seminar Report

SEMINAR REPORT 2013 PLASMA ASSISTED MILLING OF HRSA

CHAPTER 1

INTRODUCTION

The objective of the assisted machining techniques is to improve the cutting process by

acting on the chip removal mechanism. Nowadays, based on their feasibility and industrial

expectations there are two most techniques used, they are Jet Assisted Machining (JAM) and

Thermal Enhanced Machining (TEM). In Jet Assisted Machining a high velocity jet stream of

materials is directed Thermal Enhanced Machining- conventional cutting process in which an

external energy source is used to enhance the chip-generation mechanism to impact on the work

piece surface to achieve the required machining process. The cutting process takes place at

temperatures ranging from 400° to 700°C, and therefore the shear strength of the material is

significantly lower than the room temperature strength, resulting in considerably lower cutting

forces.

The technique is economically feasible only when the machinability of the material being

processed is limited and therefore the cutting parameters depth of cut, feed per tooth and cutting

speed are very low. . This is the case of the heat resistant alloys, such as the Ni-base alloy

Inconel 718 (standard AMS 5596, UNS N07718, 52Ni-19Fe-18Cr-5(Cb1Ta) -3Mo-0.9Ti-0.5Al )

and the Co-base alloy Haynes 25 (standard AMS 5537, L605, UNS R30605, 51Co-20Cr-15W-

10Ni-1.5Mn-0.10C-3Fe-0.4S).These materials maintain their mechanical properties as well as an

excellent corrosion resistance even over 600°C. Because of these properties, above materials

have been used in the manufacturing of the turbine components of both commercial and military

airplane engines.

DEPARTMENT OF MECHANICAL ENGG Page 1


CHAPTER 2

TECHNOLOGY OF PLASMA ASSISTED MILLING

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. The following figure shows a schematic drawing of the plasma-machine system

layout.

Fig 1 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 . The heat distribution on the surface

is supposed to follow a Gauss distribution. 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.



CHAPTER 3

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 amongst the

materials with lowest machinability. This 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, as well as to the low thermal

conductivity of these materials.

Ductility-the machining of ductile alloys requires very sharp cutting edges with a positive

rake angle. For the operation to be a cutting process rather than a plowing action.

However, these materials must be machined with strong tools with low rake angles

because of their high specific cutting energy. The high ductility is responsible of the

some frequent chip type named ``chipfoot,'' when the tool exits from the work piece; at

this point the material is stretched rather than cut. This produces an uncontrolled chipping

of the tool edge. Figure below shows an Inconel chip-foot and the burrs on the edges of a

Haynes 25 work piece that also related with material ductility.

Fig 2 Chip foot produced with conventional machining



Strain hardening-This phenomenon is caused by the cold working of the material during

the plastic deformation inherent to the cutting process. It is closely related to the

metallurgical structure of materials. In this case related to the austenitic matrix of Inconel

718 alloy and the g phase of Haynes 25. In order to reduce it, very small feed, high

cutting speed and worn tools must be avoided. Cold working of the machined surfaces

related to the increase in hardness makes future operations more difficult, causing

premature notch wear of the tool at the point of the edge intersecting the work surface

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. 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

weldability 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. The alloy Ti6Al4V is an

Alfa-beta alloy used in the cold parts of turbines.



CHAPTER 4

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 following figure shows the machine setup of PAM process.

Fig 3 PAM machine setup

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, avoiding the zones of the work piece previously machined.



The temperature of the ionized gas Argon is well over 15,000 K. The velocity of the plasma jet

about 5 mm away from nozzle is 400 m/s and at this is the speed at which the plasma jet impacts

on the material surface. The energy is transferred by convection to the work piece and produces

the heating of the work surface at temperatures ranging from 400 to 1,000°C. 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 in case of milling because of

the location of the nozzle fixed with respect to the milling tool. The velocity is equal to the

machine-tool linear feed F .

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 (Al2 O 3 1CSiw).

In Plasma Assisted Milling different technical inputs must be taken into account to adequately

select the cutting parameters

The process parameters f z and a e 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 a

p depends of the temperature gradient under the surface due to the plasma heating.

The cutting conditions f z , Vc , a p and a e have a direct influence on the tool behavior

and process performance. There is a cross relationship between the machine parameters

( F , S , a p and a e ) and the heating parameter F . The relation of these parameter is

shown as F=( fz 1000 V c z)/ (∏ D), 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

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. An adequate selection of z allows the selection of a wide range of values for the

feed per tooth f z and the cutting speed V c.



