Structural transformations and the thermomechanical effects in machining and tribology.

1. Structural Transformations and the
Thermomechanical Effects in
Machining and Tribology- A Review
Prof. Padmanabhan Krishnan
Department of Manufacturing Engineering
School of Mechanical Engineering
VIT-University, Vellore- 632014, India.
Email: padmanabhan.k@vit.ac.in

2. The Background and Motivation
• Machining and Tribology are two secondary disciplines where
the investigators appear to grossly neglect metallurgical
effects putting productivity as a priority before the actual
short term and long term consequences that arise from
these frictional operations. The thermomechanical and
metallurgical effects in dry and wet machining are studied to a
lesser extent. The importance of structural transformations
and thermomechanical effects in dry and wet machining is
often overlooked in favour of productivity. These studies
assume greater significance in dry machining without the use
of coolants as it is considered eco-friendly.

3. The Background and Motivation….
• Very few direct and simulated studies are reported on
this subject and they don’t involve metallurgical effects
and rather focus on geometry and operational
parameters alone. For productivity’s sake the models
adopted for machining or wear optimization follow
Anova, Grey relationship, Orthogonal arrays, Taguchi
models, ANN, Response Surface Methodology and ANFIS,
to name a few. These models are parametric,,operational
and none of them consider the structural
transformations or thermomechanical effects from a
metallurgical point of view

4. Aim and Objectives
• This review papers purports to correlate the machining and wear
operations with metallurgical phenomena like phase diagrams, ,
phase transformations, equilibrium and martensitic
transformations, microstructural design, Hall -Petch relationships,
polymorphism and allotropy, spinodal decomposition, Cottrell
atmospheres, Orowan looping and Ostwald ripening , Ageing,
Guinier -Preston zones, diffusion and inter-diffusion, precipitation
and particle coarsening, Interfaces of surfaces and sub-surfaces,
recovery, recrystallization and grain growth, theory of thermally
activated growth, transformation kinetics, isomeric and chiral
transformations in polymers, molecular weight and structural
transformations and hygrothermal effects of soaking and cooling ,
to name a few that are of great significance in the machining
processes and structural health monitoring of machined parts. The
consequent strengthening and weakening mechanisms are also
discussed.

5. Scope of This Presentation
• This paper effectively brings out the importance of
metallurgical effects due to machining and wear and
demonstrates the necessity of these inclusive studies that
create a safe and consistent procedure for structural health
monitoring and life cycle assessment of machined and
worn material systems. It emphasizes the need to
incorporate structural transformations and
thermomechanical effects in the machining/wear
processes to enable post- operative failure prevention
initiatives and assist in a feedback quality control that
would improve machining surface and structural integrity
rather than an merely exist on an ignorant productivity
based approach.

6. The Contents
• Correlation of machining and wear operations with
metallurgical phenomena, structural transformations
and thermomechanical effects is discussed.
• Case Studies that have intense requirements on
correlational consequences between structure and
machining/tribology of biomedical composites, low
melting materials , polymer composites, high melting
materials, gas turbine blades and low tolerance
systems are highlighted with evidence.
• Useful conclusions are drawn based on the presented
investigations and case studies.

7. Machining Operational Parameters
• Cutting Tool Geometry
• Feed Rate
• Depth of cut
• Rake angle
• Spindle speed in RPM
• Orthogonal cutting
• WJ and AWJ parameters
• Laser or EBM parameters
• To name a few …….

8. Tribological Parameters
• Wear, Rate, Volume, Ratio, Specific….
• Friction, Coulomb, Static, Stick-Slip, Dynamic,
Coefficients of Friction…
• Lubrication: Air, Gas, Water, Oil, Grease,
Paints, Dispersions, Suspensions, Emulsions…
• Disc on Disc, Pin on Disc, Pad on Disc, LRT, Ball
mill wear, Bearing tribometers….

9. Machining Optimization
• Taguchi
• Orthogonal arrays
• ANOVA, Regression, Genetic algorithms
• ANN
• ANFIS
• Grey Relational Approach
• Often aimed at maximizing MRR for a set of
optimum machining parameters.
• Materials Properties or Structural Health of the
Work Piece is never a priority.

