BRAZING: Process, Joint Design, Materials, Methods & Filler Materials


Brazing process:

Brazing is a process, in which a liquid metal flows into narrow gaps between two parts and solidifies to form a strong and permanent bond.
Ferrous and nonferrous filler metals normally have melting temperatures above 450 deg C, but below those of the two base materials, which do not melt during the joining process.
Almost all metals, and some ceramic alloys, can be brazed using filler metals, such as aluminum and silicon, copper and its alloys, gold and silver and their alloys, and magnesium and nickel alloys.

Joint Design in Brazing:

Pre application of filler metal
Fig. 1 Pre application of filler metal

The metal filler is normally applied to a preheated joint through the melting and deposition of a rod or a wire. However, commonly, the brazing metal can be placed in the immediate vicinity of the joint prior to heating, in the form of preformed rings, disks, slugs, etc.
Bond integrity and strength depend on joint geometry, clearances, and surface cleanliness.
Butt and lap joints are the two primary brazing joint configurations.
Butt joints are simple in design and preparation.
However, in such joints, all of the load is transmitted in the undesirable tensile stress form, where the thinnest section of the joint dictates the strength of the joint.
Lap joints strengths do not depend on the cross sections of the components, and the load is normally transmitted in the desirable shear stress form.
In both configurations, however, the clearance must be carefully controlled beyond an optimal gap, the capillary forces may not be enough to uniformly distribute the filler metal fluid through the joint.
Although joint clearances are functions of the filler and base materials, the empirical, ideal gap has been traditionally defined as 0.05 to 0.15 mm (up to 0.25 mm for precious metals).

Materials and Environment in Brazing:

The brazing of all metals and ceramics depends on the wetting of the filler material at relatively high temperatures.
Formation of unwanted oxides at such high temperatures, however, may impede the ability of the filler material to wet the joint surfaces.
Use of a suitable flux complemented with an inert gas atmosphere dissolves and/or prevents the formation of oxides and promotes wetting by lowering the surface tension of the filler metal.
Common fluxes used for brazing include chlorides, fluorides, borates, and alkalis.

Brazing Methods:

Brazing joint configuration
Fig. 2 Brazing joint configuration

Manual torch brazing is the most common technique used, primarily reserved for one of a kind (repair or prototyping) jobs.
Brazing technique:
Furnace brazing: Parts with pre placed filler segments are brazed in (electric, gas, or oil-heated) furnaces that typically employ a conveyor for (time-controlled) continuous through motion of the parts to be joined.
Induction and resistance brazing: Electrical resistance is utilised to melt pre placed filler materials quickly.
Dip brazing: Complete immersion of small parts into a (constant temperature) molten filler material vat provides wetting and filling of the joints.

Filler materials commonly used in brazing are:

Aluminum–silicon: This group of alloys can be used as fillers for aluminum (and its alloys) base materials at melting temperatures of 500–600 deg C.
Copper and copper–zinc: This group of alloys can be used as fillers for ferrous-base materials at melting temperatures of 750 to 1700 deg C.
Nickel and nickel alloys: This group of alloys can be used as fillers for stainless steel, nickel, and cobalt based alloys at melting temperatures of 950 to 1200 deg C.

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