5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

IIT Guwahati, Assam, India

536-1

APPLICATIONOFPUREWATER JET MACHINING FOR IMPROVING

SURFACE FINISH OF PARTS FABRICATED BY ABRASIVE WATER

JET MACHINING

Vijay Kumar Pal1*, S.K. Choudhury2

1*Ph.D. Scholar, Indian Institute of Technology Kanpur, Kanpur, 208016, Email: vijayp@iitk.ac.in

2Professor, Indian Institute of Technology Kanpur, Kanpur, 208016, Email: choudhry@iitk.ac.in

Abstract

Fabrication of 3-D features is a major research interest in Abrasive Water Jet (AWJ) process, but the poor

surface quality of machined components restricts the process for being widely used. Present work initially

focuses on fabrication of micro channels by AWJ and analyses the effect of process parameters namely pressure,

traverse speed and stand-off distance on depth and surface finish of samples machined. Experiments were

performed on Ti-6Al-4V alloy of 1 mm thickness sheet and Central rotatable Composite Design (CCD) test

matrix with an alpha value of 1.68 was used for design of experiment. The correlations between the process

parameters and responses like depth and surface roughness were established by multiple linear regression

models. Experimental observations show that the depth is affected most by pressure, followed by traverse speed.

The combination of high pressure and fast traverse speed results in a quite smooth surface because high pressure

provides sufficient jet energy for smooth fracture. Second part of this paper presents an innovative path strategy

to improve surface quality of machined samples. Here, AWJ was used for rough/stock removal of material

followed by pure water jet (PWJ) along the same path (movement of the nozzle) as a final cut to improve

surface quality of machined samples. 3-D optical profilometer with objective lens (5x) and field of view (FOV

2x) along with the SPIP software was used to measure geometry and profile of slots. Digital microscope of 230x

and a scanning electron microscope (SEM) were used to observe and analyse the micro structure of the

machined pockets. The SEM investigation demonstrated that for all the samples machined by PWJ (as finishing

pass), the material removal mechanism is uniform and surface was found smoother than in case of AWJ and

embedded particles were also removed to a certain extent. Keywords: Abrasive water jet (AWJ), pure water jet (PWJ), Traverse speed, 3-D optical profilometer, SEM

1 Introduction

Pure water jet machining (PWJ) was traditionally

used for cutting soft materials, cleaning and removal

of coating in early 70s. Abrasive particles were mixed

with high velocity water jet to improve the efficiency

of the process in terms of material removal rate and

making it possible to cut a wide variety of materials

ranging from soft to hard. Now, Abrasive water jet

machining (AWJM) has become a significantly

emerging manufacturing process with its enormous

capabilities of machining different materials and high

speed of cutting. Initially AWJM technique was used

only for shape cutting (through cuts) of different

materials. It is a non-conventional machining process

in which a mixture of abrasive particles with high

pressure water was converted to a high velocity jet for

cutting. The high speed abrasive water jet machining

employed the erosion phenomenon for material

removal when the abrasive particles along with high

velocity water hit the target surface as explained by

Finnie, (1960). Less fixture requirements and almost

no heat affected zones due to non-contact between the

cutting tool and work piece are some of the major

advantages of this technique. Process primarily

depends on the following input parameters – abrasive

flow rate, traverse speed, standoff distance (SOD),

water jet pressure, shape and size of abrasive

particles. This process is well established for through

cutting and most of the works reported was based on

through cutting by AWJM.

Nowadays, researchers have also started

experimenting on generating blind features using

AWJM. For generating blind features like pockets and

channels, several authors used the multiple passes

linear traverse cutting as milling strategy. This

principle is based on the superposition of several

passes to obtain a cavity of defined geometry. The

APPLICATION OF PURE WATER JET MACHINING FOR IMPROVING SURFACE FINISH OF PARTS FABRICATED BY

ABRASIVE WATER JET MACHINING

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lateral distance between the single kerf/passes is the

main parameter in this process Laurinat et al., (1993),

which is kept less than the diameter of the jet (d).

