Introduction to welding
Welding: usually refers to metal welding.
It is by heating or pressurizing to make two separate objects into one integral form.
Classification: according to the different heating degree and process characteristics in the welding process, the welding method can be divided into three categories.
(1) Fusion Welding
Heat the workpiece to a molten state and form it into a molten pool (usually filled with filler metal), the weld seam will be formed after cooling crystallization, then the welder part will be integrated into one whole piece.
The common fusion welding methods include gas welding, arc welding, electro-slag welding, plasma arc welding, electron beam welding, laser welding and so on.
(2) Pressure Welding
Whether heating or not in the welding process, the pressure welding is required.
Common pressure welding including resistance welding, friction welding, cold pressure welding, diffusion welding, explosive welding.
When the melting point is lower than the solder (filler metal) of the weld metal, fill the joint gap and connect with the weld metal diffusion.
The welder does not melt during brazing, and there is generally no plastic deformation.
Characteristics of welding production:
(1) Save metal materials and with a light structure.
(2) Make large size into a small one; manufacture heavy and complex machine parts; simplify casting, forging and cutting process and get the best technical and economic results.
(3) The welded joint has good mechanical properties and tightness.
(4) It can make full use of the properties of materials to manufacture bimetallic structure.
Introduction of various welding technologies
01 Electric Arc Welding
It is a strong and persistent gas discharge phenomenon. The positive and negative electrodes have a certain voltage, and the gas medium between the two electrodes should be in the ionization state.
Usually, two electrodes are connected to the power supply when the welding arc is ignited (one electrode is workpiece, the other one is filler metal wire or electrode), for brief contact and rapid separation. A short circuit occurs when the electrodes are in contact with each other, and then forming an arc.
This is called a contact arc.
After the arc is formed, the arc will keep burning as long as the power supply maintains a certain potential difference between the electrodes.
Electric Arc characteristics:
It features 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.
The current in the arc can from dozens of amps to thousands of amps to meet the welding requirements of different workpieces.
The temperature of the arc can reach above 5000K, and is able to melt all kinds of metal.
The arc consists of three parts: cathode area, anode area, and arc column.
Arc welding power source:
The power supply used in welding arc is called arc welding power supply, which can be divided into four types:
- Ac arc welding power source.
- Dc arc welding power source.
- Pulse arc welding power supply.
- Inverter arc welding power source.
Direct current connection:
A dc welding machine is used when the workpiece is connected to the anode and the electrode is connected to the cathode. This is called direct current connection.
At this time, the workpiece is heated and suitable for welding thick workpiece;
Direct current reverse connection:
When the workpiece is connected to the cathode and the electrode is connected to the anode, it is called dc reverse connection.
At this time, the workpiece is hot and small, and it is suitable for welding thin small workpiece.
When welding with ac welding machine, there is no positive or inverse problem because of the alternation of polarity.
Welding metallurgy process
In the process of arc welding, liquid metal, slag and gas interact with each other, which is the process of metal re-smelting.
However, due to the particularity of welding conditions, the process of welding chemical metallurgy is different from that of the general smelting process.
First, the welding metallurgy temperature is high, the phase boundary is large, the reaction speed is fast.
When there is air intrusion in an arc, the liquid metal reacts with strong oxidation, nitridation, and a large amount of metal vaporization.
The water in the air, the oil, rust, and water in the welding materials and workpiece are decomposed into hydrogen atoms at high temperature, which can be dissolved in liquid metal. The plastic and toughness of the joints are reduced (hydrogen embrittlement), which will result in cracks.
Secondly, the welding pool is small, the cooling is fast, and the various metallurgical reactions are difficult to reach the equilibrium state.
The chemical composition of the weld is not uniform, and the gas and oxides in the molten pool are too late to surface. It is easy to form pores, slag and other defects, sometimes even crack.
The following measures are usually adopted during the arc welding process:
(1) in the process of welding, mechanical protection of molten metal is made to separate it from the air.
There are three types of protection: gas protection, slag protection and gas – slag joint protection.
(2) For metallurgical treatment of welding pool. A certain amount of deoxidizer (mainly ferromanganese and ferrosilicon) and alloy elements are added to the welding material (electrode coating, welding wire and flux).
The FeO in the molten pool was eliminated during the welding process, and the damage of the alloy element was compensated.
Common shielded metal arc welding (SMAW) method:
1 Manual metal-arc welding
Manual metal-arc welding is one of the earliest and most widely used welding methods.
It takes the coated filler rod as the electrodes and added metal, the arc is burned between the end of the electrode and the surface of the welder.
On the one hand, the coating can produce gas to protect the arc, on the other hand, the molten slag can be deposited on the surface of the molten pool to prevent the interaction between the molten metal and the surrounding gas.
One of the most important roles of slag is to produce physical and chemical reactions with molten metal or to add alloy elements to improve weld metal energy.
