U.S. patent application number 12/672344 was filed with the patent office on 2011-05-26 for bonded structure of dissimilar metallic materials and method of joining dissimilar metallic materials.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Akira Fukushima, Shigeyuki Nakagawa, Hiroshi Sakurai, Chika Sugi, Sadao Yanagida.
Application Number | 20110123825 12/672344 |
Document ID | / |
Family ID | 40052409 |
Filed Date | 2011-05-26 |
United States Patent
Application |
20110123825 |
Kind Code |
A1 |
Sakurai; Hiroshi ; et
al. |
May 26, 2011 |
BONDED STRUCTURE OF DISSIMILAR METALLIC MATERIALS AND METHOD OF
JOINING DISSIMILAR METALLIC MATERIALS
Abstract
Disclosed herein are bonded structures and methods of forming
the same. One embodiment of a bonded structure comprises first and
second metallic layers and a bonding interface between the first
and second metallic layers formed by diffusion and comprising a
layer of at least one intermetallic compound. The intermetallic
compound layer is formed in an area 52% or greater of an area of
the bonding interface and has a thickness of 0.5 to 3.2 .mu.m.
Inventors: |
Sakurai; Hiroshi; (Kanagawa,
JP) ; Nakagawa; Shigeyuki; (Kanagawa, JP) ;
Fukushima; Akira; (Kanagawa, JP) ; Yanagida;
Sadao; (Kanagawa, JP) ; Sugi; Chika; (Toyko,
JP) |
Assignee: |
NISSAN MOTOR CO., LTD.
Yokohama-shi, Kanagawa
JP
|
Family ID: |
40052409 |
Appl. No.: |
12/672344 |
Filed: |
August 7, 2008 |
PCT Filed: |
August 7, 2008 |
PCT NO: |
PCT/IB08/02081 |
371 Date: |
February 5, 2010 |
Current U.S.
Class: |
428/650 ;
219/148; 228/194 |
Current CPC
Class: |
B23K 2101/34 20180801;
B23K 11/115 20130101; B23K 2103/18 20180801; B23K 2103/10 20180801;
B23K 11/20 20130101; Y10T 428/12736 20150115; B23K 26/22 20130101;
B23K 2103/20 20180801; B23K 2103/14 20180801; B23K 2103/24
20180801; B23K 26/323 20151001 |
Class at
Publication: |
428/650 ;
228/194; 219/148 |
International
Class: |
B32B 15/18 20060101
B32B015/18; B23K 28/00 20060101 B23K028/00; B23K 11/20 20060101
B23K011/20 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 10, 2007 |
JP |
2007-209742 |
Apr 18, 2008 |
JP |
2008-108663 |
Claims
1. A bonded structure of dissimilar materials made from metals
comprising: a first metallic sheet; a second metallic sheet
overlying the first metallic sheet; and a bonding interface between
the first and second metallic sheets formed by diffusion and
comprising at least one intermetallic compound layer, wherein the
intermetallic compound layer is formed in an area 52% or greater of
an area of the bonding interface and has a thickness of 0.5 to 3.2
.mu.m.
2. The bonded structure according to claim 1 wherein the
intermetallic compound layer is formed in an area 70% or greater of
the area of the bonding interface.
3. The bonded structure according to claim 1 wherein the
intermetallic compound layer has a thickness of 0.6 to 2.8
.mu.m.
4. The bonded structure according to claim 1 wherein the
intermetallic compound layer has a thickness of 0.8 to 2.5
.mu.m.
5. The bonded structure according to claim 1 wherein the
intermetallic compound layer is comprised of a crystal grain with a
longitudinal diameter of 1.0 .mu.m or less.
6. The bonded structure according to claim 1 wherein one of the
first metallic sheet and the second metallic sheet comprises an
iron-based alloy and the other comprises an aluminum-based
alloy.
7. The bonded structure according to claim 6 wherein the
intermetallic compound comprises an Fe--Al based compound.
8. A method of bonding dissimilar materials made from metals
comprising: overlying a first metallic sheet with a first melting
point and a second metallic sheet with a second melting point to
form a bonding interface, wherein the first melting point is lower
than the second melting point; rapidly heating the first and second
metallic sheets; rapidly cooling the first and second metallic
sheets; and forming a compound layer by diffusion comprising at
least one intermetallic compound at the bonding interface by heat
treating the first and second metallic sheets at a heat treatment
temperature equal to or greater than a temperature at which
dislocation loops and voids formed by atomic vacancies resulting
from the rapid cooling are at least partially eliminated by a main
component metal of the first metallic sheet.
9. The method of claim 8 wherein the heat treatment temperature is
equal to or lower than 1/2 of a melting point in absolute
temperature of the at least one intermetallic compound whose
melting point is lowest among the at least one intermetallic
compounds.
10. The method of claim 8 wherein the heat treatment temperature is
equal to or higher than 1/2 of a melting point in absolute
temperature of a main component metal of the first metallic
sheet.
11. The method of claim 8 wherein the heat treatment temperature is
equal to or lower than one of: 1/2 of a melting point in absolute
temperature of the at least one intermetallic compound whose
melting point is lowest among the at least one intermetallic
compounds, and a temperature at which the first metallic sheet is
softened by disappearance of a precipitation strengthening phase or
recrystallization.
