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Solution Wear
• in cutting ferrous alloys at high speeds Carbide cutting tools (not HSS steel tools)
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Mechanical Behavior Dominated Wear
• Friction and wear controlled by mechanical behavior of materials – Friction due to plowing and asperity deformation
– Delamination wear – Abrasive wear (Even some cutting tools -- HSS, Alumina)
– Erosive wear – Fretting wear – Wear by plowing of the surface by wear
particles and asperities
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Friction and wear controlled by Chemicalbehavior of materials
– Solution wear (e.g., carbide tools)– Friction due to adhesion (at high sliding speeds
and high loads)
– Adhesive wear (at high sliding speeds and high loads)
– Oxidative wear (rare)
– Wear due to melting of the sliding interface (mostly in polymers)
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Consider the wear of a cutting tool
• When steel is cut at high speeds using a cutting tool, what is the wear mechanism?
• How can we make the tool last longer?
• How can we cut the workpiece at a higher speed?
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The Causes of Solution Wear
• Chemical reaction
• Formation of a solution between the materials in contact to decrease the free energy
– Wear of tungsten carbide tools – Diamond in cutting ferrous metal
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Introduction to Metal Cutting
Chip Formation
Tool
α
βφ
Chip
Workpi ece
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Cutting Forces & Chip Formation
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Tool Wear
Diagram removed for copyright reasons.See Figure 8.8 in [Suh 1986]: Suh, N. P. Tribophysics. Englewood Cliffs NJ: Prentice-Hall, 1986. ISBN: 0139309837.
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Chip Formation
Diagram removed for copyright reasons. See Figure 8.2 in [Suh 1986].
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Introduction to Metal Cutting
Rness
Surface Roughness vs Cutting Speed
Cutting Sp ee d
Su rf ough
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Introduction to Metal Cutting
FRs of Cutting Tool Materials
FR1 = Wear Resistance FR2 = Fracture Resistance FR3 = Resistance to Plastic Deformation FR4 = Stiffness
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Introduction to Metal Cutting
Cutting Tool Materials Plain carbon steel (early 1800’s) High carbon steel (mid 1800’s) High Speed Steel Tool (HSS) Cemented tungsten carbide tools Cemented W/Ti/Tacarbide (800 feetmin) Aluminum oxide (>2000 ft/min) Cubic boron nitride (CBN) Coated tools (TiN, TiC/TiN, HfC)
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Introduction to Metal Cutting
Cutting speed increased steadily during the 20th century due to the advances in
cutting tool materials.
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Introduction to Metal Cutting -Tool Wear
Crater on the rake face Wear land formation on the “flank” face
Wear Crater
Flank w ear
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Introduction to Metal Cutting
Tool Wear
At high cutting speed, crater wear dominates. Its wear rate is very sensitive to temperature and thus, cutting speed.
Tools fail when the crater depth is about 0.005 inches.
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Introduction to Metal Cutting
Tool Wear
At low cutting speed, flank wear dominates. Its wear rate is not sensitive to temperature and thus, cutting speed.
When the length of the wearland reaches about 0.030”, tools fail catastrophically.
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Tool Wear Mechanisms
Abrasive Wear Delamination wear (HSS) Solution wear Chemical reaction Wear due to diffusion of elements
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Solution Wear
What is the definition of solution wear?
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Cutting Temperature
How sensitive is the solution wear rate to temperature?
How is the maximum temperature under a typical cutting condition, e.g., cutting AISI 4340 steel at 800 ft/minute?
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Cutting Temperature
Temperature distribution in cutting zone (as determined from an infrared photo). Free-cutting mild steel: speed 75 ft/min; width of cut b=0.25 in., rake angle α=30°, relief
angle=7° , initial workpiece temperature To=611°C. (After Boothroyd, 1963). 20
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Solution Wear
y
x
Tool
Workpiece
Chip
Depth of cut
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Solution Wear
Conservation of Mass
∂ v x
∂ x +
∂ v y
∂ y = 0
Transport rate
1 A
∂ m ∂ t
= cv y − D ∂ c ∂ y
⎛ ⎝ ⎜ ⎞
⎠ y = 0
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Solution Wear
Conservation of Mass
∂vx +∂v y = 0
∂x ∂y
Since ∂vx < 0, ∂x
∂vy > 0, v y = constant > 0∂y
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Solution Wear
Wear Rate of the material being dissolved
dV = ∫ K ⎛⎜ − D ∂c + c v ⎞ dAs ydt ⎝ ∂y ⎠
V = Wear rate of the material being dissolved
K = Ratio of the molar volumes of the tool material being dissolved and the deforming workpiece material, respectively.
D = Diffusivity of the slowest diffusing solute constituent in the
c
material being cut
c = concentration of the dissolving material in the chip
s = equilibrium concentration of the tool material in chip material
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Solution Wear
Wear Rate of the material being dissolved
dV = ∫ K ⎛⎜ − D ∂c + c v ⎞ dAs ydt ⎝ ∂y ⎠
Local Crater Wear Rate v = Kc vc s y
Relative Wear Rates of Two Different Materials
v( ) ( Kc )c 1 = s 1
( ) ( Kc )v c 2 s 2
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Determination of Solubility
The Gibbs Free Energy
The Gibbs Free Energy is defined as
G = H − TS For formation of a solution at constant pressure and temerpature
dG = 0 dT = 0 dP = 0 For an irreversible, spontaneous process at constant T and P,
dG < 0
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The Gibbs Free Energy
G = H − TS
The physical significance of the Gibbs free energy :
It is the maximum useful work that can be done by a system at constant pressure and temperature.
