Metal founding – Process – Shaping a forming surface
Reexamination Certificate
2002-05-10
2003-07-08
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping a forming surface
C164S045000, C164S113000, C164S137000
Reexamination Certificate
active
06588485
ABSTRACT:
FIELD OF THE INVENTION
Cost considerations have prevented the use of titanium compressor wheels in automotive air boost devices. The present invention concerns 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 into the volute-shaped chamber of a compressor housing. In order to accomplish these three distinct functions with maximum efficiency, 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 (MPS) 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 “rise” 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-to-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 rake angle (angle of surface relative to center line), (b) an angle offset from radial, and/or 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 expelled in this way has not only high flow, but permits recovery of high pressure over a wide flow range.
Accordingly, functional considerations dictate the complex shape of a compressor wheel.
Recently, tighter regulation of engine exhaust emissions has led to an interest in even higher pressure ratio boosting devices. Current aluminum compressor wheels are not capable of withstanding repeated exposure to higher pressure ratios (>3.8). While aluminum is a material of choice for compressor wheels due to low weight and low cost, the temperature at the blade tips, and the stresses due to increased centrifugal forces at high RPM, exceed the capability of conventionally employed aluminum alloys. Refinements have been made to aluminum compressor wheels, but due to the inherent limited strength of aluminum, no further significant improvements can be expected. Accordingly, high pressure ratio boost devices have been found in practice to have short life, to be associated with high maintenance cost, and thus have too high a product life cost for widespread acceptance.
Titanium, known for high strength and low weight, might at first seem to be a suitable next generation material. Large titanium compressor wheels have in fact long been used in aircraft jet engines that power aircraft from the B-52B/RB-52B to the F-22. However, titanium is one of the most difficult metals to work with, and currently the cost of production associated with titanium compressor wheels is so high as to limit wide-spread employment of titanium. It is also well known that titanium is highly reactive in the molten state, making it particularly difficult to cast titanium into thin molds without significant mold/metal reaction. This reaction layer must be removed at significant expense. Thin sections aggravate the problem of obtaining a sound casting free of this reaction layer.
The automotive industry is driven by economics. While there is a need for a high performance compressor wheel, it must be capable of being manufactured at reasonable cost. There are presently no known cost-effective manufacturing techniques for manufacturing automobile or truck industry scale titanium compressor wheels having the optimal design described above.
That is, while titanium compressor wheels per se are known, the methods by which they are manufactured are economically prohibitive. For example, it is known to manufacture titanium compressor wheels from solid titanium stock, using computer-aided manufacturing (CAM) equipment, also known as numerically-controlled cutting equipment. However, due to the difficulty of working with titanium, and due to the large amount of material which must be removed, this technique does not come into consideration as an economical means for production of titanium compressor wheels.
Casting techniques are also known, and can be classified into “rubber mold” techniques and “investment casting” techniques.
U.S. Pat. No. 6,019,927 (Galliger) entitled “Method of Casting a Complex Metal Part” teaches a method for casting a titanium gas turbine impeller which, though different in shape from a compressor wheel, does have a complex geometry with walls or blades defining undercut spaces. A flexible and resilient positive pattern is made, and the pattern is dipped into a ceramic molding media capable of drying and hardening. The pattern is removed from the media to form a ceramic layer on the flexible pattern, and the layer is coated with sand and air-dried to form a ceramic layer. The dipping, sanding and drying operations are repeated several times to form a multi-layer ceramic shell. The flexible wall pattern is removed from the shell, by partially collapsing with suction if necessary, to form a first ceramic shell mold with a negative cavity defining the part. A second ceramic shell mold is formed on the first shell mold to define the back of the part and a pour passage, and the combined shell molds are fired in a kiln. A high temperature casting material is poured into the shell molds, and after the casting material solidifies, the shell molds are removed by breaking.
It is apparent that the Galliger gas turbine flexible pattern is (a) collapsible and (b) is intended for manufacturing large-dimension gas turbine impellers for jet or turbojet engines. This technique is not suitable for mass-production of automobile scale compressor wheels with thin blades, using a non-collapsing pattern. Galliger does not teach a method which could be adapted to in the automotive industry.
“Investment casting”, on the other hand, involves: (1) making a wax pattern of a hub with cantilevered airfoils, (2) casting a refractory mass about the wax pattern, (3) removing the wax by solvent or thermal means, to form a casting mold, (4) pouring and solidifying the casting, and (5) removing the mold materials.
There are however significant problems associated with the initial step of forming the compressor wheel wax pattern. Whenever a die (comprised of retractable die inserts) is used to cast the wax pattern, the casting die must be opened (die inserts retracted) to release the product. However, since the blades of a compressor wheel have a complex shape as discussed above, the complex geometry of the compressor wheel, with undercut rece
Borg-Warner Inc.
Dziegielewski Greg
Lin Kuang Y.
Pendorf & Cutliff
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