12. The method of claim 8, wherein the rapid heating and the
subsequent rapid cooling occurs by electric resistance joining by
energization heating.
13. The method of claim 8, wherein the rapid heating and the
subsequent rapid cooling occurs simultaneously with baking in a
painting process.
14. A method of increasing bond strength between a dissimilar metal
of an iron-based alloy and a dissimilar metal of an aluminum-based
alloy, the method comprising: heat treating the dissimilar metals
bonded by rapid heating and rapid cooling, the heat treating
occurring at a temperature ranging from 130.degree. C. to
440.degree. C., thereby forming a compound layer by diffusion at a
bond interface containing at least one Fe-Al based intermetallic
compound.
15. The method of claim 14 wherein heat treating occurs at a
temperature equal to or higher than 190.degree. C.
16. The method of claim 14 wherein heat treating occurs at a
temperature equal to or lower than 410.degree. C.
17. The method of claim 14 wherein the iron-based alloy is a
zinc-plated steel sheet and wherein, prior to heat treating, the
method further comprises: eutecticly fusing zinc of the zinc-plated
steel sheet and aluminum of the aluminum-based alloy, thereby
forming a low melting point fused material; removing an oxide
coating on a surface of the aluminum-based alloy at the bonded
interface together with the fused material; and bonding by rapid
heating and rapid cooling new surfaces of both dissimilar metals
from which a zinc-plating layer and the oxide coating are
removed.
18. The method of claim 17 wherein the rapid heating and the
subsequent rapid cooling occurs by electric resistance joining by
energization heating.
19. The method of claim 17 wherein the rapid heating and the
subsequent rapid cooling occurs simultaneously with baking in a
painting process.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from Japanese Patent
Application Ser. No. 2007-209742, filed Aug. 10, 2007, and Japanese
Patent Application Ser. No. 2008-108663, filed Apr. 18, 2008, each
of which is incorporated herein in its entirety by reference.
TECHNICAL FIELD
[0002] The invention relates to bonding techniques for joining
dissimilar metallic materials such as steel and aluminum alloy,
steel and titanium alloy, and aluminum alloy and titanium alloy, at
a bonding interface between which materials is formed an
intermetallic compound. The present invention also relates to the
resulting structures.
BACKGROUND
[0003] Summary of Lectures at Japan Welding Society Meeting, 77th
series, pages 320 to 321, by Japan Welding Society, September 2005
discloses, for example, that when joining dissimilar metals
consisting of steel and aluminum, if silica and oxygen exist at
suitable densities in an intermetallic compound reaction layer
formed at a joining interface, excess growth of a reaction layer
can be suppressed and joining strength can be increased. In
particular, by using a steel sheet adjusted so that an inner
oxidation is 1.5 .mu.m, a cross tensile strength of 1.4 kN
(maximum) can be obtained by electric resistance spot welding and
by combining the steel sheet with an aluminum alloy sheet (A6022)
1.6 mm thick.
[0004] Summary of Lectures at Japan Welding Society Meeting, 78th
series, pages 162 to 163, by Japan Welding Society, April 2006
describes that at the time of spot welding 980 MPa grade alloyed
molten zinc-plated steel sheets 1.2 mm thick with an aluminum alloy
sheet (A6022) 1.0 mm thick, two-step energization stimulates
softening and melting of a plated layer, whereby a wedge-shaped
Al.sub.3Fe.sub.2 intermetallic compound is formed in a reaction
interface layer resulting in a high cross tensile strength of 1.2
kN.
BRIEF SUMMARY
[0005] Disclosed herein are bonded structures and methods of making
the same. One embodiment of a bonded structure comprises a first
metallic layer, a second metallic layer overlying the first
metallic layer, and a bonding interface between the first and
second metallic layers formed by diffusion and comprising at least
one intermetallic compound layer. The intermetallic compound layer
is formed in an area 52% or greater of an area of the bonding
interface and has a thickness of 0.5 to 3.2 .mu.m.
[0006] A method of bonding dissimilar materials made from metals as
disclosed herein comprises overlying a first metallic layer with a
first melting point and a second metallic layer with a second
melting point to form a bonding interface, wherein the first
melting point is lower than the second melting point. The first and
second metallic layers are rapidly heated and subsequently rapidly
cooled. A compound layer is formed by diffusion comprising at least
one intermetallic compound at the bonding interface by heat
treating the first and second metallic layers at a heat treatment
temperature equal to or greater than a temperature at which
dislocation loops and voids formed by atomic vacancies resulting
from the rapid cooling are at least partially eliminated by a main
component metal of the first metallic layer.
