P LACE AND A PPLICATIONS OF E LECTRON B EAM W ELDING The basic advantage of power beam welding is the small heat input, which means

minimal and easily controlled bead-width, heat-affected-zone and weld-distortion. In addition, the range of combination of the joining materials is wide including those with high melting points and widely different physical properties. When selecting a process for a specific joining application, a number of questions such as joint preparation, cleaning, inert gas or vacuum shielding, depth of penetration, weld joint accessibility, productivity, and cost must be answered. Comparison of various aspects associated with electron, laser and plasma beams are listed in Table 3.

It is impossible to state with conviction, which welding process should be used for maximum efficiency in a given application. Both electron beam and laser are good choices for critical, heat sensitive weld joints and widely dissimilar materials. Electron beam is the indisputable candidate for penetration beyond 6 mm without preparation of the weld joint. For not very high volume welding (of order of thousand or tens thousands of small component assemblies, laser offers the best approach. It should be mentioned that the ability of the lasers to be transported to inaccessible areas using optical fibbers makes it particularly useful in hazardous work. For maximum flexibility, immediate use, lower critical joint tolerances, and low capital investment, plasma arc and gas tungsten arc are the dominant choices.

EB welding have benefits in mass production (hundred thousands pieces). Vacuum as shielding environment is 35 times cheaper (if not include capital costs) than pure gas shielding of molten pool. At welding of lightweight metals EB not need anti-reflex coatings.

High voltage EB (of order of 150 kV) can be brought out the vacuum chamber in air environment, but radiation protection of the operator is need. An intermediate evacuating by differential pumps space and Helium flow are used at such EB welding at atmospheric pressure.

Table 3. Comparison of Welding Processes

Parameter E-BEAM LASER PLASMA

Penetration Thickness[ mm] 0.5-200 0.5-50 0.1-10

Welding Speed Fast Fast Medium to Fast

Distortion V. Low Low Moderate

Power Density [W/m²] 10⁹-10¹² 10¹¹-10¹³ 10⁸-10¹⁰

Maximum Power [kW] 100 10 15

Equipment Size V. Large Small Medium

Cost Comparison 5 -10 10 1

Operational Constraints HV, X-Rays Optical Ultraviolet Difficult Locations V. Poor V. Good Fair

Figure 1 present a comparison between cross-sections of welds (at equal depth) obtained after 1) EB welding; 2) micro-plasma welding and 3) Ar arc welding. The heat input in the sample is proportional to area of the melt zone. So the distortions of welded sample and the need of position-fixing equipment for welded pieces is lowered or avoided.

In the last few decades, EB welding of the refractory metals and alloys, of heterogeneous metal junctions and of heavy engineering components were wide spread. The high joining rate, the deep and narrow weld (Figure 2 and Figure 3) and the minimal heat affected zone are basic advantages leading to the most often use of this process.

Figure 1. Cross-sections of various welds

Figure 2. EB weld with deep penetrating beam with power density 1011 W/m2; (165 kV, 320 mA , 3.5 mm/s)

Process Parameter Optimization and Quality Improvement at Electron Beam Welding 107

Figure 3. Metallographic photographs of the transverse cross-section of the EB welded junction of two plates with thickness of 78 mm. A deep and narrow molten zone and two heat affected zones are shown. The beam power is 15 kW, welding speed is 1 cm/s, the beam is focused 60 mm below the sample surface

The development of new high-intensity heat sources such as electron beams (EB) has facilitated welding of refractory metals and alloys, of heterogeneous metal junctions and of heavy engineering components. Electron beam welding (EBW) of materials has a number of decisive advantages over conventional techniques. The focused electron beam is one of the highest power density sources and that way high processing speed are possible, narrow welds with very narrow heat affected zone can be produced accurately. The weld cross-sections may have a "knife" shape. This is one the main advantages of the EBW method over the conventional methods of welding - the lower energy needed for the formation of a joint with equal width. The narrow heat affected zone allows the welding of materials and components near the weld zone that are not suitable for such processing. The crystal structure near the welded area is preserved unchanged, which on the other hand leads to preserving of the physical and mechanical properties of the welded materials. The thermal deformations are minimal, i.e. less are the cavities in the zone around the weld. The welded details may be thin or wide, and also can have different thermal conductivity. EBW is suitable for the welding of chemically active at high temperatures metals (Zr, Ta, Ti, Hf, Mo, W, Be, V etc.) and their alloys due to the fact, that the process is held in vacuum.

The Change of the weld and thermal affected zones at opening of the key-hole from the back side of work-piece is presented in Figure 4.

a) b)

Figure 4. Change of the weld and thermal affected zones at opening of the key-hole from the back side of work-piece

Figure 5. Some typical applications of EBW technology. a) Automatic CVT gear. Planetary and drive gear, welded with 4 EB-welds b) Aircraft - stator ring assembly with more than 300 EB-welds to join the vanes to the ring and the ring to flanges c) Industry - nozzle guide vanes for large turbines

Another characteristic of the welded seams is its hardness. In the area of the weld the hardness is usually higher than in the non-welded areas that can do it brittle. The crystal structure of not-melted metal is changed only in the narrow thermal affected zone.

Some applications of EBW technology are shown in Figure 5.