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Brazing

Updated Thursday, 8th March 2018

The workpiece material is not melted as in welding, but metal parts are joined using filler metals that have a melting point above 450°C, but below the melting point of the materials being joined. The molten filler metal is drawn into the weld gap by capillary attraction across the joint, eventually solidifying to form the bond. There are various ways to apply the heat.

Brazing collar onto tube

Images to demonstrate 'Brazing' - see article Copyrighted  image Icon Copyright: The Open University

Methods of pre-placing the brazing alloy for furnace or induction brazing

Images to demonstrate 'Brazing' - see article Copyrighted  image Icon Copyright: The Open University
  1. Surfaces to be joined are degreased, abrasive grit blasted and pickled.
  2. Components are assembled or jigged, very often with a ring of braze metal charge placed in position.
  3. Components are heated by torch, in a conveyor furnace or by induction, to required temperature. Joint is filled by capillary action.

Manufacture:

  1. Surface preparation as soldered joints (see L02).
  2. Assembly/jigging: Jigs restrain thermal expansion, absorb heat, deteriorate and are expensive. If possible joints should be self-locating. Jigs should be easy to load, have minimum contact with parts, have a low thermal mass, keep alignment, and be heat resistant. During assembly/jigging flux is positioned as a paste, and brazing material as a “shaped preform” of wire, or foil, or as a paste.
  3. Heating:
    • torch  most common hand-held or fixed heating method
    • induction  mainly for ferrous parts
    • furnace – batch or continuous. Can be done under Ar, N2, H2, cracked NH3, or vacuum with no flux
    • resistance – heating is local, and electrodes can act as jigs
    • dip (salt bath)  assembly is immersed in molten flux.
  4. Finishing – flux or excessive braze material is removed before inspection.

Holding components together without jigs (BS 1723)

Images to demonstrate 'Brazing' - see article Copyrighted  image Icon Copyright: The Open University

Materials:

  1. Selection of brazing material depends on the parent metal, operating temperature, economics and the fit of the joints.
  2. Brazing alloys available as wire, foil, powder, clad sheet, deposited coatings, paste and specially-prepared preforms.
Alloy
Forms
Joint gap (mm)
Braze temp. (˚C)
Properties and uses
Ni Cr P Powder, paste 0 - 0.1 960 Fills tight joints. Little alloying with parent material

Ni Cr Si B Fe

Powder, paste 0.03 - 0.3 1040 High strength joints in heavy steel sections
Cu Zn Powder, paste, wire, shim 0 - 0.1 ≤1120 Fills tight joints
Cu Ni Si Powder, paste, wire shim 0.03 - 0.2 1100 Good gap-filling. Used for tungsten carbide-steel joints
Ag Cu Powder, paste, wire, shim 0.03 - 0.2 820 Used on thermionic equipment
Au Ni Powder, paste, wire, shim 0.03 - 0.2 980 Good oxidation resistance at high temperature
Ag Cu Pd Powder, paste, wire, shim 0.03 - 0.2 850 Good resistance to crevice corrosion
  1. Alloying often occurs between the brazing material and the parent metal(s). The finished joint may be a new, higher strength alloy.
  2. Fluxes increase the wettability of the joint surfaces by dissolving oxide. Flux is active at ±50˚C, around the melting temperature of the brazing material.
Type of flux
Applications
Borax and fluoro-borate General use above 750˚C except with refractory oxides
Fluoride General use below 750˚C except with refractory oxide
Alkali-halide All processes involving the brazing of aluminium and aluminium-based alloys, with Zn, Cd or Al-Si systems
Fluorine/boron compounds 600 - 1000˚C particularly with refractory oxides

Design:

  1. Joint clearance is critical, depending on brazing materials, surface condition, degree of alloying and length of joint.
  2. Recommended joint clearances vary from interference fit to 0.6 mm.
  3. Lap joints are preferred to butt joints wherever possible. Strength and reliability depends on metal penetration by capillary attraction, rather than on external fillets as in welding. Consequently, joint design for brazing differs from that for welding, and large fillets are avoided except when brazing aluminium alloys, which are half fillet and half lap with wider joint gaps.
  4. Joint design depends on how brazing material is applied.
Images to demonstrate 'Brazing' - see article Copyrighted  image Icon Copyright: The Open University Images to demonstrate 'Brazing' - see article Copyrighted  image Icon Copyright: The Open University

See Also: Adhesive bonding, Soldering and Fasteners

This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.

 

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