Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2001-01-22
2003-04-22
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S236000, C075S245000, C075S229000, C419S012000, C419S013000, C419S014000, C419S020000
Reexamination Certificate
active
06551371
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a titanium-based composite material, which can be utilized for high-stress component members of a variety of machines, and a process for producing the same. In particular, it relates to a titanium-based composite material, which is suitable for engine valves for automobiles, etc., which are required to exhibit heat resistance, and a process for producing the same.
BACKGROUND ART
Since titanium alloys exhibit high specific strength and good toughness, they are used in various machinery component members. For example, with the U.S.A. and the U.K. as the central figure, titanium alloys have been used mainly in the fields of military, space and aircraft. Further, in these fields, heat resistant titanium alloys exhibiting good heat resistance have been developed energetically. However, since these heat resistant titanium alloys have been developed while being emphasized on their performances, they are expensive and lack mass-producing capability. Furthermore, it is difficult to melt and form them, and their yield rates were poor. Accordingly, these titanium materials were used only in limited fields.
However, recently, as the high-performance and light-weighting requirements of machinery are increased, titanium materials, especially titanium materials which are good in terms of heat resistance, have been given attention again in general machinery fields, such as automobile, etc. As one of the examples of the titanium materials, which are good in terms of the heat resistance, an automotive engine valve is hereinafter described.
Conventionally, engine valves are disposed in inlet ports and outlet ports of an engine, and they are an important component part which determines the performance of the engine, such as the fuel consumption, the efficiency, the output, and so on. Further, the engine valves become high temperatures exceeding 600° C. In particular, the valves (exhaust valves) in the exhaust system become considerably higher temperatures than the valves (intake valves) in the intake system. For instance, even in a mass-produced engine, since the exhaust valves are subjected to a higher temperature, there may be a case where the exhaust valves become at around 800° C. Therefore, the exhaust valves are required to exhibit good heat resistance. The conventional exhaust valves for mass production have used a heat resistant steel, such as SUH35, etc., as per JIS standard.
However, when the heat resistant steel, such as SUH35, is used in a reciprocating component part, like the valve, its inertial weight increases, because the specific gravity is large. Consequently, the maximum number of the revolutions is limited, further, since it is necessary to increase the spring load, the friction enlarges, and the engine is inhibited from being high performance.
Hence, it is considered to apply a titanium material, which is good in terms of the specific strength, etc., to the engine valve. Since the titanium material is light weight, and since it is superb in terms of the mechanical properties, it is a very attractive material. When the titanium material is applied to the engine valve, it is possible to reduce the inertial weight, to make it produce a higher output, and to improve the fuel consumption. Accordingly, titanium materials have been employed earlier for engine valves for racing cars.
However, in view of the costs, the titanium materials have not been employed for mass-produced engine valves. In particular, since the conventional titanium material has a working limit temperature of about 600° C., it is difficult to employ it to the component members, like exhaust valves, which are used in elevated temperature ranges.
Next, the heat resistance of titanium materials will be investigated. Generally, the heat resistance of titanium alloys is governed by the structure. The structure is determined by the alloy composition, the processing temperature, the processing degree and the heat treatment conditions after processing. In particular, the structure is affected greatly by the processing temperature.
For example, there is a case where the heat resistance of titanium materials is enhanced by containing silicon in the titanium materials. In this case, by taking the relationship between the &bgr; transformation temperature and the solid solution temperature of a silicon compound (silicide) into consideration, it is necessary to determine the processing temperature. Specifically, in the case that the &bgr; transformation temperature is higher than the solid solution temperature of the silicide, when a titanium alloy (for example, Ti—Al—Sn—Zr—Nb—Mo—Si-based titanium alloy) is processed by hot working at a high temperature of the &bgr; transformation temperature or more, coarse acicular microstructure has been formed. This acicular microstructure is unpreferable, because it becomes the causes of the casting breakage, the deterioration of elongation and the degradation of low cycle fatigue property.
While, the processing at the &bgr; transformation temperature or less is generally difficult, because the deformation resistance is large. It is understood from this example that the processing ability decreases when it is intended to improve the heat resistance of titanium material. Accordingly, it is difficult to obtain the compatibility between the heat resistance and the processing ability.
In order to solve such assignments, and to further improve the heat resistance, etc., of titanium materials, various proposals have been made, for instance, as follows.
{circle around (1)} Japanese Examined Patent Publication (KOKOKU) No. 4-56,097 (registered No. 1,772,182), an Al—Sn—Zr—Nb—Mo—Si-contained alloy, in which a trace amount of C is contained, is disclosed. This titanium alloy is enhanced in terms of the heat resistance, the heat treating property and the hot working property by adding a trace amount of C so that the &agr;+&bgr; region, which is the temperature range of the heat treatment and the hot working, is enlarged.
However, in the case of this titanium alloy, the temperature (working limit temperature), at which a sufficiently high temperature tensile strength and fatigue property are obtained, is 600° C. approximately. Further, this titanium alloy is produced by melting, casting and forging, which are regarded as basic processes. Hence, the costs go up, and accordingly it is not suitable for mass-produced articles, such as automotive component parts, which are required to be low costs.
Furthermore, although the &agr;+&bgr; region is enlarged, the solid solution temperature of the silicide is lower than the &bgr; transformation temperature. Consequently, when hot working is carried out at a temperature higher than the &bgr; transformation temperature, coarse acicular microstructures have been formed. In order to avoid this, in the publication, eventually, the processing is carried out at a temperature of the &bgr; transformation temperature or less. Therefore, although the titanium alloy forms the balanced bi-modal structure in view of the material properties, it still exhibits large processing resistance, and the hot working property is not fully improved.
{circle around (
2
)} In Japanese Unexamined Patent Publication (KOKAI) No. 4-202,729, there is disclosed an Al—Sn—Zr—Nb—Mo—Si-contained alloy, in which Mo is added in an especially large amount, is disclosed. Thus, the heat resistance of the alloy is improved to about 610° C.
However, even in this case, similarly to the titanium alloy of Japanese Examined Patent Publication (KOKOKU) No. 4-56,097, the heat resistance is insufficient. In addition, the addition of Mo in a large amount is unpreferable, because it causes the deterioration of the high temperature tensile strength.
Further, a titanium alloy is disclosed which further contains at least one member selected from the group consisting of C, Y, B, rare-earth elements and S in a total amount of 1%. Thus, the heat resistance, specifically, the creep resistance is improved.
However, even in this case, a sufficient creep property
Furuta Tadahiko
Saito Takashi
Takamiya Hiroyuki
Yamaguchi Toshiya
Kabushiki Kaisha Toyota Chuo Kenkyusho
Mai Ngoclan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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