Choosing the Right Electron Beam Welding Equipment: A Comprehensive Guide

Composition of the Electron Beam Welding Machine

There are various structural designs for electron beam welding machines, but the basic principles remain generally the same. Figure 1-5 illustrates the typical composition of a vacuum electron beam welding machine.

As shown in the diagram, the main components of a vacuum electron beam welding machine are the electron beam gun, vacuum welding chamber, high voltage power supply and electrical control systems, vacuum systems, workbench, and auxiliary devices.

Figure 1-5 Vacuum Electron Beam Welding Machine Components

Electron Beam Gun

(1) Structure of the electron beam gun

The device in the electron beam welding machine that generates and focuses electrons into a beam is called the electron gun. It is a core component of the electron beam welding equipment.

The electron gun mainly consists of a cathode, an anode, a grid, and a focusing coil. Electron guns can be categorized into two types: diode and triode; modern electron beam welding machines mostly use triode guns. Figure 1-6 illustrates the structure of a triode electron gun, whose electrode system is composed of a cathode, a bias electrode, and an anode.

The cathode is at a high negative potential, forming an accelerating electric field for the electron beam with the grounded anode. The bias electrode is negatively charged relative to the cathode, and by adjusting its negative potential, as well as changing the shape and position of the bias electrode, the size of the electron beam current and the shape of the electron beam can be adjusted.

The focusing coil, also known as the electromagnetic lens, focuses the electron beam onto the seam of the workpiece, thereby increasing the working distance of electron beam welding and making it easier to control and adjust. The deflection coil aligns the spot of the electron beam with the joint of the workpiece to be welded or makes regular periodic motions in the weld seam area.

The direction and amount of deflection can be adjusted by changing the direction and size of the current in the deflection coil.

Figure 1-6 Schematic Diagram of the Structure of a Triode Electron Gun

1—Cathode
2—Bias Electrode
3—Anode
4—Focusing Coil
5—Deflection Coil
6—Workpiece
Ub—Acceleration Voltage
UB—Bias Voltage

(2) Cathode in the electron beam gun

The cathode in the electron gun should be made of materials that have strong thermionic emission capabilities and are not easily “poisoned”, such as tungsten, tantalum, and lanthanum hexaboride (LaB6).

Cathodes can be divided into directly heated and indirectly heated types according to their heating methods. The direct heating method heats the cathode directly, characterized by its simple structure, easy manufacturing, and relatively low cost. The downside is that the geometric shape of the emission surface is prone to deformation. Indirect heating uses conduction radiation or electron bombardment to heat the cathode.

The advantage is that the cathode surface is an equipotential surface, and the emission current density is relatively uniform. The disadvantage is that the manufacturing process is more complicated, and the cost is relatively high. Depending on the required beam current value, the cathode shape can be made into point emission or surface emission.

Generally, tungsten wire is commonly used to make needle-shaped or disc-shaped cathodes; tungsten tape is used to make V-shaped direct-heated cathodes; tungsten blocks or tungsten rods are used to make indirectly heated cathodes. Tantalum sheets are also commonly used as directly or indirectly heated cathodes; lanthanum hexaboride is generally made into indirectly heated cathodes, as shown in Figure 1-7.

Figure 1-7 Common Cathode Shapes

a) Direct-heated Tungsten Tape Cathode
b) Needle-shaped Tungsten Wire Cathode
c) Disc-shaped Tungsten Wire Cathode
d) Direct-heated Molybdenum Cathode
e) Indirect-heated Lanthanum Boride Cathode
f) Indirect-heated Tantalum Cathode
g) Indirect-heated Tungsten Rod Cathode

(3) Commonly used electron guns in electron beam welding machines

The Pierce electron gun is the most commonly used in the country, as shown in Figure 1-8. This type of electron gun has the advantage of high efficiency, with 99.9% of the beam current passing through the anode aperture. However, the electrode shape is complex, and the machining requirements are high, suitable for low and medium voltage situations below 70kV.

Pierce guns have diode and triode types, and their cathode emission surface and focusing pole are both spherical. The focusing pole and cathode of the diode gun are at the same potential, as shown in Figure 1-8a.

Figure 1-8 Schematic Diagram of the Pierce Gun

a) Diode Gun
b) Triode Gun

The focusing pole of the triode gun is replaced by a control grid, and a negative bias is added between the grid and the cathode, making the cathode work under space charge limitation, making the current density within the entire electron beam cross-section relatively uniform, as shown in Figure 1-8b. The triode gun adjusts the beam current by changing the negative bias of the control grid.

High Voltage Power Supply and Control System

(1) High Voltage Power Supply

The high voltage power supply provides the electron gun with acceleration voltage, control voltage, and filament heating current. The control principle of the high voltage power supply is shown in Figure 1-9. The power supply should be sealed in an oil tank to prevent harm to the human body and interference with other control parts of the equipment.