[0007] Also disclosed herein is a method of increasing bond
strength between a dissimilar metal of an iron-based alloy and a
dissimilar metal of an aluminum-based alloy. The method comprises
heat treating the dissimilar metals bonded by rapid heating and
rapid cooling, the heat treating occurring at a temperature ranging
from 130.degree. C. to 440.degree. C. A compound layer is thereby
formed by diffusion at a bond interface containing at least one
Fe--Al based intermetallic compound.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The description herein makes reference to the accompanying
drawings wherein like reference numerals refer to like parts
throughout the several views, and wherein:
[0009] FIG. 1 is a graph showing a relation between a thickness and
a cross tensile strength of a metallic compound layer formed at a
bonding interface of steel and aluminum alloy;
[0010] FIG. 2 is a schematic of a spot welding device used for
joining dissimilar metallic materials disclosed herein;
[0011] FIG. 3 is a plan view showing the shape of a test piece for
cross tensile testing that is used for evaluation of a joining
strength;
[0012] FIG. 4 is a graph showing a relation between an area ratio
and a joining strength within a nugget of an intermetallic compound
layer of the thickness ranging from 0.5 to 3.2 .mu.m;
[0013] FIG. 5 is a graph showing a relation between a maximum value
of a crystal grain size and a joining strength of an intermetallic
compound layer;
[0014] FIG. 6 is a cross-sectional view showing a joining structure
of a zinc-plated steel sheet and an aluminum alloy, which is
obtained as disclosed herein;
[0015] FIG. 7A is a transmission electron microscopy photograph
showing a state of a joining structure before heat treatment of a
first embodiment;
[0016] FIG. 7B is a transmission electron microscopy photograph
showing a state of a joining structure after heat treatment of a
first embodiment;
[0017] FIG. 7C is an outward appearance photograph showing a
fracture state of a test piece by a cross tensile test of a first
embodiment;
[0018] FIG. 8 is a transmission electron microscopy photograph
showing a joining structure obtained by a comparative example
1;
[0019] FIG. 9A is an outward appearance photograph showing a
fracture state of a test piece by a cross tensile test of a
comparative example 2;
[0020] FIG. 9B is an outward appearance photograph showing a
fracture state of a test piece by a cross tensile test of a second
embodiment;
[0021] FIG. 9C is an outward appearance photograph showing a
fracture state of a test piece by a cross tensile test of a third
embodiment;
[0022] FIG. 9D is an outward appearance photograph showing a
fracture state of a test piece by a cross tensile test of a
comparative example 3;
[0023] FIG. 10A is a schematic showing an outline of joining of
dissimilar metallic materials according to a fourth embodiment,
together with a spot welding device;
[0024] FIG. 10B is a cross-sectional view showing a joining
structure of a zinc-plated steel sheet and an aluminum alloy, which
is obtained by the fourth embodiment;
[0025] FIG. 10C is a transmission electron microscopy photograph
showing a joining structure obtained by the fourth embodiment;
and
[0026] FIG. 11 is a transmission electron microscopy photograph
showing a joining structure obtained by a fifth embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0027] The conventional methods discussed above for bonding or
joining of dissimilar metallic materials inevitably produce
inefficient joining conditions that are industrially unsuitable. If
efficiency of conditions is attempted, a thick compound layer
containing an intermetallic compound having a large grain size
results without achieving the necessary strength. In view of this,
an effort has been made to join members of dissimilar metals with
high joining strength at the intermetallic layer at the joining
interface in an efficient manner.
[0028] Hereinafter, a bonded structure of dissimilar metals and a
method of bonding dissimilar metals according to the invention will
be described further in detail and concretely with respect to
certain embodiments.
[0029] FIG. 1 depicts the relationship between a thickness and a
cross tensile strength of a metallic compound layer formed at a
joining interface between a steel sheet of the thickness of 0.55 mm
and a 6000 series aluminum alloy sheet. As used herein, the terms
bonding and joining are interchangeable. The sheets are joined
together by using an alternate current type spot welding device
shown in FIG. 2 under various joining conditions, such as electric
current of 20000 to 30000 A, pressurizing force of 150 to 600 kgf,
and an energization time of 250 milliseconds or less. Thereafter,
the sheets are heat treated under varied conditions, such as a
treatment temperature of 140 to 500.degree. C. and a treatment time
of 20 minutes to 7 hours. For FIG. 1, the data was obtained under
conditions wherein 90% or more of the intermetallic layer was 1
.mu.m or less in thickness so that the thickness of the
intermetallic compound layer formed at the bonding interface is as
uniform as possible after welding. The cross tensile test was
performed by using a test piece shaped and sized as shown in FIG. 3
and according to the method prescribed in Japanese Industrial
Standard JIS Z3137 (entitled Specimen dimensions and procedure for
cross tension testing resistance spot-and embossed
projection-welded joints).
[0030] As apparent from FIG. 1, for obtaining a high strength
bonded structure of dissimilar metals, it is necessary to control
the thickness of a metallic compound layer formed at a constant
area ratio by diffusion joining. It will be understood that for
attaining a bonding strength of an aluminum alloy sheet material
having a tensile strength of 210 MPa, i.e., a cross tensile
strength of 0.6 kN or more, a control latitude of the thickness of
the intermetallic compound layer should be set within the range
from 0.5 to 3.2 .mu.m. For attaining a cross tensile strength of
0.9 kN or more, the control latitude should be set within the range
from 0.6 to 2.8 .mu.m. For attaining a high strength of 1.2 kN or
more, the thickness of the intermetallic compound layer should be
set within the range from 0.8 to 2.5 .mu.m. Namely, when the
thickness of the intermetallic compound layer exceeds 3.2 .mu.m,
its contribution to the strength is lowered, and a sufficient
strength cannot be obtained when the thickness is less than 0.5
.mu.m. Accordingly, the thickness of the intermetallic compound
layer should be set within the range at least from about 0.8 to
about 2.5 .mu.m.
