Joining two metallic surfaces is not a new thing in industrial applications. Various metal-joining processes are especially developed to create a strong bonding of two surfaces. These processes range from welding and mechanical fastening to adhesive bonding. Various governing factors affect the selection of these processes. We are going to discuss one such process called brazing.
Brazing is one of the simple forms of metal joining techniques. In this technique, two similar or dissimilar metal surfaces are bonded with the help of a filler material. Brazing is one of the most ancient technologies that date back to before 2000 BC. Since then, the process has stuck to its principle with advancements in filler materials. Today, we will discuss this technique in detail.
What Is The Copper Brazing Technique?
The copper brazing process is a metal bonding process that involves high temperatures. It is similar to the welding technique with one key difference: the use of a copper filler. The copper filler is melted by supplied heat above its melting temperature. This molten copper is poured between two surfaces that need to be joined. Upon cooling, a strong joint is produced with good load-bearing properties.
This technique might sound familiar with another metal joining process called soldering. Both brazing and soldering processes share the same working principle. However, the soldering process is usually done at temperatures below 400 degrees Celsius. On the other hand, copper brazing is done at a temperature range of 1050 to 1550 degrees Celsius. The annealing of copper at this temperature range creates a strong joint that offers continuous load distribution.
Necessity Of Copper Brazing
A common question may arise: Why is copper brazing used even with the availability of various metal joining processes? The answer lies in various advantages offered by brazing over other techniques. Other techniques involve a broader heat-affected zone which distorts the surrounding metal properties. This heat-affected zone in the brazing process is very small. Hence, the molten copper filler does not burn through thin surfaces.
Brazing is ideal for joining small workpieces at a faster rate. It also saves overall costs due to a low power requirement. The joint created by brazing does not need any type of heat treatment. Brazing helps in joining two dissimilar metal workpieces as the bonding is initiated by filler and not parent metals.
Flux In Copper Brazing
Flux is a substance used in applications involving molten metal. The flux acts like a cleaning agent which keeps the brazing spot free from any type of impurities. It is also responsible for creating good-quality brazed joints by eliminating oxides. Flux is selected to be used at the same temperature as a brazing process. In appearance, it looks like a white paste or glue. It is applied around the spots to be brazed.
The flux paste is a combination of fluorides and chemicals mixed at the required composition. The selection of flux is dependent on the base metal and filler material. Different types of water-based flux are available that possess properties suitable for brazing.
Aluminum-Magnesium Flux: This flux combination offers good-quality chemical reactions at lower temperatures. It helps in dealing with the oxidation of aluminum workpieces.
Borax Flux: This flux is popular for brazing as well as the welding process. The flux tends to react with the material’s oxide layer and lowers the melting point of the filler material. Hence the filler becomes more flowable and penetrates gaps easily.
Alkaline Flux: This flux is usually used for very high-temperature applications. The alkaline borates help in maintaining the temperature of the base metals.
Silver Flux: Silver flux is a combination of potassium borate and other chemicals. It is used for low-temperature applications involving metals like cadmium-silver alloys.
Working Of Copper Brazing Process
The copper brazing process is one of the simple metal joining processes. The overall governing factors are low and hence the working of the brazing process is easy to understand.
During the preparation phase, the brazing setup is created, and all necessary equipment is assembled. Proper marking of the workpiece is done to guide the filler wire. The surface is cleaned with abrasive paper to remove any type of burr and dirt. The flux paste is introduced in this stage. Proper coating of the surfaces with flux is done to ensure smooth brazing. After this step, the workpieces are fixed at their respective positions.
The surface to be brazed is preheated with the help of a blowtorch. The heating phase is not overdone to keep the flux layer from burning. When temperature is favorable for brazing, the filler wire is held between the maintained gap of the workpieces. The wire and torch come in contact to start melting filler metal.
The flux helps in increasing the flow of filler and creates a protective layer against oxidation. By maintaining the flame and wire movement, the gap is filled with filler material. As soon as the gaps are filled, the heat source is removed and the joint is kept for natural cooling. The flux on the surface is cleaned and the joint is inspected for any visible cracks.
