Welding Training Series:
- Welding Training 101: Welding Method (1)
- Welding Training 101: Welding Materials (2)
- Welding Training 101: Welding Defects, Symbol, Deformation, Cracks, Inspection (3)
1. Welding description
1. What is welding?
Welding is the process of joining the same metal or different metals.
2. What is a weld seam?
After the welding operation, the polymer of metal and non-metal formed in a certain area is called weld seam.
2. Welding method
According to the characteristics of the welding process, it can be classified into fusion welding, pressure welding and brazing.
Fusion welding:
The welding method in which the metal at the joint is melted under the action of high temperature, etc.
Pressure welding:
Welding method completed under certain pressure.
Use the metal whose melting point is lower than the melting point of the base metal as the filler metal to heat the weldment and filler metal to the temperature between the melting point of the base metal and the melting point of the filler metal, so that the filler metal is liquid and wets the base metal, fills the joint gap and diffuses with the base metal to achieve welding.
Specific classification:
2.1 Common welding methods
2.1.1 Manual electrode arc welding
What is an arc?
The strong and lasting gas discharge between the workpiece and electrode is called arc.
To put it bluntly, it is just a gas discharge phenomenon.
However, the arc includes three parts: arc column area, cathode area and anode area.
Arc temperature:
- Cathode area 2100 ℃,
- Arc column 5700~7700 ℃
- Anode area 2300 ℃
Heat in arc zone:
- Anode accounts for 43%
- Cathode area accounts for 36%
- Arc column area accounts for 21%
What is arc welding?
What is manual arc welding? To put it simply, arc welding is a welding method that uses arc combustion to provide heat source.
The electric arc welding method using manually operated welding rods is called manual electrode arc welding, which is called manual arc welding for short.
How is manual electrode arc welding performed?
(1) The arc burns between the electrode and the weldment, and the heat of the arc makes the workpiece and the electrode melt into a molten pool at the same time;
(2) The electric arc melts or burns the coating of the electrode, producing slag and gas, which protect the molten metal and molten pool;
(3) When the arc moves forward, the subsequent molten pool cools and solidifies while new molten pool is continuously generated, thus forming a continuous weld.
Advantage of manual electrode arc welding:
Simple equipment, flexible operation and strong adaptability.
Disadvantage of manual electrode arc welding:
The production efficiency is low, the labor intensity is high, and the quality of welding joints is not easy to guarantee.
Application of manual electrode arc welding:
It can weld most metals, suitable for various welding positions, and can weld both thin and thick plates.
2.1.2 Automatic submerged arc welding
What is the commonly said submerged arc automatic welding?
Automatic welding – The welding action is automatically completed by the mechanical device.
Submerged arc welding – Welding in which an arc burns under a granular flux layer.
How does automatic submerged arc welding process?
(1) The flux flows out of the funnel and is evenly stacked on the welded part of the workpiece to form a flux layer (30-50mm);
(2) The continuously fed welding wire generates an arc between the welding wire and the weldment under the flux layer, melting the welding wire, workpiece and flux, forming a metal molten pool, and isolating them from the air;
(3) As the welding machine moves forward automatically, the arc continuously melts the metal, wire and flux of the weldment in front, while the edge behind the molten pool begins to cool and solidify to form a weld, and the liquid slag subsequently condenses to form a hard slag shell.
Why should a layer of flux be spread on the surface of the welding position?
It has the following three functions:
(1) Protective effect
Flux melts to form slag and gas, effectively isolating air, protecting droplet and molten pool, and preventing the burning of alloy elements;
(2) Metallurgical action
In the welding process, it plays the role of deoxidation and supplementing alloy elements;
(3) Improve welding process
Keep the arc burning steadily and make the weld beautiful.
What are the characteristics of automatic submerged arc welding?
What areas are they mainly used in?
Advantage of automatic submerged arc welding:
High production efficiency (5~times higher than manual arc welding);
Good welding quality (less porosity and slag inclusion);
Low cost (labor, time and material saving);
Good working conditions (no spatter, low labor force).
Disadvantage of automatic submerged arc welding:
Not suitable for welding complex welds and narrow spaces;
The equipment is complex and the required welding current is large (small current will cause unstable arc);
Poor adaptability (applicable to flat welding, long straight weld and circumferential weld with larger diameter).
