Vjti Presentation Dr Pawade

A presentation on

Machining of ‘Difficult-to-machine’ Materials: Superalloy - Inconel 718

By

Dr. Raju S. Pawade Department of Mechanical Engineering

Dr. Babasaheb Ambedkar Technological University, Lonere, Raigad 402 103 MS Email: [email protected]

Superalloys: Introduction • High temperature heat resisting alloys - Group VIII A elements • Complex physical metallurgy • Developed for turbo superchargers and aircraft turbine engines • Two types: Solid solution strengthened and Precipitation strengthened • Inconel 718: Precipitation hardened Ni-base superalloy: most demanding and most difficult to machine

Classification of superalloys Nickel based, Cobalt based and Iron based superalloys Nickel based superalloys Type

composition

Inconel 625

Ni 61 Cr 21.5 Mo 9 NbExcellent strength from –253 °C to 705 °C, 3.6 Fe 2.5 excellent oxidation resistance to 980 °C, excellent high temperature properties, Ni 54 Cr 18 Fe 18.5 Nb higher toughness and higher ductility 5.1 Mo 3

Inconel 718

properties

Incoloy 800

Ni 6325 Cr 21 Fe 46Resistance to oxidizing, carburising, High C0.05 1.4 creep and rupture properties,

Nickel 200

Ni 99.6 C 0.08

Nickel 222

Ni 99.6Mg 0.075

Good mechanical properties, excellent resistance to many corrosives

Importance of nickel base superalloy in aerospace engines

Higher yield strength of Inconel 718

Aero-engine components Jet engine

Compressor valves

Vanes profile Burner cans

Turbine blade

Applications of Superalloys Applications

Components

Gas turbine

Blades, disks, shafts, casing, engine mounts, fasteners, wheels, buckets

Rocket motors

Exhauster, thrust reverser

Pumps

Impellers, bodies

Space shuttle

Engine parts, turbochargers

Space station

Pressure vessels of nickel-hydrogen batteries

Cryogenics

Cryogenic tanks

Nuclear power

Nuclear fuel element spacers

Tooling

Hot extrusion tooling

Heat treatment

Equipments for heat treatments

exhaust

valves,

Medical application Dentistry uses, prosthetic devices

hot

plugs,

Applications of Superalloys

Fan case

Fan compressor

Nozzle

Compressor rotor

Shaft

Turbine disc

Turbofan

Pump impeller

Thrust reverser

Afterburner

Cryogenic tank

Microstructure of superalloys – Inconel 718 γ ′ Gama prime

Plate like shaped of γ ′ ′ gama double prime

MC, M2C grain boundary carbides of Cr, Mo, Ti, Fe

δ phase –Ni3Nb Orthorhombic Phase

Microstructural phases FCC Nickel (Austenite) – Gama (γ )

Ni3-Al

BCT DO22 crystal structure – Gama double prime (γ ′ (650-900 °C)

Ni3Nb

′)

FCC Gama prime (γ ′ ) - Ni3(Ti, Al) crystal structure

CCT Diagram

Strengthening Mechanisms Coherency strain effects Principle obstacle to motion of dislocation pairs - coherency of tetragonally distorted DO22 γ ′ ′ particles on {1 0 0} planes of FCC matrix • More prominent than order strengthening in γ ′ ′ hardened alloys – superior strength at low to intermediate temperatures Ordered strengthening • Independent motion of dislocation pairs- stress concentration effects- increment in critical resolved shear stress

• More prominent in γ ′ hardened alloys – superior strength at higher temperatures (> 800 °C) • Compact morphology of γ ′ and γ ′ ′ phases, (Ti + Al)/Nb ratio 0.9-1.0 - improved thermal stability • Higher (Ti + Al)/Nb content favoured γ ′ formation over γ ′ ′ Stacking fault energy • Determines slip mode and strength at low to intermediate temperature in γ ′ ′ L12 structure ∀ γ ′ phase Inhibits deformation by a stacking fault mode of shear

Microstructural elements and their effect Microstructure element

Characteristics

Effect

Parent gamma (γ )

[FCC] -solid solution of Fe, Cr and Mo in Ni.

