Definition of Brazing
According to AWS A3.0, brazing is defined as:
“A group of joining processes producing the bonding of materials by heating them to the brazing temperature in the presence of a brazing filler metal having a liquidus above 450°C [840°F] and below the solidus of the base metal. The brazing filler metal is distributed and retained between the closely fitted faying surfaces of the joint by capillary action.”
Another way to understand is that filler metals have to melt while base materials should not.
In addition to this, filler metal is distributed by capillary action in the gap formed by the joint surfaces after melting.
In order to clarify the definition, it is worth emphasizing that brazing is a non-mechanical joining method that deserves to be distinguished from welding and soldering.
Differences from welding
In brazing:
- The add-on (consumable) material is melted at a temperature lower than the melting temperature of the base materials.
- Due to the above point, the base materials do not melt.
- The consumable fills the gap between the base materials by capillary effect.
Differences from soldering
In soft (or weak) soldering:
- A non-ferrous consumable is used (lead, for example);
- The consumable has a melting point of less than 450°C.
History
Reports of the use of brazing are very old.
It is speculated that the braze may have been accidentally discovered in a furnace around 4000 BC.
The first evidence found was a gold and silver jewel in the tomb of the Egyptian queen Pu-abi (Dated to approximately 2500 BC).
The uses of Brazing
Brazing is widely used in numerous applications due to:
- Ability to join materials of very different nature, such as (metals and ceramics) or (titanium and stainless steel);
- Small thicknesses. Welding could deform them excessively;
- Heat-treated materials. In order to prevent the loss of the heat treatment (before welding).
For these reasons, brazing is used for joints in the auto parts, refrigerators, heat exchangers, aeronautical and aerospace components, electronic components, etc.
Typical applications of components for the refrigeration and auto parts industry:
Brazing of aluminum heat exchangers (They are used in passenger car cooling systems):
Brazing of copper and graphite (They are used in the nuclear industry):
Benefits
- Allows the union of very different and normally non-weldable materials.
- Components can be processed in bulk.
- Brazing can be more economical and productive;
- Deformation or distortion is minimized or even eliminated;
- The dilution with the base metal is minimal;
- Thermal cycles are predictable;
- Allows the union of materials with very different thicknesses
Disadvantages
- Lower joint strength when compared to a welded joint;
- The brazed joint will likely have a lower strength than the base metal;
- High temperatures can destroy or weaken brazed joints.;
- Some applications require high control of joint cleanliness and precise use of flux;
- The final color of the joint is often different from the base metal (Undesirable visual appearance).
Heat sources
There are basically 5 heat sources for brazing. Each type is suitable for a part style, geometry, material, or volume to be brazed.
- (a) Torch or torch
Suitable for small parts, produced in small quantities.
- (b) By induction
Suitable for parts that require greater temperature control
- (c) Continuous oven
Suitable for small parts, produced in large quantities.
- (d) Batch oven
Suitable for large and complex parts.
- (e) Vacuum oven
Suitable for reactive materials or materials that cannot be oxidized.
Types of Joints
These are the configurations in which the base materials will be brazed. There are the following types of brazed joints:
- (a) Top
- (b) Overlapped
- (c) and (d) Top and overlapping variations
- (e) Angled
Properties
A brazed joint must achieve certain properties to meet its objectives:
- Mechanical resistance;
- Shear strength;
- Fatigue resistance;
- Toughness;
- Corrosion resistance;
Designers consider not only the strength of the alloy to be brazed, but also the strength area or minimum overlap length required to maintain mechanical properties.
Concepts
Wettability
Wetability is the ability of a liquid phase to spread over a solid substrate.
In brazing, the liquid phase is represented by the molten filler metal and the solid substrate by the base material.
A schematic graphic representation of this concept can be seen in the picture below. It has 3 distinct cases of wettability:
In the first case above, the filler metal does not show any tendency to spread over the base material.
It remains in the form of a drop which does not wet the surface.
In this case, there is no physical contact between the liquid phase and the substrate, so there will be no possibility of bonding occurring.
In the second case, the filler metal is spread over the base material, however at a limited level.
In this case, it is said that the wettability is moderate. There is physical contact between the liquid phase and the substrate, which allows them to unite.
In the third case, the filler metal spreads completely over the base material, forming almost a coating. It is said then that the wettability is excellent.
