Process for lap joining two kinds of metallic members having different melting points

A circular blank made of a steel plate is lapped onto an Al-based plate in lapped areas of the Al alloy plate and another steel plate. Then, the three members are pressed by a pair of electrodes, and a welding current is allowed to flow between both the electrodes, thereby sequentially performing the melting of a current-supplied portion of the Al alloy plate and a portion near the current-supplied portion, the elimination of a molten portion by a partial bulgy deformation of the blank toward the Al alloy plate, the abutment of the bulgy deformed portion against the steel plate, and the resistance-welding between the bulgy deformed portion and the steel plate.

FIELD OF THE INVENTION 
The present invention relates to a process for lap-bonding of two metal 
members having different melting points, and particularly, to such a lap 
bonding process which includes lapping a first metal member and a second 
metal member having a melting point higher than that of the first metal 
member onto each other, and bonding the lapped areas to each other. 
BACKGROUND ART 
If a spot welding process using a large electric current is utilized to 
bond lapped areas of two metal members having different melting points, 
e.g., an Al-based member (aluminum having a melting point of 660.degree. 
C.) and an Fe-based member (iron having a melting point of 1,540.degree. 
C.), a nugget is formed on the Al-based member following melting of the 
latter due to a difference in melting points between both the members, but 
a phenomenon occurs that the Fe-based member is hardly molten. 
If the strength of weld zone of such different members is examined, it can 
be seen that the weld zone shows a strength substantially equal to that of 
the weld zone of Al-based members, namely the same type of members in a 
tensile shearing test, but shows a strength, for example, of only about 
one sixth of that of the same type of the members in a U-tensile test. 
Therefore, it is a conventional practice to employ a process in which a 
clad material comprised of an Al alloy layer and a steel layer is 
interposed between the lapped areas of the Al-based member and the 
Fe-based member, with the Al alloy layer located on the side of Al-based 
member and the steel layer located on the side of Fe-based member (see 
Japanese Patent Application Laid-open No.111778/1993). 
However, the prior art process suffers the following problems: In a case 
where the lapped areas have a complicated shape such as an arcuate shape, 
the accommodatability is poor, and a gap is produced between the Al-based 
member and the Fe-based member in the lapped areas depending upon the 
thickness of the clad material and as a result, the places where this 
process can be applied are largely limited in respect of the design. In a 
case where the clad materials are dotted between the lapped areas, the 
air-tightness of the weld zone is injured by the gap. On the other hand, 
in a case where the clad material is mounted over the entire length of the 
lapped areas, an increase in weight is caused. In addition, the clad 
material is relatively expensive and hence, an increase in manufacture 
cost of the bonded product cannot be avoided. 
A further attempt has been made to provide a solid-phase bonding between 
Fe-based and Al-based members by decreasing a welding current. 
For example, Japanese Patent Application Laid-Open No. 7-214338 discloses a 
technique for bonding an Fe-based metal material and an Al-based metal 
material by a resistance welding with use of a pin made of an Fe-based 
metal material having a substantially T-shaped section. However, in the 
case of this prior art process, the pin which is pressed by an electrode 
to penetrate through at least one of the materials has a complicated 
shape. For this reason, there are problems that the manufacture cost for 
the pin is increased, and in the bonding operation, labors are required by 
positioning and handling of the pin, resulting in a poor efficiency. 
Further, the surface of the Al-based member is covered with a firm oxide 
film and for this reason, an enhancement in a bond strength to be provided 
by the solid phase bonding is hindered by the oxide film. 
To avoid this, it is necessary to subject the Al-based member to an oxide 
film removing treatment, e.g., a brushing using a wire brush. However, the 
carrying-out of such a treatment is undesirable, because it increases the 
operating steps and the operating cost. 
Furthermore, Japanese Patent Publication No. 52-2378 teaches a technique 
for bonding materials by a spot welding, which comprises preparing a hard 
material having a relatively large hardness and a high melting point and a 
soft material having a relatively small hardness and a low melting point, 
forming at least one of the materials into a rounded bar-like shape, and 
lapping the materials onto each other to bond them to each other, while 
pressing them from above and below by the pair of electrodes. With this 
process, an oxide film generated in the surface of the soft material 
formed by an Al alloy, for example, can be destroyed by a plastic 
deformation, and therefore, there is an advantage of enhancing the welding 
strength. In addition, a recessed groove positioning the hard material in 
a predetermined position is provided in one of the electrodes pressing the 
hard material, and therefore, there is an advantage that any deviation in 
the relative positional relationship between both the materials can 
effectively be prevented. However, the hard material opposed to the 
electrode in which the recessed groove is formed is limited to ones having 
such a shape that can be engaged into the recessed groove, and therefore, 
there is a demerit that the utilization is limited. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a bonding 
process of the above-described type, wherein even in a case where lapped 
areas have a complicated shape, the accommodatability is good, and the 
generation of a gap in the lapped areas can be avoided and the workability 
is enhanced and moreover, the manufacture cost of the bonded product can 
be reduced. 
