Induction/high-frequency heating is a complex brazing method, particularly suitable for high-volume welding. This article provides guidance on this and includes guidance on other brazing methods.

Brazing is the process of filling the gap between workpieces with molten filler metal under capillary action to form a joint. This welding method must be performed at temperatures above 840 °F and below the melting temperature of the base material. A widely accepted statement is that the electrons shared by the workpiece and filler metal create a permanent joint that results in a strong connection.

In recent years, brazing systems and technologies have developed rapidly. Using modern testing equipment, users can discover new products and technologies to enhance the characteristics, versatility, and economy of this ancient metallurgical joining technique. Over the past 100 years, many brazing products and systems have been developed, including oxy-acetylene torches, controlled atmosphere furnace brazing, and vacuum furnace brazing. Today, brazing is widely used in various manufacturing systems, including air conditioning and refrigeration, household tools, automotive parts, tools and machinery, electronic components, marine aviation facilities, agricultural irrigation, and commercial machinery.

Brazing Procedure

Effective welding means heating the correct part of the workpiece to reach the optimal brazing temperature at the lowest cost. This includes not only heating methods but also appropriate heating techniques to ensure optimal flow of the filler metal. This article will focus on the applications and techniques of induction brazing. The unique characteristics of this type of brazing allow it to be widely used in current metal assemblies.

Induction Method

Induction brazing is similar to resistance brazing; heat is generated from resistance to current. However, there are differences: most fillers can be used in the induction process; since heat is generated by current passing through coils rather than the base material, the latter may be partially insulated from the filler.

Induction heating can be selective; it allows users to operate in cases where a smaller assembly or overall heating of the workpiece is not possible.

The temperature for induction brazing can typically be reached in seconds, achieving high productivity.

Induction heating uses magnetic fields to generate resistance heat for heating the base material. Iron has magnetic properties and higher resistance, making it heat up faster and easier than copper. All metals can undergo induction brazing. In recent years, redesigns of complex forging and stamping processes have enabled batch production processes; these new designs are a major factor in cost reduction.

Induction heating has proven to be a valuable aid in welding. It allows for rapid localized heating with minimal strength loss when connecting high-strength components. Accurate heating control enables continuous welding effectively. The adaptability of induction heating to production lines allows for strategic layout of workpieces during assembly; if necessary, heating and control can also be done via electronic remote control, such as footswitch control.

Process

Brazing is not just about melting filler metal in the joint gap; joints must be properly designed and meet required tolerances to allow normal flow of filler metal; oxides on the base material must be completely removed to provide sufficient wettability and prevent re-oxidation of its surface; appropriate filler metals must be selected for normal melting to ensure capillary flow. The heat for induction brazing is achieved through resistance heat generated by induced currents in an alternating magnetic field. For example, the brazing area of the workpiece is located within a high-frequency alternating magnetic field generated by a circulating heating induction coil, where resistance heat from induced currents heats the workpiece. Each welding operation requires high-frequency current and appropriate induction coils.

When performing operations, the inducer controls the entire cycle heating system, turning brazing into a button-operated welding process. Often only a few seconds of heating are needed for the workpieces to be quickly joined together; thus, induction brazing is suitable for mass production but also applicable in certain special cases due to its heating characteristics. Filler alloys melt at high temperatures to produce high-strength joints. Metals that can be joined using induction brazing include carbon and alloy steels, stainless steel, cast iron, copper and copper alloys, nickel and nickel alloys as well as alloys containing certain amounts of aluminum.

Most forms of filler materials are wire, strip, and powder; additionally, ductile preform alloys can also exist as washers and rings. To control the amount of filler used, preform alloys allow for pre-assembly which saves alloy and produces uniform joints. A good preform alloy should make thorough contact with the workpiece to ensure smooth melting occurs at appropriate temperatures.

Importance of Coil Design

When designing coils, it is essential to consider compatibility with the metallic properties of the workpiece while adhering to the geometry of the welded area. When brazing using induction heating, special attention should be paid to how heat is applied, how preform filler alloys are set up, tolerances of matching workpieces, thermal conductivity as well as expansion properties of workpieces. Testing tensile strength near joints in base materials often fails; therefore when selecting base materials one must consider not only weight but also thermal/electrical conductivity, corrosion resistance and other properties along with tensile strength.

The geometry of joints also plays an important role in strength and economy. All factors are equal; larger welding surfaces produce greater shear strength than smaller surfaces but require more base material and filler metal. The engineer's goal is to meet strength requirements with minimal welding surface area possible. As for heating methods, coil design is equally important; it must ensure that all areas near joints exceed melting temperatures when reaching uniform temperatures. It also requires that joint areas reach welding temperatures first to prevent filler from flowing into higher temperature areas where heat may dissipate.

To achieve better results with preform fillers, they should not form a closed loop with coils creating inductive coupling. It is also required that electromagnetic protection for preform fillers be maximized at all times to avoid premature melting before reaching welding temperatures on joint surfaces; this can be accomplished by placing fillers inside assembled workpieces or hiding them under certain components. The gap between welded workpieces determines the thickness of fillers and significantly impacts joint strength. If maximum strength is required for joints, then gaps must be sufficiently large to allow molten fillers through while enabling gases produced during heating to escape.

The ideal joint gap is 0.002-0.005 inches, and in many cases, 0.006 or 0.008 is also acceptable. The joint gap should be avoided below 0.001 inches or above 0.008 inches as this not only leads to weak joint strength but also incurs higher manufacturing costs. When determining the joint gap for similar materials to be brazed, the thermal expansion factors of the workpieces must be considered, and sufficient space should be left so that the filler can fill the joint gap at welding temperatures.

Relatively speaking, when designing brazing, both the strength of the base material and the filler itself must be considered simultaneously. To achieve maximum strength, the brazed joint should be designed with a larger shear area rather than an overlap area. Finally, it is recommended to perform a simple inspection of the joint when pre-setting the filler.