Manual metal arc welding equipment is simple, light and flexible.
It can be used for the welding of short joints in maintenance and assembly, especially for areas which are difficult to reach.
The welding electrode of manual metal arc welding can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and alloy.
2 Submerged arc welding
Submerged arc welding is a fusion electrode welding method which takes granular flux as a protective medium and the arc is hidden under the flux layer.
The welding process of submerged arc welding consists of three parts:
- Apply enough granular flux to the welding seam;
- Conductive mouth and welding parts are connected to the welding power level to produce a welding arc;
- Automatically feed the wire and move the arc to perform welding.
The main features of submerged arc welding are as follows:
- Unique arc performance.
- the weld quality of high slag is good for isolating air protection.
The main components of the arc are CO2.
The nitrogen content and oxygen content in the weld metal are greatly reduced.
The welding parameters are automatically adjusted, the arc walking is mechanized, the metallurgical reaction is sufficient, the anti-wind ability is strong, so the weld composition is stable and the mechanical property is good.
- good working conditions, welding slag isolation arc is good for the welding operation; low labor intensity.
- The electric field intensity of arc column is relatively high, it has following features compared with that of the fusion electrode.
- the equipment has good regulation performance.
Due to the high electric field intensity, the sensitivity of the automatic regulating system is higher, which makes the welding process more stable.
- high minimum welding current.
- High production efficiency due to the shortening of wire conduction length, significant increase in current and current density, so that the penetration ability of arc and the deposition rate of welding wire are greatly improved;
Due to the thermal insulation of flux and slag, the total thermal efficiency is greatly increased and the welding speed is greatly improved.
The scope of application:
Because of the depth of submerged arc welding, high productivity and high mechanical operation, it is suitable for long weld with thick plate structure welding.
It has been widely used in shipbuilding, boiler and pressure vessel, bridge, the structure of the overweight machinery, plant, marine structures, the manufacturing sector etc.
In addition to the connection of the components in the metal structure, the submerged arc welding can also be deposited on the surface of the base metal.
As welding metallurgy and welding materials production technology development, the weld material of submerged arc welding have developed from carbon structural steel to low alloy steel, stainless steel, heat-resistant steel, etc, and some non-ferrous metals, such as nickel base alloy, titanium alloy, copper alloy, etc.
Due to its own characteristics, its application has some limitations, mainly include:
(1) limitation of the welding position.
Because of the reason of the flux retention, if special measures are not adopted, the submerged arc welding is mainly used for welding of horizontal position welding, which cannot be used for horizontal, vertical and inverted welding.
(2) limitations of welding materials.
It cannot weld metal and alloys such as aluminum and titanium, which are mainly used for welding black metal.
(3) only suitable for long seam welding, and can’t weld seam with limited space.
(4) no direct observation of arc.
(5) not suitable for the thin plate and small current welding.
3 Tungsten electrode gas shielded arc welding.
This is an arc welding without melting polar gas.
The weld is formed by melting the metal with an arc between the tungsten electrode and the workpiece.
During welding, tungsten is extremely infusible, and only the electrode is used.
It is also protected by argon or helium gas from the nozzle of the torch.
Additional metals may be added as needed.
It is known internationally as TIG welding.
Tungsten electrode gas shielded arc welding is a very good method for connecting sheet metal and backing welding because it controls heat input well.
This method can be used for almost all metal connections, especially for metals such as aluminum and magnesium that can form refractory oxides and reactive metals like titanium and zirconium.
The weld quality of this welding method is high, but its welding speed is slow compared with other electric arc welding method.
4 Gas metal arc welding
Arc welding is a fusion welding method which USES arc as a heat source.
The arc is based on continuous welding stick and a molten pool mixed with a parent metal and formed by molten metal.
After the arc heat source is removed and crystallized to form the weld, the separated parent material is connected through metallurgy.
Characteristics of CO2 welding:
(1) CO2 will be decomposed into CO, O2 and O under high temperature of the welding arc, which has a strong compression effect on the arc.
Thus, the arc form of the welding method has a small diameter of arc column.
The arc and the area are small and often difficult to cover the end of the welding tip.
Therefore, the transition resistance (speckle force) of the droplet is larger and the droplet is coarse, and the transition path is less axial and the splash rate is large.
(2) good protection of the welding zone.
The density of CO2 is the most commonly used to protect the gas.
After the thermal decomposition of CO2 gas, the volume increases, so the protection is better.
(3) relatively concentrated energy and large penetration ability;
(4) low production cost and energy saving.
(5) the process and technology also have good visibility in the welding area, which is convenient for observation and operation;
The welding heat affected zone and welding deformation are small;
The molten pool has a relatively high crystallization speed and good welding performance.
Low sensitivity to rust.
(1) when oxidized CO2 welding of the alloy element, CO2 will be decomposed into CO, O2 and O under the action of arc temperature.
