The Magic of Induction Brazing

Figure 1: A range of induction coils

Induction brazing uses the same basic principles as an induction hob you may find in your kitchen or employed in heat-treating steel. The article which follows will look into the benefits and limitations of induction brazing as well as examine the principles and best practices to ensure a sound braze joint.

Principle of Induction Heating

Induction heating works on the theory that eddy currents are generated within electrically conductive materials by applying electromagnetic induction. These eddy currents coupled with the electrical resistance within the material gives rise to Joule heating.  The induction heater consists of an electromagnet through which a high frequency alternating current is passed.  The frequency of the AC current is just one of the important factors in determining the degree of heating and the depth of penetration of the heat.

What Materials Can Be Brazed?

Induction heating works on electrically conductive materials, usually this limits the process to metals but there are ways of circumventing these limitations.  The ease with which a material can be heated by induction depends on the electrical resistivity and their magnetism.  Ferrous metals are therefore much easier to heat than aluminium.  Likewise carbon steels will heat more readily than stainless steels.  Ceramics are not usually brazed by induction but if they are being brazed to a ferrous metal then heat transfer via conduction into the ceramic may allow a user to tackle this challenge.

The Equipment

Induction brazing equipment consists of a generator which is rated by power and frequency and an induction coil which is usually water-cooled.  There will be fixturing to hold parts and depending on the application, a facility to shroud the components in an inert gas to prevent oxidation. If the process is automated the physical size can be significant although smaller table top versions are also available on the market.

Induction Brazing Parameters

  • Coil design - The design of the coil is critical to ensure even and efficient heating of the components to be brazed.  The coil is made from copper tubing through which cooling water is fed.  It is designed to selectively heat only the part of the component that needs to be brazed.  They may be as simple as a single round section copper coil to much more complex shapes as shown in Figure 1.  They should be individually designed for each braze joint in order to maximize the efficiency.  The inverse square law governs the energy imparted to the joint. Hence the closer the coil can be to the joint the more efficient the energy transfer will be.
  • Frequency - This is dependent on the geometry of the braze joint and the properties of the material.  The depth of penetration of the heat will be greater with a lower frequency but rest assured if your joint section is very thick as conduction will also play a part in bringing the braze joint up to temperature. 
  • Time – The speed at which the components can become red hot remains one of the most fascinating aspects of induction brazing.  Cycle times of as little as 10 seconds are normal!  Larger parts will take longer to reach the brazing temperature as will materials with lower electrical resistivity and magnetism. 
  • Fluxes – Most induction brazing occurs in air.  A suitable flux is required to prevent oxidation and to allow the braze alloy to flow.
  • Atmosphere – An alternative to brazing with flux is to braze in an inert atmosphere.  The most suitable atmosphere to use will depend to a certain degree on the materials being brazed and the braze alloy being employed.  Refer to a previous technical article for full details on atmospheres for induction brazing.

What else needs consideration?

Temperature control

Optical pyrometers are the most effective way to control temperature during induction brazing.  A feedback control loop integrated with the induction generator allows instant and accurate control.  If a shroud is to be used around the component to permit the use of an inert or oxidizing atmosphere then the pyrometer must have visibility of the braze joint.  This can be facilitated by using quartz glass which allows the pyrometer to read the temperature accurately.

Fixturing

Any fixturing within 50mm of the coil could be affected by the electromagnetic current and so should be made from non-magnetic materials such as aluminium or austenitic stainless steel.  Ceramics are often used as bases for supporting components.

Automating the process

Being such a repeatable process, induction brazing lends itself to automation.  Carousels, pick and place, turntables can all be incorporated to automate or semi-automate the process.

Pros and Cons of Induction Brazing

Limitations:

  • Not all materials are suitable for induction brazing
  • Low volume process compared to furnace brazing
  • Not always suited to brazing multiple joints together
 

Benefits:

  • High speed process
  • Low cost
  • Localized heating only where it is needed
  • No fumes hence safer than flame brazing
  • Good accuracy and repeatability
  • Flexibility – interchangeable coils

Typical Applications

  • Tungsten Carbide to Tungsten Carbide for Tool Manufacture
  • Joining pipes for refrigeration and air conditioning
  • Brazing PCD to Carbide for cutting tools

Case Study:  Joining tungsten carbide to tungsten carbide using a Palladium based braze alloy (VBC Alloy 4059) with a liquidus temperature of 1013°C. 

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 pre-assembled 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 induction 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.

Case study:  Joining copper to carbon steel – dissimilar materials.  One of the problems engineers will come across frequently is the difference in coefficients of thermal expansion when joining different materials. 

In this case, a copper pipe of diameter 50mm with a CTE of ~16.5 ppm/°C to be joined to a carbon steel pipe (CTE ~11 ppm/°C) of similar diameter. The copper pipe is expanded allowing the steel pipe to slide in with a clearance of 0.1mm. This gap allows the braze alloy to flow into the joint.  The filler metal to be used in this case is silver-based VBC Alloy 4102 (Bag-7) which has a brazing temperature of 675°C -760°C.  It is used in wire formed into a ring which is placed at the joint.   

A brazing flux (complying to EN1045 FH10) is used to prevent oxidation during the heating cycle to allow the molten alloy to flow into the joint. This low temperature brazing alloy is a suitable alternative to cadmium containing alloys. Due to the ductile nature of silver/copper based alloys it will be capable of accommodating the difference in thermal expansion of the two parent metals. 

The assembly is placed into a suitably sized induction coil which is powered by a 30KW generator.  The frequency is set to 100 kHz.  The cycle time is 60 seconds during which the components heat, the braze alloys melts and flows into the joint and is then allowed to cool.

After brazing the corrosive flux residue needs to be removed with hot water and mechanical brushing.

In summary, Induction Brazing should be considered as an attractive alternative to flame or furnace brazing due to the speed and relative low cost of the process.  Please contact tech@vbcgroup.com with any queries relating to Induction Brazing.  See http://www.vbcgroup.com/brazing-materials  for our full list of braze alloys.