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By Dan Kay
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| Fig. 1 -- Induction coil used to heat a metal post. Photo courtesy of GH Induction Atmospheres. |
A number of companies who are currently using vacuum-furnaces for many of their brazing processes are also using induction-brazing equipment to join some of their other production parts. 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.
This article is written without a lot of complex language in order to make this process as simple and easy to understand as possible, and to therefore encourage people to use it more. For a deeper, more thorough engineering-study of the principles and theory of induction heating, the reader is referred to other technical books and articles on the subject. This current article will give you a good, basic understanding of induction brazing, and how to apply it in your brazing shops.
Figure 1 shows a typical example of an induction-heating setup, in which a small region of the metal bar is being heated inside a tubular copper coil (water is flowing through the copper tubing to keep it cool).
Some of the reasons why people use induction-brazing processes are:
1. They do not want (or need) to heat the entire component to brazing temperature, but only desire to braze one small location on that component. Because furnace brazing would heat the entire component, induction brazing becomes an excellent option to heat only that part that needs to be heated for brazing, thus potentially saving a lot of money compared to the time and cost of heating the entire part in a furnace.
2. They cannot heat other portions of the component because high heat would damage those other portions, etc.
3. 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.
How does induction heating actually work?
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| Fig. 2 – Right Hand Rule showing relationship between current flow through a conductor and the generated electromagnetic field around that conductor. |
Back to the copper coil shown in Fig. 1. For induction heating purposes, there is a rapidly reversing electrical current flowing through the copper coil which causes (induces) large amounts of heat to be generated inside the vertical metal bar that you see inside the copper coil loops. The reader 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 as compared 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.
Right-Hand Rule
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 electro-magnetic field that is generated around that conductor because of that electrical flow. As shown in Fig. 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.
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| Fig. 3 -- Diagram of electromagnetic field around copper-coils, according to the “right-hand rule”. |
Let’s see how the “right-hand rule” is used in induction brazing. Please look at the drawing in Figure 3, which shows the cross-section of a single induction coil with four (4) 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 the Fig. 3 drawing, the electricity is flowing toward us on the left side of each coil loop in the illustration, and, as it continues to move around each loop, 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. Try to picture 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, and you can easily see this.
How does induction heating really work?
In actual practice, if a steel bar were placed in the center of the coil, as 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 electro-magnetic 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 re-orient themselves in the opposite direction in order to be properly aligned with the polarity of that reversed electro-magnetic field. And then, once that occurs, if the electrical current is then reversed again the magnetic domains in the steel will again have to try to re-orient 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 hand together, back and forth. Then, as you increase the frequency of the rubbing back and forth, notice how your palms get warmer as you generate increased friction between the palms.
In a similar fashion, 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, but instead 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, whereas 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.
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| Fig. 4 -- coil spacing is important for uniform heating. If the coil loops are placed too far apart (as shown in the drawing on the left), it could lead to a “barber-pole” heating pattern on the part being brazed. |
Induction Brazing Variables
So, assuming that inductive heating is a viable brazing option for you, what principles of inductive heating are necessary to consider if we want to use it successfully for brazing? Here’s six (6) important items you must consider:
1. Coil spacing. This refers to the vertical distance between each loop of the induction coil (such as the coil shown in Fig. 1). If the vertical distance between each loop is quite small, as shown in the drawing on the right side of Figure 4, then the steel bar lowered down inside the coils will heat uniformly. However, if the vertical spacing between each loop of the coil were large, as shown in the drawing on the left side of Fig. 4, it would be possible to create a “barber-pole” type of heating pattern on that steel bar. So, if you want uniform, even heating, keep the coil-spacing distance small.
2. Coupling distance. This is the width of the gap between the outside diameter (OD) of the steel bar shown in Fig. 1 and the inside-diameter (ID) of the copper coil itself. This is more clearly illustrated in Fig. 5.
Notice the heating pattern (dark semi-circular patterns in the bar) shown in Fig. 5, as well as the horizontal line in the middle of the bar representing the joint that is to be brazed. When the coupling-distance is small, the heating pattern is concentrated on the surface of the bar, and doesn’t penetrate too deeply into the bar itself. Thus, the outside edges of the braze-joint get very hot, but the inside of the joint remains cool. However, as the coupling-distance gets wider and wider (looser and looser), the inductive-heating of the bar becomes less efficient, and the surface of the bar doesn’t get quickly overheated, allowing heat to sink deeper into the braze-joint when the induction cycle is held a bit longer at brazing temperature. This is helpful for brazing, since we want the heat to steadily go deeper into the bar to adequately heat the entire braze-joint up to brazing temp, rather than heating just the outside surface of the bar.
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| Fig. 5 – Coupling distance is the gap-distance between the OD of the part being brazed, and the ID of the induction-coil itself. |
VERY IMPORTANT – Keep the coupling-distance as even as possible around the braze-joint! It is ESSENTIAL that the coupling distance be carefully controlled during induction heating/brazing, since it will strongly affect the heating of the metal placed inside the induction coil. If the part being brazed is held too close to one side of the coil (tight coupling distance) and thus quite far from the other side of the coil (loose coupling distance), the heating of the metal part will not be uniform, and can easily and quickly result in overheating of the metal surfaces that have the tight coupling distance, leaving the surface with the loose coupling distance cool by comparison. This could easily result in uneven melting of any applied brazing filler metal (BFM), as well as uneven flow of the BFM through the joint.
RECOMMENDATION: It is probably best to use a semi-automated setup when induction brazing, so that the part to be brazed can be mechanically raised and lowered into and out of the coil, thus keeping the part accurately centered in the induction coil, with as even a coupling distance around the part as possible.
