Electric heating – Metal heating – Bonding
Reexamination Certificate
1999-03-26
2001-11-27
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
Bonding
C219S162000, C219S149000, C219S156000, C228S194000, C228S248500
Reexamination Certificate
active
06323458
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
1. Field of the Invention
The present invention relates to a joined metal member comprising two metal members joined with each other by press and heat generated on passing an electrical current through the metal members, and a method of joining said member.
2. Prior Art
Various methods are known for joining metal members in such cases as joining a valve seat of an engine cylinder head onto the periphery of an opening of air intake or exhaust port of the cylinder head body.
For example, Japanese Patent Publication JP-A 8-100701 discloses a method of joining a valve seat with an Al-based cylinder head body by brazing using an Al—Zn brazing material and a fluoride flux.
Also such a method is known as disclosed in Japanese Patent Publication JP-A 58-13481, where metal members are joined by resistance welding which utilizes the heat generated by contact resistance between mating surfaces of the two members. With regard to the resistance welding, it has been proposed to increase the amount of heat generated in the mating surfaces by infiltrating pores in a valve seat made of a sintered material with an infiltration metal, thereby decreasing the amount of heat generated inside the sintered material as disclosed in Japanese Patent Publication JP-A 6-58116, or to form a film on the valve seat surface and fuse the film when welding the valve seat with the cylinder head body as disclosed in Japanese Patent Publication JP-A 8-270499.
Further, as disclosed in Japanese Patent Publication JP-A 8-200148, it is proposed to make solid-phase diffusion joining (pressure welding) of the valve seat and the cylinder head body while forming a plastic-deformed layer on the joining surface of the cylinder head body without forming a molten reaction layer
When metal members are joined by press and heat generated on passing an electrical current through the metal members as in the resistance welding or solid-phase diffusion joining process, in case the one of the metal members is a sintered metal such as valve seat, it is desired to infiltrate the pores inside of the sintered material with a metal having a high electrical conductivity such as Cu as proposed in Japanese Patent Publication JP-A 6-58116. This makes it possible to lower the bulk resistivity of the sintered material by means of the infiltrating metal and inhibit the heat generation inside the sintered material when passing a current, thereby effectively generating heat in the mating surfaces and achieving better joint.
However, impregnation of an infiltrating metal into the sintered material requires an additional step in the process, which leads to increased production cost. Also, because the infiltrating metal generally increases the thermal conductivity, a valve seat or the like to be used at a high temperature suffers such a problem that the heat dissipation efficiency becomes too high due to the combined effects of both the increased thermal conductivity and the joining effect, thus making it difficult to form an oxide film on the surface, which may lead to poor wear resistance on the valve sheet.
SUMMARY OF THE INVENTION
An object of the present invention provides a method of joining two metal members by pressurizing and heating due to generated heat on passing electrical current through the metal members using a proper sintered material for the first metal member, in order to reduce the production cost by eliminating the infiltration step and, particularly in the case of joining the valve seat to the cylinder head, to improve the wear resistance of a valve seat by inhibiting an increase in thermal conductivity.
In order to achieve the object described above, the present invention employs, as a first metal member, a sintered material made by sintering a powdery material containing particles having high electrical conductivity dispersed therein.
With this scheme, although the first metal member includes pores therein unlike an infiltrated material which is infiltrated with Cu or the like, the first metal member is sintered with the particles having high electrical conductivity dispersed therein beforehand and, therefore, resistivity of the bulk of the first metal member can be maintained at a somewhat high level comparable to that of the infiltrated material. This makes it possible to inhibit the heat generation when passing a current, despite the presence of the pores, and achieve a better joint without the need for infiltrating the material with Cu or the like after sintering.
On the other hand, the thermal conductivity can be kept lower than that of the infiltrated material due to the insulating effect of the pores, and therefore in the case where the first metal member is a valve seat to be used at a high temperature, an oxide film is apt to be formed on the valve sheets while in use because of the heat sink efficiency being maintained at a proper level, then, allowing the wear resistance of the valve surface of the sheet to be improved. Thus it is made possible to reduce the production cost while achieving satisfactory joining and improve the wear resistance of the first metal member when the joined metal member is used at a high temperature.
Particles having high electrical conductivity herein are defined as a substance having an electrical resistivity of not higher than 3×10
−8
&OHgr;m.
According to the present invention, copper is used as the high electrical conductivity. This makes it possible to effectively decrease the resistivity of the bulk of the first metal member while keeping the production cost low.
In the present invention the first and second metal members previously are located into surface contact with each other before a heat and pressure are applied thereto.
According to this invention, it is made possible to effectively prevent the first metal member from overheating during joining, because the first metal member has a lower thermal conductivity than the infiltrated material.
Generally, in the case of supplying a current and applying a pressure after bringing the base metals into point or linear contact with each other, heat generated in the mating surfaces in the early stage of current flow is concentrated significantly in the vicinity near the contact point or line. As a result, the first metal member is likely to be overheated due to the low thermal conductivity. Moreover, since the first metal member has relatively lower strength due to porosity, the surface to be joined of the first metal member is likely to deform.
However, according to the present invention, the amount of generated heat in the joined surface in surface contact is optimized, and overheat of the first metal member can be prevented despite the low thermal conductivity. Consequently, the pressure can be surely transmitted across the interface of the mating surfaces while preventing the mating surfaces from deforming, and achieve joining more satisfactorily. Area of the surface contact herein may desirably be in a range of 40 to 200 mm
2
(more preferably 40 to 100 mm
2
).
The first metal member preferably may have electrical conductivity higher in the portion thereof on the side opposite to the second metal member than in the portion on the side toward the second metal member.
This scheme makes it possible to control the heat generated in the joined surface to an optimum level while minimizing the heat generated in the first metal member, and make the portion of the first metal member in the vicinity of the joined surface to have a high strength, thus more effectively preventing the first metal member from deforming due to overheat in the joined surface during joining. On the other hand, it is easily to make the first metal member have different values of electrical conductivity in the portion thereof on the side opposite to the second metal member and in the portion on the side nearer to the second metal member. For example, those portions may be made of different materials and temporarily sintered separately, and then put together and subject to final sintering. Thus ease of joining can be im
Nanba Satoshi
Nomura Seiji
Elve M. Alexandra
Mazda Motor Corporation
Nixon & Peabody LLP
Studebaker Donald R.
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