and reference depth at 450 kHz=
d=3,160
d=.029"
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 the ability for current to flow beyond the curie temperature for 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 temperature (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 throughout 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 a 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 good joint quality.
In many cases, parts have finished subassembly work that may be damaged by conducted 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:
Figure 5 (Above)- A typical block diagram of a brazing system is shown.
Figure 4 (Below)- Outlined in this table are the minimum diameters that can be efficiently heated at a given frequency.
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