Pure transformer oil can serve as an insulating medium and a heat transfer medium, transferring heat from electrical components to the outer wall of the tank. All electrical components are mounted on a frame, which is fixed on the cover of the oil tank for easy maintenance and adjustment.

In recent years, semiconductor high-frequency high-power switch power supplies have been applied to electron beam welding machines. Their working frequency has greatly increased, and with a small filter capacitor, a very small ripple coefficient can be obtained. Such power supplies release very little electrical energy when discharging, reducing their potential harm.

Moreover, the on-off time of the switched power supply is much shorter than that of a contactor. When used in conjunction with a highly sensitive micro-discharge sensor, it provides a powerful means to suppress discharge. Such power supplies are small in size and light in weight, for example, the external dimensions of a 15kW high voltage oil tank are 1100mm×500mm×100mm, and it weighs only 600kg.

Figure 1-9 Schematic Diagram of High Voltage Power Supply Control Principle

(2) Control System

The control system of early electron beam welders was limited to controlling the decrement of the electron beam, the scanning of the electron beam, and the switching of the vacuum pump valve. Currently, programmable controllers and computer numerical control systems have been applied to electron beam welders, greatly improving the control scope and accuracy.

In addition to controlling the vacuum system and welding program of the welding machine, the computer numerical control system can also control the electronic parameters, the movement trajectory and speed of the worktable in real time, and achieve automatic tracking of the electron beam scanning and welding seam.

Vacuum System and Vacuum Chamber

(1) Vacuum System

The vacuum system is used to vacuum the electron gun and vacuum chamber. Figure 1-10 shows the composition of a vacuum system for a generic high vacuum electron beam welder.

Vacuum systems mostly use three types of vacuum pumps: one is a low vacuum pump, also known as a piston or vane mechanical pump, which can draw the electron gun and workspace from atmospheric pressure to about 10Pa; another is an oil diffusion pump, used to reduce the pressure of the electron gun and workspace to below 10-2Pa.

The oil diffusion pump cannot start directly under atmospheric pressure and must be combined with a low vacuum pump to form a high vacuum pumping unit. Another type is a turbo molecular pump, which is an extremely high-speed high vacuum pump. Unlike the oil diffusion pump, it does not need preheating and also avoids oil contamination.

It is often used in the vacuum system of the electron gun. The shape and depth of the weld seam obtained at different vacuum degrees are different. The current trend is to use turbo molecular pumps because their ultimate vacuum degree is higher, there is no oil vapor pollution, no need for preheating, and vacuuming time is saved.

The vacuum degree of the workspace is in the range of 10-1~10-3 Pa. Lower vacuum can be obtained with a mechanical pump, and high vacuum is achieved with a mechanical pump and diffusion pump system.

Figure 1-10 Vacuum System for Electron Beam Welding

1- Vacuum Chamber
2- Large Mechanical Pump
3- Small Mechanical Pump
4- Diffusion Pump
V1~V6– Vacuum Valves
S1~S5– Vacuum Gauges

(2) Vacuum Chamber

The design of the vacuum chamber (workspace) should meet the requirements for airtightness; on the other hand, it should meet the rigidity and strength indices required to withstand atmospheric pressure and the requirements for X-ray protection.

The vacuum chamber can be made of low carbon steel plate to shield the external magnetic field from interfering with the trajectory of the electron beam. The inner surface of the workspace should be nickel-plated or undergo other surface treatments to reduce surface adsorption gases, splashing and oil stains, etc., shorten the vacuuming time, and facilitate cleaning of the vacuum chamber.

The vacuum chamber usually opens one or several windows for observing the internal workpieces and welding conditions. Low-pressure electron beam welders (acceleration voltage less than 40kV) can prevent X-ray leakage by adjusting the thickness of the workspace steel plate and rationally designing the workspace structure.

The electron gun and workspace of medium and high-pressure electron beam welders (acceleration voltage greater than 60kV) must have a tight lead plate protective layer. The size and shape of the vacuum chamber should be determined according to the purpose of the welder and the size of the parts. The vacuum chamber volume of a general-purpose electron beam welder is large.

The specialized electron beam welder is designed according to the workpieces to be welded. Especially for high-efficiency welders, to reduce the vacuuming time, the vacuum chamber volume should be minimized as much as possible. Because the range of applications for electron beam welding is wide, the structure of the vacuum chamber of the electron beam welder varies greatly, as do the sizes.

To avoid problems such as X-ray leakage and vacuum chamber deformation, users of electron beam welders are not allowed to arbitrarily remodel the vacuum chamber.

