HOT DIFFUSION WELDING

Abstract: A first member of copper alloy and a second member of another matrial are joined with an insert between them by hot diffusion welding with heat and pressure. The insert material is a copper plating formed on the joining surface of the second member to be joined, whereby the thermal conductivity on the joining surfaces between the two members is improved to effect electric resistance heating smoothly, so that continuous working is made possible without polishing for removing oxide from the electrically conductive surface of a pair of electrodes. Furthermore, when the insert material interposed between the joining surfaces of the both members as described above is of a normal brazing material, an even number of superposed bodies, each consisting of a first member, an insert material and a second member to be joined, are superposed on one another in such a manner that the respective first members are brought into abutting contact with each other, and subjected to electric resistance heating. In this case, the second member having a high thermal capacity is abutted against the pair of electrodes, whereby there is no need for polishing to remove oxide from the electric conductive surfaces of the pair of electrodes, so that the continuous working is made possible by hot diffusion welding method according to the present invention.

Description: The invention relates to a hot diffusion welding method according to the preamble of claim 1 (see JP-A-59-24592). BACKGROUND ART In order to produce machine parts capable of serving their individual, characteristic functions, a casting method has been conventionally and extensively employed in joining the first and second parts which are formed of metallic materials having different properties (in this specification, the first part is a part to which another part is to be joined and the second part is a part to be joined to the first part). However, such a casting method is limited in its application when the second part is made from an alloy. Because it is inevitably required in the casting method to completely melt the second part, and in the case that the second part is made from an alloy and this alloy especially contains a metal having a low-boiling point, such a metal vaporizes during the melting of the entire alloy. This causes such a phenomenon that when the second part is made from an alloy of copper and zinc for example, zinc vaporizes just before the melting point of the alloy and forms a layer of zinc or zinc oxide on the surface of the second part. This phenomenon reduces the bonding strength of the second part at the joint face of the first part, or causes a difference between the copper-zinc composition ratio of the second part and that of the casting material. Therefore, it has been thought to be very difficult to unite the second part made from an alloy to the surface of the first part by means of the casting method. Diffusion welding is a method of uniting parts by not melting but applying pressure to the second part so as to be joined to the first part. Like the ultra-high pressure welding method and the HIP method, in which ultra-high pressure (hydrostatic pressure in the case of the HIP method) is applied to the first and second parts which have been superposed, in the coaxial superposed direction thereof, diffusion welding is suitably employed for the second part made from one of the above-mentioned materials that are difficult to unite by casting. Of the above welding techniques taken in a solid phase condition, the hot diffusion welding method, in which an insert material is interposed between the first and second parts and each part is subjected to heating and pressurizing so that they are united, can be the most promising in achieving excellent bonding effects by suitably selecting an insert material. In the hot diffusion welding method, it is common to utilize resistance heat as heating means, the resistance heat being generated by supplying a current. One example of such a technique conventionally used can be seen in the welding of a sliding member for a hydraulic apparatus. According to this conventional technique, as shown in the Figure, a brazing filler metal of normal type 33 serving as the insert material is interposed between a first part 31 such as a shoe formed of a low alloyed steel (hereafter referred to as SCM material) and a second part 32 such as a sliding member formed of a copper alloy (hereafter referred to as PBC material), thereby forming a body in which the first part 31, the brazing filler metal 33 and the second part 32 are superposed. Then, the hot diffusion welding method is performed in such a way that while a pressure 38 being applied to the superposed body thus obtained, the superposed body is heated by supplying a current from a power source 39 through a copper electrode 34 and another copper electrode 35. In the above-described conventional hot diffusion welding method, heat is not easily transferred between the first part 31 and the second part 32 because of the interposition of the normal brazing filler metal 33. Further, since the second part 32 is generally thin in its shape and has a small thermal capacity, Joule heat generated at the second part 32 is transmitted to the copper electrode 34 when the second part 32 is brought into direct contact with the copper electrode 34 which has been water-cooled, with the result that the joint faces cannot be heated to a temperature which is high enough to carry