10. Status of Present Publications
• There is no single publication on machining or
tribological optimization that takes metallurgical
effects in to account and incorporates them in to
the models.
• ANN, Grey relational, Taguchi or ANFIS are
considered by thousands for optimization of
machining and geometry only like removal rates
and surface roughness.
• The specimen workpiece to be treated like a
patient for SHM ( Structural Health Monitoring).
• The machining clinic not a barbershop !

11. A Negative Case Study 1: Abrasive
but………
Abrasive machining of porcelain and zirconia with a dental handpiece
Authors
L Yin, S Jahanmir, LK Ives, Publication date, 2003/9/30, Journal Wear, Volume 255
Issue 7, Pages 975-989, Publisher, Elsevier, Description
The machining characteristics and material removal mechanisms of two dental ceramics—
feldspathic porcelain and yttria-stabilized tetragonal zirconia—were investigated using a
dental handpiece and diamond burs with different grit sizes. The material removal rates
were measured as a function of total machining time using a constant load of 2N on the bur,
consistent with clinical cutting conditions. As the diamond grit size was increased from
ultrafine (UF)(10μm) to fine (F)(41μm) and coarse (C)(172μm), the removal rate and the ...

12. A Neagative Case Study 2: Precise in
Geometry But…..

14. Thermomechanical Effects and
Metallurgical Transformations

15. Spinodal Decomposition during
Machining
• Spinodal decomposition of a structure is one into regions of high
and low concentrations .
• Kinetics of decomposition is controlled by mobility of atoms only,
i.e. by lattice diffusion.
• It is primarily a diffusion problem in a thermodynamically unstable
solid solution.
• Face milling, lapping, planetary polishing, flat surface grinding and
honing over an area is less adiabatic and near isothermal.
• Spinodal decomposition at the surface can severely affect fatigue
properties due to local concentration differentials that might
produce surface cracks in service.
• Pre or Post machining heat treatment must take this in to account
• Post failure studies often erroneously point to fatigue failures of
components that failed in service and ignore machining
abnormalities.

16. Consequences of Spinodal
Decomposition
• When an area is being face
milled/ honed, isothermal
conditions can prevail giving
rise to spinodal
decomposition.
• Aluminium alloys, Al-Zn
systems and glasses have all
shown spinodal
decomposition.
• Machining optimization must
be metallurgical and not
operational and productivity
based.
• Courtesy: Romesh C Sharma, IITK, Phase
Transformations in Materials, CBS, 2002.

17. Order-Disorder Transformations
• Order-disorder transformations occur in solid solutions
that have negative enthalpy of formation, i.e. attractive
interaction between unlike atoms. eg: BCC based CuZn,
CoFe and FCC based Fe3Al, Cu3Al,AuCu, AgTi etc. that
are called Super-lattice structures.
• In many machining or wear operations these materials
exhibit order-disorder transformations that show very
high specific heat due to ordering at high temperatures
and thermal conductivity discontinuities.
• Dry eco-friendly processes aggravate these effects.
• Courtesy: Bragg-William Theory, Bethe’s theory.

18. Consequences of Order-disorder
Transformations
• Thermomechanical and electrical properties get
altered appreciably over a case depth .
• This affects the structural health of these
components unless otherwise corrected through
optimal heat treatment procedures that are
aware of this.
• Thin sections and micro-machined parts for
micro-testing exhibit maximum influence as there
is a threat of the entire structure getting
transformed over smaller case depths.

19. Precipitation and Grain Coarsening
• Precipitation and grain growth is common among Fe-Cu
and Al-Cu alloys.
• Particle coarsening ( Oswald Ripening ) occurs in Al-Cu
alloys due to super saturation. GP zones form.
• During face milling or grinding operation where heating is
not local, the observed microstructural changes are
significant on the surface of aluminium alloys.
• Their mechanical properties are severely altered by the
choice of machining processes and heat treatment that
follows.
• No machining optimization method takes structural
transformations in to account

20. Recovery, Recrystallization and Grain
Growth
• Any machining process optimization should
consider recovery, recrystallization and grain
growth.
• Static and dynamic recrystallization phenomena
occur depending on the heat domain and
kinetics of the process. Prof. YVRK Prasad’s
deformation mechanism maps will be useful in
machining process optimization.
• In superalloys, titanium based alloys, steels and
brasses these phenomena are significant.

