(1) selection of brazing joint type

The types of brazing joint include butt joint, lap joint, t-joint, flanging joint and socket.Commonly used are lap joints, sockets, t-joints and flanged joints.The brazing joint of the butt joint has lower strength than the base material and is only suitable for welding unimportant or low-load welding parts.But sometimes must use the butt type, can take the oblique butt type, in order to increase the contact area of the joint.The size of the contact area shall be determined according to the thickness of the weld and the workpiece conditions.

(2) selection of brazing joint clearance

The size of brazing joint gap is related to the parent metal and the type of filler metal and brazing agent, as well as the brazing method, brazing temperature and the placement of filler metal.The influence of metal expansion coefficient should also be considered when brazing heterogeneous metal joints.

In a certain range, the densification of drill joint can be improved by reducing the clearance of drill joint.However, if the clearance of brazing joint is too small, the densification will decrease because of the filling difficulty of filler metal.If the gap is too large, the filler will not be able to fill the brazing joint due to the gap capillary action.In short, the size of brazing joint clearance has an important effect on the density and strength of brazing joint.Suitable brazing joint clearance enables the joint to obtain its maximum strength.The clearance of common metal material brazing joint is shown in the following table.

(3) selection of filler metal and brazing agent

In flame brazing, the suitable filler and brazing agent should be selected according to the use requirements of brazing joint and the type of base material.When selecting filler metal, the operating temperature and strength requirements of the brazing joint should be considered, and the corrosion resistance, electrical conductivity, thermal conductivity, wettability of the filler metal to the base metal, and the interaction between the filler metal and the base metal should also be considered.When choosing brazing agent, not only the kind of brazing metal (base material), but also the type of solder used and brazing method should be considered.For common metal flame brazing the choice of solder and brazing agent see the following table.

(4) clean before welding

Before brazing, if the brazing is not clean, there is dirt in the brazing joint, will produce filler filler fill brazing joint and poor bonding and other defects, so that the brazing joint strength, therefore, before brazing must be cleaned.The grease on the welding surface can be cleaned by acetone, alcohol, gasoline or carbon tetrachloride.In addition, the use of hot alkali solution to remove oil stains can also achieve good results.Such as iron, cu-ni alloy parts can be immersed in an aqueous solution of 10% sodium hydroxide (NaOH) at 80 ~ 90℃ for 8 ~ 10min or immersed in an aqueous solution of 10% sodium carbonate (Na2CO3) at 100℃ for 8 ~ 10min to achieve the purpose of oil removal.For small, complex, high-volume parts, ultrasound can also be used to clean.Rust spots and oxides on the welding surface are usually removed by means of file, emery, emery cloth, abrasive wheel, sand blasting or chemical etching.The chemical etching method is suitable for mass production.When using the method of chemical etching to prevent excessive corrosion of welding parts surface, chemical etching should be immediately after the neutralization treatment, and then in cold or hot water rinse clean, and dry.

Do not touch the joint by hand after the joint surface treatment. The cleaned joint should be brazed as soon as possible to avoid oxidation of the welded parts at room temperature.

In order to ensure the brazing joint gap, there should be reasonable surface roughness requirements for the brazing joint joint surface, which should generally reach above Ra6.3. If the roughness requirements for the brazing joint surface are too low, the joint gap may be too large.If the roughness of the joint surface is too high, not only the processing is difficult, but also the joint clearance is too small.

(5) gas welding flame brazing operation technology

1.Heat the welding part with the outer flame of a slightly carbonized flame, and the flame core is 15 ~ 20mm away from the surface of the welding part to increase the heating area.

2.When the brazing is heated to a temperature close to the solder melting temperature, the brazing agent can be applied immediately and the brazing agent can be melted by heating with an outer flame.

3.When the brazing agent is melted, immediately contact the solder with the welded part heated to a high temperature, and let it melt into the brazing gap.When the liquid solder flows into the gap, the distance between the flame core and the solder should be increased to 35 ~ 40mm to prevent the solder from overheating.

4.In order to increase the melting and diffusion capacity between base metal and filler metal, the brazing temperature should be increased appropriately.However, if the temperature is too high, will cause the brazing joint overburning, so the brazing temperature should be generally controlled at 30 ~ 40℃ above the solder melting point.At the same time, according to the size of the welding parts, the heating duration should be properly controlled.

5.After brazing, the flux and slag should be removed quickly to prevent corrosion.For welding parts prone to crack after brazing, it is necessary to conduct thermal insulation and slow cooling or low temperature tempering immediately after brazing.