Brazing is a process used to join metal surfaces by allowing molten metal to flow into the joint. This method, one of the oldest, differs from welding in that it does not melt the base metal during joining. It requires very close-fitted parts and uses a higher temperature than other joining methods. The filler metal flows into the parts through capillary action, preventing workpieces from melting.
Well, in this reading, we’ll explore what brazing is, its diagram, techniques, types, and how it works. We’ll also discuss brazing flux, filler metals, and their difference from soldering and welding.
Let’s get started!
What is Brazing?
The technique of brazing involves melting and depositing a filler metal—which has a lower melting point than the metal into the joint to connect two or more metal objects.
Brazing is not the same as welding in that the work parts are not melted throughout the process. Brazing is different from soldering in that it involves considerably closer-fitting pieces and a higher temperature.
By capillary action, the filler metal enters the space between closely fitting pieces during the brazing process. The filler metal is heated just above its melting point, or liquidus, under the protection of an appropriate atmosphere—typically a flux.
After that, in a procedure called wetting, it pours over the base metal and cools to fuse the work parts together. The ability to combine metals—either the same or different—with significant strength is one of brazing’s main benefits.
Brazing parts is crucial for achieving high-quality joints, requiring clean, free-of-oxides base metal surfaces. Joint clearances of 0.03 to 0.08 mm are recommended for optimal capillary action and joint strength. Cleanliness of brazing surfaces is essential for proper wetting, as contamination can cause poor flow.
Cleaning methods include chemical and abrasive or mechanical cleaning, with mechanical cleaning requiring proper surface roughness to ensure wetting on rough surfaces is more readily occurring. Joint strength is also significantly influenced by surface cleanliness.
Diagram
Related: How Does Submerged Arc Welding (SAW) Works?
Brazing Techniques
There are two heating methods of brazing such as localised and diffuse heating.
The Localized Heating Method
Torch brazing is a technique used for heating and melting filler metals, primarily used in small production assemblies with unequal metal weights. This technique requires flux to protect joints and requires post-cleanup. Induction brazing uses high-frequency alternating current to heat the workpiece and melt filler material. Resistance brazing generates heat from the electrical resistance of the brazing alloy, ideal for creating simple joints between highly conductive metals.
The Diffuse Heating Method
Furnace brazing involves gas firing or heating elements to heat the furnace to the desired temperature, applying brazing filler metal to join surfaces. This process allows for accurate control of heating and cooling cycles, often performed in a vacuum to protect the alloy from atmospheric conditions. Dip brazing involves immersing the workpiece or assembly in a bath of molten filler metal or molten salt, applying brazing flux to prevent oxidation and solidifying the assembly.
How Does Brazing Works?
To acquire a high-quality joint in brazing, the parts should be closely fitted, and the base metals should be clean and free from oxides. The gap between the workpiece should have a clearance of 0.03 to 0.08mm for better capillary action and joint strength. Though it is not expected to have a joint of such clearance in some operations.
Surface cleaning in brazing operation is very essential as contamination can lead to poor wetting (flow). In brazing, the cleaning of parts is done in two ways such as chemical cleaning and abrasive or mechanical cleaning. In the case of mechanical cleaning, maintaining the proper surface roughness as wetting on a rough surface takes place much more than on a smooth surface of the same geometry.
Discussing the temperature and time on the quality of brazed joints, generally, the brazing temperature must be above the melting point of the filler metal. As the temperature of braze alloy increased, the alloying and wetting action also increases.
In some situations, a higher temperature must be implied to accommodate some factors like, to allow the use of different filler metals, to control metallurgical effects, or to sufficiently remove surface contamination. The best temperature usually used are:
- Be the lowest possible braze temperature
- Minimize any heat effects on the assembly
- Minimize filler metal/base metal interaction
- Maximize the life of any fixtures or jigs used
The effect of time on the brazed joint will primarily affect the extent to which these effects are present. However, the production process selected is expected to minimize brazing time and the associated costs. But, time and cost are secondary to other joint attributes (e.g., strength, appearance) in some non-production settings.
For brazing operations to be carryout within an inert or reducing atmosphere environment (i.e. vacuum furnace), a flux known as borax must be implied to prevent oxides from forming during the heating stage. This flux also cleans the contamination left on the brazing surfaces.
What are Brazing Flux?
