The Principle and Characteristics of Electron Beam Welding

Electron Beam Welding (EBW) is a welding method that uses the thermal energy generated by a concentrated high-speed electron flow striking the welding seam to fuse the welded metal. It operates in both vacuum and non-vacuum environments.

EBW has been in industrial use for over 60 years, its inception and initial application mainly catering to the welding requirements of the nuclear and aerospace industries.

Today, EBW applications have expanded to aviation, space exploration, shipbuilding, automotive, electric motors, electronics, machinery, medical devices, petrochemicals, energy, and other fields. Over the decades, EBW has generated substantial economic and social benefits.

Principles of Electron Beam Welding

EBW is a high-energy beam welding method. An electron beam of a certain power, once focused by an electron lens, has a current range of 20-1000mA, a focal diameter of Φ0.1-Φ1mm, and power density reaching over 106W/cm2. This power density is 100-1000 times higher than that of a regular arc.

The electron beam is generated by an electron gun. The cathode in the electron gun emits electrons outward through thermionic emission or field emission. Under the influence of an acceleration voltage of 30-150kV, the speed of the electrons accelerates to 30%-70% of the speed of light, possessing high kinetic energy.

These high-speed electrons converge into a high-density electron beam through the electrostatic lens and magnetic lens in the electron gun. Figure 1-1 illustrates the principle of electron beam generation.

Principle Diagram of Electron Beam Generation
Figure 1-1: Principle Diagram of Electron Beam Generation

When the electron beam strikes the surface of the weldment, kinetic energy transforms into thermal energy, causing the metal at the joint to rapidly melt and evaporate. Under the action of high-pressure metal vapor, the melted metal is displaced, allowing the electron beam to continue striking the solid metal deep within the weldment. This process quickly “drills” a keyhole in the weldment, as shown in Figure 1-2, with the hole surrounded by liquid metal.

Figure 1-2: Principles of Weld Seam Formation in Electron Beam Welding

a) Local melting and vaporization of the joint
b) Metal vapor displaces liquid metal, allowing the electron beam to “drill” into the base material, forming a keyhole
c) The electron beam penetrates the workpiece, with the keyhole surrounded by liquid metal
d) A weld seam forms behind the electron beam

As the electron beam moves relative to the weldment, the liquid metal flows toward the rear of the molten pool around the hole, gradually cooling and solidifying to form a weld seam. During EBW, a keyhole always exists in the welding molten pool.

The presence of the keyhole fundamentally changes the heat conduction pattern of the welding molten pool, turning EBW from a “conduction weld” of the general molten welding method into a “penetration weld”. This is a common feature of high-energy beam welding, including laser welding and plasma arc welding.

In high-powered EBW, the power density of the electron beam can reach 106-108W/cm2, sufficient for strong penetration effects and a significant depth-to-width ratio. When the power density of the electron beam is less than 105W/cm2, its penetrating ability is relatively small, and the metal melting process is similar to arc welding, resulting in shallower weld penetration.

The welding quality of EBW is closely related to beam intensity, acceleration voltage, welding speed, electron beam spot quality, and the thermophysical properties of the welded material.

Features of Electron Beam Welding

The electron has a very small mass, only 9.1×10-31kg, and its charge-to-mass ratio is as high as 1.76×10-11C/kg. Both electric and magnetic fields can quickly and accurately control the electrons. Therefore, as a welding heat source, the electron beam can be precisely controlled and react quickly in addition to having high energy density.

In this regard, EBW significantly outperforms laser beam welding, which can only be controlled by lenses and mirrors and reacts slowly. Table 1-1 summarizes the features of EBW compared to other traditional welding methods.

Table 1-1: Features of Electron Beam Welding

Strong penetration ability of the electron beam, resulting in a large depth-to-width ratio of the weld seamThe small size and high power density of the electron beam can achieve deep and narrow welding with a high depth-to-width ratio (up to 60:1). It can weld through stainless steel plates with thicknesses ranging from 0.1mm to 300mm in one go. Compared to conventional arc welding, it saves a significant amount of filler material and reduces energy consumption.
Rapid welding speed, yielding high-quality weld seam structureThe concentrated energy leads to rapid melting and solidification processes. For example, when welding a 125mm thick aluminum plate, the welding speed can reach 40cm/min, which is 40 times faster than argon arc welding. The short high-temperature action time reduces alloy element burn-off, prevents grain enlargement, and improves joint performance and weld seam corrosion resistance.
Minimal thermal deformation of the weldmentThe high power density results in less heat input to the weldment, minimizing deformation. Post-welding, the workpiece can still maintain a high degree of dimensional accuracy, making it suitable for the final connection process of precision machined parts.
High purity of the weld seam, resulting in excellent joint qualityVacuum electron beam welding not only prevents molten metal from being contaminated by harmful gases such as hydrogen, oxygen, and nitrogen, but it also aids in degassing and purifying the weld seam metal. Therefore, it is particularly suitable for welding chemically active metals like titanium and its alloys. It can also be used for welding vacuum-sealed components, maintaining a vacuum state inside the component after welding.
Good reproducibility and strong process adaptabilityWelding parameters can be independently adjusted over a wide range, making it easy to achieve mechanization and automatic control. The process has good repeatability and reproducibility, providing stable product quality. By controlling the deflection of the electron beam, automatic welding of complex seams can be achieved: the electron beam can reach a distance of about 500mm in a vacuum, allowing welding of seams that are difficult to access, offering wide adaptability to weld structures.
Capability to weld a wide variety of materialsIn addition to welding joints of metal and dissimilar metal materials, it can also weld non-metallic materials, such as ceramics and quartz glass.
Simplifies the manufacturing processComplex or large integral structures can be divided into easy-to-process, simple, or small parts, which can then be welded into a whole using electron beam welding. This approach reduces processing difficulty, saves materials, and simplifies the process.
LimitationsThe equipment is complex, requires a large initial investment, and is relatively expensive. The joint position must be accurate with small and uniform gaps. Strict requirements are imposed on joint processing and assembly before welding. The size and shape of the weldment are often limited by the workspace. Special welding fixtures need to be processed, and both the fixtures and the weldment must be non-magnetic materials. Otherwise, complete demagnetization treatment is required.

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