In general, the choice of brazing flux should consider the following factors:

1. Base metal and solder

The type of base metal and solder is the first factor to consider when selecting brazing flux.

For example, when brazing brazing copper and copper alloys, dehydrated borax (hardener) is often used, and boric acid is sometimes added.

2. Brazing method

Different brazing methods place different requirements on the flux. When using resistance brazing, the flux should have a certain conductivity; when dip brazing, the flux should remove moisture to avoid boiling and explosion.

3. Brazing temperature

The brazing temperature is determined by the base metal and the brazing material. When selecting the flux, the melting temperature should be adapted to the brazing temperature.

4. Brazing shape

For brazed joints with complex brazing shapes, since the flux and its residue are not easily removed after brazing, the corrosive agent with small corrosivity and easy removal should be selected.

Analysis of the causes of internal defects in brazed joints

1. slag inclusion

The reasons causing:

(1) Brazing flux used more or less;

(2) The density of the flux is too large;Improper gap selection

(3) Improper gap selection;

(4) The solder fills the joint from both sides of the joint at the same time

(5) The solder does not match the melting temperature of the flux

(6) Flux viscosity is too large

(7) Uneven heating

2. Stomata

The reasons causing:

(1) Improper cleaning of base metal before brazing

(2) Solder overheating

(3) Precipitated gas in base metal and solder

(4) Poor selection of flux, weakening of flux removal and protective gas deoxidation

(5) Improper joint clearance

(6) Too much flux