Patent Publication Number: US-2023158629-A1

Title: Method for machining ceramic workpiece with composite vibration

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present disclosure claims priority to U.S. Provisional Application No. 63/281,156 filed Nov. 19, 2021. 
    
    
     BACKGROUND 
     Airfoils and other components in a turbine section of a gas turbine engine are typically formed of a superalloy and may include thermal barrier coatings to extend temperature capability and lifetime. Ceramic materials, such as monolithic ceramics, ceramic matrix composites, and combinations of these, are under consideration to replace superalloys. Among other attractive properties, ceramic materials have high temperature resistance. Ceramic materials, however, typically cannot be directly substituted for a superalloy. Rather, there are manufacturing and design factors that are unique to ceramics and which challenge practical implementation. 
     SUMMARY 
     A method for machining a ceramic workpiece according to an example of the present disclosure includes providing a sonotrode that has a transducer and a horn arranged along an axis. The horn has helical slots and terminates at a tip. The tip is brought into proximity of the ceramic workpiece and an abrasive media is provided to a work zone around the tip. The transducer produces ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip. The helical slots convert a portion of the axial vibration to torsional vibration at the tip. The axial vibration and the torsional vibration cause the abrasive media to abrade the ceramic workpiece in the work zone and thereby remove a localized portion of the ceramic workpiece. 
     In a further embodiment of any of the foregoing embodiments, the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on the second section. 
     In a further embodiment of any of the foregoing embodiments, the second section is cylindrical and has a solid core. 
     In a further embodiment of any of the foregoing embodiments, the second section has a diameter and each of the helical slots has a constant depth, and a ratio of the diameter to the constant slot depth is 5:1 to 10:1. 
     In a further embodiment of any of the foregoing embodiments, the second section has a diameter and each of the helical slots has a slot length, and a ratio of the diameter to the slot length is 1:1 to 1:4. 
     In a further embodiment of any of the foregoing embodiments, each of the helical slots has a constant depth and a slot length, and a ratio of the slot length to the constant slot depth is 5:1 to 20:1. 
     In a further embodiment of any of the foregoing embodiments, the second section has a diameter and each of the helical slots has a slot length and a constant slot depth, and a ratio of the slot length to the constant slot depth divided by the diameter is 1:1 to 1:2. 
     In a further embodiment of any of the foregoing embodiments, each of the helical slots defines a first slot end that is distal from the tip and a second slot end that is proximal to the tip, the first slot ends are located at a first common axial position, and the second slot ends are located at a second common axial position. 
     In a further embodiment of any of the foregoing embodiments, the second common axial position is no more than 12.7 millimeters from the tip. 
     In a further embodiment of any of the foregoing embodiments, the first slot end and the second slot end are circumferentially offset by 45° to 135°. 
     In a further embodiment of any of the foregoing embodiments, the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on the first section. 
     In a further embodiment of any of the foregoing embodiments, each of the helical slots defines an angle of 30° to 60° with the axis. 
     In a further embodiment of any of the foregoing embodiments, the horn is a step horn. 
     In a further embodiment of any of the foregoing embodiments, the ceramic workpiece is a ceramic matrix composite. 
     An ultrasonic machining system according to an example of the present disclosure includes a sonotrode that has a transducer and a horn arranged along an axis. The horn has helical slots and terminates at a tip. Upon operation, with the tip in proximity of a ceramic workpiece, the transducer produces ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip. The helical slots convert a portion of the axial vibration to torsional vibration at the tip, and the axial vibration and the torsional vibration cause an abrasive media in a work zone around the tip to abrade the ceramic workpiece and thereby remove a localized portion of the ceramic workpiece. 
     In a further embodiment of any of the foregoing embodiments, the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on either the first section or the second section. 
     In a further embodiment of any of the foregoing embodiments, the helical slots are on the second section, the second section has a diameter, each of the helical slots has a constant depth, each of the helical slots has a slot length, a ratio of the diameter to the constant slot depth is 5:1 to 10:1, and a ratio of the diameter to the slot length is 1:1 to 1:4. 
     In a further embodiment of any of the foregoing embodiments, a ratio of the slot length to the constant slot depth is 5:1 to 20:1. 
     In a further embodiment of any of the foregoing embodiments, each of the helical slots defines a first slot end that is distal from the tip and a second slot end that is proximal to the tip, the first slot ends are located at a first common axial position, and the second slot ends are located at a second common axial position. 
     In a further embodiment of any of the foregoing embodiments, the second common axial position is no more than 12.7 millimeters from the tip, the first slot end and the second slot end are circumferentially offset by 45° to 135°, and each of the helical slots defines an angle of 30° to 60° with the axis. 
     The present disclosure may include any one or more of the individual features disclosed above and/or below alone or in any combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG.  1    illustrates an example sonotrode for ultrasonic machining of ceramic material. 
         FIG.  2    illustrates a portion of the horn of the sonotrode. 
         FIG.  3    illustrates another example sonotrode. 
         FIG.  4    illustrates an ultrasonic machining system during operation to produce a hole in a ceramic workpiece. 
     
