Determining a practical approach to the application
By Rich Detty
Induction heating is heating by electrical energy that is induced through an alternating electromagnetic field. It makes use of the transformer effect, in which alternating current (AC) voltage placed on one winding will produce an AC voltage on another winding.
The work coil in an induction heater is the primary winding of a transformer. The load acts as a short-circuited secondary winding. The voltage established on the load makes current flow in the load. That current flowing against the inherent electrical resistance in the material of the load generates heat. Induction heating is in reality resistance heating; in essence, it is electrical current flowing against inherent electrical resistance.
A material which is to be inductively heated must be at least a fair electrical conductor. This is why the process is applicable to all metals, and selective or continuous heating is the primary goal.
Induction heating is unique in its ability to produce heat within a charge material without physical contact, without a heat source, and without producing products of combustion.
Induction has no source-related temperature limitations and is applicable at energy levels or power densities much higher than those of any indirect electric or fuel-fired method.
When induction heating is applied to a brazing, soldering, or joining process, the following considerations must be reviewed to determine a practical approach to the application:
1. Parts to be brazed: material, size and weight, thickness, special characteristics (holes, corners, mass uniformity), production rate, and required temperature
2. Power supply: required kilowatts, frequency, water cooling requirements, and coil designs
3. Process considerations: smoke and fume removal, part quality and fit, cleanliness, fluxing and flux removal, alloy selection, and cost of operation/savings
4. Material handling concepts: manual station, two-station operation, conveyorized/in-line systems, rotary indexing, and rotary continuous
Parts to be brazed
Brazing is a method of joining two similar or dissimilar metals or ceramics at temperatures above 840 degrees Fahrenheit with a suitable filler metal that becomes liquidous at a temperature less than the parent materials being joined. Soldering shares many of the same characteristics but is performed at lower temperatures. Welding differs from these practices and the filler material must be melted to join the assembly.
Brazed assemblies and applications are common in everything from hand tools to abrasive cutters, jewelry, lighting, aerospace, and automotive industries. Each part has industrial service considerations for joint strength, aesthetic appearances, and for part quality, including resistance to corrosion, stress, and temperature or pressure cycling.
Power Supply Considerations
To determine the appropriate power supply for an application, the kilowatts required to achieve a production rate must be determined first.
The formula representing the kilowatts required is expressed as:
For example, a hydraulic fitting and steel tube (see figure 1) require a temperature of 1,300 degrees Fahrenheit for a silver brazing application. The production rate is six parts per minute, and the weight of the heated area is .25 pound. Therefore the equation is:
The efficiency of the application involves the following issues when applied to a general mass per kilowatt calculation:
1. Power supply conversion efficiency
2. Induction work coil efficiency
3. Transmission losses to the coil
4. Thermal losses throughout the mass of the part being heated.
Conversion Efficiencies. Typical line-to-load conversion efficiencies of today’s solid-state power supplies range from 60 to 90 percent. Those of earlier vacuum-tube power supplies and motor generator systems may be 50 percent. Applying this to the previous example of 3.88 kW and knowing the power supply conversion efficiency is 90 percent shows that the power supply requirement is now 4.31 kW.
Coil Efficiency and Transmission Losses. Determining coil efficiency and transmission losses from the power supply to the work coil are the next steps in establishing the final kilowatt requirement of the application.
Solenoid coils, sometimes referred to as helical coils, are efficient designs because they encompass the part for maximum coupling of the magnetic field to the part being heated. Typical efficiencies of solenoid cells are from 85 to 95 percent when close-coupled to the part.
Coil coupling is the air gap between the workpiece and the induction work coil. As the air gap increases between the coil and the workpiece, the coupling efficiency decreases, as the two are inversely proportional (see Figure 2).
With other coil designs such as pancake or conveyor-style, parts are proximity-heated. Additional input power is required to maintain the desired rate.
Referring to the original example of 3.88 kW having a 90 percent conversion efficiency (4.31 kW), having a conveyor-style work coil of 50 percent efficiency would require a power supply with an output rating of no less than 8.62 kW.
Induction heating equipment manufacturers have computer programs to calculate the slightest change in coil design to optimize the equipment selections and application efficiencies (see Figure 3).
The operating output frequency of the power supply can be critical to an application’s success. Typically, smaller diameters or thin-wall parts are heated with higher-frequency equipment, while larger masses or heavy-wall parts are heated with frequencies as low as 3 kHz.
Once again, referring to the previous example of the tube and fitting assembly, calculating the depth of penetration of the magnetic lines of flux will establish the current flow in the hex head fitting at a lower or higher frequency by using the following formula:
Therefore, reference depth at 10 kHz =
And reference depth at 450 kHz =
In this case, the lower frequency would prevent overheating of the edges of the hex and allow a deeper depth of current penetration to braze the assembly.
In some cases in which small-diameter solids are being heated, a phenomenon known as “cancellation” can occur that is also frequency-dependent. Cancellation is the point at which the layers of current penetration overlap and nullify the ability for current to flow beyond the curie temperature of most magnetic metals or 1,335 degrees F. For optimum efficiencies in heating a solid, multiply the reference depth (d) by 4.5 when heating the part in an encompassing or helix coil to avoid cancellation. If heating from one side, multiply by 2.25. This results in the minimum diameters that can be efficiently heated at a given frequency (see Figure 4).
