Flux for hard soldering

Various metals and alloys Will try to react with elements or substances in the atmosphere, especially when the temperature rises. In welding wire The most common reactions are: Oxidation can sometimes form nitrides and carbide. These reactions will make the solder more difficult.

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To prevent such reactions, flux, gas atmosphere, and vacuum are required to cover the work while soldering. but must be chosen appropriately. This is because some gas atmospheres can make work brittle, such as titanium, zirconium, columbia, and tantalum. It becomes brittle when solidified in an atmosphere mixed with hydrogen, oxygen and nitrogen. And in the case of brazing, copper work must be careful with hydrogen embrittlement.

Effective protection of soldering surfaces is The solder wire must be covered with flux throughout the soldering area until the soldering temperature is reached. and the flux must remain effective throughout the soldering cycle.

While soldering, the solder replaces the flux. Therefore, the important properties of a flux are viscosity, surface tension and surface energy between the flux and work. Therefore, the flux must be selected according to the work metal and the soldering temperature.

Some solid soldiers are added with deoxidizer elements such as phosphorus, lithium. and other elements that combine well with oxygen, making the solder as a self-fluxed solder can be used without external flux or atmospheric manipulation. But if using flux with or controlling the atmosphere as well It will make the solder have better wetting ability.

2.3.1 Classification and selection of fluxes

Fluxes are divided into five major groups, each of which have different suitability The flux must be reactive throughout the temperature and soldering time. If the soldering time is long, it should be used. The flux is less reactive but lasts a long time. If the buckling time is short, a flux with a short but intense reaction life should be used. If there are several fluxes with similar properties, a more economical and safer type must be considered.

Choosing the right flux for the job and the hardening method There are the following considerations:

  1. Dip brazing requires a flux that is free from water or moisture.
  2. Resistance brazing requires a conductive flux to allow current to flow through it. Therefore, wet flux should be used.
  3. Choose a flux that is easy to clean.
  4. Opt for a flux that is neither corrosive nor minimal.

2.3.2 Flux use

Soldering fluxes are available in powder, cream and liquid form, which must be selected according to the type of work, and the method of brazing. Generally, cream type is preferred because it is easy to use and has good adhesion to the solder surface. Cream or dry powder flux particles should be uniformly small. In the case of cream, it should be warmed before use.

Powder flux can be used in a number of ways, for example:

  1. Used in dry powder condition.
  2. Mix with water and alcohol to make cream.
  3. Preheat the solder wire and dip it in the flux tin, where the flux will adhere to the solder wire surface. (In the case of soldering with a gas welding nozzle)

The liquid flux is the solution. Use the spray method or apply to the soldering area. If you want to fill the solder joints to be soldered, you must use the powder solder mixed with the flux.

When using flux in solidification, the following precautions must be taken.

  1. After cleaning grease or oil and/or before soldering, do not touch the workpiece. or, if necessary, minimal exposure.
  2. The powder flux must be completely dry before use.
  3. If the flux is separated from the container for use, the remainder must be kept clean. if not sure The flux will be clean, discard it. and use new ones to be more economical

2.3.3 Residual Flux Removal

After solid soldering The residual flux on the workpiece must be removed quickly. and complete, if there is residual flux, it can easily cause the work to corrode. If the joints are well designed and soldered with the correct method The solder will completely replace the flux. The flux is expelled to cover the solder surface which when solidified. It will be brittle and look like glass coated on solder joints.

Flux removal from work is generally accomplished by rinsing in hot water and air drying. If the solder joint is tolerant of some thermal shock, the flux should be removed by immersing the hot work in cold water where the flux will easily chip away from the work.

If the flux is firmly attached, it can be removed by dipping it in chemicals or using mechanical methods such as polishing with a wire brush, firing with steel beads or extracting it. Which must be considered according to the nature of the metal work is important, for example, stainless steel work should be polished with a stainless steel welding wire brush. Aluminum and copper work must be cleaned in a way that does not damage the surface. If the solder is alloyed with silver or copper, it should not be washed with nitric acid.