First, let’s define what soldering is: “The formation of a permanent connection with interatomic bonds by heating the materials being joined below their melting point, wetting them with solder, flowing solder into the gap and its subsequent crystallization” (GOST 17325-79).
Soldered joints are not mechanical: in accordance with GOST and IPC, they are only subject to the requirements of ensuring electrical contact between the surfaces being soldered. Mechanical strength is standardized for welded joints. Soldering pipes produces a welded joint, and a welding flux is used. We will look at fluxes for soldering.
The main task of the flux is to remove the oxide film from the surface of the contact pads of the printed circuit board and the leads of the mounted elements.
Why do you need flux when welding?
The use of fluxes provides the following advantages in welding.
- In both electric arc and gas welding, the welding flux provides more intense melting of the metal (at high currents or high oxygen concentrations, respectively). Thanks to this, there is no need to cut the edges of the future weld in advance.
- In the weld area and on the surfaces adjacent to it, it is possible to avoid metal waste - its losses due to oxidation and evaporation .
- Arc burning has higher stability , which is especially important for complex seam configurations
- Energy losses of the current source for heating the metal are reduced, and its efficiency increases accordingly.
- The consumption of filler material is optimized .
- More convenient work for the welder, because the flux shields some of the arc flame.
What is it needed for
The chemical activity of the area where the parts are joined increases significantly during the welding process when high temperatures occur. Under the influence of air, slag and oxides begin to enter the metal, which leads to deterioration in the quality of the weld.
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Welding fluxes create a protective environment that isolates the welding zone from the negative influence of air. Flux in welding is a non-metallic component that is involved in the process of joining products and has a positive effect on this process.
Flux in welding adds additional benefits to this process:
- makes metal melting more intense;
- there is no need for pre-processing of product edges;
- metal losses due to evaporation are reduced;
- arc burning becomes stable;
- metal splashing and spark formation are reduced;
- the efficiency of heating the metal increases by reducing energy consumption for this process;
- filler material consumption is reduced to the optimal value.
Flux for welding shields part of the arc flame, which for the welder is an element of safety for the work he performs. Thus, welding flux is a substance that protects the weld pool from interaction with the surrounding air and prevents carbon from being displaced from the base material.
Conditions for using welding fluxes
The purpose of the flux is to stabilize metallurgical processes while maintaining the required productivity of the electrodes. To do this, certain conditions must be observed during the welding process.
- The flux should not react chemically with the metal of the rod and the base metal.
- The weld pool area must remain isolated throughout the welding process.
Residues of flux associated with the slag crust as a result of welding should be easily removed upon completion of work. Moreover, up to 80% of the material can be reused after cleaning.
Operating principle
First, to understand the principle of flux action, you need to understand what a typical welding zone consists of:
- An area of an arc column with an internal temperature of 4-5 thousand degrees Celsius.
- An area of a gas bubble that is formed due to intense atomic evaporation of components in an oxygen environment.
- An area with molten slag located in the upper part of the gas cavity.
- A layer of molten metal at the bottom of the cavity.
- A slag crust that forms a hard boundary to the welding zone.
In addition to the areas mentioned above, the welding wire is no less important, it also affects the chemical reactivity.
Now that we understand what the welding zone consists of, we move on to flux. During welding, the surface of the part is actively oxidized and a slag crust is formed. These processes can be avoided if an easily melting inert material enters the welding zone. This type of material is welding flux. It will protect the part from oxidation and contribute to the formation of a high-quality seam.
To effectively use fluxes in your work, you must meet the following conditions:
- The material should stabilize the speed of work, not slow it down.
- It must not react chemically with the surface of the parts being welded or the welding wire.
- The gas bubble must be isolated from the environment throughout the entire operation.
- If all recommendations are followed, flux residues should be easily removed after welding. In this case, most of the removed material can be reused (after cleaning).
In practice, it turns out that meeting these requirements is not so easy. The flux may vary in composition, as well as the technology for feeding it into the welding zone, so you need to consider what kind of metals you are welding and what type of welding you use.