Application:
It is used for long straight weld and circumferential weld with large diameter of production and medium plate structure, such as boiler, pressure vessel, ship, etc.
In order to obtain reasonable weld formation and good welding quality, as well as improve the welding speed, multi wire submerged arc welding has emerged.
It is widely used in applications requiring high welding efficiency, such as wind power, marine engineering, shipbuilding, pressure vessels, heavy machinery, pipelines and other industries.
Multi power series multi-wire submerged arc welding:
It is mainly used for pipeline welding.
According to the application, it can be divided into external welding of long straight longitudinal seam of pipeline, internal welding of long straight longitudinal seam of pipeline, external welding of spiral welded pipe, internal welding of spiral welded pipe and other different types.
Single power parallel multi-wire submerged arc welding:
Greatly improve the production efficiency, increase the welding speed, reduce the flux consumption, reduce the heat input and deformation, save energy, make the primer more efficient and cover more smoothly.
2.1.3 Gas shielded welding
The arc welding using gas as the arc medium and protecting the arc and welding area is called gas shielded arc welding (GMAW).
In other words, gas shielded welding uses gas as the medium and protection.
What specific types does it have?
Classified according to whether the electrode is melted and the type of shielding gas:
- Non melting electrode (tungsten electrode) inert gas welding (GTAW/TIG);
- GMAW/MAG;
- MIG welding.
Shielding gas:
- Argon → argon arc welding
- CO2 → CO2 gas shielded welding
Electrode:
- Non melting electrode (tungsten electrode)
- Melting electrode
Differences between consumable electrode and non consumable electrode gas shielded welding:
MIG/MAG: Directly feed the welding wire, which is both electrode and filler metal.
TIG: Tungsten electrode is the electrode and does not melt; The welding wire is fed separately and used only as filler metal
Argon arc welding
What are the characteristics of argon arc welding?
What are the main applications?
Tungsten electrode, manual wire feeding.
The welding wire is only the filler metal.
The welding wire does not pass the current, so there is no splash.
However, in order to avoid tungsten electrode melting and burning, which may cause tungsten contamination in the molten pool, the welding current is not easy to be too high.
Only thin plates less than 4mm can be welded.
Melting electrode, automatic wire feeding.
The welding wire is both an electrode and a filler metal.
There is no melting and burning loss of tungsten electrode.
The welding current can be adjusted in a wide range, so thin and medium-thick plates can be welded.
Advantage of argon arc welding
It can be welded in all directions;
Easy for observation and automatic control;
Good protection effect, stable arc and good welding quality;
It can weld almost all metals.
Disadvantage of argon arc welding
High argon cost;
The current density of argon arc welding is high, the light emitted is relatively strong, and the ultraviolet radiation generated by the arc is large, causing greater harm to the body;
The heat affected zone of argon arc welding is large, and the workpiece is prone to crack, pinhole, wear, scratch, undercut and other defects after repair;
Application of argon arc welding
It is mainly used for welding nonferrous metals and alloy steels that are easy to be oxidized (mainly welding of Al, Mg, Ti and their alloys and stainless steels).
CO2 gas shielded arc welding
The principle of CO2 gas shielded arc welding is the same as that of MIG welding, except that the shielding gas is CO2.
The volume of CO gas in the molten pool and droplet expands rapidly and bursts, resulting in splashing.
It can oxidize metal and burn alloy elements, and can not weld nonferrous metals and alloy steel.
Advantage of CO2 gas shielded welding
Low cost (40%~50% of submerged arc welding and manual arc welding);
High efficiency (high current density, large penetration and fast welding speed);
Good welding quality (air flow cooling, small heat affected zone, small deformation);
Able to weld in all positions.
Disadvantage of CO2 gas shielded welding
Poor weld formation and large spatter;
Burn out alloy elements, easy to produce pores;
Poor wind resistance during welding, suitable for indoor operation.
Application of CO2 gas shielded welding
It is applicable to the welding of low carbon steel and low alloy steel sheet (0.8~4mm).
The welding wire containing deoxidizer must be used, and the DC connection must be reversed;
In addition, there shall be no wind during welding, and indoor welding is preferred.
2.1.4 Electroslag welding
Electric slag welding is a welding method that uses the resistance generated by the current passing through the slag to heat melt the welding wire and base metal to form a weld.