Thermal Losses
Considerations of how the heat is going to flow through the part are important. Induction heating is chosen for its selective, rapid localized heating of the masses for joining. Although more critical to aluminum brazing, the heat flow through the part after the alloy is in a liquid state may change the assembly’s unheated areas into heat sinks. This may cause the alloy to solidify prematurely before good capillary flow can occur.
Conversely, in assemblies such as a forging with heavy wall thickness, additional heating on exterior surfaces may be required to obtain proper temperatures for the alloy and joint. The proper power supply and good coil design with adequate time at temperature are necessary to ensure joint quality.
In many cases, parts may have finished subassembly work that may be damaged by conducted heat after the joining process such as plastic seals or O-rings. Solutions include rapid cooling after the alloy has solidified or shielding the part’s delicate area with a heat sink or even submersing it in liquid during the heating process.
All conventional induction heating installations require water cooling of internal power devices, especially the coil assembly and where high currents are present. Typically, a closed-loop recirculating system is provided with ample capacity to provide a continuous flow of cooling water to the power supply and its related components within the system.
Three basic methods of exchanging the heat buildup within a closed-loop recirculator exist:
1. Water-to-air exchanger: Exchanging the heat to a dry or evaporative radiator-style cooler
2. Water to water exchanger: Exchanging the heat to a plant or city water source
3. Refrigeration exchange: Exchanging the heat to a refrigeration compressor-condenser unit
Depending on the kilowatt size, duty cycle, and installation, one of these methods should be sufficient, and it should be chosen based on the existing availability of a plant cooling source.
Process Considerations
The choice of brazing or joining of any two or more parts necessitates good part design to maintain a repeatable process. The joint tolerances within the braze area should be maintained with close proximity from part to part to avoid poor alloy flow or dissimilar characteristics in heating the assembly.
The ultimate goal is to reduce the variables to a minimum to obtain a repeatable assembly process. The induction system will provide a repeatable method to heat the assembly but cannot compensate for large variances in a part’s design.
The assemblies must be clean and free of metal burrs, grease, machining lubricants, and rust inhibitors. These materials will burn off the assembly and, in some cases, prohibit proper flow of the alloy.
Alloys can be dispensed by a round perform, paste, wirefeed, or any combination of the three. In most cases using induction heating, the alloy is prepositioned in the work area via a split perform ring.
Flux is required to act as a “getter” and clean the alloy before the alloy becomes liquidous. Whether the application is high-temperature brazing or low-temperature soldering, flux is required in any oxidizing environment to prevent surface oxidation on the material at an elevated temperature.
In some cases of atmosphere brazing within a contained environment, hydrogen may be used in combination with other inert gases to create an active environment that will provide a “scrub” and brighten the metal while preventing oxidation allowing alloy to flow.
Fume extraction is recommended when brazing to remove vapors caused by flux activation, residual burnoff of coatings, or rust inhibitors still on the part at heating.
Material Handling Concepts
Once adequate power has been selected to achieve the temperature and anticipated rate of the application, the next step is to determine the method of material handling that is both cost-effective and operator-friendly (see Figure 5).
The most conventional approach is a manual brazing station in which the operator is responsible for loading and unloading the assembly. A manual station can incorporate pneumatic lift/lower mechanisms to assist the operator in positioning the parts within the coil.
Other methods include shuttle tables to free the work area or adjustable tables to accommodate a variety of assemblies that may be brazed at the same station. To increase throughput or capacity of a workcell, two or more parts may be brazed simultaneously, depending on how much time is required to assemble the parts versus how long it takes to braze all of the parts.
In cases in which the assembly time is equal to the heat cycle, a two-station nonsimultaneous operation of two independent brazing coils may be an option to maximize the duty cycle of the power supply. As the operator loads and unloads the first station, the second is heating and vice versa.
If the production requirements of the application require additional automation to increase throughput, rotary or in-line processing may be an acceptable alternative. The most common approaches in automation are through rotary indexing and a continuous rotary dial which allows the assemblies to index or pass through an induction coil.
Rotary indexing allows the operator to load and unload the assemblies away from the brazing station and is usually coupled with a lift mechanism at the brazing station. As the dial indexes, the workpiece is lifted by a pneumatic cylinder into the coil and heat is applied. Once the heat cycle has ended, the cylinder releases, allowing clearance to index to a cooling station before unloading.
A continuous rotary system uses a conveyor or “open-ended” work coil, allowing the part to pass progressively through the inductor without interruption.
Whether using a rotary indexing or continuous rotary dial, the equipment paces throughput of the operation, guaranteeing continuous work flow through the work cell.
Rotation or puddling of the alloy through mechanical drives or vibration equipment enhances the alloy flow. In manual or automatic systems, companion equipment for alignment or rotation may be necessary to enhance the alloy flow or remove flux from within the liquidous braze medium. This reduces the potential of voids within the joint by breaking the mechanical surface tension in the joint before it solidifies.
In addition, air cooling, misting, or water cooling of the part can be added to cool the assembly and remove fluxes before the parts discharge from the equipment.
Conclusion
Induction heating offers repeatability when brazing, soldering and joining. As in any manufacturing system, the proper selection of equipment, combined with good manufacturing practices can lead to safe long-term, trouble-free operation.