In the past, as the methods of bonding metal materials with each other, welding methods had mainly been used. In recent years, however, use of the liquid phase diffusion bonding method as new industrial bonding technology for replacing this has been spreading.
The liquid phase diffusion bonding method is the technology of interposing between bonding faces of bonded materials, that is, the bevel faces, an amorphous alloy foil with a melting point lower than the bonded materials, specifically a multimetal alloy foil having at least 50% of its crystal structure amorphous, containing an element having the ability to form a bonded joint through a diffusion-limited isothermal solidification process, for example, B or P, and comprising a base material of Ni or Fe, then heating and holding the joint at a temperature of at least the melting point of this amorphous alloy foil so as to form a joint by an isothermal solidification process.
This liquid phase diffusion bonding method enables bonding with lower heat input compared with ordinary welding methods, so is characterized in that almost no residual stress of the weld occurs along with heat expansion and contraction and no excessive buildup of the weld such as with welding methods occurs, so the bond surface is smooth and a precision bonded joint can be formed.
In particular, since liquid phase diffusion bonding is facial bonding, the bonding time does not depend on the area of the bonding faces and is constant. Further, the bonding is completed in a relatively short time. From these viewpoints, this is bonding technology of a concept completely different from the conventional welding methods. Therefore, there is the advantage that if a joint can be held for a predetermined time at a temperature of at least the melting point of the amorphous alloy foil inserted between the bevel faces of the bonded materials, bonding between the surfaces can be realized without having to select the bevel shape.
The applicant has already proposed a method for producing a metal machine part provided with a pipeline inside it using this liquid phase diffusion bonding method in Japanese Unexamined Patent Publication (Kokai) No. 2003-181651 and Japanese Unexamined Patent Publication (Kokai) No. 2001-321963.
However, the liquid phase diffusion bonding disclosed in these patent publications enables the bonding time to be made a relatively short time, but the isothermal solidification proceeds limited by diffusion. In so far as this is the case, in order for diffusion atoms in the amorphous alloy foil to diffuse and disperse in an amount sufficient for raising the melting point of the joint, when using an amorphous alloy foil of a thickness of 10 μm, it is necessary to isothermally hold the foil at about 900 to 1300° C., corresponding to a temperature of at least the melting point of the alloy foil, for at least about 60 seconds.
By making the amorphous alloy foil used for the liquid phase diffusion bonding thinner, the bonding time can be shortened to a certain extent, but the effect of the precision of working of the bevel faces of the bonded materials on bonding defects and the deterioration of the joint strength becomes greater, so there are also limits to the reduction in thickness of the alloy foil. In actuality, the concentration of the diffusion atoms is raised in order to lower the melting point of the bonding foil or the chemical composition of the bonded materials is relied upon to induce melting of the parent material at the time of bonding. As a result, the actual thickness of the bonding alloy foil quite often exceeds 50 μm.
Further, even if raising the pressing stress in liquid phase diffusion bonding, while it is possible to shorten the bonding time to a certain extent, raising the pressing stress makes the bonded materials more susceptible to buckling deformation, so there are limits to increasing the pressing stress.
Accordingly, in the methods for producing metal machine parts using liquid phase diffusion bonding disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2003-181651 and Japanese Unexamined Patent Publication (Kokai) No. 2001-321963, improving the productivity of the metal machine parts and reducing the production costs by shortening the bonding time while maintaining the joint quality in liquid phase diffusion bonding has become an issue in the industry.
On the other hand, electrical resistance welding is known as a bonding technique frequently used for bonding metal machine parts in the past.
Electrical resistance welding is a method of utilizing the heat of resistance produced by passing a current through metal, giving a large current to make the bevels of the bonded materials instantaneously melt, and pressing the bevels to form a bonded joint.
For example, when melt bonding a thermocouple to a measured object for measurement of its temperature, bonding steel plate to a frame member of an automobile, and in other cases where the relative bonding area is small and a high bonding strength is not required, spot welding, projection welding, upset welding, and other electrical resistance welding methods are frequently used as simplified bonding methods. Conversely, when bonding large bevels with relatively large bonding areas, flash pad welding and continuous electrical resistance welding able to apply a large current and a high pressing force are utilized—such as for seam welding of metal pipe.
However, when using these resistance welding methods to produce metal machine parts, sometimes fluctuations in the bonding conditions will cause for example bonding defects due to residual oxide-based inclusions at the bonds or insufficient welding current will cause so-called “cold welding” or welding defects due to insufficient melting. Further, the pressing at the time of bonding causes large deformation to occur, fine cracks occur at the welded parts, and the bevel ends remain unbonded, which become causes of a reduction in joint performance, particularly fatigue strength. In particular, when at least one bonded material is a cylindrical metal material, the drop in the joint fatigue strength tends to become remarkable. As measures against this, in the past, for example, changes in the material design or post-processing for improving the shape of the weld was required and there were problems such as the limitations on the freedom of the joint design, increase of costs, etc.
In addition to this, in resistance welding, sometimes the weld width is extremely narrow and bevel deformation occurs, so quality assurance by nondestructive testing was difficult. Due to this and other reasons, improvement of the bonding quality in resistance welding in bonding joints where reliability is particularly required is an issue in industrial technology.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-90619, Japanese Unexamined Patent Publication (Kokai) No. 11-90620, and Japanese Unexamined Patent Publication (Kokai) No. 11-90621 disclose a method and apparatus for bonding metal members making joint use of liquid phase diffusion bonding and conduction type resistance welding in the bonding of Al-based cylinder head members and Fe-based valve seats, but in each case the technique is just simple primary bonding resistance welding with the interposition of a brazing material.
That is, no isothermal solidification diffusion treatment is being performed to convert the incomplete isothermal solidification structures of the resistance welds occurring in primary bonding in the methods disclosed in the above patent publications to liquid phase diffusion bonding structures, so it is difficult to sufficient raise the quality of the bonds.
Further, in the above art, a brazing material is pressed out until becoming extremely thin. The steps up to this are treated as part of the production process. Therefore, homogenization of the bond structure is not being considered. Further, these disclosed art are technologies for forming joints for heterogeneous bonding of nonferrous metals such as Al. There is no description at all regarding the bonding of ferrous materials, in particular iron base materials. Of course, ordinary welding can be used for iron base materials and use of ordinary welding technology is difficult for bonding heterogeneous joints. Therefore, technology for bonding iron base materials is not described in the above patent publications.