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Brazing is a process used to join metals and sometimes ceramics in which a molten filler metal called a braze alloy is used for joining. The melting point of the braze alloy is usually above 450°C, which is much lower than the melting temperatures of the joining parts.
The filler metal is lightly heated over its melting point. Once the molten filler cools down, the workpieces will bind together to form a strong joint between the metals.
The brazing process is carried out in different atmospheres, including normal air, ammonia, combusted fuel gas, hydrogen, nitrogen, inorganic vapors, noble gases, and a vacuum. The metal joining technique can also use different heating sources, such as an induction coil, a furnace, or a torch.
The metallurgical compatibility between the filler and the materials to be joined is essential to achieve a good brazed joint. In addition, the gap where the molten filler will be distributed also contributes to the quality of the joint.
Brazing is a commercially accepted and commonly used process for joining dissimilar metals. Its flexibility and the high integrity of the joints make brazing a reliable metal joining method under many different conditions.
Types of Braze Filler Materials
Braze filler materials are available in various forms that you can choose from depending on your specific production requirements. Some of the popular forms of braze filler materials include:
Braze Powder: These are homogenous, uniform particles containing all the elements of a particular alloy.
Braze Paste: This type of filler material is available in the form of a paste composed of one or more braze alloy powders and a neutral, flux-free binder.
Braze Tape: This form of filler comes in the form of a uniform roll of material and binder, which is made to order with a specified thickness and width.
Braze Preforms: These are custom shapes cut from braze tape with an adhesive back that you can easily apply over the workpiece when brazing.
Braze Foil: This is a flexible braze material containing no binder or flux. You can cut the foil into your desired shape and tack weld it onto a part for brazing.
Ideal Brazing Temperature
Temperature plays a vital role in getting a good quality brazing joint. The pieces to be joined are normally heated above 450°C (840°F). Most brazing processes happen at temperatures between 800°F and 2,000°F. For the braze joint to be strong, the metals joined together must be near the same temperature.
Temperature also has other impacts on brazing. As the temperature of the braze alloy increases, the wetting action and alloying of the filler metal becomes better as well. Hence, the ideal brazing temperature should be above the melting point of the filler metal used.
However, several other factors influence the selection of an ideal brazing temperature. An ideal temperature to join two pieces is usually selected to:
- Minimize the effects of heat on the assembly
- Be the lowest possible temperature to melt the filler metal
- Minimize the interaction between the filler metal/base with metal
- Maximize the life of the jigs or fillers used
Sometimes, a higher temperature is preferred to accommodate additional factors, such as using a different filler metal, removing surface contamination, or controlling metallurgical effects.
Some of the commonly used alloys and their brazing temperatures are listed below:
|Silver with Lithium
Basic Brazing Procedure
Brazing must be carried out carefully to achieve the best quality results. Following are some valuable tips that you can follow to achieve perfect brazing joints.
Determine the Spacing of the Joint: The correct spacing is essential for the capillary action of the filler metal. The space needed for the filler metal to fill the joint area is usually between 0.25 mm and 0.127 mm.
Use of the Right Brazing Alloy: Commonly used filler metals include silver, aluminum alloys, and copper. Silver is frequently selected because of its relatively low melting point. Copper has a higher melting point than silver but is more economical. Alloys can be made in the form of a stick, paste, or preform, depending on the application.
Eliminate Contaminants and Grease: The proper flow of the brazing material is necessary for brazing, and this flow can be restricted if there are rust particles, dirt, or grease present in the path. Therefore, all contaminants must be removed before brazing. A degreasing solvent, chemical bath, steel wire brush, or emery cloth can be used for this process.
Add Flux or Carry Out the Process in a Suitable Atmosphere: The use of flux helps prevent oxidation when the metal is heated, protects the alloy, and improves its flow rate. To eliminate the use of flux, you can carry out the brazing process in a protected atmosphere such as hydrogen, nitrogen, or disassociated ammonia. The absence of oxygen in such an atmosphere prevents oxidation and hence prevents the need for flux.
Proper Positioning of the Parts: Ensure that the joining parts are correctly positioned and brazed before applying heat for accurate alignment. You can use jigs and fixtures such as clamps, weights, vises, and supports to lock the positions of the parts.
Start Heating: The brazing process is carried out between temperatures of 800°F and 2,000°F. For a strong bond, the metals you plan to join together must be close to the same temperature.
Clean the Joint: Once brazing is done in the open atmosphere, you must remove the flux residues, as they are chemically corrosive and have the potential to weaken the joints. You can remove these residues using hot water or specific acid solutions such as hydrochloric acid or sulphuric acid.
Of all the joining methods, brazing is considered one of the most versatile. The joint, after brazing, will have high tensile strength and is often stronger than the metal pieces joined together. Brazed joints also have excellent properties like gas and liquid repulsion, resistance to vibration and shock, and resistance to normal temperature changes. Finally, the metal pieces joined together will retain their original metallurgical properties since the joining process doesn’t involve melting the base metal.