Abstract:
A process for joining a turbine wheel and a turbine shaft of a turbocharger comprising the steps of: providing a turbine wheel; providing a turbine shaft; holding the turbine shaft in a welding device; contacting the turbine shaft to the turbine wheel; energizing a pilot current; lifting the shaft a predetermined height from the turbine wheel to draw a pilot arc; energizing a weld arc current locally melting the shaft weld end and forming a weld pool on the wheel; plunging the shaft toward the wheel into the weld pool; turning off the current; and removing the welding device from the welded shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority benefit of U.S. provisional patent application No. 61/138,580 filed on Dec. 18, 2008 and is herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to joining of a turbine shaft and a turbine wheel. 
       BACKGROUND OF THE INVENTION 
       [0003]    Turbochargers may be utilized in an internal combustion engine to compress intake air in order to achieve higher thermal efficiencies, power outputs, torque and fuel economies for the engine. Turbochargers may be utilized in various engines in automotive as well as in aeronautical applications. Generally a turbocharger may include a turbine wheel that rotates at a high velocity such as up to 200,000 rpm. It is powered by exhaust air at elevated temperatures. There is therefore a need to use high temperature materials, especially metals when constructing a turbine wheel. The turbine wheel may be welded to a shaft that is coupled to a compressor wheel. The joining of the shaft to the turbine wheel allows the compressor wheel to rotate within a housing to compress intake air at ambient temperature into a high density and low velocity air known as diffusion. Due to the high rotational velocity it is essential to maintain the balance, axial symmetry and concentricity in an accurate manner as well as provide a high strength joining of the components. 
         [0004]    Current prior art turbo charger wheel and shaft may be joined using an inertia friction welding technique in which a shaft may be coupled to a fly wheel that accumulates kinetic energy from rotation at a fixed speed and is forced together with a stationary workpiece such as the turbine wheel. Friction heat is generated to rub the two surfaces together to form a bond. Various limitations included in the inertia friction welding process include the generation of flash coat that must be removed through post welding machining. Additionally, the flash may be trapped inside a cylindrical joint requiring a greater effort to balance the wheel shaft assembly after the joining operation. Further, high thrust pressures in a range of from 2800 kg per cm 2  requires the use of large, rigid and expensive machinery. 
         [0005]    It is additionally known in the prior art to utilize an electron beam welding process to join a turbine wheel and shaft assembly with less post-weld machining and possibly less balancing than inertia friction welding. Electron beam welding utilizes a high power density beam which is focused on a joint in a vacuum. The electron beam produces a deep narrow fusion zone with little weld distortion. Due to high quality weld with little distortion and less work for post-weld machining, EB is often chosen for high stress turbocharger applications. However, electron beam (EB) welding machines typically require a cycle time such as greater than one minute which may further be lengthened if a fixture is used to weld multiple assemblies. Further, EB welding equipment requires high capital investment costs as well as requires the process being carried out in a vacuum. 
         [0006]    It is additionally known that gas lasers such as CO 2  lasers and solid state lasers such as Nd:YAG lasers are used in welding torque converters and the like, and can be used for welding a turbine wheel and shaft made of titanium. The CO 2  laser has a wavelength that necessitates the use of expensive helium shielding gas to reduce plasma from material interaction that absorbs the beam power and has poor beam quality (multiple TEM mode). The YAG laser needs an expensive pump (either diode or lamp) with a short life. Both CO 2  and YAG lasers are less energy efficient converting electricity into light. 
         [0007]    There is therefore a need in the art for an improved joining process for joining a turbine wheel to a shaft. There is also a need in the art for a joining process that allows high strength and quality joints in an economic manner. Further, there is a need in the art for a welding operation that does not require a vacuum while providing a high strength and accurate joining operation with less post-weld operations. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect there is disclosed a process for joining a turbine wheel and a turbine shaft of a turbocharger comprising the steps of: providing a turbine wheel; providing a turbine shaft; holding the turbine shaft in a welding device; contacting the turbine shaft to the turbine wheel; energizing a pilot current; lifting the shaft a predetermined height from the turbine wheel to draw a pilot arc; energizing a weld arc current locally melting the shaft weld end and forming a weld pool on the wheel; plunging the shaft toward the wheel into the weld pool; turning off the current; and removing the welding device from the welded shaft. 
