Simulation of Thermal Effects for the Analysis of Micro Laser Assisted Machining

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Simulation of Thermal Effects for the Analysis of
Micro Laser Assisted Machining

• By
• Saurabh R Virkar
• Under guidance of Dr. John A Patten
• ICOMM 2010
• Venue University of Wisconsin, Madison

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Introduction

• Silicon Carbide (SiC) is an advanced engineered
  ceramic and an alternative to semiconducting
  Silicon (Si) for operation at elevated
  temperatures and high power applications. Some of
  SiCs beneficial properties include chemical
  resistance, high temperature resistance, extreme
  hardness and high stiffness
• Hardness of SiC 26 GPa
• The machining of SiC is difficult due to its high
  hardness and brittle nature.
• Ductile mode µ-LAM has been studied to replace
  grinding and polishing processes and to increase
  the material removal rates and maintaining the
  workpiece surface quality

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• In µ-LAM, the laser beam passes through the
  diamond tool, thus heating the surface just below
  the tool tip in the chip formation zone

Diamond tool
Schematic of µ-LAM
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High Pressure Phase Transformation

• The ductile material removal can be attributed to
  a High Pressure Phase Transformation (HPPT) at
  the tool-chip interface and the resultant phase
  is metallic or amorphous
• The HPPT occurs due to contact between the sharp
  tool and workpiece at or below critical depth of
  cut, i.e., below the ductile to brittle transition

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Why Simulations?

• Silicon Carbide (SiC) is very expensive
  semiconductor
• Measurement of temperatures at nano-scale is
  practically not possible
• Also the rate of heat transfer and pressures on
  tool and workpiece can be studied
• There is a metallic phase at tool chip interface
  due to high pressure phase transformation
• Software used AdvantEdge version 5.4
• Commercial software for machining solutions in
  metals developed by Third Wave Systems Inc.

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Objective

• To simulate different heating conditions over a
  temperature range for studying the laser heating
  effect
• To study the change in chip formation, cutting
  forces and pressures with changes in
  heating/temperature conditions

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Mathematical model

• Drucker Prager Yield Criterion
• (1)
• (2)
• Where I1 is first invariant of stress tensor
• (3)
• Where J2 is second invariant of deviatoric stress
  tensor
• Hence initial yield stress is given by
• (4)
• For uniaxial stress, s2 s3 0 and also sc s1
  H 26 GPa
• st H/2.2 11.82 GPa (for ceramics)
• Hence, ? 16.25 GPa and Drucker-Prager
  coefficient (a) 0.375

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Simulation Model
Workpiece Material properties
Material properties Value Units
Elastic Modulus, E 330 GPa
Poissons ratio 0.212 -
Hardness, H 26 GPa
Initial yield stress, s0 16.25 GPa
Reference plastic strain, e0p 0.049 -
Accumulated plastic strain, ep 1 -
Strain hardening exponent, n 50 -
Low strain rate sensitivity exponent, m1 100 -
High strain rate sensitivity exponent, m2 100 -
Threshold strain rate, etp 1E7 sec-1
Drucker-Prager coefficient (DPO) 0.375
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Thermal Softening Curve
Workpiece Thermal properties
Properties Value
Thermal Conductivity (W/cm K) 3.21
Thermal Cutoff temperature ( C) 1500
Melting temperature ( C) 2830
Initial reference temperature ( C) 20
Note The values for temperature from 20 C to
1500 C which is thermal cutoff temperature are
estimated based on various references. The value
for melting temperature of SiC is also estimated
from a reference.
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Tool parameters and geometries
Tool geometry
Cutting Edge Radius, r, (nm) 100
Rake angle, a - 45º
Relief angle, ß 5º
Width of tool (µm) 20
The -45 rake angle creates a high pressure
sufficient to accommodate the HPPT, thus the chip
formation zone is conducive for ductile
deformation
Tool Properties
Thermal Conductivity, W/m C 1500
Heat Capacity, J/kg C 471.5
Density, kg/m³ 3520
Elastic Modulus, GPa 1050
Poisson's ratio 0.2
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Simulated Thermal Effect Conditions

• Tooltip Boundary Condition
• Rake and Clearance face Heated Boundary Condition
• Workpiece Boundary Condition

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Tooltip Boundary Condition
A thermal boundary condition was provided on the
tool tip about 2µm on rake and clearance face
from cutting edge
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Workpiece Boundary Condition
A thermal boundary was provided on the workpiece
top surface
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Simulation parameters
Parameters Values
Feed (nm) 500
Cutting speed (m/s) 1
Width of cut (mm) 0.02
Co-efficient of friction 0.3

• Temperature range of the simulation work

• 20 C, 700 C, 1500 C, 2200 C and 2700 C where
  1500 C is the thermal cutoff point in the
  material model.
• From 20 C till 1500 C, the thermal softening
  curve has a 3rd order polynomial fit in the
  material model
• From the thermal cutoff point (1500 C) till
  melting point (2830 C) the curve is linear

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Constraints

• AdvantEdge does not provide for the direct
  incorporation of the laser heat source, thus the
  heating effect is modeled with these thermal
  conditions
• For this study, the crystalline dependency of
  the brittle behavior of SiC is not included in
  the model
• Note The temperature scale changes in each
  figure, as the minimum temperature is set
  slightly above and below the boundary condition
  temperature

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Tooltip Boundary Condition at 20 C
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Tooltip Boundary Condition at 2200 C
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Rake and Clearance face at 20 C
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Rake and Clearance face heated at 2200 C
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Workpiece Boundary Condition at 20 C
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Workpiece Boundary Condition at 2200 C
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Results
Temperatures ( C) Cutting Force (mN) Thrust Force (mN) Chip formation Pressure (GPa)
Tooltip Boundary Condition simulation 20 500 900 Yes 50
Tooltip Boundary Condition simulation 700 460 890 No 46
Tooltip Boundary Condition simulation 1500 370 610 No 37
Tooltip Boundary Condition simulation 2200 200 300 No 20
Tooltip Boundary Condition simulation 2700 80 130 Yes 8
Workpiece Boundary Condition simulation 20 470 1040 Yes 47
Workpiece Boundary Condition simulation 700 450 1000 No 45
Workpiece Boundary Condition simulation 1500 390 570 No 39
Workpiece Boundary Condition simulation 2200 200 260 No 20
Workpiece Boundary Condition simulation 2700 30 40 No 3
Toolface Boundary Condition 20 500 1060 Yes 50
Toolface Boundary Condition 700 450 1000 No 45
Toolface Boundary Condition 1500 380 620 No 38
Toolface Boundary Condition 2200 200 300 No 20
Toolface Boundary Condition 2700 60 90 Yes 6




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Force plots (All simulation conditions)
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Cutting Pressure Plots (All Boundary Conditions)
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Conclusions

• The thermal effects successfully simulated the
  laser heating effect
• Decrease in cutting forces and pressures is
  studied with increase in temperature
• The change in chip formation due to change in
  temperature above and below the thermal cutoff
  point is studied is studied

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On-going work

• To determine the effect of interaction between
  temperature and compressive stress on the cutting
  forces and pressures from room temperature till
  melting point of SiC
• 3D scratch test simulations for comparison with
  experiments

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Acknowledgement

• Support from NSF (CMMI-0757339)
• Support from ThirdWave Systems