Overview of brazing basics

2.1 Concept

Brazing: A welding method in which a filler metal having a lower melting point than a base material (referred to as a brazing filler metal) is melted by heating, a base material is wetted with a liquid brazing material, a joint gap is filled, and a base material is mutually diffused to achieve a joint.

Compared with fusion welding, the base metal does not melt during brazing, and only the brazing material melts;

Compared to pressure welding, no pressure is applied to the weldment during brazing.

The weld formed by brazing is called a braze.

The filler metal used for brazing is called a brazing filler metal.

Brazing process:

The surface-cleaned workpieces are assembled together in a lap type, with the solder placed near the joint gap or between the joint gaps. When the workpiece and the brazing material are heated to a temperature slightly higher than the melting point of the brazing filler metal, the brazing material melts (the workpiece is not melted), and is sucked into and filled with the gap between the solid workpieces by capillary action, and the liquid brazing material and the workpiece metal mutually diffuse and dissolve. A brazed joint is formed after the cold.

2.2 Welding materials

2.2.1 Brazing material: that is, a material used as a filler metal during brazing.

2.2.1.1 Basic requirements for solder:

1 is lower than the melting point of the workpiece metal;

2 has sufficient wettability (the performance of the solder flowing into the gap);

3 has the ability to properly dissolve and diffuse with the workpiece metal;

4 welded joints should have certain mechanical properties and physical and chemical properties.

2.2.1.2 Classification

According to the melting point, the solder is divided into solder and brazing filler metal.

1 solder: solder with a melting point lower than 450 ° C, tin-lead-based, lead-based (T <150 ° C, generally used for brazing copper and copper alloy, good heat resistance, but poor corrosion resistance) An alloy such as a cadmium base (one of the best heat resistance in solders, T = 250 ° C).

Solder is mainly used for welding workpieces with low force and low working temperature, such as the connection of various electrical wires and the brazing of instruments and instrument components (mainly used for soldering of electronic circuits).

Commonly used solders are: tin-lead solder (the most widely used, with good processability and electrical conductivity, T <100 ° C), cadmium silver solder, lead silver solder and zinc silver solder.

Soldering: Brazing using solder. Brazed joints have low strength (less than 70 MPa).

2 brazing filler metal: a brazing material having a melting point higher than 450 ° C, and an alloy such as aluminum, copper, silver or nickel.

Brazing fillers are mainly used for welding workpieces with high force and high working temperature, such as bicycle frames, carbide tools, drilling drills, etc. (mainly used for welding of mechanical parts and components)

Commonly used brazing filler metals are: copper-based brazing filler metal and silver-based brazing filler metal (the most widely used type of brazing filler metal, which has good mechanical properties, electrical and thermal conductivity, and corrosion resistance. It is widely used for brazing low carbon steel, Structural steel, stainless steel, copper and copper alloys, aluminum-based solders (mainly used for brazing aluminum and aluminum alloys) and nickel-based solders (mainly used in the aerospace sector).

Brazing: Brazing using brazing filler metal. The brazed joint has a high strength (greater than 200Mpa).

2.2.1.3 Number of solder material

National standard: B (Braze) + chemical element symbol (basic component of the surface solder) + number (mass score (%) of the basic component of the table) + element conforms (other components of the table solder) , sorted by content, not standard content (maximum of no more than 6))----Other characteristics mark (some characteristics of the surface solder, such as "V" means vacuum grade solder, "R" means can be done The brazing material can also be used as the copper and zinc content of the gas welding wire).

Such as: B (solder code) Ag72Cu (silver-based solder WAg = 72%, and contains copper) - V (vacuum grade solder)

Department logo:

(1) Ministry of Metallurgy Ministry:

"H1 (indicating solder) + element symbol (table brazing base component) + element symbol (table brazing material main component) + number (table content except main component of basic component) --- number ( The content of other components except the basic and main components in the brazing filler metal)"

For example, H1SnPb10 date means tin-lead solder Wpb=10%

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

(2) Ministry of Machinery

"HL (table solder) + number (indicating the chemical composition type of the solder → '1' means copper-zinc alloy; '2' means copper-phosphorus alloy; '3' silver alloy; '4' aluminum alloy; '5 'Table zinc alloy; '6' table tin-lead alloy; '7' table nickel-based alloy) + number + number (representing different grades in the same type of solder)"

Such as HL605 - Table No. 5 tin-lead solder.

2.2.2 Brazing flux

Flux: This is the flux used in brazing.

2.2.2.1 Effect of flux:

(1) Removal of oxides and other impurities on the surface of the base metal and solder

(2) Covering the surface of the workpiece metal and solder with a liquid film, the isolation air acts as a protection—protecting the solder and the weldment from oxidation.

(3) Improve the wettability of the liquid solder to the workpiece metal and increase the filling ability of the solder.

2.2.2.2 Classification:

The flux is generally classified into three categories: solder, hardener, and flux for aluminum, magnesium, and titanium.

