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→Resistance Brazing
==== Resistance Brazing====
In this process the resistive heating under electric currents is the source ofthermal energy. For contact applications two methods are used for resistancebrazing''(Fig. 3.8)''.
During Direct Resistance Brazing the electric current flows straight through the joint area composed of the contact tip, brazing alloy, flux, and the contact carrier. These components are secured between the electrodes of a resistance brazing machine and heated by an electrical current until the brazing alloy liquefies. In Indirect Resistance Brazing the current flows only through one of thecomponents to be joined (usually the non-precious contact carrier). Thisprocess allows to move the contact tip (“puddeling”) when the brazing alloy is inits liquid stage and this way remove residue bubbles from the heated and boilingflux and increase the percentage of the bonded area.Two different kinds of electrodes are used for resistance brazing:
*Electrodes from poorly conducting carbon containing materials (graphite)<br />The heat is created in the electrodes and thermally conducted into the joint area<br />
*Electrodes from higher conductive and thermally stable metallic materials<br />The heat is created by the higher resistance in the joint area which, through selected designs, creates a constriction area for the electrical current in addition to the resistance of the components to be joined.<br />
Graphite electrodes are mainly used for indirect resistance brazing and for jointarea > 100 mm<sup>2</sup>. For contacts tips with a bottom area < 100 mm<sup>2</sup> which arealready coated with a phosphorous containing brazing alloy the heating time canbe reduced to a degree that the softening of the contact carrier occurs only veryclosely to the joint area. For this “short-time brazing” specially designed metalelectrodes with compositions selected for the specific assembly componentpairings are used. The bond quality for normal resistance brazing with the application of flux rangesfrom 70 to 90% of contact size, for short-time welding these values can beexceeded significantly. Fig. 3.8: Resistance brazing (schematic)
====Induction Brazing====