TWI Knowledge Summary

Induction brazing

by Sue Dunkerton

The process

Brazing is the process of joining metallic or ceramic materials using a molten filler metal, drawn into the joint by capillary action or preplaced with preform. Usually, the filler metal becomes molten well below the melting point of the materials being joined, although in some cases, the melting points are close but again the filler is the lower melting point material.

Various heat sources are used to heat the substrates and filler metal, and this summary provides background on use of induction heating.

Induction heating

High frequency induction heating for brazing involves use of induction coils to induce localised heat in the materials being brazed. Heating occurs primarily as a result of the resistance to flow of current induced in conductive materials when placed in a magnetic field produced by a rapidly alternating current. The alternating current in the parts to be brazed is generated by the induction coils the geometry of which dictates the electromagnetic field.

Equipment used for brazing is typically in the frequency range 5-500kHz, with powers of 1kW to 100s of kW. The higher the frequency, the greater the skin effect (i.e. preferential heating of the surface). By using lower frequencies and slowing the heating rate, uniform heating through the thickness is possible via conduction within the material.

The benefits of induction include the ability to provide localised heating, and precise control of both heating rate and final temperature. Also, the method can be mechanised, and have multi-brazing stations for one power source.

The difficulties of the process include the fact that there is no easy theory behind coil design to deliver the best heating profile to the workpiece. Except for welding simple geometries (e.g. tube butt or scarf joints), specialised assistance is necessary to design coils. However, manufacturers of equipment have many years of experience in this area and have demonstrated that quite complex geometries can be efficiently heated.

Materials

Induction brazing is suitable for many metallic materials, with magnetic materials being heated more readily. Where ceramic materials are involved, heating will most likely occur by conduction from surrounding metallic parts, or the use of a susceptor.

Silver based brazing alloys are used extensively, but copper is also widely used.

Applications

The applications are varied, demonstrating the versatility of the technique. They range from heavy mining equipment (attaching hard facing surfaces), to tools, to plumbing, to many electrical consumer and electronic components. Where production volumes merit it, the process will be mechanised and may have multi-work stations.

Benefits

The main benefits for this process over other brazing methods are the selective and rapid heating, precise heat control and suitability for large volume production.

Precautions

Precautions necessary with induction brazing include those usually applied for mechanical and electrical equipment, and handling of hot components. Specifically, care is necessary with respect to the high frequency electromagnetic field, ensuring it is not released to the general work environment.

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.