out diffusion welding with the melting behaviour of the brazing filler metal 33 interposed between the joint faces. It is, therefore, unavoidably necessary to provide an auxiliary electrode 36 of doughnut shape serving as a heat insulating material between the copper electrode 34 and the second part 32, as well as an electrode such as a silver-tungsten electrode 37 for compensating for the electric conductivity of the auxiliary electrode 36. In the resistance heating means in the conventional technique as mentioned above, even the silver-tungsten electrode 37 is oxidized at the surface thereof as the number of welding operations increases, so that the deformation rate of the silver-tungsten electrode 37 which is in contact with the surface of the second part 32 increases as well with the increased welding operations. This causes such a trouble that it becomes necessary to polish the silver-tungsten electrode 37 to remove the oxide film formed thereon when the deformation rate exceeds a certain level. Furthermore, electrodes other than the copper electrodes 34 and 35, namely, the auxiliary electrode 36 and the silver-tungsten electrode 37 need to be prepared for every size of the second part 32, and this increases the complexity of the apparatus for carrying out hot diffusion welding. JP-A-64-53763 discloses a welding method, wherein in welding of SS41 with SKD11 or SS41 with SUS304 (steel materials) a brazing filler metal such as Si, B, P, Cr, etc. is interposed between these materials as an insert material and the joint faces are heated with resistance heat generated by supplying a current. JP-A-58-97485 discloses a welding method, wherein in welding Inconel625 as a heat resisting superalloy Ti or BN¹⁻², and in welding IN738 as a heat resisting superalloy BN¹⁻² is used as a brazing filler metal. The joint faces are heated also in this case with resistance heat generated by supplying a current. In the conventional hot diffusion welding method where resistance heating is utilized as heating means, there are limitations in using a brazing filler metal as the insert material. Taking amorphous metals used as the brazing filler metal in the hot diffusion welding method for example, those metals available in the market are limited in their quality and characteristics and therefore often fail in achieving a satisfactory bonding strength. Concretely, amorphous metals have the following disadvantages:- (a) most of them are in the form of metal foils having low melting points and high ductility, so that the bonding strength decreases when heat generated by friction or shear force is added; (b) although some amorphous metals containing components such as tungsten and molybdenum have high melting points, they still have disadvantages in wettability for the adaptability with the first part and/or second part; and (c) in the first part and/or second part, they create fragile intermetallic compounds between metals contained as impurities and metals constituting the parts. Therefore, brazing filler metals unavoidably exhibit a poor bonding strength when used in the welding of machine parts such as sliding members of a hydraulic apparatus where high heat resistance is required and a great shear force is applied. JP-A-59-24592 discloses a welding method, wherein a plating layer of Ni, Cu, Cr etc. is used as an insert material by welding superhard alloy such as cermet etc. and metal such as carbon steel, stainless steel, copper, copper alloy, nickel, nickel alloy etc. However, according to this welding method it is required that the heat needed for welding is supplied by an external heater. DISCLOSURE OF THE INVENTION In order to overcome the foregoing various problems, the invention aims to provide a hot diffusion welding method that is capable of not only uniting the second part made from materials difficult to unite by casting methods, but also obtaining stable resistance heat with the use of simplified electrodes by employing an insert material in specified forms interposed between the first and second parts. In order to achieve this object, the hot diffusion welding method according to the invention comprises: (a) a plating layer forming step of forming a plating layer as an insert material on a joint face of a first part to which a second part is to be jointed; (b) a superposing step of superposing a second part on the first part with the plating layer between; (c) a heating and pressurizing step of heating a joint of a superposed body which has been obtained in the above superposing step, and pressurizing the superposed body in the superposed direction thereof to unite the first and second parts at the joint; wherein said second part is a copper metal part made from pure copper or a copper alloy, and said insert material is a copper plating layer made of pure copper or a copper alloy, characterized in that the heating is made by resistance heat generated by supplying a current through said superposed body by means of a pair of electrodes being arranged on said first and second parts, respectively. The above copper metal part may be obtained by continuous casting, extrusion molding or similar methods, and may be made from a copper metal of any composition including copper alloys such as Cu-Zn alloys, Cu-Sn alloys, Cu-Al alloys and Cu-Pb alloys. As the first part, iron materials such as SCM and mild steel (hereafter referred to as SS material) are generally used, but it is obviously possible to employ any kinds of pure metals and alloys. In a