Electron beam welding of thin-walled parts

Crosssection of an membrane bellows welded by the electron beam

When some devices (e.g. vacuum or cryogenics) are manufactured, the need to weld thin-walled parts is often met. Application of electron beam for this purpose may bring, compared with micro-plasma or laser) some valuable advantages balancing its higher costs. But good results could only be achieved if certain conditions, which will be explained in the following article, are fulfilled.

Welding of membranes

Fig. 1: Welding membrane M to a ring K.Fig. 1: Welding membrane M to a ring K. Fig. 2: Miniature membrane welded between two rings.Fig. 2: Miniature membrane welded between two rings. Fig. 3: Welding membrane boxesFig. 3: Welding membrane boxes

Rather often, thin membranes are to be welded to massive parts or among themselves. Their profile, which may be important for their proper function, from the point of view of welding is not significant. Only flat membranes should be avoided, as the weld contractions can cause „crack effect“, in most cases inadmissible.

Fig. 1 shows how to weld a membrane M to a massive ring K. The perimeter of the membrane is pressed down to the ring between two flanges F1, F2. The electron beam penetrates through the membrane into the massive ring, producing a vacuum tight joint.

The membrane can also be welded between two metal rings, as shown in Fir. 2.

Fig. 3 shows how membranes could be welded to form a closed box. For this purpose the membranes are pressed together between rings, made preferably of copper, leaving only a narrow fringe on the outer circumference free for welding.

Fig. 4, showing metallographic sample of two stainless steel membranes, proves that optimal welds can be made by electron beam.

Membrane bellows

Fig. 4: Electron beam welded membranes.Fig. 4: Electron beam welded membranes. Fig. 5: Welded membrane  bellowsFig. 5: Welded membrane bellows Fig. 6: Welded bellows made of 400 membranes.Fig. 6: Welded bellows made of 400 membranes.

By welding annular membranes alternately on inner and outer edge, as shown in Fig.5, „welded membrane“ bellows can be manufactured. Compared with bellows made by pressing, the welded bellows are rather expensive, but they are unreplaceable where high travel or flexibility is needed (see Fig. 6).

As the first step to make a bellows, couples of annular membranes are welded together on inner circumference using some appropriate welding aid, e.g. such as shown in Fig. 7. All these couples are tested to be vacuum tight before they are welded together on their outer circumference to form the bellows. In this way even rather long, very flexible bellows can be manufactured. Fig. 6 shows a bellows made by welding 400 membranes of 30/16 mm diameter. Fig. 8. shows how a membrane M can be welded to a flange F. Examples of membrane bellows welded in ISI are demonstrated by photos on Fig. 9 and 10.

Fig. 7: Membrane welding aid.Fig. 7: Membrane welding aid. Fig. 8: Welding  membrane to flangeFig. 8: Welding membrane to flange Fig. 9: Welded membrane bellows: diameter  10, 13 a 16 mm.Fig. 9: Welded membrane bellows: diameter 10, 13 a 16 mm. Fig. 10: Various bellows welded in ISI Brno.Fig. 10: Various bellows welded in ISI Brno.

Manufacture of thin-walled tubes

Thin-walled tubes made of stainless steel with low thermal conductivity are often needed in high vacuum or cryogenics constructions. If needed in great lengths and quantities, they are to be, of course, purchased from one of many industrial producers. If only a few, short tubes will do, it may be favourable to manufacture them from a metal sheet bended to a full circle and welded. If this procedure should be successful, the sheet of appropriate length must be perfectly sharp cut and an appropriate welding aid must be used. The essential parts of such an aid are shown in Fig. 11. A tube of 90 mm diameter made from 0,20 thick sheet by welding is shown in Fig. 12. 

Fig. 11: Aid for welding thin-walled tubes.Fig. 11: Aid for welding thin-walled tubes. Fig. 12: Tube manufactured from a thin sheet by welding.Fig. 12: Tube manufactured from a thin sheet by welding. Fig. 13: Joining thin-walled tubes.Fig. 13: Joining thin-walled tubes. Fig. 14: Joining tubes 125 mm in diameter, 0,25 mm wall thickness.Fig. 14: Joining tubes 125 mm in diameter, 0,25 mm wall thickness.

In our welding practice we have met the demand to join tubes 125 mm in diameter with 0,25 mm thick wall. We have prepared short tubes of this dimensions, using the above described procedure and applied electron beam to weld the tubes joined in three different ways, as shown in Fig. 13. For butt joint an aid shown in Fig. 13a) was inevitable. For joint shown in Fig. 13b) no welding aid is needed, but the beam with appropriate power must be well focused, and the welding speed must not be low. The third type shown in Fig. 13c) also was successfully welded with only a simple welding aid, as shown in the drawing. Welding of all three types was successful, as can be seen in Fig. 14. 

Welding thin-walled tubes to massive parts

Special precautions must also be taken if a thin-walled tube should be welded to a massive component. No welding aids are usually necessary if proper preparation and welding procedure are applied. The tube wall must be in good contact with the other part, particularly if it is on the outside. Slightly conical shape of the joint helps to make the contact tight as needed. When the joint is designed as shown in Fig. 15, the weld can be realized without directly heating the thin tube. If for some reason the weld must be realised by electron beam penetrating through the thin wall, (Fig. 16b and Fig. 17) the beam must be well focused and the welding speed must not be low. When the thin tube is situated inside the massive part, as shown in Fig. 18, the weld could be made at the edge of the tube (EB1) or through the wall (EB2).

Fig. 15: Types of thin-walled tube joints.Fig. 15: Types of thin-walled tube joints. Fig. 16: Welding through the wall.Fig. 16: Welding through the wall. Fig. 17: Welding pressed bellows to massive parts.Fig. 17: Welding pressed bellows to massive parts. Fig. 18: Thin tube insideFig. 18: Thin tube inside 

Disc rotors of servomotors

Challenging, and so interesting, is the welding of flat copper conductors of disc rotors for servomotors. Such rotor is made of four layers of flat conductors, mechanically joined and electrically insulated by a thin layer of glass-laminate. To make the proper winding of the motor, the tips of conductors in separate layers are to be electrically perfectly connected, which is possible only by welding. Electron beam is in this case the best tool for such purpose. An appropriate welding aid is of course inevitable. We have used copper discs about 5 mm thick, provided with holes of 1 mm in diameter, exposing the conductor tips to the electron beam. The welding was realized by rotation the workpiece in front of the gun, with uninterrupted, adequately deflected beam. The quality of the welds is documented by the detailed photo of a part of the commutator in Fig. 21.

Fig. 19: Disc rotor of servomotor: conductor thickness 0,2 mm, insulation 0,1 mm, diameter 28/115 mmFig. 19: Disc rotor of servomotor: conductor thickness 0,2 mm, insulation 0,1 mm, diameter 28/115 mm Fig. 20: Welding aid.Fig. 20: Welding aid. Fig. 21: Detail of commutator