Under welding conditions, CO is insoluble in metal and does not participate in the reaction.
Both CO2 and O have strong oxidizing properties that oxidize Fe and other alloy elements.
(2) alloying of deoxygenation and weld metal.
Usually, a certain amount of deoxidizer is added to the wire to deoxidize.
In addition, the residual deoxidizer is retained as the alloy element in the welding seam to make up for the loss of oxidation loss and guarantee the chemical composition of the weld.
(1) short circuit transition (short arc, thin wire, small current) suitable for all position welding of the sheet;
(2) fine grain transition, thick wire, long arc, large current welding;
(3) the transition from submerged arc droplet (rarely used).
The scope of application:
At present, CO2 gas protection welding is widely used in locomotive manufacture, shipbuilding, automobile manufacturing, coal mining machinery manufacturing etc.
Suitable for welding low carbon steel, low alloy steel, low alloy high strength steel, but not suitable for welding non-ferrous metal, stainless steel.
Although the data show that CO2 gas shielded welding can be used for welding stainless steel, it is not the first choice for welding stainless steel.
5 Plasma arc welding
Water cooling nozzle and other measures can reduce the arc column area. The temperature, energy density and plasma flow rate of the arc are significantly increased. This arc, which is compressed by an external constraint, is called a plasma arc.
Plasma arc is a special kind of arc, which is an arc with high energy density, it is still a gas conduction phenomenon.
Plasma arc welding is a method to heat and melt the workpiece and the mother material by using the heat of the plasma arc.
Classification: Perforated plasma arc welding and microbeam plasma arc welding.
Perforated plasma arc:
The welding current is 100 ~ 300A, the joint need not to open the groove, and will not leave clearance.
In welding, the plasma arc can completely melt the welding parts and form a small through-hole. The molten metal is pushed around the hole. When the arc is moved, the holes move, and the weld is formed at the rear position, then the single-side welding is forme.
The maximum welding thickness of plate with this method is:
carbon steel 7mm, stainless steel 10mm.
Microbeam plasma arc:
The welding current is 0.1 ~ 30A and the welding thickness is 0.025 ~ 2.5mm.
In addition, there is weld fusion plasma arc welding for copper and copper alloys.
It can be used for thick plate deep fusion welding or thin plate high-speed welding and surfacing welding, which can solve the problem of the ac (variable polarity) plasma arc welding and aluminum alloy plasma arc welding.
The main technical parameters of plasma arc welding are welding current, welding speed, shielding gas flow, ion gas flow, welding nozzle structure and aperture, etc.
Plasma arc cutting:
The metal part of the incision is melted so that it evaporates with the high-temperature arc flow of the plasma arc. An incision is made by blowing the molten material away from the base with the help of a high-velocity stream or current.
(1) the high energy density of the plasma arc, the high temperature of arc column and strong penetrating ability. 10 ~ 12mm thick steel without open groove, can weld through a double – face forming. Fast welding speed, high productivity and low-stress deformation.
(2) the weld section is in a wine glass shape and no finger-like penetration problem.
(3) the arc straightening is good, and the fluctuation of the molten pool is small due to the influence of long arc length fluctuation.
(4) the arc stability is 0.1a, still has a relatively flat static characteristic, and the constant current source is used, and the welding of thin plate can be very well (0.1mm).
(5) tungsten internal shrinkage to prevent the weld tungsten.
(6) using small hole welding technology to achieve single side welding and double side forming.
(7) the equipment is more complex, the gas consumption is large, and it is only suitable for indoor welding.
The accessibility of the welding gun is worse than that of TIG.
(8) the arc diameter is small, and the welding gun axis and the seam center line are more accurate for alignment.
Metallurgical reaction: single, only evaporated.
Steep drop power, dc positive connection;
When welding aluminum magnesium, use ac, steep drop power supply, and require a steady arc measure.
Welding material: protective gas, tungsten electrode.
Widely used in industrial production, especially in aerospace and other military and advanced industrial technology. It’s applied in copper and copper alloy, titanium and titanium alloy, alloy steel, stainless steel, molybdenum and some other types of metal welding, such as missile shell of titanium alloy.
6 Flux-cored welding
The tubular wire arc welding is also used to weld the combustion arc between the welding wire and the workpiece.
It can be considered a type of gas metal arc welding.
The wire used is tubular wire, which contains various components of flux.
The welding uses protective gas, mainly CO2.
The flux is decomposed or melted, and it is used to protect the molten pool, the alloy and the arc.
In addition to the advantages of arc welding, the tube welding electrode has advantages in metallurgy because of the role of the flux in the pipe.
The tubular wire arc welding can be applied to the welding of most black metal joints.
The tube welding wire arc welding has been widely used in some advanced industrial countries.
“Tubular wire” is now known as “flux cored wire”
02 Fusion welding
1 Gas welding
A fusion welding method that USES the heat generated by combustible gas to burn in oxygen and melts the parent material.