    
    
     DETAILED DESCRIPTION 
     One challenge to implementing ceramic materials in place of superalloys is that ceramic materials must be processed differently than superalloys. The processes used to form ceramic materials into the desired geometry of a functional component have unique limitations. For instance, for a superalloy, cooling holes, slots, and the like can be formed during casting or, for relatively small dimensions, by precision machining after casting. Ceramic materials, however, are hard and brittle in comparison to superalloys. As a result, there is considerable difficulty in efficiently machining holes, slots, or other small features, and doing so with a desired degree of accuracy. Ultrasonic machining (“USM”) is one technique that is under consideration for forming these features. USM generally involves mechanical vibration at approximately 20 kHz or more in the presence of an abrasive media to cause removal of material. When used on ceramics, however, USM yields low material removal rates that are insufficient for practical implementation on ceramics. In this regard, as will be discussed herein, the present disclosure provides a method and system for USM that facilitates increased material removal rates on ceramic materials. 
       FIG.  1    illustrates an example sonotrode  20  for facilitation of increased material removal rates in USM systems. The sonotrode  20  is operable to provide a composite axial-torsional vibrational mode in order to enhance material removal. The sonotrode  20  has a transducer  22  and a horn  24  that are generally arranged along a central axis (A). The transducer  22  may include one or more piezoelectric elements that, when activated with an electric current, produces vibrational waves that propagate axially (i.e., axial vibration V 1 ). 
     The horn  24  is mechanically coupled to the transducer  22  and includes several sections. As shown, the horn  24  is a step horn, although it is to be understood that the type of horn is not necessarily limited to step horns. The horn  24  includes a first section  26  and a second section  28 . A least a portion of the first section  26  tapers in cross-section, to focus the vibration. In the illustrated example, the initial portion of the first section  26  adjacent to the transducer  22  is cylindrical but then transitions to conical. The second section  28  has a uniform cross-section and terminates at a tip  30 . In this example, the second section  28  is cylindrical. Both the first section  26  and the second section  28  are solid and may be formed from an alloy or steel, such as but not limited to an aluminum alloy or steel. 
     The horn  24  further includes helical slots  32 . In this example, the helical slots  32  are on the second section  28 . The helical slots  32  serve to convert a portion of the axial vibration (V 1 ) to torsional vibration V 2 , while limiting excitation of undesirable bending modes. The degree and manner to which the helical slots  32  do this can be controlled via the slot geometry. 
     As shown in representative  FIG.  2   , each slot  32  defines a first slot end  32   a  that is distal from the tip  30  and a second slot end  32   b  that is proximal to the tip  30 . The first slot ends  32   a  are located at a first common axial position A 1 , and the second slot ends  32   b  are located at a second common axial position A 2 . Thus, as the axial vibration V 1  propagates down the horn  24  it encounters, and is acted upon by, all of the slots  32  at once. 
     As also shown in  FIG.  2   , the second section  28  has a diameter D, each of the helical slots  32  has a constant depth d, a slot length L, and a circumferential offset C. The depth d is the distance from the surface of the second section  28  to the floor of the slot  32 . The slot length L is the linear axial distance from the first slot end  32   a  to the second slot end  32   b , and the circumferential offset C is the length of the arc segment in degrees between the first end  32   a  and the second end  32   b.    
