In this post, we will analyze 10 different types of welding for you.
1. Electrode arc welding
(1) Welding arc
Arc is a persistent and intense gas discharge phenomenon between two charged conductors.
(1) Short circuit between welding rod and workpiece
In case of short circuit, individual contact points with dense current are heated by resistance heat q = 12Rt, and the electric field intensity of very small air gap is very high.
① A small amount of electrons escape.
② Individual contact points are heated, melted, even evaporated and vaporized.
③ There are many metal vapors with low ionization potential.
(2) Lift the welding rod at an appropriate distance
Under the action of thermal excitation and strong electric field, the negative electrode emits electrons and makes high-speed directional movement, hitting neutral molecules and atoms to excite or ionize them.
The gas in the air gap is ionized rapidly. In the process of impact, excitation and recombination of positive and negative charged particles, its energy is converted to emit light and heat.
Arc structure and temperature distribution
The arc consists of three parts, namely, cathode area (generally white bright spots on the end face of the electrode), anode area (thin bright area in the bath corresponding to the end of the electrode on the workpiece) and arc column area (air gap between two electrodes).
Conditions for stable arc combustion
(1) There shall be a power supply that meets the electrical characteristics of welding arc
a) When the current is too small, the gas ionization between air gaps is insufficient, the arc resistance is large, and a higher arc voltage is required to maintain the necessary ionization degree.
b) With the increase of current, the ionization degree of gas increases, the conductivity increases, the arc resistance decreases and the arc voltage decreases.
However, when it is reduced to a certain extent, in order to maintain the necessary electric field strength and ensure the emission of electrons and the motion energy of charged particles, the voltage must not change with the increase of current.
(2) Do a good job in cleaning and select electrodes with appropriate coating.
(3) Prevent partial blowing.
(4) Polarity of electrode
In welding, when DC welding machine is used, there are two methods: positive connection and reverse connection.
AC arc welding equipment is widely used, and the polarity of the electrode changes frequently, so there is no polarity problem,
1) Positive connection
The weldment is connected to the positive pole of the power supply and the electrode is connected to the negative pole.
Normal connection method is adopted for general welding operation.
2) Reverse connection
The weldment is connected to the negative pole of the power supply and the electrode is connected to the positive pole.
Generally, when welding thin plates, in order to prevent burn through, the reverse connection method is used for welding.
(2) Welding process of electrode arc welding
1). Welding process
2). Welding rod arc welding heating characteristics
- High heating temperature and local heating. The metal near the weld is heated very unevenly, which may cause workpiece deformation, residual stress, uneven microstructure transformation and performance change.
- The heating speed is fast (1500 ℃ / s), the temperature distribution is uneven, and the microstructure and defects that should not appear in the heat treatment may appear.
- The heat source is moving, and the heating and cooling areas are constantly changing.
(3) Metallurgical characteristics of arc welding
- The high temperature in the reaction zone makes the alloy elements evaporate strongly and burn out by oxidation.
- The metal molten pool is small in volume and in liquid state for a short time, resulting in uniform chemical composition, gas and impurities do not have time to float out, which is easy to produce defects such as pores and slag inclusion.
(4) Welding rod
Composition of welding rod manual arc welding rod is composed of welding core and coating.
1) Welding core
① As an electrode of arc welding, it conducts electricity with the weldment to form an arc;
② During the welding process, it melts continuously and transits to the moving molten pool, which crystallizes with the molten base metal to form a weld;
2) Electrode coating
① Effect of drug skin
- Effective gas slag joint protection for molten pool;
- Deoxidize and desulfurize the molten metal in the molten pool and infiltrate alloy into the molten pool metal to improve the mechanical properties of the weld;
- Play the role of arc stabilization to improve the welding process.
② Composition of drug skin
- Arc stabilizer: mainly use potassium, sodium and calcium compounds that are easy to ionize.
- Slag forming agent: form slag to cover the surface of the molten pool, prevent the atmosphere from invading the molten pool, and play a metallurgical role.
- Gasifier: decomposes gases such as CO and H2and surrounds the arc and molten pool to isolate the atmosphere and protect molten droplets and molten pool.
- Deoxidizer: it is mainly used to remove oxygen from the molten pool by ferromanganese, ferrosilicon, ferrotitanium, ferroaluminum and graphite.
- Alloying agent: mainly used in ferroalloys such as ferromanganese, ferrosilicon, ferrochromium, ferromolybdenum, ferrovanadium and ferrotungsten.