Gas welding is a kind of welding method with gas flame as the heat source.
Most of the applications are the oxygen-acetylene flame, which is made of acetylene gas.
Because the equipment is easy to operate, the heating speed and productivity of gas welding are low, the thermal influence area is large, and it is easy to cause large deformation.
Gas welding can be used for welding of many ferrous metals, nonferrous metals and alloys.
Combustible gas: Acetylene, liquefied petroleum gas, etc.
Acetylene, for example, the combustion in oxygen flame temperature is up to 3200 ℃.
There are three types of oxyacetylene flame:
The mixture ratio of oxygen and acetylene is 1:1.2, and acetylene is fully burned, suitable for welding carbon steel and non-ferrous alloy.
The mixture ratio of oxygen and acetylene is less than 1, and acetylene is excessive, suitable for welding high carbon steel, cast iron and high-speed steel.
The mixture ratio of oxygen and acetylene is greater than 1.2, with excess oxygen, suitable for brazing of brass and bronze.
The disadvantages are obvious, such as oxyacetylene flame temperature is low, slow heating, wide heating area and welding heat-affected zone, the welding deformation is large, and the welding process, the protection of the molten metal is poor, welding quality is not easy to guarantee, so it has limit application.
However, gas welding has the advantages of no power supply, simple equipment, low cost, convenient movement and strong universality, so it has practical value in no-power situation and fieldwork.
At present, it is mainly used for the welding of thin steel plate (0.5-3mm thickness), copper and copper alloy and cast iron.
2 Air pressure welding
Air pressure welding is the same as gas welding.
When welding, heat the end of the two butt joints to a certain temperature, then apply enough pressure to obtain a strong connection.
It is a solid phase welding.
Air pressure welding without filler metal, often used in rail welding and steel reinforcement welding.
3 Electroslag welding
Electroslag welding is the welding method of the thermal energy of slag.
The welding process is carried out in the position of vertical welding and in the gap between the two workpiece ends and the water-cooled copper sliding blocks on both sides.
In welding, the end of the workpiece is melted by the resistance heat generated by the current through the slag.
According to the electrode shape used in welding, electroslag welding is divided into wire electrode electroslag welding, plate electrode electroslag welding and welding nozzle electroslag welding.
Characteristics of electroslag welding:
In the welding process of electroslag welding, there is a stable electroslag process in addition to the initial stage, which is fundamentally different from submerged arc welding.
The advantages of electroslag welding are:
Weldable workpiece thickness (from 30mm to over 1000mm), high productivity.
It is mainly used for welding of butt joint and t-connector.
Electroslag welding can be used for welding of various steel structures, and can also be used for welding of castings.
Due to the slow heating and cooling of the electroslag welding head, the thermal influence zone is wide and the microstructure is thick and tough, so it is generally required to conduct normal fire treatment after welding.
Limitations of electroslag welding:
(1) due to the large welding pool, the heating and cooling are slow, and it is easy to overheat in the weld and heat affected areas to form a large organization.
Therefore, electroslag welding is usually used to remove the coarse grain in the joint after welding.
(2) electroslag welding is always carried out in a vertical welding mode. Flat welding is not allowed. Electroslag welding is not suitable for the workpiece with a thickness below 30mm, and the weld should not be too long.
Classification and application of electroslag welding.
Classification of electroslag welding:
Electroslag welding, plate electrode electroslag welding, electroslag welding and tube electrode slag welding.
Electroslag welding is the most commonly used method.
The electrode is made of the wire electrode. According to the thickness of the welding parts, one or more wires can be used.
Single wire welding can be welded with a thickness of 40 ~ 60mm. When the thickness of the welding part is greater than 60mm, the welding wire shall be horizontal swing.
Three wire swing can weld up to 450mm thick.
Wire electrode slag welding is mainly used for welding parts with welding thickness of 40 ~ 450mm and welding parts of the long weld, which can also be used for girth weld of large welding parts.
It is mainly used in heavy machinery manufacturing, forging, and casting.
For example, the thickness of welding parts is 40 ~ 450mm for heavy-duty machine base and high-pressure boiler etc, and the material is carbon steel, low alloy steel, stainless steel etc.
4 Electron beam welding
Electron beam welding is the method of welding the heat generated by the concentrated high-speed electron beam bombardment of the workpiece surface.
When the electron beam is welded, the electron beam produces electron beam and accelerates.
The commonly used electron beam welding are: high vacuum electron beam welding, low vacuum electron beam welding and non-vacuum electron beam welding.
The first two are done in a vacuum chamber.
The welding preparation time (mainly the vacuum time) is longer and the workpiece size is limited by the vacuum chamber size.
Compared with electric arc welding, the main features of electron beam welding are the weld depth, weld width and weld metal purity.
It can be used for the precision welding of very thin materials and can be used for welding very thick plate (the thickness of 300mm).