(1) solder

According to its composition, it can be divided into inorganic solder (which has high chemical activity and strong ability to remove oxides. It can significantly promote the wetting of the base metal by the liquid solder. The components are inorganic acids and inorganic salts. The ferrous and non-ferrous metals, including stainless steel, heat-resistant steel and nichrome, can be used, but the residue is corrosive, must be cleaned after welding, and organic solders.

According to the corrosion effect of the residue on the brazed joint, it can be divided into three categories: corrosive, weak corrosive and non-corrosive. Among them, the inorganic solder is a corrosive flux; the organic solder belongs to the latter two categories.

Commonly used solders are phosphoric acid aqueous solution (only used below 300 ° C, is a suitable flux for brazing Cr-containing stainless steel or manganese bronze), zinc chloride aqueous solution and rosin (can only be used for brazing surface oxidation below 300 ° C. Severe gold, silver, copper and other metals).

(2) Hardener:

Commonly used brazing agents are borax and boric acid (the active temperature is high, both above 800 ° C, can only be used with copper-based solder, poor deoxidation ability, can not remove oxides of Cr, Si, Al, Ti, etc.) KBF4 (potassium fluoroborate, low melting point, strong deoxidation ability, suitable flux for silver-based solders with melting point below 750 °C).

2.3 Connector form

The bearing capacity of the brazed joint is related to the size of the joint. Therefore, the brazed joint generally uses a lap joint or a socket joint. As shown in Figure 6-3-17:

Figure 6-3-17 Example of brazed joint

When designing the brazed joint, the assembly position of the brazed parts and the placement of the brazing material should be considered. When assembling, the assembly clearance should be uniform, flat and appropriate. The gap is too small, which will affect the penetration and wetting of the solder, and will not reach all the solder joints; if the gap is too large, the solder will be wasted and the strength of the solder joint will be reduced. Generally, the gap of the brazed joint is taken to be 0.05 to 0.2 mm.

2.4 Heating method:

Brazing heating methods include soldering iron heating, flame heating, resistance heating, induction heating, immersion heating, and furnace heating.

Soldering irons have a lower heating temperature and are generally only suitable for soldering.

The type of impregnation heating is salt bath heating and metal bath heating, which itself provides flux or solder, which is fast heating and the joint is clean.

Heating in the furnace: the atmosphere, the furnace temperature can be controlled, the heating is uniform, and the weldment deformation is small.

Both immersion heating and furnace heating can be used to weld multiple pieces or multiple brazing joints at the same time, which is especially suitable for welding parts with complex shapes and multiple brazing seams.

2.5 Characteristics and application of brazing

Features:

(1) Brazing heating temperature is low, the joint is smooth and flat, the structure and mechanical properties change little, the deformation is small, and the workpiece size is accurate.

(2) Weldable dissimilar metals can also be welded to dissimilar materials, and there is no strict limit on the thickness difference of the workpiece.

(3) Some brazing methods can weld multiple weldments and multiple joints at the same time, and the productivity is high.

(4) The brazing equipment is simple and the production investment cost is small.

(5) The joint strength is low, the heat resistance is poor, and the pre-weld cleaning requirements are strict, and the brazing material is expensive.

application:

Brazing is not suitable for the welding of general steel structures and heavy-duty, dynamic-loaded parts. Mainly used in the manufacture of precision instruments, electrical components, dissimilar metal components and complex sheet structures, such as sandwich components, honeycomb structures, etc., also used to braze all kinds of different wire and carbide tools.
Third, brazing

1. Brazing characteristics and application Brazing uses an alloy with a melting point lower than that of the base material as a brazing filler metal. When heated, the brazing material melts and fills and remains in the joint gap by wetting and capillary action, while the base metal is in solid state, relying on The interdiffusion between the liquid solder and the solid base material forms a brazed joint. Brazing has little influence on the physical and chemical properties of the base metal, and the welding stress and deformation are small. The dissimilar metals with different welding properties can complete multiple welds at the same time. The joint appearance is neat and tidy, the equipment is simple, and the production investment is small. However, the brazed joint has low strength and poor heat resistance.

Applications: Carbide tools, drill bits, bicycle frames, heat exchangers, ducts and various types of containers; brazing is the only possible connection method in the manufacture of microwave waveguides, tubes and electronic vacuum devices.

2. The brazing filler metal and the brazing filler metal are filler metals that form the brazed joint, and the quality of the brazed joint depends largely on the brazing filler metal. The brazing filler metal should have a suitable melting point, good wettability and caulking ability, and can diffuse with the parent metal. It should also have certain mechanical properties and physical and chemical properties to meet the performance requirements of the joint. Brazing is divided into two categories according to the melting point of the solder: soldering and brazing.

(1) Brazing solder solder melting point below 450 ° C is called soldering, the common solder is tin-lead solder, which has good wettability and electrical conductivity, widely used in electronic products, electrical appliances And auto parts. The joint strength of soldering is generally 60 to 140 MPa.

