Introduction to Basic Manufacturing Processes and ... - always yours

336 Introduction to Basic Manufacturing Processes and Workshop Technology
8Al + 3Fe O = 4Al O + 9Fe
3 4 2 3
1
2 3
8
4
7
6
5
7
1. Crucible,
2. Slag basin,
3. Runner,
4. Wax pattern,
5. Sand plug,
6. Preheating,
7. Workpiece,
8. Riser.
Fig. 17.29 Thermit welding process
17.10.1 Electron Beam Weldinig (EBW)
In EBW process, the heat is generated when the electron beam impinges on work piece. As
the high velocity electron beam strikes the surfaces to be welded, their kinetic energy
changes to thermal energy and hence causes the workpiece metal to melt and fuse.
A schematic setup of the electron beam welding is shown in Fig. 17.30. This process
employs an electron gun in which the cathode in form of hot filament of tungsten or tantalum
is the source of a stream of electrons. The electrons emitted from filament by thermionic
emission are accelerated to a high velocity to the anode because of the large potential
difference that exists between them. The potential differences that are used are of the order
of 30 kV to 175 kV. The higher the potential difference, higher would be the acceleration. The
current levels are low ranging between 50 mA to 1000 mA. The electron beam is focused by
a magnetic lens system on the workpieces to be welded. The depth of penetration of the weld
depends on the electron speed which in turn is dependent upon the accelerating voltage.
When the high velocity electron beam strikes the work-piece all the kinetic energy is converted
to heat. As these electrons penetrate the metal, the material that is directly in the path is
melted which when solidifies form the joint. Electron beam welding has several advantages
which may not be found in other welding processes. The penetration of the beam is high. The
depth to width ratios lies between 10:1 to 30:1 can be easily realized with electron beam
welding. It is also possible to closely control this penetration by controlling the accelerating
voltage, beam current, and beam focus. The process can be used at higher welding speeds
typically between 125 and 200 mm/sec. No filler metal or flux needs to be used in this process.
The heat liberated is low and also is in a narrow zone, thus the heat affected zone is minimal
as well as weld distortions are virtually eliminated. It is possible to carry out the electron
beam welding in open atmosphere. For welding in vacuum, the work-piece is enclosed in a
box in which the vacuum is created. When electron beam moves in the normal atmosphere,
the electrons would be impinging with the gas molecules in the atmosphere and would thus