of Dissimilar Materials
Many applications and emerging applications are dependent on dissimilar
material joints. Due to the different chemical, mechanical and thermal
behaviors of materials, dissimilar materials joining presents challenges
significantly different than similar materials joining. Over the
years, dissimilar materials have been joined via:
|· Mechanical Interlocking
· Mechanical Connection (Screw/Nut & Bolt/Rivet
· Gluing or Chemical Bonding
· Welding Methods (friction and diffusion)
· Brazing Procedures
· Soldering Processes
These methods can work for varying applications;
however, when joining for electronics, thermal management and
high reliability, many of these joining methods do not apply and
only specialized welding, brazing and soldering are used. Brazing
is accomplished above 450°C while soldering is done below
that temperature. This is the major distinction between these
two joining methods. Both work very well when joining similar
metals. However, when joining dissimilar materials, including
ceramics and/or composite materials, soldering, brazing and welding
become limited. In response to these limitations, MRi has developed
technologies, products and technical services for dissimilar materials
Traditional Soldering & Brazing
Traditionally, joining of metals to ceramics with brazing or soldering
techniques has relied on pre-metallizing (the pre-coating of the
materials to be connected by conventional techniques such as evaporation
of metal in a vacuum furnace) and/or the removal of surface oxides
by chemical flux prior to applying the brazing alloy. Not only
are such processes multi-step and cumbersome, but the strength
of the resulting joints directly dependent on the quality of the
surface pre-treatments, and on the effectiveness of the fluxing
agents used in the process. Chemical fluxes are generally corrosive
to the base materials as well as being hazardous to the environment.
Figure 1 depicts a conventional brazing process.
Conventional soldering and brazing works by using metals such
as a lead and tin based solders or silver, copper, nickel or other
precious metals and/or alloys, that melt at a lower temperature
than either of the materials being joined [Figure 1]. The joining
alloy fuse into the surfaces of the materials being joined, forming
a metallurgical bond without significantly melting either of the
two materials. When brazing or soldering in air, fluxes are used
to react with oxide scales and to shield the joint area with gases
and/or liquids, which acts as a barrier to prevent diffusion of
oxygen into the joint being formed. Specifically, for joining
ceramics to metals, thin layers of deposited metals are usually
used prior to brazing to facilitate the braze bond.
Figure 1 shows a typical active brazing process
Active brazes utilize titanium (or other reactive
metal such as Hf and/or Zr) to react with oxide films on parts
to be joined during the brazing process[Figure 2]. Such reactions
reduce the oxide films and provide new compounds resulting in
a metallurgical bond. However, the titanium is so active that
the oxygen atmosphere needs to be eliminated by mechanical means
e.g. a vacuum furnace. For these brazes the titanium reaction
is activated only at temperatures greater than 8500C. For joining
ceramics and some metals to ceramics, thin layers of deposited
metals are sometimes required prior to the brazing process to
facilitate the braze bond.
Figure 2 shows a typical active brazing/soldering process.
Dissimilar materials joining requires consideration
- compositional compatibility for joining material
on both interfaces
- differences in coefficient of thermal expansion
- differences in melting points.
- wetting of ceramic (non-metallic) surfaces
Different metals such as aluminum and stainless
steel are good examples of this. Aluminum melts well below any
compatible stainless steel brazes, hence eliminating brazing.
Meanwhile, the conventional solders that can bond to aluminum
cannot bond to stainless steel. The joining of copper to stainless
steel, while they can be brazed, present thermal expansion mismatches
that fracture the braze joint.
The high temperature joining of ceramics to metals [for example
(Al2O3, SiC, Si3N4, ZrO2, carbides
) to (copper, aluminum,
steel, nickel alloys
) ] raise major thermal expansion mismatch
problems since the brazing requires temperatures over 1900°F
which on cooling can crack many ceramics. Additionally, active
brazes (containing Ti, Hf, and/or Zr) are usually required to
get bonding reactions on the ceramic side of the interfaces. Low
temperature joining (soldering) can be successful in achieving
ceramic/metal joints, but requires the ceramics to be premetallized
by plating with nickel, gold evaporative coatings, or the Mo-Mn
(oxide-metal frit firing) deposition processes. These metallizing
techniques leave metals on the ceramic side of the interfaced
to which solders can wet.
