Welding technology of metal materials
What is welding technology?
Welding: usually refers to the welding of metal. It is a forming method by heating or pressing, or both at the same time, so that two separate objects produce the binding force between atoms and form a whole body.
Classification of welding
According to the different heating degree and process characteristics, welding methods can be divided into three categories.
- (1) Fusion welding. The welding part of the workpiece is heated to the melting state locally to form a molten pool (usually with filler metal added). After cooling and crystallizing, the weld is formed, which is combined into an inseparable whole by the welder. The common welding methods include gas welding, arc welding, electroslag welding, plasma arc welding, electron beam welding, laser welding, etc.
- (2) Pressure welding. In the welding process, whether heated or not, pressure welding method is required. Common pressure welding includes resistance welding, friction welding, cold pressure welding, diffusion welding, explosion welding, etc.
- (3) Brazing. The filler metal (filler metal) with melting point lower than the metal to be welded is used to fill the joint gap and diffuse with the weld metal to realize the connection. In the process of brazing, the parts to be welded are not melted, and generally there is no plastic deformation.
Features of welding:
- (1) It can save metal material and light structure weight.
- (2) In order to make heavy and complex machine parts and simplify casting, forging and cutting process, the best technical and economic effect can be obtained.
- (3) The welded joint has good mechanical properties and sealing property.
- (4) It can make bimetallic structure and make full use of the properties of the material.
Application: welding technology is widely used in machine manufacturing, shipbuilding industry, construction engineering, power equipment production, aviation and aerospace industry.
Disadvantages: welding technology also has some shortcomings, such as the welding structure can not be disassembled, which brings inconvenience to the maintenance; there will be welding stress and deformation in the welding structure; the microstructure and properties of welded joints are often uneven, and welding defects will be produced.
Introduction of various welding technologies
Arc: a strong and lasting gas discharge phenomenon, there is a certain voltage between the positive and negative electrodes, and the gas medium between the two electrodes should be in the ionized state. When the welding arc is ignited, the two electrodes (one is the workpiece, the other is filled with metal wire or electrode) are connected to the power supply for a short time and separated rapidly. When the two electrodes contact each other, the short circuit occurs and the arc is formed. This method is called contact arcing. After the arc is formed, as long as the power supply maintains a certain potential difference between the two poles, the arc combustion can be maintained.
- Arc characteristics: low voltage, high current, high temperature, high energy density, good mobility, etc. generally, the voltage of 20 ~ 30V can maintain the stable combustion of the arc, and the current in the arc can be from tens of amperes to several thousand amperes to meet the welding requirements of different workpieces. The temperature of the arc can reach above 5000K, which can melt various metals.
- Arc composition: cathode area, anode area and arc column area.
- Arc welding power source: the power source used for welding arc is called arc welding power source, which can be generally divided into four categories: AC arc welding power source, DC arc welding power source, pulse arc welding power source and inverter arc welding power source.
- DC positive connection: DC welding machine is used. When the workpiece is connected to the anode and the electrode is connected to the cathode, it is called DC positive connection. At this time, the workpiece is heated and suitable for welding thick workpieces.
- DC reverse connection: when the workpiece is connected with the cathode and the electrode is connected with the anode, it is called DC reverse connection. At this time, the heat of the workpiece is small, which is suitable for welding thin and small workpieces. When welding with AC welding machine, because the polarity of the two poles changes alternately, there is no problem of direct connection or reverse connection.
The process of welding metallurgy
In the process of arc welding, liquid metal, slag and gas interact, which is the process of metal remelting. However, due to the particularity of welding conditions, welding chemical metallurgy process has different characteristics from general smelting process.
First of all, the welding metallurgy temperature is high, the phase boundary is large, and the reaction speed is fast. When there is air invasion in the arc, the liquid metal will have strong oxidation and nitriding reaction, and a large amount of metal will evaporate. The water in the air and the hydrogen atoms decomposed from the oil, rust and water in the workpiece and welding materials can dissolve into the liquid metal under the high temperature of the arc, which will reduce the plasticity and toughness of the joint（ Hydrogen embrittlement) to produce cracks.
Secondly, the welding pool is small and the cooling is fast, which makes it difficult for various metallurgical reactions to reach the equilibrium state. The chemical composition in the weld is not uniform, and the gas and oxide in the weld pool can not float out in time, which is easy to form defects such as pores, slag inclusion, and even cracks.
In order to ensure the quality of welding seam, the following measures are usually taken in the process of arc welding:
- (1) During welding, the molten metal is mechanically protected from the air. There are three protection methods: gas protection, slag protection and gas slag joint protection.
- (2) The metallurgical treatment of welding pool mainly includes adding a certain amount of deoxidizer (mainly ferromanganese and ferrosilicon) and a certain amount of alloy elements into the welding materials (electrode coating, welding wire and flux), so as to eliminate the FeO in the welding pool and compensate the burning loss of alloy elements.
