Abstract:
A frictional welding process for joining a titanium aluminide turbine to a titanium alloy shaft is disclosed. The disclosed process includes preheating the turbine to a designated temperature, providing a specially-designed joining interface geometry at the distal end of the shaft and optimizing the frictional welding parameters. The frictional welding is carried out in multiple steps but, while the shaft is being spun by a rotating chuck, two different pressures and two different time periods are used until the narrower portions of the distal end of the shaft have been fused onto the welding surface of the turbine. Then, an additional forging step with yet another engagement pressure between the shaft and the turbine is carried out without rotation of the shaft.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to turbochargers for engines, and more particularly, to turbochargers fabricated with titanium aluminide that are welded to shafts fabricated with titanium. 
       BACKGROUND 
       [0002]    Turbochargers can increase the power of engines by providing additional air to the engine cylinders. An exhaust-gas driven turbine connected to a compressor may be used to produce the additional air. However, turbocharger lag, which occurs while turbocharger turbines develop adequate rotational speed, can be a problem. One method for reducing turbocharger lag is to decrease the weight of the turbocharger&#39;s rotating parts, including the turbine and the shaft attached to the turbine. 
         [0003]    Titanium-aluminide constitutes a lightweight, strong material that may be used to produce turbocharger turbines. However, the use of titanium-aluminide can complicate joining of the turbine to the turbocharger shaft, which is often made with steel. Titanium-aluminide and steel have different thermal expansion properties and may produce undesirable phase transformations at their material interfaces. Therefore, because turbochargers experience significant temperature variations, a titanium-aluminide turbine and a steel shaft may be unsuitable for joining directly to one another. 
         [0004]    One method of joining titanium-aluminide turbines to steel shafts is disclosed in U.S. Pat. No. 6,291,086. The method describes the use of an interlayer material disposed between a titanium-aluminide turbine and steel shaft. The interlayer material is welded to both the titanium-aluminide turbine and steel shaft. Therefore, although this method may provide a suitable connection between the turbine and shaft, two welds must be made and an additional material must be used, which can add significant time and cost to production. Further, use of a steel shaft adds significant weight to the turbocharger, which can increase turbocharger lag. Thus, turbocharger shafts fabricated from lighter materials such as titanium or titanium alloys are preferable as indicated in U.S. Patent Application Publication 2006/0067824, which discloses titanium aluminide turbine boned to a titanium shaft by various means such as gas tungsten-arc welding, gas metal-arc welding, resistance welding, laser welding, plasma arc welding, electron-beam welding, friction welding, brazing, and soldering. 
         [0005]    However, bonding a turbine made from a titanium aluminide to a shaft made from a titanium alloy is challenging because of three reasons: high local thermal stress involved with bonding process; formation of brittle intermetallic phases at the bonding interface; and inherent low room temperature ductility of titanium aluminide alloys. Because of these reasons, the bonding interface between a titanium aluminide turbine and a titanium alloy shafts and even the titanium aluminide turbine itself are prone to crack during or after the bonding process. 
         [0006]    In some applications, because of specific geometry or a large size of the titanium aluminide turbine, the local thermal stresses can become extremely high and therefore render the bonding process even more challenging. For example, in turbine rotor applications, the bonding interface is fairly close to rear face of the turbine, and the geometry of the turbine hub changes rapidly. This rapid change in geometry, in addition to the large thermal mass of turbine wheel, may cause a steep temperature gradient, and therefore, may cause large thermal stress which may exceed the strength of the titanium aluminide in the vicinity of the bonding interface with the titanium alloy shaft. 
         [0007]    One method of bonding a titanium aluminide component to a titanium alloy component involves the use of friction welding as disclosed in US2008/0000558. The titanium aluminide component is heated to a temperature between 300° C. and 800° C. The titanium alloy component is rotated relative to the titanium aluminide component. The titanium aluminide and titanium alloy components are pressed against each other while the titanium alloy component is rotated. The rotation is then stopped, and the two components are pressed against each other again as a forging step after the rotation is stopped. 
