Induction brazing is an excellent way to quickly heat up a localized area of a large assembly in order to permanently join them together. Let’s take a brief look at the induction-brazing process to see what it is and how it can be effectively used by brazing shops today to meet some of their production needs. We’ll present this topic in two different parts (the second part will appear in the August issue).
This article does not contain any complex language or mathematical equations because I want to make the induction-brazing (IB) process as simple and easy to understand as possible, which will encourage people to use it more. For a more in-depth engineering study of the principles and theory of induction heating, the reader is referred to other technical books on the subject. This article will give you a good, basic understanding of IB and how to apply it in your brazing shops.
Figure 1 shows a typical example of an induction heating setup for brazing, in which a small region of a copper-tube-to-fitting assembly is being heated inside a copper coil. It should be noted that the copper coils are in the form of tubing, through which water is flowing to keep the coils cool.
Reasons for Using Induction Brazing (IB)
The following are reasons for using induction heating for brazing.
- The induction coils will heat only that portion of an assembly that needs to be heated for brazing. This potentially saves a lot of money compared to the time and cost of heating the entire part up to brazing temperature, which would be the case if the part were to be heated in a furnace.
- Perhaps there are portions of an assembly that are not allowed to be heated because heating such a component would damage them. By using induction heating, only the part that needs to be brazed is heated. The rest of the part can be kept at room temperature (or cooler).
- Perhaps the part is just too big to fit into a furnace, and methods must be found in which only a local area will be effectively heated for brazing.
Basic Principles of Induction Heating
Simply put, induction heating uses a rapidly reversing electrical current flowing through the inductor coil (such as through the copper coil shown in Fig. 1), which causes (induces) large amounts of heat to be generated inside the vertical metal tube that you see inside the copper coil loops. You might compare this internal heating of a metal bar to the microwave cooking of food, in which heat is generated inside the food by microwaves to the heating of food in a standard convection oven, where heat is generated outside the food and then conducted into the food by the heated air on the outside of the food.
In a similar fashion, IB causes metal parts positioned inside the induction coils to “heat up from within.” Conventionally, parts are heated by either external radiation of heat onto the surface (such as in a vacuum brazing furnace) or via conduction of heat to the part through a heated gas in a sealed container (controlled-atmosphere brazing, or CAB, furnace).
So, with that very brief intro, let’s take a closer look at how IB works.
As many people know, the so-called “right-hand rule” is a guide that helps us to understand the relationship between the flow of electricity through a conductor and the related electromagnetic field that is generated around that conductor because of that electrical flow. As shown in Figure 2, let your right thumb represent the direction of flow of the electricity through the conductive metal. The other four fingers of your right hand represent the direction of the electromagnetic field flow generated around that conductor.
Copper is a metal that has very high thermal and electrical conductivity. For this reason, induction coils are made from copper.
Let’s see how the right-hand rule is used in IB. Please look at the drawing in Figure 3, which shows the cross section of a single induction coil with four loops. The coil is made from hollow copper tubing, through which water flows to keep the tubing cool. When the induction equipment is powered up for brazing, electrical current is sent to the induction coil, which, in turn, generates an electromagnetic field around the coil loops.
For the electromagnetic field shown in Figure 3, the electricity is flowing toward us on the left side of each coil loop in the illustration. As it continues to move around each loop, it would then be heading away from us on the right side of each loop in that diagram. Notice the heavy concentration of the electromagnetic flux field in the center of the coil that results from this. You can see this by moving your right hand (with your thumb pointing in the direction of the current flow) around each loop as it curves from the left side to the right side.
How does induction heating really work?
In actual practice, if a steel bar were placed in the center of the coil shown in Fig. 1, the electromagnetic field around the copper tubing will cause (induce) a reaction in the steel bar. Using a very simplistic approach to this (in order to help the reader more easily understand the basic principles of induction heating/brazing), it’s as if the electromagnetic field around the copper tubing forces the magnetic domains in the steel bar to try to line up with (conform to the polarity of) the lines of the electromagnetic field in that region of the bar.
Now, what will happen when the electrical current flowing through the copper coils is suddenly reversed? The magnetic domains in the steel, so to speak, must now reorient themselves in the opposite direction in order to be properly aligned with the polarity of that reversed electromagnetic field. Once that occurs, if the electrical current is then reversed again, the magnetic domains in the steel will again have to try to reorient themselves in the direction of the changed field, etc. As this happens, friction is being developed in the steel. A way to picture this is to slowly rub the palms of your hands together, back and forth. As you increase the frequency of the rubbing back and forth, notice how your palms get warmer as you generate increased friction.
In induction heating/brazing, the direct-current flow of electricity is alternated at increased frequency to generate more and more friction in the metal. This so-called “alternating direct current” doesn’t just reverse direction a few times per second, it does it many thousands of time per second, which can generate huge amounts of heat inside the steel bar due to the induced friction.
Obviously, the ability of a metal to resist or conduct heat (caused by this friction) is an important consideration in deciding if induction heating/brazing is viable for your specific application needs. The poorer the metal’s conduction, the greater the friction that can be generated in that metal by inductive heating. Thus, steel will heat up much faster than copper or aluminum, since steel is a very poor conductor of thermal energy and both copper and aluminum are excellent conductors of heat. It would take huge amounts of energy to be able to create enough friction in copper and aluminum parts to be able to braze them via induction heating.
Where is the brazing filler metal (BFM) placed for good IB?
Because of the electrical field within and around the induction coil, it is often convenient (although not required) to pre-place the BFM in or next to the joint prior to heating it (Fig. 4) rather than try to hand-feed the BFM while the IB operation is taking place. Obviously, an important advantage of using pre-placed preforms is that exactly the same amount of BFM is applied to each of the joints. Consequently, this also removes the variability in the amount of BFM that is applied when different workers are hand-feeding the BFM into the joint using a brazing rod or wire.
Flux or Inert Atmosphere
It will be necessary to use brazing flux or an inert atmosphere to keep the surfaces to be brazed from oxidizing during the IB operations. IB is typically conducted in open air. Because of this, a brazing flux would need to be used to protect the metal surfaces from oxidation during heating (this includes the filler metal). The exception to this is when brazing pure copper to pure copper using a phosphorus-copper BFM, since the phosphorus in the BFM will act as a flux to absorb oxygen and keep the surfaces clean for brazing. But if either of the two metal components being brazed is not pure copper, it will be absolutely necessary to use a brazing flux to keep the surfaces clean enough to be brazed, or, alternatively, the components could be brazed in an inert (oxygen-free) atmosphere. IB can also be done in vacuum chambers, and equipment is readily available for such operations.
Induction brazing (IB) is a wonderful tool that many shops may wish to use for certain parts that need to be brazed quickly, are too large to fit inside a brazing furnace or perhaps have areas that cannot tolerate high heat since damage might result to those areas if heated up to brazing temperature. IB is safe, fast and very reliable when proper procedures are followed.
In my next article, I’ll take a closer look at the six variables that need to be followed in order to effectively IB. These are coil spacing, coupling distance, frequency setting, induction-machine power, coil design (which is the job of your induction equipment supplier) and optional use of flux concentrators. So hold onto this first part of the article until then.