You will come across many applications where dissimilar metals or metals and nonmetals need to be joined, but fusion welding is not practical for multiple reasons. In such situations, torch brazing can be a good option if the working temperature and joint strength requirement are not severe.
This article is about torch brazing, types of torch brazing, different types of filler metals, and applications of a torch brazing.
What is torch brazing?
As the name indicates, torch brazing uses a hot burning gas or torch to join two or more base metals by brazing. The base metals or workpieces are joined with a braze joint made from molten filler metal.
There are two essential things you have to note here:
- The filler metal selected has a melting point much less than the base (workpiece) metal.
- The base metal will remain in the solid-state during the brazing process, and the molten filler metal fills the joint due to capillary action and joins the two workpieces together with the braze joint.
The torch brazing process is simple compared to welding, can be learned easily, and does not need much investment. Torch brazing is suitable for joints that do not undergo severe force and severe temperature.
Torch brazing can braze a combination of dissimilar metals or metals and nonmetals and is also called a solid-liquid process.
Torch brazing is a versatile process; it can be used for DIY projects, for small production batches, and mechanized for large production batches.
Fuel gas used for torch brazing
Depending on the temperature required for brazing, you can use a combination of a fuel gas (acetylene or propane) and air, compressed air, or oxygen. You can use a torch with a single tip or a torch with multiple tips; multiple tips help you to provide uniform heat at a faster rate.
You need less heat for brazing small components or components with thin sections, and a mixture of air and propane or air and acetylene can be used for such applications.
MAPP gas torches are small brazing kits suitable for DIY kinds of works and are used for manual brazing. They are ideal for brazing at low temperatures, and thin section brazing works.
Oxyacetylene torches are the most popular and versatile type used in a home garage or small workshops. You need two cylinders containing oxygen and acetylene, a torch, and a set of regulators and hoses to connect the cylinders to the torch.
You can obtain the required type of flame by varying the flow of oxygen and acetylene, and the preferred flame for torch brazing is neutral or reducing type. The gas torch has interchangeable tips, and propane can also be used in place of acetylene.
Braze joints
Both lap and butt joints are used. To increase the area of the joint, sometimes, the butt joint is prepared with slanting edges instead of straight edges. For the same workpiece thickness slanting, edges give a joint with a higher area.
Braze Flux
Fluxes help remove the leftover surface contaminants, oxides, oil, and grease and prevent further oxidation of the base and filler metal. It improves the flowability of the molten filler metal by reducing its surface tension (improves wettability) and dissolves the oxides, and brings them to the surface as slag.
The flux can be in the form of powder, paste, a mixture of power and water, or a paste containing flux and filler metal. The flux used for brazing (in any form) should be chemically compatible with the base and filler metals. The slag and the traces of flux left after the brazing should be cleaned since it can corrode the base metal and the joint. Borax and boric acid are the common fluxes used.
Torch brazing is done in the following steps:
Step 1: The workpieces or base metals are thoroughly cleaned to remove contaminants like oil, grease, dirt, and also oxide coating. You can use a file, sandpaper, and a degreasing solution to do this. Ensure the brazing surfaces are perfectly clean.
Step 2: Ensure a clean brazing surface and maintain a recommended joint clearance (0.065 to 0.125 mm). Both of these are important for good capillary action. Contamination affects the wettability and flow of the molten filler metal, and narrow or excessive joint clearance affects the capillary action.
Step 3: Place the brazing assembly in the fixture. Apply the flux. You can pre-place the filler metal (in the form of a ring, washer, formed strip, or powder), or it can be fed during brazing.
Step 4: Preheat the base metals to the brazing temperature using a suitable torch. Feed the brazing rod (if not pre-placed). The base metal temperature is normally sufficient to melt the filler metal, if not provide extra heat. The molten filler metal fills the joint by capillary action and solidifies to a brazed joint.
Step 5: Take the brazing assembly out of the fixture and clean it with a wire brush and hot water to eliminate the slag and leftover flux.
The brazing temperature should minimize adverse thermal effects on the brazed workpieces, improve the life of the brazing fixtures, and minimize any reaction between the filler metal and the base metal. In the production of brazed parts, the brazing temperature is usually set to maximize the production rate.
Brazing is typically done at around 800º F/426º C or more. However, the brazing temperature can be slightly less for some brazing operations (like aluminum brazing with aluminum and zinc filler rods).
Most of the metals can be torch brazed with a few exceptions like titanium, zirconium, and some types of stainless steel (the reason can be they are highly reactive).