[0031] In the joining nugget formed by a spot welding device, the
intermetallic compound is not always formed at the entire area of
the nugget (joining surface), but a compound thickness distribution
was seen within the nugget. The area ratio of the
[0032] intermetallic compound layer formed with the above-described
thickness and strength was obtained by observing the central cross
section of the joining portion, measuring the length by which the
thickness of the intermetallic compound is within the
above-described range, assuming that donut-shaped joining zones are
formed concentric with the nugget, and calculating, from the
distance of the center of the nugget, the ratio of the sum of the
areas of the joining zones, to the nugget area (joining area). The
result is shown in FIG. 4.
[0033] According to FIG. 4, if the ratio of the area at which the
intermetallic compound layer was formed so as to have the thickness
in the range from 0.8 to 2.5 .mu.m to the joining area was 52% or
more, a good joining strength was exhibited even at the lower limit
of variations. When the area ratio increased to 70% or more, the
joining strength further increased and a sufficient cross tensile
strength of 0.9 kN or more could be attained.
[0034] FIG. 5 shows the influence of a maximum crystal grain size
as measured along the longitudinal diameter of an intermetallic
compound on a joining strength. It is seen that even if the
thickness of the intermetallic compound layer becomes a little
larger, the joining strength is increased if the crystal grain is
small.
[0035] The rapid heating process and rapid cooling process
introduces lattice defect and crystal restoration during the
joining process. The structural variation of the crystal grain at
the joining interface of the intermetallic compound was refined by
the heat treatment subsequent to the joining process. For the
crystal grain to grow or disappear, atoms need to move in the
material, that is, diffusion is necessary. The structure formed by
the rapid heating process and the rapid cooling process has a high
system energy. For example, the rapid cooling process introduces
composition inclination, many voids and dislocation. A metallic
structure in such a high-energy condition is changed by heat
treatment, reducing the structure to a low energy condition.
Representative examples are phenomena such as uniformalization of
composition inclination, growth or disappearance of crystal grain,
disappearance of dislocation loops and voids that are formed by
gathering of atomic vacancies, an changing of crystal grain
boundaries to low energy grain boundaries.
[0036] Under high temperature conditions, a number of atomic
vacancies are contained in a lattice, but if the metallic system is
rapidly cooled from such a high temperature condition, vacancies
exist in the cooled lattice with a high density. Such
supersaturated vacancies increase the system energy. In case of
heat treatment of a normal structure, since there exist vacancies
in only the amount that is admitted thermodynamically, diffusion
caused by the heat treatment is restricted by the amount of
vacancies and the speed of movement of vacancy. But by heat
treatment of the structure having an excess amount of vacancies due
to rapid cooling, diffusion in excess of that caused by normal heat
treatment can occur. To cause growth and disappearance of crystal
grains, it is necessary to elevate the temperature for a prolonged
time to attain a sufficient amount of vacancies and atomic
movement. However, because a sufficient amount of vacancies exist
in the crystal lattice, sufficient diffusion and reaction can occur
even at a relatively low temperature.
[0037] Refinement of crystal grain, which is a feature of a
structural change of the embodiments disclosed herein, can occur
under a heat treatment condition in which the number (frequency) of
crystal nucleus formations is high and the grain growth speed is
not so high. The low-temperature heat treatment of the rapidly
cooled structure can increase the number (frequency) of nucleus
formations due to the excess amount of vacancies and can avoid,
during the grain growth thereafter, coarsening of the crystal
grains.
[0038] For performing heat treatment under such a condition,
high-temperature heat treatment leading to coarsening of the
crystal grain should be avoided, and the temperature sufficient to
move the vacancies introduced by the rapid heating process and the
rapid cooling process should be set to the lower limit value of the
heat treatment temperature. Namely, for restoration of the metallic
structure, heat treatment should be performed not at the
temperature at which the electric resistance is restored but at the
temperature at which disappearance of the voids or dislocation
loops formed by gathering of atomic vacancies occurs. Regarding
crystal restoration, a detailed description is found in "Theory of
Dislocation", pp. 229-235, edited by Japan Metal Society and issued
by Maruzen Co., Ltd. It can be said that, for disappearance of
voids that are formed by gathering of atomic vacancies or
distinction of dislocation loops, heat treatment is performed at
the temperature for the "V step" or more of restoration of a metal
structure.
[0039] For the crystal grain growth of the intermetallic compound
in the diffusion joining interface between metallic materials of
different melting points, sufficient diffusion of either base metal
needs to occur. Since the lower melting point base metal causes
diffusion at the lower temperature, the grain growth of the
intermetallic compound at the interface is ruled by the diffusion
of the lower melting point base metal. For example, in the joining
of iron and aluminum, refinement of crystal grains by heat
treatment of a rapidly heated and rapidly cooled structure,
represented by electric resistance welding, starts to occur when
the V step of restoration of an aluminum-base alloy is reached.
This is because aluminum is a base material of a lower melting
point as compared with an iron-based alloy, i.e., at the
temperature of 127.degree. C. (400 K) or more. This temperature
coincides with the temperature for causing disappearance of
dislocation loops and voids that are formed by gathering of the
atomic vacancies of the rapidly cooled structure of the
aluminum-base metal as the base metal. The structure caused by the
rapidly heating process and the rapidly cooling process changes
into a stable structure.