As for how electroslag welding is carried out, please see the following analysis:
(1) At the beginning, short-circuit the welding wire and the starting groove for arc starting;
(2) Continuously add a small amount of solid flux to melt it with the heat of the arc to form liquid slag;
(3) When the molten slag reaches a certain depth, increase the feeding speed of the welding wire and reduce the voltage, so that the welding wire is inserted into the slag pool and the arc is extinguished, thus turning into the electroslag welding process.
What are the characteristics of electroslag welding?
For what welding fields?
Advantage of electroslag welding
High productivity, no need to consider the thickness of weldment;
Few weld defects and good welding quality;
No groove is required, and the cost is low.
Dis advantage of electroslag welding
The heat input is large, and it is easy to overheat near the weld;
The weld metal has coarse crystalline as cast structure and low impact toughness;
The weldment shall be subject to normalizing and tempering heat treatment after welding.
Application of electroslag welding
It is applicable to vertical welding and welding of 40~450mm thick plates, generally used for straight seam or circumferential seam welding, thick plate splicing, vertical welding of blast furnaces in steel plants, welding of large castings, forgings, etc.
2.1.5 Resistance welding
To put it simply, the welding under pressure using resistance heat as the welding heat source is called resistance welding.
Advantage of resistance welding
Low welding voltage, high current and high productivity;
Low cost, no need for filler metal and other welding materials,
The welding deformation is small, and there is no need for correction and heat treatment after welding;
Good working conditions, simple operation, easy to realize automatic production.
Disadvantage of resistance welding
The welding equipment is complex and the investment is large;
There are certain restrictions on the thickness of weldment and joint form;
At present, there is no simple and reliable nondestructive testing method.
Resistance welding can be divided into spot welding, seam welding and butt welding.
Spot welding
Seam welding
Resistance butt welding
Flash butt welding
1. Spot welding
Spot welding diagram
The weldment is assembled as a lap joint, and the cylindrical electrode is pressurized and energized to generate resistance heat to melt the weldment metal and form nuggets (welding spots).
Spot welding process and shunting phenomenon
Spot welding process: compression → power on (nugget formation) → power off (freezing) → removal pressure
Shunt phenomenon: when welding the second point, the existing welding point will conduct current, causing power loss. The current at the welding point is reduced, which affects the welding quality.
Prevent shunt: there should be a certain distance between two welding points.
The distance between two adjacent welding heads shall not be too small, and the minimum point distance is shown in the following table.
| Workpiece thickness/mm | Point pitch/mm | ||
|---|---|---|---|
| Structural steel | Heat-resisting alloy | Aluminium alloy | |
| 0.5 | 10 | 8 | 15 |
| 1.0 | 12 | 10 | 15 |
| 1.5 | 14 | 12 | 20 |
| 2.0 | 16 | 14 | 25 |
| 3.0 | 20 | 18 | 30 |
| 4.0 | 24 | 22 | 35 |
Application of spot welding
It is mainly used for sheet metal stamping structure and reinforcement components;
It is applicable to the welding of overlapping structural parts of thin plates with sealing requirements or high joint strength requirements, such as oil tank, water tank, etc.
2. Seam welding
The weldment is assembled into overlapping or diagonal joints and placed between two roller electrodes.
The roller pressurizes the weldment and rotates to drive the weldment to move forward.
The weldment is powered continuously or intermittently, and the weldment metal is melted by resistance heat to form a series of welding points.
Classification of seam welding
| Form | Electric current | Electrode | Characteristic | Application |
| Continuous seam welding | Continuous conduction | continuous spin | Simple equipment and high productivity, but electrode wear is serious. | Small power welding non important structure (cylinder making, barrel, etc.) |
| stitch-and-seam welding | Intermittent continuity | continuous spin | Wide application (ferrous metal) | |
| Step seam welding | Intermittent continuity | Intermittent rotation | Complex equipment, high requirements, low electrode wear and high welding quality. | It is mainly used for seam welding of magnesium and aluminum alloys. |
Characteristic of seam welding
Solder joints overlap each other and flow diffluence is serious.
The current is about 1.5~2.0 times of that of spot welding;
The pressure is about 1.2~1.6 times of that of spot welding;
Therefore, high-power welding machine is required.
Apply pressure and use the roller as the electrode.