Welding Worktable and Fixtures

The worktable, turntable, and welding fixtures play a crucial role in maintaining the position of the electron beam and the seam, the stability of the welding speed, and the repeat accuracy of the seam position during the welding process. Most electron beam welders use a fixed electron gun and move the workpiece in a linear or rotational manner to achieve welding.

For large vacuum chambers, the welding can also be done by keeping the workpiece stationary and moving the electron gun. To improve the productivity of electron beam welding, a dual worktable or multi-position fixture can be employed. When using a dual worktable, one worktable is used for welding within the vacuum chamber, while the other is used for loading and unloading workpieces outside the chamber.

The advantage of this worktable is that assembly and welding can proceed simultaneously, the volume of the vacuum chamber is small, and multiple parts can be welded after vacuuming once, resulting in high productivity. For large and medium-sized welders, the worktable can be moved out of the vacuum chamber for easy loading and unloading of workpieces.

The control of the worktable can be either manual or automatic. Modern welder worktables mostly use numerical control for complex seams and even space welding.

Electrical Control System

The electrical control system of the electron beam welder mainly completes the power supply of the electron gun, the programmatic opening and closing of the vacuum system valves, the constant speed movement of the transmission system, the closed-loop control of the welding parameters, and the program control of the welding process.

Selection of Electron Beam Welder

Electron beam welders can generally be classified according to vacuum status and acceleration voltage. According to the vacuum status, they can be divided into vacuum type, local vacuum type, and non-vacuum type. Based on the high and low acceleration voltage of the electron gun, they can be divided into high voltage type (60~150kV), medium voltage type (40~60kV), and low voltage type (<40kV).

When choosing an electron beam welder, factors such as the material to be welded, thickness, shape, and batch size of the product should be considered. Generally, high-vacuum welders should be chosen for welding chemically active metals (such as W, Ta, Mo, Nb, Ti, etc.) and their alloys. Low vacuum welders should be chosen for welding easily evaporating metals and their alloys.

High voltage welders are suitable for welding large workpieces, and medium voltage welders for medium thickness workpieces. Special welders should be chosen for batch production, and general welding equipment for a variety of small batches or single-piece production. Table 1-5 lists the technical parameters of some domestic electron beam welders.

Table 1-5 Technical Parameters of Some Domestic Electron Beam Welders

ModelEZ-60/100EZ-60/200EZ-150/75ES1-2
Power SupplyVoltage/V380380380380
Phase3333
Frequency/Hz50505050
Acceleration Voltage/kV20 ~ 600 ~ 600 ~15030
Electron Beam Current/mA1 ~ 1670 ~2000 ~ 750 ~ 30
Electron Beam Focus Diameter/mmΦ0. 5Φ1 ~Φ1. 5Φ0. 5 ~ Φ1
Workpiece Thickness (Stainless Steel)/mm203050
Weld Seam Depth-Width Ratio15:110:115:1
Welding SpeedLongitudinal/(m/min)0.2~1.20. 1 ~3
Rotation/(r/min)0.25 ~820 ~75
Vacuum Pumping Time/min20 ~ 3030
Chamber PressureHigh Vacuum/Pa1. 333 ×10-2High VacuumHigh Vacuum1. 3 × 10-7
Low Vacuum/Pa6. 666
Chamber VolumeLength/mm9008305220
Width/mm6006005210
Height/mm7005002840
Features and ApplicationsThis is a versatile model, capable of both high and low vacuum operations. It’s suitable for welding small to medium-sized parts made of stainless steel, aluminum, copper, and other hard-to-melt materials.Perfect for manual welding of active and hard-to-melt metals such as aluminum, titanium, molybdenum, tungsten, tantalum, zirconium, stainless steel, and high-strength steel in linear and circular weld seams.With a higher acceleration voltage than the EZ-60/200, it can weld thicker workpieces.A specialized welding machine for automatic circumferential seam welding of slender thin-walled tubes and end plugs. Features a three-stage electron gun and is monitored using industrial television.

The major manufacturers of electron beam welders in the world include Cambridge Vacuum Engineering (CVE) in the UK, Techmeta Electron Beam Welding Company in France, Pro-beam Electron Beam Welding Technology Co., Ltd. in Germany, and Paton Welding Research Institute in Ukraine, etc. Since the 1980s, China has made significant progress in the development of vacuum electron beam welders.

The main professional manufacturers in China include Shanghai Electric Welding Machine Factory, Chengdu Electric Welding Machine Research Institute, Shenyang Electric Welding Machine Factory, Guilin Shida, and Zhongke Electric, etc. Currently, medium power vacuum electron beam welders have formed a series.

Welders of 50kV and 60kV have been widely used in actual production, and some welding equipment has adopted microcomputer control. Figure 1-11 shows the exterior photo of the HDZ-10B type vacuum electron beam welder.

Figure 1-11 Exterior Photo of HDZ-10B Vacuum Electron Beam Welder

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