Ductility, Rapid work hardening, toughness, Abrasiveness

Gamma double prime [BCT] -metastable precipitate Primary strengthening when Nb is (γ ′ ′ ) (Ni3Nb)- ellipsoidal disc shaped > 5%, creep/rupture properties Gamma prime (γ ′ )

[FCC] -fine ordered metastable Good heat and creep resistance precipitate Ni3(Al, Ti)- spheroids

Delta (δ )

Ordered orthorhombic Ni3Nb (A2B-compound)- large white rods (plates)

η phase

[HCP] intra-granular array of Ni3Ti, Brittle

MC Carbides of Ti, Mo, Nb, Fe, Cr

Grain boundaries quantities TiC,NbC

Imparts hardness

plateletLoss of ductility, reduced impact and creep strength in

smallRupture strength at elevated temperature

Mechanical and Thermal properties, Composition Mechanical properties Hardness

Thermal properties

412HV/ Thermal conductivity,WM-1 K-1 331 BHN

15.0

Young’s modulus, GPa 206

Density, g cm-3

8.47

Yield strength, Mpa

Specific heat, J Kg -1 K-1

461

Tensile strength, Mpa 1276

Thermal expansion, x 10-6 K -1

11.5

Elongation, 50 mm%

10-12

Melting temperature, °K

1550

Composition

Ni52.2 Fe18.5 Cr19 Mo3.05 Nb+Ta5.3 Ti0.9 Al0.5 C0.04 Mn0.2 Si0.3 B0.005

1034

Mechanical properties and their machinability Mechanical properties

Machining difficulties

•

High temperature strength

Softening of cutting tools

•

High shear strength

High dynamic shear strength

•

High toughness

•

Rapid work hardening

Tough chips causing seizure and cratering Excessive tool wear at DOCN

•

High abrasive resistance

Excessive tool wear

•

Poor thermal diffusivity

High machining temperatures

Machining difficulties

Micro-chipping along the main cutting edge

Tearing of surface layer

Galling and pitting on rake face (Adhesion)

Severe chipping and galling on flank

Thin layer of Smeared workpiece materialsevere galling (after 5 s at 75 m/min)

Coating peeling and adhesion

Coated ceramic tool

Fractured coatings, and exposed carbide substrate of coated tools

Notching wear

Chipping wear

Machinability of superalloys Machinability of superalloys

Physical properties

Mechanical properties

Microstructure

Strength

Composition

Ductility

Thermal properties

Work hardening

Material condition

Hardness

Effect of mechanical/thermal properties on machinability Material condition • Proper heat treatment to protect inter-granular corrosion and surface contamination • Common treatments: solution treating, solution treating and aging, stress relieving, and annealing • Optimum material conditions for improved machinability Alloy group

Material condition

A

Cold drawn

B

Cold drawn, stress relieved

C

Hot finished

D-1, D-2

Annealed

E

Cold drawn, stress-relieved

• Solution annealed and rapid air quenched : soft condition- suitable for drilling, tapping and threading • Solution treated precipitation strengthened alloys: strength formability • Fully age hardened: too hard for rough machining and weak cutting edges • Fully age hardened: finish machined with high surface finish and close tolerances • Full annealed: residual stresses during machining

Thermal conductivity • Low values of (Kρ C) - localization of temperature at tool tips – high thermal gradients in the cutting edge Strength • Higher cutting forces – high shearing and frictional forces produces higher machining temperatures (> 760 °C) Work hardening • Higher flow stress( 1.5 – 2.5 GPa) and temperature (800-1200 °C) causes rapid work hardening • Heavy chips formed Ductility • Higher ductility- increased chip-tool contact area- higher cutting forces • To prevent - use sharp cutting edge and positive geometry

Hardness • Retain hardness at higher temperature- higher cutting forces and friction – higher temperatures in the tool nose region • Rate of catastrophic shear reduced- machined surface hardness 15 –20 points higher than base material

Hv

Distance from top surface in mm

Cutting speed ranges for materials

Measure to asses machinability of superalloys Mechanism of machining

Measures to Asses machinability of superalloys

Mechanics of machining Tool-life and tool wear Machined surface characteristics

Machinability Indicators Machining Machining characteristics characteristics Inconel718 718 ofofInconel

In-process In-process characterization characterization

Cutting force AE variables

Post-process Post-process characterization characterization

Chip analysis Surface roughness Surface damage Microhardness change Residual stress