The physical contact between the liquid phase is the greatest possible, so the union between them is easily obtained.
The wettability of a filler metal on a base material will depend on several factors:
- (a) Filler metal deposited on the prepared surface of the base material (before brazing);
- (b) Present conditions allow the consumable to spread on the base material;
- (c) Poor conditions impede the flow of filler metal;
- (d) Conditions here were so bad that the consumable ran away or regressed from the base material.
Capillary or capillary effect
It is a physical phenomenon that occurs when a liquid phase wets a substrate and can be better understood by observing the figure below.
If wettability exists, the liquid phase tends to rise above the normal level through the capillary effect.
The height reached is proportional to the gap's size.
On the other hand, when there is no wettability, the gap is not even filled, and the height of the liquid phase remains below its normal level.
Note: The gap will only be filled when the molten filler metal wets the base materials. The filling will be easier with smaller the gaps.
Brazing, therefore, is nothing more than the filling of a gap between the base materials by a molten filler metal
And this filler metal necessarily has wettability on the base materials.
A schematic representation of brazing can be seen below, where the evolution of filler metal can be followed.
Brazing gap
It has been shown that filling the gap between the base materials depends on the ability of the molten filler metal to wet the base material.
Furthermore, filling occurs more easily in smaller gaps.
One could then imagine that the gap to be used should be as small as possible, as this would facilitate its filling.
Unfortunately, this concept is wrong. Excessive gap reduction makes it harder for the flux.
The flux does not perform well in very small spaces.
Just as the gap should not be too small, it should not be too large.
A small gap will give a small capillary effect, which makes filling difficult.
It is concluded, therefore, that the gap to be used must be within a certain range.
A range where it is known that the flux and the capillary effect is satisfactory, thus ensuring an adequate filling of the gap.
The gaps to be used are generally in the range of 0.05 to 0.20 mm.
It depends on filler metal, type of flux and type of joint used.
In any case, the consumable supplier should be consulted to recommend the required gap.
Flux (Cleaning agents)
Cleaning is simply essential for brazing.
We need to clean the surface of the base materials before brazing. They must be free of oil or grease.
This is because the oil or grease, when heated, produces residues that are left on the surface of the materials.
These residues prevent the filler metal from wetting the base materials, making brazing unfeasible.
They are usually eliminated through a degreasing operation, carried out by industrial solvents.
Flux Functions
- Eliminate the layer of surface oxides from the base materials, thus enabling the occurrence of wettability;
- Prevent base materials from oxidizing during heating in brazing. This is necessary as heat tends to accelerate chemical reactions, including oxidation reactions;
- Protect the filler metal until it melts, thus allowing wettability to occur;
- Do not attack or react with base material (Flux);
- Deoxidize the base material surface before the start of the filler metal fusion (at least 50°C below the working temperature), keeping it deoxidized until the end of the brazing (Flux);
- Provide good wettability and fluidity on the base material, properly spreading over the surfaces to be brazed (Flux);
- Be easily removed after brazing (Flux).
Flux can be solid, liquid or gaseous.
Filler Metals
Proper selection of filler metal to be used is often the key to success.
In general, these materials must have some important characteristics for the brazing to occur properly, such as:
- Provide good wettability on the base materials to be brazed;
- Adequate melting temperature (or melting temperature range) in relation to the base materials and fluidity that allows the molten metal to adequately penetrate the joints by capillary effect;
- Present the required properties for the brazed component. For example: adequate mechanical strength, required electrical conductivity, etc;
- Do not overreact with the base material, causing erosion or forming fragile phases;
- Not showing a high tendency to liquidation (partial fusion).
It is customary to classify filler metals according to the chemical elements that compose them.
In general, we say that there are different families of filler metals, with each family characterized by containing the same (or almost the same) elements.
These families of filler materials differ (from each other) mainly by melting temperatures.
This characteristic is of fundamental importance in brazing. Lower the melting temperature means less heating required, so the brazing will be the cheaper and faster.
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Citation
When you need to include a fact or piece of information in an assignment or essay you should also include where and how you found that piece of information (Brazing).
That gives credibility to your paper and it is sometimes required in higher education.
To make your life (and citation) easier just copy and paste the information below into your assignment or essay:
Luz, Gelson. What is Brazing (Concepts, Properties and Consumables). Materials Blog. Gelsonluz.com. dd mmmm. yyyy. URL.
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