Abinitio Computational Approach to

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Ab-initio Computational Approach to Laser
Micro-machining of Structural Ceramics
Anoop N. Samant, Narendra B. Dahotre
Laboratory for Laser Materials Synthesis
Fabrication Department of Materials Science
Engineering, University of Tennessee, Knoxville,
TN
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OUTLINE

• Objectives
• Structural Ceramics
• Methodology
• Laser Machining of Structural Ceramics
• Physical Phenomena in Machining
• Data Analysis
• Contribution of current work
• Significance of research
• Future Work
• Conclusions

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OBJECTIVES

• Demonstrate feasibility of laser machining of
  structural ceramics.
• Understand material removal mechanisms (MRM)
• Develop an ab-initio computational model based on
  MRM.
• Use model for advance predictions of laser
  processing conditions to attain desired
  attributes.
• Save considerable energy and time.

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STRUCTURAL CERAMICS

• Properties
• Low thermal and electrical conductivity
• High hardness
• Chemical stability
• High thermal resistance
• Applications
• Machine tools, valves, bearings, rotors
• Optical and Electronic devices
• Hazardous Waste Disposal
• Examples
• Silicon Carbide, Alumina, Silicon Nitride,
  Magnesium Oxide, Zirconia

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LASER MACHINING

• An operation similar to laser drilling
  subsequently conducted on neighboring locations.
• Advantages
• Non contact processing
• Capability of automation
• Reduced manufacturing costs
• Efficient material utilization
• Reduced heat-affected zone (HAZ)
• High productivity

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LASER MACHINING
Fig.3 Types of Laser Machining 2
1Kalpakjian, Serope and Steven R. Schmid,
Manufacturing Engineering and Technology, Upper
Saddle River, New Jersey Prentice Hall, Inc,
2001. 2Samant and Dahotre, Journal of European
Ceramic Society, 29(2009) 969.
Fig.2 Laser Machining 1
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METHODOLOGY

• Come up with optimum pulse width, pulse energy,
  and pulse repetition rate to develop sufficient
  laser-ceramic interaction.
• Vary processing parameters to machine cavities of
  different dimensions.
• Develop 3D-thermal model to generate temperature
  profiles.
• Incorporate different physical phenomena into the
  developed model.
• Correlate predicted attributes of machined
  cavities with observed features.

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SILICON CARBIDE MACHINING
Table 1. Laser Parameters (JK 701 pulsed NdYAG
laser )

• 25 and 125 pulses machined 2 and 3 mm plates at 6
  J, 0.5 ms and 50 Hz.

Fig.4 Through holes in 2mm and 3mm SiC plates 1.
1 Samant et. al., International Journal of
Advanced Manufacturing Technology, in press.
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TEMPORAL EVOLUTION

• Fouriers Second Law for maximum surface
  temperature
• Radiation losses at the surface

(1)
(2)

• Convection taking place at the bottom

(3)
e - emissivity , k(T) - thermal conductivity, T0-
initial temperature, h(T) - heat transfer
coefficient, H plate thickness, a
absorptivity (1 due to multiple reflections. 1)
1 Mazumdar et. al., J.Appl.Phys., 51(2), 1980
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TEMPORAL EVOLUTION

• Temperature during pulse OFF time 1

(4)
Ti - Temperature during heating of pulse i, toff
- OFF period between successive pulses, erf -
error function, a(T) - thermal diffusivity

• Temperature during pulse ON time 1

(5)
Ti-1 - temperature during cooling of the
earlier pulse, ton - pulse duration, P
incident beam power
1 Konstantinos et.al. , Journal of Materials
Processing Technology 183 (2007), 96.
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TEMPORAL EVOLUTION

• Temperature drops during the OFF time and rises
  during the ON time of the laser giving the
  heating curve a meandering nature.
• Maximum surface temperature reached while
  processing 3mm thick plate is higher than that
  reached while processing 2 mm plate.
• High temperatures exist for extremely short time
  and rapidly drops due to self quenching.

Fig.5 Heating curves for a) 2mm and b) 3mm thick
SiC plate
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EVAPORATION LOSSES

• Rate of evaporation

(6)
mv - mass of vapor molecule, Ts-surface
temperature, k - Boltzmann Constant , p(Ts) -
saturation pressure, p0 ambient pressure

• Material loss

(7)

• Corresponding drop in temperature

(8)
Lv - latent heat of evaporation
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DISSOCIATION ENERGY LOSSES

• Possible dissociation species
• Si(l), C(s), C(g), Si(g), Si3(g), Si2(g),
  SiC2 (g), Si2C(g) and Si(s)
• Most likely reaction at 3103 K

(9)

• Volume of hole

(10)
(assuming cylinder of diameter
(Volume/ 22.4 x 10-3) moles)
zava available melt depth ztotal - zeva
melt depth from surface - evaporated depth

• Energy Loss

(11)
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RECOIL PRESSURE

• Cause Evaporation of the melt surface exposed
  to laser.
• Effective melt depth zeff Portion remaining
  after expelling a fraction of the total melt
  depth.
• Recoil pressure pr Is a function of the
  material properties, maximum surface temperature
  and input energy 1