How fluxes work
- Before welding, a thick (40-60 mm) layer of flux is applied to the joints.
- The electrode is inserted into the welding zone and the arc is ignited.
- Under the influence of high temperatures (up to 6000 °C), the flux with its low density quickly melts in a gas bubble, isolating the weld pool from above, blocking access to gas, water vapor and other chemicals.
- Having a high surface tension, in the same way the flux melt prevents intense splashing of the metal.
- This allows you to significantly increase the arc current (up to 1000-2000 Amperes) without serious loss of electrode material and while maintaining good weld quality.
- Under the influence of flux, thermal power is concentrated in the arc zone - as a result, the melting of the metal occurs faster.
- In this case, all joints are filled with metal, regardless of the condition of the edges.
- The material balance of the weld changes - 60-65% of it is the metal of the parts being welded, and only the rest is the metal of the welding electrode.
Functions of flux additives
Most metals are highly reactive, so they are covered with a layer of oxides. The oxygen content in the air (21%) is quite enough for the oxidation reaction.
When working with metals, an oxide film inevitably gets into the contact area. Even if you removed it the day before using some method, it will form again very quickly.
Oxidation reactions occur especially easily on aluminum surfaces. It is almost impossible to weld them using conventional methods. It is necessary to use fluxes and an inert gas environment.
Oxides entering the weld pool disrupt the process of weld formation. Flux components can prevent metal contact with oxygen and remove a layer of oxidation products. The resulting cloud of gases reduces the consumption of the electrode and prevents splashing of the welding mass.
For high-quality welding you need a constant arc. Gases formed from fluxes stabilize the arc combustion process.
The weld seam is formed under normal conditions without defects. Flux components interact with the molten metals, improving the properties and external surface of the joint.
The choice of flux is determined by the composition of the metal and welding conditions in each production situation.
Welding fluxes - classification
The classification of fluxes is extremely broad. They are distinguished by appearance and physical state, chemical composition, method of production, and purpose. So, for example, for surfacing or arc welding, as a rule, granular or powder fluxes with certain electrical conductivity indicators are used, and for gas welding, gases, powders, and pastes are used.
According to the method of obtaining composites
There are fused and unfused fluxes.
Fused welding flux is widely used not only in welding, but also in surfacing. It demonstrates high efficiency in cases where the surface of the weld metal, by adding additional chemical elements, must obtain higher technical characteristics - for example, increased resistance to corrosion or a very even and smooth weld.
Submerged Surfacing
Fused fluxes are obtained in the following way: the components are ground, mixed, then melted in flame or electric furnaces in the complete absence of oxygen. The heated particles are then passed through a continuous stream of water, hardening and thus turning into granulate. The particle size varies - the thinner the welding rod, the smaller the granules should be.
Unmelted fluxes (ceramic) for welding are made by mixing crushed particles of a mixture of ferroalloys, minerals, and slag-forming materials without subsequent melting. The particles are mixed with glass and then sintered.
Among their advantages:
- low consumption,
- possibility of repeated use,
- high quality of the resulting seam.
An example is UF grade ceramic welding flux (UF-01, UF-02, UF-03), which is used in energy and civil engineering for welding metal structures made of high-strength low-alloy steels.
Chemical composition of fluxes for welding
Chemical composition is an important component in the characteristics of fluxes. The material must be chemically inert at very high temperatures. In addition, it must ensure effective diffusion of individual elements (for example, alloying elements) into the weld metal.
The largest mass fraction (from 35...80% of the total volume) in welding flux is usually (but not all) silicon dioxide (silica) - an acidic oxide, a colorless transparent crystalline mineral. Silicon prevents the process of carbon formation , thereby reducing the risk of cracks and pores in the weld metal.