Application of seam welding
Seam welding is mainly used for thin-walled structures with regular weld seams and a thickness of less than 3mm and with sealing requirements;
Such as aircraft and automobile fuel tanks, various containers, steel radiators, etc.
3. Butt welding
A type of resistance welding method that uses resistance heat to weld two workpieces together along the entire end face is called butt resistance welding, or butt welding for short.
Butt welding can be divided into resistance butt welding and flash butt welding.
3.1 Resistance butt welding
Resistance butt welding refers to the method of pressing the end faces of two workpieces all the time, heating them to plastic state with resistance heat, and then rapidly applying upset pressure (or only maintaining the pressure during welding without upset pressure) to complete welding.
Characteristic of resistance butt welding
Simple operation and symmetrical joint shape;
The requirements for welding surface cleaning before welding are high;
The end face processing requirements are high, otherwise the welding quality is difficult to ensure.
Application of resistance butt welding
It is suitable for welding low-carbon steel bars and pipes with compact welding section, low strength requirements, diameter (or side length) less than 20mm, or non-ferrous metal bars and pipes less than 8mm (welding of rod shaped parts with similar end face shape and size).
3.2 Flash butt welding
The weldment is assembled as a butt joint, the power is connected, and its section is gradually moved closer to local contact.
The metal in the section is melted and flies out under resistance heating, forming a flash.
When the end reaches the predetermined temperature within a certain depth range, the upsetting force is rapidly applied to complete the welding.
Process: pressurization — energization — flash — upset forging
Characteristic of flash butt welding
Less slag inclusion in the joint, good welding quality and high joint strength;
The requirements for cleaning the end face of the weldment before welding are not strict;
Flash butt welding has large metal loss, and the joint burrs after welding need to be processed and cleaned. The labor conditions are poor.
Application of flash butt welding
In principle, all metal materials that can be cast can be welded by flash butt welding.
For example, low carbon steel, high carbon steel, alloy steel, stainless steel;
Aluminum, copper, titanium and other non-ferrous metals and alloys;
It can also weld dissimilar alloy joints.
2.1.6. Brazing
(Soldering iron brazing)
(Flame brazing)
(Induction brazing)
Use the metal with a melting point lower than the weldment as the filler metal, heat the weldment (base metal) and filler metal to a temperature higher than the melting point of the filler metal and lower than the melting temperature of the base metal, melt and wet the base metal, fill the joint gap, and diffuse with the base metal to form a welded joint, which is called brazing.
Brazing process
Solder melting — liquid solder flowing into joint gap — mutual diffusion between solder and base metal — filling gap — forming joint after solidification
Characteristics of brazing
1) The melting point of the filler metal is lower than that of the base metal, and the base metal will not melt during brazing;
2) The composition of filler metal and base metal is very different;
3) The melted filler metal is sucked in by wetting and capillarity and kept in the base metal gap;
4) Metallurgical bonding is formed by mutual diffusion between liquid solder and solid base metal.
Advantage of brazing
The structure and properties of the base metal do not change (the base metal does not melt, only the filler metal melts);
Simple equipment, less production investment;
The heating temperature is low, the deformation is small, and the joint is smooth;
High productivity, can weld multiple weldments and joints at the same time;
It can be used to weld dissimilar metals or dissimilar materials, with no limit on the thickness difference of the workpiece.
Disadvantage of brazing
The brazed joint has low strength and poor heat resistance.
Classification of brazing
| Type/characteristic | Soft soldering(tin, lead solder) | Brazing(Copper based and silver based solders) |
| Solder melting point | ≤ 450℃ | ＞ 450℃ |
| Performance Characteristics | Joint strength ≤ 100MPa, low working temperature | Joint strength ＞ 200MPa, high working temperature |
| Application | It is used for welding instruments and conductive components with little stress. | It is used for welding of components, tools and tools with large force. |
Application of brazing
It is mainly used for manufacturing precision instruments, electrical components, dissimilar metal components and welding of complex sheet structures, such as sandwich components, honeycomb structures, etc.
2.2 Other welding methods
2.2.1 Friction welding
Friction welding is a pressure welding method that uses the heat generated by the friction between the surfaces of the weldment to make the end face reach the thermoplastic state, and then rapidly upsets to complete the welding.
