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Brazing-nickel:how to avoid pitfalls.SOLUTIONS with Effective, Practical Advice
Brazing-nickel needs a reminder of conditions that may hinder proper processing and also of elements that may embrittle the joint. If due attention is paid to avoid impact from negative factors, nickel base alloy are brazed without excessive problems. Nickel and nickel alloys used for their corrosion resistance are brazed with high silver base brazing alloys. The following: sulfur or low melting metals like zinc, lead, bismuth and antimony should be absent. A Table of some silver base filler metals used for Brazing-nickel can be found in our page on Brazing Stainless Steel. Brazing nickel or alloys selected for their resistance to elevated temperatures can be done with nickel base filler metals. A Table showing several such filler metals can be found in the same page mentioned above. These filler alloys can be applied as preforms (if the filler is available as foil or wire) or as powder or slurry. An Article on Brazing with Nickel AWS BNi-2/AMS 4777 was published (3) in Issue 105 of Practical Welding Letter for May 2012. An Article on Nanoparticles Doping of a MoNi Brazing Filler was published (4) in Issue 162 of Practical Welding Letter for February 2017.
Filler metals that contain in their composition temperature depressants (like boron, silicon and chromium) may gain a higher remelt temperature if the brazing cycle can be extended enough in time, about one hour (without adverse effects). This will permit the diffusion of depressants away from the joint. If this purpose is achieved, heating again the brazement in service at brazing temperature will not remelt the filler metal alloy, by now deprived of temperature depressants. If the resistance to elevated temperatures is important and if the stresses in the Brazing-nickel joints are not excessive, other filler metal alloys can also be used that were successfully employed in gas turbine engines and in rocket engines. Gold nickel filler metal alloys are an example, like AMS 4787 with 82% Au and 18% Ni, (AWS BAu-4) melting at 949 °C (1740 °F). This material, although providing joints weaker than those made with nichel-chromium-boron-silicon brazing alloys, is resistant to stress corrosion and oxidation, can fill relatively large gaps, is very ductile and does not erode base metal. Also palladium bearing brazing alloys have been developed and used in electronic devices and also in high temperature applications. A few of these compositions are listed in a Table in the following commercial publication. See Palladium Alloys. Several non standard filler alloys of various compositions used also for Brazing-nickel are listed in the following commercial publication. See Praxair Braze Filler Metal Products (4 pages).
Nickel materials or parts stressed or cold worked may be subject to stress cracking when coming in contact with molten filler metal. To avoid such cracking, stress relieving should be performed either before or during Brazing-nickel. In investigating cracked nickel brazed joints one should be advised that utmost care should used while cutting the brazements for preparing metallographic specimens. It was reported that good joints could crack under harsh cutting conditions. Therefore it is important to question the preparation technique before jumping to erroneous conclusions relative to cracking of Brazing-nickel joints. The recommended clearances in the joints are quite normal, around those used for brazing stainless steel, namely between 0.0127 and 0.127 mm (0.0005" and 0.005"). Fluxes are needed, and their residuals should be thoroughly removed after brazing to avoid corrosion, except if Brazing-nickel is performed under highly reducing protective atmosphere. Vacuum furnaces can also be used. With a proper flux even oxyacetylene torch brazing can be done. An Article on Measuring Heating Rates in Vacuum Furnaces was published (3) in Issue 140 of Practical Welding Letter for April 2015. Precipitation hardenable nickel base superalloys containing readily oxidized aluminum and/or titanium (interfering with wetting) should be nickel plated before brazing. Precipitation treatment can preferably be combined with the Brazing nickel cycle.
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