Equipment for Experiments Response variable

Equipment used

Cutting force components

Cutting force dynamometer (Piezoelectric type) KISTLER model 9257A

AE parameters (counts, AE System Physical Acoustic Corporation energy, frequency) system MISTRAS UK with32 bit EDSP software and sensor R15 Chip morphology

Microscope Olympus, SEM

Chip dimensions

Measuroscope NIKON, Japan, MM-22

Surface roughness

Surface roughness tester (Perthometer) Mahr M2 Germany

Surface alterations

Scanning electron Microscopy (SEM) Hitachi

Microhardness

Microhardness tester SHIMADZU Japan MV2

Surface residual stresses X-ray diffractometer PHILIPS, PANalytical PRO MRD System X’pert Stress

Analysis of Cutting Force Components 700 600

400 300

400 300

200

200

100

100

0

0

1

251

501 751 1001 No. of Counts

1251

Force (N)

500

500

Cutting Force Radial Force Feed Force

350 300 250 200 150 100 50

1

251

501 751 No. of counts

1001

V =300 m/min, f = 0.15 mm/rev, ap = 0.75 mm, CW2 Vc =125 m/min, f = 0.15 mm/rev, ap = 1.00 mm, CW2c



Cutting forces (2-3 times) higher than feed and radial forces



Reduced cutting (1.5 times), feed (2 times), radial (3-4 times) forces at 475 m/min-significant heat generation-excessive melting of work material



Higher radial (thrust) forces – large friction – tool wear



450 400

Cutting Force Radial Force Feed Force

600 Force (N)

Force (N)

700

Cutting Force Radial Force Feed Force

Ac = f(d, f) - At low f and d - lower cutting forces

0 1

251

501

751

1001 1251 1501

No. of counts Vc =475 m/min, f = 0.10 mm/rev, ap = 0.75 mm, CW2

Material blockage due to chamfered edge

Statistical Analysis of Cutting Force ANOM

ANOVA

Source

DF

SOS

MS

F-ratio

P-value

Vc

2

27.435

13.7176

6.14

0.035

f

2

96.806

48.4032

21.66

0.002

ap

2

79.469

39.7345

17.78

0.003

E

2

0.421

0.2107

0.09

0.911

Vc × f

4

25.364

6.3411

2.84

0.122

Vc × ap

4

5.746

1.4366

0.64

0.652

f × ap

4

4.993

1.2484

0.56

0.702

Error

6

13.406

2.2343

Total

26

253.642

SN Ratio =



η =− 10log

10

Most significant factors – Vc, f, ap



Vc (125-300 m/min) – reduced CF – thermal softening- reduced material strength



f (0.05 – 0.10 mm/rev) – increased CF – larger c/s area in deformation



ap (0.50 -1.00 mm) – increased CF – larger deforming volume and MRR



CH Edge geometry – lower CF

1 n  2 n ∑yi  t =1

Cutting Forces in Turning The cutting forces produced during machining are usually 2-3 times higher than feed and radial force components. Radial forces are higher when the cutting tool is subjected to more flank wear. The cutting forces decrease as the cutting speed increases in the high-speed machining regime (above 60 m/min), however, they increase with an increase in the cutting speed in low speed machining range (below 60 m/min).

Cutting force Thrust force

Fig. (a-b): (a) Effect of cutting speed on force components for plain carbide tools (ο natural chip-tool contact length; • 0.15mm chip-tool contact length) [Sadat, 1987], and (b) cutting force components for coated CBN insert (Vc = 350 m/min, ap = 0:5 mm, f = 0:2mm/rev) for 45% TiN [Devilez et al, 2007]

Fig. (a-b): Effect of cutting speed on cutting forces using (a) CBN tool [Arunachalam et al. 2004], and (b) Nanostructured TiAlN coated carbide tool, ap = 1.5 mm [Devilez et al, 2007]

Cutting Forces in Turning Cutting forces increase with an increase in feedrate and depth of cut. Coated carbide tools experience lower cutting forces during machining than plain carbide tools. Among coated carbides, multicoated and nano-structured coated tools produce lower cutting forces compared to single layer coated tools. Chamfered and honed cutting edge geometry on cutting tools produces lower cutting forces than the chamfered edge geometry.