To achieve the above object, according to the present invention, there is 
provided a lap bonding process which includes lapping a first metal member 
and a second metal member having a melting point higher than that of the 
first metal member onto each other, and bonding resulting lapped areas to 
each other, the process comprising the steps of lapping a plate shaped 
third metal member onto the first metal member in the lapped areas, the 
third metal member having a melting point higher than that of the first 
metal member and being capable of being plastically deformed and 
resistance-welded to the second metal member; pressing the first, second 
and third metal members by a pair of electrodes and allowing a welding 
current to flow between both the electrodes, thereby sequentially 
performing a melting of a current-supplied portion of the first metal 
member and a portion of the first metal member near the current-supplied 
portion, an elimination of a molten portion produced in the first metal 
member by a partial bulgy deformation of at least one of the second and 
third metal members, and a resistance welding of the second and third 
metal members through a bulgy deformed portion of the at least one member. 
With the above process, the first and second metal members are firmly 
bonded to each other through a bulgy deformed portion. 
The plate-shaped third metal member may be a blank produced by punching and 
hence, has a large degree of freedom in the shape. As a result, even when 
the lapped area has a complicated shape, It is possible to easily 
accommodate the complicated shape. 
Further, the plate-shaped third metal member is lapped onto the first metal 
member in the lapped area and hence, a gap cannot be produced between the 
first and second metal members. 
Moreover, a plate-shape member is used as the third metal member and hence, 
increases in manufacture cost and weight of a bonded product due to use of 
the third metal member are suppressed. 
It is another object of the present invention to provide a bonding process 
of the above-described type, wherein in the course of welding the Al-based 
member to the Fe-based member, various shapes of the members are 
applicable. 
To achieve the above object, according to the present invention, there is 
provided a lap bonding process for lapping a first metal member and a 
second metal member having a melting point higher than that of the first 
metal member onto each other, and bonding resulting lapped areas to each 
other, the process including the steps of selecting an Al-based member 
having a planar portion as the first metal member and an Fe-based member 
having a planar portion as the second metal member; lapping the first and 
second metal members onto each other at the planar portions; pressing the 
lapped areas by a pair of electrodes and supplying an electric current 
between the electrodes, thereby forming a recess on a bonded surface of 
the Al-based member by a deformation of the Al-based member through medium 
of a pressed and current-supplied portion of the Fe-based member; and 
bonding the pressed and current-supplied portion and the Al-based member 
at the recess. 
In the above process, by lapping the first and second metal members onto 
each other at the planar portions, and pressing and supplying a current to 
the lapped areas, a recessed portion is formed in the Al-based first metal 
member due to its deformation. And the welding is performed utilizing the 
recessed portion, thereby enabling the process to be applied in infinitely 
wide fields.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVEVTION 
EXAMPLE I 
Referring to FIGS. 1 and 2, a bonded product 1 includes an Al alloy plate 
(or an Al plate) 2 as an Al-based member which is a first metal member, 
and a steel plate (an Fe alloy plate or an Fe plate) 3 as an Fe-based 
member which is a second metal member having a melting point higher than 
that of the Al alloy plate 2, with lapped areas 4 of the plates 2 and 3 
being bonded to each other. 
For the lap bonding process, a circular blank 5 made by punching from a 
third metal member having a melting point higher than that of the Al alloy 
plate 2, e.g., a steel plate, is used, and a spot welding as a resistance 
welding is utilized. 
A bonded structure produced by the lap bonding process is such that a bulgy 
deformed portion 8 resulting from that plastic deformation of a central 
portion of the circular blank 5 which has been produced by pressing the 
members by upper and lower electrodes 6 and 7 made by O. F. C. and by 
supplying a welding current is spot-welded to the steel plate 3 to form a 
nugget 9, with a molten portion of the Al alloy plate 2 being eliminated, 
and an outer peripheral portion 10 of the circular blank 5 is in pressure 
contact with the Al alloy plate 2. 
The lap bonding process will now be described in detail. 
Referring to FIGS. 3 and 4, an upper electrode 6 of a spot welding machine 
11 is comprised of a rod-like electrode body 12 which is circular in 
section, and a truncated conical protrusion 14 provided on a lower end 
face of the electrode body 12 to project therefrom and having a draft 13. 
Therefore, the protrusion 14 has a circular section within a plane 
intersecting an axial direction of the electrode. A rounded portion 16 is 
provided at a peripheral edge of a smaller end face 15 of the protrusion 
14. A JIS R-type electrode is used as a lower electrode 7, but a JIS 
CF-type electrode or a CR-type electrode may also be used. In Figures, 
reference numeral 17 is a transformer, and 18 is an inverter-type 
controller. 
(a) As shown in FIG. 5, one end of the Al alloy plate 2 is lapped onto one 
end of the steel plate 3 and then, the circular blank 5 is lapped onto the 
Al alloy plate 2 in the lapped area 4. 
(b) As shown in FIG. 6, the circular blank 5, the Al alloy plate 2 and the 
steel plate 3 are disposed between both the electrodes 6 and 7 with the 
circular blank 5 located on the side of the upper electrode 6, and then, 
those members 5, 2 and 3 are pressed by both the electrodes 6 and 7, and 
at the same time, a welding current is allowed to flow between both the 
electrodes 6 and 7. 
