Introduction to common brazing welding

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Welding generally refers to the welding of metals. It is a forming method that uses heating or pressure, or both at the same time, to create an inter-atomic bonding force between two separate objects and connect them into one body. Depending on the degree of heating and process characteristics during the welding process, welding methods can be divided into three major categories. They are fusion welding, pressure welding, and brazing welding, which are the three main welding methods. Today’s article mainly introduces the brazing welding:

1.What is the concept of brazing welding?

brazing welding is a metal joining process that heats and melts a metal or alloy called brazing material (or flux) so that it flows into the seams of the connected parts, and then forms a strong connection after cooling. Unlike welding, brazing welding operates at temperatures below the melting point of the parts being joined, so it does not melt the parts being joined.

The solder usually has a lower melting point than the metal being joined and will form a wetting and lubricating effect between the parts being joined, promoting the flow of the solder into the joint. During the brazing welding process, the parts being joined are usually kept at or near room temperature to ensure that their structure is not subject to thermal deformation.

2.Classification of brazing welding

According to the different melting points of solder, brazing welding is divided into brazing and soft soldering. Soft soldering: The melting point of soft soldering solder is lower than 450°C, and the joint strength is low (less than 100MPa). Soft soldering is mostly used for soldering conductive, airtight and watertight devices in the electronics and food industries. Soldering using tin-lead alloy as solder is the most commonly used. Soft solder generally requires the use of flux to remove the oxide film and improve the wetting properties of the solder. brazing: The melting point of brazing solder is higher than 450°C, and the joint strength is higher (greater than 200 MPa). Brazing joints have high strength and some can work at high temperatures.

（1）Soft soldering

Soft soldering is a type of connection method that uses solder with a melting point not exceeding 450°C and is heated to a soft soldering temperature that is lower than the melting point of the base material and higher than the melting point of the solder. The solder is spread on the closely fitting connection surface by capillary action or on the workpiece surface by wetting.

Soft solder is a solder used for soft soldering whose liquidus temperature (melting point) does not exceed 450°C. Solder is usually an alloy that does not contain iron. The temperature of 450℃ is the dividing point between brazing welding and soft soldering. Most of the process parameters and influencing factors involved in brazing welding also apply to soft soldering. In fact, industrial terms such as soft soldering, brazing or silver soldering are also used to distinguish soft soldering and brazing welding.

①Soft soldering features

Soft soldering can be performed with common heat sources such as soldering irons and blowtorches for brazing welding, which is easy to operate;

The heating temperature is low, and the structural properties of the base metal change little. Copper and aluminum components can be welded in any way, and the possible expansion, strength change and deformation are small;

Brazing welding has high productivity and can weld dozens to thousands of welds at one time, making it easy to realize automated production;

Due to the low melting temperature of the solder used, the flux is not easily scorched during brazing welding and there is a wide range of suitable flux compounds to choose from.

②Soft soldering classification

Soft soldering

Dip soft soldering (DS) is a soft soldering method that uses a metal bath of molten solder to heat. Solder can be kept molten using any heat source. Solder dipping equipment is usually automated and programmed to control the pre-cleaning, flux addition, preheating and dipping processes.

Soft soldering in the furnace

Furnace soft soldering (FS) is a soft soldering method in which the workpiece to be welded is placed in the furnace for heating. During soft soldering in the furnace, the welding parts must be assembled and fixed in place. Solder must be preloaded into the joint. The furnace may be heated using any suitable fuel or energy source.

Induction Soft soldering

Induction soft soldering (IS) is a soft soldering method that uses resistance heat generated by induced current in the workpiece for heating. It is similar to induction brazing welding.

Infrared Soft soldering

Infrared soft soldering (IFS) is a soft soldering method that uses infrared radiation heat for heating. It is similar to infrared brazing welding.

Soft soldering

Soldering iron soft soldering (INS) is a soft soldering method that uses a soldering iron to heat. A soldering iron has a component called a heating tip that heats and transfers heat to the solder. The heating tip is made of copper. The heating head of the soldering iron can be heated in different ways. For example, the electric soldering iron uses an internal resistance coil to heat; there are also flame heating and furnace heating. Soft soldering welding guns are also used in industry. This type of welding gun uses resistance heat and its heating head is a high resistance element. Soft soldering soldering guns are widely used in electronic component assembly. Soldering iron Soft soldering generally adopts manual operation.