(12)
1 Anisimov , Sov. Phys. JETP 27 (1968), 182
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SURFACE TENSION

• Surface tension effect Modifies pressure on
  melt, thus affecting depth of machined cavity
  1.
• Effective beam radius Beam radius changes with
  change in machined depth 2 .
• Velocity of expulsion 3

(13)
(13)
(14)
(14)
t - surface tension coefficient of liquid Si , t
time, reff effective beam radius, ? - density
1 Han et.al., Journal of Heat Transfer, 127
(2005),1005. 2 Salonitis et.al., Journal of
Materials Processing Technology 183 (2007),
96 3 Matsunawa et.al., J. Phys. D Appl. Phys.
30(1997),798.
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MACHINED CAVITY DEPTH

• Machined cavity Depth of cavity formed due to
  expulsion of available melt depth 1

(15)
zexpelled Depth expelled at different
time instants
(16)
zt Total cavity depth formed at a certain time
instant
Fig.6 Cavity Evolution

• Predicted pulses 21 and 103 pulses for machining
  through a 2 and 3 mm plate.

1 Semak et. al., J. Phys. D Appl. Phys. 39
(2006),590.
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MACHINED CAVITY DEPTH

• Till t2, material expelled in upward direction.
• Around t3, direction of material expulsion
  reversed due to least resistance to the recoil
  pressure by small mass of supporting material at
  the bottom.
• At t4, all the rest of molten material expelled
  and a clean through cavity formed.

Fig.7 Stages of Cavity Evolution
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ALUMINA MACHINING

• Applications Substrate in hybrid circuits,
  aerospace industry.
• Dissociation at 3250K
• Machining mechanism Dissociation, melt expulsion
  by recoil pressure and evaporation.
• Predicted pulses 3, 7, 16 and 19 pulses for
  machining 0.26, 0.56, 3.23 and 4.0 mm
  respectively at 0.5ms, 4J and 20Hz.

(17)
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ALUMINA MACHINING
Fig.10 Evolution of cavities 1
Fig.9 Cavities in alumina 1
1 Samant and Dahotre, Int. Journal of Machine
Tools and Manufacture, 48 (2008), 1345.
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MAGNESIUM OXIDE MACHINING

• Applications Refractory and brake linings, thin
  film semi-conductors.
• Dissociation at 3123K
• Machining mechanism Dissociation followed by
  evaporation.
• Predicted machining times 0.11, 0.2, 0.25 and
  0.8 s for machining 0.25, 0.86, 1.54 and 3mm
  respectively at 0.5ms, 4 J and 20 Hz.

(19)
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MAGNESIUM OXIDE MACHINING
Fig.14 Evolution of cavities with time 1
Fig.13 Cavities in MgO 1
1 Samant and Dahotre, Optics and Lasers in
Engineering, 47(2009),570.
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PHYSICAL PHENOMENA IN DIFFERENT CERAMICS
Table 2. Physical Phenomena Governing Machining
in Different Ceramics 1
Physical Process
Material
1 Samant and Dahotre, Ceramics International,
in press.
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FLOW CHART
Fig.8 Flowchart for computations

• Stepwise procedure to achieve final machining
  parameters starting with material properties and
  process parameters.

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DATA ANALYSIS
Table 3. Comparison between experimental and
predicted attributes of machined cavities
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CONTRIBUTION OF CURRENT WORK

• Prior work conclusions a) Machining comprises of
  melting and material removal by expulsion 1.
• b) Machining takes place by single step
  evaporation without melting 2.
• c) Effect of multiple reflections neglected.
• Current work conclusions a) Material removal
  occurs by a combination of melt expulsion,
  dissociation and evaporation.
• b) Multiple reflections affect the amount
  of absorbed energy.

1 Salonitis et.al, Journal of Materials
Processing Technology, 183(2007) 96. 2 Atanasov
et. al, Journal of Applied Physics, 89(2001) DOI
10.1063/1.1334367
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SIGNIFICANCE OF RESEARCH

• Proposed systematic approach is an advancement of
  existing computational approach to ceramic
  machining.
• Advance prediction of number of pulses/ pulse
  duration/ pulse energy possible.
• Developed model can be extended to two and three
  dimensional laser machining.
• A system with optimum machining rate can be
  generated.

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FUTURE WORK

• Laser Machining in 2 D (Laser Cutting) and 3D
  (engraving complex shapes).
• Effect of multiple passes on machined depth by
  considering preheating effect.
• In-situ temperature measurements and absorptivity
  predictions using thermocouples.

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SUMMARY

• Structural ceramics were successfully machined.
• Theoretical model incorporating several vital
  effects encountered during machining was
  developed.
• Model predictions compared well with experimental
  observations for machining.
• Model aids to provide an advance estimate of
  number of pulses required for machining required
  depth or the depth machined after applying
  certain number of pulses.
• Laser Fluence and machining time could also be
  predicted.

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THANK YOU