New developments in advanced welding

Basics of the process

7.2.1 Electron beam generation and guiding

Generation of the electron beam

Triode systems for beam generation are generally applied in modern electron welding machines as shown in Fig. 7.2. These systems are composed of anode, cathode and the control electrode (Wehnelt cylinder). The electrons that are necessary for beam generation are emitted from the cathode by thermionic emission. The cathode is made from a material such that the work that must be performed by the electron to leave is comparatively low. The cathode material must show a high electron emission rate, be resistant to high temperatures and guarantee a relatively long cathode life. Appropriate materials are tungsten and tantalum. The heating of the cathodes may be carried out directly or indirectly. Indirectly heated cathodes are heated by electron bombardment from an auxiliary cathode; current passes through those cathodes that are directly heated and therefore they are heated by Joule resistance. A high voltage electric field is applied which supplies the electrons with kinetic energy in order to emit them from the electron cloud and subsequently accelerate them. Depending on the strength of the applied voltage, the electrons may be accelerated up to two-thirds of the speed of light. An acceleration voltage generates an electric field between cathode and anode, situated directly opposite each other, (Schiller et al., 1977). By the application of a control voltage between the cathode and a control electrode, (Fig. 7.2) a barrier field is generated in the triode system that forces the emitted electrons back to the cathode. Thus the beam current is controlled by alterations of the control voltage because through its decrease more electrons pass the barrier field towards the anode. Owing to its particular shape, similar to a concave mirror, the control electrode affects the electrostatic focusing of

the electron beam. After passing the anode the electrons have achieved their final speed and the electron beam is focused and deflected by means of electromagnetic focusing lenses. The focusing effect leads to the constriction of the electron beam, the so-called cross-over.

Beam manipulation

The electron beam diverges slightly after passing the pierced anode and is then focused to a spot diameter of between 0.1 and 1.0 mm by a beam manipulation system to reach the necessary power density of 106 to 107W/ cm2. The beam is first guided through the alignment coil onto the optical axis of the focusing objectives. One or several electromagnetic lenses direct the beam onto the workpiece inside the vacuum chamber. Deflection coils that are positioned at various parts of the electron beam generator assist in the deflection or oscillating motions of the electron beam. A diagrammatic representation of an electron beam welding machine is depicted in Fig. 7.3.

7.2.2 Deep penetration effect

When the electrons strike the surface of the workpiece their kinetic energy is converted into thermal energy. Although the electron mass is very low (approximately 9.1 x 10-28g) electrons have a high electric voltage potential which, at an accelerating voltage of 150kV, allows electron acceleration up to a speed of approximately 2 x 108m/s. Not all beam electrons penetrate the workpiece and release their energy to the material. Some of the striking electrons are emitted in other forms: back-scattered electrons, thermal radiation, secondary electrons or X-ray radiation as shown in Fig. 7.4.

Because of their low mass, the electrons that penetrate the material do so to only very shallow depths (of up to 150 mm), another process is needed in order to obtain large weld depths, the so-called deep-penetration effect. The material is melted and vaporised in the centre of the beam and this happens so quickly that the heat dissipation into the cold material has almost no effect. The resulting vapour is superheated to temperatures of above approximately 2700 K. The vapour pressure is sufficiently high to press the molten metal upwards and to the sides. A cavity develops where the electron contacts the yet unvaporised metal and heats this further. This leads to a vapour cavity which in its core consists of superheated vapour and is surrounded by a shell of fluid metal. This effect is maintained as long as the pressure from the developing vapour cavity and the surface tension of the molten pool are in equilibrium. The diameter of the vapour cavity corresponds approximately with the electron beam diameter. With a sufficiently high energy supply, the developing cavity penetrates through the entire workpiece (Schultz, 2000). The relative motion between workpiece and electron beam causes the material

which has been molten at the front of the electron beam to flow around the cavity and to solidify at the rear. The formation of the vapour cavity is depicted in Fig. 7.5 (Schiller et al., 1977; Schultz, 2000).

The pressure and temperature conditions inside the cavity are subject to dynamic changes over time. Under the influence of the constantly changing geometry of the vapour cavity, welding faults such as shrinkage cavities may occur when the welding parameters have been chosen unsuitably. It is possible to avoid these faults by a suitable choice of welding parameters and, in particular, by the selection of suitable oscillation characteristics; examples are circular, sine, rectangular and triangular functions.

