Precision Induction Brazing

Brazing is a heating process in which two or more like or unlike materials are joined together by means of another metal alloy with a lower melting point. Braze joints can be made exceptionally strong, sometimes stronger than the two metals being joined. (The term 'silver soldering' is sometimes used to refer to silver brazing.)

Braze joints are liquid- and gas-tight, can withstand shock and vibration, are unaffected by normal temperature changes, provide good electrical conductivity and can be easily plated using conventional processes. Typical brazing temperatures vary between 800°F and 2150° F.

Modern induction heating provides reliable, repeatable, non-contact and energy-efficient heat in a minimal amount of time without flame. Solid state systems are capable of heating very small areas within precise production tolerances, without disturbing individual metallurgical characteristics. For larger volume applications and/or quality-dependent processes, parts can be brazed with induction under a controlled atmosphere without flux or any additional cleaning steps.Typical RF power supplies for induction brazing range from 1 to 20kW, depending on the parts and application requirements.

First, the two metals to be joined are cleaned by coating them with flux. The braze paste or preform is then put in position and heat is applied until the braze flows creating a solid joint.

If the braze is being stick fed, the parts are first brought up to temperature; then braze is introduced into the joint area by hand. The appropriate temperature depends on the type of braze that is being used.

Braze: The alloy can be in paste, preform or stick form, depending on the application. A wire preform is generally preferred because it ensures uniform distribution and promotes joint-to-joint consistency. Different braze alloys have different heating characteristics; silver is frequently used for induction brazing because of its low melting point. Silver-copper eutectic brazes have melting temperatures between 1100°F and 1650°F. Aluminum braze, the least common, has a melting temperature of 1050°F to 1140°F. Copper braze, the least expensive, has a melting temperature of 1300°F to 2150°F.

Flux: The functions of flux are to dissolve the oxides formed during the heating process, shield the alloy and joint from oxidation, provide clean surfaces to promote even spreading of the alloy, and to promote alloy flow by capillary action. There are many different types of fluxes available for use at different temperature ranges. Black flux is used for high temperatures (up to 1800°F) and is good for steel brazing. White flux is most often used for lower temperature (1100°F to 1500°F) applications. Ideally, the flux should have a lower melting point than the base metal, and should be entirely liquid before the braze alloy melts.

Heat source: Fast, precise induction heat works best.

Braze does not flow consistently each time the joint is made.
Parts may crack after the braze is complete.
If the parts to be joined are at different temperatures, braze will not stick to the colder part.
The high temperatures required can cause distortion of the metals being bonded.

Both metals that are being joined together need to be of equal temperature during the brazing process for a successful braze joint.
A slower heating cycle is better then a faster one. If productivity rates are critical, it is better to process multiple parts at the same time.
When using a braze preform ring for induction brazing, make sure there is good physical contact between the part and the ring. A loose ring will need much higher temperature to melt and will also cause non-uniform joints.
The parts being brazed must be clean at the joint area.