     In one example, a ratio of the diameter D to the constant slot depth d is 5:1 to 10:1. In a further example, a ratio of the diameter D to the slot length L is 1:1 to 1:4. In a further example, a ratio of the slot length L to the constant slot depth d is 5:1 to 20:1. In a further example, a ratio of the slot length L to the constant slot depth d divided by the diameter D is 1:1 to 1:2. In a further example, each of the slots  32  has an angle G with respect to the axis A that is from 30° to 60°. In a further example of any of the above examples, the first slot end  32   a  and the second slot end  32   b  are circumferentially offset by 45° to 135°. In a further example of any of the above examples, the second common axial position A 2  is also no more than 12.7 millimeters from the tip. 
     The sonotrode  20  with the above features, or combinations thereof, facilitates adaptation of USM for the machining of ceramic material. For instance, most of the material removal is due to the axial vibration V 1 . Therefore, the portion of the axial vibration V 1  that is converted into the torsional vibration V 2  can be limited via the above prescribed ranges. Moreover, the cycles of vibration should be in sync such that the peak amplitude of the axial vibration V 1  coincides with the peak amplitude of torsional vibration V 2 . Also, the torsional vibration V 2  can be primarily induced at or near the tip  30  by placing the slots  32  near the tip  30  per the above range. In one alternative shown in  FIG.  3   , however, the helical slots  32  are located on the conical portion of the first section  26 . 
       FIG.  4    illustrates an example of a USM system during operation to machine a ceramic workpiece  40 . The ceramic material of the workpiece  40  is not particularly limited and may be a monolithic ceramic, a ceramic matrix composite (CMC), or combinations of monolithic and CMC. The monolithic ceramic may be, but is not limited to, silicon nitride or silicon carbide. The ceramic matrix composite may be, but is not limited to, a SiC/SiC ceramic matrix composite in which SiC fiber tows are disposed within a SiC matrix. Alternatively, the fibers and/or matrix may be Si 3 N 4 . 
     With the tip  30  in proximity of the ceramic workpiece  40 , the transducer  22  ( FIG.  1   ) produces ultrasonic vibration that axially propagates down the horn  24  and causes axial vibration V 1  at the tip  30 . The aforementioned helical slots  32  convert a portion of the axial vibration V 1  to torsional vibration V 2  at the tip  30 . The axial vibration V 1  and the torsional vibration V 2  cause an abrasive media  42  containing abrasive particles  44  in a work zone Z around the tip  30  to abrade the ceramic workpiece  40  and thereby remove a localized portion of the ceramic workpiece  40 . For instance, at the peak amplitude of the axial vibration the abrasive particles  44  are driven to penetrate into the exposed surface of the ceramic workpiece  40 . Simultaneously, the torsional vibration acts to drive the abrasive particles  44  sideways across the exposed surface, causing the cutting off of “microchips” of ceramic and smoothing of the surface. The simultaneous penetration, cutting, and smoothing facilitates an increase in material removal rate and accuracy in comparison to using only axial vibration, thereby enabling more practical application of USM for ceramic material. During the remaining vibration cycle, the horn  24  and the ceramic workpiece  40  are separated and there is thus little material removal. 
     The tip  30  of the sonotrode  20  can be advanced into the ceramic workpiece  40  as material is removed in order to form a deeper hole and/or translated along the surface of the ceramic workpiece  40  to produce a slot. Additionally, a mass element  25  ( FIG.  1   ) may be provided on the opposite axial side of the transducer  22  from the horn  24 . The displacement the tip  30  is larger than at the back side of the transducer  22  because the mass element  25 , which may be made from steel, is of relatively higher impedance than the horn  24  (which may be made from aluminum). This prevents the backward propagation of the axial vibration to improve the output amplitude at the tip  30 . 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.