Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to the materials being joined. Welding is a fabrication process that joins materials usually Metals or Thermoplastics by causing coalescence. A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same Velocity (speed and direction The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The workpieces melt as the kinetic energy of the electrons is transformed into heat upon impact, and the filler metal, if used, also melts to form part of the weld. The kinetic energy of an object is the extra Energy which it possesses due to its motion The welding is often done in conditions of a vacuum to prevent dispersion of the electron beam. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. The process was developed by German physicist Karl-Heinz Steigerwald, who was at the time working on various electron beam applications, perceived and developed the first practical electron beam welding machine which began operation in 1958. [1]
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Operation
As the electrons strike the workpiece, their energy is converted into heat, instantly vaporizing the metal under temperatures near 25,000 °C. The Celsius Temperature scale was previously known as the centigrade scale. The heat penetrates deeply, making it possible to weld much thicker workpieces than is possible with most other welding processes. However, because the electron beam is tightly focused, the total heat input is actually much lower than that of any arc welding process. Arc welding uses a Welding power supply to create an Electric arc between an electrode and the base material to melt the metals at the welding point As a result, the effect of welding on the surrounding material is minimal, and the heat-affected zone is small. The heat-affected zone (HAZ is the area of base material either a Metal or a Thermoplastic, which has had its microstructure and properties altered by Distortion is slight, and the workpiece cools rapidly, and while normally an advantage, this can lead to cracking in high-carbon steel. Carbon steel, also called plain carbon steel, is Steel where the main alloying constituent is Carbon. Almost all metals can be welded by the process, but the most commonly welded are stainless steels, superalloys, and reactive and refractory metals. In Metallurgy, stainless steel is defined as a Steel Alloy with a minimum of 11 A superalloy, or high-performance alloy, is an Alloy that exhibits excellent mechanical strength and creep resistance at high temperatures good surface Refractory metals are a class of Metals that are extraordinarily resistant to Heat and Wear. The process is also widely used to perform welds of a variety of dissimilar metals combinations. However, attempting to weld plain carbon steel in a vacuum causes the metal to emit gases as it melts, so deoxidizers must be used to prevent weld porosity. A deoxidizer is a chemical used in a reaction or process to remove Oxygen. [2] Electron Beam Welding is a very similar process to Laser Beam Welding, except that electrons are focussed instead of photons in the case of lasers. The advantage of using an electron beam is that the beam does not have a tendency to diverge as laser beams do when they contact the workpiece. Some of the uses of EB welding include making aerospace and automotive parts, as well as semiconductor parts and even jewelry.
The amount of heat input, and thus the penetration, depends on several variables, most notably the number and speed of electrons impacting the workpiece, the diameter of the electron beam, and the travel speed. Greater beam current causes an increase in heat input and penetration, while higher travel speed decreases the amount of heat input and reduces penetration. The diameter of the beam can be varied by moving the focal point with respect to the workpiece—focusing the beam below the surface increases the penetration, while placing the focal point above the surface increases the width of the weld. [2]
The three primary methods of EBW are each applied in different welding environments. The method first developed requires that the welding chamber be at a hard vacuum. Material as thick as 15 cm (6 in) can be welded, and the distance between the welding gun and workpiece (the stand-off distance) can be as great as 0. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International Inches redirects here To see the Les Savy Fav album see Inches. 7 m (30 in). While the most efficient of the three modes, disadvantages include the amount of time required to properly evacuate the chamber and the cost of the entire machine. As electron beam gun technology advanced, it became possible to perform EBW in a soft vacuum, under pressure of 0. 1 torrs. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. This allows for larger welding chambers and reduces the time and equipment required to attain evacuate the chamber, but reduces the maximum stand-off distance by half and decreases the maximum material thickness to 5 cm (2 in). The third EBW mode is called nonvacuum or out-of-vacuum EBW, since it is performed at atmospheric pressure. The stand-off distance must be diminished to 4 cm (1. 5 in), and the maximum material thickness is about 5 cm (2 in). However, it allows for workpieces of any size to be welded, since the size of the welding chamber is no longer a factor. [2] A schematic drawing may be helpful [1]
Equipment
The electron beam gun used in EBW both produces the electrons and accelerates them, using a hot cathode emitter made of tungsten that emits electrons when heated. Hot cathode is also a name for a Hot filament ionization gauge, a vacuum measuring device Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy The electrons are then accelerated to a hollow anode inside the gun column by means of a high voltage differential. An anode is an Electrode through which Electric current flows into a polarized electrical device They pass through the anode at high speed (approx 1/2 the speed of light) and are then directed to the workpiece with magnetic forces resulting from focusing and deflection coils. These components are all housed in an electron beam gun column, in which a hard vacuum (about 0. Cathode rays (also called an electron beam or e-beam) are streams of Electrons observed in Vacuum tubes i An electron gun (also called electron emitter) is an electrical component that produces an Electron beam that has a precise Kinetic energy and is most often 00001 torr) is maintained. [2]
The EBW power supply pulls a low current (usually less than 1 A), but provides a voltage as high as 60 kV in low-voltage machines, or 200 kV in high-voltage machines. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. High-voltage machines supply a current as low as 40 mA, and can provide a weld depth-to-width ratio of 25:1, whereas the ratio with a low-voltage machine is around 12:1. The beam power of a power supply is an indicator of its ability to do work, and determines the power density (generally 40-4000 kW/cm² or 100-10,000 kW/in²). In Engineering, the term specific power can refer to power either per unit of Mass, Volume or Area, although power per unit of The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. [2]
For the hard vacuum and soft vacuum EBW methods, the welding chamber used must be airtight and strong enough to prevent it from being crushed by atmospheric pressure. It must have openings so that the workpieces can be inserted and removed, and its size must be sufficient to hold the workpieces but not significantly larger, as larger chambers require more time to evacuate. The chamber must also be equipped with pumps capable of evacuating it to the desired pressure. For a hard vacuum, a diffusion pump is necessary, while soft vacuums can often be obtained by less costly equipment. Diffusion pumps use a high speed jet of vapor to direct gas Molecules in the pump throat down into the bottom of the pump and out the exhaust [2]
See also
References
- ^ Schultz, Helmut (1993). In electron beam technology an Electron beam is applied to do useful work Electron beam welding. Woodhead Publishing/The Welding Institute : Cambridge, England. ISBN 1-85573-050-2.
- ^ a b c d e f Cary, Howard B and Helzer, Scott C (2005). Modern Welding Technology. Pearson Education: Upper Saddle River, New Jersey, 202-206. New Jersey ( is a state in the Mid-Atlantic and Northeastern regions of the United States. ISBN 0-13-113029-3.
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