Liquid phase diffusion bonding using high diffusivity element as insert material

A liquid phase diffusion bonding method using an insert material such as B,C,Si and Hf having a high diffusivity and a melting point higher than that of the base metal is disclosed. During the bonding, the insert material is not melted, but the insert material and the base metal are reacted with each other in such a manner that the diffusion bonding can be carried out under a non-oxidizing atmosphere at a temperature lower than the melting point of the insert material.

FIELD OF THE INVENTION 
The present invention relates to a diffusion bonding method for use in the 
bonding of gas turbine blades and nozzles, and to the bonding of Fe-base 
alloys and nonferrous alloys, in which a transient liquid phase bonding 
method (to be called hereinafter "TLP") for heat resistant super alloys is 
applied. In contrast to the existing method, the insert materials used 
according to the present invention have the characteristics that their 
diffusivity is high and their melting points are higher than those of the 
base metal. 
BACKGROUND OF THE INVENTION 
The TLP bonding method has been developed since the 1970s in order to 
improve the bonding strength of heat resistant super alloys (U.S. Pat. No. 
3,678,570). The existing TLP bonding method is carried out in such a 
manner that an insert material is inserted into between two base metals to 
be bonded, and then, it is held for a long time at a temperature (i.e., at 
the bonding temperature), higher than the melting point of the insert 
material, so that the insert material in liquid phase is isothermally 
solidified, thereby bonding the two base metals. According to this bonding 
method, there is almost no distinction between the base metals and the 
bonding zone thereby improving the bonding strength greatly. 
The existing TLP bonding method consists of: a step of melting the insert 
material, melting the base metals, isothermal solidification, and 
homogenizing the bonding zone and the base metals. The most important 
factor affecting the bonding process is the insert material, and there 
have been many much efforts to develop superior insert materials. The 
existing bonding method uses insert materials in the form of alloy 
powders, alloy films and an alloy layer on the bonding surface, which 
contain lower diffusivity elements and melt at the bonding temperature. 
The TLP bonding method (U.S. Pat. No. 4,122,992) developed by Duvall et al, 
uses a brazing foil and requires a long time (e.g. reportedly up to 100 
hours) at a high temperature for the homogenization of the bonding zone 
and the base metals. This causes a lower productivity, and the 
deterioration of the base metals. 
Meanwhile, the high energy beam method (U.S. Pat. No. 4,691,856) and the 
boron packing method, in both of which an alloy layer is formed on the 
bonding surface, involve the problems of forming the alloy layer in a high 
temperature vacuum or in an inert gas atmosphere. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a liquid phase 
diffusion bonding in which an insert materials (such as B,C,Si and Hf 
sheets or powders) having high diffusivities and melting points higher 
than those of the base metal have only to be inserted into the bonding 
joint at atmospheric pressure and at the room temperature, thereby 
simplifying the process. 
It is another object of the present invention to provide a liquid phase 
diffusion bonding in which the bonding mechanism is different from that of 
existing methods, in such a manner that, while the insert material is 
melted at the bonding temperature according to the existing method, the 
insert materials according to the present invention such as B,C,Si and Hf 
are not melted at all at the bonding temperature, but only the portions of 
the base metals reacted to the insert material are melted. 
It is still another object of the present invention to provide a liquid 
phase diffusion bonding method in which the bonding time (including the 
time for the homogenization) is greatly shortened (to about 1 hour) 
compared with the existing TLP bonding method due to the use of high 
diffusivity insert materials. 
The present invention is a liquid phase diffusion bonding method for 
bonding super-alloys, Fe-base alloys and nonferrous alloys, and is capable 
of homogenizing the bonding zone and the base metals within a short period 
of time. 
Further, diffusivity of the insert material (such B,C,Si and Hf) is very 
high and the base metals themselves are solidified after melting, and 
therefore, there is no need for a homogenization treatment after 
isothermal solidification with the result that the bonding time is greatly 
shortened compared with the conventional TLP bonding method.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a comparison between a conventional bonding process and 
the bonding process of the present invention. According to the present 
invention, in contrast to a conventional TLP process only base metal is 
melted at the bonding temperature, and the melting of the base metals and 
the isothermal solidification occur during the homogenizing process of 
boron. According to U.S. Pat. No. 4,691,856 in which an alloy layer is 
formed on the bonding faces by means of high energy beams, some of the 
elements are evaporated from the base metal, thereby requiring a lengthy 
time for the homogenization. 
According to the present invention, however, the bonding process is carried 
out under atmospheric pressure and ambient temperature, with the result 
that elements are not lost from the base metals, and the homogenization is 
achieved simultaneously with the isothermal solidification. FIGS. 2, 2a, 
and 2b illustrates a summary of the differences between the present 
invention on the one hand and the conventional TLP method, the high energy 
beam method, and the boron packing method on the other hand. 
The high energy beam method has disadvantages in that a laser has to be 
used under a vacuum or an inert gas atmosphere, and some elements are 
evaporated from base metal during high energy beam treatment. 
Secondly, is reported that the boron packing method requires a treatment at 
a high temperature (e.g., 700.degree.-850.degree. C.) for a long time 
(e.g., 3.5 hours), and also requires about 20 hours for achieving the 
homogenization during the bonding. 
According to the present invention, however, a high melting point high 
diffusivity element has only to be inserted into the bonding zone at room 
temperature atmospheric pressure, and therefore, there is no need for 
heating the base metals to a high temperature prior to carrying out the 
bonding, and no need for the time for forming an alloy layer. Further, 
there is no element loss from the base metals which is liable to occur 
during a high temperature treatment, and therefore, the homogenization is 
achieved simultaneously with the isothermal solidification. 
Thirdly, for the present invention a high diffusivity insert element such 
as boron has only to be diffused, and therefore, the bonding time 
(including the time for the homogenization) is only 1 hour, thereby making 
it possible to reduce the deterioration of the materials and to improve 
the productivity greatly. 
EXAMPLE 
Pieces to be bonded were sufficiently polished, and washed with acetone. 
The washed pieces were dried, and then, a high melting point high 
diffusivity insert material (e.g., boron sheet) was inserted between the 
pieces to be bonded. 
Here, instead of the boron sheet, boron powders can be used, and when boron 
powders are used, the boron powders and alcohol are mixed as a solution, 
and doped on the bonding surface. After the alcohol is evaporated from the 
doped surface, a boron film will remain on the bonding surface. Such a 
procedure is carried out under room temperature and atmospheric pressure. 
For the bonding, the pieces with the insert material were held under a 
vacuum of below 10.sup.-4 torr at a temperature of 
1150.degree.-1250.degree. C. for about 1 hour for the case of a Ni-base 
super alloy (e.g., Rene 80) or a stainless steel (e.g., AISI 304). FIG. 3 
is a picture showing the microstructure of the cross section of the bonded 
zone for the case where Rene 80 was used as the base metal, boron powders 
as the insert material, and held for 1 hr at 1600.degree. C. FIG. 4 shows 
the tensile strengths of Rene 80 and AISI 304 stainless steel in the case 
where boron powders were used as the insert material.