All metals and alloys welded with other welding methods can be welded by electron beam.
It is mainly used for welding of high-quality products.
It can also solve the welding of heterogeneous metals, oxidized metals and refractory metals.
But it is not suitable for mass products.
Electron beam welding machine:
The core is the electron gun, which is the device that completes the generation of electrons, the formation of the electron beam and the convergence of the electron beam.
It is mainly composed of filament, cathode, anode and focusing coil.
The filament heats up while charged with electricity and heats the cathode,
The electron is emitted when the cathode reaches about 2400K. Between high voltage electric field of cathode and anode, the electrons are accelerated (about half the speed of light), through the anode hole injection, then by focusing coil, it will converge into 0.8 ~ 3.2 mm diameter of electron beam toward the weldment, and the kinetic energy is converted into thermal energy on the surface of the welding piece so that the joint of the welding parts can be melted rapidly and the weld is formed after the cooling crystallization.
According to the vacuum degree of welding workshop (place of welding), the electron beam welding can be divided into:
（1）High vacuum electron beam welding.
The studio is in the same room with the electron gun. The vacuum is 10-2 ~ 10-1pa. Suitable for precision welding of refractory, active, high purity metal and small parts.
（2）Low vacuum electron beam welding.
The studio and the electron gun are divided into two vacuum Chambers. The vacuum degree of the studio is 10-1 ~ 15Pa. Suitable for larger structural parts and refractory metals with less sensitive to oxygen and nitrogen.
（3）Non-vacuum electron beam welding.
The distance between the welding parts and the electron beam outlet should be controlled at about 10mm to reduce the scattering caused by the collision between the electron beam and the gas molecules.
Non-vacuum electron beam welding is suitable for welding of carbon steel, low alloy steel, stainless steel, refractory metal and copper, aluminum alloy, etc.
Advantages of vacuum electron beam welding:
(1) the electron beam has a high energy density and can reach up to 5 x 108W/cm2, which is about 5000 ~ 10,000 times of ordinary arc. Heat concentration, high thermal efficiency, small thermal influence area, narrow weld bead, welding deformation is very small.
(2) welding in a vacuum environment, the metal does not interact with the gas phase, and the joint strength is high.
(3) the focus radius of the electron beam can be adjusted to be large, flexible and adaptable. Can weld the thin piece of 0.05mm, also can weld 200 ~ 700mm thick plate.
It is especially suitable for welding refractory metal, active or high purity metal and thermal sensitive metal.
But the equipment is complex, the cost is high. The size of the welding parts is limited by the vacuum chamber, the assembly precision is high, and the X-ray is easily excited. Long welding auxiliary time, low productivity.
These weaknesses limit the widespread use of electron beam welding.
5 Laser welding
Laser welding is a kind of laser beam which is focused by a large power coherent monochromatic photon flow.
This welding method usually has 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 a vacuum. The disadvantage is that the penetration is not as strong as the electron beam welding.
Precise energy control can be carried out during laser welding, so the welding of precise microdevices can be realized.
It can be applied to many metals, especially soldering of refractory metals.
when the material is stimulated, the wavelength, frequency and direction of the laser beam are exactly the same.
Having the characteristics of good monochromaticity, good direction and high energy density.
When the laser is focused by transmission or reflector, it can obtain a beam with a diameter of less than 0.01mm and a power density of 1013W/cm2.
It can be used as a heat source for welding, cutting, drilling and surface treatment.
The material that produces the laser has solid, semiconductor, liquid, gas.
Amongst them, yttrium aluminum garnet (YAG) solid laser and CO2 gas laser are mainly used for welding, cutting processes.
The main advantages of laser welding are:
(1) the laser can be transmitted by optical fiber, prism and other optical methods.
Suitable for welding of small parts and other welding methods which are difficult to reach.
It can also be soldered through the transparent material.
(2) high energy density. High-speed welding can be achieved. Heat affected area and welding deformation are very small, especially suitable for welding of heat-sensitive materials.
(3) the laser is not affected by the electromagnetic field and does not produce X-ray, without the need for vacuum protection. It can be used for welding large structures.
(4) the insulation conductor can be welded directly without the need to remove the insulating layer in advance; Also can weld the heterogeneous material with a large difference of physical properties.
The main disadvantages of laser welding are:
Expensive equipment, low energy conversion rate (5% ~ 20%).
The machining, assembling and positioning requirements of welding parts are very high.
At present, it is mainly used for welding of microdevices in the electronic industry and instrument industry, as well as welding of silicon steel sheet and galvanized steel plate.
03 Pressure Welding
1 Resistance welding
This is a kind of welding method with resistance heat as energy, including electroslag welding with slag resistance heat as energy and resistance welding with solid resistance heat as energy.
Resistance welding general is to make the workpiece in a certain electrode pressure, and by using resistance heat produced by an electric current pass through the workpiece to melt the contact surface between the workpiece and welding method to realize the connection.
