Metal working – Method of mechanical manufacture – Impeller making
Reexamination Certificate
2003-07-08
2004-06-29
Rosenbaum, I Cuda (Department: 3726)
Metal working
Method of mechanical manufacture
Impeller making
C029S888024, C029S557000
Reexamination Certificate
active
06754954
ABSTRACT:
FIELD OF THE INVENTION
The compressor wheel is the life-limiting component in turbochargers currently produced for commercial diesel engines. Changing the wheel material from aluminum to titanium alloy is one technical solution. However, compressor wheels have highly complex shapes and must be manufactured with high dimensional accuracy. The difficulty of working with titanium has inhibited the adoption of titanium compressor wheels in automotive air boost devices. The invention provides an economical process for the manufacture of titanium compressor wheels.
DESCRIPTION OF THE RELATED ART
Air boost devices (turbochargers, superchargers, electric compressors, etc.) are used to increase combustion air throughput and density, thereby increasing power and responsiveness of internal combustion engines.
The blades of a compressor wheel have a highly complex shape which is design-optimized for (a) drawing air in axially, (b) accelerating this air centrifugally, and (c) discharging air radially outward with elevated energy (velocity/pressure) into the volute-shaped chamber of a compressor housing. In order to accomplish these three distinct functions with maximum efficiently and minimum turbulence, the blades can be said to have three separate regions.
First, the leading edge of the blade can be described as a sharp pitch helix, adapted for scooping air in and moving air axially. Considering only the leading edge of the blade, the cantilevered or outboard tip travels faster than the part closest to the hub, and is generally provided with an even greater pitch angle than the part closest to the hub (see FIG.
1
). Thus, the angle of attack of the leading edge of the blade undergoes a twist from lower pitch near the hub to a higher pitch at the outer tip of the leading edge. Further, the leading edge of the blade generally is bowed, and is not planar. Further yet, the leading edge of the blade generally has a “dip” near the hub and a “risen ” or convexity along the outer third of the blade tip. These design features are all engineered to enhance the function of drawing air in axially.
Next, in the second or transitional region of the blades, the blades are curved in a manner to change the direction of the airflow from axial to radial, and at the same time to rapidly spin the air centrifugally and accelerate the air to a high velocity, so that when diffused in a volute chamber after leaving the impeller the energy is recovered in the form of increased pressure. Air is trapped in airflow channels defined between the blades, as well as between the inner wall of the compressor wheel housing and the radially enlarged disc-like portion of the hub which defines a floor space, the housing-floor spacing narrowing in the direction of air flow.
Finally, in the third region, the blades terminate in a trailing edge, which is designed for propelling air radially out of the compressor wheel. The design of this blade trailing edge is generally complex, provided with (a) a pitch, (b) an angle offset from radial, and/or (c) a back taper or back sweep (which, together with the forward sweep at the leading edge, provides the blade with an overall “S” shape). Air induced and expelled in this way produces not only high flow, but also efficiently generates high pressure when diffused into a collecting duct or scroll.
Accordingly, functional considerations dictate the complex shape of a compressor wheel. The compound and highly complex curvatures of a turbocharger compressor wheel are most advantageously and economically obtained by a casting process wherein the wheel hub section and blades are integrally formed desirably from a lightweight material, such as aluminum or aluminum alloy chosen for its relatively low rotational inertia for achieving the further advantage of rapid accelerative response during transient operating conditions.
Recently, tighter regulation of engine exhaust emissions has led to an interest in even higher pressure ratio boosting devices. Aluminum compressor wheels are however not capable of withstanding repeated exposure to higher pressure ratios (>3.8), and have a relatively short, finite fatigue life. When a compressor wheel is rotated at operating tip speeds of 500 m/s or more, cast aluminum is subjected to relatively high tensile loading particularly in the hub region of the wheel which must support the wheel mass. Unfortunately, the hub region of any cast wheel is a site of metallurgical imperfections, such as dross, inclusions, and voids, which inherently result from the casting process. The presence of these imperfections in the vicinity of the central bore, which acts as a stress riser, renders the wheel highly susceptible to fatigue fracture in the hub region.
Accordingly, while economical to manufacture, cast compressor wheels are liable to failure. Failure of a compressor wheel necessitates at least replacement of the turbocharger, and may even cause damage to a vehicle engine. Thus, there is a need for a compressor wheel manufactured by a technique other than casting.
It is known that fatigue failures in compressor wheels can be significantly reduced by machining the compressor wheel from raw stock material, thereby avoiding the internal imperfections inherently resulting from a casting process. However, the complex machining requirements to form the impeller blades with the desired aerodynamic contours from wrought aluminum renders such a method for manufacture of aluminum compressor wheels impractical from a cost standpoint.
Titanium is much more difficult to work than aluminum, and material removal rates are low. Accordingly, the machining of titanium compressor wheels from wrought titanium—generally beginning with a billet or form in the shape of a bell—is out of the question due to both high cost and amount of time required to produce the final net shape.
U.S. Pat. No. 4,850,802 (Pankratz et al) entitled “Composite compressor wheel for turbochargers” attempts to side-step the flaws inherent in casting, and teaches a composite compressor wheel comprising a cast shell and a noncast hub insert. The cast compressor wheel shell is formed from relatively lightweight, low inertia material, such as aluminum or, a selected aluminum alloy, and includes aerodynamically contoured impeller blades and a hub section having a recess in the base. A hub insert of a non-cast material, e.g., billet, is secured into this recess, and is sized and shaped to occupy regions within the compressor wheel subjected to relatively high stress during operation. Since the hub insert substantially occupies high stress regions within the wheel, wheel fatigue life is improved.
The above technique has been applied to the manufacture of hybrid compressor wheels for gas turbine engines. See U.S. Pat. No. 4,335,997 (Ewig) entitled “Stress resistant hybrid radial turbine wheel” teaching a turbine rotor with radially extending blades for a gas turbine engine, wherein the hub may be forged titanium alloy, and wherein a shell of cast titanium alloy may be HIP bonded thereto to form a compressor wheel. This technique has, in practice, not even proven itself practical in the manufacture of gas turbine engine turbines, and certainly is much too costly and time consuming to be applied to mass production of small compressor wheels as employed in the automotive industry. Further, this technique requires separate manufacture then joining of two separate parts, and the integrity of the bond between the two parts is questionable.
There is thus a need for a simple and economical process for mass producing titanium compressor wheels, which process avoids the dimensional and structural imperfections such as dross, voids, and inclusions which inherently occur during a casting process, and which process also avoids the high cost associated with machining of titanium from blank. The process must be capable of reliably producing compressor wheels with high dimensional accuracy.
SUMMARY OF THE INVENTION
The present inventor investigated the above-described technical problems in the manufacture of titanium compressor wheels,
Borg-Warner Inc.
Cuda Rosenbaum I
Dziegielewski Greg
Pendorf & Cutliff
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