(c) As shown in FIG. 7, the circular blank 5, the Al alloy plate 2 and the 
steel plate 3 are heated by a contact resistance as a result of supplying 
of the current in the state in which they have been pressed, and then, the 
current-supplied portion 19 of the Al alloy plate 2 having a lower melting 
point and a portion near the current-supplied portion 19 are molten, while 
the current-supplied portions of the circular blank 5 and the steel plate 
3 and portions near them are softened. 
(d) As shown in FIG. 8, the central portion of the circular blank 5 pressed 
by the truncated conical protrusion 14 of the upper electrode 6 is 
deformed to be bulgy toward the Al alloy plate 2 to form a truncated 
conical shape, whereby the molten portion is eliminated and moved to a gap 
between the Al alloy plate 2 and the steel plate 3. Therefore, a smaller 
end 20 of the bulgy deformed portion 8 is put into abutment against the 
steel plate 3. 
(e) As shown in FIG. 9, the smaller end 20 of the bulgy deformed portion 8 
and the steel plate 3 abutting against the smaller end 20 are supplied 
with the current in the state in which they are by the electrodes 6 and 7. 
Therefore, the smaller end 20 and the steel plate 3 are spot-welded to 
each other to form the nugget 9, thereby forming the welded zone in the 
same-type materials. 
After such spot-welding, the truncated conical protrusion 14 of the upper 
electrode 6 is easily withdrawn from the bulgy deformed portion 8, because 
it has the draft 13. 
With the above-described process, the Al alloy plate 2 is firmly bonded to 
the steel plate 3 with a rivet coupling-like fastened structure provided 
by the outer peripheral portion 10 and the bulgy deformed portion 8 of the 
circular blank 5. 
In addition, the circular blank 5 made by punching has a larger degree of 
freedom in the shape and as a result, even when the lapped area 4 has a 
complicated shape, it is possible to easily accommodate this. 
Further, the circular blank 5 is lapped onto the Al alloy plate 2 in the 
lapped area 4 and hence, a gap cannot be produced between the Al alloy 
plate 2 and the steel plate 3. 
Moreover, the circular blank 5 is of a single-plate structure and hence, 
increases in manufacture cost and weight of the bonded product 1 due to 
the use of the circular blank 5 are inhibited. 
In the above-described lap bonding example, the bulgy deformed portion 8 
can be formed on the steel plate 3 using the lower electrode 7 having the 
same shape as the upper electrode 6. Alternatively, bulgy deformed 
portions 8 can be formed on both of the circular blank 5 and the steel 
plate 3, respectively. 
As shown in FIG. 10, an adhesive 22 is applied to the entire periphery of 
that surface 21 of the circular blank 5 which is opposed to the Al alloy 
plate 2, whereby the circular blank 5 can be reliably retained at a 
predetermined position in the lapped area 4 to enhance the bonding 
operability. 
As shown in FIG. 11, the adhesive 22 may be replaced by a sealing material 
23, and the sealing material 23 may be interposed in an annulus between 
the Al alloy plate 2 and the steel plate 3 in the lapped area 4, whereby 
the corrosion resistance of the bonded zone including a bore 24 (see FIG. 
9) produced in the Al alloy plate 2 by the bonding operation can be 
enhanced. 
FIGS. 12 to 14 show three examples of bonded products 1, wherein a third 
metal member is formed utilizing the plastically deforming ability of the 
steel plate 3. 
The example shown in FIGS. 12A and 12B was produced in the following 
manner: One end 25 of the steel plate 3 was folded back. The folded-back 
portion 5.sub.1 is used as a third metal member. Then, one end of the Al 
alloy plate 2 was inserted between the steel plate 3 and the folded-back 
portion 5.sub.1, so that both the planes of both the Al alloy plate 2 and 
the steel plate 3 were parallel to each other, and the directions of 
extensions of the plates 2 and 3 crossed each other at 90.degree.. 
Thereafter, a bonding process similar to that described above was carried 
out. 
The example shown in FIG. 13 was produced in the following manner: A 
plate-like protrusion 26 was provided at one side edge of an end of the 
steel plate 3, so that the plate-like protrusion 26 and the steel plate 3 
are located on the same plane. The plate-like protrusion 26 was folded 
back, and the folded-back portion 5.sub.1 was used as a third metal 
member. Then, one end of the Al alloy plate 2 was inserted between the 
steel plate 3 and the folded-back portion 5.sub.1, so that both the planes 
of the Al alloy plate 2 and the steel plate 3 were parallel to each other 
and the plates 2 and 3 extended in the same direction. Thereafter, a 
bonding process similar to that described above was carried out. 
The example shown in FIG. 14 was produced in the following manner: 
Substantially half of a plate-like folded portion 27 formed by folding one 
end of the steel plate 3 at right angle was folded back, and the 
folded-back portion 5.sub.1 was used as a third metal member. Then, one 
end of the Al alloy plate 2 was inserted between the steel plate 3 and the 
folded-back portion 5.sub.1, so that both the planes of the Al alloy plate 
2 and the steel plate 3 were in a right angle relation to each other and 
the directions of extensions of the plates 2 and 3 crossed each other at 
90.degree.. Thereafter, a bonding process similar to that described above 
was carried out. 
Particular examples will be described below. 