A large current is usually used.
In order to prevent an arc on the contact surface and to weld the weld metal, pressure should always be applied during welding.
When this type of resistance welding is carried out, the surface of the welder is of primary importance to the stable welding quality.
Therefore, the electrode and the workpiece and the contact surface between the workpiece must be cleaned before welding.
1) when the molten core is formed, it is always surrounded by the plastic ring. The molten metal is separated from the air and the metallurgical process is simple.
2) the heating time is short, the heat concentration is concentrated, the thermal effect area is small, and the deformation and stress are small, and it is usually not necessary to arrange the calibration and heat treatment process after welding.
3) no filler metal, such as welding wire, welding rod, and welding materials such as oxygen, acetylene and argon, low welding cost.
4) simple operation, easy to realize mechanization and automation, and improve working conditions.
5) high productivity and no noise or harmful gas. In mass production, the assembly line can be woven together with other manufacturing processes.
But flash butt welding is required to be isolated because of sparks.
1) there is currently no reliable non-destructive testing method. Welding quality can only be checked by the destructive testing of process samples and workpieces, as well as by various monitoring techniques.
2) the lap joints of joint and welding seam not only increase the weight of the components but also cause the tensile strength and fatigue strength of the joint due to the angle between the two plates.
3) high power of equipment and the high degree of mechanization automation, making equipment more expensive and difficult to maintain. And the common high-power single-phase ac welding machine is bad for the normal operation of the power grid.
It is widely used in automobile, airplane, instrument, electrical appliance, steel bar and other industries. It is widely used in materials, but it is only slightly different in resistance welding.
It is mainly used for thin plate components with a thickness of less than 3mm.
All kinds of steel, aluminum, magnesium and other non-ferrous metals and their alloys, stainless steel can be welded.
2 Friction welding
Friction welding is the solid phase welding of mechanical energy.
It USES the heat generated by mechanical friction between two surfaces to achieve a metal connection.
The heat concentration of friction welding is concentrated at the joint surface, so the thermal influence area is narrow.
Pressure is applied between the two surfaces, most of which increase the pressure at the end of the heating process so that the hot metal is combined with the top forging, and the normal combination surface does not melt.
Friction welding is highly productive, and almost all metals that can be hot-wrought can be welded together.
Friction welding can also be used for welding of heterogeneous metals.
It is suitable for the workpiece with a diameter of 100mm.
By using the heat generated by the friction between the contact ends of the welding parts, the end face reaches the thermoplastic state, and then the top forging force is rapidly applied to realize the welding method of solid phase welding.
Friction welding has the following advantages:
(1) the welding quality is stable, the welding parts have high dimensional accuracy, and the scrap rate of the joint is lower than the resistance welding and flash butt welding.
(2) high welding productivity, 5 ~ 6 times higher than flash butt welding.
(3) suitable for welding heterogeneous metals such as carbon steel, low alloy steel and stainless steel, high-speed steel connection, copper – stainless steel, copper – aluminum, aluminum – steel, steel – zirconium, etc.
(4) low processing cost, power saving, no special cleaning for welding parts.
(5) easy to realize mechanization and automation, simple operation, no spark, arc and harmful gas in the welding work site.
It is difficult to weld non-circular sections by rotating the workpiece.
Disc and thin-walled pipe fittings are difficult to weld because they are not easily clamped.
The maximum cross-section of friction welding is 20000mm2, which is limited by the motor power of the main shaft of the welding machine.
Friction welding machine one-time investment cost large, suitable for mass production.
Heterogeneous metals and exotic steel products, such as copper – aluminum transition joints in the power industry, high-speed steel-structural steel tools for metal cutting, etc.
Structural steel products, such as power station boiler coil, valve, tractor bearing, etc.
3 Diffusion welding
Diffusion welding is usually a solid-phase welding method with indirect heat energy as energy.
Usually in a vacuum or a protective atmosphere.
When welding, the surface of two welders is exposed to high temperature and under great pressure for a certain period of time, so as to reach the distance between atoms, which is combined with the simple mutual diffusion of atoms.
Before welding, it is necessary not only to clean the oxide and other impurities on the surface of the workpiece, but also the surface roughness is less than a certain value to ensure the welding quality.
Diffusion welding under the protection of the vacuum or protective atmosphere, to a certain temperature (lower than the parent metal melting point) and pressure conditions, make the mutual contact of the flat is bright and clean after welding surface micro plastic flow and close contact. The atoms spread out, and after a long period of time, the original interface disappeared, and the welding method of complete metallurgy was achieved.
Diffusion welding has the following advantages:
(1) the welding of various kinds of materials and heterogeneous materials can be achieved in the condition that the properties of the welded materials are rarely damaged, and can be used to make double or multi-layer composite materials.
(2) can weld the workpiece with a complex welding structure and large thickness difference.
