Abstract:
A method of monitoring laser shock peening of a material includes forming an ablative layer on the material, directing the laser beam at the ablative layer to produce an acoustic wave in the material, converting the acoustic wave in the material to thermal energy external to the material and measuring the thermal energy.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to treating metal and, in particular, to monitoring laser metal treatments. 
         [0002]    Laser peening, or laser shock peening (LSP), is a process of inducing beneficial residual compressive stresses into a material (usually metal) by using a powerful laser. As used herein, a material being processed shall be referred to as a “treated material.” An ablative coating, usually black tape or paint, is applied to the treated material to absorb the energy from a laser. Short energy pulses from the laser are then focused to explode the ablative coating, producing a shock wave. The process may be repeated in multiple locations. A translucent layer, usually consisting of water, is required over the coating and acts as a tamp, directing the shock wave into the treated material. 
         [0003]    A piezoelectric sensor is normally used for real time (on-line) monitoring of LSP processing. The piezoelectric sensor converts the stress (acoustic) waves created by LSP into an electric signal proportional to the strength of the wave. The electric signal may then be used to monitor the LSP process. The piezoelectric sensor, however, is often destroyed because it is in direct contact with the treated material. The one-use destruction of these gauges requires the use of multiple gauges for wherever multiple laser pulses are needed. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to one aspect of the invention, a laser shock peening (LSP) measurement device for measuring energy provided to a treated material during an LSP process is provided. The device of this embodiment includes an energy converter configured to receive acoustic energy from the treated material and to convert the acoustic energy to thermal energy. The device of this embodiment also includes an energy measurement device coupled to the energy converter that produces an electrical output based on the thermal energy proportional to the acoustic energy. 
         [0005]    According to another aspect of the invention, a method of monitoring laser shock peening of a material is provided. The method includes forming an ablative layer on the material, directing the laser beam at the ablative layer to produce an acoustic wave in the material, converting the acoustic wave in the material to thermal energy external to the material and measuring the thermal energy. 
         [0006]    According to yet another aspect of the invention, a turbine is provided. The turbine of this embodiment is prepared by a process comprising: forming an ablative layer on a blade of the turbine; directing the laser beam at the ablative layer to produce an acoustic wave in the material; converting the acoustic wave in the material to thermal energy external to the material; and measuring the thermal energy. 
         [0007]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  shows an LSP system according to one embodiment of the present invention; and 
           [0010]      FIG. 2  shows a detailed example of an energy converter according to one embodiment of the present invention. 
       
    
    
       [0011]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    As described above, utilizing piezoelectric sensors in LSP processing requires the use of multiple sensors because the sensors can be destroyed by one use. In addition, monitoring the LSP process by measuring the acoustic waves directly results in operator discomfort due to the high sound levels produced. Indeed, some LSP processes create sound levels ranging from 100 to 140 dB. 
         [0013]    Embodiments of the present invention may convert the acoustic waves to another form of energy to reduce the sound level associated with LSP processing. In addition, conversion of the waves to another form may increase the lifetime of sensors used to monitor LSP processes. 
         [0014]    Embodiments of the present invention may allow for sensors utilized in online LSP monitoring to be reused because they are not in direct contact with material being processed. In one embodiment, this may be accomplished by modifying the measured energy from acoustic to thermal through the use of magnetic fields and an electrically conductive medium. In particular, the acoustic wave is transmitted away from the treated material by an acoustic coupler having the same or similar acoustic impedance as the treated material. This acoustic wave is then converted to heat. The conversion may occur, for example, in an electrically conductive medium. In one embodiment, coils that generate a magnetic field may surround the medium. 
         [0015]    The electrically conductive medium and the coils may be enclosed inside a housing or container made of a material having a different acoustic impedance than the treated material in order to prevent acoustic energy loss due to transmission to the outside environment. In one embodiment, the temperature rise of the medium inside the container heats a thermally conductivity plate that contacts the container. The resultant temperature increase of the thermal plate is converted into an electric voltage by a sensor. 
         [0016]    In an alternative embodiment, the thermally conductive plate may be omitted and temperature changes in the medium may be directly measured using techniques known in the art. For instance, temperature changes may be measured using an IR detector or a radiometer. 