3. Frequency setting. The frequency setting of the induction machine, simply put, tells you how frequently the direction of the electrical flow is reversed. Whereas the alternating current (AC current) we use in our homes in the US may operate at a 60-cycle frequency, that of an induction brazing machine is much higher, on the order of tens of thousands of times per second, or higher (hundreds of thousands per second, and even millions of times per second).
It is interesting to note that the higher the frequency, the more intense will be the surface heating rate. This may, or may not, be good for brazing, depending on the thermal conductivity of the metal being joined. The frequency level chosen is best determined by experimentation, so as to optimize the frequency range best suited for your particular design and base metals being used.
4. Power of induction-machine.The larger the part that is to be brazed, the greater should be the available power in the induction machine in order to be able to heat that greater mass in a reasonably short period of time. For small parts, a one-kilowatt (1-kw) or a 5-kw desk-top machine may be quite adequate, allowing parts to come up to temperature in a matter of seconds. Typical induction brazing cycles might range from about 15-seconds all the way up to a few minutes. As you can see, the induction-brazing cycle time is much shorter than would be the case if the same parts were being furnace-brazed.
For induction-brazing of large parts, large power units that sit on the floor may be needed. Such units may have power ratings from about 10-kw up to 50-kw or higher in order to be able to supply sufficient electrical power for large brazing needs.
5. Coil Design.Designing an appropriate coil for the part you wish to braze is not a simple matter. In my opinion, you should always have the induction-machine manufacturer (from whom you purchased the equipment) design and build the induction-brazing coils for you. That is their job! Do NOT merely buy the equipment from the manufacturer and then try to make your own induction-brazing coil. That is truly being “penny wise and pound foolish” as the British phrase goes. Study some of the coil-designs shown in Fig. 6.
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| Fig. 6 -- Some induction coil designs used for brazing. Some coils are suitable for one part to be brazed at a time, whereas other coil designs allow for high-speed brazing of parts moving continuously through the coils. (Courtesy of Lepel Corporation). |
Fig. 6 illustrates just a small sampling of the many possible coil designs available to the induction-brazer today. Note the complex design of the induction coil at the bottom left of the diagram, used for brazing a bicycle frame. Next to that on the bottom row is shown a special induction coil used to heat-treat the inside surfaces of a hinge assembly.
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| Fig. 7 – One (1) induction coil, with seven loops, to braze joint between two metals with different heat-conductivities. (Drawing courtesy of Lepel Corporation). |
Fig. 7 shows a seven-loop induction coil design used to braze a low-conductivity metal shaft into a high-conductivity copper fitting containing a silver-based BFM ring. Note that the poorer conductivity metal shaft at the top uses a larger coupling distance and only two coil loops, whereas the more massive lower fitting made from copper (high conductivity) required closer coupling-distance (and more coil loops) to be able to adequately heat the copper fitting in order to melt the BFM ring, and then allow the molten BFM to flow up by capillary action along the surface of the shaft until a small fillet of BFM is visible on the outside of the joint.
One induction-equipment manufacturer I visited showed me one of their research-lab’s coil storage cabinet which contained several hundred different coils, every one different in design from each of the others. So, unless you need just a very simple circular coil, I strongly recommend that you work with your induction-equipment supplier to have them design any type of complex coil that you might need.
REMEMBER -- you might, as an example, need a coil which can apply lots of heat to one part of a braze-joint or metal, with a lot less heat being applied simultaneously to another portion of the joint, and this can all be achieved by varying the coupling distance and coil-loop spacing of a complex induction coil so that it correctly fits around different parts of the joint area to be brazed for proper heating.
6. Flux concentrators. Electromagnetic fields around induction coils can extend outwards a great distance from the coils, thus potentially causing concern about unneccesarily heating items positioned near the brazing-coils that you don’t want to heat. A “flux concentrator” is a putty-like substance, sometimes containing a lot of iron powder for example, which can be molded around the outside of some of the induction coils, as shown in Fig. 8, in order to pull-in, i.e., focus the electromagnetic field so that it is “short-circuited” through the putty, thus keeping the electromagnetic field away from places that it shouldn’t go.
CONCLUSION Induction brazing is a wonderful tool that many shops may wish to use for brazing certain parts that need to be brazed quickly, or perhaps are too large to fit inside a brazing furnace, or perhaps have areas on them that cannot tolerate high heat since damage might result to those areas if heated up to brazing temp.
The principles of induction brazing are not that hard to understand, and in quick summary might be listed as:
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| Fig. 8 – Flux concentrator allows the electromagnetic field to be “short-circuited” to keep it away from sensitive places, items, or people. |
1. Coil design is extremely important to induction-brazing, and is rightfully the responsibility of the induction-equipment manufacturer.
2. Coupling-distance should be uniform around the braze-joint.
3. Be sure the induction-equipment has enough power to handle the mass of the parts being brazed.
4. Use an induction-frequency that will allow you to heat the entire braze-joint area effectively and efficiently and not just the outside surface of the braze-joint.
EXAMPLE 1: If I wanted to harden the surface of a steel automotive crankshaft by induction heat-treating, what coupling distance and induction-frequency should I use? Generally speaking, you’d probably opt for close coupling distance (to more intensely heat only the outside surface of the part), as well as high frequency (since once again that would tend to keep the heating-intensity on the outside surface of the part).
EXAMPLE 2: If I wanted to braze the ends of two round bars together (such as shown in the horizontal braze-joint illustrated in Fig. 5), what coupling distance and induction-frequency should I use? Generally speaking, you’d probably opt for a wider coupling distance (to allow the heat to more slowly heat the part and let the heat penetrate into the joint), and a lower frequency (since that, too, will allow the heat to sink into the bar better than would high-frequency heating).
Call or e-mail me if you have questions about the application of induction brazing to your particular needs.
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Dan Kay - Tel: 860-651-5595: - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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