Plasma assistad machining of Heat resistant super alloys

2. Outline
1. Introduction
2. Review of literature
3. Objectives
4. Working Principle of PAM
5. Conclusions
6. References

3. Introduction
• Ever since their introduction many years ago, heat resistant super alloys (HRSA)
have been difficult to machine.
• Materials such as titanium, Inconel and nickel alloys are being increasingly used in
aerospace, automobile and medical equipment.
• HRSA materials are metallurgic-ally composed to have high strength at high
temperatures, the stresses that are generated when machining also are high.
• The unique capability of these nickel, iron or cobalt-based super alloys to perform
close to the melting point of their basic metal gives them varied but generally poor
machinability.
• About twice as much power is needed to machine HRSA materials as is needed for
low-alloy steel, and the specific cutting force is 4,000 N/sq-m for HRSA compared
with 2,500 N/ sq-m for steel.
• These alloys are ductile, their fatigue resistance, hardness and toughness at high
temperatures combine to develop a number of wear mechanisms for cutting tools.
• The edge of a cutting tool is exposed to considerable mechanical stress, strain and
heat in machining these alloys.

4. Review of literature
• Sung-Ho Moon, Choon-Man Lee “ A study on the machining characteristics using
plasma assisted machining of AISI 1045 steel and Inconel 718”
International Journal of Mechanical Sciences 142–143 (2018) 595–602
 The local application of an external heat source increases the temperature of the
materials to a high temperature, which reduces the mechanical strength of the
difficult to cut materials, making them more easily machined with lower
mechanical energy.
 Experiments were carried out with the PAM system on AISI 1045 steel and
Inconel 718 using the deter- mined preheating temperature and depth of cut
(DOC).
 It was found that PAM reduced cutting force and improved the surface quality.

5. Review of literature
• E.O. Ezugwu* “Key improvements in the machining of difficult to cut aerospace
super alloys” International Journal of Machine Tools & Manufacture 45 (2005)
1353–1367
 This paper presents an overview of major advances in machining techniques
that have resulted to step increase in productivity, hence lower manufacturing
cost, without adverse effect on the surface finish, surface integrity, circularity
and hardness variation of the machined component.
 The application of new techniques and recently developed cutting tool
materials in the machining of nickel base and titanium alloys have resulted in
several fold increase in tool life without compromising the surface finish and
integrity of the machined components.
 Alloys that are difficult to cut at room temperature can be easily machined at an
elevated temperature, up to an optimum temperature level, using hot machining
principles.

6. Review of literature
• L. N. López de Lacalle; J. A. Sánchez; A. Lamikiz; A. Celaya “Plasma Assisted
Milling of Heat Resistant Super alloys” J. Manuf. Sci. Eng. 2004; 126(2):274-285.
doi: 10.1115/1.1644548
 It has been identified that the use of PAM for Inconel 718 using whisker
reinforced ceramic tools (Al2O3 1CSiw) leads to an increase in the tool life of
approximately 200%. The reason is that notch wear, which is the most
important wear mechanism in conventional milling, nearly disappears.
 For Ti6Al4V, PAM leads to localized melting of the material, and to the
formation of a new metallurgical structure with lower mechanical properties,
especially in service. So it is not recommended.
 The problem of the plasma jet orientation just ahead of the cutting tool, and the
dimensional variation induced by the localized material heating are the reasons
for which this technique should be recommended only in roughing operations.

7. Objectives
• The objective of the assisted machining techniques is to improve the cutting process
by acting on the chip removal mechanism.
• This process, known as PAM (Plasma Assisted Milling) has been applied to the
machining of three very low machinability materials: a Ni-base alloy (Inconel 718),
a Co-base alloy (Haynes 25), (both belonging to the group of the heat-resistant
alloys) and the Ti-base alloy Ti6Al4V.