Hashish (1994) used the principles of rotary table and

masking to perform a controlled depth milling of iso-

grid structures. Fowler et al., (2005) have developed

the process of controlled depth milling (CDM) and

studied the effects of various parameters like traverse

speed, jet impingement angle, milling direction, grit

size, etc. on surface characteristics while machining

titanium alloy. Pal and Tandon, (2011) studied the

role of machinability in AWJ-CDM for materials like

AL 6061 alloy, AL 2024, Brass 353, Titanium AISI

304 (SS) and Tool Steel. The authors observed that,

time taken to mill increases as the depth of milling

increases non-linearly due to loss of energy of jet and

increase in stand-off distance (SOD).

Axinte et al. (2010) introduced a plain water jet

milling on titanium material to make pockets and

observed plain water jet machining gives a better

surface finish than abrasive water jet machining and

there is no side effect were formed like corrosion etc.

because in PWJ and AWJ process the mechanism is

almost same. However, no comprehensive work has

been reported, to the best of author’s knowledge,

trying to study the effect of PWJ on different

materials after performing AWJM. In this work,

Experiments were performed to generate micro slot

by using AWJ process on Ti-6Al-4V alloy of 1 mm

thickness sheet. Experiments with AWJ were carried

out by varying the process parameters namely,

pressure, traverse speed and stand-off distance.

Central rotatable Composite Design test matrix with

an alpha value of 1.68was used to analyse responses

on depth and surface finish. To analyse the effect of

PWJ on machined sample by AWJ, experiments were

performed on the same path used in AWJ followed by

PWJ.

2 Experimental details

Experiments were performed to generate micro

channels by using AWJ process on Ti-6Al-4V alloy of

1 mm thickness sheet. The analysis were carried out

by varying the process parameters namely, Pressure

(A), stand-off distance (B) and traverse speed (C).

Central rotatable composite design (CCD) test matrix

with an alpha value of 1.68 was used to analyse

responses on depth and surface finish. Each input

parameter was varied at five different levels within a

given domain. In this way, 20 experiments were

performed to develop depth and surface finish models.

The range of process parameters and their levels are

given in Table 1. In the given setup, the optimum

values of parameter like abrasive flow rate was

considered based on standard operating conditions of

AWJ machine available.

The experiments were conducted with a commercial

abrasive water jet machine (OMAX Corp.) with

integrated software of the controller. It consists of a

machine bed with underneath catcher tank and the

specimen mounted on the bed.

Table 1 process parameters

Parameter lowest Highest

Pressure (MPa) 103 241

Standoff distance (mm) 2 6

Traverse speed (mm/min) 2000 5000

Fig.1 shows the experimental set-up of AWJ

system. The nozzle assembly consists of an orifice

with a diameter of 0.3 mm; which converts the highly

pressurized water into a collimated jet, mixing

chamber, abrasive feeding tube and a focusing nozzle

of internal diameter of 0.762 mm. All the trials were

conducted at 90o impingement angle (perpendicular to

the work surface) and abrasive flow rate was kept

constant in all the experiments without changing the

tungsten carbide mixing tube and sapphire orifice

during the experimentation.

Fig. 1 Experimental setup

2.1 Methodology adopted in the present work to

improve surface finish material and cutting insert

In this methodology we have used pure water jet for

the improvement of surface quality of the machined

specimens which were manufactured by AWJ. The

first half of the figure (Fig. 2) shows the channels

manufactured by AWJ and the later half shows that

machining was performed with AWJ as in the first

pass and on the same path PWJ (second pass) was

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

IIT Guwahati, Assam, India

536-3

used to improve its surface morphology. The nozzle

path was same in both the cases.

AWJ machining was used for rough/stock removal

of material because of its high MRR followed by pure

water jet (PWJ). In PWJ process, the pressure energy

of water with no abrasive particles is converted into

kinetic energy when the high pressure water passes

through an orifice and hit the surface. Since PWJ is

not suitable for machining hard materials because of

low MRR, therefore in the present methodology PWJ

was used only for finish pass (last pass) at higher

pressure (35 Kpsi).