         [0008]    However, despite these recent advances, the joint between a titanium aluminide turbine and a titanium alloy shaft may crack during or after the joining process. The friction welding process imposes substantial thermal stresses. Because titanium aluminide and other intermetallic phases formed at the welding interface have a limited ductility, such components or joints can be prone to crack under stress. Further, the stresses are often increased because of the geometry of the turbine and shaft and the size of the two components. Thus, the joining of a titanium aluminide turbine to a titanium alloy shaft is very challenging. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    In satisfaction of the aforenoted needs, a friction welding method is disclosed for joining a first component fabricated from titanium aluminide to a second component fabricated from a titanium alloy. The disclosed method includes preheating the first component made from titanium aluminide, rotating the second component and pressing the second component against the first component at a first pressure for a first time period. After the first time period expires, the method also includes rotating the second component and pressing the second component against the first component at a second pressure for a second time period. After the second time period expires, the method further includes stopping the rotation of the second component and pressing the second component against the first component at a third pressure for a third time period as a forging step. 
         [0010]    In another aspect, a method of joining a shaft to a turbine of a turbo charger is disclosed. The method includes providing a titanium alloy shaft having an end having a first diameter and that is connected to a distal end having a second diameter that is smaller than the first diameter. The smaller distal end extends distally from the larger end of the shaft. The method also includes providing a titanium aluminide turbine with a welding surface for receiving the end of the shaft and having a third diameter greater than the second diameter. The method also includes preheating the turbine and placing the shaft into rotating chuck and placing the turbine in a stationary chuck that is axially aligned with the rotating chuck so that the shaft is in axial alignment with the welding surface of the turbine. The method then includes rotating the shaft and pressing the distal end of the shaft against the welding surface of the turbine for a first time period and at a first engagement pressure. After the distal end of the shaft has been substantially displaced or fused into the welding surface of the turbine, the method further includes continuing to rotate the shaft and pressing the end of the shaft against the welding surface of the turbine for a second time period and at a second engagement pressure. After the second time period has ended, the method further includes stopping rotation of the shaft and continuing to press the end of the shaft against the welding surface of the turbine for a third time period and at a third engagement pressure as a forging step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a partial side and cross sectional view of a turbine and shaft prior to the connection of the shaft to the turbine. 
           [0012]      FIG. 2  is another side and partial sectional view of the shaft and turbine as the shaft is being frictionally welded to the turbine; 
           [0013]      FIG. 3  is yet another side and partial sectional view as the shaft is being frictionally welded to the turbine. 
           [0014]      FIG. 4  is yet another side and partial sectional view of a shaft and turbine during the final forging step of the disclosed frictional welding process. 
           [0015]      FIG. 5  is a flow diagram illustrating a disclosed frictional welding process. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , a turbocharger turbine  10  is shown inside a heating apparatus  11  which may be used to preheat the turbine  10  prior to conducting the remaining steps of the disclosed frictional welding process. The turbine  10  may include a plurality of vanes  12  connected to a turbine rotor wheel  13 . The rotor wheel  13  may include a hub portion  14  that features an end surface or welding surface  15 . The turbine  10  may also include a distal extension  16  which may be used to hold the turbine  10  in place in the stationary chuck  17  shown schematically in  FIG. 2 . 
         [0017]    Returning to  FIG. 1 , the shaft  17  may include an end  18  that is connected to a further distal end  19  by a beveled portion  26 . The smaller distal end  19  engages the turbine  10  first during the frictional welding process as illustrated in  FIG. 2 . Still referring to  FIG. 1 , the shaft  17  may be held in a rotating chuck  21  that can rotate the shaft  17  about its axis  22  as indicated by the arrow  23 . The axis  22  of the shaft  17  is in alignment with the axis  24  of the turbine  10 . 
         [0018]    The turbine  10  may be preheated to a temperature ranging from about 300° C. to about 700° C., more preferably from about 400° C. to about 600° C. and still more preferably about 500° C. Before, after or during preheating, the turbine  10  can be mounted to the stationary chuck  21  as illustrated in  FIG. 2 . Rotation of the shaft  17  begins and the shaft  17  may be pressed against the welding surface  15  of the turbine  10  as illustrated in  FIG. 2 . More specifically, the distal end  19 , which has a smaller diameter than the shaft  17  and a smaller diameter than the welding surface  15 , may be pressed against the welding surface  15  as the rotating chuck  21  rotates the shaft  17  for a predetermined time period t 1  and with an engagement pressure P 1  between the distal end  19  and the welding surface  15 . 