A. U-tensile Strength 
As shown in FIG. 15, first halves 28 for a plurality of U-tensile test 
pieces were made from the Al alloy plate 2, and second halves 29 for a 
plurality of U-tensile test pieces were made from the steel plate 3, both 
according to JIS Z 3137. Further, the steel plate 3 was subjected to a 
punching to provide a plurality of circular blanks 5. 
The material for the Al alloy plate 2 is JIS 5182 and had a thickness 
t.sub.1 set at 1.0 mm. On the other hand, the material for the steel plate 
3 is JIS SPCC and had a thickness t.sub.2 set at 0.7 mm. In this case, 
t.sub.1 =(2.sup.1/2).times.t.sub.2 is established between the thickness 
t.sub.1 of the Al alloy plate 2 and the thickness t.sub.2 of the steel 
plate 3. This is for the purpose of ensuring that the plates 2 and 3 have 
substantially the same rigidity. The circular blank 5 had a diameter 
D.sub.1 set at 15 mm. 
As shown in FIG. 3, in the upper electrode 6, the diameter D.sub.2 of the 
electrode body 12 is set at 16 mm; the taper angle .theta. of the 
truncated conical protrusion 14 is set at 90 degrees; the length L is set 
4 mm; and the radius R.sub.1 of the rounded portion 16 at the peripheral 
edge of the smaller end face 15 is set at 1 mm. The smaller-end diameter 
D.sub.3 is varied in a range of 4 to 7 mm. 
In the lower electrode 7, the diameter D.sub.4 is set at 16 mm; and the 
radius R.sub.2 of a spherical tip end face 30 is set at 80 mm. 
A plurality of U-tensile test pieces 31 as shown in FIG. 16 according to an 
embodiment were produced by carrying out a bonding process similar to that 
described above (see FIGS. 5 to 9), except that the first and second 
halves 28 and 29 and a circular blank 5 were used and the welding 
conditions and the upper electrode 6 were changed. 
Then, a U-tensile test piece 32 shown in FIG. 17 according to a comparative 
example 1 was produced by carrying out a spot welding using the first and 
second halves 28 and 29 and using two lower electrodes 7 as upper and 
lower electrodes, respectively. 
Further, a U-tensile test piece 33 shown in FIG. 18 according to a 
comparative example 2 was produced by carrying out a spot welding using 
the two first halves 28 and using two lower electrodes 7 as upper and 
lower electrodes, respectively. 
Thereafter, the U-tensile test pieces 31 to 33 were subjected to a tensile 
test. 
Table 1 shows the smaller-end diameter D.sub.3 of the upper electrode 6, 
the welding conditions, the amount of expulsion and surface flash and the 
U-tensile strength for the U-tensile test pieces 31 to 33. 
TABLE 1 
__________________________________________________________________________ 
Smaller- 
end Welding conditions 
diameter D.sub.3 Press- 
Current 
(mm) of Welding ing supplying Amount of U-tensile 
upper current force time expulsion and strength 
electrode (kA) (kgf) (cycle) surface flash (kgf) 
__________________________________________________________________________ 
Example 1 
4 10 200 20 smaller 
105 
Example 2 5 10 200 20 smaller 130 
Example 3 6 12 200 20 slightly 150 
larger 
Example 4 7 14 200 20 larger 200 
Comparative -- 16 200 4 smaller 15 
example 1 
Comparative -- 24 400 4 smaller 95 
example 2 
__________________________________________________________________________ 
As apparent from Table 1, it can be seen that the U-tensile strength of the 
test pieces according to Examples 1 to 4 is largely enhanced and exceeds 
the strength of bonding of the Al alloy plates according to the 
comparative example 2. It can be seen that the U-tensile strength of the 
test piece according to comparative example 2 is approximately one sixth 
of that of the comparative example 1. 
FIG. 19A is a photomicrograph showing the metallographic structure of a 
section of the bonded zone of the test piece which is Example 1, and FIG. 
19B is a reduced tracing of the photomicrograph shown in FIG. 19A. It can 
be seen from FIGS. 19A and 19B that the nugget 9 was formed between the 
smaller end 20 of the bulgy deformed portion 8 and the second half 29, 
whereby the first and second halves 28 and 29 were firmly bonded to each 
other. 
If the diameter D.sub.3 of the smaller end of the upper electrode 6 is 
equal to or larger than 6 mm as in Examples 3 and 4, the U-tensile 
strength is higher, but an expulsion and surface flash is generated. 
Then, U-tensile test pieces 31 to 33 similar to those described above were 
produced by carrying out a bonding process similar to that described 
above, except that the thickness t.sub.1 of the first half 28 was changed 
to 1.2 mm; the thickness t.sub.2 of the second half 29 was changed to 0.8 
mm and further, the welding conditions were partially changed. 
Table 2 shows the smaller end diameter D.sub.3 of the upper electrode 6, 
the welding conditions, the amount of expulsion and surface flash and the 
U-tensile strength for the U-tensile test pieces 31 to 33. 
TABLE 2 
__________________________________________________________________________ 
Smaller- 
end Welding conditions 
diameter D.sub.3 Press- 
Current 
(mm) of Welding ing supplying Amount of U-tensile 
upper current force time expulsion and strength 
electrode (kA) (kgf) (cycle) surface flash (kgf) 
__________________________________________________________________________ 
Example 5 
4 10 200 4 smaller 
181 
Example 6 5 12 200 20 smaller 205 
Example 7 6 14 200 20 slightly 240 
larger 
Example 8 7 14 200 20 larger 260 
Comparative -- 16 400 4 smaller 20 
example 3 
Comparative -- 24 200 4 smaller 180 
example 4 
__________________________________________________________________________ 
It can be seen that a tendency similar to that in Table 1 is recognized 
even in the case of Table 2. 