Fig. 2 Methodology to improve surface quality

3 Measurement

3.1 Measurement of slot geometry

The geometry of the machined slots by AWJ was

measured through a 3-D optical profilometer

embedded with the Scanning Probe Image Processing

(SPIP) software. Optical profilometer, having a large

scanning range, generates high resolution 3D and 2D

images. Objective lens (5X) and FOV (2X) were used

in the optical profilometer. Scanned 3-D image of the

machined slot is shown in Fig. 3. The Surface

roughness of the samples was measured in three

different places along the length of the slots. Digital

microscope of 230x was used to observe the nature of

the machined surface and details were studied by

scanning electron microscope (SEM), Model-FESEM

SUPRA 40 VP CAL ZEISS. Experimental matrix and

results of depth of channels and surface roughness

when machined through AWJ is shown in Table 2.

Fig. 3 Optical image (3-D) of channel (Ti-6Al-4V)

Table 2 Experimental matrix showing depth and

surface finish

Pressure

(MPa)

Stand-

off

distance

(mm)

Traverse

speed

(mm/min)

Depth

of slot

(µm)

Ra

(µm)

131 2.81 2608 210 6.2

213 2.81 2608 235 6.9

213 2.81 4391 183 7.8

131 2.81 4391 170 7.2

131 5.19 2608 200 6

131 5.19 4391 153 6.8

213 5.19 4391 168 7.1

213 5.19 2608 200 7.6

172 4 3500 172 7.3

172 4 5000 178 8

172 4 3500 184 7.3

241 4 3500 198 7.6

172 4 3500 180 7

103 4 3500 146 5.6

172 4 2000 248 6.1

172 4 3500 177 7.2

172 4 3500 169 7.4

172 2 3500 175 7.5

172 6 3500 153 6.9

172 4 3500 175 7.3

3.2 Depth and surface roughness models:

Regression equation of depth and surface roughness

was developed based on experimental data. The

coefficients involved in the equation were calculated

by design expert software. Equations developed for

depth achieved and surface roughness for channels

machined by AWJ is given below:

(a) Depth of channel (Machined by AWJ)

2 2 2

175.96 10.28 8.35 21.14

2.87 0.38 1.63

0.11 2.94 14.39 ..............(1)

Depth P SOD TS

P SOD P TS TS SOD

P SOD TS

= + × − × − ×

− × × + × × + × ×

− × − × + ×

(b) Surface roughness (Machined by AWJ)

2 2 2

7.25 0.51 0.081 0.40

+0.075 0.17 0.20

0.24 0.048 0.048 ...........(2)

Ra P SOD TS

P SOD P TS TS SOD

P SOD TS

= + × − × + ×

× × − × × − × ×

− × − × − ×

Where, P represents pressure, SOD represents stand-

off distance and TS represents traverse speed. The

adequacies of developed model were checked by

Analysis of Variance (ANOVA) method. In present

work, the values of R-squared are closer to 1; adjusted

and predicted R-squared values are in the reasonable

agreement.

AWJ channels AWJ + PWJ channels

AWJ passes

PWJ passes

APPLICATION OF PURE WATER JET MACHINING FOR IMPROVING SURFACE FINISH OF PARTS FABRICATED BY

ABRASIVE WATER JET MACHINING

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4 Result and discussions

In this section, effect of process parameters of AWJ

on depth and surface roughness is discussed based on

developed regression equations (Sec. 3.2). 3-D

surfaces for interaction effects on depth and surface

roughness of process parameters in different

responses are plotted.

4.1 Effect of parameters on depth of channels cut

by AWJ

Fig. 4 depicts the 3-D surface plots of depth of

channels machined by AWJ. The interaction effects of

process parameters namely, pressure (A), stand-off

distance (B) and traverse speed (C) on depth can be

seen. It can also be seen from the figure that depth of

the slots decreases rapidly with increase in traverse

speed because at high traverse speed the exposure

time on machined area is less.

As the pressure increases, the depth of the pocket

increases due to high kinetic energy of the jet (Fig.

4(b)). Depth of the slot is slightly affected by the SOD

with the increase in the SOD, initially the depth of the

slot increases and then starts decreasing with the

further increase in the SOD. This can be explained by

the fact that, increase in SOD beyond a certain value

divergence of the jet increases and the jet losses some

of its kinetic energy affecting the process.