         [0019]    Returning to  FIG. 3 , after the shaft  17  has been rotated against the turbine  10  for the first predetermined time period t 1  and that the first predetermined engagement pressure P 1 , the distal end  19  of the shaft  17  has become at least partially fused into the welding surface  15  of the turbine  10  in combination with some outward radial displacement of the distal end  19  on the welding surface  15 . At this stage, the rotating chuck  21  continues to spin the shaft  17  but at a different pressure P 2  for a second predetermined time period t 2 . Generally, as the size of a component increases, the engagement pressure should increase. Therefore, P 2 , which is applied after the distal end  19  has been removed (compare  FIGS. 2 and 3 ), may be greater than P 1 . However, the first time period t 1  may be longer than the second time period t 2  due to the need for the generation of heat during the first time period t 1 . 
         [0020]    Returning to  FIG. 4 , after the second time period t 2  has passed, the combination of the engagement pressure P 2 , the heat generated by the engagement between the end  18  of the shaft  17  and the welding surface  15  causes the beveled portion  26  of the shaft  17  to be at least partially fused into the welding surface  15  of the turbine  10  as shown by a comparison of  FIGS. 3 and 4 . 
         [0021]    Returning to  FIG. 4 , the distal end  19  and the beveled portion  26  had been fused on to the welding surface  15  of the turbine  10 .  FIG. 4  also illustrates that the diameter D 1  of the shaft  17  may be about the same or slightly greater than the diameter D 2  of the welding surface  15 . It will also be noted that the diameters D 1  and D 2  ( FIG. 4 ) are both larger than the diameter D 3  ( FIG. 1 ) of the distal end portion  19  of the shaft  17  as illustrated in  FIG. 1 . 
         [0022]      FIG. 4  illustrates a forging step of the disclosed frictional welding process. The rotating chuck  21  is stopped and rotation of the shaft  17  is discontinued. However, an engagement pressure P 3  may be applied between the end  18  of the shaft  17  and the welding surface  15  of the turbine  10 . The engagement pressure P 3  for the forging step may be greater than the pressure P 2  used to weld the beveled portion  26  and end  18  of the shaft  17  to the welding surface  15  of the turbine  10  and greater than the pressure P 1  used to weld the distal end portion  19  of the shaft  17  to the welding surface  15  of the turbine  10 . 
         [0023]    Thus, in general, the second engagement pressure P 2  may be greater than the first engagement pressure P 1  because of the increase in size of the beveled portion  26  and end  18  of the shaft  17  as compared to the distal end portion  19 . However, primarily because of increased heat that has been generated by the time the process reaches the step illustrated in  FIG. 3 , the second time period t 2  may be shorter than the first time period t 1 . Because of the larger size or diameter of the end  18  of the shaft  17 , the third pressure P 3  during the forging process may be greater than the first two engagement pressures P 1  and P 2 . The forging step should not be rushed; it should be carried out for a third time period t 3  that may be longer than both the first and second time periods, t 2  and t 1 . 
         [0024]    In summary, the disclosed process may include preheating the turbine  10  to a designated temperature, providing an inner phase geometry that includes a welding surface  15 , a shaft  17  with a smaller diameter distal end  19  or nose  19  and using a shaft diameter  17  that is similar in diameter to the welding surface  15 . The welding parameters include using lower pressure P 1  for the first step, a higher pressure P 2  but a shorter time period t 2  for the second step and a higher pressure P 3  and a longer time period t 3  for the final forging step. 
         [0025]    In previous designs, thermal stresses during the friction weld process were causing cracking. In particular, with a larger diameter and larger pressure, the temperature gradient between a point at the center of the shaft, which has zero velocity, and a point at the outer edge of the shaft, which has the highest velocity, was too much. By reducing the diameter at the distal end  19  of the shaft  17 , the pressure P 1  can be reduced and the difference in velocity between the axial center of the shaft  17  or end  19  and the outer perimeter of the end  19  is reduced (i.e. smaller radius). Thus, the temperature gradient across the shaft  17  is reduced resulting in less thermal stress. Another benefit is that once the nose has been “ground away”, the higher pressure P 2  time period t 2  can be shortened (versus the prior art approach) since the welding process is well underway in the first phase (P 1,  t 1 ). 
