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Joining 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 joining.


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.

Conventional Brazing
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.

Active Brazing
Figure 2 shows a typical active brazing/soldering process.

Dissimilar materials joining requires consideration of:

  • compositional compatibility for joining material on both interfaces
  • differences in coefficient of thermal expansion (CTE).
  • 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 Technologies

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 elements
  • 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®

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 by:

  • Lowering CTE mismatch with low joining temperatures, < 450°C.
  • 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:


WideGap™ Joining
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
 
MRi Technologies
Conventional Joints
Features
S-Bond®
(low temperatures)
WideGap™
(high temperatures)
Active Braze
(high temperatures)
Pre-metallizing (solder or braze)
Application Some fixturing, no masking & no post cleaning Requires vacuum furnace Requires vacuum furnace and pre-metallizing
Slow, multi-step
Processing Open air application and low temperature Vacuum equipment & batch processing Vacuum equipment & batch processing Metallizing & vacuum equipment
Mechanical 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.
Environmental Good, no flux Good, enclosed Good, enclosed Good, enclosed
Processing Requires mechanical activation Vacuum furnaces, Pre-placement needed. Vacuum, pre-placement needed. Multi-step, requires pre-coating. Excellent bonds
Characteristics

•strengths to 10,000 psi
•electrically conductive
•thermally conductive
•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
•narrow gaps
•high CTE mismatches

•typical solder strengths (3,500 psi) ·
•electrically conductive
•thermally conductive
•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…

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Home | About Materials Resources International (MRi) | MRi Technologies for Joining Dissimilar Materials | Product Applications
S-Bond® : Fluxless Low Temperature Joining

WideGap™ Joining : Powder Preform Technology for Joining Dissimilar Materials

BrazeCoatŪ : Metallurgically Bonded Carbide Coatings/Claddings

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Materials Resources International
811 W. Fifth Street
Lansdale, PA 19446 USA
Tel: (215) 631-7111 Fax: (215) 631-7115

Web: www.MaterialsResources.com
e-mail: rsmith@materialsresources.com