Brazing
Brazing is a versatile joining process. Through the use of heat, a brazing alloy (in its molten state) is drawn into a capillary gap between the metals being joined. The filler metal has a melting temperature above 840 °F (450 °C) and below the metals being joined. Brazing alloys are used to join similar and dissimilar metals as well as specialty metals with strong and ductile joints.
Advantages of brazing process
Brazing is probably the most versatile method of metal joining and it has several advantages over other processes.
Such as:
- High quality joints produced through brazing are strong – frequently they are stronger than parent metals;
- Ductile joints: able to withstand considerable shock and vibration;
- Brazing allows the joining of metals with dissimilar sizes and masses. It also allows the joining of dissimilar metals such as ferrous to non-ferrous, ceramic, carbides, and others;
- Post-brazing processes, such as finishing, are hardly required because brazing produces joints with smooth, neat fillets;
- Leak tight: brazing is heavily used by the HVAC industry because joints are leak tight;
- Electrical Conductivity: joints offer good electrical conductivity and are heavily used in industries that require this property.
Technical Terms Involved in Brazing
Parent Metals / Base Metals: The materials being joined.
Brazing Alloy: A filler metal with melting temperature above 840 ºF (450ºC) and below the parent metals.
Flux: It is a chemical compound applied to the joint surfaces. It prevents oxide formation during heating cycle and it is essential in the brazing process (except on controlled atmosphere environments).
Liquidus: The lowest temperature at which the alloy is completely liquid.
Solidus: The highest temperature at which the alloy is completely solid.
Melting Range: The temperature range at which the alloy melts.
Eutectic: These brazing alloys have a single melting point, rather than a melting range.
Joint Gaps: Gap between the parent metals being brazed. It will be filled with the brazing alloy.
Capillary Attraction: Force that draws the molten brazing alloy to the joint gap during the brazing operation.
Wetting: When a molten brazing alloy has the ability to flow over and adhere to the surface of the parent metal.
Preforms: Filler metals engineered to a format specific for an application, such as rings, washers, shims, and others.
Brazing Paste: These products are manufactured from the addition of filler metal in powder form to a neutral binder and/or a flux (if necessary). Brazing Paste allows reductions to the overall brazing process and also the ability to fully automate it.
Neutral Binder: It holds flux and filler metal in stable suspension.
Six steps for successful brazing
Joint design and gap clearance
Joint´s strength depends on capillary attraction, which in turn varies according to gap clearance and joint design.
Joint clearance at brazing temperature needs to be adequate.
Different coefficients of thermal expansion need to be taken into account at brazing temperature.
Use an overlap of 3-4 times the thinnest base metal when designing a lap joint.
Pre-brazing cleaning
Parent metals need to clean prior to brazing. Surfaces need to be free of oil, grease and oxides.
Oil, grease and other organic residues can be cleaned by using a degreasing solvent. Hot water and detergent can also be efficient.
Surface oxides can be removed mechanically.
Fluxing parts
Choosing the correct flux is just as important as choosing the correct brazing alloy. More information can be found throughout this catalog. Ideally, both surfaces are fluxed before assembling and heating.
Flux should be active from 122 °F (50 °C) below the solidus temperature of the brazing alloy to at least 122 °F (50 °C) above the liquidus temperature of the brazing alloy, ensuring the flux is effective during the entire brazing operation.
Assembling and Fixturing
Now that parts are properly cleaned and fluxed, we need to make sure they stay in position during the entire brazing process.
It is important that parts are aligned throughout the heating and cooling cycles in order for capillary action to occur.
Prefer using fixtures that are good thermal isolators, such as refractory materials.
Heating and applying the alloy
Different heating methods can be applied to brazing and the most common are: torch brazing, induction, resistance welding and furnace brazing.
It is important that heating is applied broadly to the base metals and when brazing temperature is reached the filler metal is then brought into contact with heated parts, melting instantly. Capillary action draws the filler metal into the joint.
Molten brazing alloys always flow to the hottest part of the joint and heating should continue as the alloy is penetrating the gap between the base metals.
Post-brazing cleaning
During the cooling cycle care must be taken in order for parts not to crack or damage due to thermal shock.
If flux was used, it is important to clean its residues after brazing.
This can be done through cleaning processes such as water quenching, mechanical action or chemical treatment.