A significant part is manganese . As an active deoxidizer, this component of welding fluxes reduces the formation of oxides in the weld pool area, reacting first with oxygen in iron oxides, then with silicon oxide. The result of a complex reaction is manganese oxide, which is insoluble in steel and subsequently easily removed. In addition, manganese reacts with sulfur, which is harmful to the weld metal—it binds with it to form sulfide, which is then also removed from the surface of the weld.
Also among the chemical elements of fluxes are alloying additives - in addition to silicon and manganese, these are molybdenum, chromium, titanium, tungsten, vanadium and others . The task is to restore the primary chemical composition of the metal, and in some cases, by alloying, to replenish the burnt-out main impurities of the steel and provide the weld metal with additional special properties. Usually in flux they are represented by compounds with iron - ferroalloys (ferrochrome, etc.).
Types of fluxes for welding by purpose
Their choice of chemical composition directly depends on the purpose of welding fluxes.
- For welding low-carbon steels, fluxes with a high content of silicon and manganese are used in combination with low-carbon steel wire without alloying additives. The second option is a small proportion of manganese (or no manganese at all) in the flux, but alloying additives are present in the steel of the welding rod.
- For welding low-alloy steels, fluxes with high chemical inertness are used - higher than for low-carbon steels. This results in a more ductile weld. An example is flux for welding steel AN-46.
- For welding high-alloy metals, fluxes with minimal chemical activity are used. Silicon, like manganese, is practically not used - it is replaced by fluorite (fluorspar), due to which easily separated fusible slags are formed. Also, such fluxes usually contain aluminum oxide and quicklime.
- To weld active metals (such as titanium), salt fluxes are used - as a rule, these are chloride and fluoride salts of alkali metals. The admixture of oxygen is completely absent in them, since it reduces the plasticity of the seam.
Acid (active) group
Active fluxes are made on the basis of hydrochloric or orthophosphoric acid, in rare cases - hydrofluoric acid; their composition may include zinc chloride, chloride or fluoride metals. Such fluxes are also called corrosive fluxes.
Active flux dissolves the oxide film well, but its residues can corrode the metal. For soldering radioelements and circuit boards, such compounds must be used with caution, carefully removing the remaining film. They are usually used for corrosion-resistant steels, copper and its alloys, galvanized iron, nickel, nichrome.
Purpose of welding flux - examples
| Fused fluxes | Unmelted fluxes | ||
| AN-348-A, AN-348-AM, AN-348-V, AN-348-VM, OSTS-45, OSTS-45M, AN-60, FC-9 | Mechanical welding and surfacing of low-alloy and carbon steels with low-alloy and carbon welding wire | ANK-35 | Welding low-carbon steels with low-carbon wire Sv-08 and Sv-08A |
| AN-8 | Electroslag welding of carbon and low-alloy steels; welding of low-alloy steels with carbon and low-alloy welding wire. | ANK-46 | Welding low-carbon and low-alloy steels |
| AN-15M, AN-18, AN-20S, AN-20P, AN-20SM | Automatic arc welding and surfacing of high- and medium-alloy steels | ANK-30, ANK-47 | Welding seams of high cold resistance |
| AN-22 | Electroslag welding and automatic arc surfacing and welding of low and medium alloy steels | ANK-45 | Welding of high alloy steels |
| AN-26S, AN-26P, AN-26SP | Automatic and semi-automatic welding of stainless, corrosion-resistant and heat-resistant steels | ANK-40, ANK-18, ANK-19 | Surfacing with low-carbon welding wire Sv-08 and Sv-08A; |
| AN-17M, AN-43 and AN-47 | Arc welding and surfacing of carbon, low and medium alloy steels of high and high strength | ANK-3 | As an additive to flux brands AN-348A, OSTS-45, AN-60 to increase the resistance of seams to pore formation |
3.2. Properties of solders
Brazing
carried out by electric contact method, graphite or copper electrodes or using arc welding. Small parts are soldered using an autogen. With the electric contact method, solder is placed in advance between the parts to be joined or introduced into the joint during the soldering process; welding is carried out without metal additives by fusing the ends of the parts being connected.