Stresses and Strains • Very high dynamic shear strength- difficulty in machining

τ ρ [ ( FC Cosφ − Ft Sinφ ) Sinφ ] / wo to =

σ N = ( FC Cos α − Ft Sin α ) / ContactAre a

τ s = ( FC Sinα + Ft Cosα ) / ContactArea

•High shear strains in secondary zone ( 20-50)- failure of cuting tool due to plastic deformation

Temperature • Smaller temperature gradient in the area of chip-tool contact and toolnose • Higher temperature in the region near cutting edge than any other regions • High temperature in the flank region exceeds than the rake face and hence develops wear land Mean rake face temperature inversely proportional to kρ c At higher cutting speed- temperature decreases but again increases with increases in cutting speed Peak temperature 1200 °C at 150 m/min Local tool temperature vs. cutting speed

Tool wear mechanisms • Tool wear is a complex function of • Cutting tool materials • Coating on cutting tool materials • Cutting parameters • Machining duration • Machining processes

Progressive wear mechanism At low cutting speed Initial wear

Tool failure

Galling-Micro-Chipping-Pitting-Chip adhesion-Coating delamination

Galling – Progressive chipping

At high cutting speed Initial wear

Tool failure

Galling- Severe chipping-Pitting- Coating delamination- Flaking

Galling – Breakage - Flaking

Summary of Tool wear mechanisms Cutting Tool Material and their typical compositions

Cutting Tool Material and their typical compositions

Carbides: Depth of cut wear notch, high flank wear, crater wear on face. WC + Co; WC+TiC+TaC. Diffusion of carbide particles into Co binder by grain boundary diffusion. Plastic deformation of the cutting edge due to their poor thermo-mechanical stability. Coated carbides TiC; TiN, Al2O3, CrN, TiAlN, TiN, TiCN. Ceramics Uncoated: Al2O3+SiCw Si3N4, Al2O3 + TiC CBN

Depth of cut notch wear, abrasive flank wear, Adhesion as tool wear mechanism, peeling of coatings, adhesive flaking and chemical wear, micro-chipping, flaking and edge breaking

Depth of cut notch wear due to adhesion of work material to the tool due to diffusion, trailing edge wear on round insets, attritional wear, thermal cracking, plucking of tool particles, cutting edge fracture, chipping and fracture (in milling). Flank, nose and crater wear, plastic deformation of the cutting tool nose, chip adhesion ob the tool face.

Foot formation phenomenon

Severe abrasive wear of nitride based ceramic tool

Severe abrasive wear on cutting edge

Flank wear observed on d) uncoated insert e) CrN/TiN nanolayer coated insert f) TiN/AlTiN nanolayer coated insert

Chip Morphology f (mm/rev)

0.05

0.10

0.15

Highly burnt chip

Vc (m/min)

125 m/min Long ribbon thin and snarled

Helical coiled

Short washer type helical and loose Vc: 125m/min; f: 0.15 mm/rev; ap: 0.75 mm; CW1

300 m/min

Highly strained chip Long ribbon

Snarled helical washer

Long washer type helical

Snarled washer type and helical

Short washer, helical and loose arc

475 m/min

Snarled ribbon

Vc: 475m/min; f: 0.05 mm/rev; ap: 1.00 mm; CW2

Chip Formation Mechanism 



Temperature softening in pdz > strain hardening – heat dissipation in the concentrated zone- the chip becomes serrated

Serrated chip

Cause: Thermoplastic instability and the initiation and propagation of cracks inside pdz Free surface of the chips show cracks extended up to ½ to ¾ th width of the chips



Chip segmentation frequency is more at higher cutting speed 475 m/min than at 300 m/min



Chip segmentation frequency increases with increase in feedrate

max

hch hch



hch

Pitch, P

max

min : Maximum chip

height

hch

min

: Minimum chip

height

Vc =475 m/min, f = 0.15 mm/rev, ap = 0.50 mm, CW2

Chip Formation Mechanism ∆ Schip1 = 57 µm ∆ Schip2 = 86 µm

FChS =

∆ Schip1

100 Vc f a p 6 ∆Schip

∆ Schip3 = 43 µm ∆ Schip4 = 55 µm ∆ Schip2 ∆ Schip3 ∆ Schip4

Vc = 300 m/min; f = 0.10 mm/rev; ap = 0.50 mm; CW2 Vc = 475 m/min; f = 0.10 mm/rev; ap = 0.75 mm; CW2