Resistor Soft soldering

Resistive soft soldering (RS) welds using the resistive heat generated when current flows through the weldment. This method is slightly different from resistance brazing welding. It usually requires the use of hand-held tools for welding, and requires the introduction of low voltage and high current into the weldment. This method is commonly used in the electrical equipment manufacturing industry. The figure below shows an application example of connecting the lug connector to the welding cable using resistive soft soldering. Solder is usually added manually. This method is also used for brazing welding industrial copper pipe fittings.

Flame Soft soldering

Flame soft soldering (TS) is similar to flame brazing welding, except that the brazing welding used has a lower temperature and uses air instead of oxygen as the combustion gas. Use a small propane cylinder that becomes a handle when you attach a torch head to the cylinder. The solder is added manually. Braze Soft soldering is widely used in the pipe installation industry to brazing weld pipes and copper fittings.

Crest Soft soldering

Crest brazing welding is an automated soft soldering method mainly used to install electronic components on printed circuit boards. Soldering is performed by sliding the soldering surface of the circuit board over a wave of molten solder. First insert the electronic component leads into the circuit board holes and secure them to the printed circuits on the back of the printed circuit board. The printed circuit board is then placed over a bath of molten solder and brought into contact with the crests of the solder, thereby soldering it to the leads of the electronic component. This is a fully automated method and results in high-quality welds. This method is widely used in the electronics industry.

（2）Brazing

Brazing belongs to the high temperature brazing welding process. Most brazing temperatures are 1200~1400F°, (much higher than soft soldering, but much lower than fusion soldering). Like soft soldering, brazing relies on capillary action to fill the seam with solder. There are many different types of braze alloys that can be used to weld almost any kind of metal or metal alloy.

The American Welding Society divides braze metals into several types based on the types of metallic elements they contain. For example, BCu stands for brazing copper solder, and BAISi is aluminum-silicon hard solder. For each brazing material, different technical parameters are sometimes given to describe the composition and content of each brazing material alloy. The mass fraction of the alloy component is indicated at the end of its type name in the form of numbers or letters, such as 4 in BAg-4. Which braze alloy is used depends on the base metal being welded. The choice of flux depends on the selected brazing alloy. The flux and brazing alloy must be matched to each other to obtain good welding results.

brazing features

High-strength connection: brazing uses high melting point solder, which usually has good mechanical properties and high-strength connection. This gives the brazing connection excellent stability and durability in environments with high stress and vibration.

High-temperature processes: Brazing is usually performed at higher temperatures, usually between 450°C (842°F) and 1,300°C (2,372°F). This makes brazing suitable for connections in high-temperature environments, such as car engine parts or equipment inside furnaces.

Good sealing performance: Since the solder used in brazing usually has high fluidity and wettability, brazing connections usually have good sealing performance and are suitable for applications requiring strict sealing requirements.

Suitable for a variety of materials: brazing can be used to join different types of metals, as well as metals and non-metals. This multi-material connection feature makes brazing a great advantage in some special applications.

Low deformation: Since the operating temperature of brazing is relatively high but still lower than the melting point of the metal being connected, the deformation of the connected parts is usually small. This helps keep the shape and dimensions of the connected parts stable.

Corrosion resistance: Brazing connections usually have high corrosion resistance, especially in high temperature and high pressure environments. This makes brazing suitable for some applications that require long-term stable operation under harsh environmental conditions.

3.Process characteristics of brazing welding

brazing welding has several notable features:

(1) Low temperature: The melting point of the solder used in brazing welding is lower than the melting point of the metal to be connected, so the metal to be connected will not be melted during the joining process. What is melted is the filler metal, so the required heating temperature is relatively low.

(2) No damage to the surface of the original part: Due to the low melting point of the solder, the surface quality of the connected metal will not be damaged during the connection process, and the appearance and performance of the original part can be maintained.

(3) Wide applicability: brazing welding can be used to connect different types of materials, such as dissimilar metal combinations, metal and non-metal connections, and can also connect fibers and dispersion reinforced composite materials, making it widely used in many industries and fields. application.

(4) Reliable strength: The brazing welding process can provide a strong and durable connection, making the connected parts have high strength and durability.

