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Abstract
With
increased applications of metal cutting in the creation of small objects or
small features on large objects, there is a compelling need to understand the
mechanics of micro-cutting is envisaged. As the uncut chip thickness decreases,
the specific cutting energy in micro-cutting increases nonlinearly. This is
also known as ‘size effect’ or ‘scaling effect’ in metal cutting, which occurs
due to several factors such as material strengthening, subsurface deformation,
tool geometry, fracture phenomenon, workpiece microstructure, etc. The first
three phenomena are well studied and reported in the literature, whereas the
gross fracture, microcrack formation and workpiece microstructure have received
very little attention in the literature. Therefore, the major objective of this
work is to model the role of fracture phenomena in the specific cutting energy
and evaluate their contribution to the size effect in micro-cutting. The work
further aims at evaluating the contribution of workpiece microstructure to the
size effect in micro-cutting. Accordingly, comprehensive simulations using FEA
and SPH formulations have been performed in orthogonal micro-cutting on AISI
1215 and AISI 1045 steels. At the same time, elaborate experimentation has been
done to validate the fundamental physics of machining process as observed from
the simulations, and to validate the simulations.
To model the microcrack formation in
the shear zone, a failure strain based mathematical correlation considering
normal and shear stresses on the shear plane have been developed and used in
FEA simulations of AISI 1215 and AISI 1045 to identify number of microcracks
along the shear plane. As the number of microcracks increases, their
contribution to the specific shear zone energy or the size effect appear to
increase by 0-20% under various cutting conditions. Apart from microcracks
formation, fracture energy associated with gross fracture phenomena ahead of
tool-tip is quantified in terms of specific work of fracture (R) by using two different analytical
models during micro-cutting with sharp tool (Atkins’s model) as well as rounded edge tools (Karpat’s model) using FEA simulations. The analytically estimated R
value is also of the same order of magnitude as obtained from the simulations. The
contribution of fracture in the specific cutting energy is in the range of
8-36% in micro-cutting of AISI 1215 steel. Further, it is noted that R encompasses several other energies
than fracture energy. Therefore, to accurately evaluate the fracture energy and
its contribution to the size effect in the micro-cutting, J-integral is evaluated as a function of processing parameters
ahead of too1-tip by defining a contour path which covers majority of the
plastically deformed region. The contribution of fracture to the specific
cutting energy in terms of J-integral
is in the range of 4-24% under various parametric conditions and a higher
contribution is observed at lower uncut chip thicknesses.
To model microstructural aspects
i.e. grain and grain boundaries in size effect in micro-cutting, SPH
simulations have been used. In general, the cutting forces obtained with the
simulation are inversely proportional to the grain size, which is consistent
with the Hall-Petch effect. It is also seen that cutting becomes difficult when
material grain size is smaller and cutting edge radius is larger as this
increases the size effect. The contribution of microstructure to the size
effect is up to 39% depending on the grain size and cutting conditions.
Keywords: Micro-cutting, Size effect, Specific cutting energy, Microcracks, Gross fracture, Microstructure