(2) Brazing of a brazing filler metal having a melting point higher than 450 ° C is called brazing, and the usual brazing filler metal is a brazing filler metal and a silver-based brazing filler metal. The joints made of silver-based brazing filler metal have high strength, electrical conductivity and corrosion resistance. The melting point of the brazing filler metal is low and the processability is good. However, the price of the brazing filler metal is relatively high, and it is often used for high-precision weldments. Brass solder is used. Brazing is often used for steel and copper alloy workpieces with high forces and brazing of tools. The strength of the brazed joint is 200 to 490 MPa.

Note: The contact surface of the base metal should be very clean, so use flux. The function of the flux is to remove oxides and oil impurities on the surface of the base metal and the solder, to protect the contact surface of the solder and the base material from oxidation, and to increase the wettability and capillary fluidity of the solder. The melting point of the flux should be lower than that of the solder, and the flux residue should be less corrosive to the base metal and the joint. The flux commonly used for soldering is a rosin or zinc chloride solution, and the usual flux for brazing is a mixture of borax, boric acid and basic fluoride.

3. Brazing heating method Almost all heating sources can be used as a brazing heat source, and the brazing is classified accordingly:

Flame brazing: heating with a gas flame for brazing of carbon steel, stainless steel, hard alloys, cast iron, copper and copper alloys, aluminum and aluminum alloys.

Induction brazing: A resistance-heat-welded weldment that uses an alternating magnetic field to generate an induced current in a part for use in a weld with a symmetrical shape, particularly for tube-shaft brazing.

Dip-dip brazing: The part or the whole of the weldment is immersed in the melt of the molten salt mixture or the molten metal, and the brazing process is realized by the heat of the liquid medium, which is characterized by rapid heating, uniform temperature and small deformation of the weldment.

Brazing in the furnace: The electric resistance is used to heat the weldment, and the electric resistance furnace can protect the weldment by vacuuming or using a reducing gas or an inert gas.

In addition, there are soldering iron brazing, electric resistance brazing, diffusion brazing, infrared brazing, reactive brazing, electron beam brazing, laser brazing, and the like.

4. The brazed joint is shown in Figure 3-31. Brazing is usually in the form of sheet lap joint and sleeve insert. This can compensate for the lack of strength of the brazing filler metal by increasing the joint surface between the weldments and ensure the bearing capacity of the joint. This type of joint also facilitates the control of the joint gap, and the proper gap allows the solder to be evenly distributed in the joint for optimum brazing. The gap of the brazed joint is generally in the range of 0.05 to 0.2 mm.

Sixth, brazing

Brazing refers to the use of a metal material having a lower melting point than the base material as a brazing filler metal, heating the weldment and the brazing material to a temperature higher than the melting point of the brazing filler metal and lower than the melting point of the base material, and moisturizing the base material with a liquid brazing filler metal to fill the joint. The gap and the mutual diffusion of the base material realize a method of joining the weldments. As shown below.

During brazing, after the contact surface of the brazed workpiece is cleaned, it is assembled in an overlapping manner, and the brazing material is placed near the joint gap or directly into the joint gap. When the workpiece is heated with the solder to a temperature slightly above the melting temperature of the solder, the solder will melt and wet the surface of the weld. The liquid solder spreads along the joint by capillary action. Then, the brazing metal and the brazing material are mutually dissolved, and mutually infiltrated to form an alloy layer, and after condensing, a brazing joint is formed.

In order to make the brazing joints firmly connected, the adhesion of the brazing filler metal is enhanced, and a flux is used for brazing. Its function is to remove the oxides on the surface of the solder and the base metal, protect the weldment and the liquid solder from oxidation during the brazing process, and improve the wettability of the liquid solder to the weldment.

There are generally two types of commonly used brazing filler metals. One type is a brazing material, and the melting point is above 450 ° C. The commonly used brazing filler metal is a copper-based, silver-based, aluminum-based, nickel-based alloy. The flux is usually borax, boric acid, chloride, fluoride, and the like. The heating sources for brazing include torch flame, electric resistance heating, induction heating, salt bath heating and furnace heating. Brazed joints have high strength and are suitable for brazing workpieces with high force or high working temperature, such as carbide cutters, bicycle frames, etc. These brazing are usually referred to as brazing; the other is Solder, melting point below 450 ° C, the most widely used solder is tin-based alloy, most solders suitable for soldering temperature of 200-400 ° C, flux is rosin, rosin alcohol solution, zinc chloride solution, The heating method is usually heated by a soldering iron. Brazing joints are low-strength and are suitable for workpieces with low receiving force or low operating temperatures, such as containers, instrument components, etc. These brazing are often referred to as soldering.

The characteristics of brazing are that the surface of the joint is smooth, airtight, stable in shape and size, and the microstructure and properties of the weldment are not changed much, and the same or different metals and some non-metals can be connected. When brazing, it is also possible to heat the entire workpiece and weld a lot of welds at a time, which improves productivity. However, the strength of the brazed joint is relatively low, and the lap joint is often used to increase the joint strength by increasing the lap length; in addition, the preparation work before brazing is required to be high.