(5) The same material is not required: Compared with other welding methods, brazing welding does not require that the metal to be connected and the solder have the same composition. These characteristics make brazing welding an important metal joining technology that is widely used in many fields.

4.Selection and classification of brazing welding materials

In the brazing welding process, in addition to the base material having a great impact on the strength of the brazing welding, the selection of the brazing welding filling material is also very important. With the correct selection of brazing filler metal and matching brazing welding method, brazing welding can use almost any commercially available filler material to provide a strong joint. During the brazing welding process, the process in which the solder penetrates into the base metal and forms an alloy with the base metal is called diffusion. Under high-temperature and high-stress conditions, filler materials that are easy to diffuse and can form alloys with the base metal should be given priority. When the component is constructed from an extremely thin base metal, filler materials with low diffusivity are often chosen. The strength of the joint depends on the joint design, stress state, brazing welding temperature, brazing welding amount, heating rate, holding time at peak temperature and many other factors to consider. When selecting filling materials, the operating temperature must first be considered. Very few fillings Materials have different melting points. Solder materials whose solidus and liquidus lines are very close usually do not show a strong tendency to separate, and they are relatively fluid.Secondly, the degree of alloying of the filling material and the base metal must be considered; alloying can improve the mechanical properties of the joint and increase the remelting temperature of the joint. Finally, the placement of the filler material is also an important consideration, not only because the placement method requires access to the joint, but also because the filler material must remain in place until it melts during the heating process. Filling materials come in different forms. The most common forms of filling materials are: wire, powder, foil, solder paste, strip, gasket, etc. The choice of material form mainly depends on the design of the specific joint. In order to obtain satisfactory application results in brazing welding, the brazing material must have the following characteristics:

1.Appropriate fluidity at the brazing welding temperature to ensure flow through capillary action and provide uniform alloy distribution; 2. Avoid premature release stability of low melting point elements in the filler metal; 3. The solder has wetting properties The ability of the base metal joint surface; 4. The low volatility of the brazing alloy elements at the brazing welding temperature; 5. The ability to alloy or combine with the parent metal to form an alloy with a higher melting temperature; 6. Operation requirements at brazing welding Within the scope, the controllability of cleaning or corrosion between the filler metal and the base metal; 7. The ability to generate or avoid interaction between the base metal and the filler metal according to the use requirements.

(1) Soft materials

Commonly used solders are tin-lead based, lead-based (T<150℃, generally used for brazing welding copper and copper alloys, good heat resistance, but poor corrosion resistance), cadmium-based (the most resistant solder material) One of the best thermal properties, T=250℃) zinc-based alloys. Soft solder is mainly used for welding workpieces with low stress and low operating temperature. It is widely used in electronic products, electrical appliances and auto parts, such as the connection of various electrical wires and the brazing welding of instruments and instrument components. Tin-lead material is the most widely used solder material. When the high-tin-lead alloy contains Sn1.9%, a eutectic with a melting point of 183°C is formed. Most of the existing low-temperature fillers are tin-based alloys, such as binary Sn-Ag, Sn-Cu and Sn-Bi, or ternary Sn-Bi-Zn and Sn-Ag-Cu.

(2) Hard materials

Commonly used hard solders include: copper-based solder, silver-based solder, aluminum-based solder (mainly used for brazing welding aluminum and aluminum alloys), nickel-based solder, molybdenum-based solder, titanium-based solder, etc.

(3)01 copper base

Due to its good economy, copper-based solder has been widely used in brazing welding of steel, alloy steel, copper and copper alloys. The main materials are copper-phosphorus and copper-based alloys. Copper-phosphorus alloys are mainly composed of copper and 5-7.5wt-% phosphorus. Other small additions include silver (up to 20wt-%) and tin (up to 7.5wt-%). The main advantage of copper-phosphorus alloys compared to other filler metals is that when pure copper is joined in air, they do not require flux, saving time and cost. Phosphorus reacts with oxygen in the atmosphere to form phosphorus pentoxide, which reacts with surface copper oxide to form fusible slag, which will not cause corrosion. Current research and development into copper and copper-phosphorus-based filler metals is primarily focused on heat exchangers in the automotive industry. OKC600 (Aurubis) is a copper-based filler metal containing 4.2wt-% nickel, 15.6wt-% tin and 5.3wt-% phosphorus. It is the emergence of this filler material that makes copper heat exchangers once again Start competing with aluminum radiators.