8. Plasma Assisted Milling
• The plasma arc machining process was introduced to the industries in 1964 as a
method of bringing better control to the arc welding process in lower current
ranges.
• Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas
to melt and displace material in its path.
• Gases are heated and charged to plasma state.
• Plasma state is the superheated and electrically ionized gases at approximately
5000⁰C.
• These gases are directed on the Work-piece in the form of high velocity stream.
• The machine consists of two unit’s i.e. the plasma production unit and the milling
unit.
• The high energy plasma is utilized for thermal softening of the material followed
by milling operation.
• Plasma assisted milling Heated spot by the plasma jet is a circle of maximum
heating, where the temperature is between 500°C and 1,000°C.The energy is
transferred to the work piece by convection process and the adjacent zone, where
the heat is transferred by conduction .

9. Plasma Assisted Milling
• The nozzle is focused at a distance of about 8 ±10 mm ahead of the milling tool in
the direction of the feed. This distance is high enough to prevent the tool body from
being directly affected by the plasma jet.
• The nozzle is placed at a height of 5-6 mm over the work piece and thus the electric
arc responsible for the ionization of the channel known as transferred arc can be
activated. The diameter of the heated spot is about 4-5 mm.
• The spot must be located just exactly at the material to be removed thus avoiding
the zones of the work piece previously machined. The geometry of the work piece
must be simple with geometrical features that do not involve sharp changes in the
feed direction, since the plasma spot must be located ahead of the tool during the
whole process.
• The ionized gas produces material surface heating by convection. The result is a
phenomenon known as thermal softening, which is related to the reduction of the
cutting forces.

10. Main components of the plasma assisted milling system.
Below, top view of the plasma spot and the milling tool.

11. Plasma Power Generator
• The plasma power equipment is a commercial welding one, providing transferred
arcs direct current at a maximum intensity of 250 A.
• The plasma torch is a copper nozzle of 2 mm diameter. Tungsten electrode cathodes
with 30° taper angle are used.
• The plasma gas is Argon with a flow of 0.5 l/min, while the shielding gas is a
mixture of Argon and 5% of Hydrogen, with an approximate flow of 11 l/min.
• The nozzle serves as anode when used with nonconductive materials, while the arc
is transferred to the work-piece in the case of conductive pieces.
• The nozzle is placed 5-6 mm over the work piece. The heating of the work piece
depends primarily on two operating parameters one is the intensity of the
transferred arc I and the translational velocity of torch over the work surface.
• Plasma is a superheated, electrically ionized gas flow. The plasma power generator
consists of a power supply, an arc starting circuit and a torch.
• The arc starting circuit is a high frequency generator circuit that produces an AC
voltage of 5,000 to 10,000 volts at approximately 2 megahertz. This voltage creates
a high intensity arc inside the torch which ionize the gas and thereby producing the
plasma.

12. Plasma Power Generator
• Once the gas flow is stabilized the high A.C voltage breakdown is applied and thus
producing the arc between the electrode and nozzle. The flow of the gas forces this
arc through the nozzle orifice and thus creating the pilot arc.
• when the pilot arc comes in contact with the work piece surface the system
shutdown the A.C supply and the pilot arc is maintained with a D.C supply .Thus
the process of heating the work piece surface using plasma jet is carried out.
Cooling Mechanism
•Hot gases continuously comes out of nozzle so there are chances of its over heating.
•A water jacket is used to surround the nozzle to avoid its overheating.

13. Picture Of The Plasma Torch

14. Recommended Materials for PAM
• Plasma assisted milling is recommended for the machining of low-machinability
alloys, and especially those whose mechanical properties decrease only over a
certain temperature.
• In these materials a high mechanical strength is related to high shear strength and
therefore machining is difficult.
• The Ni-base and Co-base alloys are considered to be the materials with lowest
machinability.
• Low machinability depends mainly on the following factors.
 The cutting forces and the temperature at the cutting zone are extremely high. This is due
to the heat generated by the high deformation energy.
 Ductility-the machining of ductile alloys requires very sharp cutting edges with a
positive rake angle.
 Strain hardening-This phenomenon is caused by the cold working of the material during
the plastic deformation inherent to the cutting process.