Frequently Asked Questions:
Now that we have discussed the brazing process in detail, some confusion might arise in your mind. To address these confusions, we have answered some common FAQs below. Going through each question will surely help you clear every single doubt related to copper brazing.
Ans: When the filler metal is melted, its color represents the actual temperature. This helps in maintaining the filler temperature within the brazing range. The melted filler metal flows into the gap with the help of flux. At this stage, the annealing of the filler material takes place, creating an alloy joint between the workpiece. After cooling, this filler converts into a brazed joint.
Ans: Flux is a combination of chemicals and fluoride, compatible with the base metal. When brazing or soldering, the flux increases the flowing ability of filler material. It also helps in maintaining the heat-affected area limited to the heated point. This leads to the elimination of impurities and creates a smooth brazed joint.
Ans: In brazing, the heat source plays an important role. A neutral flame is ideal for brazing in which perfect combustion of the propane gas takes place. A continuous motion of the propane torch is required to ensure the flux layer is not burnt. Since the propane torch also provides heating to a high-temperature range, it is suitable for brazing.
Ans: Due to its small heat-affected zone, brazing is suitable for copper pipes and tubes. By ensuring proper use of flux and heating, the copper surface can be brazed without damaging material composition. Even though the thickness of copper pipes and tubes is less, the brazing process is effective for them.
Ans: One of the plus points of copper brazing is its compatibility with other metals. Copper brazing can be done between similar as well as dissimilar metals, thanks to the wider choice of filler materials. With the help of suitable filler composition, the brazing of these two metals is possible.
Safety Measures For Brazing Process
The brazing process is one of the safest metal-joining processes. Due to the elimination of risky elements like high-voltage electricity, the overall setup becomes user-friendly. Still, certain safety measures must be taken to avoid any hazards. These measures also help in keeping the brazing process free of defects.
- A gas torch is used as the heat source for brazing. Checking for gas leakage must be done before lighting the torch.
- The brazing process emits burnt gasses and toxic fumes that are harmful to humans. Proper ventilation for quick removal of these fumes is equally important.
- Cleaning of the surfaces ensures the removal of any foreign particles. These particles can cause creeps in the brazed joints, weakening the internal composition.
- Flux must be applied in good quantity. It ensures the proper flow of filler material and helps in cleaning the joint while brazing.
- Before brazing, proper research on the filler and base metal must be done. The selection of flux, brazing temperature, and various important factors are dependent on this information.
Difference Between Uncoated And Flux-coated Filler Rods
As discussed earlier, flux plays an important role in the brazing process. By using compatible flux, a non-defective brazed joint can be achieved. The flux is no longer limited to being a paste or powder. Upgradation in technology led to the discovery of flux-coated filler roads.
These rods are a good alternative to the regular flux paste. Both filler rods share the same application. A comparison chart is presented below to learn about some key differences.
| Uncoated Filler Rods | Flux-coated Filler Rods |
| These filler rods do not come with a flux coating. The flux application process is manual. | These rods are treated with a flux layer. Hence, it does not need extra flux. |
| It can be covered with flux suitable with the base metal. | The pre-coated flux layer needs to be scrapped off in case the metal composition fails to match. |
| Overheating of the flux layers burns it. Scraping this burnt layer is easy. | Overheating causes the formation of hard residues that are not easy to remove |
| They are not compatible with automation setups. | These rods can be used with automation machines. |
| These rods are cheaper. | The flux-coated rods are expensive. |
Conclusion:
The copper brazing process is a simple yet effective process for joining metallic workpieces. The brazed joints possess very good strength and can be used with dissimilar metals. By controlling important factors, this process becomes a cheaper alternative to other metal joining processes. With this article, every aspect of copper brazing is already covered. Along with this information, the working process and FAQ sections were also presented. We hope all this information will enhance your knowledge about this technique.