[0040] As having been described above, movement of the vacancies in
the low melting point base metal lattice is a heat treatment
condition of the embodiments herein. But for efficiently obtaining
a target structure within a short time, it is desirable to perform
heat treatment at the temperature that causes sufficient diffusion.
A sufficient diffusion effect can be expected by setting the heat
treatment temperature so as to be equal to or higher than 1/2 of
the melting point expressed by absolute temperature of the low
melting point side base material.
[0041] On the other hand, when heat treatment is performed at the
temperature equal to or higher than 1/2 of the melting point
expressed by absolute temperature of the low melting point base
material and at a higher temperature in case of a precipitation
strengthened alloy for instance, the temperature at which the
precipitation phase disappears is reached. Further, in case the
strength is obtained by work hardening, the strength of the base
material is lowered by recrystallization. If the precipitation
phase, which strengthens the base material, disappears, the joining
strength itself cannot be improved even if the strength of the
joining portion is increased. Accordingly, the heat treatment after
joining must allow the strength of the base material to be
maintained at the strength necessary for a structured body. Namely,
heat treatment is performed at the temperature equal to or lower
than the temperature at which the low melting point base material
is softened due to disappearance of the precipitation strengthening
phase or recrystallization.
[0042] Diffusion of the low melting point side base metal needs be
sufficient, but if diffusion within intermetallic compound grains
is excessive, it is possible that the strength cannot be attained
due to the coarsening of the grains of the intermetallic compound.
To avoid such coarsening of the grains of the intermetallic
compound, it is necessary to perform heat treatment at a
temperature that causes diffusion within the intermetallic compound
grains of the joining portion sufficiently, i.e., that enables the
vacancies to move easily within the intermetallic compound. This
temperature can be the temperature equal to or lower than 1/2 of
the melting point expressed by absolute temperature of the
intermetallic compound whose melting point is lowest. A desired
temperature range is one that causes movement of the vacancies
within the low melting point side lattice and does not coarsen the
metallic compound grains, thereby attaining a sufficient strength
of the base material. It has been discovered that it is
industrially most effective to perform heat treatment at a highest
possible temperature within that temperature range. Namely, the
heat treatment needs to be performed at the temperature equal to or
lower than one of: 1/2 of the melting point expressed by absolute
temperature of the intermetallic compound whose melting point is
lowest among the formed intermetallic compounds and the temperature
at which the low melting point side metallic material is softened
by disappearance of precipitation strengthening phase or
recrystallization.
[0043] By energy dispersive x-ray (EDX) analysis of Al-Fe based
intermetallic compounds, a component ratio of the Al/Fe atomic
ratio was observed to be close to 3:1 or 5:2, and it was judged
from this component ratio that the intermetallic compounds were
Al.sub.3Fe and Al.sub.5Fe.sub.2. The melting points of those
intermetallic compounds are higher than the melting point of
aluminum, 660.degree. C. The melting points of Al.sub.3Fe and
Al.sub.5Fe.sub.2 are 1160.degree. C. and 1169.degree. C.,
respectively. The aimed temperature for causing movement of the
vacancies within the intermetallic compound structure can be set at
1/2 of the melting points expressed by absolute temperature, i.e.,
about 440.degree. C. To change the structure formed by the rapid
heating process and the rapid cooling process into a stable
structure without causing coarsening of the intermetallic compound,
it is preferable to elevate the temperature equal to or higher than
the temperature that eliminates the dislocation loops and voids
that are formed by gathering of the atomic vacancies of the rapidly
cooled structure of the low melting point side base material, more
preferably 1/2 of the melting point expressed by absolute
temperature of the main component metal of the low melting point
side metallic material. If heating is carried out at a temperature
exceeding this temperature, coarsening of the intermetallic
compound grains was caused.
[0044] In diffusion joining, the joining strength becomes
insufficient if the intermetallic compound is too thin. If the
process temperature is elevated to achieve higher production
efficiency, the resulting intermetallic compound grains are liable
to be coarsened, possibly causing a fragile characteristic. In
diffusion joining, since the joining is generally performed in a
temperature range in which the diffusion speed is sufficiently
large, the intermetallic compound crystal grains are liable to be
coarsened. If it is tried to refine the crystal grains, a long
processing time is necessitated and an impracticable processing
time is required for application to industrial production. In
welding, since the temperature becomes high enough to reach the
melting point, coarsening of the intermetallic compound occurs in
almost all cases.
[0045] As disclosed herein, by once causing electric resistance
heating at the joining interface, forming a structure of a
composition in an unequilibrium state and of a uniform thickness
and thereafter performing heat treatment at the temperature equal
to or higher than the temperature for causing disappearance of
dislocation loops and voids that are formed by gathering of the
atomic vacancies of the rapidly cooled structure of the low melting
point metal, the intermetallic compound layer having a uniform
thickness at the time of joining of dissimilar metallic materials
is thereby obtained. In addition to refining the compound structure
of an unequilibrium composition, a strong intermetallic compound
layer of refined crystal grains and moreover having a uniform
thickness can be formed.
