New developments in advanced welding

Non-vacuum electron beam welding

In the early stages of electron beam technology development, research was carried out in Germany on methods to guide the beam from the vacuum environment of the beam generator to the atmosphere. This became the basis of the non-vacuum electron beam welding (NV-EBW) process. The substantial weld depths which characterise vacuum electron beam welding (as a result of the power density of the beam) are not achievable with the NV-EBW method. The strong point of NV-EBW lies mainly in high-speed production. Achievable welding speeds reach up to 60 m/min when welding aluminium sheets and up to 25m/min when welding steel plates.

Industrial researchers in the United States recognised the potential of NV - EBW early and advanced it to a further universally applicable joining method. For better energy coupling to the workpiece, beam generators with an accelerating voltage of 175 kV are used (Dilthey and Behr, 2000; Draugelates et al., 2000). While the NV-EBW technology was taken up immediately in the United States and applied successfully, the method attracted less attention in other countries. The car industry has recently, due to the high efficiency of the electron beam and also to the high achievable welding speed, become interested in this method.

7.5.1 Technology

Vacuum-related restrictions can be overcome by guiding the vacuum-generated electron beam that has exited the beam generator to the atmosphere, over a multi-stage orifice assembly and nozzle system. The pressure chambers have a correspondingly high pressure (10-2 up to 1mbar). They are connected after the beam generator chamber (vacuum 10-4mbar), are evacuated separately and are separated from each other by pressure nozzles. The electron beam is focused on the exit nozzle which has an inner diameter of 1-2mm. After its exit to the atmosphere the electron beam collides with the air molecules and expands, Fig. 7.15 (Schultz, 2000; Behr, 2003).

The scattering of the electron beam is reduced by increasing the accelerating voltage and by the application of helium as a working gas. For the effective utilisation of the helium gas flow, a coaxial gas jet is applied at the beam exit outlet. The effect of the electron scattering at the gas molecules is additionally attenuated by the strong heating of the gas in the electron path. This reduces

gas density and decreases scattering (Dilthey and Behr, 2000; Draugelates et al., 2000; Schultz, 2000; Behr, 2003).

As in laser beam welding, plasma formation occurs in NV-EBW. However, in NV-EBW the corpuscular character of the beam causes the plasma to be ‘transparent’ to the electron beam. For this reason the reflection properties of the material to be processed do not play a role during beam coupling (Behr, 2003).

After their exit from the orifice assembly, the electrons of the focused beam impinge on the material surface with a high speed and transmit their kinetic energy to the material lattice. This causes an increase in the kinetic energy of the lattice atoms and, when the power density in the focal spot is sufficiently high, the material temperature rises and even exceeds the boiling point of the material to be welded. However, not all beam electrons participate in the conversion of kinetic energy to thermal energy. The collision of the accelerated electrons with the mass particles of the air and of the joining materials causes, depending on the acceleration voltage and the density of the material, X-ray radiation. This radiation must be shielded. In addition, the ionisation of the air causes the generation of ozone, which must be neutralised. A radiation-proof working chamber should be designed with suitable materials and dimensions. Limits to the component size are, therefore, not much higher in NV-EBW than they are in Nd:YAG laser beam welding.

During welding, the power density may be varied either via the beam current or via the working distance and/or the gas atmosphere. However, the power density is not the only decisive factor for the welding result: for each material, several parameters must be taken into account, alone or in combination. Such parameters include working distance, welding speed, beam current, electron beam incident angle, possible gas supply and possible wire supply. For a successful application of NV-EBW it is necessary to know what parameters affect the process and how their interactions influence it (Fig. 7.16). Only then can the potential user consider the limits and possibilities of any given method for a particular joining task.

NV-EBW can allow significant increases in productivity. In the course of several years’ research and development acitivities in the field of NV-EBW it has been shown several times over that the characteristic properties of this welding method are hardly known in industry. This lack of awareness complicates the introduction of NV-EBW (where the disadvantages are known) as it does for many manufacturing processes. The development of X-ray radiation can initially limit the introduction of a new welding method. This criterion seems to characterise NV-EBW as a particularly hazardous process. However, there are strict radiation protection guidelines and when these are followed electron beam equipment will be radiation tight and fitted with several safety measures. Accidents involving radioactive contamination from the NV-EBW machines have not been heard of up to the time of writing.

7.16 Process parameters in NV-EBW.

7.5.2 Applications of NV-EBW

A high-power and out-of-vacuum electron beam is the ideal tool for welding conventionally manufactured sheets and sheet metal parts. The upper bead of the weld is similar to that of an arc weld and thus cannot be compared with the typically narrow deep geometry of vacuum electron-beam welded joints. The method is characterised by high energy efficiency; its high available beam power yields a high power density even when the beam is expanded and allows high welding speeds (Fig. 7.17).

The application of NV-EBW is particularly recommend when high weld speeds and short cycle times with smaller weld depths are required at the same time. The main application field is thin sheet welding (thicknesses from 0.5 mm up to 10 mm).

A further field of NV-EBW is the welding of tailored blanks which is today applied extensively in the car industry and in terotechnology. The made-to-measure plates are produced through joining different plates with varying thickness, qualities and surface coatings to suit their future loads during application. The plates thus produced may afterwards be formed into a component and then welded to a shell. Manufacturing of tailored blanks shows the advantages of NV-EBW: cost saving through the reduction of parts and tools and through a substantial reduction of materials and assembly time. The component and/or the car as the final product may therefore be reduced in weight and its fuel consumption will be lower.

A classical application field of NV-EBW is the welding of components

7.17Working range of NV-EBW with an acceleration voltage of 150kV.

where several plates which form a flange weld are joined, Fig. 7.18. Non­vacuum electron beam welding is very suitable for this application. The broad beam fuses several plate edges simultaneously which leads to a gas - tight and even joint. The flange weld and the lap joint are particularly suitable for components where, after a very rough weld preparation, the desired result is a gas - and liquid-tight weld.

The materials that have been tested up to now with the NV-EBW method are uncoated and coated steels, light metals such as aluminium and magnesium and non-ferrous metals, such as brass or copper. Material combinations, like for instance, that of steel and copper may also be realised with results comparable to vacuum EBW, without, however, achieving the higher weld depths of vacuum EBW. Supplementary application tests on the use of filler wire in NV-EBW have been carried out.

7.5.3 Summary

Non-vacuum electron beam welding is an efficient, reliable and well-known tool for material processing in welding. NV-EBW is, compared with other fusion welding methods, characterised by a lower thermal load on the point of effect and by high process speeds. Non-vacuum electron beam welding has the advantage that the process is not dependent on a vacuum chamber. The application of the NV-EBW method is recommended whenever high welding speeds and short cycle times with smaller weld depths are required

at the same time. Non-vacuum electron beam welding directly competes with laser beam welding with a beam power of up to 20kW. With an equipment efficiency of approximately 60 % the non-vacuum electron beam is clearly the more efficient tool.

As a joining technique, NV-EBW is gaining in importance and NV-EBW may, in future, provide significant competitive advantages through its specific properties.