Common ARC welding methods
Manual arc welding
Manual arc welding is one of the earliest developed and widely used arc welding methods. It uses the coated electrode as electrode and filler metal. The arc burns between the end of the electrode and the surface of the workpiece to be welded. On the one hand, the coating can produce gas to protect the arc; on the other hand, slag can be produced to cover the surface of molten pool to prevent the interaction between molten metal and surrounding gas. The more important role of slag is to produce physical and chemical reaction with molten metal or to add alloy elements to improve the energy of weld metal. Manual arc welding equipment is simple, light and flexible. It can be used in the welding of short seam in maintenance and assembly, especially in the welding of difficult parts. Manual arc welding with corresponding electrodes can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys.
Submerged arc welding
Submerged arc welding (SAW) is a melting electrode welding method, in which granular flux is used as protective medium and the arc is hidden under the flux layer. The welding process of submerged arc welding consists of three parts:
- 1. Sufficient granular flux is uniformly deposited at the joint to be welded;
- 2. The conductive nozzle and the weldment are respectively connected with the welding power supply to generate welding arc;
- 3. The welding wire is automatically sent in and the arc is moved for welding.
The main characteristics of submerged arc welding are as follows:
1. The arc performance is unique.
- (1) the weld quality is high, and the slag insulation air protection effect is good CO2, nitrogen content and oxygen content in weld metal are greatly reduced, welding parameters are automatically adjusted, arc walking is mechanized, molten pool exists for a long time, metallurgical reaction is sufficient, and wind resistance is strong, so weld composition is stable and mechanical properties are good;
- (2) good working conditions, slag isolation arc light is conducive to welding operation; mechanized walking, labor intensity is low.
2. Compared with GMAW, arc column has the following characteristics:
- (1) Good regulation performance of equipment, high sensitivity of automatic regulation system, and high stability of welding process;
- (2) Higher lower limit of welding current.
3. High production efficiency, because the conductive length of the welding wire is shortened, the current and current density are significantly improved, so that the penetration capacity of the arc and the deposition rate of the welding wire are greatly improved; and due to the heat insulation effect of flux and slag, the total thermal efficiency is greatly increased, and the welding speed is greatly improved.
- Metallurgical reaction: the flux participates in the metallurgical reaction, Si and Mn are reduced, and part C is burned, which limits the removal of H from impurities s and P, and prevents hydrogen blowholes.
- Droplet transfer: slag wall transition.
- Power supply: DC power supply is used for small current, constant speed wire feeding, self arc regulation; high current is generally AC power supply, variable speed wire feeding (SAW welding wire is generally thick), arc voltage feedback arc regulation. Welding materials: welding wire and flux. The selection of welding wire and flux must ensure the high quality welding joint, reduce the cost as much as possible, and pay attention to the applicable current type and polarity.
Scope of application: because of its deep penetration, high productivity and high degree of mechanical operation, it is suitable for welding long welds of medium and heavy plate structures. It is widely used in shipbuilding, boiler and pressure vessel, bridge, overweight machinery, nuclear power plant structure, ocean structure, weapons and other manufacturing sectors. It is one of the most commonly used welding methods in today’s welding production. Submerged arc welding is not only used for the connection of components in metal structure, but also can be overlaid with wear-resistant or corrosion-resistant alloy layer on the surface of base metal. With the development of welding metallurgy technology and welding material production technology, submerged arc welding materials have developed from carbon structural steel to low alloy structural steel, stainless steel, heat-resistant steel and some non-ferrous metals, such as nickel base alloy, titanium alloy, copper alloy, etc. Due to its own characteristics, its application also has certain limitations, mainly as follows:
- (1) The limitation of welding position, due to the reason of flux retention, if no special measures are adopted, submerged arc welding is mainly used for horizontal and vertical welding, but not for horizontal, vertical and overhead welding;
- (2) Due to the limitation of welding materials, aluminum, titanium and other highly oxidizing metals and alloys can not be welded It is suitable for welding ferrous metal;
- (3) It is only suitable for long weld cutting, and cannot weld the weld with limited space position;
- (4) It can not directly observe the arc;
- (5) It is not suitable for thin plate and low current welding.
Gas tungsten arc welding
This is a kind of gas shielded arc welding (GMAW), which uses the arc between the tungsten electrode and the workpiece to melt the metal and form a weld. In the welding process, the tungsten electrode does not melt and only acts as an electrode. At the same time, argon or helium is sent into the nozzle of welding torch for protection. Additional metals can be added as needed. It is called TIG welding in the world. Gas tungsten arc welding (GMAW) is an excellent method for joining sheet metal and backing welding because it can control heat input well. This method can be used for joining almost all metals, especially for welding aluminum, magnesium, which can form refractory oxides, and active metals such as titanium and zirconium. The welding quality of this welding method is high, but compared with other arc welding, its welding speed is slower.
Gas metal arc welding
GMAG is a fusion welding method using electric arc as the heat source. The arc is established between the continuously fed welding wire and the molten pool. The molten weld pool is formed by mixing the molten wire metal and the base metal. After the arc heat source is removed, the weld is crystallized and the separated base metal is connected by metallurgical means.
