Brazing 101: Everything You Need to Know for Beginners

What is brazing?

The brazing filler metal is heated to a suitable temperature (above 450 ℃, higher than the liquidus temperature of the brazing filler metal, but lower than the solidus temperature of the base metal), which is conducive to the wetting of the liquid filler metal on the surface of the base metal and the filling of the brazing joint by capillary action, forming the connection between the same or different materials.

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Brazing characteristics

(1) The melting point of the filler metal is lower than that of the base metal, and the base metal will not melt during brazing;

(2) The composition of filler metal is very different from that of base metal;

(3) The melted filler metal is sucked in and kept in the base metal gap by wetting and capillarity;

(4) The metal bonding is formed by mutual diffusion of liquid solder and solid base metal.

Decomposition of brazing process

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Brazing advantages and disadvantages

Brazing advantages:

1) The brazing temperature is lower than the base metal, which has little influence on the structure and properties of the base metal;

2) Small stress and deformation, suitable for connection of high-precision, complex parts or structures;

3) High productivity, many joints can be completed at one time;

4) Extensive applicability, can weld metal, nonmetal and dissimilar metals;

5) The joint surface is of good quality.

Brazing disadvantages:

1) The joint has low strength and poor heat resistance;

2) Multi purpose lap joint wastes metal, increases structure weight and easily produces stress concentration;

3) High requirements for preparation before welding, especially surface quality and assembly joint clearance;

4) Some brazing process methods have large investment and high cost.

Types of Brazing

1) Classification according to solder melting point

Below 450 ℃ – Soldering

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 heat source type

Flame brazing, soldering iron brazing, furnace brazing, induction brazing, resistance brazing.

Terms and definitions related to brazing

Liquidus: the lowest temperature at which solder is completely liquid;

Solid phase line: the highest temperature at which the filler metal is completely solid;

Wetting effect:

Wetting The phenomenon of adhesion between a liquid object and a solid object after contact.

It can be divided into immersion wetting, adhesion wetting and spreading wetting.

When a liquid is in a free state, it will try to stay spherical!

When the liquid contacts with the solid, if the cohesion>adhesion, the liquid cannot adhere to the solid surface without wetting!

When the adhesion of liquid is greater than its cohesion, the liquid can adhere to the solid surface once wetting occurs.

The wettability of liquid to base metal can be measured by the contact angle between liquid phase and solid phase.

During brazing, the wetting angle of filler metal shall be less than 20 °.

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Capillary action:

When two mutually parallel metal plates are vertically inserted into the liquid solder, it is assumed that the parallel metal plates are infinite and the amount of solder is infinite.

Due to the capillary effect, if the solder can wet the metal plates, the situation shown in Fig. (a) will occur; otherwise, the situation shown in Fig. (b) will occur.

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Brazing filler metal-Soldering

Sn based and Pb based soft solders:

They have good wetting and spreading ability for copper and other metals, and are most widely used in the electronic industry.

Cd based solder:

Mainly cadmium silver alloy, with good heat resistance and corrosion resistance.

Zn based solder

Au based soft solder

Other low melting point soft solders.

Include:

① In (indium) based solder

② Bi (bismuth) base solder

③ Ga (gallium) based solder

Lead-free solder

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Brazing filler metal-Brazing

Due to its relatively high strength, brazing filler metal can be used to braze stressed components.

The 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 widely used brazing filler metal.

Cu based filler metal:

Copper brazing filler metal: brazing carbon steel and low alloy steel.

Copper zinc solder: various brazing methods are used to weld various metals.

Copper phosphorus filler metal: mainly used for brazing copper and copper alloys, widely used in motor manufacturing and refrigeration equipment.

W-based filler metal:

Excellent performance, can weld various metals.

See also:

Brazing filler metal-Flux

Function and performance requirements of brazing flux:

1) Remove the oxide film to create conditions for wetting and spreading;

2) The liquid flux covers the base metal and the solder surface for protection;

3) It plays an interface active role to improve wetting and spreading.

Necessity of removing film during brazing

The oxide film on the metal surface seriously affects the wetting and spreading and must be removed.

The denser the oxide film is, the stronger its bond with the metal matrix is, and the higher its thermal and chemical stability is, the harder it is to remove.

Solder flux, gas medium, mechanical method and physical method can be used for removal.

Solder flux can not only prevent the oxidation of workpiece and solder, but also remove its oxide film.

It can also reduce the surface tension to promote the flow of solder, and cooperate with the formation of solder joint.

Table 1 Formation Rate of Oxide Film in Dry Air

Metal1 minute1 hour1 day
Stainless steel101010
Iron202433
Aluminum2080100
Copper335050
Thickness of oxide film (10-8 cm)
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Flux classification

Brazing gas medium and its function

The neutral gas used in brazing is mainly argon, and nitrogen is also used in some cases.

Argon is an inert gas, mainly for protection, and has no ability to directly remove oxide film.

Some oxide films are removed by the decomposition of oxides and the reduction, dispersion and dissolution of oxide film strength in the solder under the adsorption of liquid solder.