X

View along width of chip X-X Cracks on the free surface Chip

X F Saw tooth edge

G

H

III II C B’ B X

I

D

E

Tool

2 1

A’ A Workpiec e

J

Effect of cutting speed on chip segmentation frequency

When σ c < τ y in pdz - fracture initiates at the free surface  Crack runs from B to A  Region A’AED - with shear localized deformation  Region EFGD - with no deformation  A’B’ - subsequent fracture plane  Top surface of chip - brittle deformation  Lower surface of chip - ductile deformation 

Serrated chip formation mechanism

Chip Formation Mechanism •Low thermal diffusivity 20% and thermal contact number 21% of steel – shear localization – Catastrophic shear instability Region 2

Region 1

1. Undeformed surface 2. Part of catastrophically shear failed surface 3. Intense shear bands 4. Intense sheared surface in contact with tool 5. Localized deformation in primary zone 6. Machined surface

Fig. Schematic of shear localized chip formation in machining of Inconel 718 [Hou and Komanduri, 1997]

Chip Formation in Turning The chips formed during machining of this material are of shear localized type due to a predominant thermal softening over the strain hardening at higher cutting speeds. Tough, continuous, abrasive chip (10-60 m/min) Serrated, composite chip at higher speeds ( above 61 m/min) At lower feedrates, the chips are of continuous type, whereas, an increase in feedrate changes them to isolated segmented type. In dry machining, long continuous chips are produced. However, the chips have continuous and tubular shapes with the application of flood coolant during machining. Further, they become short tubular type, when the high-pressure coolant is used.

Surface Roughness • Geometrical damages (roughness, waviness) metallurgical damages (cracks, residual stress, micro-hardness variation) • Feed rate and depth of cut

Surface roughness

• High speed and low feed, High tool nose radius : better surface finish • Coarse and Fine scale damages: microcraks, cavities, deformation, fractured areas, short and long grooves : BUE, sliding effect • Tensile and compressive residual stresses, tearing of surface layer, more surface damage at low cutting speed

Surface roughness Coated carbide tool

2.5

Average surface roughness (R a ) in microns

V Tearing and plowing of the material due to BUE fragments roughness V T softening of material and less flank wear roughness

f = 0.050 mm/rev f = 0.088 mm/rev f = 0.125mm/rev

2

1.5 1 V = 28 m/min

0.5

V = 36 m/min

V = 45 m/min

Effect of 0

0 cutting speed

20

40

Cutting s peed (m /m in)

60

Surface roughness (Ra) in microns

2.5 2

1.5 1

0.5

feed marks feed surface roughness

0 0

0.05

0.1

Feed rate (mm/rev) Effect of feed rate

0.15

Surface roughness CBN tool

ANOVA

Source

DF

SOS

MS

F-ratio

P-value

Vc

2

19.327

9.664

1.62

0.273

f

2

66.645

33.322

5.59

0.043

ap

2

6.838

3.419

0.57

0.592

E

2

19.602

9.801

1.64

0.269

Vc × f

4

89.641

22.410

3.76

0.073

Vc × ap

4

69.233

17.308

2.90

0.118

f × ap

4

52.633

13.158

2.21

0.184

Error

6

19.327

9.664

Total

26

66.645

33.322

ANOM 

Feedrate: 0.05 mm/rev – Ra higher - 0.10-0.15 mm/rev – Ra reduced



More contact between tool and work -Severe strain hardening



Depth of cut: 0.50 -0.75 mm – Ra higher - Higher machining deformation Cutting speed: 300-475 m/min – Ra decreased - Higher thermal influence- discontinuities get wiped out



Surface roughness in Turning Machined surfaces show lower roughness at higher cutting speeds. Higher values of feedrate and depth of cut both produce higher surface roughness. Use of plain carbide tools show higher machined surface roughness than the ceramic coated carbide tools. Use of mixed oxide ceramic tools give lower surface roughness as compared to pure oxide ceramic tools. Round insert produce lower surface roughness since it has larger contact length relative to square shaped inserts. Chamfered and hone cutting edge inserts produce lower surface roughness than the sharp cutting edge inserts. Positive rake tools show lower values of surface roughness.

