on the bottom of the tenon groove, which also affects the effect of LSP. However, the bottom of the tenon groove is a part that is easy to fail, and surface peening is especially desired. 
       BRIEF SUMMARY 
       [0006]    The object of this disclosure is to provide a laser shock peening method and device for the bottom surface of a tenon groove of an aircraft blade, which can avoid the “plasma shielding” phenomenon and create a uniform and stable water confinement layer, and thereby enhance the LSP effect. 
         [0007]    To solve the technical problem described above, according to the geometric characteristics of the bottom of a tenon groove, a light path conversion system is utilized in this disclosure to change a circular facula of a laser beam into a strip-shaped facula with high-power density, so as to carry out laser shock peening for the bottom surface of the tenon groove. In addition, a flow-guiding injection device and a water pumping device are arranged, respectively, on the two end surfaces of the bottom of the tenon groove to create a uniform and stable water confinement layer. The specific technical scheme is as follows: 
         [0008]    A laser shock peening method for the bottom surface of a tenon groove of an aircraft blade, the method comprising: utilizing a light path conversion system to change a laser beam with a circular facula into a laser beam with a strip-shaped facula and high-power density, so as to carry out laser shock peening for the bottom surface of a tenon groove that is covered by an absorbing layer, according to the geometric characteristics of the bottom of the tenon groove; meanwhile, utilizing a flow-guiding injection device and a water pumping device and controlling the water flow parameters at the water inlet end and water outlet end on the bottom of the tenon groove, respectively, to create a uniform and stable water confinement layer having a thickness of 1 mm-1.5 mm. 
         [0009]    The laser beam with a strip-shaped facula has a width of 0.5 mm-1 mm and a length of 7 mm-14 mm, with 5 J-12 J of pulse energy and 10 ns-30 ns of pulse width. 
         [0010]    A device that implements the laser shock peening method for the bottom surface of a tenon groove of an aircraft blade, the device comprising: a laser  10 , a laser control unit  11 , a light path conversion system  13 , a five-axis working platform  19 , a first fixture  23 , a second fixture  22 , a third fixture  21 , a water tank  27 , a flow-guiding injection device  26 , a water supply hose  25 , an injection head  24 , a water pumping header  16 , a water pumping hose  20 , a water pump  17 , and a water trough  18 ; the first fixture  23 , second fixture  22 , and third fixture  21  being mounted on the five-axis working platform  19 , with the second fixture  22  disposed between the first fixture  23  and the third fixture  21 , the injection head  24  being mounted on the top of the first fixture  23 , and connected to the flow-guiding injection device  26  via the water supply hose  25 , the water inlet tube of the flow-guiding injection device  26  being connected to the water tank  27 , the water pumping header  16  being mounted on the top of the third fixture  21 , and being connected to the water pump  17  via the water pumping hose  20 , the water outlet of the water pump  17  being connected to the water trough  18 , the laser  10  being disposed right above the five-axis working platform  19 , and the light path conversion system  13  being disposed between the laser  10  and the five-axis working platform  19 . 
         [0011]    The flow-guiding injection device  26  is designed to control the pressure and flow of the water stream, so that the water pressure is controlled within a range of 0.1 MPa and ˜0.3 MPa, and the inlet flow rate is controlled within a range of 0.8×10 −5  m 3 /s and ˜2.0×10 −5  m 3 /s. 
         [0012]    The outlet flow rate of the water pump is within a range of 0.8×10 −5  m 3 /s and ˜2.0×10 −5  m 3 /s, so that the water inflow rate is equal to the water outflow rate in the water confinement layer on the bottom of the tenon groove, to ensure a uniform water film. 
         [0013]    The injection head  24  is flat-shaped, and the water outlet in the end surface of the injection head  24  has a width of 1.5 mm and a length of 10 mm. 
         [0014]    The water pumping header  16  is flat-shaped, and the water outlet in the end surface of the water pumping header  16  has a width of 1.5 mm and a length of 10 mm. 
         [0015]    The device can be used through the following steps:
       Apply a black paint or aluminum foil absorbing layer on the bottom surface of a tenon groove of a blade, and then mount the blade onto the second fixture  22  and keep the bottom surface of the tenon groove in level state;   Mount the injection head  24  onto the top of the first fixture  23  in a way that the end surface of water outlet of the injection head  24  closely abuts one end surface of the bottom of the tenon groove, and the bottom surface of the water outlet of the injection head  24  is in the same plane as the bottom surface of the tenon groove, and the two sides of the water outlet are at the outer side of the side surfaces of the bottom of tenon groove;   Connect the injection head  24  and flow-guiding injection device  26  with the water supply hose  25 , and then connect the water inlet tube of the flow-guiding injection device  26  to the water tank  27 ;   Mount the water pumping header  16  onto the top of the third fixture  21  in a way that the end surface of water inlet of the water pumping header  16  closely abuts the other end surface of the bottom of the tenon groove, the bottom surface of the water inlet of the water pumping header  16  is in the same plane as the bottom surface of the tenon groove, and the two sides of the water inlet are at the outer side of the side surfaces of the bottom of the tenon groove;   Connect the water pumping header  16  and water pump  17  with the water pumping hose  20 , and then connect the water outlet tube of the water pump  17  to the water trough  18 ;   Set the facula diameter, pulse energy, and pulse width of the laser by the laser control unit  11 ;   Change the laser beam with a circular facula into a laser beam with a strip-shaped facula and high-power density via the light path conversion system  13 , and ensure the laser beam with a strip-shaped facula is perpendicular to the horizontal plane and the focal spot of the laser beam is on the bottom surface of the tenon groove;   Adjust the parameters of the flow-guiding injection device  26  and water pump  17 , to form a uniform and stable water confinement layer with appropriate thickness on the bottom of the tenon groove;   Switch on the laser and start laser shock peening for the bottom surface of the tenon groove, and complete the peening for the entire bottom surface of the tenon groove by translating the working platform.       
 
