Patent Application
20100154214
This invention relates to joining of a turbine shaft and a turbine wheel.
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.
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 cm2 requires the use of large, rigid and expensive machinery.
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.
It is additionally known that gas lasers such as CO2 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 CO2 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 CO2 and YAG lasers are less energy efficient converting electricity into light.
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.
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.
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.
Referring to
In one aspect, and as detailed in
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.
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,
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.
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 mm2 and provide a weld joint having a tensile value of above 179 kilonewton; and join an effective area of 198 mm2 and provide a weld joint having a tensile value of 100 kilonewton.
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 O2 or CO2 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 mm2 and having a tensile value of greater than 97 kilonewton may be produced.
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.
In another aspect, the drawn arc welding process of the first embodiment may include a ring joint design, as shown in
Referring to
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.
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.
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.
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.
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.
In another aspect, the process of the second embodiment may include welding a cavity shut such as in the depicted embodiment of
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61138580 | Dec 2008 | US |