Surface topography Coated carbide tool •Broken chip fragments •Few tool digging marks

• Few cavities

• Less feed marks • Less cavities • Fractured areas

• Severe broken chip fragments

• Some cavities

• Less broken chip • Large broken chip

• Tool digging

fragments

marks • Feed marks • Few cavities

45

Cutting Speed (m/min)

36

28

•Severe cavities

fragments

Feed rate (mm/rev)

V = 36 m/min, f =0.125 mm/rev

V = 36 m/min, f =0.05 mm/rev

V = 36 m/min f = 0.125 mm/rev

SEM Micrographs

V = 36 m/min f = 0.05 mm/rev

V = 45 m/min f = 0.088 mm/rev

V = 45 m/min, f =0.088 mm/rev

Coated carbide tool

Surface Topography

CBN tool Deep groove

Fractured particles

Micro particle deposits

Smeared layer Feed marks

V = 125, f = 0.10 mm/rev, d = 0.75 mm, CH

Weight %

V = 125, f = 0.10 mm/rev, d = 1.00 mm, CW1

C

O

Al

21.63

40.42

37.95

Specimen 5 Vc= 125 m/min, f = 0.10 mm/rev, ap = 0.75 mm, CH    

Fractures along the depth of cut line due to DOCN wear Scattered micro particle deposits 25-30 microns in size Smeared layers 30-50 microns Deep grooves 30-40 microns wide along the feed marks (100 microns apart) due to digging of worn out tools Micro-pits 10-20 microns size Feed marks are covered by the smeared layer Specimen 6

Vc = 125 m/min, f = 0.10 mm/rev, ap = 1.00 mm, CW1 

Scattered micro particle deposits 2- 3 microns in size over entire surface Light feed marks

Surface Topography

CBN tool Smeared layer

Weight %

Micro particle deposits

C Chip fragments

O

Si

Fe

N

70.79 23.16 1.78 3.63 0.63 Material side flow Micro pits

Shallow grooves

V= 300, f = 0.15 mm/rev, d = 1.00 mm, CH

V= 300, f = 0.10 mm/rev, d = 0.50 mm, CH

Specimen 13 Vc = 300 m/min, f = 0.10 mm/rev, ap = 0.50 mm, CH   

Shallow grooves of 25 microns wide at feed marks location uniformly located after every 100 microns on entire surface Few micro particle deposits 1-2 microns Chip fragments 3-5 microns size Significant smeared layers or material side flow about 40-50 microns wide Feed marks after every 15 microns covered by smeared layer Specimen 18

Vc = 300 m/min, f = 0.15 mm/rev, ap = 1.00 mm, CH   

Heavy material side flow up to 20-40 microns from the feed marks Fractured particles adhered to surface at feed marks location 150 microns apart Micro particle deposits of 50 microns size Micro-pits Accumulation of debris size 30-50 microns at feed marks location

CBN tool

Surface Topography

Micro particle deposits

Micro chip fragments

Light feed marks

Weight % V= 475, f = 0.05 mm/rev, d = 1.00 mm, CW2

V=

475, f = 0.10 mm/rev, d = 0.75 mm, CW2

B

C

O

Al

Si

Ti

Co

2.61

64.12

31.03

0.01

0.11

0.48

1.65

Specimen 21 Vc = 475 m/min, f = 0.05 mm/rev, ap = 1.00 mm,  CW2  

Few micro particle deposits 15 microns in size Light feed marks after every 50 microns Tool abrading grooves spaced 50 microns apart Few micro pits Minor chip abrading marks Specimen 23

Vc = 475 m/min, f = 0.10 mm/rev, ap = 0.75 mm,  CW2

Few shallow pits Light feed marks at spacing of 5 microns Few micro particle deposits 0.5 to 1 micron in size

Surface Topography in Turning

(a) (b) (c) (d) Fig.(a-d): Deformation of machined surface of Inconel 718 (a-b) cracked carbide particle in the deformed layer, (c) surface tearing and cavities with worn tool, and (d) surface layer with new tool [Sharman et al. 2004]

Surface Topography in Turning The surfaces machined at lower cutting speeds show severe surface damage, however, the damages reduce considerably at higher cutting speeds. The machined surfaces produced using controlled contact length tools show lesser damage than the natural contact length tools. The use of coolant is effective in reducing surface damage at lower cutting speeds, but is found ineffective at higher cutting speeds. Machining with worn out tools show more grain boundary deformation as compared to a new tool. Rhomboid shaped inserts cause more surface damage than the round inserts. The surfaces machined with coated carbide tools show relatively more damage than CBN and ceramic tools.