CHAPTER 5

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. A

picture of the plasma torch is shown in the following figure.

fig 4 Plasma power generator



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 in the case of milling, because of the

location of the nozzle fixed with respect to the milling tool.

Plasma is a superheated, electrically ionized gas flow. The plasma power generator consists

of a power supply, an arc starting circuit and a torch. Plasma arc generated between electrode in

torch and anode work piece the plasma flows through water-cooled or gas cooled nozzle that

constricts and directs stream to desired location for heating. 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. 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.



CHAPTER 6

EXPERIMENTAL ANALYSIS

The materials selected for the experimental analysis are Haynes 25, Inconel 718 and

Ti6Al4V.The work piece material are supplied in the form of plates of width from 30 to 50 mm,

and they are rigidly bolt onto a Kistler 9255BR dynamometer. Properties that are analyzed

through the experiment using plasma assisted milling are

Heating of the Alloy

Cutting Force Reduction

Tool Wear

Structural Integrity of the Material

For the measuring of Cutting forces the work pieces are bolt onto a Kistler 9255B measuring

device, where Fx , Fy and Fz are obtained. The force signals were sampled at more than 10 KHz.

The device measures the tangent force and thrust force based on Merchant circle shown below

Fig 5 Calculation of Ft and Fv



In the above figure Xm and Y m are the coordinate axes of the Kistler measurement device,

where the cutting forces ( Fx and Fy ) are measured. Xt and Y t are the tool coordinate axes,

where the tangential ( Ft ) and thrust force ( F v) components must be calculated. A procedure to

get the Ft and F v cutting force components from the measured Fx and Fy must be carried out.

In some of the work pieces, small diameter holes have been EDM'ed in order to introduce

K-type thermocouples 1.5 mm diameter down to 1 mm below the heated work surface. An

infrared camera Nikon Laird-S270 has been used to mea-sure the maximum heating of the work

surface just after being exposed to the plasma jet. The camera has a range of 210 to 1,200°C

with two different lens and accuracy of 62% . The scanning frequency is 2 images per second.

These are employed for finding out the temperature on both inside and surface of the work

material.

CHAPTER 7



RESULTS IN CO-BASED ALLOYS

The co-based alloy used for the study is Haynes 25 which is very difficult to machine. The

results obtained from the study are


The first objective was the evaluation of the temperature in the material just after the plasma

heating. If we go deeper into the material below the work surface, the temperature decreases

rapidly at high gradient. In any case, the temperature of the surface and the subsurface layers that

will be removed by the machining process will be different. The plasma jet must heat the

materials as high as possible, but never melt its surface. If this were the case, a welding bead

would appear. This drives to an unstable process since the milling tool may find unexpected re-

solidified material on its path, perhaps with a very different and harder metallurgical structure.

Molten material may also adhere to the tool. This can be produced by a too high plasma arc

intensity or/and small linear feed rate.

Fig 6 Temperature at a point 1 mm below the work surface

The figure above shows the temperature at a point located 1 mm below the work surface. The

peak value corresponds to the plasma jet impacting just on the work piece surface. . The peak

value corresponds to the plasma jet impacting just on the work piece surface. The table included

in shows the maximum values of this temperature as a function of tool linear feed and plasma



intensity. It is appreciated that for the same intensity, the maximum temperature decreases when

feed is increased, whereas for a constant feed rate the temperature seems to increase with the

plasma intensity.

Cutting Force Reduction

The machining tests on Haynes 25 were carried out using a 12 mm diameter milling tool. The

tool material was sintered tungsten carbide CW micro grain grade K10 which was coated with a

single layer of TiAlN (3 mm). The value of the plasma arc intensity has been varied throughout

the tests, as shown in Table below

Table 1 Tangential cutting force reduction as a function of the plasma intensity

The reduction of the cutting forces has been measured. Thus, the tangential component Ft was calculated

from the experimentally measured Fx and Fy Its values being gathered in table above. Based on these

data it can be concluded that the reduction of the forces is enough high when the plasma intensity is over

30 A. From room temperature to 500° the material strength decreases slowly, at this temperature the slope

of strength-temperature curve is more pronounced. Between 30 and 50 A the reduction of the cutting

forces is approximately constant. This is due to the no uniform distribution of temperatures into the work

piece and on work piece surface while some parts of the chip section do not reach high temperatures and

will still maintain its high mechanical features. But at arc intensities above 60A a weld bead appears.



