Laser Machine Project

8/13/2019 Laser Machine Project

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1) Abstract2) Introduction of the problem undertaken3) Machine and workpiece specifications4) Literature review5) Experimental Analysis6) Conclusions7) Future scope8) References


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The present study is an effort to understand material removal mechanism in laser

ablation of metals by focusing on laser drilling. The change in material removal

mechanism with variation in power density of laser was studied.

Experiments were conducted to study the effect of input power on MRR and the

sample was then analyzed under a 5X microscope to find entry hole diameter, exit

hole diameter and taper angle in continuous and pulsed laser drilling of titanium.


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The basic concept of laser involves providing the lasing material (in our case glass

fibre) with energy so as to excite electrons to higher energy states and when these

electrons come back to their ground state they emit photons. These photons arereflected back and forth by the mirrors at the two ends of glass fibre thus exciting

more electrons. Finally, photons with sufficient energy escape through the end

producing a narrow laser beam.

Laser ablation is the process of removing material from a solid surface by

irradiating it with a laser beam. Usually, laser ablation refers to removing material

with a pulsed laser, but it is possible to ablate material with a continuous wave

laser beam if the laser intensity is high enough.

In this paper we have tried to investigate laser ablation by focusing on laser drilling

in continuous-wave and pulsed forms of the laser. The approach was as follows:

Understanding basic material removal mechanism in laser drilling and variousfactors affecting it by performing a literature review for the same.

Further studies were conducted by referring to published papers to know theaffect of machine parameters such power, frequency and duty cycle on process

parameters such as MRR, taper of drilled holes etc.

Experiments were conducted to verify the results of the referred journals.


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MACHINE SPECIFICATIONS:Glass fibre laser

Max power: 100 WMax frequency: 100 kHz

Pulse width: of the order of s

Intensity type: Gaussian

WORKPIECE SPECIFICATIONS:Workpiece material: TitaniumWorkpiece thickness: 500 m

Specific heat (C) : 560 J/kg.K

Latent heat of melting : 360 kJ/kg

Melting temperature: 1950 K


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MATERIAL REMOVAL MECHANISM

Laser drilling is based on the absorption of the laser energy by the workpiecematerial and the conversion of the photon energy into thermal energy. When the

temperature exceeds that of the melting and/or vaporization, the workpiece

material changes phase and the hole geometry is formed. If the laser irradiance is

kept below a certain threshold (typically 106 W/cm2 for steels) the workpiece

material is melted and not vaporized. In that case, the hole is formed due to

ejection of the melted material with the use of an assisting gas jet. For laser

irradiance values beyond the threshold value, the material is removed mainly due

to vaporization.

MATERIAL REMOVAL RATE IN CW LASER DRILLING

In continuous-wave laser drilling, as power increases more energy is provided to

the surface and melting temperature is attained faster. Thus, it is obvious that with

increase in input power, material removal rate (MRR) in CW laser drilling

increase.

NO. OF PULSES REQUIRED TO DRILL IN PULSED LASER DRILLING

In case of pulsed laser, rather than MRR, number of pulses for melting the sampleis a better measure. A fixed amount of energy needs to be supplied for drilling to

occur .If we assume that cooling can be neglected then, at a fixed duty cycle and

frequency, as power increases the number of pulses required for drilling will

decrease.

HOLE ENTRANCE DIAMETER

We are using a Gaussian beam in this experiment, which has an intensity equation:

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The rise in temperature at point (x,y,z) using such a beam in drilling if laser is

located at (0,0,0) is given by:

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Hence, when power is increased a larger area attains sufficient temperature above

melting temperature and therefore as power increases hole entry diameter

increases.

HOLE TAPER

Holes drilled using lasers are not perfect and have taper present in them. This is

due to the fact that as laser beam penetrates the material the energy is conducted tosurrounding metal and a smaller area attains a temperature greater than that of

melting. Therefore, laser drilled holes are tapered.

dent=entry diameter, dexit=exit diameter

!= Taper angle, t=thickness of material

At high input power, time required for laser to go through and through the

workpiece decreases and hence very less amount of heat spreads to the surroundingmaterial. Thus, the taper angle decreases with increase in input power.


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The machine and workpiece specifications have been already mentioned. The

following experiments were carried out to verify the literature review given in the

previous section:

! CONTINUOUS WAVE LASER DRILLING Material Removal Rate VS Power

Laser drilling was carried out on the workpiece (titanium) and time for

drilling was measured. After each drilling operation workpiece weight was

taken. Using difference of weight after respective drills, density of titanium

and time measured, MRR was calculated.