Features of CO2 welding:
- (1) Under the action of high temperature of welding arc, CO2 will decompose into CO, O2 and O, which has strong compression effect on the arc. Therefore, the arc shape of this welding method has the characteristics of small diameter of arc column, small area of arc heel and difficult to cover all droplet at the end of welding wire. Therefore, the transfer resistance (spot force) of droplet is affected The larger the droplet coarsens, the axial transition path becomes worse, and the spatter rate is higher;
- (2) The welding zone is well protected, and the density of CO2 is the largest among the commonly used shielding gases. In addition, the volume of CO2 gas increases after thermal decomposition, so the protection is better;
- (3) The energy is relatively concentrated and the penetration capacity is large;
- (4) Low production cost and energy saving.
- (5) It also has the advantages of good visibility of welding area, easy observation and operation, small welding heat affected zone and welding deformation, small volume of molten pool, fast crystallization rate, good all position welding performance and low sensitivity to rust.
- (1) Oxidation of alloying elements during CO2 welding, CO2 will decompose into CO, O2 and O under the action of high arc temperature. Under the welding conditions, CO is not soluble in metal and does not participate in the reaction. However, both CO2 and o have strong oxidation, which can oxidize Fe and other alloy elements.
- (2) Deoxidation and alloying of weld metal? Usually, a certain amount of deoxidizer is added to the welding wire to deoxidize. In addition, the remaining deoxidizer is left in the weld as alloy elements to make up for the oxidation loss and ensure the chemical composition requirements of the weld.
- (1) Short circuit transfer (short arc, fine wire, small current) is suitable for all position welding of thin plate;
- (2) Fine particle transition, thick wire, long arc, high current welding;
- (3) Submerged arc jet transfer (rarely used).
Power supply: flat characteristic power supply (single knob adjustment), DC reverse connection, constant speed wire feeding
Welding materials: CO2 gas and welding wire
Scope of application: at present, CO2 gas shielded welding is widely used in locomotive manufacturing, ship manufacturing, automobile manufacturing, coal mining machinery manufacturing and other fields. It is suitable for welding low carbon steel, low alloy steel and low alloy high strength steel, but it is not suitable for welding non-ferrous metal and stainless steel. Although some data show that CO2 gas shielded welding can be used for stainless steel welding, it is not the first choice for welding stainless steel.
Plasma arc welding
With the help of water-cooling nozzle and other measures, the cross-sectional area of the arc column area can be reduced, and the temperature, energy density and plasma velocity of the arc can be significantly increased. This kind of arc with external restraint to compress the arc column is called plasma arc.
Plasma arc is a special form of arc. It is a kind of arc with high energy density. It is still a phenomenon of gas conduction. The plasma arc welding method is used to heat the workpiece.
Classification: keyhole plasma arc welding and micro plasma arc welding.
- Keyhole type plasma arc: welding current is 100 ~ 300A, there is no need to groove the joint and leave no gap. During welding, plasma arc can completely penetrate the weldment and form a small through-hole. The molten metal is squeezed around the small hole, and the arc moves, and the small hole moves along with it, forming a weld in the rear, so as to realize one-sided welding and two-sided forming. The upper limit of plate thickness that can be welded by this method is 7mm for carbon steel and 10mm for stainless steel.
- Micro plasma arc: welding current is 0.1 ~ 30a, welding thickness is 0.025 ~ 2.5mm. In addition, there are also fusion plasma arc welding for copper and copper alloy welding, which can be used for deep penetration welding of thick plate or high-speed welding of thin plate and melting electrode plasma arc welding of surfacing welding, which can solve the AC (variable polarity) plasma arc welding process of aluminum alloy plasma arc welding. The main process parameters of plasma arc welding include welding current, welding speed, shielding gas flow, ion gas flow, nozzle structure and aperture of welding gun.
- Plasma arc cutting: using the high temperature and high-speed arc flow of plasma arc, the metal in the incision is partially melted and evaporated, and the molten material is blown away from the matrix with the help of high-speed air flow or water flow to form a cutting method.
- (1) Plasma arc has the advantages of high energy density, high temperature of arc column and strong penetrability. The steel with thickness of 10-12mm can be welded through and formed on both sides at one time. It has the advantages of high welding speed, high productivity and small stress deformation.
- (2) The cross section of the weld is in the shape of wine cup without finger penetration.
- (3) Arc straightness is good, and the fluctuation of molten pool is small due to the influence of arc length fluctuation.
- (4) The arc is stable for 0.1A, still has a relatively flat static characteristics, with constant current source, can be very good for thin plate welding (0.1 mm).
- (5) Tungsten electrode shrinks inward to prevent tungsten inclusion in weld
- (6) The keyhole welding technology is used to realize single side welding and double side forming.
- (7) The equipment is relatively complex and the gas consumption is large, so it is only suitable for indoor welding. The accessibility of welding gun is worse than that of TIG.
- (8) Because of the small arc diameter, it is necessary to align the welding torch axis with the weld center line more accurately.
- Metallurgical reaction: single, only evaporation.
Power supply: steep drop power supply, DC positive connection; AC and steep drop power supply are used for welding aluminum and magnesium, and arc striking and arc stabilizing measures are required.
Welding materials: shielding gas, tungsten electrode.
Scope of application: widely used in industrial production, especially in the welding of copper and copper alloy, titanium and titanium alloy, alloy steel, stainless steel, molybdenum and other metals used in military industry and advanced industrial technology, such as titanium alloy missile shell, some thin-walled containers on aircraft, etc.
