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A. Roy et al.
small scale. Additionally, there is a need to improve the current state of conventional machining (CM) processes as they pose growing challenges in efficient and
economical cutting of newer materials [1–3]. As a result, various hybrid machining
techniques were proposed to enhance and improve CM. One such hybrid technique
that showed to yield tangible benefits in machining of advanced materials is ultrasonically assisted machining (UAM) [4, 5]. In UAM, high-frequency electrical energy
is converted into mechanical vibration, which is superimposed on movement of a
cutting tool with a specific intensity and in a specific direction during the machining
process [3]. Such vibration is typically of high frequency (~20 kHz) and relatively
low amplitude (12–50 μm). Due to the nature of imposed vibration, the cutting tool
periodically loses contact with the workpiece in UAM, transforming a machining
process into a micro-chipping one. UAM can lead to a considerable reduction of
average cutting forces when compared to CM [3, 4]. For example, the cutting force
in UAM was demonstrated to be reduced in excess of 50% relative to CM for nickel
alloys [6] and titanium [2, 7] alloys. Additionally, better surface quality was obtained
for workpieces in UAM [1].
Although UAM exhibits tractable benefits when compared to CM, there is a
need to understand the main micro-mechanisms that drive plasticity at small length
scales under superimposed constrained dynamic rapidly changing loading states. The
temporal scale in such a process is essentially very small. UAM results in extreme
deformation conditions in its process zone, where strain levels can exceed 2, accompanied by strain rates of up to 10
5 s
−1 and temperatures in excess of 700–800 °C.
Additionally, extremely high strain gradients are observed in this zone. Hence, the
influence of machining parameters (e.g. depth of cut and cutting speed) in UAM on
the machining-quality characteristics of workpieces needs a careful study [1, 3, 4,
8, 9]. Consequently, finite-element (FE) simulations were widely used to study the
machining processes of various metallic materials [10, 11]. An increase in demand
for high-precision micro-featured components requires a thorough understanding
of grain-level deformation of metallic materials [9, 12]. To better understand local
deformation at a tool–workpiece interface, single-crystal plasticity (SCP) is usually
employed to study micromechanics at the smallest practical length scale. Zhang
et al. [13] adopted an SCP model to describe the mechanical behaviour of grains at
mesoscale in cutting simulations of titanium alloys. In the work of Kota and Ozdoganlar [14], a significant lattice rotation due to machining-related plastic deformation
was observed in single-crystal aluminium. An initial crystal orientation of a workpiece with respect to the cutting direction was found to have a significant effect on
chip formation and a cutting force of single-crystal copper in the micro-cutting simulation of Abolfazl et al. [15] and Tajalli et al. [16]. Moreover, the work of Demiral
et al. [17] and Pal and Stucker [18] indicated that inhomogeneous plastic deformation
could affect machinability of a workpiece based on strain-gradient crystal-plasticity
simulations.
In contrast to extensive investigations of micromechanics of the CM process,
similar fundamental studies for UAM are scarce. In this paper, CM and UAM at
micro-scale were studied using SCP simulations based on our previous work [11]. A
special attention is paid to the influence of ultrasonic vibration on the cutting force
A. Roy et al.
small scale. Additionally, there is a need to improve the current state of conventional machining (CM) processes as they pose growing challenges in efficient and
economical cutting of newer materials [1–3]. As a result, various hybrid machining
techniques were proposed to enhance and improve CM. One such hybrid technique
that showed to yield tangible benefits in machining of advanced materials is ultrasonically assisted machining (UAM) [4, 5]. In UAM, high-frequency electrical energy
is converted into mechanical vibration, which is superimposed on movement of a
cutting tool with a specific intensity and in a specific direction during the machining
process [3]. Such vibration is typically of high frequency (~20 kHz) and relatively
low amplitude (12–50 μm). Due to the nature of imposed vibration, the cutting tool
periodically loses contact with the workpiece in UAM, transforming a machining
process into a micro-chipping one. UAM can lead to a considerable reduction of
average cutting forces when compared to CM [3, 4]. For example, the cutting force
in UAM was demonstrated to be reduced in excess of 50% relative to CM for nickel
alloys [6] and titanium [2, 7] alloys. Additionally, better surface quality was obtained
for workpieces in UAM [1].
Although UAM exhibits tractable benefits when compared to CM, there is a
need to understand the main micro-mechanisms that drive plasticity at small length
scales under superimposed constrained dynamic rapidly changing loading states. The
temporal scale in such a process is essentially very small. UAM results in extreme
deformation conditions in its process zone, where strain levels can exceed 2, accompanied by strain rates of up to 10
5 s
−1 and temperatures in excess of 700–800 °C.
Additionally, extremely high strain gradients are observed in this zone. Hence, the
influence of machining parameters (e.g. depth of cut and cutting speed) in UAM on
the machining-quality characteristics of workpieces needs a careful study [1, 3, 4,
8, 9]. Consequently, finite-element (FE) simulations were widely used to study the
machining processes of various metallic materials [10, 11]. An increase in demand
for high-precision micro-featured components requires a thorough understanding
of grain-level deformation of metallic materials [9, 12]. To better understand local
deformation at a tool–workpiece interface, single-crystal plasticity (SCP) is usually
employed to study micromechanics at the smallest practical length scale. Zhang
et al. [13] adopted an SCP model to describe the mechanical behaviour of grains at
mesoscale in cutting simulations of titanium alloys. In the work of Kota and Ozdoganlar [14], a significant lattice rotation due to machining-related plastic deformation
was observed in single-crystal aluminium. An initial crystal orientation of a workpiece with respect to the cutting direction was found to have a significant effect on
chip formation and a cutting force of single-crystal copper in the micro-cutting simulation of Abolfazl et al. [15] and Tajalli et al. [16]. Moreover, the work of Demiral
et al. [17] and Pal and Stucker [18] indicated that inhomogeneous plastic deformation
could affect machinability of a workpiece based on strain-gradient crystal-plasticity
simulations.
In contrast to extensive investigations of micromechanics of the CM process,
similar fundamental studies for UAM are scarce. In this paper, CM and UAM at
micro-scale were studied using SCP simulations based on our previous work [11]. A
special attention is paid to the influence of ultrasonic vibration on the cutting force