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Difference Between PAM and IBM – Plasma Arc Machining and Ion Beam Machining

Pintu — Mon, 11 May 2020 12:45:51 +0000

Different forms of energy (such as mechanical, thermal, electrical, chemical, electro-chemical, light, etc.) are directly utilized in advanced machining processes to realize material removal from the workpiece for fabricating intended 3-D feature following the subtractive manufacturing approach. Plasma Arc Machining (PAM) is one such advanced machining process where thermal energy (heat) is primarily used to melt down and vaporize material from the workpiece. A high temperature jet of thermal plasma is first obtained by intensely heating a suitable gas (such as air, helium, nitrogen, argon) with the help of an electric arc. This high velocity plasma jet is then directed towards the workpiece using a nozzle. PAM plasma temperature can reach 20,000°C or even more. When the plasma jet having such a high temperature strikes the work surface, the workpiece material at that spot quickly melts down and vaporizes owing to this extreme temperature of the plasma. While a smaller fraction of the material is removed in the form of vapour of the concerned workpiece material, majority of the material in molten state is blown away by the high velocity plasma jet. Thus material removal for plasma cutting or machining occurs in both the ways. PAM can be applied to electrically conductive as well as non-conductive materials; however, the electric arcing fashion will be different (transferred plasma arc and non-transferred plasma arc). Although PAM offers a relatively higher material removal rate, its cut quality is affected by wider kerf, wider heat-affected zone, stray cutting, and deformation. The PAM process is incessantly noisy, but it does not require a vacuum chamber for its operation.

The Ion Beam Machining (IBM) process is another advanced machining process; however, its working principle is not based on thermal energy. Rather, it is one mechanical energy based process where momentum transfer takes place in atomic level. In IBM, ample ions are first generated by glow-discharge across two electrodes having very high potential difference (around 100 kV). Such ions are then constricted in the form of a beam and accelerated towards the workpiece. When the narrow beam of high velocity ions strikes the workpiece, it can dislodge atoms from the work surface without heating or melting the substrate. The corresponding mechanism of material removal is terms as “sputtering”, where high velocity ions strike the atoms of a solid surface to knock them off by momentum transfer. Accordingly, material removal occurs in atomic form, and thus, it generates highly finished surface. Moreover, IBM process is free from thermal damages, but it requires a soft-vacuum chamber for its operation (this increases the length of mean free path, and thereby reduces the chance of collision between air molecules and ions). Several similarities and differences between PAM and IBM are given below in table format.

Similarities between PAM and IBM

Both PAM and IBM are considered as advanced machining processes (AMP) or non-traditional machining (NTM) processes.
Physical contact between a solid tool and workpiece does not exist in either of these two processes. Accordingly, these processes are free from burr formation, progressive wear, mechanical residual stress, etc. However, thermal residual stress may develop in PAM due to thermal cycle.
Process capability for both the cases is independent of mechanical and chemical properties of the workpiece material.
Both the processes can be applied to electrically conductive and non-conductive materials. Note that the arcing fashion for PAM will be different for conductive and non-conductive materials.
Both the plasma beam and ion beam are made of charged particles.
Electrodes are indirectly used in both the processes to generate plasma or ions.
These processes are commonly integrated with computer control system to facilitate precise control.

Differences between PAM and IBM

Plasma Arc Machining (PAM)	Ion Beam Machining (IBM)
PAM is one thermal energy based advanced machining process.	IBM is one mechanical or kinetic energy based advanced machining process.
Mechanism of material removal in PAM is a combination of (i) vaporization, and (ii) blowing away in molten state.	Mechanism of material removal in IBM is sputtering (dislodging by bombarding ions).
A constricted jet of intensely hot plasma is used to supply thermal energy for material removal.	A beam of high velocity ions is used to transfer momentum for dislodging atoms from the working surface.
Material removal in PAM takes place in the form of vapor of the concerned workpiece material. However, considerable fraction of the material is blown away in molten form.	In IBM process, atom (or a cluster of atoms) is removed directly from the workpiece surface. No such melting or vaporization occurs here.
Power density of the plasma jet is relatively higher (10² to 10³ W/mm²).	Power density of ion beam is significantly less (10^-2 to 10⁰ W/mm²).
Plasma forming gases (air, argon, nitrogen, hydrogen) must be continuously pumped into the high pressure gas chamber. This gas gets converted to thermal plasma when it flows coaxially with the high temperature arc, and finally comes out of the chamber in the form of a high velocity plasma jet.	A gas (argon) is introduced in a vacuum chamber at a relatively lesser flow rate. This gas is then ionized by striking with energized free electrons (electron ionization). The ionized particles are then manipulated to form a narrow but high velocity ion beam.
PAM process does not require a vacuum chamber for its operation. It can be operated in open atmosphere.	IBM process necessitates a soft vacuum chamber having pressure in the order of 10^-3 to 10^-5 atm.
PAM process is associated with excessive thermal damage of the workpiece. It produces a wide heat affected zone.	Thermal damage in IBM process is mostly neglected (a very narrow zone of atomic level may get affected by impact).
PAM tends to deform the object owing to very high temperature non-uniform heating and subsequent cooling of a wide area.	IBM process is free from deformation as heating is mostly negligible.
PAM process is usually very noisy. So personal protective equipment (such as ear muffler) are indispensably necessary.	IBM process is not noisy.
PAM can be used to cut thick objects (even up to 50 mm).	IBM is not suitable for deep cutting. It is preferred for micro-fabrication, smoothening and surface contouring.
PAM usually does not generate highly finished surface. Concerned roughness is in the order of 0.5 – 2.0 μm.	IBM is especially suitable for generating highly finished surfaces having nanometric roughness (10 nm or even lower).