For electrical contact soldering with silver solders
Borax is usually used as a flux. Soldering with self-fluxing solders, which contain phosphorus, and welding in a protective atmosphere are carried out without the use of flux.
Solders containing phosphorus cannot be used for soldering steel and cast iron and joints subject to shock and vibration due to the brittleness of the soldered seam. The classification and chemical composition of soft and semi-hard solders are given in table. 3.1.
Table 3.1
Classification and chemical composition of soft and semi-hard solders
| Solder | Chemical composition, % | |||||||
| View | Brand | Tin | Antimony | Cadmium | Copper | Lead | Silver | Indium |
| Tin | O2 | 99,9 | – | – | – | – | – | – |
| Antimony-free | POS61 | 60–62 | – | – | – | Rest | – | – |
| POS40 | 39–41 | – | – | – | – | – | ||
| POS10 | 9–10 | – | – | – | – | – | ||
| POS61M | 60–62 | – | – | 1,5–2,0 | – | – | ||
| POSK50-18 | 49–51 | – | 17–19 | – | – | – | ||
| Low antimony | POSSu61-0.5 | 60–62 | 0,2–0,5 | – | – | Rest | – | – |
| POSSu40-0.5 | 39–41 | – | – | – | – | |||
| POSSu30-0.5 | 29–31 | – | – | – | – | |||
| POSSu18-0.5 | 17–18 | – | – | – | – | |||
| Antimony | POSSu95-5 | 94–96 | 4–5 | – | – | Rest | – | – |
| Silver | PSrO10-90 | Rest | – | – | – | – | 10±0,5 | – |
| PSrOSu8 (VPr-6) | – | – | – | – | – | 8±0,5 | – | |
| PSrMO5 (VPr-9) | – | – | – | 2±0,5 | – | 5±0,5 | – | |
| PSrOS3.5-95 | – | – | – | – | 3,5±0,4 | – | ||
| PSrOS3-58 | 57,8±1,0 | – | – | – | – | 3±0,4 | – | |
| PSr3 | – | 3±0,3 | – | |||||
| PSr3Kd | – | – | 95–97 | – | – | 3,0–4,0 | – | |
| PSrO3-97 | Rest | – | – | – | – | 3±0,3 | – | |
| PSr2.5 | 5,0–6,0 | – | – | – | 91–93 | 2,2–2,7 | – | |
| PSr2.5S | – | – | – | – | – | 2,5±0,2 | – | |
| PSr2 | 30±1 | 2±0,2 | – | |||||
| PSrOS2-58 | 58,8±1,0 | – | – | – | – | 2±0,3 | – | |
| PSr1.5 | 15±1 | – | – | – | – | 1,5±0,3 | – | |
| PSr1 | 35±1 | – | – | – | – | 1±0,2 | – | |
| Indium | POSI30 | 42 | – | – | – | 28 | – | 3 |
| PSR3I | – | – | – | – | – | 3 | 97 | |
The physical and mechanical properties of soft and semi-hard solders are given in Table. 3.2.