         [0017]      FIG. 1  shows an LSP system  100  according to one embodiment of the present invention. The LSP system  100  includes a laser  102  and a treated material  104 . In one embodiment, the treated material  104  is a metal. In a specific embodiment, the treated material may be metal the forms the blades of turbine. Accordingly, the system  100  may be used to create turbines. Of course, the system  100  may be utilized in other contexts as well. 
         [0018]    The treated material  104  may include an ablative layer  106  affixed to a first side  105  thereof. The ablative layer  106  may be formed of a black tape or paint and is applied to absorb the energy imparted by the laser  102 . On a second side  107  the treated material  104  may include an acoustic coupler  108  in intimate contact therewith. The acoustic coupler  108  may have the same or substantially the same acoustic impedance as the treated material  104 . 
         [0019]    In operation, the laser  102  is directed at the ablative material  106 . Short energy pulses are then focused to explode the ablative material  106 , producing a shock waves  110 . The shock waves  110  may have detrimental effects if allowed to reflect inside the treated material  104 . Reflected waves are indicated by reference numeral  112 . The reflected waves  112  can propagate cracks due to their tensile nature and reduce the life of the treated material  104 . 
         [0020]    It has been discovered that placing the acoustic coupler  108  in intimate contact with the treated material  104  may reduce or eliminate reflected waves  112 . In particular, use of an acoustic coupler  108  transmits the shock waves out of the treated material  104 . The waves transmitted out of the treated material  104  may be used for online monitoring of LSP processes. 
         [0021]    One embodiment of the present invention includes an energy converter  114  coupled to the acoustic coupler  108 . The energy converter  114  may convert the acoustic shock waves  110  to another form of energy. In one embodiment, the energy converter  114  converts the acoustic waves into thermal energy. 
         [0022]    An energy measurement device  116  may measure the converted energy output by the energy converter  114 . In one embodiment, the energy measurement device  116  may include a thermally conductivity plate that contacts the energy converter  114 . The resultant temperature increase of the thermal plate is converted into an electric signal (current or voltage) by a sensor forming part of the energy measurement device  116 . Such an energy measurement device  116  (LSP sensor) may be utilized more than once because it is not destroyed by direct contact with a shock wave as in the prior art. In an alternative embodiment, the energy measurement device  116  may measure temperature changes in the electrically conductive material directly. For instance, an energy measurement device  116  implemented as an IR detector or a radiometer may directly measure temperature changes. 
         [0023]      FIG. 2  shows a more detailed example of an energy converter  114  according to one embodiment of the present invention. The energy converter  114  is coupled between an acoustic coupler  108  and an energy measurement device  116 . The energy converter  114  converts acoustic energy (waves) to thermal energy in one embodiment. 
         [0024]    The acoustic coupler  108  may include a plunger  202 . The plunger  202  directs and transmits the shock wave  110  to the energy converter  114 . In particular, the plunger  202  directs and transmits the shock waves  110  to a piston  204  of the energy converter  114 . 
         [0025]    When an electric conductor is made to move in a magnetic field, an electromotive force is produced. The electric conductor can be replaced by any electrically conducting medium (such as a fluid or a gas) that is a good conductor of electricity. The medium can be made electrically conducting by ionization or by other means known in the art. 
         [0026]    In one embodiment, the energy converter  114  includes an electrically conducting medium  206  disposed within a container  208 . The electrically conducting medium  206  may be set into motion by shock waves  110  being transmitted to piston  204 . 
         [0027]    In more detail, the shock waves  110  generated during LSP may cause motion in the electrically conducting medium  206  after traversing through the treated material and the acoustic coupler  108 . In the event the acoustic coupler  108  is formed of a material with the same acoustic impedance as the treated material, the transmission ratio may be increased. Forming the acoustic coupler  108  such that it includes the plunger  202  may focus the shock wave. 
         [0028]    In one embodiment, the acoustic coupler  108  generally, and plunger  202  in particular, may be coupled to piston  204 . In one embodiment, the piston  204  may be made of acoustically similar material to the acoustic coupler  108 . The acoustic waves from the piston  204  then traverse through the electrically conducting medium  206  within the container  208 . The container  208  may be made of a material having a dissimilar acoustic impedance from the treated material  104  so that there is no loss of acoustic energy inside the container  208  due to transmission of the waves to the outside. In one embodiment, the container  208  may include a casing  210  disposed on its outer walls. The casing  210  may be made of material having a dissimilar acoustic impedance to prevent any acoustic transmissions and reflect all the energy inside the container  208 . 
