Application Cases

Experiment Name: Application of High-Voltage Amplifier in Research on Focus and Stigmation Control Technology for Electron Beam Additive Manufacturing

Research Direction: Additive Manufacturing

Experiment Objective:
Electron beam selective melting technology, also known as electron beam 3D printing technology, is a branch of metal additive manufacturing. This technology uses an electron beam as the heat source. Under computer control, the beam scans and melts metal powder along a predetermined trajectory, stacking layers upon layer to form dense three-dimensional parts. This technology can be used to manufacture porous metal structures for medical applications, such as artificial joints. Pore morphology, structure, and other parameters can be conveniently adjusted based on the computer model to match the mechanical properties of human bone. To ensure high precision in the fabrication of porous structures, the electron beam needs to maintain sufficient thermal power density across the entire processing range. This means keeping the beam diameter small for a fixed accelerating voltage. However, due to the influence of electron optical aberrations, the beam spot diameter typically increases with the deflection angle, and its shape gradually changes. This can lead to reduced processing accuracy for large-scale workpieces and may also cause defects such as slag inclusions and porosity. Therefore, adjusting the beam spot quality is necessary.

Testing Equipment: Host computer, A/D and D/A converters, ATA-4315 high-voltage amplifier, filament, alignment coils, stigmator coils, main focusing coil, auxiliary focusing coil, deflection coils, vacuum chamber, substrate, lifting stage, secondary electron detector, etc.

Figure: Control System of Electron Beam Additive Manufacturing Equipment

Experimental Procedure:
The structure of the electron beam column and the control system for the electron beam 3D printing equipment is shown in the figure above. The coils used are all electromagnetic coils. The electron beam is emitted by the filament. Under the influence of the magnetic fields generated by the various coils, it is focused, deflected, and ultimately converges onto the workpiece surface. The main focusing coil has a relatively large inductance and is controlled by a constant current source. The dynamic adjustment is performed by an auxiliary focusing coil, which is a hollow coil with much smaller inductance. The controller receives commands from the host computer to control the electron beam to scan along a predetermined trajectory. While changing the electron beam position, the controller queries the focus/stigmation correction table and dynamically adjusts the current supplied to the auxiliary focusing coil and the stigmator coils, achieving dynamic focusing and stigmation. The secondary electron detector collects reflected electrons for imaging, and the clarity of the resulting image can reflect the quality of the beam spot.

In the field of electron optics, aberration theory is commonly used to analyze the trajectories of electrons in electric and magnetic fields. The deviation between the actual trajectory and the Gaussian (ideal) trajectory is the aberration. Aberrations are categorized into spherical aberration, coma, field curvature (defocus), astigmatism, distortion, and chromatic aberration. In practice, the electron beam diameters in 3D printing equipment are on the order of hundreds of micrometers. Therefore, lower-order aberrations that have a significant impact on the beam diameter, such as defocus and astigmatism, should be considered. The expressions for defocus and astigmatism are as follows:

Figure: Expressions for Defocus and Astigmatism

From these equations, it is evident that defocus and astigmatism are influenced by the deflection distance and direction. When there is no deflection, there is no defocus or astigmatism, and the beam spot quality is optimal. When the electron beam is deflected, the magnitude of defocus and astigmatism changes with the deflection position. Therefore, it is necessary to correct the electron beam at different positions.

Experimental Results:

Figure: Experimental Comparison Images

(1) The use of dynamic focusing and stigmation control technology can effectively improve beam spot quality. After calibration, the electron beam additive manufacturing equipment could achieve clear imaging within a 12° deflection. The EOG (Edge Orientation Gradient) value of the image was successfully increased from 0.81 to 1. At large deflection angles, the electron beam shape is distorted due to astigmatic aberration. As a result, the electron image may achieve very high resolution in one direction but appears blurred in other directions. After stigmation correction, the clarity of the electron image becomes consistent in all directions, and the beam spot shape is improved.

(2) Dynamic focusing alone cannot effectively improve the beam spot shape, but it can reduce the beam spot diameter and enhance the clarity of the electron image in a specific direction.

Aigtek ATA-4315 High-Voltage Amplifier:

Figure: Specifications of the ATA-4315 High-Voltage Amplifier