Frequency => 10 kHz

SR. NO. POWER (W) MRR (mm3/min)

1. 40 3.25

2. 60 3.35

3. 80 4.00

4. 100 4.24

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Hole entry diameter VS PowerThe drilled hole was observed under 5X microscope and hole entry

diameter was measured.

Frequency = 10 kHz

SR. NO. POWER (W) ENTRY DIA. (m)

1. 40 29.55

2. 60 32.01

3. 80 38.49

4. 100 43.22

Taper VS PowerHole entry and exit diameters were measured by observing the drilled hole

under 5X microscope and following formula was used to find taper angle:

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Frequency = 10 kHz

SR. NO. POWER (W) Entry dia. (m) Exit dia. (m) Taper (deg.)

1. 40 29.55 15.22 0.82

2. 60 32.01 20.63 0.65

3. 80 38.49 29.77 0.5

4. 100 43.22 36.60 0.38

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!PULSED LASER DRILLING No. of pulses to melt VS Power

Holes are drilled using pulsed laser and time is measured. Then, using the

frequency of machine number of pulses are calculated.

Frequency = 100 Hz , Duty cycle = 50%

SR. NO. POWER (W) TIME (s) No. of Pulses

1. 10 145.22 14522

2. 20 42.9 4290

3. 40 10.68 10684. 60 3.85 385

5. 100 0.94 94

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Hole entry diameter VS PowerIt is performed in the same way as described in the CW laser drilling

section.

SR. NO. POWER (W) ENTRY DIA. (m)1. 20 58.21

2. 40 61.91

3. 60 64.02

4. 80 65.33

Taper VS PowerUsing the same formula as in the CW case;

SR. NO. POWER (W) Entry dia. (m) Exit dia. (m) Taper (deg.)

1. 20 58.21 27.00 1.79

2. 40 61.91 34.18 1.59

3. 60 64.02 40.07 1.37

4. 80 65.33 46.29 1.09

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As predicted from theory, with increase in input power, MRR increases since more

amount of material gets the minimum threshold energy required for laser drilling.

At constant duty cycle, with increase in input power, the number of pulses required

to melt the material for laser drilling decreases which is obvious since more

amount of energy is contained in each pulse at high input power. Ideally, the graph

of number of pulses vs. input power should be hyperbolic but some energy is lost

because of cooling of material between the pulses. Hence, the number of pulses

required at low input power is quite high.

As the laser beam strikes the surface, the material on surface absorbs the energy

and melts and provides a way for laser to go further deep, thus creating a hole onthe surface. With increase in input power, more material gets melted and thus the

entry-hole diameter increases.

As the laser goes further deep in the workpiece, its energy spreads into the

surrounding material and hence, less amount of material gets the minimum

threshold energy required for melting and so, the hole diameter decreases resulting

in slightly tapered hole (1 ~ 2 degrees).

The trend of taper angle is also in agreement with literature read.


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More insight into laser ablation can be obtained by:

1) Study of heat affected zones.

2) Using uniform circular/rectangular laser beam instead of Gaussian circular beam

3) Study of the effect of duty cycle on MRR.

4) Performing laser cutting instead of laser drilling.

5) Study of machined surface using interferometer.

6) Performing experiment with varying laser beam diameter.

7) Study of the impact of melting and boiling temperature of workpiece on MRR.


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1.A theoretical and experimental investigation on limitations of pulsed laserdrilling-

Konstantinos Salonitis, Aristidis Stournaras, George Tsoukantas, Panagiotis

Stavropoulos, George Chryssolouris

2. Comparative statistical analysis of hole taper and circularity in laserpercussion drilling

M. Ghoreish, D.K.Y. Low, L. Li

3.Laser drilling of high aspect ratio holes in femtosecond, picosecond andnanosecond pulses

A. Weck, T.H.R. Crawford, D.S. Wilkinson, H.K.Haugen, J.S. Preston

4.A study of thermal and mechanical effects on materials induced by pulsedlaser drilling A. Luft, U. Franz, A. Emsermann, J. Kaspar

5.Parametric study to improve laser hole drilling process-B.S. Yilbas

6.Notes on laser micromachiningProf. Ramesh Singh

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Laser Machine Project