WideGap™ Joining Solutions

Problem: Ceramic/Composite High Temperature Joining

Ceramics (Al₂O₃, Si₃N₄, SiC, and/or ZrO₂) in addition to ceramic composites and refractories such as carbon:carbon and silicon:silicon carbide are seeing increased use for aerospace, aero-engines and industrial heating equipment. However, many times they must be joined to a metallic component for toughness or for functional reasons. The only way to join these materials, outside of mechanical means, is by "active" brazing. Such active brazes contain high activity elemental additions such as Ti, Hf, and/or Zr to react with the very stable ceramic surfaces. The problem is that even though conventional active brazes work, their joints are very narrow in order to prevent brittle phase formation. Such narrow joints can, however, lead to severe localized thermal stresses (from brazing or service) resulting from large thermal expansion mismatches. These mismatches stem from the large differences in CTE between ceramic/refractory composite materials and metals. Therefore, reliable braze techniques which can operate near the service temperatures of the ceramics, (1000 to 2000⁰C), which accommodate interfacial thermal stresses and do not form detrimental intermetallic phases are needed.

Solution: WideGap^™

MRi’s engineered powder preform braze technology utilizes mixes of compatible, active braze filler metals and refractory, low expansion ceramic and/or metallic parent material powders to withstand service temperatures from 650 to 2000⁰C. The non-melting "parent material" particles fill the gaps, provide capillary filling, reduce the formation of brittle intermetallics (typical in wide brazes), and their design produce CTE matched composite braze joints.

Specific WideGap^™ braze preforms, materials systems and techniques been developed and others under development for industrial/commercial applications while new applications are continually being sought. Preforms include i)pastes, ii)tapes, iii) pressed/sintered shapes, or iv) hot isostatically pressed (HIP’d) shapes, dependent on the required final density and costs. During joining, the preforms are placed in the metal/ceramic joints and parts are heated to slightly above the braze alloy matrices’ melting temperatures. Preforms generally produce braze gaps from 0.020 to 0.080", depending on the differing CTE’s of the materials being joined. However, WideGap™ formulations could be tailored for larger gaps if larger gradations are desired.

 

Problem: Joining Copper Conductors or Electrical Contacts

Copper conductors and/or other metal or graphite contacts are used in a wide variety of applications including buss bars used to carry high currents and windings of large motors generators, and copper conductors used to conduct electricity for a range of power generating SCR devices. Normal braze joining of such components requires close tolerance machining to allow a 0.002 to 0.005" gap between the components. The narrow gaps are used to drive capillary action and to prevent brittle phases from forming, as they do in wider braze gaps. Such precision gaps increase costs and lowers the reliability of subsequent brazed copper joints. Also for dissimilar material contacts and/or differential (current) heating, CTE driven mismatch stresses can cause joint failures. WideGap™ brazes can alleviate such mismatch strains. Thus, to lower costs, increase manufacturability, or increase joint performance, brazing processes that permit wider gaps to be brazed are needed. In related applications, more robust, high quality brazing techniques are also needed for copper joining in the field for the installation of heavy copper conductors.

Solution: WideGap^™

MRi’s WideGap™ brazing is a process which relies on powder metallurgy techniques using parent metal powders with conventional brazing powders to yield a preform which can be placed in joints for subsequent brazing. WideGap brazing of copper conductors utilizes OHFC copper powder mixed in preforms with either Cu-Ag or Cu-Sn alloy powders. WideGap braze preforms are placed in the copper conductor or contact joints and the parts are subjected to brazing temperatures, slightly above the braze alloy melting temperatures. The braze alloy powders (typically 20 to 40 volume %) melt and then diffuse into the still solid OFHC copper particles which are contained in the WideGap preform. The braze alloy matrices in the WideGap preforms, while molten, infiltrates and bonds the copper conductor or electrical contact interfaces to the preform powders via "liquid phase" sintering producing a solid, well bonded joint, typically from 0.020" to 0.080" thick. WideGap braze maintains its quality by braze joining the millions of narrow gaps that form between the preform’s powder particles.

For more real world applications of WideGap joining contact by e-mail: solution@mri-bluebell.com