Tubular wire arc welding
Tubular wire arc welding is a kind of GMAW, which uses the burning arc between welding wire and workpiece as heat source. The welding wire used is tubular welding wire, which is filled with flux of various components. During welding, shielding gas, mainly CO2, is added. The flux decomposes or melts when heated, which plays the role of slag forming, bath protection, alloying and arc stabilization. In addition to the advantages of GMAW mentioned above, tubular wire arc welding has more advantages in metallurgy due to the effect of flux in the pipe. Tubular wire arc welding can be applied to the welding of most kinds of ferrous metal joints. Tubular wire arc welding has been widely used in some advanced industrial countries. “Tubular wire” is now known as “flux cored wire.”
Gas welding: a fusion welding method that uses the heat generated by the combustion of combustible gas in oxygen to melt the base metal weld and realize the connection. Gas welding is a welding method using gas flame as heat source. The most widely used is the oxyacetylene flame which uses acetylene as fuel. Due to the simple equipment and convenient operation, but the heating speed and productivity of gas welding are low, the heat affected zone is large and easy to cause large deformation. Gas welding can be used for welding many ferrous metals, nonferrous metals and alloys.
Combustible gas: acetylene, liquefied petroleum gas, etc. Taking acetylene as an example, the flame temperature of acetylene in oxygen can reach 3200 ℃. There are three kinds of oxyacetylene flame:
- ① Neutral flame: the volume mixing ratio of oxygen and acetylene is 1-1.2, acetylene is fully burned, suitable for welding carbon steel and non-ferrous alloy.
- ② Carbon flame: oxygen and acetylene volume mixing ratio is less than 1, acetylene excess, suitable for welding high carbon steel, cast iron and high speed steel.
- ③ Oxidation flame: the volume mixing ratio of oxygen and acetylene is greater than 1.2, with excess oxygen, which is suitable for brazing brass and bronze.
Gas welding flame temperature is low, heating speed is slow, heating area is wide, welding heat affected zone is wide, welding deformation is large, and in the welding process, the protection of molten metal is poor, and the welding quality is not easy to guarantee, so its application is few. However, gas welding has the characteristics of no power supply, simple equipment, low cost, easy to move and strong versatility, so it is of practical value in no power supply situation and field work. At present, it is mainly used for the welding of thin steel plate (thickness 0.5-3mm), copper and copper alloy, and repair welding of cast iron.
Gas pressure welding
Gas pressure welding and gas welding, gas pressure welding is also a gas flame as the heat source. During welding, the ends of the two butted workpieces are heated to a certain temperature, and then sufficient pressure is applied to obtain a firm joint. It is a solid phase welding. Gas pressure welding without filler metal, commonly used in rail welding and steel welding.
Electroslag welding (ESW) is a welding method which takes the resistance heat of slag as the energy source. The welding process is carried out in the vertical welding position and in the assembly gap formed by the end faces of two workpieces and the water-cooled copper sliding blocks on both sides. During welding, the end of the workpiece is melted by the resistance heat generated by the current passing through the slag. According to the shape of electrode used in welding, electroslag welding can be divided into wire electrode electroslag welding, plate electroslag welding and nozzle electroslag welding.
Characteristics of electroslag welding: in the welding process of electroslag welding, except for an arc process in the initial stage, the rest are stable electroslag processes, which are essentially different from submerged arc welding.
Advantages of electroslag welding: the thickness of weldable workpiece is large (from 30mm to 1000mm), and the productivity is high. It is mainly used for the welding of butt joint and T-joint in section. Electroslag welding can be used in the welding of various steel structures and the assembly welding of castings. Due to the slow heating and cooling, wide heat affected zone, coarse microstructure and toughness of electroslag welding joint, normalizing treatment is generally required after welding.
Limitations of electroslag welding:
- (1) Due to the large weld pool, slow heating and cooling, the weld and heat affected zone are easy to overheat and form coarse structure. Therefore, normalizing treatment is usually used to eliminate coarse grain in the joint after welding.
- (2) Electroslag welding is always carried out by vertical welding, and flat welding is not allowed. Electroslag welding is not suitable for workpieces with thickness less than 30mm, and the weld joint should not be too long.
Classification and application of electroslag welding
Classification of electroslag welding: wire electroslag welding, plate electroslag welding, nozzle electroslag welding and pipe electrode electroslag welding.
Wire electrode electroslag welding is the most commonly used electroslag welding method. It uses welding wire as electrode. According to the thickness of the weldment, one or more welding wires can be used. The thickness of the weldment that can be welded by single wire welding is 40-60mm. When the thickness of the weldment is greater than 60mm, the welding wire should be swung horizontally; the three wire swing can weld 450mm thick weldments. Wire electroslag welding is mainly used to weld weldments with a thickness of 40-450 mm and longer welds, as well as girth welds of large weldments.
Application of electroslag welding: it is mainly used in heavy machinery manufacturing industry to manufacture forging welding structural parts and cast welding components, such as heavy machine tool base, high pressure boiler, etc. the thickness of welding parts is generally 40 ~ 450 mm, and the materials are carbon steel, low alloy steel, stainless steel, etc.