(3) the joint components and tissues are uniform, reducing the stress corrosion tendency.
(4) the welding deformation is small and the joint precision is high, which can be used as the final assembly connection method of the part.
(5) it can be carried out simultaneously with other processing technologies (such as vacuum heat treatment, etc.), and the welding of multiple joints can be completed simultaneously, thus improving productivity.
The high requirement for surface processing and cleaning of welding parts, long welding time, low productivity, high cost and large investment in equipment.
Welding between different melting points or incompatible metals in metallurgy.
Welding of metal and ceramics.
Welding of structural parts of titanium, nickel and aluminum alloy.
It is not only applied in the fields of advanced technology such as atomic energy, aerospace and electronics industry but also has been extended to the general machinery manufacturing industry.
Brazing energy can be chemical reaction heat or indirect heat energy.
The brazing filler metal is made of a metal with a melting point lower than the melting point of the welded material and melted by heating. The capillary tube will insert the solder into the gap between the contact surface of the connector and wet the metal surface. The solder joint is formed by the mutual diffusion between the liquid phase and the solid phase.
Therefore, brazing is a kind of welding method of solid phase and liquid phase.
- Characteristics and application of brazing.
The effect of brazing on the physical and chemical properties of the parent material is small.
Welding stress and deformation are minor, able to weld different kinds of metal with different welding properties.
Multiple weld lines can be completed simultaneously. And the connector is beautiful and tidy.
The equipment is simple, the production investment is small.
But the strength of brazing head is low, the heat resistance is poor.
Carbide cutter, drill bit, bicycle frame, heat exchanger, catheter and various kinds of containers.
Brazing is even the only possible connection method in the manufacture of microwave waveguides, vacuum tubes and electronic vacuum devices.
- Brazing filler metal and flux.
The filler metal is the filler metal forming the brazing head, and the quality of the brazing head is largely determined by the solder.
The solder should have the proper melting point, good wettability and joint ability, and can spread with the parent material.
It should also have certain mechanical properties and physical and chemical properties to meet the use performance requirements of joints.
According to the solder melting point, brazing is divided into two types: soft brazing and hard brazing.
Solder melting point is lower than 450 ℃ soldering called soft soldering, solder is commonly used tin-lead solder.
It has good wettability and conductivity.
Widely used in electronic products, electrical appliances and auto parts.
The joint strength of soft soldering is usually 60 ~ 140MPa.
Solder is higher than the melting point of 450 ℃ soldering called hard braze welding, the commonly used brazing filler metal includes solder brass solder and silver based solder.
The joint with silver – based filler has high strength, conductivity and corrosion resistance. The solder melting point is low and the technology is good, but the solder price is higher.
It is used for welding parts with higher requirements. Generally, brazing solder is used.
Hard soldering is used for heavy steel and copper alloy workpieces, as well as tool brazing.
The strength of the brazing joint is 200 ~ 490MPa.
The contact surface of the parent material should be very clean, so it is necessary to use the flux.
The role of the flux is to remove oxides and oil stains on the surface of the mother and the solder, to protect the solder and the parent material from oxidation, and to increase the wettability and capillary fluidity of the solder.
The melting point of the flux should be lower than that of the solder.
The commonly used solder for soft soldering is rosin or zinc chloride solution, and the commonly used flux is borax, boric acid and alkaline fluoride mixture.
According to the heat source or heating method different brazing can be divided into: flame brazing, induction brazing, brazing, dip brazing, resistance brazing, etc.
Due to the low heating temperature, the performance of the workpiece material is less affected and the stress deformation of the welding parts is smaller.
But the strength of brazing head is generally low, the heat resistance is poor.
Heating method of brazing:
Almost all heat sources can be used as brazing heat sources, and the soldering is classified accordingly.
Heating with the gas flame for hard brazing of carbon steel, stainless steel, hard alloy, cast iron, copper and copper alloys, aluminum and aluminum alloy.
Thermally heated welds of resistance heat generated by the alternating magnetic field in the parts. It’s applied to welding parts with symmetrical shapes, especially for pipe shafts.
Immerse the welding part or whole into the molten salt mixture or solder melt, the brazing process is achieved by the heat of the liquid medium.
It is characterized by rapid heating, uniform temperature and small deformation of welding parts.
Brazing in the furnace:
The resistance furnace can be used to protect the welding parts by vacuuming or reducing the gas or inert gas.
In addition, there are soldering iron brazing, resistance brazing, diffusion brazing, infrared brazing, reaction brazing, electron beam brazing, laser brazing, etc.
Soldering can be used to weld carbon steel, stainless steel, high-temperature alloy, aluminum, copper and other metal materials.
It can also be connected to heterogeneous metals, metals and non-metals.
Suitable for welding joints with little or no room temperature, especially suitable for precision, micro – and complicated soldering.
05 Other welding introductions
1 High-frequency welding
High-frequency welding is the energy of solid resistance heat.
