Inert and Reducing Gases used in Induction Brazing
You may instinctively associate VBC with Vacuum Brazing technologies but did you know we also technically support Induction brazing?
Induction brazing is an efficient and clean method of joining metallic and ceramic components which can be carried out in air but there are further benefits to be obtained when brazing in inert or reducing atmospheres.
The benefits of induction brazing in a suitable atmosphere over vacuum brazing include significantly reduced capital investment, much faster heating cycles, localised heating only which reduces the risk of distortion of parts and removing the need for fluxes.
Parts that are brazed using this method avoid risks of oxidation if the correct gas and set up is used. Fixturing plays an important part of the set up. It must allow the gas to completely surround the brazed joint throughout the heating cycle to prevent oxidation. Certain alloys, most notably the active braze alloys such as VBC Alloy 4033, a Silver/Indium based alloy used in brazing ceramics for example will not wet the surface if any oxygen is present. The gas should flow over the part to remove any moisture and oxygen away from the joint.
It is often desirable to be able to see the braze joint up close during the braze cycle. This is not possible in furnace brazing. With induction brazing quartz glass can be designed into the fixtures in the fixtures to allow the operator to see the braze flow. Quartz glass is advisable especially if an optical pyrometer is to be used as the beam can be aimed at the part through the glass without any loss in accuracy of the temperature reading. The induction coil can be inside or outside the glass though keeping the distance between the coil and the work-piece to a minimum is preferred for maximum energy transfer.
There are two types of gases used in brazing, inert gases that inhibit oxidation and reducing gases that react with any undesirable films on the parts being brazed or the braze alloy and thereby remove them from the braze joint.
Oxidation is the formation of oxides on the surface of the metal when it reacts with the oxygen present in air. These oxides may prevent the braze alloy from wetting the surface. This reaction is enhanced when the metal is heated.
Atmospheres which inhibit oxidation are inert and their purpose is to purge the braze zone of any oxygen or other gases which may react with the metals being joined. These atmospheres include argon and helium and mixes of either.
Nitrogen is sometimes used as a shielding gas and can be regarded as inert for most cases but it is not suitable for brazing titanium with which it forms titanium nitrides. Likewise titanium will react with hydrogen to form titanium hydrides. For brazing titanium and its alloys argon or helium should be used with a Silver/copper based active braze alloy such as VBC Alloy 4011 for example.
Reducing atmospheres are chemically active and will react with surface contaminants on the parts or the braze alloy and will remove them from the braze joint. Hydrogen is commonly used as a reducing gas and will reduce most metal oxides.
Occasionally a combined gas may be used, for example Nitrogen/Hydrogen or Hydrogen/Argon (Hygon). Gases that are commonly used as reducing or deoxidising agents also include mixtures of nitrogen & hydrogen, nitrogen/methanol mixtures and argon or argon/hydrogen mixes. The choice of gas will be determined by the materials being brazed, the required quality and the cost implications.
The dew point (or moisture content) of the gas is an important consideration as this will have a significant influence on which oxides will be reduced. Ideally the dew point should be at a minimum as any water vapour will tend to cause oxidation. One exception to this is when brazing carbon steels where some degree of moisture is required as the braze alloy can flow excessively otherwise.
A further benefit of induction brazing is that the rapid cool down cycle reduces oxidation. In addition the brazed parts are usually cleaner and brighter after brazing in a reducing/deoxidising atmosphere.
Case Study: Joining tungsten carbide to tungsten carbide using a Palladium based braze alloy (VBC Alloy 4059) with a liquidus temperature of 1013⁰C. The surfaces to be joined are first carefully cleaned in an ultrasonic bath containing acetone. The braze media is a foil preform sized to ensure complete coverage of the joint once molten and is likewise cleaned. The parts are preassembled and placed into a fixture that applies the correct pressure to the components. The fixture is enclosed in quartz glass and allows a Nitrogen/Hydrogen atmosphere to flow over the parts during the induction heating cycle. The coil is designed to ensure even heating throughout the joint. The gas flows over the assembly prior to the initiation of the heating cycle and remains on until the assembly has cooled to a safe temperature where oxidation will not occur. After cooling the braze foil can be seen to have fully wet the surfaces forming a perfect bond and the external surface of the tungsten carbide has been cleaned due to the reducing effect of the hydrogen.
To summarize induction brazing in reducing and deoxidising atmospheres can be used as an efficient, clean and cost effective means of joining metal and ceramic components. The atmospheres and alloys suggested here are examples only and the choice of alloy for your application will depend on several factors, including but not exclusively the base materials being joined, the geometry of the part and economic considerations. At VBC Group we are available to advise you on all your brazing needs. Please contact us on firstname.lastname@example.org for any questions you may have on induction heating atmospheres or any other brazing topic.