         [0025]    The technical innovation of this disclosure lies in the geometric characteristics of the bottom of a tenon groove, a light path conversion system  13  utilized to change the circular facula of a laser beam into a strip-shaped facula with high-power density, so as to carry out laser shock peening for the bottom surface of the tenon groove, avoid a “plasma shielding” phenomenon, and at the same time, the processing efficiency is ensured; a flow-guiding injection device  26  and a water pumping device are utilized, and the flow parameters at the water inlet and water outlet on the bottom of the tenon groove are controlled, to ensure a uniform and stable water confinement layer on the bottom of the tenon groove. 
         [0026]    This disclosure has the following beneficial effects: by means of laser shock peening for the bottom surface of a tenon groove of an aircraft blade tenon, the occurrence and development of fatigue cracks on the bottom of the tenon groove can be inhibited, the occurrence of phenomena such as blade looseness, vibration, or unbalance, etc., can be avoided, and the engine safety and reliability can be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a schematic diagram of an aircraft blade; 
           [0028]      FIG. 2  is a schematic diagram of “plasma shielding;” 
           [0029]      FIG. 3  is a schematic diagram of laser shock peening for the bottom surface of a tenon groove of an aircraft blade; 
           [0030]      FIG. 4  is a schematic diagram of the end surface of water outlet of an injection head; 
           [0031]      FIG. 5  is a schematic diagram of the end surface of water inlet of a water pumping header; 
           [0032]      FIG. 6  is a schematic diagram of the processing path of laser shock peening; and 
           [0033]      FIG. 7  is a schematic diagram of sampling points for residual stress testing of the bottom surface of a tenon groove. 
       
