electron beam welding abstract

8/2/2019 Electron Beam Welding Abstract

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Synopsis of Electron beam

weldingGeneral Arrangement :-

Electron beam welding (EBW) is a welding process which produces coalescence of metals

with the heat obtained from a concentrated beam composed primarily of high-velocity

electrons impinging upon the surfaces to be joined. Heat is generated in the work piece as it is

bombarded by a dense stream of high-velocity electrons. Virtually all of the kinetic energy-

the energy of motion-of the electrons is transformed into heat upon impact.

The electron beam welding process had its inception in the 1950s in the nuclear field. There

were many requirements to weld refractory and reactive metals. These metals, because of

their affinity for oxygen and nitrogen of the air, are very difficult to weld.

The original work was done in a high vacuum. The process utilized an electron gun similar to

that used in an X-ray tube. In an X-ray tube the beam of electrons is focused on a target of

either tungsten or molybdenum which gives off X-rays. The target becomes extremely hot

and must be water-cooled. In welding, the target is the base metal which absorbs the heat to

bring it to the molten stage. In electron beam welding, X-rays may be produced if the

electrical potential is sufficiently high.


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As developments continued, two basic designs evolved: (1) the low-voltage electron beam

system, which uses accelerating voltages in 30,000 volts or (30 kV) to 60,000-volt (60 kV)

range and (2) the high-voltage system with accelerating voltages in the 100,000- volt (100

kV) range. The higher voltage system emits more X-rays than the lower voltage system.

In both systems, the electron gun and the work piece are housed in a vacuum chamber. Thereare three basic components in an electron beam-welding machine. These are (1) the electron

beam gun, (2) the power supply with controls, and (3) a vacuum work chamber with work-

handling equipment. The electron beam gun emits electrons, accelerates the beam of

electrons, and focuses it on the work piece.

Recent advances in equipment allow the work chamber to operate at a medium vacuum or

pressure. In this system, the vacuum in the work chamber is not as high. It is sometimes

called a "soft" vacuum. This vacuum range allowed the same contamination that would be

obtained in atmosphere of 99.995% argon. Mechanical pumps can produce vacuums to the

medium pressure level.

One of the major advantages of electron beam welding is its tremendous penetration. This

occurs when the highly accelerated electron hits the base metal. It will penetrate slightly

below the surface and at that point release the bulk of its kinetic energy which turns to heat

energy. The addition of the heat brings about a substantial temperature increase at the point of

impact. The succession of electrons striking the same place causes melting and then

evaporation of the base metal. This creates metal vapors but the electron beam travels

through the vapor much easier than solid metal. This causes the beam to penetrate deeper into

the base metal. The width of the penetration pattern is extremely narrow. The depth-to-width

can exceed a ratio of 20 to 1. As the power density is increased penetration is increased.

The heat input of electron beam welding is controlled by four variables: (1) the number of

electrons per second hitting the work piece or beam current, (2) the electron speed at the

moment of impact, the accelerating potential, (3) the diameter of the beam at or within the

work-piece, the beam spot size, and (4) the speed of travel or the welding speed. The first two

variables, beam current and accelerating potential, are used in establishing welding

parameters. The third factor, the beam spot size, is related to the focus of the beam, and the

fourth factor is also part of the procedure.

Since the electron beam has tremendous penetrating characteristics, with the lower heat input,

the heat-affected zone is much smaller than that of any arc welding process. In addition,

because of the almost parallel sides of the weld nugget, distortion is greatly minimized. Thecooling rate is much higher and for many metals this is advantageous; however, for high-

carbon steel this is a disadvantage and cracking may occur.

The weld joint details for electron beam welding must be selected with care. In high vacuum

chamber welding special techniques must be used to properly align the electron beam with

the joint. Welds are extremely narrow and therefore preparation for welding must be

extremely accurate.

Filler metal is not used in electron beam welding; however, when welding mild steel highly

deoxidized filler metal is sometimes used. This helps deoxidize the molten metal and produce

dense welds.


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Almost all metals can be welded with the electron beam welding process. The metals that are

most often welded are the super alloys, the refractory metals, the reactive metals, and the

stainless steels. Many combinations of dissimilar metals can also be welded.

One of the disadvantages of the electron beam process is its high capital cost. The price of the

equipment is very high and it is expensive to operate due to the need for vacuum pumps. Inaddition, fit up must be precise and locating the parts with respect to the beam must be

perfect.

electron beam welding abstract

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