Brazing is a joining process that differs from other joining processes like welding due to the fact that it does not melt the base metal during joining. Though brazing is classified among types of soldering it uses a higher temperature for joining. It also requires very close-fitted parts. The filler metal flows into the closely fitted parts by capillary action.
Today we will be looking deep into the brazing joining process; its basics, types, working principle, flux filler materials, types, advantages, and disadvantages, etc.
Types of brazing
There are many heating methods available to accomplish brazing operations. The most important factor in choosing a heating method is achieving an efficient transfer of heat throughout the joint and doing so within the heat capacity of the individual base metals used. The geometry of the braze joint is also a crucial factor to consider, as is the rate and volume of production required. The easiest way to categorize brazing methods is to group them by the heating method. Here are some of the most common:
- Torch brazing
- Furnace brazing
- Induction brazing
- Dip brazing
- Resistance brazing
- Infrared brazing
- Blanket brazing
- Electron beam and laser brazing
- Braze welding
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.
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;
- Copper-zinc (brass)
- Copper-tin (bronze)
- Nickel alloy
- Amorphous brazing foil using nickel, silicon, copper, boron, phosphorus, etc.
Advantages and disadvantages of a brazed joint
The following stated below are the benefits and limitations of brazing;
- The base metal does not melt in this joining process
- The clean joint is produce
- It allows much tighter control over tolerances
- Dissimilar metals and non-metals can be brazed
- Less thermal distortion is produced than welding
- Brazing doesn’t require secondary finishing as welding does
- the joining process can be coated or clad for protective purposes
- Brazing is easily adapted for mass production.
- Filler metals used in brazing are soft
- Joint strength is low compared to welding
- The joint is weaker than the base metal
- Brazed joint requires a high degree of base-metal cleanliness
- Joint color is often different from that of the base metal
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