special case, the first part may be made from the same material as the second part. The copper plating layer formed as the insert material may be any kind of copper plating and its typical composition is Cu-Zn alloy. This plating layer may consist of not only one kind of layer but a multiple kinds of layers. Nickel plating or copper plating may be done to provide underplate for the purpose of improving the affinity for the first part made from an iron material. Since the insert material is a plating layer as described above, the joint faces of the first and second parts are closely fitted, forming a smooth structure. Therefore, with the pressure applied in the superposed direction, the joint face of the second part can be perfectly adhered to the joint face of the first part throughout the joint area with the plating layer between. When resistance heat is generated by supplying a current to the first and second parts through the pair of electrodes which are electrically in contact with the first and second parts respectively, the substantially same heat conductivity as obtained in a single body can be maintained between the first and second parts. This facilitates the heat transmission between the first and second parts. As a result, even if the second part that is usually thin in its shape and has a small thermal capacity is in direct contact with the water-cooled electrode, the joint is well heated so that the hot diffusion welding can be smoothly carried out between the first and second parts. This enables the pair of electrodes to have a simple structure which does not need any other electrodes such as an auxiliary electrode for preventing the excessive cooling (the cooling should be prevented for the reason mentioned above) and an silver-tungsten electrode for compensating for the contact resistance especially for the second part. When a copper plating layer is employed as the insert material for the second part made from a copper metal, the copper plating layer exhibits excellent wettability so that a great bonding strength can be ensured. The pressure applied in the superposed direction of the superposed body for uniting while heating is carried out with resistance heat generated by supplying a current is in the range of 9,807 × 10⁶ to 19,61 × 10⁷ Pa (100kg/cm² to 2t/cm²), or preferably 3,92 × 10⁷ to 14,71 × 10⁷ Pa (400kg/cm² to 1.5t/cm²). Preferably, the above pressure is applied to the superposed body using a known means such as through a column of a hydraulic press. The temperature at the joint between the first and second parts, the temperature being imparted by the resistance heat generated by supplying a current, is preferably in the range of 70% to 100% of the melting point (represented by absolute temperature) of the second part. As a matter of course, the voltage and current necessary for generating the resistance heat and the applying time are set in accordance with the above required temperature at the joint. The welding by the hot diffusion welding method of the invention is preferably carried out under vacuum or in an atmosphere of an inert gas such as argon gas or nitrogen gas, but it is also possible to carry out the welding in atmospheric air in the case where the applied pressure is made high, thereby reducing the time required for the welding. BRIEF DESCRIPTION OF THE DRAWINGS The only Figure shows an example of a conventional method BEST MODE FOR CARRYING OUT THE INVENTION Concrete examples of a hot diffusion welding method according to the invention will be described hereinbelow. (EXAMPLE 1) Welding was carried out using a steel material SCM440H (this material serves as a first part to which another part is to be joined) and a wear-resistant brass material P31C (this material serves as a second part which is to be joined to the first part). This wear-resistant brass material had a composition of: Zn (26 to 30wt%); Fe (0.5 to 1.3wt%); Al(3.5 to 4.5wt%); Ni (2.0 to 4.0wt%); Si(0.5 to 1.5wt%); Co(0 to 1.0wt%); and Cu(Bal.). This welding was carried out according to the invention. First, pure Cu plating having a film thickness of about 5 µm was formed as an underplate on the SCM440H material and then Cu-Zn (brass) plating having a film thickness of about 20 µm was formed, whereby an insert material was prepared. The P31C material was then placed on the surface of the insert material, thereby preparing a superposed body in which the second part, the insert material and the first part were superposed. The temperature of the superposed body was kept at 850°C by resistance heating for 5 minutes under vacuum (1,333 Pa = 1 × 10⁻²Torr.). During the resistance heating, a pressure as indicated in Table 1 was applied for one minute from above the P31C material in the superposed direction. After welding had been carried out as described above, the weld state was checked by conducting the non-destructive test in which the ultrasonic inspection method was adopted as well as the shearing test. The result is as shown in Table 1. Table 1 also shows the weld states resulted from welding in which the same materials were used under different welding conditions. Also, the weld states were checked when welding was carried out under an atmosphere of argon gas and/or when the pressure was applied for 3 minutes. In those cases, the similar results were obtained. The SCM440H material serving as the first part and the P31C material serving as the second part were firmly adhered to each other with the plating between which serves as