         [0029]    In operation, a magnetic field is generated in the medium  206  inside the container  208  by passing a suitable current through coils  212  surrounding the medium  206 . In one embodiment, coils  212  are disposed within the container  208 . In another embodiment, the coils  212  surround the container  208 . The magnetic field can also be generated without the coils by the use of permanent magnets or electromagnets. 
         [0030]    The electrically conducting medium  206  is set into motion by the electromotive force of a magnetic field produced by the coils  212 . The magnetic field is the result of applying a current via leads  230  to the coils. In one embodiment, the flux produced by the magnetic field is aligned so that it is in a different direction than the shock wave  110  travels. In one embodiment, the flux is aligned in an opposite direction than the shock wave  110  travels. The energy of the shock wave  110  gets reduced as it gets converted to electromotive force. In particular, eddy currents are generated in the medium  206  due to the electromotive force and, thus, the shock wave  110  gets converted into thermal energy due to its electromotive force acting in opposition to the magnetic flux generated by the coils  212 . Thus, there is an increase in the temperature of the medium  206 . This temperature rise is proportional to the strength of the shock waves and the magnetic flux generated by the coils  212 . 
         [0031]    In one embodiment, the container  208  may be formed of a high-temperature material capable of withstanding the high temperatures generated. The container  208  may, as described above, may be made of material that has different acoustic impedance than the acoustic coupler  108  so that any acoustic waves that have a tendency to escape the container  208  are not allowed to do so and are contained therein. 
         [0032]    In one embodiment, the electrically conducting medium  206  is an electrically conducting fluid, an organic/inorganic fluid, or a gas, that has an acoustic impedance similar to or the same as the acoustic impedance of the acoustic coupler  108 . The impedance match can be made when the fluid used has same Bulk modulus/density values. 
         [0033]    The energy converter  108  may be coupled to energy measurement device  116 . The energy measurement device  116  may be a temperature sensor in one embodiment. In such an embodiment, the energy measurement device  116  may include a first conductive plate  220  in thermal contact with the container  208 . In one embodiment, the first conductive plate  220  directly contacts the container  208 . In one embodiment, the first conductive plate  220  is made of a material having a high thermal conductivity. The temperature increase in the electrically conducting medium  206  (and container  208 ) is thus transferred through the first conducting plate  220 . 
         [0034]    In one embodiment, the first conductive plate contacts a pyroelectric layer  222 . The pyroelectric layer  222  may be coated on both sides with suitable electrically conductive layers  224 . A temperature increase in the pyroelectric layer  222  is converted into a voltage between the electrically conductive layers  224 . The combination of the pyroelectric layer  222  and the electrically conductive layers  224 , therefore, form a pyroelectric transducer. 
         [0035]    In one embodiment, conductive wires  226  are connected to the electrically conductive layers  224  and the voltage difference may be measure by a voltage meter  228 . Of course, any means of determining the voltage difference (or a current created) due to the temperature change transmitted to the pyroelectric transducer  222  may be utilized. In one embodiment, the voltage output is displayed on a screen and may be used as a direct indication of any water or paint malfunction. Any process inadequacies translate to a reduction in the shock pressure produced which results in a lower temperature rise in the electrically conducting fluid. 
         [0036]    In one embodiment, the energy temperature measurement device  116  may be connected to a comparator  230 , which compares the measured temperature value to predetermined limits set for process parameters. For example, if the thickness of the ablative paint is not optimal or the water flow rate is not optimal, the shock wave intensity produced is much less. This results in a non-optimal laser shock peening process and lower depth of residual compressive stresses. The comparator  230  compares the deviation of the signal from the optimal process parameters and sends a defect alarm if any deviation is detected. The comparator can thus detect the exact location and send a signal to carry out the laser shock processing again with revised set of process parameters (like water flow rate, paint thickness etc.), to get the desired peening effect. Thus, real-time monitoring of the process variables can be ensured. 
         [0037]    Alternately the thermal energy can be directly measured using techniques known in the art like IR, radiometer etc instead of converting the temperature to an electric output through a pyroelectric transducer. 
         [0038]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.