MRi has focused many of its developments on solving the problems
normally associated with braze or solder joining of dissimilar
materials. The products, presented below, address the low and
the high temperature needs for many dissimilar material joints.
MRi's products address:
- wetting of all surfaces, without fluxes
- active bonding to ceramics, employing active
- CTE mismatch, using either low temperatures
or composite brazes
MRi's technology and products capable of dissimilar
materials joining are presented below.
For low temperature (<400°C) applications:
S-Bond® joins all metals, ceramics, and composites in air
and without flux and without the need for premetallization, answering
many of the issues associated with joining dissimilar materials.
MRi's patented S-Bond® alloys contain active elements to
permit active, low temperature joining. S-Bond® joining combines
its active joining alloys with processes. These processes offer
many advantages, but they are unlike soldering or brazing process
since the application and joining methods differ and are specific
to the characteristics of our patented alloys. Specifically our
alloys are active and do not have the capillarity of conventional
solders. As such, these alloys require that that S-Bond®
processes pre-place and then "mechanically activate"
the alloy in the joints. S-Bond® processes have been designed
to accommodate such features.
S-Bond® can join any combination of dissimilar materials
- Lowering CTE mismatch with low joining temperatures,
- Active joining, at low temperatures, using
patented S-Bond® alloys.
- Wets all surfaces, (without metallizing) with
the active S-Bond® alloys
For high temperature (> 600°C) applications, MRi offers:
MRi's innovative brazing technology that uses powder
metallurgy preforms to produce composite braze joints with controlled
thickness to compositionally tailor or offset CTE mismatches.
These braze powder based preforms combine non-melting temperature
"filler" particles that are infiltrated during the brazing
cycle by a lower melting temperature braze matrix. This technique
can braze across wide gaps ( ~ 0.02" to 0.1" ) where
conventional brazes are normally 0.002 to 0.006" thick. Proper
combination of the filler and braze alloy matrix provides composite
metallurgical bonds, thus controlling CTE mismatch through the
composite properties of the WideGap braze. These joints
lower thermal stresses in the joint area and enable dissimilar
materials to brazed at high temperatures.
Active elements can be added to the WideGap powder preforms,
permitting the joining of ceramic, carbon-based composites, and
graphite materials to themselves and/or to other metals. High
strength, high temperature joints result from tailored metallurgical
interfaces and the joints' composite properties that lower the
residual stresses in the joint materials.
MRi offers WideGap preforms as flexible mats (polymer
filled powder mixtures) that can be tailored to the composition
of joint materials. The mats are placed in the joints and are
processed in vacuum or hydrogen brazing furnaces.
This table offers a comparison of dissimilar joining methods and
MRi's product characteristics
Comparison of Joining
Technologies for Dissimilar Materials
||Some fixturing, no masking &
no post cleaning
|| Requires vacuum furnace
||Requires vacuum furnace and pre-metallizing
|| Open air application
and low temperature
& batch processing
& batch processing
||Joint is strong,
ductile. Small distributed intermetallics. Low joining temperatures
yield good thermal mismatch
||High brazing temp.
with thick graded perform minimizes cracking
||High brazing temp.
may lead to cracking of dissimilar & ceramic materials
||The lower temp.
flexible nature of the braze material make it more forgiving
to different Coefficients of Expansion.
||Good, no flux
|| Multi-step, requires
pre-coating. Excellent bonds
strengths to 10,000 psi
bonds to all mat'ls
|strengths over 50,000 psi
fills wide gaps
composites lower CTE mismatches
can include active braze elements
strengths over 50,000psi
high CTE mismatches
|typical solder strengths
(3,500 psi) ·
limited to certain materials
Define your needs and refer to MRi's product and technology portfolio
in the later parts of this Website. If you don't immediately identify
a product or technology for your dissimilar joining problem, please
contact us, we have range a broad of joining solutions.
Thank you for browsing
we look forward to being your source
of joining solutions. If you have any questions regarding the
technology or the products described in this Website and invite
you to contact us