    
    
       [0034]    Among the figures:  1 —blade,  2 —bottom surface of tenon groove,  3 —tenon,  4 —plasma,  5 —laser,  11 —laser control unit,  12 —laser beam with a circular facula,  13 —light path conversion system,  14 —laser beam with a strip-shaped facula,  15 —blade,  16 —water pumping header,  17 —water pump,  18 —water trough,  19 —five-axis working platform,  20 —water pumping hose,  21 —third fixture,  22 —second fixture,  23 —first fixture,  24 —injection head,  25 —water supply hose,  26 —flow-guiding injection device,  27 —water tank,  30 —end surface of water outlet of injection head,  31 —end surface of water inlet of water pumping header,  32 —processing path,  41 —point A,  42 —point B,  43 —point C,  44 —point D,  45 —point E,  46 —point F; 
         [0035]    The height H 1  of the water outlet is 1.5 mm, and the length L 1  is 10 mm. 
         [0036]    The height H 2  of the water outlet is 1.5 mm, and the length L 2  is 10 mm. 
       DETAILED DESCRIPTION 
       [0037]    Hereunder, this disclosure will be further detailed with reference to the accompanying drawings. 
       Embodiment 1 
       [0038]    During the surface peening for the bottom surface of the tenon groove of the aircraft blade  1  shown in  FIG. 1 , a laser shock peening device for the bottom surface of a tenon groove of an aircraft blade shown in  FIG. 3  is utilized to avoid the “plasma shielding” effect shown in  FIG. 2 . The device comprises: a laser  10 , a laser control unit  11 , a light path conversion system  13 , a five-axis working platform  19 , a first fixture  23 , a second fixture  22 , a third fixture  21 , a water tank  27 , a flow-guiding injection device  26 , a water supply hose  25 , an injection head  24 , a water pumping header  16 , a water pumping hose  20 , a water pump  17 , and a water trough  18 , wherein, the end surface of water outlet of injection head  24  and the end surface of water inlet of water pumping header  16  are shown in  FIGS. 4 and 5 , respectively. 
         [0039]    The first fixture  23 , second fixture  22 , and third fixture  21  are mounted on the five-axis working platform  19 , and the second fixture  22  is disposed between the first fixture  23  and the third fixture  21 , the injection head  24  is mounted on the top of the first fixture  23 , and is connected to the flow-guiding injection device  26  via the water supply hose  25 , and the water inlet tube of the flow-guiding injection device  26  is connected to the water tank  27 , the water pumping header  16  is mounted on the top of the third fixture  21 , and is connected to the water pump  17  via the water pumping hose  20 , and the water outlet of the water pump  17  is connected to the water trough  18 , the laser  10  is disposed right above the five-axis working platform  19 , and the light path conversion system  13  is disposed between the laser  10  and the five-axis working platform  19 . 
       Embodiment 2 
       [0040]    The device can be used through the following steps:
       Apply a black paint or absorbing layer on the bottom surface  2  of a tenon groove of a blade, and then mount the blade  15  onto the second fixture  22  and keep the bottom surface  2  of the tenon groove in level state;   Mount the injection head  24  onto the top of the first fixture  23  in a way that the end surface  30  of water outlet of the injection head  24  closely abuts one end surface of the bottom of the tenon groove, and the bottom surface of the water outlet of the injection head  24  is in the same plane as the bottom surface  2  of the tenon groove, and the two sides of the water outlet are at the outer side of the side surfaces of the bottom of the tenon groove;   Connect the injection head  24  and flow-guiding injection device  26  with the water supply hose  25 , and then connect the water inlet tube of the flow-guiding injection device  26  to the water tank  27 ;   Mount the water pumping header  16  onto the top of the third fixture  21  in a way that the end surface  31  of water inlet of the water pumping header  16  closely abuts the other end surface of the bottom of the tenon groove, the bottom surface of water inlet of the water pumping header  16  is in the same plane as the bottom surface  2  of the tenon groove, and the two sides of the water inlet are at the outer side of the side surfaces of the bottom of the tenon groove;   Connect the water pumping header  16  and water pump  17  with the water pumping hose  20 , and then connect the water outlet tube of the water pump  17  to the water trough  18 ;   Set the facula diameter to 3 mm, pulse energy to 5 J, and pulse width to  10  ns for the laser with the laser control unit;   Change the laser beam  12  with a circular facula into a laser beam  14  with high-power density and a strip-shaped facula having a width of 0.5 mm and a length of 14 mm via the light path conversion system  13 , and ensure the laser beam  14  with a strip-shaped facula is perpendicular to the horizontal plane and the focal spot of the laser beam  14  with a strip-shaped facula is on the bottom surface  2  of the tenon groove;   Adjust the parameters of the flow-guiding injection device  26  and water pump  17 , so that the water pressure is 0.1 MPa and both of the flow rates are 0.8×10 −5  m3/s, to create a uniform and stable water confinement layer having a thickness of 1 mm-2 mm on the bottom of the tenon groove;   Switch on the laser  10  and start laser shock peening for the bottom surface  2  of the tenon groove, and complete the peening for the entire bottom surface  2  of the tenon groove by translating the five-axis working platform  19  along the processing path  32  shown in  FIG. 6 ;   After the processing, carry out a residual stress test at the points labeled in  FIG. 7 , to evaluate the effect of laser shock peening for the bottom surface of the tenon groove of the aircraft blade.       
 
