advances in numerical modeling of manufacturing processes

TRANS. INDIAN INST. MET., VOL. 57, NO. 4, AUGUST 2004
Fig. 7 : Thermal conductivity and heat capacity of Ti-6Al-4V
known that the heat generated inside the workpiece
is concentrated along the primary deformation zone
and the secondary deformation zones, appearing as
thermal energy. A rough estimation of the tool rake
face temperature can be obtained using equation 11 34 :
1 2
⎛ vh ⎞
T f
= E⎜
⎟
(11)
⎝ k ρ c ⎠
where T f
is the mean temperature over tool rake
face, E is the cutting energy (assuming all cutting
energy is converted to heat), k is thermal conductivity,
is density, c is specific heat, v is cutting speed, and
h is depth of cut.
From the above equation it is seen that the thermal
properties significantly influence the temperature over
the tool/workpiece interface. The temperature varies
inversely with the half-power of the change of the
product of thermal conductivity k, and heat capacity
rc. Thus, higher temperatures are to be expected in
cutting stronger materials (high E) at higher speed,
especially if the workpiece material is a poor heat
conductor of low density, and low specific heat.
The density of Ti-6Al-4V can be thought as constant,
while the thermal conductivity and specific heat vary
with temperature. Both capacity and conductivity
increase with temperature 35 .
Poor conductivity of the titanium alloys (as compared
to steels) results in a larger portion of the heat
generated during machining being transferred to the
cutting tool, Fig. 8 36 . This leads to high tool
temperatures resulting in high tool softening and
wear.
Fig. 8 : Energy flow rate into tool vs. thermal conductivity
of tool 36 .
Cutting forces and interface pressure generated during
machining are directly proportional to the flow stress
of the workpiece material at the representative thermomechanical
conditions. During machining the titanium
alloy experiences high strains, very high strain rates
and temperatures close to its melting point. This
results in the following material response:
i. Rapid strain hardening at room temperature with
strain softening after a peak flow stress is reached
(saturation of slip density in the + phase).
ii.
As the temperature is raised due to heat
generation in the primary and secondary shear
zones, both the strain hardening and strain
softening responses reduce with phase
transformations, with almost rigid-perfectly
plastic behavior above beta transus.
iii. The strain softening of Ti-6Al-4V during
deformation varies with the change of
microstructure and much more marked flow
softening is observed in microstructure
compared to the + microstructure. The
softening rate depends on the volume fraction of
the and phases present below the transus
temperature and on the phase above this
transus.
iv. Strain rate hardening continues at all temperatures
with the strain rate sensitivity increasing at higher
temperatures. This increase in sensitivity has a
major influence on propagation of plastic
instability.
352