Process Capabilities of Electron Beam Machining

                  Process Capabilities of Electron Beam Machining

 

-        Introduction-

 EBM(Electron Beam machining) is a thermal machining  process wherein material is removed from the workpiece by focusing a beam of high velocity electrons. A high voltage DC power supply is used to generate electrons with high energy in this process. It is a progressive machining process for material removal. Generally, electron beam machining is carried out in a vacuum chamber , this helps in removing unnecessary scattering of electrons and also making the process more efficient. 

For micro-cutting of metals, this is the preferred process. It can also be used to bore the wide range of metals.

Parts of Electron Beam Machining:

 

Electron gun:
Electron gun is the primary part of electron beam machining. The beam of an electron is generated which is further used to remove material from the workpiece. This electron gun contains tungsten or tantalum filament which acts as cathode.

Bias Grid:
It is used to control the flow of electrons that are generated by an electron gun.

Anode:
To accelerate the electrons to very high velocity, the anode is used

Magnetic Lens:

The magnetic lens is made up of magnet and the main function of itis to concentrate the beam of electrons.

Aperture:
An aperture is similar to the aperture of the camera but different in purpose, this aperture is used to capture the stray electrons so as to pass  only focused and concentrated beam of electrons through the aperture.

Electromagnetic Lens:
The electromagnetic lens is then used to focus the electrons’ beam on the workpiece.

Diffusion Pump:
The diffusion pump is used to maintain the vacuum within the electron beam chamber. The level of vacuum in this chamber ranges from 10 to the power minus 4 to 10 to the power minus 6 torque.

Deflector Coil:
If in case a proper hole is not being created by an electron beam, the deflector is used to deflect the electrons’ beam by a small amount.

Optical Viewing System:
Optical Viewing System is used by the operator to check whether the process is under control or not.
This optical viewing system consists of a telescope and illumination system.

Slotted Disc:
The slotted disc is used to remove the vapour and fumes to avoid obustruction of optical windowsthat may occur while machining the workpiece by using electron beam machining. This slotted disc is in synchronization with the electron beam.

-        EBM process :

                Mechanism of Material Removal in EBM 

        Firstly, a vacuum is created and maintained using the condensation pump within the nonparticulate radiation chamber. Followed by that, a possible difference is applied across the tungsten filament so that  temperature of the filament is around  2500°C. As the  negative terminal of the DC power supply is connected to the present tungsten filament , the tungsten filament acts as a cathode. There is a resultant emission of thermo-ionic electrons from the tungsten filament at high temperature of 2500 °C and under vacuum. As  the cathode is negatively biased, the electron emitted from the cathode is  removed from it. Once the electrons are emitted from the electron gun, these electrons are passed through the bias grid which controls the flow of the electrons. And to avoid the gathering of electrons on top of this grid it is also negatively biased. As the electrons pass the bias grid they reach up to  the anode and since they are negatively charged and there is a potential difference between the anode and the cathode, the electrons are accelerated as they pass the anode. These electrons attain a velocity which is nearly half the velocity of light when they pass through this anode section. The electrons pass through the magnetic lens after passing the anode.The beam of electrons gets much more concentrated and focused as the beam passes through the magnetic lens. Some of the electrons diverge from their way and the rest of them moves toward the workpiece as a beam. The diverged electrons are known as stray electrons. The electron beam after passing through the magnetic lens, the electron beam passes through the aperture. The aperture captures the stray electrons when the stray electrons are available near the aperture and only focused and concentrated beam of electrons passes through this aperture.The beam becomes very concentrated and focused as soon as the beam emerges from this section and there are no stray electrons. After that, an electromagnetic lens finally focuses the electron beam on the workpiece. Before reaching to the workpiece, the electron passes through deflector coil if needed and this coil deflects the beam by a small amount . This deflector coil is used only when we are not getting a proper hole or to improve the shape of the machined holes. To impinge on the workpiece with a spot size of 10-100 μm the highly focused electron beam is made. To check if the beam is rightly placed or not, a telescope and illumination system is used and if not, the deflector coil is used to align the electron beam properly. As the electrons strike the workpiece high velocity or kinetic energy of electrons are converted into heat energy and due to high power density of the charge , the workpiece melts and vaporizes instantly. A single pulse is needed to drill a hole in a thin sheet, whereas multiple pulses are required for as the thickness goes on increasing. If the vacuum is not used in this process then the electrons will not be able emit from the cathode and even if the emission happens the electron will not be able to achieve that acceleration due to collision with the air molecules.

