by saurabh r virkar under guidance of dr. john a patten icomm 2010 venue: university of wisconsin,...
TRANSCRIPT
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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 SiC’s 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
SiC
Schematic of µ-LAM
Diamond tool
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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, σ2 = σ3 = 0 and also σc = σ1 = H= 26 GPa
σt = H/2.2 = 11.82 GPa (for ceramics)
Hence, К= 16.25 GPa and Drucker-Prager coefficient (α) = 0.375
213
232
2212 6
1 J
ct
ct
2
03 12 IJ
3211 I
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Simulation Model
Material properties Value UnitsElastic Modulus, E 330 GPaPoisson’s ratio 0.212 -Hardness, H 26 GPaInitial yield stress, σ0 16.25 GPa
Reference plastic strain, ε0p 0.049 -
Accumulated plastic strain, εp 1 -Strain hardening exponent, n 50 -
Low strain rate sensitivity exponent, m1 100 -
High strain rate sensitivity exponent, m2 100 -
Threshold strain rate, εtp 1E7 sec-1
Drucker-Prager coefficient (DPO) 0.375
Workpiece Material properties:
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Thermal Softening Curve
Properties ValueThermal Conductivity (W/cm K) 3.21*Thermal Cutoff temperature ( C)⁰ 1500*Melting temperature ( C)⁰ 2830Initial reference temperature ( C)⁰ 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.
Workpiece Thermal properties:
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Tool parameters and geometries:
Cutting Edge Radius, r, (nm) 100
Rake angle, α - 45º
Relief angle, β 5º
Width of tool (µm) 20
Tool geometry:
Thermal Conductivity, W/m C⁰ 1500Heat Capacity, J/kg C⁰ 471.5Density, kg/m³ 3520Elastic Modulus, GPa 1050Poisson's ratio 0.2
Tool Properties:
The -45 rake angle creates a high pressure sufficient to accommodate the HPPT, ⁰thus the chip formation zone is conducive for ductile deformation
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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
700 460 890 No 46
1500 370 610 No 37
2200 200 300 No 20
2700 80 130 Yes 8
Workpiece Boundary Condition simulation
20 470 1040 Yes 47
700 450 1000 No 45
1500 390 570 No 39
2200 200 260 No 20
2700 30 40 No 3
Toolface Boundary Condition
20 500 1060 Yes 50
700 450 1000 No 45
1500 380 620 No 38
2200 200 300 No 20
2700 60 90 Yes 6
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Force plots (All simulation conditions)
20 700 1500 2200 27000
200
400
600
800
1000
1200
Cutting and Thrust Forces
Tooltip Cutting force (mN)
Tooltip Thrust force (mN)
Toolface BC Cutting Force (mN)
Toolface BC Thrust force
WBC Cutting Force (mN)
WBC Thrust force (mN)
Temperature (° C)
For
ces
(mN
)
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Cutting Pressure Plots (All Boundary Conditions)
20 700 1500 2200 27000
10
20
30
40
50
60
Cutting Pressures
Tooltip Cutting Pressure (GPa)Toolface BC Cutting Pressure (GPa)WBC Cutting Pressure (GPa)
Temperature (° C)
Cut
ting
Pre
ssur
e (G
Pa)
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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