Induction Heating Consultant

Induction Heating

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
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Stanley Zinn

Induction Brazing and Soldering    View as PDF

by STANLEY ZINN, Induction Heating Consultants • June 2010

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Coil Design

As in torch brazing, the parts to be joined must reach the same temperature at the same time to provide capillary action. Hotter areas will cause the alloy to move toward the hottest part of the assembly. This is further complicated not only by the largest mass in the assembly but by the material as well. Because of the differences in resistivity of materials and the fact that heating with a high frequency current is based on I2R, different materials will heat differently in an induction field. The same current that flows in a steel part may not bring a similar piece, made of copper, to the equivalent temperature in the same interval of time. Therefore, the coil must be designed to deliver more current to the greater mass/ lower resistivity material, while still bringing the mating part to temperature at the same time.

This change in design of the coil requires placing either more turns at the greatest mass, or decreasing the coupling distance to the larger part so it receives more magnetic flux from the coil.

Typically, in a tool-brazing system where the steel has a lower resistivity than the tungsten carbide insert, as well as a greater mass, the coil is positioned lower on the tool to deliver more heat to the steel shank. See Figure 4.

The strongest flux field and therefore the most heat is generated inside the coil. However, in many instances this is not feasible and it requires the coil to be designed in a “U” or channel coil shape (see Figure 5) so that it has accessibility to the assembly and can be removed after the brazing application. Though it is not the most efficient design, the end result, requiring more power, justifies the means.

Figure 6 shows a typical portable system for brazing electrical connections on large copper bars for motors. The open “U” inductor is easily placed and removed from the components for brazing in the field. Split or scissor type coils can be used but are limited to one or two turns due to mechanical limitations.

Most commercial brazing applications do not require high power and small variations in temperature are normal. The fixed mass of the components and therefore the temperature is normally controlled by either power or time. This allows multiple parts to be brazed simultaneously if sufficient power is available. Figure 7 shows the construction of a typical multi-place brazing coils. By modification of the coupling distance (opening the coupling on smaller parts), simultaneous brazing of non-similar components can also be achieved.

Vacuum or Atmosphere Brazing

As noted, many steels are preferentially brazed in vacuum, hydrogen, forming gas (90% N2 and 10% H2), Nitrogen or Argon. The purpose is to utilize the vacuum or gas as a deoxidizing agent while preventing the balance of the component from oxidizing due to heat transfer. The additional benefit of this technique is the elimination of flux resulting in a clean part that does not require cleaning subsequent to the joining operation.

The induction brazing of stainless steels can be accomplished in a vacuum or dry hydrogen atmosphere A typical system is shown in Figure 8. If an atmosphere is used, a dew point of –60°F or lower is required to prevent the chromium from being oxidized. This same approach may be used with Type 200 stainless steels if the dew point is kept very low. Precipitation–hardening stainless steels can also be brazed in dry hydrogen if there are no more than trace amounts of titanium or aluminum. These steels may also be brazed in vacuum if neither of these elements is in the steel.

Vacuum or gas atmospheres for brazing are generally used in a batch furnace but have the disadvantage of heating the entire component during the brazing cycle. In addition to affecting the metallurgical properties of the part, this process is time consuming and uses considerable space and energy as well.

Since the induction field can be highly localized, using induction in a vacuum chamber can minimize these problems. In practice, the chamber is evacuated to a suitable level (10-6 Torr where possible) and the chamber is backfilled with the desired gas. A partial pressure system maintains the gas at acceptable levels during the brazing cycle. Parts emerge from the system as clean bright assemblies after brazing.

Vacuum brazing chambers can be fitted with numerous automated fixtures to move the coil or part so that the joint area is within the coil. Rotary tables and lift rotate assemblies are sometimes provided for handling multiple parts. To minimize down time, where multiple assemblies must be processed, air locks at the entry and exit of the chamber, allow parts to be brought into, and exit from, the chamber, without contaminating the atmosphere. An example is shown in Figure 9.

Alloy Selection
There are a number of basic requirements for any filler metal (alloy) used in brazing:

  1. The alloy must have a melting temperature below the melting point of the components being brazed.
  2. The alloy must be able to “wet” to the materials being joined.
  3. Flow characteristics of the filler material must be compatible with the joint clearance and be able to utilize capillary action for filling the joint area.
  4. The alloy must be able to provide the strength and anti-corrosion resistance of the joint for the service required.

One of the most important characteristics of the filler metal is its ability to match the Thermal Coefficient of Expansion of the mating parts. The braze material must be sufficiently ductile to have a thermal expansion comparable to the metals being joined. These filler alloys then can tolerate a wider gap between components with minimal formation of porosity or brittle phases. When there is a difference in thermal expansion between the base metal and the filler metal, stress concentration and high residual stresses may result.

Subsequent to the brazing operation, the parts must remain in place until the alloy solidifies. Premature movement, while the alloy is still molten, may result in crystallization and a weak joint.

Typically, these stresses occur in the brazing of carbides to steel as required for cutting tools. In this case, stresses can be avoided by using a filler composed of a copper strip clad with the brazing alloy on both sides. This method minimizes stresses due to the low yield strength of the intermediate copper.

Brazing materials are available in a wide range of forms and must be selected to match the requirements of the manufacturing process. Typical forms are:

  • Ingot/shot/pellets
  • Wire (0.030-0.100”)
  • Sheet
  • Foil (0.003-0.010”)
  • Preforms of any of the above
  • Powder
  • Pastes (a mixture of alloy and flux)

There are a large number of soldering alloys utilized for low temperature applications. These range from the standard lead-tin Eutectic, through gold bearing filler metals, or those with constituents of antimony, bismuth, cadmium, indium, lead, silver, zinc or tin. Each solder has a specific use and is tailored to the specific application. Brazing alloys, on the other hand, are generally comprised of silver, gold, palladium, aluminum, etc.

The filler metals used to braze low carbon steels are those listed in the AWS (American Welding Society) Bag and Bcu series. The silver bearing fillers are used because they melt and flow at comparatively low temperatures. Nickel-base alloys can also be used, particularly where greater joint strength and corrosion resistance are required. The copper bearing fillers melt at higher temperatures but are considerably less expensive. These are used primarily in joining copper or brass assemblies, or for steel components in a reducing atmosphere. The copper bearing alloys are also sluggish in their flow characteristics when melted, and tend to fill larger gaps and voids more easily than the silver-bearing alloys.

When the joint members are stronger than the braze material, stress concentration and high residual stresses may become critical factors. This is also a problem when differential contraction takes place, as when dissimilar materials are brazed. This is commonly the problem when brazing tungsten carbides for cutting tools. An intermediate layer of copper (which may be clad on both sides with the braze material) can minimize these problems due to plastic deformation in the copper.

Vacuum brazing requires filler metals that will not volatilize at brazing temperatures. Fillers with high vapor-pressure components, such as zinc or cadmium should not be used in these circumstances.

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