Table 3.2
Physical and mechanical properties of soft and semi-hard solders
| Solder grade | melting point, °C | approximate soldering temperature, °C | density, kg/m³ | specific electrical resistance, μm m | limit of mechanical tensile strength, MPa | |
| solidus | liquidus | |||||
| O2 | 232 | 232 | 280 | 7310 | – | 25 |
| POS61 | 183 | 190 | 240 | 8500 | 0,139 | 43 |
| POS40 | 183 | 238 | 290 | 9300 | 0,159 | 38 |
| POS10 | 268 | 299 | 350 | 10800 | 0,200 | 32 |
| POS61M | 268 | 192 | 240 | 8500 | 0,143 | 45 |
| POSK50-18 | 142 | 145 | 185 | 8800 | 0,133 | 40 |
| POSSu61-0.5 | 183 | 189 | 240 | 8500 | 0,140 | 45 |
| POSSu50-0.5 | 183 | 216 | – | 8900 | 0,149 | – |
| POSSu40-0.5 | 183 | 235 | 285 | 9300 | 0,169 | 40 |
| POSSu35-0.5 | 183 | 245 | – | 9500 | 0,172 | – |
| POSSu30-0.5 | 183 | 265 | 306 | 9700 | 0,179 | 36 |
| POSSu25-0.5 | 183 | 266 | – | 10000 | 0,182 | – |
| POSSu18-0.5 | 183 | 277 | 325 | 10200 | 0,198 | 36 |
| POSSu95-5 | 234 | 240 | 290 | 7300 | 0,145 | 40 |
| POSSu40-2 | 185 | 229 | – | 9200 | 0,172 | – |
| POSSu33-2 | 185 | 243 | – | 9400 | 0,179 | – |
| POSSu30-2 | 185 | 250 | – | 9600 | 0,182 | – |
| POSSu25-2 | 185 | 260 | – | 9800 | 0,183 | – |
| POSSu18-2 | 188 | 270 | – | 10100 | 0,206 | – |
| POSSu15-2 | 184 | 275 | – | 10300 | 0,208 | – |
| POSSu10-2 | 268 | 285 | – | 10700 | 0,208 | – |
| POSSu8-3 | 240 | 290 | – | 10500 | 0,207 | – |
| POSSu5-1 | 275 | 308 | – | 11200 | 0,200 | – |
| POSSu4-6 | 244 | 270 | – | 10700 | 0,208 | – |
| PSrO10-90 | – | 280 | – | 7600 | 12,9 | – |
| PSrOSu8 (VPr-6) | – | 250 | – | 7400 | 19,7 | – |
| PSrMO5 (VPr-9) | – | 240 | – | 7400 | 16,3 | – |
| PSrOS3.5-95 | – | 224 | – | 7400 | 12,3 | – |
| PSrOS3-58 | – | 190 | – | 8600 | 14,5 | – |
| PSr3 | – | 315 | – | 11400 | 20,4 | – |
| PSr3Kd | 300 | 325 | 360 | 8700 | 8,0 | 54 |
| PSr2.5 | 295 | 305 | 355 | 11000 | 21,4 | – |
| PSr2.5S | – | 306 | – | 11300 | 20,7 | – |
| PSr2 | – | 238 | – | 9500 | 16,7 | – |
| PSrOS2-58 | – | 183 | – | 8500 | 14,1 | – |
| PSr1.5 | – | 280 | – | 10400 | 19,1 | – |
| PSr1 | – | 235 | – | 9400 | 26,0 | – |
| POSI30 | 117 | 200 | 250 | 8420 | – | – |
| PSR3I | 141 | 141 | 190 | 7360 | – | – |
Fluxes for gas welding
For welding aluminum and other non-ferrous metals, cast iron, tool steels, and certain grades of thin sheet steel, a protective gas atmosphere is used. It is provided by gaseous, paste, and powder fluxes. They can be applied:
- on the edges of the parts being connected;
- directly into the weld pool;
- to the filler rod.
Depending on the physical state of the material, fluxes for welding are supplied to the work area in different ways. Powdered composites cause some difficulty - they must be evenly and accurately introduced into the melt, without allowing the gas flow to blow up the powder. The compositions in the form of pastes are fed to the connection area. To supply gaseous fluxes, flow meters are used - with their help, gas is dosed into the working area.
Electromagnetic flow meter
An important point: for gas welding, the composition of the flux is selected depending on the oxides formed during welding. If they are acidic, the fluxes must be alkaline (basic); on the contrary, if they are alkaline oxides, acidic fluxes should be chosen.
The fluxes most widely used in gas welding are:
- copper, brass, bronze - for welding they use acidic fluxes containing boron-containing compounds (boric acid, etc.) - for example, brands such as MB-2 or BM-1;
- cast iron - for its welding, fluxes containing various compounds of alkali metals - sodium and potassium - are usually used;
- aluminum - compositions containing potassium, lithium and sodium fluorides, as well as chlorides, are used here. In this case, the most widely used welding flux is AF-4A.