References

Nonconventional Machining by P. K. Mishra (Narosa Publishing House).
Allen et al. (2009). Ion beam, focused ion beam, and plasma discharge machining. CIRP Annals. https://doi.org/10.1016/j.cirp.2009.09.007

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Difference Between Transferred Arc and Non-Transferred Arc Plasma Torch

Pintu — Sat, 09 May 2020 12:49:59 +0000

Thermal plasma is the ionic form of matter that is obtained by heating suitable gas to a very high temperature. Plasma consists of excited ions of gaseous atoms and free electrons (thus plasma can conduct electricity). Localized temperature of plasma can reach 30,000°C or even more. Such a high temperature can virtually melt and vaporize any material regardless of its physical state. An artificially created controllable jet of high temperature plasma can be utilized for several purposes including cutting or machining, welding, coating, heat treating, etc. All these processes utilize the heat of the plasma jet in different ways to fulfill the intended purpose. However, the basic requirement for such activity is a constricted jet of high temperature plasma flowing at high velocity. In order to artificially generate plasma, a plasma-forming gas (can be air, hydrogen, argon, or nitrogen) is introduced into a gas chamber at a high flow rate (1 – 5 m³/h). The gas chamber contains a tungsten electrode, which is connected with the negative terminal (cathode) of a DC power source. The positive terminal of the power source can be connected either with workpiece or with the nozzle of the gas chamber. Based on this connection, plasma torch (plasmatron) can be classified as transferred arc plasma torch and non-transferred arc plasma torch.

In transferred arc plasma torch, the workpiece is made an integral part of the electrical circuit. Therefore, the positive terminal of the DC power source is connected to the workpiece (while the electrode remains connected with the negative terminal). No need to mention that the workpiece must be electrically conductive. When sufficient voltage (around 200 V) is applied across two terminals, a long electrical arc forms between the electrode and workpiece through the small nozzle opening. As it is difficult to establish the arc directly between the electrode and workpiece (because of 5 – 10 mm gap), an auxiliary arc is established between the electrode and nozzle at the beginning of the work for a very short period. The plasma-forming gas that is pumped into the gas-chamber comes out through the small nozzle opening surrounding the electric arc. Owing to the high arc temperature, the gas automatically gets converted to plasma and emerges out of the nozzle in the form of a jet to finally strike the workpiece. The transferred arc plasma torch is also known as direct arc plasma torch as electrical connection is made directly between the electrode and workpiece. The problem with this arrangement arises when the workpiece is not electrically conductive. In such cases, the copper nozzle is connected to the positive terminal (anode) of the DC power source, while no connection is made with the workpiece. Such an arrangement is known as non-transferred arc plasma torch or indirect arc plasma torch. Here electric arc forms between the electrode and the nozzle. However, the plasma-forming gas forcefully directs the arc into the small nozzle opening, while itself gets converted to plasma and comes out of the nozzle as a high temperature high velocity jet. Various similarities and differences between transferred and non-transferred plasma torch are given below in table format.

Similarities between transferred and non-transferred arc plasma torch

In both the cases, an electrode is indispensably used to liberate electrons. This electrode is given negative polarity (cathode).
Both the arc systems are based on DC (direct current). The voltage remains around 200 V, while current can be as high as 1,000 A.
Plasma forming gas (like air, hydrogen, argon, or nitrogen) is also required to pump into the gas chamber continuously regardless of the type of plasma torch used.
In both the cases, an electric arc supplies necessary heat to form plasma.
Irrespective of the torch type, plasma beam operations are very noisy. Accordingly, proper personal protection must be used while operating plasma machines.

Differences between transferred and non-transferred arc plasma torch

Transferred Arc Plasma Torch	Non-transferred Arc Plasma Torch
The electric arc is constituted between an electrode and the workpiece. However, an auxiliary arc is established between the electrode and nozzle at the beginning of the work for a very short period.	The electric arc is constituted between an electrode and the nozzle, and the same arc is continued for the entire operation.
Here the workpiece is made anode (positive terminal of DC power source), whereas the nozzle is kept electrically neutral. Cathode is always a copper electrode.	Here the workpiece is kept electrically neutral, whereas nozzle is made anode. As usual, cathode is always a copper electrode.
Direct arc plasma torch can be applied to electrically conductive workpieces only.	Indirect arc plasma torch can be applied to every workpiece regardless of electrical conductivity. However, it is preferred for non-conducting materials.
Direct arc has relatively higher electro-thermal efficiency (85 – 95%).	Indirect arc has comparatively low electro-thermal efficiency (65 – 75%).
Direct arc is overwhelmingly used for machining (or cutting), welding, hardfacing, remelting, and spraying.	Indirect arc is preferred for flame spraying, spheroidizing heat treatment, ore processing, etc.
Transferred arc plasma torch is also known as “Direct Arc Plasma Torch” because the arc is maintained directly between the electrode and workpiece.	Non-transferred arc plasma torch is also known as “Indirect Arc Plasma Torch” because the arc is not maintained between the electrode and workpiece though the workpiece receives the heat.

References

Modern Arc Welding Technology by Ador Welding Limited (Oxford and IBH Publishing Company Pvt. Ltd.).
Nonconventional Machining by P. K. Mishra (Narosa Publishing House).

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