Tool Wear

Flank wear has been measured during the cutting tests, as shown in Figure below.

Fig 7 Flank wear in four teeth, with and without assisting plasma

These tests have been per-formed using a cutting speed of 70 m/min. When conventional milling

is done the tool wear after a cut length of 500 mm is 0.5 mm with 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.


It is observed that the effect of heating on the structure of the metal is very limited since no

variations can be identified in the austenitic phases of this alloy. The plasma heating does not

affect negatively and it can even be concluded that its effect is to delay the allotropic

transformation, favoring thus the stability of the structure.



Fig 8 Strain hardening in Haynes 25 after PAM

It can be concluded that the plasma heating has a beneficial effect on the stability of the Co-base

alloy. This is in accordance with the existing data referring to the good weldability of this alloy,

and the stability of the material below 900°C in service conditions.

CHAPTER 8



RESULTS IN NI-BASED ALLOYS

The ni-based alloy used for the study is Inconel 718 and the following results were observed with

PAM process


The heating of the work piece surface previous to machining and after PAM process is shown

below

Fig 9 Temperature of work piece surface before and after PAM

Just half a second after switching the plasma off, when the maximum temperature

reaches 810°C, and therefore it implies that half a second before the temperature was in the range

between 850 and 900°C. The second figure shows the heating of the material just after

undergoing the PAM process, with an axial depth of cut of 3.175 mm. In this case the maximum

temperature is about 400°C. Therefore the temperature of the re-moved layer is between 400°

and 900°, high enough to achieve the reduction of the cutting forces.



Tool Wear

Figure below shows the different tool wear patterns that occur in conventional milling and in

PAM processing.

Fig 10 Tool wear patterns with and without plasma assistance

In conventional milling a larger notch wear in the zone of the insert in contact with the work

piece surface than flank wear in the lower part of the insert appears. At 110A both mechanisms

exhibit a great reduction and deep notching nearly disappears.


Inconel 718 has been analyzed in three different states as supplied, after conventional machining,

and finally after PAM are as shown below. None of them present carbide precipitation or the

existence of new metallurgical phases. After conventional machining the work surface exhibits

important strain hardening which is inherent in materials with austenitic structure.



Fig 11 Strain hardening in Inconel 718 with and without plasma assistance

Due to thermal softening suffered by Inconel 718 after applying plasma heating the strength of

the material to be machined is smaller and the layer affected by strain hardening is smaller both

in depth and value. However, the strain hardening is still present even after applying PAM. This

is due to the plowing effect associated to the little chip thickness in the low axial point of the

inserts which also results in a high localized flank wear and to the negative axial and radial rake

angles of the cutting ceramic inserts.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.

CHAPTER 9



RESULTS IN TITANIUM ALLOYS

Ti6Al4V, a difficult to cut material used in aircraft engines is used for the study of PAM process

and the results obtained are


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.


The metallurgical structures of Ti6Al4V after heating and after PAM heating and machining

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.

CHAPTER 10



CONCLUSION

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. 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.

CHAPTER 11



REFERENCE

Weinert, K., 1994, ``Relation between Process Energy and Tool Wear when Turning

Hardfacing Alloys,'' CIRP Ann., 43~1!, 97-100.

Novak, J. W., Shin, Y. C., and Incropera, F. P., 1997, ``Assessment of Plasma Enhanced

Machining for Improved Machinability of Inconel 718,'' ASME J. Manuf. Sci. Eng., 119,

pp. 125±129.

Kitagawa, T., and Maekawa, K., 1990, ``Plasma Hot Machining for New En-gineering

Materials,'' Wear, 139, pp. 251±267.

Leshock, C. E., Kim, J. N., and Shin, Y. C., 2001, ``Plasma Enhanced Machin-ing of

Inconel 718: Modelling of Workpiece Temperature With Plasma Heating and

Experimental Results,'' Int. J. Mach. Tools Manuf., 41, pp. 877± 897.

KoÂnig, W., CronjaÈger, L., Spur, G., ToÂnshoff, H. K., Vigneau, M., and Zdebe-lick,

W. J., 1990, ``Machining of New Materials,'' CIRP Ann., 39~1!, pp. 673± 681


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