21. Kinetics of Transformation
• Isothermal and non- isothermal transformations like
IT-TTT and CCT transformations.
• Johnson-Mehl and Avrami models for isothermal
transformations.
• Diffusion controlled and non-diffusion controlled
transformations.
• In diffusion controlled processes the rates are not
constant and complete transformation does not occur .
During and after dry machining this can happen.
• Depending on the dynamics of machining operation
these can be relevant.

22. Thermally Activated Growth
• Accompanied by high Coefficient Of Friction.
• Interface controlled and diffusion controlled
transformations.
• Widmanstatten plate growth inward from grain
boundaries or allotriomorphs are thermally activated.
• Growth of Duplex Structures due to eutectoid
transformations.
• Discontinuous Precipitation
• Diffusional, massive and martensitic transformations.
Fe-Ni and Cu-Zn exhibit massive transformations.

23. Case Studies That Involve
Thermomechanical Effects and
Structural Transformations

24. Case Study 1: Thermomechanical
Effects in Turning of Aluminium alloys
• Turning is an adiabatic process.
• Finite Element Simulation conducted on 2024,
6061 and 7075 alloys based on the
Lagrangian formulation using the John-Cook
Model for thermomechanical effects.
• As the frictional heating and measurement
cannot be monitored precisely at the point of
contact between the tool and the workpiece,
FEA methods help.

25. Thermomechanical effects of turning
in Aluminium
• The maximum workpiece and tool temperature profiles and
stress and strain distribution for all the three alloys were
computed.
• These help in predicting structural changes in the three Al
alloys.
• John-Cook model, however, predicts instant adiabatic heating
in milliseconds and not in real time seconds, as discontinuities
in Sp. Heat and thermal conductivity at higher temperatures
are not taken in to account. Besides, the disadvantage is that
Eulerian time integration scheme is not used in this explicit
analysis, and hence the simulation is not unconditionally
stable. So, very small time steps are normally used to know
the thermal effects.
• Courtesy: Saraschandra M, Prudvi Krishna M, Bharghav Avinash, Padmanabhan. K,
ICCMM 2017, MARCH 9-10, VIT UNIVERSITY.

26. Thermomechanical Effects
Temperature distribution on cutting tool
after machining of AA6061 work piece
Shear stress on the AA7075 work
piece after machining b) Continuous chip c)
Pinched chip
at 0.25mS b) at 0.5mS c) at 0.75mS
Max 9210C
Min 21.890C

27. Simulation of Thermomechanical
effects in AA 2024

28. Case Study 2: Tribology of Dental
Polymer Composites

29. 29
Composites in Dentistry
Acrylics, Acrylic esters, Bis-
GMA, PMMA derivatives,
Ceramic filled composites
Can you make out the
difference ?
Ref: K. Padmanabhan ,
Comment on 'standards on restoratives'
2009 , Volume : 20 , Issue : 4 , Page : 516, 1000 + downloads

30. 30
Composites in Prosthodontics
Tooth is a functionally graded
composite material with enamel
and dentin. In the third maxillary
molar the occlusal stress can
be 2-3 MPa.
The masticatory heavy chewing
stress will be around 193 MPa.
A composite restorative must with
stand this with an FOS and with
constant hygrothermal attack.

31. 31
Multi Scale Composites for Dentistry
• Bis GMA, UDMA, Methacrylic Esters contain
glassy particles that are mostly less than 1 micron
in size
• Esters and acrylates/ceramic filler (barium
alumina silica glass, glassy microfillers, 0.1 to 10
microns size) restorative composites.
• Multi scale composites are also useful as luting
cements, crown and bridge materials and
cements and veneer materials.

32. 32
Wear Data for Restoratives
• Aesthetics, Shade, Reliability
• Most of the composites show a logarithmic wear
rate, linear wear rate is unwelcome !
• Wear of less than 200 microns in 10 years is
acceptable.
• Wear volume will be 0.5 to 0.8 cu. mm per annum,
enamel vs restoratives.
• Coeff of Friction is ~ 0.1 to 0.35.