- Binder: potassium and sodium silicate are commonly used.
3) Types of electrode coating
- Acid electrode
The drug skin contains a large amount of acidic oxides, such as SiO2, TiO2, Fe2O3, etc.
- Alkaline electrode
The drug skin contains a large amount of alkaline oxides, such as CaO, FeO, MnO, Na2O, MgO, etc.
Type of welding rod
Electrodes are divided into ten categories:
Namely structural steel electrodes, low-temperature steel electrodes, molybdenum and chromium molybdenum heat-resistant steel electrodes, stainless steel electrodes, surfacing electrodes, cast iron electrodes, nickel and nickel alloy electrodes, copper and copper alloy electrodes, aluminum and aluminum alloy electrodes and special-purpose electrodes.
Selection principle of welding rod
- Select electrodes with the same or similar chemical composition as the base metal
- Select electrodes with the same strength as the base metal
- The type of electrode coating shall be selected according to the service conditions of the structure.
(5) Changes of metal structure and properties of welded joints
Change and distribution of temperature on weldment
The metal in the weld zone begins to be heated at a higher temperature in a steady state, and then gradually cools to room temperature.
Changes in microstructure and properties of welded joints (taking low carbon steel as an example)
Main defects of welded joints
Blowholes are holes formed when bubbles in the weld pool fail to escape during weld solidification.
a) Dry the welding rod and carefully clean the welding surface of the weldment and the nearby area;
b) Use appropriate welding current and operate correctly.
(2) Slag inclusion – slag inclusion is the slag remaining in the weld after welding.
a) Carefully clean the welding surface;
b) During multi-layer welding, the slag shall be thoroughly removed between layers;
c) Slow down the crystallization rate of molten pool.
(3) Welding crack
a) Hot crack
Hot crack is a welding crack generated when the metal of the welded joint cools to the high temperature zone near the solidus during welding.
Reduce structural stiffness, preheat before welding, reduce alloying, select low hydrogen electrode with good crack resistance, etc.
b) Cold crack
A welding crack that occurs when a welded joint is cooled to a lower temperature.
a) Use low hydrogen electrode, dry and remove oil stain and rust on the surface of weldment;
b) Preheat before welding and heat treatment after welding.
(4) Incomplete penetration
Incomplete penetration is the phenomenon of incomplete penetration at the root of the welded joint.
Too small groove angle or gap, too thick blunt edge, unclean groove, too thick electrode, too fast welding speed, too small welding current and improper operation.
(5) Incomplete fusion
Incomplete fusion is the phenomenon of incomplete fusion between weld and base metal.
Causes: unclean groove, excessive electrode diameter and improper operation.
Undercut is the phenomenon of groove or depression along the base metal part of the weld toe.
Excessive welding current, too long arc, improper electrode angle, etc.
(6) Welding deformation
Causes of welding stress and deformation
Local heating during welding is the root cause of welding stress and deformation.
Basic form of welding deformation
Process measures to prevent and reduce welding deformation
(1) Inverse deformation method
(2) Plus margin method
(3) Rigid clamping method
(4) Select reasonable welding process
Process measures to reduce welding stress
(1) Select reasonable welding sequence
(2) Preheating method
(3) Post weld annealing
2. Automatic submerged arc welding
The method in which the arc burns under the flux layer for welding is called submerged arc welding.
The arc striking and electrode feeding of submerged arc welding are generally completed by automatic assembly, so it is also called submerged arc automatic welding.
(1) Welding process of automatic submerged arc welding
(2) Main features of automatic submerged arc welding
- High productivity
- High and stable welding quality
- Save welding materials
- Improved working conditions
- It is suitable for flat welding long straight weld and large diameter circumferential weld. For short weld, zigzag weld, narrow position and thin plate welding, its advantages can not be brought into play.
(3) Welding wire and flux
(4) Process characteristics of submerged arc automatic welding
- Strict requirements for preparation before welding
- Large welding penetration
- Arc striking plate and outgoing plate are adopted.
- Use flux pad or steel pad.
- Guide installation is adopted.
3. Gas shielded welding
(1) Argon arc welding
Gas shielded welding using argon as shielding gas is called pressure arc welding.
Argon is an inert gas, which can protect the electrode and molten metal from the harmful effects of air.
According to the different electrodes used, argon arc welding can be divided into molten electrode argon arc welding and non molten electrode argon arc welding.
