Metal founding – Means to shape metallic material – Including means to heat mold
Reexamination Certificate
2001-05-04
2002-06-25
Lin, Kuang Y. (Department: 1725)
Metal founding
Means to shape metallic material
Including means to heat mold
Reexamination Certificate
active
06408929
ABSTRACT:
The invention pertains to a method for the production of precision castings by the centrifugal casting, with controlled solidification, of a melt under vacuum or shield gas into a preheated mold with a central gate and several mold cavities extending toward the outside periphery of the mold, the mold cavities being surrounded by a material or a material combination with a coefficient of thermal conductivity which is lower than that of copper.
There is an increasing demand for components of titanium or alloys containing large amounts of titanium, because these materials have a low specific weight and yet are extremely strong, provided that the specific properties of titanium are taken sufficiently into account, these properties including a high melting point and a considerable degree of reactivity at high temperatures. At its melting temperature, titanium reacts not only with reactive gases, including oxygen in particular, but also with oxides and nearly all ceramics, because these usually consist at least predominantly of oxide compounds. Because titanium has a greater affinity for oxygen, oxygen is removed from the oxides, with the result that titanium oxides are formed. Some materials which have proven to be superior for use in certain areas are listed by way of example below:
pure titanium,
Ti 6 Al 4 V,
Ti 6 Al 2 Sn 4 Zr 2 Mo,
Ti 5 Al 2.5 Sn,
Ti 15 V 3 Al 3 Cr 3 Sn
Ti Al 5 Fe 2.5
50 Ti 46 Al 2 Cr 2 Nb, and
titanium aluminides.
Worthy of particular mention is the use of titanium aluminides e.g., TiAl, as materials for numerous types of components. Because of their low density, relatively high high-temperature strength, and corrosion resistance, the titanium aluminides are considered an optimum material in various areas of application. Because these materials are very difficult to shape, the only practical method of forming them is to cast them. Especially in the case of casting, however, titanium-containing metals present another set of problems, which will be discussed in greater detail below.
Some examples of ways in which titanium-containing materials are used are listed below:
valves for internal combustion engines,
turbine rotors and turbine vanes,
compressor rotors,
biomedical prostheses (implants), and
compressor housings in aircraft construction.
Both intake and exhaust valves of certain titanium alloys have been found to be extremely reliable, especially in automobile racing, with the result that thought is being given to the mass production of such valves for internal combustion machines of all types.
EP-0 443 544 B1 deals with the problem of improving the dimensional accuracy or accuracy of shape of centrifugal casting molds of copper and the removability of workpieces of titanium alloys from the molds by adding zirconium, chromium, beryllium, cobalt, and sliver as alloying elements to the copper, the sum of all alloying elements together not exceeding 3 wt. %. A comparison example in which the copper was alloyed with 18 wt. % of nickel did not lead to success. The publication in question discusses the electrical conductivity of the material but not its thermal conductivity, so that the problems involving a high quenching rate and the formation of shrinkholes and pores are not treated. On the other hand, this literature reference does discuss the disadvantages of mold materials consisting of ceramic or oxide materials.
DE 44 20 138 A1 also describes a method of the general type described above. From this document and DE 195 05 689 A1, molds for implementing such methods are known, in which at least the surfaces of the mold cavities which come in contact with the melt consist of a material selected from the group consisting of tantalum, niobium, zirconium, and/or an alloy A with at least one of these metals, i.e., materials with a thermal conductivity which is considerably less than that of copper and also with a specific heat capacity which is much less than that of copper. Insofar as base materials for these mold cavity surfaces are discussed, the base bodies consist of different metals in the case of the object of DE 44 20 138, but the condition remains fulfilled that the thermal conductivity and the heat capacity of the complete mold are lower than the corresponding values of copper. DE 195 05 689 A1 recommends materials from the group consisting of titanium, titanium alloys, titanium aluminide, graphite, and silicon nitride as base materials for the molds. These base materials have the advantage of a much lower specific weight and are therefore especially suitable for centrifugal casting molds.
With the methods and apparatuses according to DE 44 20 198 A1 an DE 195-05,689 A1, it has already become possible successfully to produce precision castings from quenching-sensitive materials on a large industrial scale. In these methods, the goal is significantly to reduce the high quenching rate, desired in the past as a way of avoiding reactions with the mold materials, and thus to reduce significantly the formation of shrinkholes, voids, pores, etc. in the castings, and especially to avoid the need for expensive reprocessing by high-pressure compaction (HIP method) and/or welding. To reduce the quenching rate even more, the two last-cited publications recommend that the molds be preheated to a minimum temperature of, for example, 800° C. For this purpose, it is provided that the mold is heated from the outside periphery; that is, the mold described in DE 44 20 138 A1 is surrounded by a heating cylinder. Because the walls of the gate must also reach the required temperature, it is necessary to heat up the entire volume of the mold to the temperature in question; and then, because the mold must also be cooled, it is necessary to cool its outside peripheryl by means of a gas with good thermal conductivity.
The known solutions are therefore energy-intensive and time-consuming, and the migration of the solidification front within the castings remains in a certain sense left to chance and/or depends to a considerable extent on the volume distribution of the castings. It is desirable for the solidification to occur in a controlled manner in the direction of the gate, so that the melt still present in that area can fill up any voids which may be forming in the casting.
The phrase “controlled solidification” is more comprehensive than the phrase “oriented solidification”, because the goal is not so much to create a certain preferential direction of the individual crystals but rather to control the direction in which the solid/liquid solidification front migrates.
The book by Kurz and Samm entitled
Gerichtet erstarrte eutektische Werk stoffe [Eutectic Materials with Oriented Solidification
], Springer-Verlag, Berlin-Heidelberg-New York, 1975, pp. 195-198, describes how relative motion can be brought about between a beating device and an individual casting mold located coaxially inside it. No heating rate is given, and the rate at which the casting mold is moved is the same as the rate at which the solidification front of the melt migrates.
The invention is therefore based on the task of providing a method of the general type described above Which makes it possible to reduce the amount of energy required and to achieve shorter cycle times and which also promotes solidification from the outside toward the inside, that is, in the direction of the gate.
According to the invention, the task described above is accomplished in conjunction with the method described above in that, before the melt is poured, the mold is heated, starting from the gate, until the gate reaches a temperature which is a function of the material being cast, the heating being carried out at a rate sufficient to produce a temperature gradient of at least 100° C. between the inside periphery and the outside periphery of the mold, the temperatures falling from the inside toward the outside.
The fundamental idea of the invention is based on a synergistic effect of the mold material and the heating direction. The use of a mold known in and of itself made of a material or a material combination with a c
Blum Matthias
Choudhury Alok
Gorywoda Marek
Jarczyk Georg
Lupton David Francis
Ald Vacuum Technologies AG
Fulbright & Jaworski L.L.P.
Lin Kuang Y.
LandOfFree
Method and apparatus for the production of precision... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for the production of precision..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for the production of precision... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2904460