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Equilibrium Concentration cs
At equilibrium, the change in free energy of compound ij at the chip/tool interface is equal to
∆Gij = ∆Gi + ∆G j where
∂G∆Gi = (
∂ni
)T , P,n j
∂G∆G j = ( )T ,P ,n i∂n j
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∆Gij = free energy of formation of the compound ij (i.e., tool material)
∆ Gi = ∆(∂G ∂ni )T ,P,n j = relative partial molar free energy of
the solution due to the change in the amount of components i in the solvent material (i.e., chip)
µi = chemical potential of component i = ∆(∂G ∂ni )T ,P,n j
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Equilibrium concentration cs
∆Gi = (∂G ∂ni
)T , P,n j = Partial molar Gibbs free energy
= (µi )T ,P ,nj = chemical potential of component i
∆Gj = (∂G ∂n j
)T ,P ,ni
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Equilibrium concentration cs
The relative partial molar free energy of solution of compoent i can be expressed in terms of the relative partial molar excess free energy of solution
xs∆Gi = ∆Gi + RT lnci
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Equilibrium concentration cs
Substituting these relationships into the equilibrium equation
ln ci + ln cj =1 (∆Gij + ∆Gi + ∆Gj )RT
⎤ci = exp⎡ 1 (∆Gij + ∆Gi + ∆G j )⎦⎣2 RT
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• .
Estimated and Reported Solubilities of Potential Tool Materials in α–Fe at 1600 K.
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Predicted Relative Solution Wear Rates of Potential Steel-Cutting Materials at 1300 K (After Kramer, 1979).
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Solubilities of the Carbides in the Iron-Group Metals at 1523 K
Carbide
WC
TiC
VC
NbC
TaC
22 (7.9)
1 (1.0)
6 (5.6)
5 (2.9)
3 (.93)
12 (3.9)
5 (4.9)
7 (6.6)
3 (1.7)
5 (1.6)
(2.2)
(2.7)
(0.53)
(0.15)
7
<0.5 (<0.5)
3
1
0.5
Cobalt
Solubility [wt % (mol %)]
Nickel Iron
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Comparison of Theoretical Predictions with Test Results: Predicted Relative Wear Rates at Various Temperatures
Carbide
Terms without parentheses are calculated on the basis of solubility estimated from thermodynamic properties. Terms in parentheses are calculated using the reported solubilities.
HfC
TiC
TiC0.75O0.25
ZrC
TaC
NbC
VC
11
8.827.65
2.412.26
4.874.44
7.196.33(10.7)(9.43)
(36.0)(31.0)10.69.13
(153.)
(12.5) (14.9)
(43.1) (52.8)12.7 15.6 23.7
(215.) (332.) (237.)(107.)
1 1 1
10.6 12.8 2.75
2.61 2.86
5.47 6.20 5.51
8.39 9.98 10.6
1500 K
Temperature
[Vwear(HfC) = 1]
1600 K 1400 K 1300 K Test Result
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Coating Techniques
• Chemical vapor deposition
• Physical vapor deposition
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Melting Point of Refractory Carbides and Oxides
Carbides
WC
Al4C3
VC
NbC
TaC
ZrC
TiC
V2O35030
TiO2(brookite)
Al2O3
MgO
ZrO2
HfO2
NbO
VN 3720
3533 CrN 2730
HfC
3578 HfN 5990
TaN 5590
ZrN 5400
TiN 5310
NbN 3720
OxidesTm (oF) Tm (
oF) Tm (oF)Nitrides
4900 + 100 3340 + 30
3720 + 40
5117 + 35
5010 + 150
5140 + 90
4950 + 150
6330 + 135
7080 + 100
6225 + 40
5550 + 30
7100 + 100
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Hardness of Oxides Hardness at Specified Temperatures of the Phases
That May Be Present in a Steel
Phase
Iron
Iron and interstitials
TiO
FeO
MgO
NiO
MnO
2745
2790
10001300
210350
220320
140200
60
100
45120
Al2O3 1300 1000 650
SiO2
ZrO2
2100700
400
300
350650
TiO2 380 250 160
MgAl2O4 1200 10501250
ZrSiO4 400 290 140
10
10
650
50
130
400 oC
Hardness, H (kgf/mm2)
600 oC 800 oC
υ
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Hardness of Carbides
Graph removed for copyright reasons. See Figure 9.40 in [Suh 1986].
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Material Properties
• The wear resistance of various carbides agree with their free energy of formation, tungsten being the worst and hafnium being the best.
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Material Properties
• Oxides have the best chemical stability. Therefore, a zirconium oxide tool should last 26 months and aluminum oxide for 23 days under a typical high speed cutting condition of steel, but they have lower melting point and hardness than carbides.
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Diffusion Wear
• This happens in machining titanium with a diamond tool.
• A new stable compound such as TiC is formed on the tool face protecting the carbide tool face. The wear rate of the tool is controlled by the diffusivity of carbon.
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