Electron beam welding
Electron beam welding (EBW) is a method of welding in which the heat energy produced by the concentrated high-speed electron beam bombarding the workpiece surface is used. In electron beam welding, the electron beam is generated and accelerated by the electron gun. Electron beam welding is commonly used: high vacuum electron beam welding, low vacuum electron beam welding and non vacuum electron beam welding. The first two methods are carried out in a vacuum chamber. The welding preparation time (mainly vacuumizing time) is long, and the size of workpiece is limited by the size of vacuum chamber. Compared with arc welding, electron beam welding has the advantages of large weld penetration, small weld width and high purity of weld metal. It can be used not only in the precise welding of very thin materials, but also in the welding of very thick (up to 300 mm) components. All metals and alloys that can be fusion welded by other welding methods can be welded by electron beam. It is mainly used for welding products with high quality. It can also solve the welding of dissimilar metals, easily oxidized metals and refractory metals. But it is not suitable for mass production.
Electron beam welding machine: the core is the electron gun, which is the device to complete the generation of electrons, the formation and convergence of electron beams, mainly composed of filament, cathode, anode, focusing coil, etc. When the cathode reaches about 2400k, electrons will be emitted. Under the action of high voltage electric field between cathode and anode, electrons will be accelerated (about 1 / 2 light speed) and emitted through anode hole. Then, through focusing coil, they will converge into electron beam with diameter of 0.8-3.2mm, which will be injected into weldment. Kinetic energy will be converted into heat energy on the surface of weldment, so that the welding joint can be melted rapidly After cooling and crystallization, the weld is formed.
According to the vacuum degree of welding room (where the weldment is placed), the classification of EBW is as follows:
- (1) High vacuum electron beam welding. The working room and electron gun are in the same room, and the vacuum degree is 10-2 ~ 10-1pa, which is suitable for the precision welding of refractory, active, high-purity metals and small parts.
- (2) Low vacuum electron beam welding. The working chamber and electron gun are divided into two vacuum chambers. The vacuum degree of the working chamber is 10-1 ~ 15pa, which is suitable for large structural parts and refractory metals which are not sensitive to oxygen and nitrogen.
- (3) Non vacuum electron beam welding. The distance between the weldment and the electron beam outlet should be controlled at about 10 mm to reduce the scattering caused by the collision between the electron beam and gas molecules. Non vacuum electron beam welding is suitable for the welding of carbon steel, low alloy steel, stainless steel, refractory metal, copper, aluminum alloy and so on.
Advantages of vacuum electron beam welding:
- (1) The energy density of electron beam is high, up to 5 × 108W / cm2, which is about 5000-10000 times of that of ordinary arc. It has the advantages of high heat concentration, high thermal efficiency, small heat affected zone, narrow and deep weld seam and minimal welding deformation.
- (2) In the vacuum environment, the metal does not interact with the gas phase and the joint strength is high.
- (3) The focus radius of electron beam can be adjusted in a wide range, flexible control and strong adaptability. It can be used to weld 0.05mm thin parts and 200-700mm thick plates.
Application: especially suitable for welding some refractory metals, active or high purity metals and heat sensitive metals. However, the equipment is complex, the cost is high, the size of the weldment is limited by the vacuum chamber, the assembly precision is high, and it is easy to stimulate X-ray, the welding auxiliary time is long, and the productivity is low. These shortcomings limit the wide application of EBW.
Laser welding is a kind of welding with high power coherent monochromatic photon beam as heat source. This welding method usually includes continuous power laser welding and pulse power laser welding. The advantage of laser welding is that it does not need to be carried out in vacuum, but the penetration is not as strong as electron beam welding. The precise energy control can be carried out in laser welding, so the welding of precision micro devices can be realized. It can be applied to many metals, especially to the welding of some difficult metals and dissimilar metals.
Laser generation: after the material is excited, the wavelength, frequency and direction of the same beam are produced.
The characteristics of laser: it has the characteristics of good monochromatic, directional and high energy density. After the laser is focused by transmission or reflector, the energy beam with diameter less than 0.01mm and power density up to 1013w / cm2 can be obtained, which can be used as heat source for welding, cutting, drilling and surface treatment. Laser materials include solid, semiconductor, liquid, gas and so on. The yttrium aluminum garnet (YAG) solid laser and CO2 gas laser are mainly used in welding, cutting and other industrial processing.
The main advantages of laser welding are as follows:
- (1) Laser can be bent and transmitted by optical methods such as optical fiber and prism, which is suitable for welding micro parts and parts difficult to reach by other welding methods, and can be welded by transparent materials.
- (2) High energy density, high speed welding can be realized, heat affected zone and welding deformation are very small, especially suitable for welding of heat sensitive materials.
- (3) Laser is not affected by electromagnetic field, does not produce X-ray and does not need vacuum protection, so it can be used in welding of large structures.
- (4) It can directly weld the insulating conductor without stripping the insulating layer in advance, and can also weld dissimilar materials with different physical properties.
The main disadvantages of laser welding are: the equipment is expensive, the energy conversion rate is low (5% ~ 20%), the requirements of welding parts interface processing, assembly and positioning are very high. At present, it is mainly used in the welding of micro devices in electronic industry and instrument industry, as well as the welding of silicon steel sheet and galvanized steel plate.
