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BRAZING RODS AND FLUXES

Introduction

When brazing, the parent metal is heated to bonding temperature. This is the minimum temperature to which the parent metal must be heated to form an alloy of the filler metal and the parent material.

When adding the brazing rod and flux we obtain a surface alloying. Brazing is a mechanical bonding.

Brazing rod with flux

Blowpipe

Brazing alloy

Base material

When brazing, the parent material is not melted, but brought to the bonding temperature.

Welding is a chemical bonding which involves structural changes in the base material.

Each brazing alloy has its own bonding temperature which is independent of the parent material. Because brazing is a surface alloy the surface must be cleaned. In most cases an oxide-film covers the surface of the parent metal and the rod before and during brazing.

A clean surface with sufficient flux allows the brazing alloy to flow out and bind to the surface

This film acts as isolation that prevents bonding.

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The function of the flux is to dissolve oxides, detach them from the the parent metal and the rod, and keep them in suspension.

The composition of the flux must be matched to type of brazing rod. The flux should liquify approx. 100°C before bonding temperature.

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Without the flux to suspend water vapour, grease and oxides, a film will prevent the brazing alloy from bonding