B. Taper Angle .theta. of Truncated Conical Protrusion of Upper Electrode 
A plurality of upper electrodes 6 each having a changed taper angle .theta. 
of a truncated conical protrusion 14 thereof were prepared. In this case, 
in the upper electrode 6, the diameter D.sub.2 of the electrode body 12 
was set at 16 mm; the smaller-end diameter D.sub.3 of the truncated 
conical protrusion 14 was set at 4 mm; the length L of the truncated 
conical protrusion 14 was set at 3 mm; and the radius R.sub.1 of the 
rounded portion 16 of the peripheral edge of the smaller end 15 was set at 
1 mm. 
In the lower electrode 7, the diameter D.sub.4 was set at 16 mm, and the 
radius R.sub.2 of the spherical tip end 30 was set at 80 mm. 
A plurality of Al alloy plates 2, a plurality of steel plates 3 and a 
plurality of circular blanks 5 each made by punching of a steel plate of 
the same type as of the steel plates 3 were also prepared. The material 
for the Al alloy plate 2 was JIS 5182 and had a thickness t.sub.1 set at 
1.0 mm. The material for the steel plate 3 was JIS SPCC and had a 
thickness t.sub.2 set at 0.7 mm. The diameter D.sub.1 of the circular 
blank 5 was set at 15 mm. 
Then, a bonding process similar to that described above (see FIGS. 5 to 9) 
was carried out to find the relationship between the taper angle .theta. 
and the mold release failure rate P, thereby giving a result shown in 
Table 3. 
The welding conditions were as follows: The welding current was 10 kA; the 
pressing force was 200 kgf; and the current supplying time was 20 cycles. 
The mold release failure rate P was determined according to an equation, 
P=(n/10).times.100 (%), wherein the number of runs of a bonding operation 
carried out using the upper electrode 6 provided with the truncated 
conical protrusion 14 having a predetermined taper angle .theta. was 10; 
and the frequency of adhesion of the truncated conical protrusion 14 to 
the inner surface of the bulgy deformed portion 8 was represented by n. 
The term "adhesion" means a mechanically fitted state to the extent which 
permits the truncated conical protrusion 14 to be removed from the inner 
surface of the bulgy deformed portion 8 by striking the bonded product 1 
by a hammer. 
TABLE 3 
______________________________________ 
Taper angle .theta. (degree) 
0 30 60 90 120 
______________________________________ 
Mode release failure rate P (%) 
100 90 80 40 0 
______________________________________ 
As apparent from Table 3, the mold release failure rate P can be remarkably 
reduced by setting the taper angle .theta. in a range of .theta..gtoreq.90 
degree. 
C. FIGS. 20 and 21 show a modification to the upper electrode 6. The upper 
electrode 6 is comprised of a rod-like electrode body 12 which is circular 
in section, and a columnar protrusion 14.sub.1 projectingly provided on a 
lower end face of the electrode body 12. Therefore, the protrusion 
14.sub.1 is a straight protrusion and has a circular section in a plane 
which intersects the direction of an electrode axis. The protrusion 
14.sub.1 serves to form a bulgy deformed portion 8 on a circular blank 5, 
and has a rounded portion 16.sub.1 provided at an edge of a tip end face 
15.sub.1, i.e., at a peripheral edge. 
To determine the relationship between the radius R.sub.3 of the rounded 
portion 16.sub.1 and the mold release failure rate P, a plurality of upper 
electrodes 6 having different radii R.sub.3 were prepared. In each of the 
upper electrodes 6, however, the diameter D.sub.2 of the electrode body 12 
was set at 16 mm; the length L of the protrusion 14.sub.1 was set at 5 mm; 
and the diameter D.sub.3 of the tip end face 15.sub.1 was set at 4 mm. 
As shown in FIG. 3, in a lower electrode 7, the diameter D.sub.4 was set at 
16 mm, and the radius R.sub.2 of the spherical tip end face 30 was set at 
80 mm. 
A plurality of Al alloy plates 2, a plurality of steel plates 3, and a 
plurality of circular blanks 5 made by punching from a steel plate of the 
same type as the steel plates 3 were also prepared. The material for the 
Al alloy plate 2 was JIS 5182 and had a thickness t.sub.1 set at 1.0 mm. 
The material for the steel plate 3 was JIS SPCC and had a thickness 
t.sub.2 set at 0.7 mm. The diameter D.sub.1 of the circular blank 5 was 
set at 15 mm. 
Then, a bonding process similar to that described above (see FIGS. 5 to 9) 
was carried out to find the relationship between the radius R.sub.3 and 
the mold release failure rate P, thereby giving a result shown in Table 4. 
The welding conditions were as follows: The welding current was 10 kA; the 
pressing force was 200 kgf; and the current supplying time was 20 cycles. 