Fluxes for gas welding are not used to join parts made of low-carbon steels, since low-melting iron oxides intensively accumulate on the surface of the molten metal.
Russian stamps
Among the domestic products, SKF is noticeable - a soldering flux made from alcohol and rosin. The prepared solution can be used immediately. All components are already mixed. It is easy to use, the residual layer can be easily removed with gasoline or alcohol.
There are many narrowly targeted soldering compounds on the market from Russian manufacturers, designated by abbreviations: LTI, TAGS, ZIL, KRS, LK and so on. They contain highly active ammonium chloride, ammonia, amines, zinc chloride, and other active reagents.
When choosing a flux, pay attention to the recommendations for use. In mixtures, every component matters.
Some types are only suitable for working with hard solders. There are types designed for specific alloys. Thus, special reactive compounds have been developed for soldering aluminum. You should carefully study the label and then select a flux.
Fluxes for automatic welding
Automatic and semi-automatic welding is most widely used when working with large structures. Thanks to high currents and flux, it is possible to weld parts of considerable thickness, without preliminary cutting of the edge. Areas of use: pipe welding, tank manufacturing, shipbuilding.
This welding method is characterized by automatic maintenance of a stably burning electric arc, the required amount of flux (with suction of unmelted), as well as continuous renewal of the molten electrode. To maintain a protective gas cloud of the required composition in the welding zone, the thickness of the flux layer should be 40-80 mm, width 50-100 mm. The brand of flux for automatic welding, as for classical arc welding, also depends on the characteristics of the metal being welded. Welding is carried out in the lower spatial position.
You can buy flux for welding of various types and brands at a profit.
Products in this category
Ceramic flux UF-03 bags TU 5929-053-00186654-2013 Ceramic flux UF-N bags TU 5929-052-00186654-2013 Ceramic flux UF-K bags TU 5929-052-00186654-2013 Flux AN 348 A mesh ki GOST 9087- 81 Flux AN 47 bags GOST 9087-81 Ceramic flux UF-01 bags TU 5929-051-00186654-2013 Ceramic flux UF-02 bags TU 5929-052-00186654-2013 Ceramic flux UF-03 bags TU 5929-053 -00186654- 2013 Ceramic flux UF-N bags TU 5929-052-00186654-2013 Ceramic flux UF-K bags TU 5929-052-00186654-2013 Flux AN 348 A bags GOST 9087-81 Flux AN 47 bags GOST 9087-81 F luce ceramic UF- 01 bags TU 5929-051-00186654-2013
Principle of operation
What welding flux is can be understood by understanding how welding occurs with its participation.
- Before starting the welding process, a thick layer of flux is applied to the future joint.
- An electrode is inserted into the welding zone and the arc is ignited.
- Flux, which has a reduced density, begins to quickly melt, isolating the weld pool from air access.
- Due to the high surface tension of the flux, strong splashing of the metal is prevented, which allows the current to be increased.
- In the arc zone, under the influence of the flux, the heat value increases, as a result of which the welding process begins to proceed faster.
- All joints are filled with molten metal.
A significant part of the remaining flux after cleaning can be used again.
The flux welding process occurs differently depending on the type of welding. When manual welding, flux in powder form is poured onto the surface of the product in a layer of up to 60 millimeters. The width is in the range of 50-100 millimeters.
Insufficient thickness can lead to defects - lack of penetration, cracks and cavities. When moving the electrode during welding, the next layer of flux is added as it moves. Depending on the granulation, the required height of the flux powder layer is determined and the current strength is selected.
In automatic and semi-automatic welding, the flux is supplied through a special tube from the machine’s hopper. Then the supply of welding wire, which has the function of an electrode, is connected. The unused part of the flux, together with the slag embedded in it, enters a container intended for this purpose. The cooled crust is removed from the surface mechanically. To work with automatic equipment, fluxes from the AN category, as well as ceramic ones, are most often used.