Non melting electrode argon arc welding
The electrode is only used to emit electrons and generate electric arc, and the filler metal is added.
Tungsten electrodes doped with thorium oxide or cerium oxide are commonly used, which are characterized by strong electron thermal emission ability, high melting point and boiling point (3700k and 5800k).
TIG welding has low current and shallow penetration. TIG welding with high productivity is often used for the welding of titanium, aluminum, copper and other alloys with medium thickness and above.
Characteristics of argon arc welding
(1) Due to the protection of argon, it is suitable for the welding of various alloy steels, non-ferrous metals easy to oxidize, and rare metals such as zirconium, tantalum and molybdenum.
(2) Argon arc welding has stable arc, small splash, dense weld, no slag on the surface, beautiful forming and small welding deformation.
(3) Open arc visible, easy to operate, easy to realize full position automatic welding.
(4) Tungsten pulsed argon arc welding can weld thin plates below 0.8mm and some dissimilar metals.
(2) Carbon dioxide gas shielded welding
Gas shielded welding using CO2 as shielding gas is called carbon dioxide gas shielded welding.
Its protective function is mainly to isolate the welding area from the air and prevent the harmful effect of nitrogen in the air on the molten metal.
Therefore, welding is carried out in CO2, CO and O2 oxidation atmosphere.
Characteristics of carbon dioxide gas shielded welding:
- High welding speed, automatic welding and high productivity.
- It is open arc welding, which is easy to control the weld formation.
- It is less sensitive to rust and less slag after welding.
- The price is low.
- Welding spatter and blowhole are still difficulties in production.
4. Electroslag welding
Electroslag welding is a method of welding by using the resistance heat generated by current passing through liquid slag.
(1) Welding process
(2) Characteristics of electroslag welding
- It can be welded into very thick weldments at one time.
- High productivity and low cost.
- The weld metal is relatively pure.
- Suitable for welding medium carbon steel and alloy structural steel.
5. Plasma arc welding and cutting
(1) Concept of plasma arc
Generally, the welding arc is a free arc, only part of the gas in the arc area is ionized, and the temperature is not concentrated enough.
When the free arc is compressed into an arc with high energy density, the arc column gas is fully ionized and becomes a state containing only positive and negative ions, that is, the fourth state of matter – plasma.
Plasma arc has high temperature (15000 ~ 30000k), high energy density (480kw / cm2) and high-speed movement of plasma flow (up to several times and sound velocity)
Three compression effects of plasma arc welding
(1) Mechanical compression effect
In the plasma gun, after high-frequency oscillation arc striking, the arc formed by gas ionization passes through the small nozzle hole and is mechanically compressed by the inner wall of the nozzle.
(2) Thermal compression effect
Due to the action of cooling water in the nozzle, the gas temperature and ionization near the inner wall of the nozzle are sharply reduced, forcing the arc current to pass only from the center of the arc column, resulting in a sharp increase in the current density in the center of the arc column and a further decrease in the arc section, which is the second compression of the arc.
(3) Electromagnetic contraction effect
Because the current density of the arc column is greatly increased, the associated electromagnetic contraction force makes the arc compress for the third time.
Due to the triple compression effect, the diameter of plasma arc is only about 3mm, while the energy density, temperature and air velocity are greatly improved.
(2) Characteristics of plasma arc welding
- High energy density, large temperature gradient and small heat affected zone. It can weld materials with strong heat sensitivity or manufacture bimetallic parts.
- Good arc stability and high welding speed. Penetration welding can be used to form the weld on both sides at one time, with beautiful surface and high productivity.
- It can be used for welding large thickness workpieces or cutting large thickness stainless steel, aluminum, copper, magnesium and other alloys.
- The arc is fully ionized and can still work stably when the lower limit of current is below 0.1A. It is suitable for welding ultra-thin plates (0.01 ~ 2mm) with micro beam plasma arc (0.2 ~ 30A), such as capsule, thermocouple, etc.
6. Vacuum electron beam welding
Vacuum electron beam welding is to use the directional and high-speed electron beam to impact the workpiece, convert the kinetic energy into heat energy and melt the workpiece to form a weld.
Characteristics of vacuum electron beam welding
When welding in vacuum, the weld is pure, smooth, mirror like, and free of oxidation and other defects.
The electron beam energy density is up to 108 w / cm2, which can quickly heat the weldment metal to a very high temperature, so it can melt any refractory metal and alloy.