This is a kind of welding method using resistance heat as energy source, including electroslag welding with slag resistance heat as energy source and resistance welding with solid resistance heat as energy source. Because electroslag welding has more unique characteristics, it will be introduced later. This paper mainly introduces several kinds of resistance welding with solid resistance heat as energy source, mainly including spot welding, seam welding, projection welding and butt welding. Resistance welding is a welding method that makes the workpiece under a certain electrode pressure and uses the resistance heat generated by the current passing through the workpiece to melt the contact surface between the two workpieces to realize the connection. A large current is usually used. In order to prevent arcing on the contact surface and to forge the weld metal, pressure is always applied during welding. In this kind of resistance welding, the surface of the workpiece to be welded is very important to obtain stable welding quality. Therefore, the contact surface between the electrode and the workpiece and between the workpiece and the workpiece must be cleaned before welding.
- 1) when the nugget is formed, it is always surrounded by plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.
- 2) Due to the short heating time and concentrated heat, the heat affected zone is small, and the deformation and stress are small. It is usually unnecessary to arrange the correction and heat treatment process after welding.
- 3) There is no need for filler metal such as welding wire and welding rod, as well as welding materials such as oxygen, acetylene and argon, so the welding cost is low.
- 4) The operation is simple, easy to realize mechanization and automation, and improve the working conditions.
- 5) High productivity, no noise and harmful gas. In mass production, it can be combined with other manufacturing processes on assembly line. However, flash butt welding needs to be isolated due to spark splashing.
- 1) At present, there is no reliable non-destructive testing method. The welding quality can only be checked by destructive test of process samples and workpieces, and guaranteed by various monitoring technologies.
- 2) The lap joint of spot welding and seam welding not only increases the weight of the component, but also reduces the tensile strength and fatigue strength of the joint due to the angle formed around the nugget between the two plates.
- 3) High power and high degree of mechanization and automation make the equipment cost higher and maintenance more difficult, and the common high-power single-phase AC welding machine is not conducive to the normal operation of the power grid.
Scope of application: it is widely used in automobile, aircraft, instrument, household appliance, building steel bar, etc. it is widely used in many industries, but the resistance welding property of oxidizable metal is poor. It is mainly used for welding thin plate components with thickness less than 3mm. All kinds of steel, aluminum, magnesium and other non-ferrous metals and their alloys, stainless steel, etc. can be welded.
Friction welding is a kind of solid phase welding with mechanical energy as energy. It uses the heat generated by the mechanical friction between two surfaces to realize the metal connection. The heat of friction welding is concentrated at the joint surface, so the heat affected zone is narrow. Pressure must be applied between the two surfaces. In most cases, the pressure is increased at the end of heating to make the hot metal combine by upsetting. Generally, the joint surface does not melt. The productivity of friction welding is high. In principle, almost all metals that can be forged can be friction welded. Friction welding can also be used for welding dissimilar metals. It is suitable for workpiece with circular cross section and maximum diameter of 100 mm.
It is a kind of solid-phase pressure welding method that uses the heat generated by the friction between the contact end faces of the weldment to make the end face reach the thermoplastic state, and then rapidly apply the upsetting force to realize the welding.
Friction welding has the following advantages:
- (1) The welding quality is stable, the dimension precision of weldment is high, and the reject rate of joint is lower than that of resistance butt welding and flash butt welding.
- (2) The welding productivity is high, which is 5-6 times higher than that of flash butt welding.
- (3) It is suitable for welding dissimilar metals, such as carbon steel, low alloy steel and stainless steel, high speed steel, copper stainless steel, copper aluminum, aluminum steel, steel zirconium, etc.
- (4) Low processing cost, power saving, no special cleaning of welding parts.
- (5) Easy to realize mechanization and automation, simple operation, no spark, arc light and harmful gas in welding workplace.
Disadvantages: it is difficult to weld non-circular section by workpiece rotation. It is difficult to weld the disc-shaped workpiece and thin-walled pipe because it is not easy to clamp. Limited by the power of the main shaft motor of the welding machine, the maximum welding section of friction welding machine is 20000 m2. Friction welding machine is suitable for mass production because of its high investment cost.
Application: dissimilar metal and steel products, such as copper aluminum transition joint in electric power industry, high speed steel structural steel tool for metal cutting, etc.; structural steel products, such as power station boiler serpentine, valve, tractor bearing bush, etc.
Diffusion welding is a kind of solid phase welding method which uses indirect heat energy as energy. It is usually carried out in a vacuum or protective atmosphere. During welding, the surfaces of the two parts to be welded are exposed to high temperature and high pressure for a certain period of time, so as to reach the distance between atoms, and then combine through simple mutual diffusion of atoms. Before welding, not only the oxide and other impurities on the surface of the workpiece need to be cleaned, but also the surface roughness should be lower than a certain value to ensure the welding quality.
Under the protection of vacuum or protective atmosphere, under the condition of certain temperature (lower than the melting point of base metal) and pressure, the smooth and smooth surface to be welded will have micro plastic rheological contact, and atoms will diffuse with each other. After a long period of time, the original interface will disappear, and the complete metallurgical bonding will be achieved.
