What is Brazing ?

Brazing is the process of joining metals through the use of a molten brazing alloy.
This process is used to join most of the metals and alloys being used in today's industries and has the capability of joining different metals with dissimilar size and mass.

The dominant metals contained in the brazing alloys are Silver, Copper, Zinc, Cadmium and Tin. The liquidus temperature of brazing alloys is above 450°C and needs to be maintained at a temperature below the melting point of the base metals being joined. A successful brazing operation takes place at a temperature between 600˚C-900˚C.

In cases where a temperature of 450°C is too high, a joining process called Soldering is employed. Unlike brazing, in soldering the alloy used has a liquidus temperature below 450°C and is maintained at a temperature below the melting point of the base metals being joined.

Contrary to soldering and brazing, a Welding process fuses together the base metals by melting them together in addition to a filler metal. Obviously, the welding temperatures are significantly higher than those of the soldering and brazing working temperatures.

How Brazing Works ?

The physical principle by which a molten brazing alloy is drawn into a joint gap is called Capillary Attraction. The capillary attraction allows the molten alloy to penetrate very tight gaps and can even flow along vertical gaps. The capillary property is based on the surface-tension principle which is caused by intermolecular forces in the molten brazing alloy.

A further significant property of brazing is the ability to create metallurgical bonding.
Brazing joins the base metals through a metallurgical bonding between the brazing alloy and the surfaces of the base metals being joined.

Successful capillary action and the bonding results from the proper use of a non-metal material called Flux containing a chemical compound - Boron (Chemical symbol B).
The flux is applied to the joint area before the base metals are uniformly heated over a broad surface area. Once the temperature of the base metals reach the active temperature of the flux, the flux dissolves Oxides caused by a chemical reaction between the Oxygen in the air and the heated based metals. The Oxides prevent the molten alloy to flow and bond between the base metals and therefore must be dissolved. Furthermore, if left untreated, the Oxides can later cause cracks in the joint area. Whenever a Phosphorus copper alloy is applied to join between Copper to Copper, the Phosphorus content in the alloy serves as a "self fluxing" agent and no flux is necessary.

Once the base metals reach the working temperature of the brazing alloy, the brazing alloy is brought in contact with the heated base metals and the brazing alloy instantly melts and flows through the gap joint by the capillary attraction. During this process, the brazing temperatures are lower than the melting points of the base metals.

The Advantages of a Successfully Brazed Joint
Strength of the joint: The tensile strength of the brazed joint often exceeds that of the base metals joined.
Ductility of the joint: The joint will withstand considerable service conditions, shocks and vibrations.
Dissimilar base metals: Brazing is perfectly suited to join dissimilar metals. Assemblies of ferrous with nonferrous are easily joined.
Working temperature: The brazing temperature is relatively low and reduces the risk of overheating and deformation of the base metal.
Single operation stage: After completing the brazing process, no further mechanical operation is usually required.
Automation: Flexibility and adaptability of the brazing materials enable factories to integrate brazing techniques into their mass
production lines.
Economical procedure: Compared to any other joining procedure, brazing is economical in terms of cost per joint.
Why Should You Choose Brazing Over Welding?

Several options should be taken into consideration when joining metal parts, including adhesive bonding and mechanical fasteners. But if strong and permanent joints are essential, there are only two practical choices: welding or brazing.

Welding is done by applying a pinpointed and intense heat directly to the joint area in order to melt and fuse the metal parts together, in addition to adding a filler metal. The joints produced by Welding are usually as strong as the metals joined or even stronger.

In cases where a temperature of 450°C is too high, a joining process called Soldering is employed. Unlike brazing, in soldering the alloy used has a liquidus temperature below 450°C and is maintained at a temperature below the melting point of the base metals being joined.

Brazing is done by applying uniform and significantly lower temperatures over a broad surface area and does not entail the melting of the base metals. Instead, a brazing alloy is melted and drawn into the gap joint through the capillary attraction. Similar to welding, the joints produced by brazing are usually as strong as the metals joined or even stronger. However, compared to welding, the low working temperatures of brazing results in fewer energy requirements, physical properties modifications, and deformation and stress in the joint area.