TABLE 4 
______________________________________ 
Radius R.sub.3 (mm) of rounded portion 
0 1 2 3 
______________________________________ 
Mold release failure rate P (%) 
100 100 20 0 
______________________________________ 
As apparent from Table 4, the mold release failure rate P can be remarkably 
reduced by setting the radius R.sub.3 of the rounded portion 16.sub.1 in a 
range of R.sub.3 .gtoreq.2 mm. 
The protrusion in the upper electrode 6 may have a non-circular section, 
e.g., a square section as shown in FIGS. 22 and 23, without having a 
circular section as described above in the plane intersecting the 
direction of the electrode axis. Namely, the protrusion 14.sub.2 shown in 
FIG. 22 assumes a truncated quadrangular pyramidal shape and has a draft 
13. The protrusion 14.sub.3 shown in FIG. 23 assumes a quadrangular 
columnar shape and has a rounded portion 16.sub.1 provided at an edge of 
the tip end face 15.sub.1, i.e., at a peripheral edge. 
If the upper electrode 6 is constructed in the above manner, the relative 
rotation between the Al alloy plate 2 and the steel plate 3 can be 
reliably prevented. 
Referring to FIGS. 24 and 25, a bonded product 1 includes an Al alloy plate 
2 and a steel plate 3, lapped areas 4 of which are bonded by utilizing a 
spot welding process as a resistance welding process, using a pair of 
upper and lower electrodes 6 and 7. In the bonded structure, a 
substantially truncated conical pressed/current supplied portion 40 bulged 
from the steel plate 3 and a substantially truncated conical recess 41 in 
the Al alloy plate 2 are in a fitted relation to each other, and a solid 
phase bonding is produced between the pressed/current supplied portion 40 
and the Al alloy plate 2 at the recess 41. Namely, the plates 2 and 3 are 
bonded by a diffusion phenomenon in a very small area of a bond interface. 
In this case, no nugget is generated, or even if a nugget is generated, it 
is extremely small and hence, little contribute to the bonding. 
The spot welding between the Al alloy plate 2 and the steel plate 3 will be 
described. 
In FIGS. 4 and 25, an inverter welding machine is used as a spot welding 
machine, and includes an upper electrode 6 which is comprised of a 
rod-like electrode body 12 which is circular in section, and a truncated 
conical protrusion 14 projectingly provided on a lower end face of the 
electrode body 12 and having a draft 13. The protrusion 14 has a rounded 
portion 16 provided at a peripheral edge of a smaller end face 15. An 
electrode of JIS R type is used as a lower electrode 7, but an electrode 
of JIS CF type or CR type may be used. 
(a) As shown in FIG. 26, one end of a steel plate 3 is lapped onto one end 
of an Al alloy plate 2. Then, lapped areas 4 are disposed between both the 
electrodes 6 and 7 with the steel plate 3 located on the side of the upper 
electrode 6, and are then pressed by both the electrodes 6 and 7, while a 
welding current is allowed to flow between both the electrodes 6 and 7. 
(b) As shown in FIG. 27, by supplying of the current in the pressed state, 
pressed and current-supplied portions 40 and 42 of the steel plate 3 and 
the Al alloy plate 2 are softened, while at the same time, the bonded 
surface 43 of the pressed and current-supplied portion 42 of the Al alloy 
plate 2 having a lower melting point is slightly molten to form a small 
molten pool 44. 
(c) As shown in FIG. 28, the pressing force of the truncated conical 
protrusion 14 of the upper electrode 6 ensures that the pressed and 
current-supplied portion 40 of the steel plate 3 is bulged into a 
substantially truncated conical shape toward the Al alloy plate 2 by the 
plastic deformation of the steel plate 3, and a substantially truncated 
conical recess 41 by the plastic deformation of the Al alloy plate 2 is 
defined in the bonded surface 43 of the Al alloy plate 2 by the pressed 
and current-supplied portion 40. The molten metal in the molten pool 44 
including an oxide film is discharged into a gap between both the plates 2 
and 3 during the recess 41 is defined. 
Since the recess 41 is defined by the melting of a portion of the Al alloy 
plate 2 and by the plastic deformation in the above manner, a cleaned 
surface is exposed in an area where the molten pool 44 has existed, as a 
result of the discharging of the molten metal, and a cleaned surf ace is 
exposed around the area where the molten pool 44 has existed, by the 
division of the oxide film by the plastic deformation of the Al alloy 
plate 2. 
Thus, a firm solid-phase bonding Is produced between these cleaned surfaces 
and the pressed and current-supplied portion 40 of the steel plate 3. 
After the above-described spot welding, the truncated conical protrusion 14 
of the upper electrode 6 is easily withdrawn from the pressed and 
current-supplied portion 40, because it has the draft 13. 
The formation of the molten pool 44 is not an essential requirement. Even 
if the molten pool 44 is not formed, the division of the oxide film is 
performed by the plastic deformation of the Al alloy plate 2 and hence, 
the cleaned surface is exposed in the recess 41. 
As shown in FIG. 29, if a foil-like Ni-insert 45 made of only nickel is 
disposed between the steel plate 3 and the Al alloy plate 2 in the lapped 
areas 4, the bond strength can be enhanced more than in a case where the 
steel plate 3 and the Al alloy plate 2 are bonded directly to each other 
in a solid phase manner. This is because the strength of solid-phase 
bonding between the steel plate 3 and the Ni-insert 45 as well as between 
the Ni-insert 45 and the Al alloy plate 2 is higher than the strength of 
solid-phase bonding between the steel plate 3 and the Al alloy plate 2. 