(4) 02 aluminum base

Aluminum-based solders: For low melting temperature aluminum alloy materials, the filler metal must have a lower melting temperature than the joining material, which is a significant limitation. Most filler metals used for brazing welding aluminum are based on aluminum-silicon systems, where the silicon suppresses the liquidus temperature to 580–630°C. Generally speaking, aluminum brazing welding requires more precise temperature control than brazing welding of other materials. Even with the addition of silicon, the gap between the melting points of the filler and the base metal is small, resulting in a narrow process window and the stable Al2O3 oxide must be removed for successful brazing welding. Many high-strength aluminum alloys cannot use Al-12Si eutectic alloys because the degree of alloying suppresses the solidus line of the base material, and the use of Al-12Si will cause performance degradation or even local melting.To avoid this problem, alloys with lower melting points found in the ternary Al-Cu-Si system were studied. Although copper suppresses the melting temperature, it also allows CuAl to form intermetallic compounds at the brazing welding interface, leading to embrittlement. To solve this problem, Sn (and Mg as a wetting agent) was introduced to form an Al-7Si-20Cu-2Sn-1Mg alloy. The researchers also added rare earth elements to the aluminum-silicon alloy to improve wettability and hardness without affecting the melting temperature. The addition of lanthanum and cerium to Al-12Si alloy increases the wettability and strength of LD2 and LD30 alloys compared to AlSiMg alloys. Aluminum-based filler metals also connect aluminum to titanium, resulting in lightweight, corrosion-resistant and cost-effective structures. The addition of germanium and rare earths to the base Al-12Si system enables successful brazing welding of 6061 aluminum onto Ti-6Al-4V at 530°C.

(5)03 silver base

Silver-based solder is the most widely used hard solder. Because the melting temperature is not very high, it can wet many metals, and has good thermal conductivity, electrical conductivity, and corrosion resistance. It is widely used in brazing welding stainless steel, copper and copper alloys, Kovar alloy, refractory metals, etc. Silver-based solder is a type of hard solder mainly composed of silver or silver-based solid solution, with added metal elements such as Cu, Zn, Ni, etc. It has a low melting point and has excellent strength, plasticity and corrosion resistance. Commonly used systems include Ag-Cu and Ag-Cu-Zn systems, among which AgCu28 and BAg45CuZn are more commonly used. AgCu28 is a eutectic solder that has good wettability to a variety of carbon steels and non-ferrous metals. It is not easy to produce intermetallic compounds and is easy to obtain welds with good performance. It is a template material for analyzing the brazing welding process. BAg45CuZn has good spreadability and gap-filling ability. Cu is the most important additive element, which can reduce the melting point and cost of solder without producing brittle phases. Zn can lower the melting point, but when the content is too high, a brittle phase will be produced in the joint. Ti can improve the wettability of silver-based solder. and ductility. In addition to the above materials, researchers have studied various ternary and quaternary systems based on silver, copper and zinc, including: Ag-Cu-Zn-Ga, silver-copper-zinc-nickel, silver-copper-indium, silver Copper-tin and silver-copper-zinc-tin, etc., tin was finally selected as a suitable substitute for cadmium, and many manufacturers used Ag-Cu-Zn-Sn quaternary filler metal. However, the cost of silver-based solder is relatively high, which limits its application.

(6)04 nickel base

Nickel-based brazing filler metal has good high-temperature strength and oxidation resistance. The brazing welding temperature is generally 1050~1250°C. It is used for brazing welding stainless steel and high-temperature alloy parts. Nickel-based solder is mainly made of nickel, with added metal elements such as B, Si, and P that can lower its melting point. It mainly exists in powder, paste and amorphous forms. They are used where high operating temperatures require good corrosion resistance, such as brazing welding of nickel- and cobalt-based superalloys and stainless steels. Nickel-based filler metals are the preferred choice for joining superalloy components in aerospace and other industries.

(7) 05 titanium base

Titanium-based solder has the advantages of high welding strength, good wetting performance, and good corrosion resistance. Titanium-based solder comes in three forms: powder, paste, and laminated sheets. The most commonly used process in the aerospace field is titanium alloy. brazing welding, especially Ti6Al4V and aluminum alloy parts such as 5A06 aluminum alloy, A6061-T6 aluminum alloy and 5052 aluminum alloy.