15. Recommended Materials for PAM
• Plasma assisted machining could be applied to Ti-base alloys such as the Ti6Al4V
alloy which is a very popular material in the aerospace industry.
• The alloy Ti6Al4V is an Alfa-beta alloy used in the cold parts of turbines.
• This material exhibits a very low machinability. But for these materials machining
problems arise from the high temperatures in the tool/ chip contact area due to the
low thermal conductivity of the alloy.
• These alloys also present high chemical reactivity at the temperatures (500°)
induced in the tool/chip interface during the cut-ting process with almost all tool
materials. These facts drive to a quick tool wear.
• Inconel 718 alloy has high corrosion resistance and high strength with outstanding
weld-ability including resistance to post weld cracking. This alloy has excellent
creep-rupture strength at temperatures up to 700°C.
• Haynes 25 has excellent high-temperature strength with good resistance to
oxidizing environments up to 980°C for prolonged exposures and excellent
resistance to metal galling and it is also very sensitive to cold working.

16. Machine Setup
• Machine setup is a three axis conventional machining center equipped with a
spindle with rotational speed below 10,000 rpm and maximum linear feed of 5
m/min.
• The NC unit controls the machining tool paths and the basic operation of the
plasma power generator. using specially programmed miscellaneous M type
functions. Thus the pilot and the transferred arcs can be switched on/off.
• The plasma power equipment is a commercial welding one, providing transferred
arcs (direct current) at a maximum intensity of 250 A.
• The plasma torch is a copper nozzle of 2 mm diameter. Tungsten electrodes
(cathodes) with 30° taper angle are used.
• The plasma gas is Argon with a flow of 0.5 l/min, while the shielding gas is a
mixture of Argon and 5% of Hydrogen, with an approximate flow of 11 l/min. The
nozzle serves as anode when used with nonconductive materials, while the arc is
transferred to the work-piece in the case of conductive pieces.
• The nozzle is placed 5-6 mm over the work-piece.
• The heating of the work-piece depends primarily on two operating parameters: the
intensity of the transferred arc “I” and the translational velocity of torch over the
work surface

17. Figure of the machine setup of PAM process
Experimental equipment of PAM. (a) Inconel 718 or Haynes 25, (b) milling tool, (c)
plasma torch, (d) Force measuring device, (e) plasma generator, (f ) 3 axes vertical
machining center, (g) torch positioning system (2 axes).

18. Machine Setup
• The most used tools for this process are sintered tungsten carbide ones (grade K5-
K10) coated with TiAlN or TiCN with very moderate cutting conditions. An
alternative solution is the use of more expensive tools such as PCBN
Polycrystalline Cubic Boron Ni-tride as well as whiskers reinforced ceramics
• In Plasma Assisted Milling different technical inputs must be taken into account to
adequately select the cutting parameters .
 The process parameters “f z” and “ae” are related to the size of the heating spot. The
machine linear feed “F” is directly related to the heating of the work surface. The axial
depth of cut “ap” depends of the temperature gradient under the surface due to the
plasma heating.
 The cutting conditions f z, Vc, ap and ae have a direct influence on the tool behavior and
process performance. There is a cross relationship between the machine parameters (F, S,
ap and ae) and the heating parameter F. The relation of these parameter is shown:
were a e radial depth of cut, a p axial depth of cut, V c cutting speed, f z
= feed per tooth, z = no of teeth of tool

19. • The values for tool diameter “D” have been selected as a function of the plasma
spot size 3-4 mm. Thus, in the case of solid carbide tools, 12 mm diameter tools
have been used. In the case of insert tools, 50 mm diameter tools with round inserts
of 12 mm diameter were selected.

20. Results in Titanium Alloys
• Heating of the Alloy
 The temperature 1 mm below the work surface has been analyzed. Thus, when
using a plasma intensity of 30 A the temperature is 171°C, whereas in the case
of 60 A the temperature is 247°C. These values are lower than those measured
in the case of Haynes 25 or Inconel 718.
 This is due to the low thermal conductivity of titanium, nearly 35% less than
the heat-resistant alloys. This results about temperature shows that clearly there
is a reduction in cutting forces associated with the PAM process on Ti6Al4V
material.
• Tool Wear
 The tool’s flank wear rate is found to be increasing with the plasma arc
intensity. The reason is again the low thermal conductivity of titanium than the
heat-resistant alloys .Due to which a high heat concentration on the surface is
formed. This is why the tool section in contact with the surface suffers a more
rapid degradation.

