What is brazing?
The brazing filler metal is heated to an appropriate temperature, typically above 450°C.
This temperature is higher than the liquidus temperature of the filler metal but lower than the solidus temperature of the base metal.
This heating process promotes wetting of the liquid filler metal on the surface of the base metal and allows the filler metal to fill the brazing joint by capillary action, thus forming a bond between two materials, which may be the same or different.
Brazing characteristics
(1) The melting point of the filler metal is lower than that of the base metal, ensuring that the base metal remains intact during the brazing process.
(2) The composition of the filler metal is significantly different from that of the base metal.
(3) The molten filler metal is drawn into and retained in the space between the base metal components through wetting and capillarity.
(4) Metallic bonding is established through mutual diffusion of liquid filler metal and solid base metal.
Breakdown of the brazing process
Brazing advantages and disadvantages
Advantages of brazing:
- The brazing temperature is lower than that of the base metal, causing minimal impact on its structure and properties.
- The brazing process results in minimal stress and deformation, making it an ideal choice for connecting complex, high-precision parts or structures.
- The brazing process is highly efficient and can produce many joints at the same time.
- Brazing has a wide range of applications and can be used to join metals, non-metals and dissimilar metals.
- The surface of the welded joint is of high quality.
Disadvantages of brazing:
- The joint has low strength and is not heat resistant;
- The multipurpose lap joint is wasteful in terms of metal usage, adds weight to the structure, and is prone to causing stress concentrations;
- There are high preparation requirements before welding, mainly in terms of surface quality and clearance of assembly joints;
- Certain brazing process methods require a large investment and have high costs.
Types of Brazing
1) Classification according to the melting point of the solder
Below 450 ℃ – Welding
Above 450 ℃ – Brazing
2) By brazing temperature
- High temperature brazing
- Medium temperature brazing
- Low temperature brazing. (relative)
High temperature brazing above 800 ℃; 550 ~ 800 ℃ is medium temperature brazing;
Low temperature brazing is conducted when the temperature is lower than 500℃.
3) By type of heat source
Flame brazing, soldering iron brazing, furnace brazing, induction brazing, resistance brazing.
Terms and definitions related to brazing
Liquid: the lowest temperature at which solder is completely liquid;
Solid phase line: the highest temperature at which the filler metal becomes completely solid;
Humectant effect:
Adhesion is the phenomenon that occurs when a liquid object and a solid object come together after coming into contact.
It can be categorized into immersion wetting, adhesion wetting and spreading wetting.
In the free state, a liquid will try to maintain a spherical shape.
When liquid comes into contact with a solid, if its cohesion is greater than its adhesion, it will not adhere to the solid surface without getting wet.
However, if the adhesion of the liquid is stronger than its cohesion, it will be able to adhere to the solid surface as soon as wetting occurs.
The ability of a liquid to adhere to a base metal can be measured by the contact angle between the liquid and solid phases.
During brazing, the wetting angle of the filler metal must be less than 20°.
Capillary action:
It is assumed that when two metal plates parallel to each other are vertically inserted into an infinite amount of liquid solder, the plates are infinite and the amount of solder is unlimited.
Depending on the wetting properties of the solder on the metal plates, the capillary effect will result in the situation shown in Figure (a) or the situation shown in Figure (b). If the solder manages to wet the metal plates, the result shown in Figure (a) will occur; otherwise, the result in Figure (b) will occur.
Brazing Filler Metal Weld
Soft solders based on Sn and Pb:
They have good wetting and spreading ability for copper and other metals and are most widely used in the electronics industry.
CD-based solder:
Mainly cadmium silver alloy, with good heat and corrosion resistance.
Zn-based solder
Au-based soft solder
Other low melting point soft solders.
Include:
① (Indium) based solder
② Bi (bismuth) base solder
③ Ga (gallium) based solder
Lead-free solder
Filler Metal Brazing-Brazing
Due to its relatively high strength, brazing filler metal can be used to weld stressed components.
Brazing filler metal includes:
AI-Based Filler Metal:
Used for brazing aluminum and aluminum alloys
Ag-based brazing filler metal:
It has excellent comprehensive performance and can be used for brazing various metals. It is the most used filler metal for brazing.
Cu-based filler metal:
Copper brazing filler metal: brazing carbon steel and low alloy steel.
Copper and Zinc Soldering: Various brazing methods are used to weld various metals.
Copper and phosphorus filler metal: mainly used for brazing copper and copper alloys, widely used in the manufacture of engines and refrigeration equipment.
W-based filler metal:
Excellent performance, can weld various metals.
See too:
- Copper Brazing: The Basics You Should Know
- Aluminum brazing: the basics you should know
Brazing filler metal-Flux
Brazing flux function and performance requirements:
1) Remove the oxide film to create wetting and spreading conditions;
2) Liquid flux covers the base metal and weld surface for protection;
3) Plays an active role at the interface to improve wetting and propagation.
Need to remove the film during brazing
The oxide film present on the metal surface significantly impacts the wetting and spreading of the weld and must therefore be removed.
The thicker the oxide film, the stronger its bond with the metal matrix and the greater its thermal and chemical stability, making it difficult to remove.
Removal can be achieved through the use of soldering flux, gaseous media, mechanical methods or physical methods.
Soldering flux not only prevents oxidation of the workpiece and solder, but also removes the oxide film. It also reduces surface tension, promoting solder flow and facilitating solder joint formation.