7.4 Fate of the electrons on meeting the workpiece.

204 New developments in advanced welding

7.2.3 Machine components

The electron beam welding machine consists of a great many individual components. The basic component of the machine is the electron beam generator where the electron beam is generated in high vacuum, influenced by electromagnetic deflection coils and then focused onto the workpiece in the vacuum chamber (see Section 7.2.1). An electron beam in air diverges strongly through collision with air molecules and thus loses power, so welding is generally carried out in a low or high vacuum inside a vacuum chamber. Different vacuum pumps are used to generate a vacuum in the beam generator and in the working chamber. In the beam generator a high vacuum (p < 10-5mbar) is necessary both for insulation and for oxidation circumvention of the cathode but possible working pressures in the vacuum chamber vary between high vacuum (p < 104mbar) and atmospheric pressure. Collision of the electrons with any residual gas molecules and consequent scattering of the electron beam is obviously lowest in a high vacuum. The beam diameter of the focused electron beam is at a minimum in high vacuum and therefore the power density in the beam is at its maximum.

A shut-off valve positioned between the electron beam generator and the working chamber allows the presence of a vacuum in the beam generator area even when the working chamber is at atmospheric pressure. Other necessary features are a high voltage supply, controls for this supply, vacuum pumps, the numerical control (NC) of the work table and operator areas. The equipment is controlled at the operator console where all relevant process parameters are set and monitored. In modern equipment, parameter selection and control may be carried out externally by a computer and corresponding software. For the determination of optimum welding parameters for process control and also for the adjustment of the electron beam on the workpiece, viewing optic systems are necessary. Simple light-optical viewing optics with a telescope or a camera system and monitor which partially represent the magnified section, or electron-optical systems are used. In these systems the electron beam scans the workpiece surface with a very low power without melting it. The back-scattered secondary electrons show, as in scanning electron microscopy (SEM), an image of the workpiece surface. Figure 7.6 shows the electron beam welding machine and peripheral equipment.

Electron beam welding machines may be classified according to the quality of the vacuum, the machine concept and the height of the maximum acceleration voltage. The acceleration voltage exerts substantial influence on the achievable welding results - the higher the acceleration voltage, the lower the beam focus diameter of the focused beam at an equal beam power. Therefore, with a high acceleration voltage, the maximum achievable welding depth increases as does the ratio between depth and width of the beam geometry. However, a disadvantage of increasing acceleration voltage is the exponentially increasing

X-ray radiation as well as increased sensitivity to flash-over voltages. In production systems a distinction is made between high-voltage machines with acceleration voltages of between 120kV and 180kV and low-voltage machines with acceleration voltages of maximum 60kV. Beam powers of up to 200 kW are used.

7.2.4 Potential of fast beam controls

The electron beam is a welding tool with virtually no mass, which is deflectable, non-contacting and almost inertia-free. It is therefore possible to oscillate the beam with extremely high frequency and by applying a control voltage the beam may be switched off between the individual oscillations. With this technique the electron beam skips between several positions with a frequency so high that the metallurgical influence on the structure is carried out at different points simultaneously, due to the thermal inertia of the structure. Through recent developments in the field of beam deflection it is now possible to vary the focus position and the beam power between the individual oscillations and the beam can be controlled in such way that up to five electron beams simultaneously process the material as shown in Fig. 7.7. This technique offers considerable potential for many applications.

This technique can be most easily applied by forcing the beam to skip between two or more positions, thus producing, at a simultaneous movement

7.7 Multi-beam technique.

of the workpiece, two or more welds. The technique has been used for several years to join saw bands for band circular saws in conveyor units. These saw bands consist of a ductile backing layer in the middle of two hardened boundary layers, i. e. two parallel welds are necessary. A further interesting field of application for multi-beam technology is the welding of axis-parallel, rotationally symmetrical bodies. As the material melts simultaneously at several points of the axis-symmetrical weld and solidifies subsequently, the shrinkage stresses also occur simultaneously and symmetrically thus avoiding disalignment of the axes. This means that the often costly and labour-consuming press fits for centring and avoiding the disalignment of axes may be dispensed with. Another application of this fast beam deflection is the joining of material combinations. The multi-beam technique allows, by varying holding times at different points, the supply of one of the joining members at the welding point with a significantly higher energy than is supplied to the second member. For example, one joining member may be molten while the other one is simply heated (diffusion welding). In this way, it is possible to join materials that do not show complete solid solubility. Without the multi-beam technique, there is only a narrow range where the beam impact point opposite the joint groove can be used to apply different energy levels to the joining members. Because this variation demands extremely precise positioning of the beam it is difficult to reproduce.