         [0009]    In another aspect there is disclosed a process for joining a turbine wheel and shaft comprising the steps of: providing a turbine wheel; providing a turbine shaft; providing a fiber laser welding device; positioning the turbine shaft relative to the turbine wheel; energizing the fiber laser and passing it about the turbine shaft and the turbine wheel joining the turbine shaft and the turbine wheel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A-F  includes side views and sectional views of a turbine wheel and shaft in first and second embodiments of joining processes; 
           [0011]      FIG. 2  is a partial sectional view of  FIG. 2B ; 
           [0012]      FIG. 3  is a partial sectional view of  FIG. 1F ; 
           [0013]      FIG. 4  includes a perspective view of a turbine shaft and wheel joined utilizing the process of a first embodiment having a drawn arc welding process and ferrule; 
           [0014]      FIG. 5  is a partial perspective view of a shaft and wheel of  FIG. 4  following a machining operation of the first embodiment; 
           [0015]      FIG. 6  is a partial sectional view of the turbine wheel and shaft following the joining operation of the first embodiment; 
           [0016]      FIG. 7  is a perspective view of a turbine wheel and shaft following a bending test of the first embodiment; 
           [0017]      FIG. 8  is a partial perspective view detailing the weld joint founed between the shaft and wheel utilizing the second embodiment of the process; 
           [0018]      FIG. 9  is a perspective view of a turbine wheel including a pedestal. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to  FIGS. 1-7 , there are shown various embodiments of the process for joining a turbine shaft  10  and wheel  15 . Referring to  FIG. 1 through 5 , there is shown a first embodiment for joining a turbine shaft  10  and wheel  15 . The first embodiment may include a drawn arc welding process for joining the turbine shaft  10  and wheel  15 . The process may include providing a turbine wheel  15  and turbine shaft  10 . The turbine shaft  10  may be held in a welding device such as a welding gun or other robotically controlled device. The shaft and turbine wheel  10 ,  15  are then abutted or contacted with each other. Next a pilot current may be energized in the welding device to flow through the contact between the wheel and the shaft. The shaft  10  is then lifted a predetermined height from the turbine wheel  15  and a pilot arc is energized between the wheel and the shaft. The welding device will then increase the current from low pilot level to a sufficiently high level, creating the main arc locally melting the shaft and wheel  10 ,  15  forming a weld pool  20 . Next the shaft  10  is plunged toward the wheel  15  and into the weld pool  20  and the arc is extinguished. The weld joint formed is allowed to cool and the current is turned off. Lastly the shaft  10  may be removed from the welding device with a weld joint formed between the turbine wheel and shaft  15 ,  10 . 
         [0020]    In one aspect, and as detailed in  FIGS. 1B-E , the turbine shaft  10  may be a solid rod  25  and the turbine wheel  15  may include a solid abutment  30 . As can be seen in the Figures, the abutment  30  may include a ramp like formation  35  formed on a back surface  40  of the turbine wheel  15 . In another aspect, the ramp like formations  35  may be replaced by a pedestal  36 , best seen in  FIG. 9  formed on the turbine wheel  15  which restricts welding heat flow to the wheel. The pedestal  36  may have a top portion  38  having a larger diameter of about 31 mm with a lower shank  42  extending to the turbine wheel  15 . The shank  42  may have a diameter of about 15 mm. In this manner a gap  44  of about 3 mm will be formed between the turbine wheel  15  and the top portion  38  of the pedestal  36 . 
         [0021]    In one aspect, the turbine shaft  10  may be formed of alloy steel such as AISI 8740 steel. The turbine wheel  15  may be formed of a nickel based alloy including the superalloy INCONEL 713. It should be realized that other materials including stainless steel and other nickel based alloys may be utilized for both the turbine wheel and shaft  15 ,  10 . 
         [0022]    In one aspect, the first embodiment of the process may include positioning a ferrule  45  about the turbine shaft  10  for containing the weld pool  20 , constricting the arc and restricting air from entering the weld area. In such an application, the shaft  10  may also include a flux load formed on the end of the shaft  10  that acts as an oxygen scavenger during the process of the first embodiment, 
         [0023]    Additionally, the process of the first embodiment may include the step of removing weld flash utilizing a machine tool following the formation of the weld joint in the drawn arc process. In one aspect, a machining tool may be integrated into the welding device. 
         [0024]    Various welding parameters may be utilized for shafts  10  having different outside diameters and profiles. In one aspect, the weld arc current may have a value of from 1,000 to 1,500 amps and may be energized for an arc duration of from 550 to 900 millisecond. In such an application, the process of the first embodiment may join an effective area of 284 mm 2  and provide a weld joint having a tensile value of above 179 kilonewton; and join an effective area of 198 mm 2  and provide a weld joint having a tensile value of 100 kilonewton. 