Brazing fluxes are available in different forms such as flux paste, liquid, powder, or pre-made brazing pastes that combine flux with filler metal powder. The most common flux is available as thinning wire and rods with a coating of flux or a flux core.
This flux flows into the joint when applied to the heated surface and melts the filler metal on the joint. The excess flux should be removed when the cycle is completed, due to the fact that the flux left can lead to corrosion. It also impedes joint inspection and prevents further surface finishing operations.
Fluxes are selected generally based on their effects on a particular base metal. The selected flux must be chemically compatible with the base metal and the filler metal being used. less active fluxes should be used on longer brazing cycles than short brazing operations
Brazing Filler Materials
Different types of alloys are used as filler metals in brazing depending on the application method and intended use. Alloys used in brazing are made up of three or more metals to obtain desired properties.
A particular filler metal is chosen for a specific project in order to obtain the desired properties. These may include withstanding the service conditions required, wetting the base metals, and melting at a lower temperature than the base metals.
Braze alloys are typically available as ribbons, rods, powder, cream, wire, and preforms. The application determines if the filler material is pre-placed at the desired location or applied during the heating process.
In manual brazing, rod or wire forms filler metals are used due to the fact that they are easier to apply. There is multiple choice to be made when trying to carry out brazing as the filler materials are of different types. It is selected depending on the operation. So, the following listed below are the type of filler materials used in brazing;
- Aluminum-silicon
- Copper
- Copper-silver
- Copper-zinc (brass)
- Copper-tin (bronze)
- Gold-silver
- Nickel alloy
- Silver
- Amorphous brazing foil using nickel, silicon, copper, boron, phosphorus, etc.
Difference Between Brazing and Soldering
Soldering is a technique that involves melting filler metal into a junction point to create strong and permanent bonds between metals. It can exhibit capillary action and is performed at a temperature below 840°F, significantly lower than the temperature used in welding. Primary solder types and materials used include lead alloys, tin alloys, and zinc alloys.
Brazing and soldering are two processes used to join two metal parts, with brazing utilizing solder at a higher temperature range of 1200-2300 degrees Fahrenheit. Both processes involve capillary action between the base metals, which are expected to be free from oil and dirt. However, brazing requires higher temperatures, leading to different joint properties in service. Brazing often results in extensive interaction between the filler metal and the workpieces, while soldering allows only minimal inter-alloying.
Joint strength and fatigue resistance are two key differences between the two processes. Brazed joints are typically stronger than solder joints, but failure can occur in solder joints if they experience stress in service. In contrast, brazing joints require a higher temperature, leading to more intense metallurgical reactions and alloying.
In terms of fatigue resistance, a good brazed joint can handle fatigue and stresses placed on the joint, while solder joints in similar conditions would fail due to low degree of alloying. Understanding the differences between brazing and soldering processes is crucial for understanding their differences and ensuring proper joint performance in service.
Difference Between Brazing and Welding
Welding is a manufacturing technique used to join metal components using heat, pressure, or a combination of both. It is commonly used for joining metals, thermoplastics, and wood. Four commonly used welding techniques are gas metal arc welding (GMAW), shielded metal arc welding (SMAW), flux-cored arc welding (FCAW), and gas tungsten arc welding (GTAW).
The major difference between brazing and welding is the addition of filler material to achieve a joint. In welding, the properties of base metals change after joining, while in brazing, they remain the same. Brazing produces stronger joints, while welding produces weaker ones. Workpieces must be cleaned before joining, while welding is not compulsory. Brazing is used to join dissimilar metals, while welding is used to join similar metals. It is suitable for small assemblies, while welding is suitable for large assemblies. Brazing joints cannot withstand high temperatures, while welding can.
Related: What is Soldering? its Applications & How it Works
Advantages and Disadvantages of Brazing
Brazing is a unique metal joining technique that offers numerous advantages over welding. It allows for tighter tolerance control and produces a clean joint without secondary finishing. It can be applied to dissimilar metals and non-metals, producing less thermal distortion than welding.
Brazed joints can be cost-effective and can be coated or clad for protective purposes. They are easily adapted to mass production and can be automated due to their less sensitive process parameters.
However, brazing has some disadvantages, such as a lack of joint strength compared to a welded joint due to the use of softer filler metals. Additionally, brazed joints can be damaged under high service temperatures and require a high degree of base-metal cleanliness. Some brazing applications require adequate fluxing agents to control cleanliness, and the joint color may differ from the base metal, creating an aesthetic disadvantage.