 

-EBM process capability                      

This process  provides  holes of  diameter  ranging from 100 μm to 2 mm with a  depth upto  15 mm. The hole can be tapered along the barrel shaped or depth . Also a  reverse taper can  be obtained  by focusing the beam below the surface. There would be an edge at the entry point along with presence of recast . Burr formation does not occur in EBM. Materials such as  stainless steel, steel, Ni and Ti super-alloys, aluminum as well as ceramics ,plastics, leathers can be machined successfully using electron beam. As the mechanism of material removal is thermal in nature and hence in electro-discharge machining, it can result into thermal damages associated with EBM. However, due to shorter pulse duration the heat-affected zone is narrow in EBM. Normally the heat-affected zone is around 20 to 30 μm. Materials like Al and Ti alloys are more readily machined, rather than machining of steel. The Number of holes drilled per second relies on the diameter of the hole, power density and depth of the hole and material type.

 EBM requires simple work holding , as it does not apply any cutting forces on the work piece and  thus enabling it to machine fragile and brittle materials   . Holes can also be drilled at a very small angle of as less as 20 to 30⁰. In the electron discharge machining,Cut formation is not observed.Also, electron beam machining gives better surface finish and thinner kerf width as compared to other thermal processes.It’s drilling rates are up to 4000 holes/sec which is also a major advantage. EBM offers No mechanical distortion and best obtainable finish, compared to the other unconventional processes used to drill precision holes and High accuracy – capability to maintain high accuracy and repeatability ±0.1mm for position of the hole and ±5% for the hole diameter. Even, Close dimensional tolerance can  be achieved as there is no resulting cutting tool wear. With high accuracy, EBM can provide holes of any shape by combining beam deflection using electromagnetic coils and the CNC table.

 

 

-  Electron Beam parameters :

The process parameters in Electron Beam Machining which directly affects the machining characteristics are:

• The accelerating voltage(V_a) 

• The beam current (I_b) 

• Pulse duration   (t_on) 

• Energy per pulse 

• Spot size   

We gone through one paper and we got following optimum parameters 

The accelerating voltage(Va) – 300 KV

• The beam current (Ib) – 14mA

• Pulse duration   (ton)  –  50 μs   to  50 ms.

• Energy per pulse – 100 J/Pulse

• Spot size   –  5.35 cm

 

-         EBM Applications :

1)  To remove small broken taps from holes EBM process is  widely used.
2) In space nuclear reactors, it is used for making fine gas orifices

3) Used for  making  turbine  blades  for  supersonic  aero  engines.

4) It  is  also  used  to  manufacture  field  emission cathodes,  integrated circuits, and computer memories.

5) It Used for making drawing dies and flow orifices.
6) It is also used for frilling synthetic jewels in the watch industry.
7)It is widely used for micro-machining of thin materials like perforating, drilling, scribing  and slotting.

8)Also used for welding small and tiny pieces of highly reactive and refractory metals.

 

-         EBM Advantage and Limitation :

 

Advantages :

When small holes with large ratios are to be drilled EBM provides very high drilling rates.It can be used to machine almost any material irrespective of their mechanical properties.The heat-affected zone is very less in this process due to shorter pulses.In this process, no mechanical cutting force applies therefore holding and fixing cost is very less.

 

Limitations :

 The high capital cost of the equipment is the fundamental limitation  . Maintenance of the equipment using a vacuum system is required regularly.
Recast layer formation takes place in this process. Various non-productive pumps are used for attaining the vacuum desired . The bottom of through hole would become cone-shaped if not properly handled .

References :

1. https://ask.lecturenotes.in/question/explain-the-construction-of-electron-beam-gun-and-diffusion-pump-in-electron-beam-machining-process-giving-neat-sketches.-rkmwws0dn

2.http://www.mechanicalwalkins.com/electron-beam-machining-parts-working-advantages-disadvantages-and-applications/

3.https://capitalcanbd.com/corn-snake-cdpdc/b306e5-in-electron-beam-machining-as-the-electrons-strike-the-workpiece

4.https://nptel.ac.in/content/storage2/courses/112105127/pdf/LM-40.pdf

5. Intelligent knowledge based system for material processing by electron beam machining (EBM) process by Author Morteza Sadegh Amalnik published in International Journal of Advanced Engineering and Technology Volume 1; Issue 1; March 2017

6. Effect of process parameters settings and thickness on surface roughness of EBM produced Ti-6Al-4V by A. Safdar, H.Z. He and Liu-Ying Wei. Rapid Prototyping Journal (2012)

7. Simulation study of electron beam spot size from thermionic electron gun using SIMION 8.1 software by Djoko Slamet Pudjorahardjo, Suprapto, Darsono, Fuad Anwar, and Andy Saktia Warseno. AIP Conference Proceedings 2014.

8. Spinney, P. S., Howitt, D. G., Smith, R. L., & Collins, S. D. (2010). Nanopore formation by low-energy focused electron beam machining. Nanotechnology

9.Martins, M., Godina, R., Pimentel, C., Silva, F. J. G., & Matias, J. C. O. (2018). A Practical Study of the Application of SMED to Electron-beam Machining in Automotive Industry. Procedia Manufacturing

10.https://learnmech.com/electron-beam-machining-ebm-advantages-disadvantages/

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