       Embodiment 3 
       [0051]    Use the same method and steps as in Embodiment 2, but change the pulse energy to 6 J, pulse width to 20 ns, width of strip-shaped facula to 1 mm, length of strip-shaped facula to 7 mm, water pressure in the flow-guiding injection device  26  and water pump to 
       Embodiment 4 
       [0052]    Use the same method and steps as in Embodiment 2, but change the pulse energy to 12 J, pulse width to 30 ns, width of strip-shaped facula to 2 mm, length of strip-shaped facula to 3.5 mm, water pressure in the flow-guiding injection device  26  and water pump to 0.3 MPa, and flow rates to 2.0×10 −5  m 3 /s. As can be seen from Table 1, the method disclosed herein can effectively introduce 200 MPa or higher residual compressive stress into the bottom surface of the tenon groove of the aircraft blade, and thereby improve the service life of the aircraft blade. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Result of Residual Stress Test of the Bottom Surface of the Tenon 
               
               
                 Groove of the Aircraft Blade in the Embodiments 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Sample 
                 Residual Stress before Laser Shock Peening (MPa) 
               
             
          
           
               
                 (Material TC4) 
                 Point A 
                 Point B 
                 Point C 
                 Point D 
                 Point E 
                 Point F 
               
               
                   
               
               
                 Embodiment 2 
                 10 
                 8 
                 9 
                 4 
                 3 
                 5 
               
               
                 Embodiment 3 
                 6 
                 9 
                 6 
                 2 
                 7 
                 8 
               
               
                 Embodiment 4 
                 8 
                 5 
                 7 
                 6 
                 4 
                 2 
               
               
                   
               
             
          
           
               
                 Sample 
                 Residual Stress after Laser Shock Peening (MPa) 
               
             
          
           
               
                 (Material TC4) 
                 Point A 
                 Point B 
                 Point C 
                 Point D 
                 Point E 
                 Point F 
               
               
                   
               
               
                 Embodiment 2 
                 −286 
                 −267 
                 −278 
                 −304 
                 −312 
                 −320 
               
               
                 Embodiment 3 
                 −346 
                 −352 
                 −349 
                 −397 
                 −406 
                 −414 
               
               
                 Embodiment 4 
                 −432 
                 −443 
                 −436 
                 −482 
                 −495 
                 −510 
               
               
                   
               
             
          
         
       
     
         [0053]    In Table 1: positive values represent tensile stress, while negative values represent compression stress.