[0046] In embodiments herein, for dissimilar metallic materials, a
combination of Fe-based alloy and Al-based alloy can be used
suitably. The Fe-based alloy is herein intended to mean an alloy
containing Fe as a main component, and more specifically rolled
steel of carbon steel, alloy steel, soft steel, high tensile steel
or the like. Further, Al-based alloy is intended to mean an alloy
containing Al as a major component, and aluminum alloys from 1000
to 7000 series can be used suitably. As used herein, "major
component" is intended to indicate a metal that is contained in the
largest amount in an alloy. There is not any limitation to the
combination of the dissimilar metallic materials used in the
embodiments herein provided that the combination is such that the
materials contain, as main components, such metals that form
intermetallic compounds at the interface by diffusion, wherein the
diffusion is caused by a processing that utilizes a rapid heating
process and a subsequent rapid cooling process due to cooling of
the materials as electric resistant joining as described above.
Examples of a combination of such metal elements are a combination
of Ti (titan) and Al (aluminum) and a combination of Ti and Fe
(iron).
[0047] In the embodiments herein, if the dissimilar metallic
materials includes a combination of Al-based alloy and Fe-based
alloy, the above-described heat treatment can be performed at the
temperature equal to or higher than the temperature that makes the
main component metal of the low melting point side metallic
material cause disappearance of dislocation loops and voids that
are formed by gathering of the atomic vacancies of the rapidly
cooled structure, i.e., 127.degree. C. and in the temperature range
from about 130 to about 440.degree. C., which temperature range is
calculated from 1/2 of the melting point expressed by absolute
temperature of the intermetallic compound. Further, from the point
of view of performing heat treatment in the temperature range equal
to or lower than 1/2 of the melting point expressed by absolute
temperature of the intermetallic compound whose melting point is
lowest amount the formed intermetallic compounds, the temperature
range equal to or higher than about 190.degree. C. is desirable.
Further, from the point of view of performing the heat treatment in
the temperature range equal to or lower than one of: 1/2 of the
melting point expressed by absolute temperature of the
intermetallic compound whose melting point is lowest among the
formed intermetallic compounds and the temperature at which the low
melting point side metallic material is softened by disappearance
of the precipitation strengthening phase or recrystallization, the
temperature range equal to or lower than about 410.degree. C. is
preferable.
[0048] A zinc-plated steel sheet can be used as the Fe-base alloy.
By causing eutectic fusion between zinc forming a plating layer and
aluminum and pressurizing the same, it becomes possible to remove
the oxide coating formed on the surface of the Al-based alloy
together with the fused eutectic material. It also becomes possible
to attain diffusion joining of new surfaces of both metallic
materials from which the zinc-plating layer and the oxide coating
are removed.
[0049] In the method of joining dissimilar metals disclosed herein,
for the joining including a rapid heating process and a subsequent
rapid cooling process, the electric resistance joining using a spot
welding device shown in FIG. 2 can be representatively employed.
However, this is an example and the invention is not limited to the
use such resistance heating. Another heating means such as laser
beam can be used. Further, the spot joining by nugget formation is
also an example and is not limiting, and, by using a roller-shaped
electrode, seam joining can be performed. In the method of joining
dissimilar metals disclosed herein, if the joined member obtained
is to be painted, the heat treatment subsequent to the
above-described joining and baking of the paint in the painting
process can be performed at the same time. By this, the heating
process can be omitted, and energy can be conserved.
[0050] For a first embodiment, by using an alternate type spot
welding device shown in FIG. 2, a Zn-plated steel sheet 2 of the
thickness of 0.55 mm and a 6000 series aluminum alloy sheet 2
having a tensile strength of 200 MPa are laid one upon the other
and resistance spot joining thereof is executed under a condition
of a pressurizing force of 300 kN, a current of 24000 A and an
energizing time of 0.2 sec. At the time of joining, aluminum and
zinc of the plating layer were once reacted at the temperature of
400.degree. C. to thereby cause eutectic fusion thereof. An oxide
coating 2a on the surface of the aluminum alloy sheet 2 was
ejected, as shown in FIG. 6, by electrode pressurization by the
welding device. The ejected oxide coating, as ejected matter D,
together with a fused eutectic material formed thereby cause a new
surface of the aluminum alloy sheet 2 while causing diffusion
reaction. Diffusion layers were formed on the steel sheet 1 and the
aluminum alloy sheet 2 without melting the aluminum alloy. Fe and
Al were reacted within the diffusion layers to form a thin
intermetallic compound layer L, and the dissimilar metallic
materials 1 and 2 were joined by way of the intermetallic compound
layer L.
[0051] As a result, the aluminum oxide coating 2a on the surface of
the aluminum alloy sheet 2 is ejected together with Zn-Al reaction
phase, i.e., an eutectic alloy, to the place around the nugget. The
thin intermetallic compound layer L having the thickness in the
range from 0.8 to 2.5 .mu.m is formed at the joining interface and
in a region equal to 48% of the joining area (nugget area), and the
intermetallic compound crystal grain of the intermetallic compound
layer L is elliptical in sectional shape, the long diameter being
about 0.3 .mu.m. A transmission electron microscopy photograph of
the joining portion is shown in FIG. 7A.
