Electron beam welding
2007 Schools Wikipedia Selection. Related subjects: Engineering
Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons are applied to the materials being joined. 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. Pressure is not applied, and a shielding gas is not used, though the welding is often done in conditions of a vacuum to prevent dispersion of the electron beam. The process was developed in France and released on November 23, 1957 in Paris by J. A. Stohr.
As the electrons strike the workpiece, their energy is converted into heat, instantly vaporizing the metal under temperatures near 25,000 °C. 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. As a result, the effect of welding on the surrounding material is minimal, and the heat-affected zone is small. Distortion is slight, and the workpiece cools rapidly, and while normally an advantage, this can lead to cracking in high-carbon steel. Almost all metals can be welded by the process, but the most commonly welded are stainless steels, superalloys, and reactive and refractory metals. 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.
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.
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. As a result, the chamber must be small to prevent it from being crushed under atmospheric pressure. 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 0.7 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. 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.
The electron beam gun used in EBW both produces the electrons and accelerates them, using an emitter made of tungsten that emits electrons when heated. The electrons are then attracted to an anode inside the tool, where they collect and are then directed 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 0.00001 torr) is maintained.
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. 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).
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.