Types of torch brazing
Torch brazing can be of three types:
- Manual torch brazing.
- Machine torch brazing.
- Automatic torch brazing.
1. Manual torch brazing
You do manual torch brazing by holding the torch in one hand and the brazing rod in the other hand, and the outcome of the brazing depends on your skill and concentration. Manual brazing is laborious work, but it is equally versatile, and you can use your skill to do many brazing jobs in challenging to reach areas that a machine cannot reach or do.
Manual brazing is highly versatile, and an experienced and skilled worker can produce strong braze joints.
2. Machine torch brazing
This is a mix of manual and automated torch brazing techniques and is used to produce repetitive brazing operations. You can load the brazing assembly in the fixture, place the filler metal and flux at one end and take out the brazed assembly at the other end manually. The actual brazing is done by a torch controlled by the machine for speed and accuracy. Depending on the type of machine used, the brazing assemblies are manually loaded with a flux and filler metal, or the machine torch brazing can use a flux coated filler rod to do the brazing.
Machine torch brazing reduces the skill requirement of the worker and increases the production rate. The quality of brazing is consistent.
3. Automatic torch brazing
Automatic torch brazing almost eliminates the human element except maybe for loading and unloading the parts to be brazed. Some systems can make even the loading and unloading automatic.
The machine cleans the base metals, places the flux and filler metal, and torch brazing. The quality is consistent and uniform, with a high rate of production.
Different types of brazing rods or filler material
The filler metal is an essential ingredient for brazing, and it can be in the form of a rod and ribbon (with or without flux coating), powder, and paste. It can be in preformed shapes like a washer, and these are used for repetitive brazing of the same parts and for machine torch brazing. Brazing rods with or without flux coating are highly suitable for manual torch brazing.
The important properties of the filler metal are:
- A melting point much lower than the base metals.
- The molten filler metal must have low viscosity, good flowability, and the capability of wetting the base metals.
- Give a braze joint of the desired strength.
The common filler metals used are an alloy of aluminum and silicon, copper and silver, nickel alloys, gold and silver, brass (copper and zinc), bronze (copper and tin), and silver. An alloy of aluminum and zinc is used for brazing aluminum.
Some examples of filler metal and its application:
- A combination of aluminum and silicon is used for brazing aluminum, and the brazing temperature is 600º C (1112º F).
- Copper is used for brazing nickel and copper.
- A combination of copper and phosphorous is used for brazing copper and copper alloys at a brazing temperature of 850º C (1562º F).
- A combination of copper and zinc (brass) is used for brazing steel, cast iron, and nickel, and the brazing temperature is 925º C (1697º F).
- A combination of gold and silver is used for brazing stainless steel and nickel, and the brazing temperature is 950º C (1697º F).
- A combination of nickel and copper is used for brazing stainless steel and nickel, and the brazing temperature is 1120º C (2048º F).
- A combination of silver and gold are noble metals and are typically used for the brazing of jewelry work.
- Gold and palladium are combined for joining superalloys and refractory metals used in high-temperature applications like jet engines.
Application of torch brazing
The different applications of torch brazing are:
- Torch brazing is popularly used in the cutting tool industry to produce braze-tipped tools where hard metal tips (tungsten carbide and ceramic) are brazed to a steel cutter or tool body. The filler metal is an alloy of silver, copper, zinc, nickel, and other metals.
- Brazing is used for connecting cables to the railway track, and the special type of brazing used for this is called pin brazing.
- For joining metals with different chemical, physical, and mechanical properties.
- For joining metals with metallurgical incompatibility (where conventional fusion welding does not produce a sound joint).
- For joining metals like aluminum to steel or aluminum to copper.
- Brazing can be an alternative for odd position joining where welding is practically impossible.
- For joining metals with poor weldability (due to their cracking when heated or reacting with the atmosphere to lose essential properties).
- For joining pipes, tubes, wires, and cables.
A process called braze welding is used for joining steel parts. The filler metal used is bronze or brass. The process is called braze welding since the joint is made without using the capillary action. Braze welding can join dissimilar metals with minimum distortion and does not need preheating of base metals.
Related Article: Welding VS Brazing | What Is The Difference?
Conclusion
Torch brazing is an important process in the manufacturing industry since it helps to join metals that are not possible by conventional fusion welding processes. A knowledge of torch brazing helps you to offer solutions for joining different metals and nonmetals that are not possible by conventional welding.
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