         [0026]    The preheating temperature for the turbine  10  can range from about 300° C. to about 700° C., more preferably from about 400° C. to about 600° C., still more preferably from about 500° C. The first engagement pressure P 1  can range from about 20 MPa or the lower limit of the machine to about 50 MPa, with one exemplary first engagement pressure being about 35 MPa. The first time period t 1  can range from about 6 to about 24 seconds, with one example being about 14 seconds. The second engagement pressure can range from about 50 MPa to about 150 MPa, with one exemplary second engagement pressure being about 100 Mpa. The second time period t 2  can range from about 1 seconds to about 7 seconds, with one exemplary time period being about 4 seconds. The third engagement (forging) pressure P 3  can range from about 150 MPa to about 500 MPa or the high limit of the machine, with one exemplary forging pressure being about 200 MPa. The forging time period t 3  can range from about 20 sec to about 5 minutes, with one exemplary forging time period being about 90 seconds. 
         [0027]    Further, instead of time control, distance control or the shortening of the axial length of the shaft  17  (including the removal of the distal end  19 ) may be used during the friction stages of  FIGS. 2 and 3 . For example, the first distance, which may be about the axial length of the distal end  19 , may range from about 3 mm to about 12 mm (instead of t 1 ), with one exemplary first distance being about 7.5 mm The second distance, which may be about the axial length of the beveled portion  26  of the shaft  17 , may range from about 2 mm to about 12 mm (instead of t 2 ), with one exemplary second distance being about 5.5 mm These estimates, of course, are dependent on the size of the components. 
       INDUSTRIAL APPLICABILITY 
       [0028]    In order to maximize the power generated by an internal combustion engine, the engine may be equipped with a turbocharger. A turbocharger includes a turbine  10  connected to a rotating shaft  17  that compresses air flowing into the engine to thereby forging more air into a combustion chamber. The increase supply of air allows for increased fueling, which may result in increased power. A turbocharged engine typically produces more power than an engine without a turbocharger. 
         [0029]    However, if the turbine  10  and shaft  17  are fabricated from heavy superalloy and steel, respectively, a phenomenon known as “turbocharger lag” may occur due to the increase torque required to get the shaft  17  and turbine  10  spinning properly. To alleviate this problem, the turbine may be fabricated from a lighter material such as titanium aluminide or various Ti 3 Al intermetallic compound-based alloys including, but not limited to Ti-45Al-2Nb-2Cr, Ti-47Al-2Nb-2Cr, Ti-47Al-2W-0.5Si, Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si, Ti-48Al-2Nb-0.7Cr-0.3Si, Ti-45Al-2Mn-2Nb-0.8 vol % TiB2 and Ti-47Al-2Mn-2Nb-0.8 vol % TiB2. Further, the shaft  17  may be fabricated from a titanium alloy including, but not limited to Ti-6Al-4V, Ti-6Al-6V-2Sn and Ti-6Al-2Sn-4Zr-6Mo. However, titanium aluminide intermetallic alloys are prone to crack during or after the bonding process because of high thermal stresses and low room temperature ductilities. This problem is compounded when a titanium aluminide turbine is joined to a titanium alloy shaft when brittle intermetallic phases are formed at bonding interface. Although friction welding has been used before for this application, the results have been unsatisfactory. 
         [0030]    Instead, the disclosed process adds an additional process step and also changes the distal diameter of the shaft  17  with dramatically improved results, particularly for resolving some intermittent cracking issues along the weld interface and in the turbine that occurs when scaling up welding components. First, the titanium alloy shaft  17  is provided with a smaller diameter distal end portion  19  which is the first portion of the shaft  17  frictionally welded to the welding surface  15  of the turbine  10 . The shaft is held in a rotating chuck  21  and the titanium aluminide turbine  10  is held in a stationary chuck  20 . The turbine  10  may be preheated before or after placement in the stationary chuck  20 . After preheating and placement in the appropriate rotating and stationary chucks  21 ,  20  respectively, the shaft  17  is rotated and the smaller diameter end portion  19  is pressed against the welding surface  15  using a first pressure P 1  and for a first time period t 1 . After the distal end portion  19  has been fused into the welding surface  15  of the turbine  10  or otherwise ground away, the chuck  21  continues to rotate the shaft  17  as the engagement pressure is increased to P 2 . At this point, the beveled portion  26  of the shaft  17  is being fused into the engagement surface  15  of the turbine  10  as illustrated in  FIGS. 3-4 . Once the beveled portion  26  has been fused into the welding surface  15 , the rotating chuck  21  is stopped and the stationary shaft  17  is pressed into the turbine  10  at a greater pressure P 3  and for a longer time period t 3 , which essentially provides a forging step.