Switching to induction for your brazing applications is easy and energy efficient. It also improves productivity, integrates well in manufacturing lines and eliminates open flames.

Man has been using fire to melt, refine, bond, shape and harden metals for thousands of years. Even today, fire in the form of gas torches is used for many metal process applications such as brazing, soldering, fusing of nickel-boron hard coating, shrink fitting, weld preheating and flame hardening. With more and more emphasis being put on employee safety and preventing burns and fire hazards in the workplace, however, manufacturers are looking for alternatives, and they are finding it in induction.

This article talks primarily about brazing and soldering. With both brazing and soldering, a filler metal is used that has a melting point lower than the solidus point of the metal parts being joined. After reaching a certain temperature, generally about 50-100°F higher than the melting temperature of the filler depending on heating rate, the surface of the base metal reacts with the filler alloy to form a metallurgical bond.

According to the American Welding Society (AWS), the only thing that differentiates soldering from brazing is temperature. If the metal-bonding process uses a filler metal that melts below 450°C (842°F), the bonding process is defined as soldering. If the filler metal melts above 450°C (842°F), however, then the bonding process is defined as brazing.^[1]

Torch Brazing

Gas torches have been used for decades for brazing and soldering, primarily because the initial capital cost is low. (Generally, torch brazing kits can be purchased for less than $2,000, depending on the size of the torch.) Besides the safety hazards associated with using gas torches, however, there are other gas-torch disadvantages.

1. A relatively high skill level is required to be able to effectively braze with a gas torch.

a. Individuals who torch braze need to be able to select the torch size and tip for the job, based on the thicknesses and type of the metals to be bonded.

b. Individuals who braze with torches need to know how to angle and move the torch around to be able to heat the entire joint to the proper temperature without overheating the joint or melting the base metals.

c.  Users of gas torches need to be able to light gas torches safely and adjust gas-flow mixtures to achieve the desired flame (carburizing, reducing, neutral or oxidizing) for the brazing operation.

d. Users of gas torches need to understand how the type of fuel used affects the brazing process.

2. Torch brazing is labor-intensive process.

3. Process variability is much larger than with other methods of brazing since no two individuals have the exact same brazing techniques.

4. Bottled gases such as butane, propane, MAPP or acetylene have fire and explosion hazards associated with them. These gas bottles need to be handled and stored properly.

5. When automating with gas torches and using natural gas as the fuel gas, the heat value of natural gas is constantly changing, which affects the process. For tightly controlled processes, adjustments must be made as the BTU content of the natural gas changes.

6. With automated gas-torch brazing processes, tip plugging constantly needs to be monitored.

However, torch brazing does work well for odd-shaped joints that are not easily heated with a specific heat pattern and poor-fitting joints where a level of skill is required to fill the joint with braze alloy.

Induction Brazing

Brazing/soldering is the most popular induction joining application at this time. Besides looking to eliminate the burn and fire hazards associated with torches, the following are reasons manufacturers are changing to induction brazing.

1. Induction brazing is a process where an AC electromagnetic field is used to generate energy within the part, causing heating at the surface and subsurface areas of the part due to Joule effect (I²R power losses: where I is current density induced in the part by the electromagnetic induction field and R is the electrical resistance of the part). Therefore, the amount of energy put into a part can be controlled very precisely, assuming the part is consistently and accurately positioned relative to the inductor, which makes for a very controlled brazing process.

2. Induction brazing is easily automated and lends itself to both single-piece flow (just-in-time processing) and high-volume brazing.

3. Induction brazing is energy efficient because power is used only during the brazing process. Depending on the material system being soldered/brazed, up to as much as 80% of the input energy goes directly to the part, which is more efficient than other traditional heating processes.

4. Induction heating when used in combination with protective-atmosphere chambers allows for fluxless brazing of part assemblies with copper, nickel and some silver-braze alloys. Typically, the only alternative to induction fluxless brazing is vacuum or controlled-atmosphere furnace brazing, which requires a significantly larger capital investment. Plus, furnace-brazing cycle times are significantly longer than induction brazing cycles (minutes to hours versus seconds with induction). Figures 1 and 2 show some automotive parts that are fluxless copper brazed using induction heating.

5. Induction brazing can be used for a wide variety of braze-joint geometries, and it is not limited to simple or tubular shapes (Fig. 1).

6. Induction brazing is capable of brazing large parts in a fraction of the time that can be achieved with gas-torch brazing. Figure 3 shows an example of a large motor rotor being silver brazed with induction heating in less than six minutes.

In spite of all the advantages associated with induction, there are some considerations when changing to induction heating.

1. Fixturing materials – In many cases, manufacturers use mild-steel brazing fixtures for torch brazing. Because induction heating uses electromagnetic energy to heat parts, changing to induction heating generally requires new nonmagnetic brazing fixtures, particularly when brazing nonferrous and nonmagnetic metals. When steel fixtures are used to hold either nonferrous or nonmagnetic metals in an electromagnetic field, the fixture will generally be heated up preferentially. Therefore, induction heating braze fixtures should be nonmagnetic and preferably not electrically conductive.

2. Fixturing design – The induction braze fixturing should have minimal contact area with the part being brazed to minimize the heat-sink effect of the fixturing.

3. Inductors – As part sizes and shapes change, inductors (coils) need to be changed to efficiently heat parts. It is the inductor shape and the gap between the inductor and part that control how the electromagnetic field couples with the part, where the joule heating occurs and ultimately the efficiency of the process.

To help manufacturers make the transition to induction heating easier, induction heating companies have been working on recent developments.

1. A variable-gap inductor by Radyne Corporation called the “InductoVise” (patent pending). The InductoVise (Fig. 4) is a general-use inductor that can be used for heat treating, brazing and soldering. The advantages of a variable-gap inductor are that it reduces the number of inductors, setup time and fixturing required for certain applications.

2. Hand-held brazing transformers allow the inductors to be moved to the part, rather than having to move the part into the inductor.

3. Faster, more-precise power-supply controls allow for consistent heating of parts and easier automation.

4. More advanced induction equipment monitoring and controlling software allows for performance-data collection and predictive analysis, which reduces downtime for high-volume production systems.

Considering recent developments, if it has been a while since you have assessed induction heating, now is a good time to re-evaluate it for your brazing needs.

For more information:  Contact Scott R. Larrabee, Induction Process Engineering Center Manager, Radyne Corporation, 211 West Boden Street, Milwaukee, WI 53207; tel: 414-481-8360 x134; e-mail: slarrabee@radyne.com; web: www.radyne.com

References

1. AWS Handbook 5th ed., p. 2, AWS 2007
2. S. Larrabee, A. Bernhard, “Design and Fabrication of Inductors for Heat Treating, Brazing, and Soldering,” ASM Handbook, Vol. 4C; Induction Heating and Heat Treatment, ASM Intl., 2014, p. 619-632