Factors to be consider before choosing between welding or brazing:
Base Metal Sizes: For large base metal assemblies, welding is the appropriate joining technique. Typically, large metal assemblies transfer the heat through the base metal therefore making it difficult to reach the appropriate working temperatures of the brazing alloy. Therefore, by applying intense heat directly to the joint area, welding excels in joining large metal assemblies
Thin Elements: Since the intense heat of welding can warp, burn and distort thin elements, brazing is the appropriate joining technique for small and thin assemblies.
Spot and Linear Joints: Both techniques meet spot (butt) joint requirements; but welding is more efficient for this joint type. On the other hand, with a linear joint, welding requires tracing along the joint while brazing does not need any tracing since the molten alloy is drawn through the linear joint by the capillary attraction.
Base Metals Melting Temperatures: With respect to dissimilar base metal melting temperatures, brazing is the preferred technique. A strong joint can be achieved through brazing without melting the base metals and with minimal changes in the metal properties when joining dissimilar metals with significant gaps in their melting points. Welding would fail to join the metals as one metal would melt completely while the other metal would remain solid.
Medium and Mass Production: For a single manual job or even for a few assemblies, one should decide between brazing and welding according to the factors being discussed above. But, if production techniques and cost efficiency are crucial, brazing is the logical choice. For high volume operations, there are plenty of techniques to improve brazing productivity. For example, a rotating table or a conveyor which is equipped with pre-heating torches and pre-placed brazing alloys which applied in a preset amount and time. Alternatively, sophisticated heavy duty welding equipment is more expensive with much less flexibility in terms of matching your automation investment to your production requirements.
Basic Steps for Successful Brazing
1. Designing of the gap joint.
In order to enhance the capillary action, gap joints should always have narrow clearance. If base metals have different thermal expansions it will affect the joint gap at brazing temperature. When choosing a lap joint, you should consider an overlap length of 4 times the thickness of the thinnest base metal part in the joint. For a tubular diameter size up to 25mm consider joint overlap of the tube diameter.
2. Selecting the appropriate brazing alloy.
In order to choose the optimal alloy, it is necessary to consider the gap design, the base metals, the brazing method and cost efficiency. If brazing Copper to Copper, Phosphorus Copper alloys are recommended. These alloys contain Phosphorus and are self-fluxing on Copper. If brazing Brass or Bronze fittings, Phosphorus Copper alloys can be used but with an appropriate flux. If brazing Iron, Steel or other ferrous metals, select one of the Cd bearing or Cd free alloys with with an appropriate flux. It is imperative that Phosphorus bearing alloys not be used as the joint may be brittle.
3. Pre-Cleaning of the components to be brazed.
The joint surface areas should be clean and free from oil, grease, or oxide contamination. Surfaces may be properly cleaned for brazing by brushing with a stainless steel wire brush or by a stiff rubbing with emery cloth. If oil or grease is present, clean with a commercial solvent. Remember to remove small foreign particles such as emery dust, by wiping with a clean dry cloth. It is extremely important that the joint surface be clean.
4. Applying flux.
When brazing a massive pieces of metal, the heating cycle will take longer and therefore more flux is necessary. You can add some additional flux by dipping the heated brazing rod in the flux. An insufficient amount of flux will quickly become saturated and lose its effectiveness. A flux that absorbs less oxides will not only insure a better joint than a totally saturated flux, but will also allow for an easier post-cleaning procedure of the brazed joint. Flux can also reduce any overhearing of the parts by serving as a temperature sensor. At a temperature of 100°C the water in the flux boils. At an approximate temperature of 300°C, the flux turns white and begins to puff up, and starts to absorb the oxides. At an approximate temperature of 400°C the flux spreads against the surface and has a milky appearance. At an approximate temperature of 600°C the flux becomes transparent and looks like water. Apply the brazing alloy to the base metal. If it melts, the metals are at the proper temperature for brazing.
5. Heating and applying the brazing alloy correctly.
Both metals in the assembly should be heated and change in appearance uniformly so they reach brazing temperature at the same time. When brazing metals with different heat transfer rates, the metal with the higher rate will need more heat than the metal with the low rate. In order to prevent overheating when brazing small elements, do not heat the brazing area directly but rather keep moving the torch around it. For large elements, heat a the area around the joint. When joining a large section to a smaller one, sometimes the surrounding heat from the flame may be sufficient to heat the small part. Place the alloy adjacent to the heated joint and a segment of it will melt and be drawn by the capillary attraction into the joint gap. Since the molten alloy always tends to flow towards higher temperature areas, it is advisable to heat the side of the assembly opposite to where the alloy will be applied. In cases of tubular assembly, apply the alloy at the gap joint. Because of the applied heat, the molten alloy tends to wet the outer surface of the tube which is hotter than the inner tube.
6. Removing flux residues.
Flux residue is chemically corrosive and can cause damage to the brazed joint and therefore needs to be removed. Most common brazing fluxes are water soluble and can be easily removed by immersing the brazed assembly in hot water (around 50°C) while it is still hot. If flux residues will not crack off, brush them with a wire brush while the assembly is still in the hot water. If an insufficient amount of flux is applied during the brazing procedure, the flux will be saturated with oxides. In this situation, the flux residues will be tough and immersing the brazed assembly in hot water will not be sufficient to remove the residue. Placing the assembly for 1-2 minutes in a 25% hydrochloric acid solution around 60°C will usually dissolve difficult residue. It is important to remember to wear a face shield and gloves when handling hydrochloric acid.
Procedure for Brazing Pipe and Tubing
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