Diffusion welding has the following advantages:
- (1) It can be used to manufacture double-layer or multi-layer composite materials without damaging the properties of the materials to be welded.
- (2) It can weld workpieces with complex structure and large thickness difference.
- (3) The composition and structure of the joint are uniform, which reduces the stress corrosion tendency.
- (4) The welding deformation is small and the joint precision is high, which can be used as the final assembly and connection method of components.
- (5) It can be carried out simultaneously with other processing technologies (such as vacuum heat treatment), and multiple joints can be welded at the same time, so as to improve the productivity.
Disadvantages: diffusion welding has high requirements for surface processing and cleaning of weldments, long welding time, low productivity, high cost and large equipment investment.
Application: welding between dissimilar metals with different melting points or metallurgical incompatibility, welding between metals and ceramics, and welding of titanium, nickel and aluminum alloy structural parts. It is not only used in the advanced technology fields such as atomic energy, aerospace and electronic industry, but also extended to the general mechanical manufacturing industry.
The energy of brazing can be chemical reaction heat or indirect heat energy. It uses the metal with lower melting point than the material to be welded as filler metal. After heating, the filler metal is melted, and the filler metal is put into the gap between the contact surface of the joint by capillary action to wet the surface of the metal to form a brazed joint by mutual diffusion between the liquid phase and the solid phase. Therefore, brazing is a solid and liquid phase welding method.
Characteristics and application of brazing
The brazing alloy with melting point lower than the base metal is used as brazing filler metal. When heated, the filler metal melts and is filled and kept in the joint gap by wetting and capillary action. While the base metal is in a solid state, the brazing joint is formed by the mutual diffusion between the liquid solder and the solid base metal. Brazing has little effect on the physical and chemical properties of base metal, welding stress and deformation are small, and dissimilar metals with great difference in weldability can be welded at the same time. The joint appearance is beautiful and neat, the equipment is simple and the production investment is small. But the strength of brazed joint is low and the heat resistance is poor.
Application: cemented carbide tools, drilling bits, bicycle frames, heat exchangers, tubes and various containers; brazing is even the only possible connection method in the manufacture of microwave waveguides, electronic tubes and electronic vacuum devices.
Solder and flux
Brazing filler metal is the filler metal which forms the brazed joint, and the quality of the brazed joint largely depends on the filler metal. The filler metal should have suitable melting point, good wettability and caulking ability, can diffuse with the base metal, and have certain mechanical and physical and chemical properties to meet the performance requirements of the joint. According to the different melting point of solder, brazing can be divided into two categories: soldering and brazing.
- (1) Soldering. Soldering with solder melting point lower than 450 ℃ is called soft soldering. Tin lead solder is commonly used in electronic products, electrical machinery and automobile parts due to its good wettability and conductivity. The strength of brazed joint is generally 60-140mpa.
- (2) Brazing. Brazing with a melting point higher than 450 ℃ is called brazing. Brass filler metal and silver based filler metal are commonly used. The joint with silver based filler metal has high strength, conductivity and corrosion resistance, low melting point and good processability. However, the price of the filler metal is high, and it is mostly used for weldments with high requirements. Generally, brass filler metal is used for weldments. Brazing is mainly used for the brazing of steel and copper alloy workpieces and tools. The strength of brazed joint is 200 ～ 490mpa.
Note: the contact surface of base metal should be very clean, so flux should be used. The function of flux is to remove oxide and oil impurities on the surface of base metal and solder, protect the contact surface between solder and base metal from oxidation, and increase the wettability and capillary fluidity of solder. The melting point of flux should be lower than that of filler metal, and the corrosion of flux residue to base metal and joint should be less. Rosin or zinc chloride solution is commonly used in soldering, while borax, boric acid and basic fluoride are commonly used in brazing.
According to different heat sources or heating methods, brazing can be divided into flame brazing, induction brazing, furnace brazing, immersion brazing, resistance brazing, etc. Due to the low heating temperature during brazing, it has little effect on the properties of the workpiece material, and the stress deformation of the weldment is also small. However, the strength of brazed joints is generally low and the heat resistance is poor.
Brazing heating method: almost all heating sources can be used as brazing heat sources, and brazing is classified according to this.
Flame brazing: heated by gas flame, used for brazing carbon steel, stainless steel, cemented carbide, cast iron, copper and copper alloy, aluminum and aluminum alloy.
Induction brazing: resistance heating weldment using alternating magnetic field to generate induction current in parts, which is used for welding parts with symmetrical shape, especially for pipe shaft.
Dip brazing: the weldment is partially or wholly immersed in the molten salt mixture or solder melt, and the brazing process is realized by the heat of these liquid media. Its characteristics are rapid heating, uniform temperature and small deformation of the weldment.
Furnace brazing: the resistance furnace is used to heat the weldment. The resistance furnace can protect the weldment by vacuuming or using reducing gas or inert gas.
In addition, there are soldering iron, resistance brazing, diffusion brazing, infrared brazing, reaction brazing, electron beam brazing, laser brazing and so on.
Brazing can be used to weld carbon steel, stainless steel, high temperature alloy, aluminum, copper and other metal materials, as well as joining dissimilar metals, metals and nonmetals. It is suitable for welding joints with low load or normal temperature, especially for precision, micro and complex multi seam weldments.