Large penetration, fast welding speed and minimal heat affected zone have little impact on the performance of the joint, and the joint is basically free of deformation.
7. Laser welding
Laser welding is a method of welding by using the focused laser beam as energy to bombard the heat generated by the weldment. Characteristics of laser welding:
- Laser welding has the advantages of high energy density, short action time, small heat affected zone and deformation. It can be welded in the atmosphere without gas protection or vacuum environment.
- The direction of the laser beam can be changed by the reflector, and there is no electrode to contact the weldment during the welding process, so it can weld the parts that are difficult to be welded by the general electric welding process.
- Laser can directly weld insulating materials. It is easy to weld dissimilar metal materials, and even weld metal and non-metal together.
- The power is small and the welding thickness is limited.
8. Resistance welding
Resistance welding is a process method that applies pressure through the electrode after the weldment is combined, and uses the resistance heat generated by the current passing through the contact surface of the joint and the adjacent area to weld.
There are many kinds of resistance welding, including spot welding, seam welding and butt welding.
(1) Spot welding
Spot welding is a resistance welding method in which the weldment is assembled into a lap joint, pressed between two electrodes, and the base metal is melted by resistance heat to form a welding spot. Spot welding is mainly used for sheet welding.
Process of spot welding:
- Preload to ensure good contact of workpiece.
- Power on to form nugget and plastic ring at the weld.
- Breakpoint forging makes the nugget cool and crystallize under the continuous action of pressure to form a solder joint with dense structure and no shrinkage cavity and crack.
(2) Seam welding
Seam welding is a resistance welding method in which the weldment is assembled into a lap or butt joint and placed between two roller electrodes, the roller pressurizes the weldment and rotates, and continuously or intermittently sends power to form a continuous weld.
Seam welding is mainly used for welding structures with regular welds and sealing requirements, and the plate thickness is generally less than 3mm.
(3) Butt welding
Butt welding is a resistance welding method to weld the weldment along the whole contact surface.
Resistance butt welding
Resistance butt welding is a method of assembling weldments into butt joints, making their end faces in close contact, heating them to plastic state by resistance heat, then cutting off power and rapidly applying upsetting force to complete welding,
Resistance butt welding is mainly used for weldments with simple section, diameter or side length less than 20mm and low strength requirements.
Flash butt welding
Flash butt welding is a method to assemble the weldment into a butt joint, connect the power supply, gradually move the end face closer to local contact, heat these contact points with resistance heat, generate flash under the action of large current, melt the end metal, and then cut off the power and quickly apply upsetting force to complete the welding when the end reaches the preset temperature within a certain depth range.
The joint quality of flash welding is better than that of resistance welding, the mechanical properties of the weld are equivalent to that of the base metal, and there is no need to clean the pre welded surface of the joint before welding.
Flash butt welding is often used for welding important weldments.
It can weld the same metal or different metal; It can weld 0.01mm metal wire or 20000mm metal bar and profile.
9. Friction welding
Friction welding is a pressure welding method that uses the heat generated by the friction between the surfaces of weldments to make the end face reach the thermoplastic state, and then quickly upsetting to complete the welding.
Features of friction welding:
- Due to friction, the oxide film and impurities on the contact surface of the weldment are cleared, so that the structure of the welded joint is dense without defects such as pores and slag inclusion.
- It can weld the same metal, which is more suitable for the welding of different metals.
- High productivity.
(1) Types of brazing
According to the melting point of solder, brazing can be divided into hard brazing and soft brazing.
Brazing with solder melting point higher than 450 ℃ is hard brazing.
Brazing filler metals include copper base, silver base, aluminum base and other alloys.
Fluxes are commonly used, such as Pengsha, boric acid, fluoride, chloride, etc.
The heating methods include flame heating, salt bath heating, resistance heating, high-frequency induction heating, etc.
The strength of brazed joint is up to 490MPa, which is suitable for workpieces with large stress and high working temperature.
Brazing with solder melting point lower than 450 ℃ is soft brazing. The commonly used soft solder is tin lead alloy.
Commonly used flux is rosin, ammonium chloride solution, etc. Commonly used soldering iron and other flame heating.
(2) Characteristics of brazing
- The heating temperature of weldment is low, the change of metal structure and mechanical properties is small, the deformation of weldment is small, the joint is smooth and flat, and the size of weldment is accurate.
- The same or different metals can be welded.
- It can weld weldments with complex shapes composed of multiple welds.
- The equipment is simple.