Another reason is that nickel has an effect of breaking the oxide film on 
the surface of the Al alloy plate 2. 
The Ni-insert 45 may be formed on the steel plate 3 or the Al alloy plate 2 
by a plating process. Alternatively, the Ni-insert 45 may be formed on a 
steel foil or an Al alloy foil by a plating process. In the former case, 
the steel foil is opposed to the steel plate 3, and in the latter case, 
the Al alloy foil is opposed to the Al alloy plate 2. 
The Al-based member is not limited to the Al alloy plate 2, and a hollow 
extrudate 46 quadrilateral in cross section or a band-like solid extrudate 
47 may be used, as shown in FIGS. 30 and 31. The Fe-based member is not 
limited to the steel plate 3, and an angle material or the like may be 
used. 
EXAMPLE 1 
As shown by a dashed line in FIG. 32, a plurality of first halves 28 for 
U-tensile test pieces were fabricated from an Al alloy plate 2, and a 
plurality of second halves 29 for U-tensile test pieces were fabricated 
from a steel plate 3, according to JIS Z 3137. The material for the Al 
alloy plate 2 was JIS 5182, and the thickness t.sub.1 of the Al alloy 
plate 2 was set at 1 mm. On the other hand, the material for the steel 
plate 3 was JIS SPCC, and the thickness t.sub.2 of the steel plate 3 was 
set at 0.7 mm. 
As best shown in FIG. 25, in the upper electrode 6, the diameter D.sub.2 of 
the electrode body 12 was set at 16 mm; the taper angle .theta. of the 
truncated conical protrusion 14 was set at 90 degrees; the length L of the 
truncated conical protrusion 14 was set at 4 mm; and the radius R.sub.1 of 
the rounded portion 16 at the peripheral edge of the smaller end face 15 
was set at 1 mm. The smaller-end diameter D.sub.3 was varied in a range of 
3 to 5 mm. 
In the lower electrode 7, the diameter D.sub.4 thereof was set at 16 mm, 
and the radius R.sub.2 of the spherical tip end face 30 was set at 80 mm. 
Using the first and second halves 28 and 29, examples 1 to 3 of U-tensile 
test pieces 31 according to the embodiment as shown by solid lines in FIG. 
32 were produced by carrying out the same process as shown in FIGS. 26 and 
28, except that the welding conditions were set uniformly, and the upper 
electrode 6 was changed. 
Then, using the first and second halves 28 and 29, examples 4 and 5 of 
U-tensile test pieces 31 were produced as comparative examples by carrying 
out the same spot welding, except that two lower electrodes 7 were used as 
upper and lower electrodes, and the welding conditions were varied. 
Thereafter, the examples 1 to 5 were subjected to a tensile test. 
Table 5 shows the smaller-end diameter D.sub.3 of the upper electrode 6, 
the welding conditions, the amount of expulsion and surface flash and the 
U-tensile strength for the examples 1 to 5. 
TABLE 5 
__________________________________________________________________________ 
Smaller-end 
Welding conditions 
U- diameter D.sub.3 
Current 
tensile (mm) of Welding Pressing supplying Amount of U-tensile 
test upper current force time expulsion and strength 
piece electrode (kA) (kgf) (cycle) surface flash (kgf) 
__________________________________________________________________________ 
Example 1 
3 10 200 10 smaller 
55 
Example 2 4 slightly 50 
larger 
Example 3 5 larger 52 
Example 4 -- 16 200 4 smaller 15 
Example 5 10 10 10 
__________________________________________________________________________ 
As apparent from Table 5, examples 1 to 3 according to the embodiment have 
a high U-tensile strength, because the solid-phase bonding was produced 
between both the halves 28 and 29 by the cleaned surface of the first half 
28. 
In the case of example 4 as a comparative example, a nugget is formed on 
the first half 28, because the welding current is raised more than that of 
examples such as example 1. As a result, the U-tensile strength is 
significantly reduced, as compared with example 1 or the like. 
In the case of example 5 as a comparative example, the solid-phase bonding 
was produced between both the halves 28 and 29, because the welding 
conditions were set in the same manner as in example 1 or the like. 
However, such solid-phase bonding was produced mainly between the oxide 
film of the first half 28 and the second half 29 and hence, the U-tensile 
strength is significantly reduced, as compared with example 1 or the like. 
It should be noted that the U-tensile strength of a U-tensile test piece 
made through a spot welding process using the two first halves 28 and the 
two lower electrodes 7 as upper and lower electrodes was 95 kgf. In this 
case, the welding current was set at 24 kA; the pressing force was set at 
400 kgf; and the current supplying time was set at 10 cycles, and both the 
halves 28 were bonded to each other through a nugget formed over both the 
halves. 
EXAMPLE 2 
Using first and second halves 28 and 29, an upper electrode 6 having a 
smaller-end diameter D.sub.3 of 3 mm and an Ni-insert 45 which are similar 
to those in EXAMPLE-1, examples 1 to 3 of U-tensile test pieces 31 
according to the embodiment as shown by solid lines in FIG. 32 were 
produced by carrying out the same process as shown in FIGS. 26 to 29. 