[0052] Then, the joined member prepared in this manner is subjected
to heat treatment at 440.degree. C. for 1.5 hours, and it was
confirmed that the entire intermetallic layer came to have the
thickness ranging from 0.8 to 2.5 .mu.m and was formed in the
region equal to 89% of the joining area. At the same time, the
intermetallic compound grains were changed in an equiaxed manner
and the crystal grains could be refined so as to have a grain size
of 0.1 .mu.m or less. A transmission electron microscopy photograph
is shown in FIG. 7B. As a result of measurement of the strength of
the joined member having been heat treated at 440.degree. C. for
1.5 hours by a cross tensile test, the outline of which is shown in
FIG. 3, the joining strength of 1.60 kN was obtained, and it was
confirmed that fracture was caused at the aluminum alloy side as
shown in FIG. 7C. In the meantime, as a result of investigation on
the above-described intermetallic compound layer L by EDS and X-ray
diffraction analysis, it was confirmed that the compound layer was
constituted by intermetallic compounds having Fe--Al component
ratios close to FeAl.sub.3 and Fe.sub.5Al.sub.2, and Zinc of the
plating layer was so scarcely contained in the intermetallic
compound as not to be found by the analysis.
[0053] For comparative example 1, the joined member was obtained
under the same condition as the first embodiment. The joined member
was heat treated at 500.degree. C. for 0.5 hours. The intermetallic
compound layer came to have the thickness exceeding 3.2 .mu.m, the
area ratio thereof was 98%, and the diffusion joining layer had the
interface having the intermetallic compound layer. However, there
was scarcely any joining layer in which the thickness of the
intermetallic compound layer was 3.2 .mu.m or less and the joining
area was 2%. Further, it was observed that the crystal grains were
changed to constitute two layers, an equiaxed portion and a
post-like portion. The crystal grains at the equiaxed portion were
in the range from 0.1 to 0.2 .mu.m, and the crystal grains at the
post-like portion extended vertically so as to have a maximum long
diameter close to 1.0 .mu.m. A transmission electron microscopy
photograph in this case is shown in FIG. 8. In the cross tensile
strength, fracture was caused at the joining interface and the
strength obtained was only about 0.09 kN.
[0054] For comparative example 2, the same spot welding device as
the above-described first embodiment was used. After a Zn-plated
steel sheet 1 of the thickness of 0.55 mm and a 6000 series
aluminum alloy sheet 2 of the thickness of 1.0 mm were laid one
upon the other, resistance spot joining was performed under a
condition of a pressuring force of 300 kN, current being made
smaller than that of the first embodiment and of 20000 A and an
energization time of 0.2 sec. for thereby joining the dissimilar
metallic materials by diffusion joining. As a result, an
intermetallic compound layer L was formed and the region of the
thickness in the range from 0.8 to 2.5 .mu.m was 46% of the joining
area. Further, the maximum long diameter of the crystal grains of
the intermetallic compound constituting the intermetallic compound
layer L was 0.06 .mu.m.
[0055] FIG. 9A shows a fracture state of a test piece in case the
cross tensile test was executed without processing the joined
member obtained in the manner as described above by heat treatment.
Although the nugget partly remained on the steel sheet side, it was
mostly fractured and peeled off from the joining interface, and the
strength of 0.67 kN was obtained.
[0056] A second embodiment was a result of heat treating the joined
member prepared under the same conditions as the comparative
example 2, at 440.degree. C. for 1.5 hours. It was confirmed that
the area of the intermetallic compound layer L having the thickness
in the range from 0.8 to 2.5 .mu.m was enlarged to 90% of the
joining area, and the crystal grains of the intermetallic compound
were changed in an equiaxed manner and refined so that the grain
size was 0.1 .mu.m. FIG. 9B shows a fracture state of a test piece
where the cross tensile test was executed after the above-described
heat treatment. Plug fracture was caused, and the cross tensile
strength reached to 1.69 kN. In this manner, a fracture strength
that was relatively high as compared with the base material
strength was obtained, though there occurred a change to a plug
fracture mode. Without being bound to theory, this is considered
due to an influence of an increase of the base material strength,
which was caused by aging of the aluminum alloy. In contrast to
fracture being a peel off type before heat treatment, fracture was
not started at the joining interface but at the base material,
signifying an increase of the strength of the joining
interface.
[0057] A third embodiment of the joined member was prepared under
the same condition as the above-described comparative example 2 but
was heat treated at 300.degree. C. for 7 hours. As a result, the
region of the intermetallic compound layer L having the thickness
in the range from 0.8 to 2.5 .mu.m was enlarged to account for 82%
of the joining area. Regarding the crystal grains of the
intermetallic compound, it was confirmed that the crystal grains
changed in an equiaxed manner and were refined so as to have the
grain size in the range from 0.05 to 0.1 .mu.m. FIG. 9C shows a
fracture state of a test piece where the cross tensile test was
executed after the above-described heat treatment. The cross
tensile strength was 1.50 kN, and it was possible to cause nugget
fracture.
