1.1 Concept

Brazing: A welding method that uses a filler metal (called brazing alloy) with a melting point lower than that of the base material. After heating and melting, the liquid brazing alloy wets the base material, fills the joint gap, and diffuses with the base material to achieve a connection.

Compared to fusion welding, in brazing, the base material does not melt; only the filler metal melts.

Compared to pressure welding, brazing does not apply pressure to the workpieces.

The weld formed by brazing is called a brazed joint.

The filler metal used in brazing is called brazing alloy.

Brazing process: The cleaned workpieces are assembled in an overlapping manner, with the filler metal placed near or between the joint gaps. When the workpieces and filler metal are heated to slightly above the melting point of the filler metal, it melts (the workpieces do not melt) and is drawn into and fills the gaps between solid workpieces through capillary action. The liquid filler metal diffuses and dissolves with the workpiece metal, forming a brazed joint upon cooling.

1.2 Welding Materials

1.2.1 Filler Metal: The material used as filler metal during brazing.

1.2.1.1 Basic requirements for filler metals:

① Lower than the melting point of the workpiece metal;

② Sufficient wettability (the ability of the filler metal to flow into gaps);

③ Appropriate solubility and diffusion capability with the workpiece metal;

④ The welded joint should have certain mechanical, physical, and chemical properties.

1.2.1.2

② Hard Filler Metal: Filler metals with a melting point above 450°C, including aluminum-based, copper-based, silver-based, nickel-based alloys, etc.

Hard filler metals are mainly used for workpieces that bear greater loads and operate at higher temperatures, such as bicycle frames, hard alloy tools, drilling bits, etc. (mainly used for welding mechanical parts).

Common hard filler metals include: copper-based filler metals, silver-based filler metals (the most widely used type of hard filler metal with good mechanical properties, electrical conductivity, thermal conductivity, and corrosion resistance. Widely used for brazing low carbon steel, structural steel, stainless steel, copper, and copper alloys), aluminum-based filler metals (mainly used for brazing aluminum and aluminum alloys), and nickel-based filler metals (mainly used in aerospace sectors).

Hard Brazing: Refers to brazing using hard filler metals. The strength of brazed joints is relatively high (greater than 200 MPa).

1.2.1.3 Filler Metal Numbering

National Standard: B (indicates brazing alloy code) + chemical element symbol (indicates basic components of the brazing alloy) + number (indicates mass fraction (%) of basic components) + element symbols (indicates other components of the brazing alloy sorted by content; no more than six components without content indication) ---- Other characteristic markings (indicates certain characteristics of the brazing alloy, such as 'V' for vacuum-grade brazing alloy, 'R' for copper-zinc content that can be used as both a brazing alloy and gas welding wire).

For example: B (brazing alloy code) Ag72Cu (silver-based brazing alloy WAg=72%, containing copper element) --- V (vacuum-grade brazing alloy)

Department Standard:

(1) Metallurgical Department Standard:

"H1 (indicates filler metal) + element symbol (indicates basic components of the filler metal) + element symbol (indicates main components of the filler metal) + number (indicates content of main components excluding basic components) --- number (indicates content of other components excluding basic and main components)"

For example H1SnPb10 indicates tin-lead filler metal Wpb=10%

H1AlCu26-4 indicates aluminum-based ternary alloy filler metal Wcu=26%, other alloy elements are 4%

(2) Mechanical Department Standard

"HL (indicates filler metal) + number (indicates chemical composition type of the filler metal → '1' indicates copper-zinc alloy; '2' indicates copper-phosphorus alloy; '3' indicates silver alloy; '4' indicates aluminum alloy; '5' indicates zinc alloy; '6' indicates tin-lead alloy; '7' indicates nickel-based alloy) + number + number (indicates different grades within the same type of filler metal)"

For example HL605 — indicates No.5 tin-lead filler metal.

1.2.2 Brazing Flux

Flux: The agent used during brazing.

1.2.2.1 Functions of Flux:

(1) Remove oxides and other impurities from the surfaces of base materials and filler metals.

(2) Cover the surfaces of workpiece metals and fillers in a liquid film form to isolate air for protection — protecting fillers and welded parts from oxidation.

(3) Improve wettability of liquid fillers on workpiece metals and increase filling capacity of fillers.

. (2) Hard Flux:

Common hard fluxes include borax, boric acid (active temperature high, all above 800°C, can only be used with copper-based fillers, poor deoxidizing ability cannot remove oxides from Cr, Si, Al, Ti), KBF4 (potassium fluoroborate, low melting point, strong deoxidizing ability suitable for silver-based fillers with melting points below 750°C), etc.

1.3 Joint Forms

The load-bearing capacity of a brazed joint is related to the size of the joining surface. Therefore, brazed joints generally adopt overlapping or socket joints.

When designing brazed joints, considerations should be given to the assembly positioning of the brazed parts and the placement of the filler material. During assembly, the gap should be uniform, flat, and appropriate. If the gap is too small, it will affect the infiltration and wetting of the filler material, resulting in incomplete bonding; if the gap is too large, it will waste filler material and reduce the strength of the brazed joint. Generally, the gap for brazed joints is taken as 0.05 to 0.2 mm.

1.4 Heating Methods:

The heating methods for brazing include soldering iron heating, flame heating, resistance heating, induction heating, immersion heating, and furnace heating.

Soldering iron heating has a lower temperature and is generally suitable only for soft brazing.

Immersion heating types include salt bath heating and metal bath heating, which provide flux or filler material themselves, heat quickly, and keep the joint clean.

Furnace heating: atmosphere and furnace temperature can be controlled, providing uniform heating with minimal deformation of the workpiece.

Both immersion heating and furnace heating can be used to simultaneously braze multiple pieces or multiple seams, making them particularly suitable for welding complex shapes with multiple seams.