An electrically heatable caulking method

Abstract: An electrical caulking method of a work which is made of magnesium or a magnesium alloy. The work includes a body and a caulking pin integrally projected from the body. The method includes a step of holding the work between opposing two electrodes in such a manner that the first electrode contacts the caulking pin and the second electrode contacts the body, a step of supplying an electric current between these electrodes and a step of exerting a pressure so as to caulk the caulking pin into a desired shape. At least the first electrode which contacts the caulking pin of the work is made of a material having a low thermal diffusivity and is difficult to alloy with magnesium.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an electrically heatable caulking method and, more particularly, to an electrically heatable caulking method adapted for caulking a work made of magnesium or any suitable magnesium alloy and having one or more caulking pins. 2. Description of the Related Art For instance, a light weight aluminum alloy or a steel plate has been conventionally and widely used as structural members of a vehicle, such as a cylinder head cover or a buffer plate. When these members are secured to another member, a so-called electrically heatable caulking method is widely used as mentioned below. First, caulking pins provided on one of the members are inserted into corresponding caulking pin holes formed in the other member. These two members are thus superimposed together and placed between two opposing electrodes, by which an alternative electric current is supplied between these two members and, in addition, a pressure force is also exerted therebetween. Also, recently, research has been conducted so as to discover a method using a magnesium or a magnesium alloy, having a specific gravity smaller than that of the above-mentioned aluminum alloy, in place of the latter. In an application of structural members of a vehicle, it is possible to use a magnesium alloy as the material of a cylinder head cover. However, in the prior art, it has been conventionally considered that, apart from a light weight aluminum alloy, it is technically difficult to use a magnesium alloy in an electrically heatable caulking method for attaching a cylinder head cover made of a magnesium alloy to a buffer plate, because a plastic working of magnesium alloy is more difficult than that of an aluminum alloy and, therefore, according to a conventional caulking method, it is difficult to work the magnesium alloy to a desired shape. SUMMARY OF THE INVENTION Under the above circumstances, according to the inventor's research, it has been found that, if the electrodes are made of a specified material and if the welding conditions, such as current ampere for welding, welding time, pressure or the like, are suitably selected, it will be possible to use a magnesium or magnesium alloy to obtain a favorable caulking. An object of the present invention is to provide an electrical caulking method that can use a magnesium or magnesium alloy. According to the present invention there is provided an electrical caulking method of a work that is made of magnesium or a magnesium alloy and comprises a body and a caulking pin integrally projected from said body, said method comprising: a step of holding said work between opposing first and second electrodes in such a manner that said first electrode contacts said caulking pin and said second electrode contacts said body, a step of supplying an electric current between said first and second electrodes and a step of exerting a pressure therebetween so as to caulk said caulking pin into a desired shape; characterized in that at least said first electrode that contacts said caulking pin of said work is made of tungsten. The above method may further comprise a second work that is to be attached together with said first work; said method comprising: a step of placing said first and second members together in such a manner that said caulking pin is inserted into said caulking pin inserting hole. Tungsten, has a low thermal diffusivity and is difficult alloying with magnesium which is advantageous. It is also advantageous for an alternate electric current to be supplied between said first and second electrodes. It is still advantageous for an electric current density to be 100-160 A/mm 2 . It is further advantageous for a pressure of 127-216 N/mm 2 (13-22 Kgf/mm 2 ) to be exerted between said first and second electrodes. It is still further advantageous for said second electrode, which contacts said body of the work to be made of a Cr-Cu alloy. In the present invention, the caulking pin of the work made of magnesium or a magnesium alloy can be appropriately plastically deformed and a good caulking effect can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view for illustrating an electrically heatable caulking method according to the present invention (before caulking); Figure 2 is a cross-sectional view similar to Fig. 1, but illustrating the state after caulking. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein Figs. 1 and 2 illustrate an electrically heatable caulking method according to the present invention. The work 1 is a member made of a magnesium alloy and comprises a plate-like body having a thickness of 6.0 mm. The body 1 is provided integrally at a predetermined position or positions with one or more caulking pins 1a projected from the body 1. The caulking pins 1a have a diameter of 6.9 mm and a height of 5.5 mm. Another work 2, which is to be attached to the work 1 by a caulking process according to the present invention, is a plate made of the same material as the member 1, i.e., a magnesium alloy. Otherwise, the plate 2 may be made of another material. The plate 2 is provided at a corresponding position or positions for the caulking pin 1a with one or more caulking pin inserting holes 2a, with a diameter of 7.9 mm. When attaching the plate 2 together with the member 1, as shown in Fig. 1, the plate 2 is first placed on the member 1 in such a manner that the caulking pins 1a are fit into the caulking pin inserting holes 2a. These members 1 and 2 are then held between opposing electrodes 3 and 4 in such a manner that the upper electrode 3 contacts the upper end of the caulking pin 1a and the lower electrode 4 contacts the member 1 at a corresponding position, respectively. At least one of the electrodes 3 and 4, i.e., the upper electrode 3 that contacts the caulking pin 1a of the member 1 of a magnesium alloy is made of tungsten with a diameter of 20.0 mm in this embodiment, because tungsten is a typical material having high melting point and hardness, low thermal diffusivity and difficulty alloying with magnesium. In this embodiment, the other electrode, i.e., the lower electrode 4 is made of Cr-Cu alloy (content of Cr: 1.0%) and has an outer diameter of 50.0 mm. When alternate electric power and pressure are exerted simultaneously between the opposing upper and lower electrodes 3 and 4, the caulking pin or projection 1a is deformed so as to close the caulking pin inserting hole 2a, as shown in Fig. 2. It has been found that a most favorable caulking result can be obtained if the caulking operation is conducted under any selected conditions within the following current and pressure ranges. Consequently, the force exerted between said upper and lower electrodes 3 and 4 is 3470-7920 N (350-800 Kgf), the current ampere is 3,000-8,000 A, and the electric power supply time is 0.2-1.6 sec. The following Tables 1 and 2 show the experimental results of the above-mentioned caulking operation. The evaluation conditions are as follows: With respect to the initial condition before deformation by caulking, the caulking pin 1a has a diameter of 6.9 mm and a height of 5.5 mm; o ○ indicates "good" samples, i.e., if the caulking pin 1a, after being deformed by the caulking process, has an outer diameter d, more than 9.8 mm, at a deformed head portion thereof, and a height H of not more than 3.0 mm, and there are no excess slits or crushes; ○ indicates "agreeable in practical use", if the caulking pin 1a, after being deformed by the caulking process, has an outer diameter d of more than 9.0 mm, at a deformed head portion thereof, and a height H of not more than 3.5 mm, and there are less slits or crushes; ▵ indicates "little difficultly in practical use", if the caulking pin 1a, after being deformed by the caulking process has an outer diameter d of less than 9.0 mm, at a deformed head portion thereof, and a height H of more than 3.5 mm; and X indicates "not agreeable in practical use", if the sample has slits or crushes and it is difficult to use. Force (N) Ampere of Current (A) Time of Current Supply (sec) 0.2 0.4 0.8 1.2 1.6 3470 (350 Kgf) 3,000 X d: 7.1 H: 5.2 4,000 X d: 7.0 X d: 8.0 H: 5.2 H: 4.3 5,000 ▵ d: 9.0 H: 3.7 4900 (500 Kgf) 4,000 X d: 7.5 ▵ d: 8.3 ○ d: 9.0 H: 4.7 H: 3.9 H: 3.5 5,000 ○ d: 9.1 o ○ d: 9.8 o ○ d:10.1 o ○ d:10.3 H: 3.5 H: 2.9 H: 2.8 H: 2.4 6,000 X d: 7.8 ▵ d:10.0 ▵ d:10.6 H: 4.4 H: 2.5 H: 2.2 7,000 X d:10.3 H: 2.6 5880 (600 Kgf) 4,000 ○ d: 9.0 o ○ d: 9.9 H: 3.5 H: 2.7 5,000 ○ d: 9.5 o ○ d:10.2 o ○ d:10.7 H: 3.1 H: 2.5 H: 2.3 6,000 ▵ d: 8.4 o ○ d:10.1 o ○ d:11.0 o ○ d:11.0 H: 4.0 H: 2.5 H: 2.2 H: 2.2 As shown in Tables 1 and 2, it was found that a most favorable caulking result can be obtained if the caulking operation is conducted under the conditions such that a tungsten electrode is used as the electrode, and with respect to the caulking pin 1a made of a magnesium alloy (the diameter thereof being 6.9 mm), the force exerted between said upper and lower electrodes 3 and 4 is 4900-7840 N (500-800 Kgf), i.e., 131-210 N/mm 2 (13.4-21.4 Kgf/mm 2 ) if represented per a unit area, the current ampere is 4,000-6,000A, i.e., the electric current density is 100-160 A/mm 2 , and the electric power supply time is 0.2-1.6 sec. As mentioned above, according to the present invention, since at least the electrode that contact the caulking pin of the work is made of tungsten, and the caulking or welding conditions are selected appropriately as mentioned above, it becomes possible to use a magnesium or a magnesium alloy as the material to be worked under a caulking process that has been considered impossible in the prior art. Therefore, the present invention can be effectively applied in various fields of the art, such as a lightening technique in articles of a vehicle. It should be understood by those skilled in the art that the foregoing description relates to only a preferred embodiment of the disclosed invention, and that various changes and modifications may be made to the invention without departing from the scope thereof, as defined in the claims.