21. Results in Titanium Alloys
• Structural Integrity of the Material
 The metallurgical structures of Ti6Al4V have been analyzed.
 The main conclusion is that material melting in the heated zone always
happens, even at low plasma intensity or high linear feed due to the very
low thermal conductivity of titanium.
 Melting of the material has been detected in all the tests arc intensities from
25 to 60 A together with a small zone of transition between the heated and
the not-heated zones.

22. Results in Co-Base Alloys (Haynes 25)
• Heating of the Alloy
 The temperature 1 mm below the work surface has been analyzed.
 Analysis of the temperature 1 mm deep in previous tests revealed that, if nozzle
height is kept between 4 and 7 mm, it has a negligible effect on the maximum
temperature. But values of 5-6 mm were fixed in all tests for this parameter.
 When using 651 mm/min feed and 60 A, the maximum temperature measured by
the thermocouple 1 mm deep below the work surface was 311°C. Since the axial
depth of cut of the test was 1 mm, the removed material was at a temperature
between 311 and 750°C
 Tool Wear
 Flank wear has been measured during the cutting tests,
 These tests have been performed using a cutting speed of 70 m/min. When
conventional milling, tool wear after a cut length of 500 mm is 0.5 mm, some of the
teeth showing chipping.
 Under the same conditions and using a plasma intensity of 60 A, tool wear is equal
or even below to 0.1 mm for the same 500 mm machined length. In this case
chipping is not observed.

23. Results in Co-Base Alloys (Haynes 25)
• Structural Integrity of the Material
 Haynes 25 is very affected by strain hardening due to the effect of cutting
processes.
 The hardness increase is due to the large amount of plastic deformation and to a
change in the crystallographic structure of the alloy.
 The plasma heating does not affect negatively and it can even be concluded
that its effect is to delay the allotropic transformation.

24. Results in Ni-Base Alloys .Inconel (718)
• Heating of the Alloy
 The temperature increase of Inconel 718 when working at F 972 mm/min and I
110 A has been measured.
 Heat treatment (H/T) did not lead to recrystallization and the desired texture
and grain morphology were preserved.
 Tool Wear
 In tests performed on a dynamometer with a plasma intensity of 110A, a
reduction of the cutting forces of up to 45% when compared with conventional
milling has been identified
 At 110A both mechanisms exhibit a great reduction and deep notching nearly
disappears.

25. • Structural Integrity of the Material
 After machining, the work surface exhibits important strain hardening
 Since the strength of the material to be machined is smaller, the layer affected
by strain hardening is smaller both in depth and value.
 The PAM processing of Inconel 718 does not affect the material integrity and
therefore this process can be recommended for industrial production of aircraft
engine components.

26. Conclusions
• For all the materials used for the study, it is found that there is considerable
reduction in cutting forces.
• This due to thermal softening of the materials resulted from the plasma jet heating.
• When using PAM in Haynes 25, tool wear is reduced if compared to conventional
milling, in which edge chipping is common.
• For Haynes 25, Inconel 718 we can see that the tool wear rate is reduced
considerably compared with the conventional type of machining, so the technique
of plasma assisted machining is very much recommended for the machining of
these two HRSA materials.
• But for the titanium alloy it is found that the PAM process shows negative results
because of the melting of work piece surface and higher tool wear rate. This due to
very low thermal conductivity of Ti6Al4V than HRSA .
• For any material the technique of PAM is economically feasible only when the
machinability of the material being processed is limited.

27. References
• Sung-Ho Moon, Choon-Man Lee “ A study on the machining characteristics using
plasma assisted machining of AISI 1045 steel and Inconel 718”
International Journal of Mechanical Sciences 142–143 (2018) 595–602
• L. N. López de Lacalle; J. A. Sánchez; A. Lamikiz; A. Celaya “Plasma Assisted
Milling of Heat Resistant Super alloys” J. Manuf. Sci. Eng. 2004; 126(2):274-285.
doi: 10.1115/1.1644548
• E.O. Ezugwu* “Key improvements in the machining of difficult to cut aerospace
super alloys” International Journal of Machine Tools & Manufacture 45 (2005)
1353–1367

28. Thanks