Table 1 Oxide film formation rate in dry air
| Metal | 1 minute | 1 hour | 1 day |
| Stainless steel | 10 | 10 | 10 |
| Iron | 20 | 24 | 33 |
| Aluminum | 20 | 80 | 100 |
| Copper | 33 | 50 | 50 |
Brazing Gas Medium and Its Function
In brazing, the neutral gas used is mainly argon, with nitrogen being used in some cases.
Argon is an inert gas that mainly serves to protect the part and does not have the ability to directly remove the oxide film.
Some oxide films can be removed through the decomposition of oxides and the reduction, dispersion and dissolution of oxide film resistance through liquid solder adsorption.
As shown in the table, the decomposition temperature of most metal oxides is higher than the melting point or even the boiling point of the metal.
It can be concluded that during the brazing process it is not possible to obtain the decomposition of the oxide simply through heating.
| Oxide | decomposition temperature (℃) |
Oxide | decomposition temperature (℃) |
| Au 2 Oh | 250 | Pbo | 2348 |
| Ag 2 O | 300 | NiO | 2751 |
| Point 2 O | 300 | FeO | 3,000 |
| CDO | 900 | MnO | 3500 |
| Ass 2 Oh | 1835 | ZnO | 3817 |
Brazing method and process
Brazing method
1. Iron welding
Features: low temperature
Scope of application:
1. It is applicable to welding (using lead-tin or lead-based filler metal) with welding temperature below 300C;
2. Soldering flux is necessary for brazing thin and small parts.
2. Torch brazing, torch welding
Features: simple, flexible and widely used
Scope of application: generally, neutral flame or light carbonization flame/general gas torch or special brazing torch (the torch can also be used for soft welding) should be used to heat the workpiece first:
1. It is applicable to the brazing of some weldments that are limited by the shape, size and equipment of the weldments and cannot be brazed by other methods
2. Automatic flame brazing can be used
3. Weldable steel, stainless steel, hard alloy, cast iron, copper, silver, aluminum, etc.
4. Common filler metals include copper, zinc, copper, phosphorus, silver base, aluminum base, and zinc and aluminum filler metals.
3. Immersion brazing, immersion welding
(Salt bath and metal bath, suitable for mass production)
4. Flux welding, wave welding, spray welding
(A variety of metal bath brazing, mainly used for brazing printed circuit boards)
5. Resistance brazing
Extremely fast heating and high productivity.
6. Induction brazing
Fast heating, less oxidation and little brazing.
Brazing Techniques
The brazing production process covers several steps, including part surface preparation before brazing, assembly, filler metal placement, brazing, post-brazing treatment and other related processes.
1. Welded joint design
When designing a welded joint, the primary consideration should be its strength, followed by process considerations such as ensuring dimensional accuracy of assembly, proper assembly and positioning of parts, weld placement, and weld joint clearance.
Lap joint is commonly used for brazing joints.
In practical production, for brazing joints made with high-strength silver-, copper- or nickel-based filler metals, the lap length is typically 2 to 3 times the thickness of the thinnest part.
For soldered joints made with soft solders such as tin-lead, the lap length may be 4 to 5 times the thickness of the thinnest part, but should not exceed 15 mm.
Types of welded joints
a) Plate brazing joint form
- 1, 2, 3 – Connection form
- 4 – Shape of the cover plate
- 5, 6 – Shape of cutting
- 7- Shape of a double lid
- 8 – Form of overlap and coverage
- 9, 10 – Fold and lock the shape
b) T-shaped joint shape and bevel brazing
- 11, 22, 13, 14 – T-Joint
- 15, 16, 17, 18, 19, 20, 21 – Beveled shape
c) Joint form of tube or bar and plate
- 26, 30 – Less use
- 27, 28, 29 – Common
- 31, 32, 33 – Multipurpose
- 34, 35, 36 – Plate thickness joint
d) Wire contact brazing joint form
- 37 – Some typical joints
- 38 – Tubular radiator connector
- 39 – Sandwich structure joint
- 40 – Honeycomb structure joint
e) Tube brazing joint form
- 22 – Same internal diameter
- 23 – Same external diameter
- 24 – Permissible difference in external diameter
- 25 – External diameter without difference
Welded joint positioning method
a) Gravity positioning b) Tight fit c) Serrated d) Flanged
e) Widening f) Spinning g) Die forging h) Narrowing
i) Bottom cut j) Groove and bend k) Fixing l) Positioning pin
m) Screw n) Riveting o) Spot welding
2. Welding surface preparation
Before the brazing process, it is crucial to completely remove any oxide, grease, dirt and paint from the surface of the part.
In some cases, it may be necessary to pre-coat parts with a specific metallic layer before brazing.
(1) Remove oil stain
Oil stains can be removed with organic solvents.
Common organic solvents include alcohol, carbon tetrachloride, gasoline, trichlorethylene, dichloroethane, and trichloroethane.
(2) Oxide removal
Before brazing, the oxide films on the surface of the part can be processed using mechanical methods, chemical etching methods and electrochemical etching methods.
3. Assembly and fixing
Weld metals are used in several brazing methods, with the exception of flame brazing and soldering iron brazing, most of which are pre-placed into the joint. Gravity and capillarity of the gap should be utilized as much as possible to encourage the filler metal to fill the gap when placed.
Paste filler metal should be applied directly to the welded joint, and powdered solder may be mixed with an adhesive before being applied to the joint.
4. Filler metal placement method
a) Placement of annular weld
- 1, 2 – Reasonable placement of materials in the shape of a ring
- 3, 4 – Placement to avoid losses along the flange plane
- 5, 6 – Placing the filler metal close to the joint
- 7, 8 – A slot for solder placement is made in a joint
b) Foil welding placement
P – applied pressure