It can be seen from the table that the decomposition temperature of most metal oxides is higher than the metal melting point or even the boiling point.

It can be seen that in the brazing process, it is impossible to obtain the decomposition of oxides simply by heating.

OxideDecomposition temperature (℃)OxideDecomposition temperature (℃)
Au2O250PbO2348
Ag2O300NiO2751
Pt2O300FeO3000
CdO900MnO3500
Cu2O1835ZnO3817

Brazing method and process

Brazing method

1. Iron soldering

Features: low temperature

Scope of application:

1. It is applicable to soldering (using tin lead or lead based filler metal) with soldering temperature lower than 300C;

2. Solder flux is required for brazing thin and small parts.

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2. Torch brazing, torch soldering

Features: simple, flexible and widely used

Scope of application: generally, neutral flame or slight carbonization flame/general gas torch or special brazing torch (torch can also be used for soft soldering) shall be used to heat the workpiece first:

1. It is applicable to brazing some weldments which are limited by the shape, size and equipment of 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. and their alloys

4. Common filler metals include copper zinc, copper phosphorus, silver base, aluminum base and zinc aluminum filler metals

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3. Dip brazing, dip soldering

(Salt bath and metal bath, suitable for mass production)

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4. Flow soldering, wave soldering, spray soldering

(A variety of metal bath brazing, mainly used for brazing printed circuit boards)

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5. Resistance brazing

Extremely fast heating and high productivity.

6. Induction brazing

 Fast heating, less oxidation and small brazing.

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Brazing techniques

Brazing production process includes surface preparation of workpiece before brazing, assembly, placement of filler metal, brazing, post brazing treatment and other processes.

1. Brazed joint design

When designing the brazed joint, the strength of the joint should be considered first, and then the process problems such as how to ensure the dimensional accuracy of the assembly, the assembly and positioning of the parts, the placement of solder, and the clearance of the brazed joint should be considered.

Lap joint is mostly used for brazing joints.

In production practice, for brazing joints with silver based, copper based, nickel based brazing filler metals with high strength, the lap length is usually 2-3 times the thickness of the thin piece;

Related reading: Types of Welding Joints

For soldered joints with soft solders such as tin lead, it can be 4-5 times the thickness of the thin piece, but the lap length is not expected to be greater than 15mm.

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Types of brazed joints

a) Joint form of plate brazing

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  • 1, 2, 3 – Connection form
  • 4 – Cover plate form
  • 5, 6 – Lapping form
  • 7- Form of one double cover plate
  • 8 – Overlapping and covering form
  • 9, 10 – Bend and lock form

b) Joint form of T-shape and bevel brazing

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  • 11, 22, 13, 14 – T-joint
  • 15, 16, 17, 18, 19, 20, 21 – Bevel form

c) Joint form of tube or bar and plate

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  • 26, 30 – Less use
  • 27, 28, 29 – Common
  • 31, 32, 33 – Multi-purpose
  • 34, 35, 36 – Plate thickness joint

d) Joint form of wire contact brazing

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  • 37 – Some typical joints
  • 38 – Tubular radiator connector
  • 39 – Sandwich structure joint
  • 40 – Honeycomb structure joint

e) Joint form of pipe brazing

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  • 22 – Same inner diameter
  • 23 – Same outer diameter
  • 24 – Allowable difference of outer diameter
  • 25 – Outer diameter without difference

Positioning method of brazed joint

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a) Gravity positioning     b) Tight fit     c) Knurling       d) Flanging

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e) Flaring          f) Spinning         g) Die forging        h) Necking

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i) Undercut     j) Slotting and bending  k) Clamping    l) Positioning pin

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m) Screw         n) Riveting        o) Spot welding

2. Surface preparation of weldment

Before brazing, the oxide, grease, dirt and paint on the workpiece surface must be carefully removed.

Sometimes, parts must be pre coated with a certain metal layer before brazing.

() Remove oil stain

Oil stain can be removed with organic solvent.

Common organic solvents include alcohol, carbon tetrachloride, gasoline, trichloroethylene, dichloroethane and trichloroethane.

(2) Oxide removal

Before brazing, oxides on the part surface can be processed by mechanical method, chemical etching method and electrochemical etching method.

3. Assembly and fixing

Solder metals are placed in various brazing methods.

Except flame brazing and soldering iron brazing, most of them are pre placed on the joint.

The gravity and capillarity of the gap shall be used as much as possible to promote the filler metal to fill the gap when the filler metal is placed.

Paste filler metal shall be directly applied to the brazed joint;

The powder solder can be mixed with adhesive and then adhered to the joint.

4. Placement method of filler metal

a) Placement of annular solder

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  • 1, 2 – Reasonable placement of one ring shaped materials
  • 3, 4 – Placement to prevent loss along the flange plane
  • 5, 6 – Placement of filler metal close to the joint
  • 7, 8 – A solder placement slot is made on a joint

b) Placement of foil solder

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P – pressure applied

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