Statistical Analysis of Residual Stress

(a) Normal probability plot and (b) Main effects plots for circumferential residual stress 





Delicate control over all parameters is necessary to control RS Higher cutting forces – Compressive plastic deformation – after unloading – TRS Lower cutting forces – Tensile plastic deformation – after unloading of deformation zone – CRS

ANOVA Source

DF

SOS

MS

F-ratio

P-value

Vc

2

541

270910

0.78

0.501

f

2

1381

690779

1.98

0.218

ap

2

847

423641

1.22

0.360

E

2

347

173862

0.50

0.630

Vc × f

4

1023

255907

0.73

0.601

Vc × ap

4

248

62100

0.18

0.942

f × ap

4

582

145704

0.42

0.791

Error

6

2090

348416

Total

26

145704

Effect of Cutting Speed and Feedrate CUTTING SPEED 

Quantum of heat dissipation – type of RS



125 m/min – TRS



475 m/min – CRS FEED RATE

Fig. Effect of cutting speed on the generation of residual stress



At 125 m min-1 , feedrate from 0.05 to 0.10 mm rev-1 – small increase in the RS. Whereas, similar change in the feedrate at 475 m min-1 – huge change in RS (–650 MPa to 250 MPa)



Small change in feedrate at 475 m min-1 – more effective in increasing the heat dissipation ability of chip – it gets dampened beyond 0.10 mm rev1 to 0.15 mm rev-1 . Fig. Interaction plots for cutting speed and feedrate for surface residual stress

Effect of Depth of Cut and Edge Geometry DEPTH OF CUT 

  



The effect of depth of cut and cutting speed both changes the VRR Cutting speed at a given doc –VRR – CRS Depth of cut – CRS for the same reasons Heat accumulation in chips - higher at 125 m min-1 – highly burnt chips Heat accumulation in chips - less at 475 m min-1 – silver chips

Vc =125 m min-1 , f =0.05 mm rev-1 , ap= 1.00 mm, CH

Fig. Interaction plots for cutting speed and depth of cut for surface residual stress

Vc =475 m min-1 , f =0.10 mm rev-1 , ap= 0.75 mm, CW2

Vc =475 m min-1 , f =0.15 mm Vc =300 m min-1 , f =0.10 mm rev-1 , ap= 1.00 mm, CW1 rev-1 , ap= 0.75 mm, CW1

(a) (b) (c) (d) Fig. Effect of edge geometry on residual stresses Fig. (a) Highly burnt chips at 125 m/min (b-c) silver chips at 475 m/min and (d) partially burnt chips at 300 m min-1

CH insert –contact area - ploughing – mechanical deformation – higher CRS  30° Chamfered insert –ploughing – lower CRS  20° Chamfered insert –cutting edge pressure on nearby material – TRS 

Fig. (a-b): (a) Influence of cutting speed (carbide tools) [Sadat, 1987], and (b) depth of cut on residual stresses (CBN tool) [Salio et al. 2006]

Fig. (a-b): Influence of (a) tool material, and (b) insert geometry on the residual stresses (C1 type CBN; 0.15 mm/rev; 0.5 mm) [Arunachalam et al. 2004]

Fig. (a-b): Effect of (a) cutting edge preparation (Honed-R; Chamfered-A1; Sharp-A2) on residual stresses [Arunachalam et al. 2004], and (b) insert shape (ap = 0.35mm and f = 0.1 mm/rev) [Coelho et al. 2004] on residual stresses

Fig.(a-b): Effect of (a) insert rake type (D-positive rake; A2-negative rake), and (b) nose radius (0.8 mm-A; 1.2 mm-B; 1.6 mm-C) on the residual stresses [Arunachalam et al. 2004]

Residual Stresses in Turning The surfaces machined at higher cutting speeds show presence of tensile residual stresses. Higher tensile residual stresses are induced using ceramic tools than with CBN tools. TiAlN coated carbide tools induce larger compressive residual stresses, whereas, the stresses are tensile type when multilayer CVD coated carbide tools are used. Sharp cutting edge tools induce tensile residual stresses, however, chamfered and honed edge tools induce compressive residual stresses. Positive rake inserts produce tensile residual stresses as compared to negative rake angle tools in which the stresses are compressive. Dry machining produces tensile residual stresses, whereas machining with coolant produces compressive residual stresses.