[0058] Comparative example 3 was a result of heat treating a joined
member prepared under the same condition as the above-described
comparative example 2, but at 500.degree. C. for one hour. It was
observed that the intermetallic compound layer L grew so as to have
a thickness exceeding 3.2 .mu.m, and the area thereof was enlarged
to almost 100% of the joining area. The crystal grains of the
intermetallic compound were changed to constitute two layers, an
equiaxed portion and a post-like portion. The grain size was in the
range from 0.1 to 0.3 .mu.m at the equiaxed portion, and the
maximum long diameter of the crystal grains at the post-like
portion was 1.8 .mu.m. FIG. 9D shows a fracture state of a test
piece in case the cross tensile test was executed after the joined
member was heat treated at 500.degree. C. for one hour. Interface
fracture was caused, and the cross tensile test result was only
0.08 kN. In this manner, a sufficient strength can be realized by
controlling the thickness and area of the intermetallic compound
layer to within a predetermined range. Where a sufficiently thin
intermetallic compound layer is formed at a joining process prior
to heat treatment, the strength can be increased by making, by
subsequent heat treatment, the thickness of the intermetallic
compound increase within the limits that enable refinement of the
grain size of the intermetallic compound. Where, to the contrary,
the intermetallic compound layer formed at the joining step becomes
relatively thick, the quality of the joining portion of dissimilar
metals can be stabilized by refining the crystal grains while
selecting a heat treatment condition that does not cause the
intermetallic compound layer to increase in thickness. In this
manner, by a process of forming a rapidly heated and cooled
structure, a typical example of which is electric resistant
joining, and a subsequent heat treatment process, a joining portion
of dissimilar metals having a high quality can be obtained stably.
In addition, by combining thereto an aging condition or the like, a
joining of dissimilar metals can result in higher strength.
[0059] As a fourth embodiment, shown in FIG. 10A, a molten zinc
plating steel sheet 1 of the thickness of 0.55 mm and a 6000 series
aluminum alloy sheet 2 having the tensile strength of 210 MPa and
the thickness of 10 mm was laid one upon the other by way of a
thermo-hardening adhesive agent S, and resistant spot joining
thereof was performed under the condition of a pressuring force of
300 kN, a current of 24000 A and an energizing time of 0.2 sec. to
thereby join dissimilar metallic materials by diffusion joining
without melting the aluminum alloy sheet 2. At this time, aluminum
and zinc of the plating layer were once reacted at the temperature
of 400.degree. C. to thereby cause eutectic fusion thereof. An
oxide coating 2a of the aluminum ally sheet 2 and the adhesive
agent were ejected by electrode pressurization by the welding
device as ejected matter, which together with a fused eutectic
material formed, thereby caused a new surface of the aluminum alloy
sheet 2. Diffusion layers were formed on the steel sheet 1 and the
aluminum alloy sheet 2. An intermetallic compound layer L was
formed at the joining interface, and, as shown in FIG. 10B, the
dissimilar metallic materials 1 and 2 were joined by way of the
intermetallic compound layer L without melting the aluminum
alloy.
[0060] Thereafter, by performing heat treatment at 170.degree. C.,
which is the hardening temperature of the above-described adhesive
agent S, the intermetallic compound L was formed in the region of
56% of the joining area so as to have a thickness in the range from
0.8 to 2.5 .mu.m. It was observed that the crystal grains of the
intermetallic compound were refined so that a maximum long diameter
was in the range from 0.05 to 1.0 .mu.m (refer to FIG. 10C). As a
result of performing the cross tensile test similarly, a joining
strength of 0.94 kN was obtained.
[0061] In this embodiment, by heat treatment at one time, the
adhesive agent S can be hardened simultaneously with refinement of
the intermetallic compound layer to thereby form an insulation
layer between the dissimilar metallic materials, thus making it
possible to improve the joining strength and the corrosion
resistant ability without increasing the number of process steps
and the amount of invested energy. In the meantime, it is needless
to say that such a technique can similarly be applied to the case
the member is painted after joining
[0062] To make a fifth embodiment, at the time of joining
dissimilar metals consisting of a steel sheet 1 and an aluminum
alloy sheet 2, the alternate type spot welding device shown in FIG.
2 was used, and the same operations as the first embodiment were
repeated except that a zinc-plating layer 2b was applied onto the
surface of the aluminum alloy sheet 2. The joined member of
dissimilar metals of the fifth embodiment is shown in FIG. 11. As a
result, an intermetallic compound layer L having the thickness in
the range from 0.5 to 3.5 .mu.m was formed at the joining
interface, and the region of the intermetallic compound layer
having the thickness in the range from 0.8 to 2.5 .mu.m was 56% of
the joining area. It was found that the maximum long diameter of
the intermetallic compound layer grains was 0.05 .mu.m. Further, as
a result of performing the cross tensile test, the joining strength
of 1.2 kN was obtained
[0063] Generally, a strong, high-melting point oxide coating exists
on the surface of the aluminum alloy 2 and its removal is a problem
at the time of diffusion joining But since in this embodiment the
oxide coating has been removed at the process of applying a
zinc-plating to the aluminum alloy sheet 2 and a new surface of the
aluminum alloy sheet 2 is exposed by melt removal, diffusion
joining of the aluminum alloy sheet with the steel sheet by
energization heating is possible.
[0064] The above-described embodiments have been described in order
to allow easy understanding of the invention and do not limit the
invention. On the contrary, the invention is intended to cover
various modifications and equivalent arrangements included within
the scope of the appended claims, which scope is to be accorded the
broadest interpretation so as to encompass all such modifications
and equivalent structure as is permitted under the law.
* * * * *