Claims: An electrical caulking method of a work that is made of magnesium or a magnesium alloy and comprises a body (1) and a caulking pin (1a) integrally projected from said body (1), said method comprising: a step of holding said work between opposing first and second electrodes (3,4) in such a manner that said first electrode (3) contacts said caulking pin (1a) and said second electrode (4) contacts said body (1), a step of supplying an electric current between said first and second electrodes (3,4) and a step of exerting a pressure therebetween so as to caulk said caulking pin (1a) into a desired shape; characterized in that at least said first electrode (3) that contacts said caulking pin (1a) of said work is made of tungsten. A method as set forth in claim 1, further comprises a second work (2) which is to be attached together with to said first work, said method comprising: a step of placing said first and second members (1,2) together in such a manner that said caulking pin (1a) is inserted into said caulking pin inserting hole (2a). A method as set forth in any one of claims 1 and 2 wherein an alternating electric current is supplied between said first and second electrodes (3,4). A method as set forth in any one of claims 1 to 3 wherein an electric current density is 100 to 160 A/mm 2 . A method as set forth in any one of claims 1 to 4 wherein a pressure of 127-216 N/mm 2 (13 to 22 Kgf/mm 2 ) is exerted between said first and second electrodes (3,4). A method as set forth in any one of claims 1 to 5 wherein said second electrode (4) which contacts said body (1) of the work is made of an Cr-Cu alloy.

Inventors: MATSUMOTO JIRO, YANAGISAWA IKUO, MATSUMOTO, JIRO, YANAGISAWA, IKUO, Matsumoto, Jiro, c/o NIKKEI TECHNO-RES., Co., Ltd., Yanagisawa, Ikuo, c/o NIPPON LIGHT METAL Co., Ltd.
Applicants: NIPPON LIGHT METAL CO, NIPPON LIGHT METAL CO., LTD.
Application year: 1993
Main classification: B21D39
IPC codes: H05B3, B21J15, B21D39, B21D53, B29C65