4.2 Effect of parameters on surface finish of

channels cut by AWJ

Fig. 5 depicts the 3-D surface plots of surface

roughness on machined channels by AWJ. In general

it was found that, as the traverse speed increases the

surface roughness increases. This can be explained by

the fact, that at high traverse speed the number of

abrasive particles impinging machined area decreases

which creates rougher surface. The combination of

high traverse speed and low pressure also creates

quite rough surface because this combination does not

provide sufficient kinetic energy on the machined

surface for smooth fracture resulting in a rough

surface.

Fig. 4. 3D surface plots for interaction effects (a)

SOD x TS (b) P x TS (c) P x TS

It can also be seen from Fig.5, that at larger SOD,

the surface finish improves but it requires sufficient

energy of jet i.e, the effect is more prominent at

higher pressure because at high SOD, the divergence

of the jet increases and the jet losses some of its

kinetic energy during the cutting which can be

compensated at higher pressure. It can also be seen

that surface roughness gets affected significantly at

low traverse speed and at higher pressure.

(a)

(b)

(a)

(c)

(b)

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

IIT Guwahati, Assam, India

536-5

Fig. 5. 3D surface plots for interaction effects (a) P

x SOD (b) P x TS (c) SOD x TS

4.3 Investigation of surface morphology of

samples performed by PWJ as a finished pass

The experiments were performed by AWJ and a

few of these samples were later made to undergo

through PWJ as a finishing pass. This section

investigates the effect of PWJ finishing pass on

Surface quality.

Fig. 6 (a) SEM images of AWJ

Different magnification SEM images were taken for

in-depth analysis. It can be seen from the images that,

for AWJ machined surfaces the erosion rate is more

and depth (penetration) increases but also generates

quite rough surface.From SEM analysis, removal of

burrs and embedded abrasive particle (Fig. 6 (a)-(b))

was also observed on most of the machined samples.

The surface morphology was observed to be

significantly different in case of by PWJ in

comparison to AWJ. It can be seen that after PWJ the

have finer surfaces because material removal was

uniform due to absence of abrasive particles while at

AWJ surfaces were of much higher roughness values

because of non-identical abrasive particles creating

uncertainty in material removal mechanism.

(c)

AWJ

AWJ

AWJ

PWJ

PWJ

APPLICATION OF PURE WATER JET MACHINING FOR IMPROVING SURFACE FINISH OF PARTS FABRICATED BY

ABRASIVE WATER JET MACHINING

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Fig. 6 (b) SEM images of PWJ

4 Conclusion

The process parameters of AWJ were analysed to

fabricate micro channels on Ti-6Al-4V alloy. The

most influential of these parameters were determined

by establishing regression models of depth and

surface roughness using multiple linear regression. To

improve the Surface quality of machined samples by

AWJ, a novel methodology has been adopted in

which pure water jet was used as a final pass. Based

on the experiments and microscopic investigation

following conclusions can be drawn:

• Depth of the slot is relatively unaffected by

the stand-off distance (SOD). With increase

in the SOD, initially the depth of the slot

increases and then starts decreasing with the

further increment in the SOD because

divergence of the jet increases and the jet

losses some of its kinetic energy.

• The proper combination of higher traverse

speed and higher pressure creates reasonably

high finished surface because this

combination provides sufficient kinetic

energy to the jet.

• Detailed SEM analysis demonstrated that,

AWJ machined surface generated quite

rough surface than PWJ because material

removal is uniform (no abrasive particles) in

PWJ while at AWJ surfaces were found to be

of higher roughness value because of non-

identical abrasive particles participating in

material removal.

References

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particles, Wear Vol. 3, 87-103.

Fowler G., Shipway PH., Pashby, IR., (2005), A

technical note on grit embedment following abrasive

water jet milling of titanium alloy, Journal of

Material Processing Technology, Vol. 159, 356-368.

Fowler G., Shipway P. H., Pashby I. R., (2005),

Abrasive water-jet controlled depth milling of

Ti6Al4V alloy – an investigation of the role of jet

workpiece traverse speed and abrasive grit size on the

characteristics of the milled material, International

Journal of Material Processing Technology, Vol. 161,

407-414.

Hashish M., (1994), Controlled-depth Milling

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(ed) 1994 Jet Cutting Technology, Mechnical

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Kong M.C., Axinte D., Voice W., (2010), Aspects of

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PWJ