         [0025]    In another aspect, the first embodiment may include a step of providing a shielding gas about the portion of the turbine shaft  10  and turbine wheel  15  that are to be joined. The shielding gas may include an inert gas such as argon or an active gas such as mixture containing O 2  or CO 2  and a weld arc current of from 1,100 to 1,500 amps for a duration of from 100 to 150 msec may be utilized. In such an application, a weld joint having an effective area of 127 mm 2  and having a tensile value of greater than 97 kilonewton may be produced. 
         [0026]    In another aspect, the process may include providing a field former that exerts force on the weld arc centering it relative to the turbine shaft  10  and the turbine wheel  15 . In this manner, a field former including an electromagnetic coil fed by either the welding current or a separate power supply exerts a force on the arc to bring it back toward the center of the shaft  10 . Alternatively, a magnetic field may be created to rotate the arc under the shaft  10  to achieve uniform melting and perpendicularity of the joining of the shaft  10  relative to the wheel  15 . 
         [0027]    In another aspect, the drawn arc welding process of the first embodiment may include a ring joint design, as shown in  FIG. 1B  that needs to be maintained during the welding process. Various welding parameters may be utilized for shafts  10  having different outside diameters and profiles. In one aspect, the weld arc current may have a value of about 2000 amps and may be energized for an arc duration of about 400 milliseconds. In such an application, the process of the first embodiment may join an effective area of about 357 mm 2  and provide a weld joint having a tensile value of about 129 kilonewtons. 
         [0028]    Referring to  FIG. 8  there is shown a turbine wheel and shaft  15 ,  10  joined utilizing a second embodiment of a process. The process of the second embodiment includes providing a turbine wheel  15  and turbine shaft  10 . Additionally, a fiber laser welding device is provided. The turbine shaft  10  is positioned relative to the turbine wheel  15 , as best shown in  FIG. 1A-B . The fiber laser is then energized and passed about the turbine shaft  10  and turbine wheel  15  joining the turbine shaft  10  and the wheel  15 . As with the previously described first embodiment, the turbine shaft  10  may be formed of steel including AISI 8740 and the turbine wheel  15  may be formed of a nickel based alloy such as INCONEL 713. 
         [0029]    In one aspect, the process of the second embodiment may include providing a shielding gas of Argon about the turbine wheel  15  and shaft  10  when the fiber laser is energized. Additionally, the process for joining the turbine wheel and shaft  15 ,  10  of the second embodiment may include energizing the fiber laser a second time with a de-focused beam to refine the joint appearance formed between the two components. 
         [0030]    In one aspect, the fiber laser may be a Ytterbium laser that has a wave length of 1,070 nm. In one aspect, the fiber laser may include a fiber of 200 μm having a collimator of 100 mm and a focus of 200 mm. 
         [0031]    Additionally, the process may include the first energizing step that has a power of 1.5 kw with a rotational speed of 20 rpm with the beam focused on the surface of the shaft and the wheel  10 ,  15 . Further, the second energizing step may include a power of 1.5 kw having a speed of 10 rpm with the beam defocused 20 mm on the surface of the shaft and wheel  10 ,  15  thereby refining the weld joint appearance. 
         [0032]    In one aspect, the second embodiment may include a shaft  10  that is hollow and that has a wall thickness of 3 mm and a diameter of 19 mm. Additionally, the shaft  10  may include a counter bore  50  formed on the end that is to be joined with the turbine wheel  15 . Additionally, the wheel  15  may include a raised abutment  55  formed thereon as with the first embodiment. The raised abutment  55  may include a counter bore  60  formed therein. A weld joint formed by the process of the second embodiment may have a tensile value of at least 90 kilonewton. 
         [0033]    In one aspect the laser may use a continuous wave or constant power. In another aspect, a periodically fluctuating power may be used to reduce the formation of a welding defect, such as porosity or blow hole. For example a square wave power may be utilized. For example a laser having an average of 1800 W-2000 W, peak-to-peak power of 500 W, 166 Hz frequency sinusoidal waveform, a welding speed of 25 inch per minute, with total weld time of 6 seconds may be utilized. Nitrogen gas at 25 psi may be used in such an operation. 
         [0034]    In another aspect, the process of the second embodiment may include welding a cavity shut such as in the depicted embodiment of  FIG. 1B . In such an application heated air may become trapped in the cavity and may cause defects such as blow holes. The process may include the step of using the laser in a focused state to drill a small vent hole on the shaft, about 0.2 mm diameter and 3 mm away from the formed joint. The same laser may be used to weld the joint, and then defocused to seal the vent hole. 
         [0035]    While specific embodiments of the first and second process have been discussed, it should be realized various power levels, times and parameters may be utilized without departing from the invention.