When welding the resistance of the heat generated by the high-frequency current within the workpiece heated to melt the surface layers of workpiece welding area or close to the state, then apply (or not applied) upset forging force and realize the combination of metals.
Therefore, it is a method of solid – phase resistance welding.
High-frequency welding according to the high-frequency current in the workpiece can be divided into contact high-frequency welding and induction hf welding.
At high-frequency welding, the high-frequency current is passed into the workpiece through contact with the workpiece.
The induction current is generated in the workpiece by the coupling of the external induction coil of the workpiece.
High-frequency welding is a highly specialized welding method, which should be equipped with special equipment.
With high productivity, the welding speed can reach 30m/min.
Welding of longitudinal or helical joints used to make pipes.
2 Explosion welding
Explosive welding is another kind of solid phase welding method with chemical reaction heat as energy.
But it USES the energy generated by the explosive explosion to connect the metal.
In the event of an explosion, two pieces of metal can be accelerated to form a metal bond in less than a second.
In various welding methods, explosive welding can be welded to the widest range of metal combinations.
It is possible to weld two metals incompatible in metallurgy with explosive welding into various transition joints.
The explosive welding is mostly used for flat covering with a large surface area. It is an efficient way to make composite boards.
3 Ultrasonic welding
Ultrasonic welding is also a kind of solid – phase welding method with mechanical energy.
In the case of ultrasonic welding, the welding workpiece is under low static pressure, and the high-frequency vibration emitted by the acoustic pole can cause the joint surface to produce strong crack friction and heat it to the welding temperature to form a bond.
Ultrasonic welding can be used for welding of most metal materials and can be used to weld metal, heterogeneous metal and welding between metal and non-metal.
It can be used for the repeated production of thin plate metal joints with wire, foil or plate in the thickness of 2 ~ 3mm.
Welding new process, new technology.
The outstanding performance of welding technology progress is the development of welding process from mechanization to automation, intellectualization and informatization.
The application of intelligent welding robot is an important symbol of high automation of the welding process.
Welding robots break through the traditional way of welding automation, making it possible to automate small batch production.
Welding robot is mostly fixed position arm type machine, there are two kinds: teaching type and intelligent type.
Through the teaching, memory welding track and welding parameters, and strictly follow the instruction program to complete the product welding.
With one instruction, the robot can accurately reproduce each step of the instruction.
This kind of welding robot is widely used, suitable for mass production and used for the assembly line.
Its function is mainly to show the teaching reappearance. Poor adaptability to environmental changes.
There is no use for small mass production of large structures on construction sites.
The starting point, space trajectory and related parameters of the weld can be determined automatically according to the simple control instruction.
According to the actual situation, it can automatically track the weld trajectory, adjust the welding torch attitude, adjust the welding parameters, and control the welding quality.
This is the most advanced welding robot, with the characteristics of dexterity, light and easy to move.
It can adapt to the welding tasks of different structures and locations, and the practical application is very few. It is still in the research and development stage.
In the welding robot, 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 controlled by the control system and the welding part.
The welding system mainly includes:
Welding controller, welding clamp, water, electrical and other auxiliary parts (underwater welding).
2 Application of computer software.
The application of computer software system in the welding field mainly includes the following aspects:
Including simulation welding thermal process, welding metallurgy process, welding stress and deformation, etc.
Welding is a complicated process involving arc physics, heat transfer, metallurgy and mechanics.
Once all the processes in the welding are simulated, the best design scheme, process method and welding parameters of welding various structures and materials can be determined by the computer system.
Traditionally, the welding process has to be determined by a series of experiments or by experience to obtain a reliable and economical welding structure.
Computer simulation proves the applicability of the numerical method to solve a problem by a small number of verification tests. A lot of screening work can be done by computer, which saves a lot of experimental 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 also determines the scope of automation welding.
In addition, computer simulation is widely used to analyze the strength and performance of welding structures and joints.
2.Database technology and expert system.
Used for welding process design and process parameter selection, welding defect diagnosis, welding cost budget, real-time monitoring, welding CAD, welder test, etc.
The database technology has penetrated into all aspects of the welding field, from raw material, welding test, welding technology to welding production.
A typical database system including welding procedure qualification, welding procedure specification, welder archives management, welding material, material composition and properties, the weldability and welding, welding CCT diagram management standard consulting system, etc.
These database systems provide favorable conditions for various data and information management in the field of welding.
The welding expert system mainly focuses on process development, defect prediction and diagnosis, computer-aided design and so on.
In the existing welding expert system, process selection and process development are the most important application fields, and the real-time control of the welding process is an important development direction.
3.Computer-aided quality control technology (CAQ)
Used for real-time monitoring of product data analysis and welding quality.
In addition, computer-aided design/manufacturing (CAD/CAM) is increasingly used in the welding process, which is mainly used in NC cutting, welding structure design and welding robot.