Friction stir welding
Continuous drive friction welding
Inertia friction welding
Welding process of friction welding:
1) Relative movement of weldment;
2) The end face contact between the two weldments produces friction heat;
3) When the section reaches the thermoplastic state, press the top section quickly to complete the welding.
Advantage of friction welding:
High welding productivity, 5-6 times higher than flash butt welding;
Stable welding quality and high dimensional accuracy of weldments;
Low processing cost, power saving, no need for special cleaning of weldments;
Easy to realize mechanization and automation, simple operation;
No spark, arc and harmful gas.
Disadvantage of friction welding:
Large investment;
It is difficult to weld non-circular sections and the welding area is limited.
Application of friction welding:
It can be used for welding the same or different metal and different steel products, such as copper aluminum transition joints in the electric power industry, high-speed steel structural steel tools for metal cutting, serpentine pipes, valves, tractor bearings, etc.
2.2.2 Laser welding
The surface to be machined is heated by laser radiation, and the surface heat is directed to the internal diffusion through heat transfer to melt the workpiece and form a specific molten pool. The weld is formed after the molten pool solidifies.
Advantage of laser welding:
High welding speed and efficiency;
High welding precision, small deformation and easy automatic control;
No electrode is needed, so there is no electrode pollution.
Disadvantage of laser welding:
Large investment, small power, and limited weldable thickness;
It is difficult to align the welding point with the laser beam gathering area;
Rapid solidification of the weld bead may cause porosity.
It is mainly used for welding parts in precision manufacturing fields such as aerospace, shipbuilding, automobile and electronic industry.
2.2.3 Plasma arc welding
What is a plasma arc?
How is plasma arc welding performed?
To put it simply: the arc in which the arc column is compressed and the energy is highly concentrated, and the gas in the arc column is fully ionized, is called plasma arc, also called compression arc.
The gas is heated by the arc and initially ionized. When it passes through the water-cooled nozzle at high speed, it is compressed, increasing the energy density and ionization degree, and forming a plasma arc.
Because of the high heat and penetration force of the plasma arc, the workpiece at the welding point is melted to form a molten pool, so as to realize the welding of the workpiece.
Advantage of plasma arc welding:
High energy density, strong arc directivity and strong penetration ability;
The depth width ratio of the weld is large, and the heat affected zone is small;
Stable arc combustion (stable even when the current is small);
The welding cost is low, and when the welding thickness is small, there is no groove and no need to fill wire;
It has stable keyhole effect, and can better realize the free forming of single side welding and double sides.
Disadvantage of plasma arc welding:
Equipment is expensive;
Poor observability of arc action area;
The weldable thickness is limited (generally less than 25mm).
Application of plasma arc welding:
It is widely used in industrial production, especially in the welding of copper and copper alloys, titanium and titanium alloys, alloy steel, stainless steel, molybdenum and other metals used in military and cutting-edge industrial technologies such as aerospace, such as titanium alloy missile shells, and some thin-walled containers on aircraft.
2.2.4 Electron beam welding
The method of welding by using the heat energy generated by the accelerated and focused electron beam bombarding the weldment placed in vacuum or non vacuum is called electron beam welding.
In other words, the heat required for welding is generated by electron beam bombarding the weldment.
Welding process:
1) The cathode of the electron gun emits electrons due to direct or indirect heating;
2) Under the acceleration of high-voltage electrostatic field, the electron beam with high energy density can be formed by focusing the electromagnetic field;
3) The high-energy density electron beam bombards the workpiece, and the huge kinetic energy is converted into heat energy to melt the weldment, forming a molten pool, and then complete the welding.
Advantage of electron beam welding:
Fast welding speed and high efficiency;
No groove, no welding wire, no flux, etc;
Low energy consumption, narrow heat affected zone, small welding deformation and excellent quality;
The penetration ability of electron beam is strong, and the depth width ratio of weld is large, which can reach 50:1.
Disadvantage of electron beam welding:
The equipment is complex and expensive;
The size and shape of weldments are often limited by the vacuum chamber;
The electron beam is easily interfered by stray electromagnetic field, which affects the welding quality.
Vacuum electron beam welding
Application of electron beam welding:
Suitable for fine welding of refractory metal, active metal and high purity metal.
It is widely used in the welding of nuclear energy, aviation, aerospace, automobile, pressure vessel, tool manufacturing and other industrial fields.