Then, using first and second halves 28 and 29 and an Ni-insert 45 which are 
similar to those in EXAMPLE-1 and using two lower electrodes 7 as upper 
and lower electrodes, respectively, a spot welding process was carried out 
with welding conditions set uniformly, thereby producing example 4 of the 
U-tensile test piece 31 as a comparative example. Thereafter, examples 1 
to 4 were subjected to a tensile test. 
Table 6 shows the construction of the Ni-insert 45, the smaller-end 
diameter D.sub.3 of the upper electrode 6, the welding conditions, the 
amount of expulsion and surface flash and the U-tensile strength for 
examples 1 to 4. 
TABLE 6 
__________________________________________________________________________ 
Smaller-end 
Welding conditions 
Amount of 
diameter D.sub.3 
Current 
expulsion 
(mm) of Welding Pressing supplying and U-tensile 
U-tensile Construction of upper current force time surface strength 
testpiece Ni-insert electrode 
(kA) (kgf) (cycle) flash (kgf) 
__________________________________________________________________________ 
Example 1 
Ni foil having a 
3 10 200 10 smaller 
98 
thickness of 
100 .mu.m 
Example 2 Ni foil having a 100 
thickness of 
50 .mu.m 
Example 3 Ni-plated layer 87 
having a thickness 
of 20 .mu.m on a steel 
foil having a 
thickness of 
100 .mu.m 
Example 4 Ni foil having a -- 10 200 10 smaller 35 
thickness of 
100 .mu.m 
__________________________________________________________________________ 
As apparent from Table 6, in examples 1 to 3 according to the embodiment, a 
solid-phase bonding by the cleaned surface of the first half 28 is 
produced and hence, the U-tensile strength is largely enhanced to be about 
2.5 or more times larger than that of example 4 as the comparative 
example. As a result of use of the Ni-insert 45, the U-tensile strength of 
each of examples 1 to 3 is about 1.6 or more times larger than that of 
example 1 in Table 5 which was produced under the same conditions as in 
examples 1 to 3, except that the Ni-insert 45 was not used. 
EXAMPLE 3 
A plurality of hollow extrudates 46 and a plurality of solid extrudates 47 
shown in FIGS. 30 and 31 were prepared. The material for both the 
extrudates 46 and 47 is JIS 6063. The size of the hollow extrudates 46 is 
30 mm in longitudinal length L.sub.1 ; 70 mm in lateral length L.sub.2 and 
4 mm in thickness t.sub.3, as shown in FIG. 30. The size of the solid 
extrudates 47 is 60 mm in width W, and 5 mm in thickness t.sub.4, as shown 
in FIG. 8(b). 
Using a second half 29, an upper electrode 6 having a smaller-end diameter 
D.sub.3 of 4 mm, extrudates 46 and 47 and an Ni-insert 45 which are 
similar to those in EXAMPLE-1, examples 1 and 2 of U-tensile test pieces 
according to the embodiment were produced by carrying out the same process 
as shown in FIGS. 26 to 29. 
Then, using a second half 29 and extrudates 46 and 47 which are similar to 
those in EXAMPLE-1 and using two lower electrodes 7 as upper end lower 
electrodes, a spot welding is carried out with the welding conditions set 
uniformly, thereby producing examples 3 and 4 of U-tensile test pieces as 
comparative examples. Thereafter, examples 1 to 4 were subjected to a 
tensile test. 
Table 7 shows constructions of the used extrudates 46 and 47 and the 
Ni-insert 45, the smaller-end diameter D.sub.3 of the upper electrode 6, 
the welding conditions, the amount of expulsion and surface flash and the 
U-tensile strength for examples 1 to 4. 
TABLE 7 
__________________________________________________________________________ 
Smaller- 
end Welding conditions Amount of 
diameter Current 
expulsion 
U- 
Construc- D.sub.3 (mm) of Welding Pressing supplying and tensile 
U-tensile Used tion of upper 
current force time surface 
strength 
test piece extrudate Ni-insert electrode (kA) (kgf) (cycle) flash 
__________________________________________________________________________ 
(kgf) 
Example 1 
Hollow 
Ni foil 
4 12 200 10 smaller 
150 
Example 2 Solid having a 142 
thickness 
of 100 .mu.m 
Example 3 Hollow -- -- 12 200 10 smaller 55 
Example 4 Solid 47 
__________________________________________________________________________ 
As apparent from Table 7, if example 1 is compared with example 3 and 
example 2 is compared with example 4, the solid-phase bonding by the 
cleaned surfaces of the hollow and solid extrudates 46 and 47 is produced 
in examples 1 and 2, and each of examples 1 and 2 has a high U-tensile 
strength, as compared with examples 3 and 4, because of use of the 
Ni-insert 45. 
In this way, according to EXAMPLE-3, not only the Al-based plate but also 
the Al-based extrudate and the Fe-based member can be firmly bonded to 
each other. 
It should be noted that in examples 1 and 2, a seam welding process and a 
projection welding process, in addition to the spot welding process, are 
included in the resistance welding process. In the projection welding 
process, a projection which is a pressed and current-supplied portion 40 
is formed on an Fe-based member.