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Brazing knowledge Summary(4)

Jan 28, 2019

Brazing is to use a metal whose melting point is lower than that of the base material as a brazing material. After heating, the brazing material melts, the weldment does not melt, the base material is wetted by the liquid brazing material, the joint gap is filled and the base material is mutually diffused, and the weldment is firmly fixed. Connected together.

Brazing is divided into soldering and brazing depending on the melting point of the solder.

Soldering

(1) Soldering: The soldered solder has a melting point of less than 450 ° C and a low joint strength (less than 70 MPa).

Soldering is commonly used in the welding of conductive, airtight and watertight devices in the electronics and food industries. Soldering with tin-lead alloy as the brazing material is most commonly used. Solder generally requires a flux to remove the oxide film and improve the wetting properties of the solder. There are many types of fluxes, and the electronics industry mostly uses rosin alcohol solution for soldering. The flux after soldering has no corrosive effect on the workpiece and is called a non-corrosive flux. The flux used for welding copper, iron and other materials is composed of zinc chloride, ammonium chloride and petrolatum. Fluoride and fluoroborate are used as fluxes for the welding of aluminum, and hydrochloric acid plus zinc chloride is used as the flux. The residue after soldering of these fluxes has a corrosive effect, called a corrosive flux, which must be cleaned after soldering.

Brazing

(2) Brazing: The brazed solder has a melting point higher than 450 ° C and a higher joint strength (greater than 200 MPa).

Brazed joints have high strength and some work at high temperatures. Brazing solders are available in a wide variety of applications, with aluminum, silver, copper, manganese and nickel based solders being the most widely used. Aluminum-based solders are commonly used for brazing aluminum products. Silver-based and copper-based solders are commonly used for brazing of copper and iron parts. Manganese-based and nickel-based brazing fillers are often used to weld stainless steel, heat-resistant steel and high-temperature alloys that work at high temperatures. Soldering materials such as ruthenium, titanium, zirconium, refractory metals, graphite, and ceramics are commonly used as solders such as palladium, zirconium, and titanium. When selecting solder, consider the characteristics of the base material and the requirements for joint performance. The brazing flux is usually composed of a chloride and a fluoride of an alkali metal and a heavy metal, or a borax, a boric acid, a fluoroborate or the like, and can be formed into a powder, a paste or a liquid. Lithium, boron and phosphorus are also added to some solders to enhance their ability to remove oxide film and wet. After soldering, the flux residue is cleaned with warm water, citric acid or oxalic acid.

Note: The contact surface of the base metal should be very clean, so use flux. The function of the flux is to remove oxides and oil impurities on the surface of the base metal and the solder, to protect the contact surface of the solder and the base material from oxidation, and to increase the wettability and capillary fluidity of the solder. The melting point of the flux should be lower than that of the solder, and the flux residue should be less corrosive to the base metal and the joint. The flux commonly used for soldering is a rosin or zinc chloride solution, and the usual flux for brazing is a mixture of borax, boric acid and basic fluoride.

Process method editing

The main process parameters of the brazing process are the brazing temperature and the holding time, that is, the amount of heat absorbed. The brazing temperature is usually chosen to be 25 - 60 ° C above the liquidus temperature of the solder to ensure that the solder fills the gap.

The brazing holding time depends on the size of the workpiece and the severity of the interaction between the brazing material and the parent metal. The holding time of large parts should be longer to ensure even heating. The solder has a strong interaction with the base metal and the holding time is short. In general, a certain holding time is necessary to promote the mutual diffusion of the brazing filler metal and the parent metal to form a firm bond. However, excessively long holding time will cause defects such as corrosion.

Ceramic metal brazing

There are many common methods of brazing, mainly based on the equipment used and the working principle. If it is divided by heat source, there are infrared, electron beam, laser, plasma, glow discharge brazing, etc.; according to the working process, there are contact reaction brazing and diffusion brazing. Contact-reaction brazing is the use of a brazing material to react with a parent metal to form a liquid phase filling joint gap. Diffusion brazing is to increase the thermal diffusion time, so that the weld and the base material are sufficiently homogenized to obtain the same joint performance as the base material. Almost all heating sources can be used as a source of brazing heat, and the brazing is classified accordingly:

Soldering iron brazing Soldering for small, simple or thin parts.

Wave soldering For assembly and soldering of high-volume printed circuit boards and electronic components. When welding, the molten solder at about 250 °C forms a peak through the slit under the pressure of the pump, and the workpiece is welded by the peak. This method is highly productive and enables automated production on the assembly line.

Flame Brazing A flame that is combusted by combusting a combustible gas with oxygen or compressed air is used as a heat source for welding. The flame brazing equipment is simple and easy to operate, and can be heated and welded simultaneously by multiple flames according to the shape of the workpiece. This method is suitable for the welding of medium and small parts such as bicycle frames and aluminum kettle mouths.

Dip-dip brazing The part or the whole of the workpiece is immersed in a solder bath covered with flux or a salt bath having only a molten salt for heat welding. This method is uniform, rapid, and accurate in temperature control, and is suitable for mass production and welding of large components. The salt in the salt bath is mostly composed of a flux. A large amount of flux remains on the workpiece after welding, and the cleaning workload is large.

Induction brazing is a welding method that uses high frequency, intermediate frequency or power frequency induced current as a heat source. High frequency heating is suitable for welding thin walled pipe fittings. Coaxial cable and split-type induction coils can be used for brazing on the ground away from the power supply, especially for some large components, such as the welding of pipe joints that need to be disassembled on the rocket.

Brazing in the furnace The workpiece with the brazing material is placed in the furnace for heating and welding. It is often necessary to add flux, or it can be protected by reducing gas or inert gas, and the heating is relatively uniform. Furnace brazing can be divided into box brazing furnace, well brazing furnace, batch brazing furnace and continuous brazing furnace. Continuous furnaces can be used for mass production.

The vacuum brazed workpiece is heated in a vacuum chamber and is mainly used for welding high quality products and oxidizable materials. 

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