the insert material, so that heat transition by resistance heating took place satisfactorily between the SCM440H material and the P31C material, with the joint heated to a sufficiently high temperature. The welding did not need to use an auxiliary electrode and the formation of an oxide film was not specially found on the contact faces of the electrodes. (EXAMPLE 2) Welding was carried out using the steel material SCM440H as the first part and phosphorus bronze PBC-2C as the second part. This phosphorus bronze PBC-2C had a composition of: Sn (9.0 to 12.0wt%); P (0.05 to 0.5wt%); and Cu (87.0 to 91.0wt%). This welding was carried out according to the invention. First, pure Cu plating having a film thickness of about 5 µm was formed as underplate on the SCM440H material and then Cu-Zn plating having a film thickness of about 20 µm was formed, whereby an insert material was prepared. The PBC-2C material was then placed on the surface of the insert material, thereby preparing a superposed body in which the second part, the insert material and the first part were superposed. The temperature of the superposed body was kept at 1000 °C by resistance heating for 5 minutes under vacuum (1,333 Pa = 1 × 10⁻² Torr.). During the resistance heating, a pressure as indicated in Table 1 was applied for one minute from above the PBC-2C material in the superposed direction. After welding had been carried out as described above, the same tests as those made in Example 1 were conducted. The result is as shown in Table 1. Also, the weld state was checked when welding was carried out under an atmosphere of argon gas, and in this case, the similar result was obtained. In this example, the SCM440H material serving as the first part and the PBC-2C material serving as the second part were also firmly adhered to each other with the plating between which serves as the insert material, so that heat transition by resistance heating took place satisfactorily between the SCM440H material and the P31C material, with the joint heated to a sufficiently high temperature. The welding did not need to use an auxiliary electrode, and the formation of an oxide film was not specially found on the contact faces of the electrodes. Welding conditions and welding status Welding conditions Weld status welding temp. pressure applied applying time non-destructive test shearing test example 1 850 °C 9.807*10⁷ Pa (1 t/cm²) 1 min. no default 22.16*10⁷ Pa (22.6kg/mm²) 850 °C 5.59*10⁷ Pa (570kg/cm²) 1 min. no default 21.77*10⁷ Pa (22.2kg/mm²) 880 °C 9.807*10⁴ Pa (1kg/cm²) 1 min. 40-70% 8.92*10⁷ Pa (9.1kg/mm²) example 2 1000 °C 9.807*10⁷ Pa (1 t/cm²) 1 min. 5% or less 19.22*10⁷ Pa (19.6kg/mm²) 1000 °C 5.59*10⁷ Pa (570kg/cm²) 1 min. 5% or less 18.53*10⁷ Pa (18.9kg/mm²) INDUSTRIAL APPLICABILITY According to the hot diffusion welding method of the invention, when the insert material interposed between the first and second parts is plating formed on the joint face of the first part, resistance heating suitable for hot diffusion welding is evenly carried out throughout the first and second parts as well as the joint thereof. Further, the plating made from a high-purity material having little contaminant is used so that an excellent bonding strength can be achieved. Plating techniques have been already established and the manufacturing cost of plating is low compared with that of a brazing filler metal which is not commonly used. A further advantage is that resistance heating can be performed in a stable condition by supplying a current through the simplified electrodes which do not require an auxiliary electrode etc. With the hot diffusion welding method of the invention, not only can machine parts in which a first part and a second part are firmly adhered at the joint be obtained with ease, but also hot diffusion welding apparatus can be simplified in order to reduce the number of parts that require maintenance and to considerably improve its productivity.

Claims: A hot diffusion welding method comprising: (a) a plating layer forming step of forming a plating layer as an insert material on a joint face of a first part to which a second part is to be joined; (b) a superposing step of superposing the second part on the first part with the plating layer therebetween; (c) a heating and pressurizing step of heating the joint of a superposed body obtained in the superposing step, and pressurizing the superposed body in the superposed direction thereof to unite the first and second parts at the joint; wherein said second part is a copper metal part made from pure copper or a copper alloy, and said insert material is a copper plating layer made of pure copper or a copper alloy, characterized in that the heating is made by resistance heat generated by supplying a current through said superposed body by means of a pair of electrodes being arranged on said first and second parts, respectively. Hot diffusion welding method as claimed in claim 1, wherein the joint between said first and second parts is heated to a temperature being in the range of 70 to 100% of the melting point of the second part. Hot diffusion welding method as claimed in claim 1 or 2, wherein the pressure applied in the heating and pressurizing step is in the range of 9,807*10⁶ Pa to 19,61*10⁷ Pa (100kg/cm² to 2t/cm²).

Inventors: INOUE SH KOMATSU LTD MANUF ENG, INOUE, SH., KOMATSU LTD. MANUF. ENGIN. RES. DEPT.
Applicants: KOMATSU MFG CO LTD, KOMATSU LTD.
Application year: 1991
Main classification: B23K20
IPC codes: B23K20, B23K35