Selection of Vacuum Brazing Fluxes

The function of brazing flux is to reduce the oxides on the tool rod and the surface of cemented carbide, so that the brazing metal can be well wetted. Generally, the melting point of filler metal is lower than that of the filler metal above 100 C, and it has better fluidity and lower viscosity. The flux melted in the brazing heating process can protect the brazing filler metal and the brazing surface, at the same time, it can reduce the oxide.

The main requirements for cemented carbide solders are as follows.

(1) The brazing filler metal should have good wettability to the brazed cemented carbide and steel matrix to ensure that the filler metal has good fluidity and permeability.

(2) One of the characteristics of cemented carbide is its high red hardness, so it is necessary to ensure that the brazing seam has sufficient strength at room temperature and high temperature.

(3) The melting point of brazing filler metal should be as low as possible to reduce the brazing stress and prevent cracking, but the melting point of brazing filler metal should be higher than the working temperature of welding seam 300 °C, so as to ensure that the tool can work normally in high-speed cutting.

(4) The brazing filler metal should not contain elements with low evaporation point, so as to avoid the influence of evaporation of elements in brazing filler metal on joint quality or harmful to human health.

4.1 The blade is made of 40Cr steel and tungsten-cobalt-titanium cemented carbide.

4.2 The filler material is BCu (Cu99.95%) with thickness of 0.1 mm, melting temperature 1083 °C and brazing temperature 1100-1150 °C.

The brazing flux is YJ1, the main component is borax, and the brazing temperature is 800-1150 °C. Hard flux QJ201 can also be used. The composition of the flux is 80% boric acid, 14.5% borax and 5.5% calcium fluoride. The brazing temperature is 850-1150 °C.

Borax is the most commonly used brazing flux for cemented carbide and steel. The application scope of various borax should be paid attention to.

(1) industrial borax (borax) should be dehydrated before use. Because industrial borax contains 10 parts of crystal water (Na2B4O7.10H2O), plus a large amount of water absorbed in the air, a large amount of foam will be generated during the brazing process, which will not only make the brazing operation difficult, but also affect the quality of the weld.

(2) Dehydrated borax (Na2B4O7) can be used to braze cemented carbides of various brands. The brazing temperature ranges from 850 to 1150 °C. It is suitable for brazing fluxes such as copper, brass, copper-zinc and silver solders, but not for brazing solders with melting point below 800 °C. When dehydrated borax is stored, attention must be paid to moisture-proof. If it is damp, it should be dried before it is used.

The dehydration treatment of borax is to place industrial borax in a steel crucible. The crucible can be heated in a resistance furnace or coke oven at 850 degrees. During the heating process, a large amount of white foam will emerge from the borax in the crucible. With the increase of heating temperature, the foam gradually decreased until the foam disappeared. When all borax melts into liquid, the heating stops. The green and transparent liquid borax in the crucible is poured into the iron box. After cooling, it will automatically break into green glass fragments. The color of dehydrated borax is related to the melting time. The longer the melting time, the darker the color of borax, but it has no effect on the quality of brazing. The melted borax blocks are crushed and sifted with sieves of 60 to 80 meshes and then put into bottles for use.

(3) The mixed flux of dehydrated borax (85%-90%) and potassium fluoride (10%-15%) is mainly used for brazing YT60 and YT30 cemented carbides. Due to the toxicity of potassium fluoride, strong ventilation equipment must be installed near the heating equipment during brazing to discharge harmful gases in time.

Fluoride and chloride have strong water absorption. When preparing fluxes containing fluoride and chloride, dehydration treatment should also be carried out. Dehydration is the heating of fluoride and chloride in a ceramic or stainless steel crucible to about 270  °C for 3 to 4 hours until no smoke is emitted. Fluoride and chloride after dehydration should be stored in sealed glass containers.

When using fluoride or chloride fluxes, attention should be paid to ventilation, and harmful gases should be eliminated in time. After brazing, the welding parts should be cleaned in time to avoid the residual flux corroding the weld seam and the base metal. Most fluxes are easy to absorb moisture, we should pay attention to sealing and preservation, so as to be available.

Flux can also be pre-prepared into paste, paste flux is prepared by adding a small amount of alcohol, rosin oil, vaseline oil and other fluxes into paste, applied to the surface of the workpiece to be welded. When using low temperature silver filler metal, the filler metal can also be used in paste form with water. The paste filler metal should be prepared now.