2.2.5 Ultrasonic welding
Ultrasonic welding is a method that uses the high-frequency oscillation of ultrasonic wave to locally heat and clean the workpiece joint, and then applies pressure to realize welding.
Welding process
1) Low frequency alternating current to high frequency alternating current;
2) Electric energy is converted into mechanical vibration energy;
3) Increase amplitude;
4) Apply pressure, and the welding head contacts the workpiece for welding.
Advantage of ultrasonic welding:
Easy operation, fast welding speed and high production efficiency;
The requirements for the cleanliness of the workpiece surface are not high;
It is not necessary to add any binder, filler or solvent.
Disadvantage of ultrasonic welding:
It is only applicable to the welding of thin parts such as wire, foil, sheet, strip and strip;
In most cases, the joint form can only be lap joint.
Application of ultrasonic welding:
It is applicable to the welding of high conductivity, high thermal conductivity materials and a variety of composite materials, and is widely used in the welding of microelectronic devices and finishing fields.
2.2.6 High frequency welding
High frequency welding is a method that uses the resistance heat generated by high-frequency current in the workpiece to heat the surface layer of the welding area of the workpiece to a molten or near plastic state, and then applies (or does not apply) upset force to achieve metal bonding.
Skin effect: When the conductor is connected with AC current, most of the current flows only along the surface of the conductor.
Proximity effect: When high-frequency current flows in opposite directions between two conductors or in a reciprocating conductor, the current will concentrate on the flow near the conductor.
To put it simply, the skin effect is “the current goes to the surface”; Proximity effect is “current shortcut”.
Advantage of high frequency welding:
High welding speed and efficiency (the current is highly concentrated in the welding area);
A wide range of weldable materials can also be used to weld dissimilar metals;
Cleaning before welding is simple (fusion welding, with pressure effect, does not need to clean the joint surface).
Disadvantage of high frequency welding:
The joint assembly accuracy is required to be high;
High voltage and high frequency current are harmful to human body and other equipment.
Application of high frequency welding:
It is suitable for welding carbon steel, alloy steel, stainless steel, copper, aluminum, titanium and other dissimilar metals.
It is widely used for welding the longitudinal seam or spiral seam when manufacturing pipes.
2.2.7 Diffusion welding
Diffusion welding refers to the welding method of closely fitting the weldment, keeping it under a certain temperature and pressure for a period of time, and making use of the mutual diffusion of atoms between the contact surfaces to form a connection.
Welding process
a) Rough initial contact
b) Phase I: deformation and interface formation
c) Phase II: grain boundary migration and micropore elimination
d) Phase III: volume diffusion, micropore elimination
Physical welding display
Advantage of diffusion welding:
Multiple joints can be welded at one time;
The joint quality is good, and no machining is required after welding;
Small deformation of weldment (low pressure, overall heating of workpiece, cooling in furnace).
Disadvantage of diffusion welding:
Large investment and high cost;
Long welding time, time-consuming and labor-intensive surface preparation, and low productivity;
There is no reliable nondestructive testing method for weld quality.
Application of diffusion welding:
It is applicable to the welding of various dissimilar materials, special materials and special structures, and is widely used in aerospace, electronics, nuclear power and other industrial fields.
2.2.8 Explosive welding
Explosive welding is a method to realize welding by using the impact force generated by explosive explosion to cause rapid collision of workpieces.
Explosive welding is also a kind of pressure welding.
For welding of small workpieces, both parallel method and angle method can be used;
The parallel method is often used for large area welding.
The gap between the front sections of the two plates is too large, which will lead to excessive acceleration of the clad plate and excessive impact energy, resulting in damage and crack at the edge of the plate, thus reducing the effective area of the clad plate and consuming the plate.
Advantage of explosive welding:
Small investment and low cost;
Especially suitable for dissimilar metals and large area welding;
The process is simple, no complex cleaning is required, and the application is convenient.
Disadvantage of explosive welding:
It can only be used for welding of plane or cylinder structure;
Open air operation has low mechanization and affects the environment.
Application of explosive welding:
Suitable for welding dissimilar metals, such as aluminum, copper, titanium, nickel, tantalum, stainless steel and carbon steel, aluminum and copper welding.
It is widely used for the welding of conductive bus transition joints, heat exchanger tubes and tube sheets, and the manufacture of large-area composite plates.