Analysis of Microhardness MH – machined specimen (323 to 533 Hv), annealed (161 Hv), hot rolled (357 Hv)  Highest MH value- Expt. 27 – severe machining conditionshighest Vc, f, ap 

Fig. Microindentation images

• MAZ: 200-300 µ m (Depth of hardened layer) • Steep hardness gradient -30 to 100 µm deep from the machined surface • Microhardness near the surface (323-533 Hv) is 1.5 times the bulk microhardness (220-361 Hv)

Fig. Microhardness profiles of the machined subsurface

Fig.(a-b): (a) Effect of cutting force on microhardness [Sharman et al. 2004], and (b) effect of tool geometry (C: 20° chamfered edge tool; M: 15° chamfered and honed edge tool) on microhardness (ap = 0.35mm, f = 0.1 mm/rev, Vc =500 m/min [Coelho et al. 2004]

Fig.(a-b): Microhardness depth profiles with (a) new tools, and (b) worn tools [Sharman et al. 2004]

Analysis of Degree of Work Hardening Degree of Work Hardening (%) It is the ratio of change in surface microhardness to the bulk microhardnes 

AOM

• Cutting speed increased (300 m/min)T - γ prime precipitates dissolvesmaterial softens- low work hardening • At 475 m/min - T - rapid tool nose wear-tool burnish the work materialmore work hardening

Effect of cutting speed ANOVA

•30° chamfer + honed insert (CH)additional ploughing by honed edge radius – higher compressive material deformation -more work hardening • 30° chamfer insert (CW1)- less dead metal zone volume-less friction-lower degree of work hardening

Effect of cutting edge geometry

Effect of Depth of Cut and Feedrate 

At 0.15 mm rev-1 , any change in doc – no effect on DWH



At 0.05 mm rev-1 – significant variation in DWH when doc changes from 0.50 mm to 1.00 mm

DEPTH OF CUT

FEED RATE 

Fig. Effect of depth of cut on degree of work hardening

At highest level of Vc, f & ap – Highest DWH

Least DWH at medium Vc = 300 m min-1  Feedrate & depth of cut – influence chip c/s area



Fig. Interaction plots between (a) feed and depth of cut and (b) speed and depth of cut (c) speed and feed for degree of work hardening

Microhardness in Turning The machined surfaces show higher microhardness than the bulk material due to severe strain hardening effect during machining. Chamfered and honed edge geometry tools produce more work hardening effect as compared to chamfered edge tools. Machining with worn out tools increases microhardness of the machined surface as compared to the new tools.

Experimental Work (Workpiece, Tooling and Setup)

KISTLER Dynamometer

Force measurement PC PC with AE Software

Tool holder Workpiece AE sensor

CBN insert

Inconel 718

Mandrel

Data acquisition system Charge amplifiers

Tool holder (Sandvik make) DCLNL 20× 20 K12

Machine CNC Turning lathe:EMCO 345

Insert (Sandvik make)

PCBN Rhombic 80° negative insert, rn = 0.4 mm

CNGA 12 04 04 T01030 AWH 7015 (CW1) CNGA 12 04 04 T01020 AWH 7020 (CW2) CNGA 12 04 04 S01030 A 7015 (CH) Tool holder

PCBN insert

Approach angle = 95°, Rake angle =-6°, Inclination angle = -6°

Measurement of Microhardness and Residual Stress σ

Microindentation area ABCD

Machined surface

Machined surface

xx

φ 60

B

A

σ

yy

σ

4 mm

Bulk

zz

Cold mount

Transverse section D

C

φ 50

σ σ σ

Bakelite fixture

Enlarged view ABCD

yy : Axial residual stress xx : Circumferential (Hoop) residual stress zz: Radial residual stress

Detector Electrolytic composition

Polishing condition

Methanol (%) Perchloric acidTime (%)

Voltage

Current

Cathode

80

20 V

0.74 A

Stainless steel

20

20 Sec

Specimen

X-Ray tube