33. Hygrothermal Behaviour
• Fibre reinforced plastics are known for
environmental attacks that reduce their function.
• Mechanical properties degrade over time !
• Moisture plays havoc at elevated temperatures, in
the presence of voids, defects and in low Tg plastics .
• Diffusion and osmotic pressure are the driving
mechanisms for hygrothermal attack
• Evaluation methods and surface preservation
• Important in marine, biomedical, aeronautical,
electronic and automobile applications

34. 34
Increasing moisure content
Tgw Tgo
Temperature
Strength/Stiffiness
Rubbery region
Glassy region
Dry
Wet
Variation of strength with temperature
Glass Transition Temperature

35. ASTM STP D 5229 M Rule
• The MOT( Maximum Operating Temperature) of
the material, device/component should be at
least 25 º Celsius lower than the lowest Tg
(normally wet) of the material attained after
hygrothermal equillibration. All the polymeric
materials and their composites must satisfy this
rule in order to qualify for certification for
reliability and durability.

36. The need to revise standards
36
K. Padmanbhan,

37. Implications of the American Dental
Association Standards
• ADA recommends a standard soaking time for
these polymer restoratives for further wear
testing or mechanical testing.
• But hygrothermal saturation and post saturation
in Saliva are a requirement for evaluation of
conditions prevailing in the mouth.
• ADA standard is deficient as it does not prescribe
guidelines that would enable us to study Life
Cycle Analysis ( LCA) of polymer restoratives that
undergo long term tribo-chemical reactions.

38. Case Study 3: Structural and
Mechanical Property Changes in the
Machining of Self Reinforced
Polymeric Composites

39. Self Reinforced Polymer Composites
• Current investigation on Polyethylene,
Polypropylene and Aromatic Polyamide based
SRPs.
• Machining mechanical test specimens using
laser, abrasive water jet and conventional
cutting.
• Structural transformations lead to a more than
five fold increase in elastic modulus of Nylon SRPs
and two fold increase in polypropylene SRPs.
Shore D Hardness doubles in laser machining
compared to conventional machining.

40. Self Reinforced Polymeric
Composites…..
• C-C single bond between carbon atoms is subjected
to changes in its bond strength due to the laser
machining methods where spot size, localization and
cooling rates differ and it is observed through FTIR
wavelengths that are emitted. The C-C bonds are
changed to other types of bonding like O-H, C=C,
C=O, C=C, C-N and C=N etc. These bonds are greater
in strength than the C-C bonds.
• Courtesy: Deepa A, Kuppan P, Padmanabhan.K

41. Case Study 4: Importance of Structural
Changes in Machining of Gas Turbine
Blades for High Dimensional Tolerance,
Strength and Stiffness.

42. The Common Ancestor, Ape and
Homo Sapiens

43. Gas Turbine Machining
• In the aerospace industry, where safety is paramount, both
material properties and high metal removal rates are
important; one does not choose one over the other.
• In the 1980s Rolls-Royce had very low removal rates on
compressor and turbine disks. The view was to extend tool
life and hence lower tool cost. However, they were very
uncompetitive compared to GE and P&W who used much
faster cutting techniques and throw away tools; the tool
cost was insignificant compared with the product cost. The
view at that time was that fast cutting would decimate
material properties. However, when they did the
experiments and had both properties and material removal
rates as constraints, they could meet both requirements.
• ----- Paul Howard Riley, Rolls-Royce.

44. Advanced Gas Turbine Blade
Machining
• Creep Feed Grinding
• Is a process where the depth of cut is increased while the
feed rate is decreased unlike the normal grinding practices.
It is used for large amounts of material removal in just a
few passes thus causing less stress and fatigue.
• Creep feed grinding is a highly accurate and efficient
method of machining intricate forms in a wide variety of
materials. Creep feed grinding has several advantages over
other machining and grinding methods, including increased
accuracy, efficiency, improved surface finishes, and the
ability to grind heat treated materials.
• Adaptive five axis machining to uniformly machine slightly
differing geometries !

45. ………Gas Turbine Blade Manufacturing
The blades produced in Rotherham
operate in the hottest part of the
engine at temperatures up to 200
degrees above the melting point of
their alloy and sit in a disc that
rotates at more than 12,000 rpm,
creating a centrifugal force
equivalent to the weight of a
London bus hanging off each blade.
They are grown in a special process
which ensures that they are created
from a single metal crystal to
maximise their strength. These casts
are then coated with a heat-
resistant ceramic and when in use
they are cooled with air that passes
through a series of precisely placed
holes in the blade.

46. Design for Machining Principle !
Do as less machining as possible or do
away with it !
Thank you .