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===3.3.2.3 Resistance Brazing===
In this process the resistive heating under electric currents is the source of
thermal energy. For contact applications two methods are used for resistance
brazing
''(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 the
components to be joined (usually the non-precious contact carrier). This
process allows to move the contact tip (“puddeling”) when the brazing alloy is in
its liquid stage and this way remove residue bubbles from the heated and boiling
flux 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)
The heat is created in the electrodes and thermally conducted into the
joint area
*Electrodes from higher conductive and thermally stable metallic materials
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.
Graphite electrodes are mainly used for indirect resistance brazing and for joint
2 2 area > 100 mm . For contacts tips with a bottom area < 100 mm which are
already coated with a phosphorous containing brazing alloy the heating time can
be reduced to a degree that the softening of the contact carrier occurs only very
closely to the joint area. For this “short-time brazing” specially designed metal
electrodes with compositions selected for the specific assembly component
pairings are used.
The bond quality for normal resistance brazing with the application of flux ranges
from 70 to 90% of contact size, for short-time welding these values can be
exceeded significantly.
Fig. 3.8: Resistance brazing (schematic)
===3.3.2.4 Induction Brazing===
During induction brazing the heat energy is produced by an induction coil fed by
a medium or high frequency generator. This creates an electromagnetic alternating
field in the braze joint components which in turn generated eddy currents
in the work piece. Because of the skin–effect these currents and their resulting
heat are created mainly on the surface of the assembly components. The
distance of the inductor must be chosen in a way that the working temperature
is generated almost simultaneously in the full joint area. For different contact
shapes the geometry of the induction coil can be optimized to obtain short
working cycles. One of the advantages of this method is the short heating time
which limits the softening of the material components to be joined.
Typical bond qualities of > 80% can be reached with this method also for larger
contact assemblies. The widely varying working times needed for the different
brazing methods are given in Table 3.1.
Table 3.1: Brazing Times for Different Brazing Methods
*Examples of brazed contact assemblies
bild
*Contact materials
als bild?
Ag, Ag-Alloys., Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO),
Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO) and Ag/C (GRAPHOR D) with 2
brazable backing, refractory materials on W -, WC- and Mo-basis
*Brazing alloys
L-Ag 15P, L-Ag 55Sn et.al. als Bild?
*Carrier materials
Cu, Cu-Alloys. et al. als Bild?
*Dimensions
Brazing area > 10 mm²
*Quality criteria
The testing of the braze joint quality is specified in agreements between the
manufacturer and the user.
===3.3.3 Welding Processes===