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Brazing knowledge summary(5)

Jan 28, 2019

Brazed joint

Brazing is generally in the form of sheet lap joints and sleeve inserts. In this way, the joint strength between the weldments can be increased to compensate for the shortage of the brazing material and to ensure the bearing capacity of the joint. This type of joint also facilitates the control of the joint gap, and the proper gap allows the solder to be evenly distributed in the joint for optimum brazing. The gap of the brazed joint is generally in the range of 0.05 to 0.2 mm.

The bearing capacity of the brazed joint is related to the size of the joint face. Therefore, brazing is generally lap joints and kits to compensate for the lack of brazing strength.

Post-weld cleaning editing

Most of the flux residue corrodes the brazed joint and also hinders the inspection of the braze joint, which often needs to be cleaned.

The rosin-containing active flux residue can be removed with an organic solvent such as isopropyl alcohol, alcohol or trichloroethylene.

A flux composed of an organic acid and a salt is generally soluble in water and can be washed with hot water. A solder composed of a mineral acid is dissolved in water and thus can be washed with hot water. A flux containing an alkali metal and an alkaline earth metal chloride such as zinc chloride can be washed with a 2% hydrochloric acid solution.

The borax and boric acid flux residues for brazing are substantially insoluble in water and are difficult to remove, and are generally removed by sand blasting. A preferred method is to place the brazed workpiece into the water in a hot state to cause the flux residue to crack and be easily removed.

The residue of the fluorine-containing potassium borate or potassium fluoride hardener (e.g., agent 102) can be boiled or removed in 10% citric acid hot water.

The aluminum residue can be removed with an organic solvent such as methanol.

The aluminum hard residue is highly corrosive to aluminum and must be removed after brazing. The cleaning methods listed below can give better results.

(1) Soak in hot water at 60-80 °C for 10 min, carefully clean the residue on the brazing joint with a brush, rinse with HNO3 15% aqueous solution in cold water for about 30 minutes, then rinse with cold water.

(2) 60-80 ° C flowing hot water rinse 5 -10min placed in 65 ~ 75 ° C, CrO32%, H3RO45% aqueous solution soaked for 15min, and then rinsed with cold water.

Quality inspection editor

The defect brazing of the brazed joint and its causes are as follows:

1. Poor interstitial, part of the gap is not filled


(1) The joint design is unreasonable, the assembly clearance is too large or too small, and the parts are skewed during assembly.

(2) The flux is not suitable, such as poor activity, the flux and the melting temperature of the solder are too different, the flux filling ability is poor, etc.; or the gas purity is low when the gas is brazed, and the vacuum is low during vacuum brazing.

(3) improper selection of solder, such as poor wetting of solder, insufficient amount of solder.

(4) Improper placement of solder.

(5) Poor preparation before brazing, such as cleaning is not clean.

(6) The brazing temperature is too low or unevenly distributed.

2. Brazing vents


(1) The joint gap is not properly selected.

(2) The parts are not cleaned before brazing.

(3) The flux removal effect of the flux or the protective gas deoxidation is weak.

(4) The brazing material precipitates gas during brazing or the brazing material is overheated.

3. Brazing joint slag


(1) The amount of flux used is too much or too little.

(2) The joint gap is not properly selected.

(3) The solder fills the joint from both sides of the joint.

(4) The melting temperature of the solder and the flux does not match.

(5) The flux has a large specific gravity.

(6) Uneven heating.

4. Brazing joint cracking


(1) Since the thermal expansion coefficients of the different parent materials are different, the internal stress formed during the cooling process is excessive.

(2) The brazing of the same material is unevenly heated, resulting in inconsistent shrinkage during cooling.

(3) When the solder solidifies, the parts are displaced from each other.

(4) The solder crystallization temperature interval is too large.

(5) The brittleness of the brazing seam is too large.

5. Solder loss


(1) The brazing temperature is too high or the holding time is too long.

(2) The solder is improperly placed so as not to have a capillary action.

(3) The local gap is too large.

6. The base metal is dissolved


(1) The brazing temperature is too high and the holding time is too long.

(2) The effect between the base metal and the brazing material is too strong.

(3) The amount of solder is too large.

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