Metal founding – Means to shape metallic material – Pressure shaping means
Reexamination Certificate
2003-10-01
2004-08-17
Stoner, Kiky (Department: 1725)
Metal founding
Means to shape metallic material
Pressure shaping means
C164S289000, C164S361000, C164S267000
Reexamination Certificate
active
06776214
ABSTRACT:
I. FIELD OF THE INVENTION
The invention relates to methods for making metallic alloys such as titanium base alloys into castings of various symmetric and asymmetric shapes, cylinders, hollow tubes, pipes, rings and other tubular products by melting the alloys in a vacuum or under a low partial pressure of inert gas and subsequently centrifugally casting the melt under vacuum or under a low pressure of inert gas in molds machined from fine grained high density, high strength isotropic graphite, the said molds either revolving around its own horizontal or vertical axis or centrifuging around a vertical axis of rotation.
II. BACKGROUND OF THE INVENTION
The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material for many applications. Titanium alloys are used for static and rotating gas turbine engine components. Some of the most critical and highly stressed civilian and military airframe parts are made of these alloys. The use of titanium has expanded in recent years from applications in aerospace structure to food processing plants and from oil refinery heat exchangers to marine components and medical prostheses. However, the high cost of fabricating titanium alloy components may limit their widespread use.
Some materials which have been found to give excellent results in certain areas of application are listed below by way of example: Pure Ti, Ti—6Al—4V, Ti—6Al—2Sn—4Zr—2Mo, Ti—5Al—2.5Fe, Ti—15V—3Al—3Cr—3Sn, Ti—46Al—2Cr—2Nb, Ti—50Al.
Another family of titanium alloys based on the intermetallic Ti—50Al compositions are being considered for various applications because of their low density, relatively high strength at high temperatures, and corrosion resistance.
While complex shapes of titanium alloys are fabricated by the casting route, somewhat simpler shapes such as seamless rings, hollow tubes and pipes are manufactured by various other thermo-mechanical processing routes. The relatively high cost of titanium components is often fabricating costs, and, usually most importantly, the metal removal costs incurred in obtaining the desired end-shape. As titanium has become a commonly used engineering material there has been a need to produce complex shapes economically. As a result, in recent years a substantial effort has been focused on the development of net shape or near-net shape technologies such as powder metallurgy (PM), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used net shape technology.
High performance titanium castings are used in large numbers in the aerospace industry while the chemical and energy industries primarily use large castings where corrosion resistance is a major consideration in design and material choice. The microstructure of as-cast titanium is desirable for many mechanical properties such as creep resistance, fatigue crack growth resistance, fracture resistance and tensile strength. Titanium castings are essentially equal in strength, fracture toughness and fatigue crack growth resistance to the corresponding wrought products.
Many titanium castings with precision and complex geometries are made by the well known investment casting process wherein an appropriate melt is cast into a preheated ceramic investment mold formed by the lost wax process, the castings are generally made in static molds. Although defects such as inclusions, gas porosity, hot tears, shrink cavities and mold/metal reactions are common to all foundry products, dealing with these problems require a different approach when casting titanium. The inability to superheat titanium melt in a cold crucible coupled with narrow liquidus/solidus temperature of molten titanium often requires the need of the centrifugal casting technique for making high quality thin walled configurations. A typical centrifugal investment casting machine spins radially symmetric molds about its own axis in a vertical orientation. Simultaneous rotation of a tree of molds located along the perimeter of a circle on a horizontal plane where melt is poured into a central sprue lying along the vertical axis of the tree creates high velocity flow of titanium melt under the action of centrifugal force. By rotation of the tree the melt flows into the mold cavities, keeping contact with one of the vertical inside walls of a gate and a mold cavity. Centrifugal force allows the melt to flow into even the most obscure crevices of the mold cavities The action of centrifugal force leads to improved mold filling and production of high quality precision castings of titanium alloys. The centrifugal force imposed on the melt enhances removal of gas bubbles and reduces the number of gaseous defects to a minimum and improves the mechanical properties.
U.S. Pat. No. 6,250,366, U.S. Pat. No. 6,408,929 and U.S. Pat. No. 6,443,212 disclose a technique and apparatus suitable of production of titanium castings via centrifugal casting in which the molds are arranged about a central axis of rotation like the spoke of a wheel, thus permitting multiple castings is also used to produce sound titanium castings. However, there are certain drawbacks associated with centrifugal casting of titanium in ceramic investment molds. During high velocity flow of melt through the mold cavities under the action of centrifugal force, ceramic walls/linings of the molds in contact with the highly reactive titanium base alloy melts are likely to cause cracking and spalling leading to formation of very rough, outside surface of the casting. The ceramic liners spalling off the mold are likely to get trapped inside the solidified titanium castings as detrimental inclusions which will significantly lower fracture toughness properties of the finished products.
Titanium alloys are fabricated in shapes such as seamless ring configurations, hollow tubes and pipes and find many engineering applications in jet engines such as compressor casings, seal and other high performance components for oil and chemical industries.
FIG. 1
shows a diagram of a turbine casing
10
and a compressor casing
20
. The compressor casing is made of titanium alloys.
FIG. 2
shows a cutway diagram of a turbofan engine and the compressor casing
30
made of titanium alloy. Seamless rings can be flat (like a washer), or they can feature higher vertical walls (approximating a hollow cylindrical section). Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall-thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing.
There are two primary processes for fabricating seamless rings of titanium alloys. In the ring forging process also called saddle-mandrel forging, an upset and punched ring blank is positioned over a mandrel, supported at its ends by saddles on a forging press. As the ring is rotated between each stroke, the press ram or upper die deforms the metal ring against the expanding mandrel, reducing the wall thickness and increasing the ring diameter.
In continuous ring rolling, seamless rings are produced by reducing the thickness of a pierced blank between a driven roll and an idling roll in specially designed equipment. Additional rolls (radial and axial) control the height and impart special contours to the cross-section. Ring rollers are well suited for, but not limited to, production of larger quantities, as well as contoured rings. In practice, ring rollers produce seamless rolled rings to closer tolerances or closer to finish dimensions.
FIGS. 3A-3G
schematically show the various steps of seamless rolled ring forging process operations.
FIG. 4
shows a ring rolling machine in operation.
FIGS. 3A-3G
show an embodiment of a seamless rolled ring forging process operation to make a ring
40
.
FIG. 3A
shows the ring rolling process typically begins with upsetting of the starting stock
42
on flat dies
44
at its plastic deformation temperature—in the cas
Ray Ranjan
Scott Donald W.
Lin I.-H.
Santoku America, Inc.
Stevens Davis Miller & Mosher LLP
Stoner Kiky
LandOfFree
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