(8)06 Jinji

Gold-based solder has the advantages of low vapor pressure, good wettability and excellent resistance to oxidation and corrosion. The main added elements are Ni, Cu, Zn, Pd, In, etc. Cu can reduce the melting point and cost of the solder. Ni can not only reduce the melting point of the solder, but also improve the high-temperature strength and corrosion resistance of the solder, and make the solder have good wettability and fluidity. Due to the high Au content in gold-based solder, the cost is very expensive, so it is only used in vacuum or inert gas furnace brazing welding in important parts of the aerospace field. It has good mechanical properties and good oxidation resistance at high temperatures.

5. Precautions for brazing welding

(1) Ventilation: Brazing welding can produce dangerous smoke and gas. Ventilation fans and exhaust hoods must be used to remove all fumes and gases from the work area. If necessary, use a supplied-air respirator to prevent inhalation of harmful fumes.

(2) Avoid using threaded fasteners: Avoid using screws or bolts during the brazing welding process because they will sinter together at high temperatures and are difficult to remove. If springs or clamps are required, they should be able to withstand the temperatures involved in brazing welding.

(3) Use isolation paint: When using metal clamps, all contact points with the brazing welding components should be coated with isolation paint. This helps prevent unnecessary solder flow and ensures proper joint formation.

(4) Clean and vacuum bake fixtures: Fixtures used during brazing welding should be cleaned and vacuum baked before use to remove all sources of contamination. This helps keep the brazing welding process clean and prevents any unwanted reactions or defects.

(5) Control cleanliness: Proper cleaning of parts to be brazing welded is crucial. Before brazing welding, every effort should be made to ensure that the parts are clean and free of oxides, contaminants, and oil. This ensures correct brazing welding flow and joint formation.

(6) Controlled atmosphere brazing welding: Brazing welding in a controlled atmosphere furnace can prevent overheating and damage to the metal, and allow the filling material to melt and flow into the joint correctly. The process takes place in a controlled atmosphere furnace such as a vacuum chamber or continuous belt furnace.

(7) Proper furnace cycle: Furnace cycle is critical to the success of a brazing welding operation. The heating and cooling speeds should be controlled to prevent deformation, uneven temperature distribution and splashing. Parts should remain stable and not quenched too quickly to avoid deformation and quenching cracking.

(8) Temperature control: The brazing welding temperature should be within the recommended range of the brazing welding specific material. Specified temperature ranges must be observed to achieve proper seam formation and prevent overheating or seam weakness.

(9) Vacuum degree: For vacuum brazing welding, the appropriate vacuum degree should be reached before heating. This helps remove water vapor and ensures a clean brazing welding environment. Vacuum safety interlocks should be programmed into cycles to ensure the required vacuum is achieved.

(10) Soaking time: Soaking at a specific temperature is very important to ensure temperature uniformity and restore vacuum. The recommended soaking time is usually around 30 minutes, but may vary based on specific brazing welding requirements.

(11) Personal protective equipment: During the brazing welding process, appropriate personal protective equipment, such as gloves, goggles and heat-resistant clothing, must be worn to prevent burns and other dangers.

In modern manufacturing, brazing welding, as an important metal joining process, plays a key role in various fields. By selecting appropriate brazing materials and fluxes, brazing welding can achieve the connection of a variety of materials and ensure that the connection has the required strength, sealing and corrosion resistance while maintaining the shape stability of the connected parts.

However, during the brazing welding process, operating procedures and safety standards still need to be strictly followed to ensure the quality and safety of the connection. For every brazing welding, you should carefully select the brazing material, control the heating temperature, prevent oxidation, and check and test the connection quality.

With the continuous development of science and technology, brazing welding technology is also constantly innovating and progressing to meet increasingly complex and demanding engineering needs. By continuously improving the accuracy, efficiency and reliability of the brazing welding process, we can better cope with the challenges in modern manufacturing and provide continuous support for the development of various industries.

Therefore, as a metal joining process with a long history and wide application, brazing welding will continue to play an irreplaceable role in various fields and make positive contributions to the development of modern engineering technology.