High frequency welding
High frequency welding is based on solid resistance heat. During welding, the resistance heat generated by high frequency current in the workpiece is used to heat the surface layer of the workpiece welding zone to the molten or near plastic state, and then the forging force is applied (or not applied) to realize the metal bonding. Therefore, it is a solid resistance welding method. High frequency welding can be divided into contact high-frequency welding and induction high-frequency welding according to the way that high-frequency current generates heat in the workpiece. When contacting with high frequency welding, the high frequency current is transmitted to the workpiece through mechanical contact with the workpiece. In induction high frequency welding, the high frequency current generates the induced current in the workpiece through the coupling effect of the external induction ring of the workpiece. High frequency welding is a highly specialized welding method, which should be equipped with special equipment according to the product. High productivity, welding speed up to 30 m / min. It is mainly used for welding longitudinal seam or spiral seam when manufacturing pipe.
Explosive welding is another solid-state welding method which uses chemical reaction heat as energy. But it uses the energy generated by the explosion of explosives to realize the metal connection. Under the action of explosion wave, two pieces of metal can be accelerated to impact in less than one second to form metal combination. Among all kinds of welding methods, the combination of dissimilar metals which can be welded by explosive welding is the widest. It is possible to use explosive welding to weld two metals which are not compatible in metallurgy into various transition joints. Explosive welding is usually used to coat flat plates with large surface area, which is an efficient method for manufacturing composite plates.
Ultrasonic welding is also a solid-phase welding method with mechanical energy as energy. In ultrasonic welding, under low static pressure, the high frequency vibration generated by the sound pole can make the joint surface produce strong crack friction and heat to the welding temperature to form a combination. Ultrasonic welding can be used for the welding of most metal materials, which can realize the welding of metal, dissimilar metal and metal to non-metal. It is suitable for repeated production of metal wire, foil or sheet metal joint less than 2-3mm.
New welding process and technology
The outstanding performance of welding technology progress is the welding process from mechanization to automation, intelligence and information development. The application of intelligent welding robot is an important symbol of highly automatic welding process. Welding robot breaks through the traditional way of welding automation and makes small batch automatic production possible.
Most welding robots are arm type machines with fixed position, which can be divided into teaching type and intelligent type.
- Teaching robot: through teaching, memorize the welding track and welding parameters, and complete the welding of products in strict accordance with the teaching procedure. Only once teaching, the robot can accurately reproduce each step of teaching. This kind of welding robot is widely used and suitable for mass production. It is used in the fixed position of assembly line. Its main function is teaching and reappearance. It has poor adaptability to environmental changes. There is no use for small batch production of large structure on site.
- Intelligent robot: it can automatically determine the starting point, space trajectory and relevant parameters of the weld according to simple control instructions, and can automatically track the weld track, adjust the torch posture, adjust the welding parameters and control the welding quality according to the actual situation. This is the most advanced welding robot, which has the characteristics of dexterity, portability and easy to move. It can adapt to welding tasks of different structures and different locations. At present, it has few practical applications and is still in the research and development stage.
Among welding robots, spot welding robot accounts for 50% – 60%, which is composed of robot body, spot welding system and control system. The degree of freedom of the robot body is 1-5, and the control system is divided into body control and welding part control.
The welding system mainly includes: welding controller, welding tongs and auxiliary parts such as underwater welding.
Application of computer software
The application of computer software system in welding field mainly includes the following aspects:
(1) Computer simulation technology includes simulation of welding thermal process, welding metallurgical process, welding stress and deformation. Welding is a complex process involving arc physics, heat transfer, metallurgy and mechanics. Once the welding process has been simulated by computer, the best design scheme, process method and welding parameters for welding various structures and materials can be determined by computer system. Traditionally, welding process is always determined by a series of experiments or experience to obtain reliable and economical welding structure. As long as the applicability of numerical method in dealing with a certain problem is proved by a small number of verification tests, a large number of screening work can be completed by computer, which saves a lot of test work, thus greatly saving manpower, material resources and time It is of great significance in the welding of new engineering structures and new materials. The level of computer simulation technology also determines the scope of automatic welding. In addition, computer simulation is widely used to analyze the strength and performance of welded structures and joints.
(2) Database technology and expert system are used in welding process design and parameter selection, welding defect diagnosis, welding cost budget, real-time monitoring, welding CAD, welder examination, etc.
Database technology has penetrated into all aspects of welding field, from raw materials, welding test, welding process to welding production. Typical database systems include welding procedure qualification, welding procedure specification, welder file management, welding materials, material composition and performance, weldability, welding CCT diagram management and welding standard consulting system. These database systems provide favorable conditions for data and information management in welding field.
The welding expert system mainly focuses on the process formulation, defect prediction and diagnosis, computer aided design and so on. In the existing welding expert system, process selection and process formulation are the most important application fields, and the real-time control of welding process is an important development direction.
(3) Computer aided quality control (CAQ) technology is used for data analysis of products and real-time monitoring of welding quality.
In addition, the application of computer aided design / manufacturing (CAD / CAM) in welding processing is also increasing, mainly used in NC cutting, welding structure design and welding robot.
Author: chemical 707
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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