Abstract
The molecular dynamics simulations were conducted to investigate the effects of random roughness on nano-cutting of γ-TiAl alloys. To simulate the actual workpiece surface, the random surface roughness was generated by a multivariate Weierstrass-Mandelbrot (W-M) function under the condition of different rake angles of cutter and depths of cutting. The equivalent height of the workpiece was used to quantify the depth of cutting. The molecular dynamics simulation results reveal that the roughness has a profound effect on the nano-cutting quality of the workpiece. Besides, the effects of roughness are also different under different cutting parameters.
Science Press
TiAl alloys have a widespread application in the aviation industry and automobile industry due to their high strength, low density, and perfect high-temperature performanc
Although the precision cutting at nanoscale can be realize
However, most MD simulations are idealized, especially for the studies of machining with the hypothesis of smooth surface, which is impossible in the practical circumstance. Thus, the effect of roughness on cutting performance through MD simulation is barely investigated. Li et a
In this research, a series of MD simulations were conducted to research the effects of roughness on nano-cutting process of γ-TiAl alloy. Firstly, the Weierstrass-Mandelbrot (W-M
As shown in

Fig.1 Crystal structure of γ-TiAl alloy
The multivariate W-M functio
(1) |
where M is the number of ridges; nmax is the frequencies of ridges; n and m are the initial values of ridge number and frequency, respectively; γ represents the radio of amplitude to frequency of the cosine shape, and γ is set as 1.
(2) |
where G is the roughness coefficient to indicate the surface amplitude. Theoretically, the fractal will be perfect if n approaches infinity. However, the value of nmax can be calculated by
(3) |
where Lmin is the minimum size of the specimen. To quantify the surface roughness, the root-mean-square (RMS) of the surface roughness can be obtained by
(4) |
where zi is the coordinate height of point i; N is the number of points.
In this research, all the simulations were implemented by Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS

Fig.2 Generation process of surface with random roughness
The nano-cutting model in

Fig.3 MD simulation model of nano-cutting process of γ-TiAl alloy
To qualitatively study the effects of roughness on nano-cutting process, three workpieces with RMS=0, 6.17, 9.59 and G=0, 1

Fig.4 Simulated rough surface morphologies with different RMS values
The accuracy of the potential function is critical to the effectiveness of the simulation. There are mainly two kinds of the potential functions in this research: embedded atom method (EAM) and Morse potential. EAM is widely performed in MD metallic system
(5) |
(6) |
where Fi is the embedding energy of atom i with an electron density ρi; φij represents the relative potential energy between atom i and atom j; rij is the distance from atom i to atom j. The interaction between Ti-Ti, Ti-Al, and Al-Al was described by EAM function in Ref.[
(7) |
where D is the intensity of action between two atoms, α represents the effective range, and r0 is the equilibrium distance of two atoms. The parameters of Morse potential to describe Ti-C and Al-C are shown in
The effects of DOC and angles of cutting edge of surfaces with different roughness were investigated under the condition of DOC varying from 1 nm to 3 nm, and the two types of rake angles were considered.
Fig.

Fig.5 Cutting processes of simulated rough surface with RMS=6.17 and equivalent DOC of 1 nm: (a) equilibration; (b) initial cutting;
(c) restoration of equilibration after cutting
The rake angle of tools was set as 30°.

Fig.6 Material responses during nano-cutting of γ-TiAl alloy with DOC=1 nm and different surface roughness: (a) RMS=0, (b) RMS=6.17, and (c) RMS=9.59
In the case of RMS=0, 6.17, the trends of defect evolution are similar. At first, only a few isolated defective atoms are generated. The bcc atoms originate from the amorphous phase transition and the hcp atoms result from the nucleation before stacking fault formation. However, the stacking fault is difficult to form due to the small DOC. The stacking fault shortly forms and soon disappears in the cutting process, because the stacking fault is quickly released by the thermal activation energy. Finally, after the restoration of equilibration, the stacking fault is formed inside the chip again, which is derived from the amorphous atoms. The effects and evolution of roughness on cutting process are quite different with RMS=9.59. It is observed that the final stacking faults are formed at different positions and in different shapes. Due to the randomness of rough surfaces, it is difficult to analyze the specific defects using conventional theories. Therefore, the theories in Ref.[
The process of phase transformation can be divided into two categories in the nano-cutting process: contact-induced amorphization (CIA) and strain-induced phase (SIP) transformation. SIP transformation includes the strain-induced amorphization (SIA) and strain-induced ordered lattice change, while CIA usually contributes to the subsurface defects in the process of the chip formation and SIP transformation. The time distribution of the number of defective atoms is shown in

Fig.7 Time distribution of the number of defective atoms under different RMS conditions with DOC=1 nm
Besides, the stacking fault can also occur in CIA region after equilibration. When DOC is 1 nm, it can be observed that defective atoms in workpieces are mainly concentrated in the chips, as shown in

Fig.8 Material responses during nano-cutting of γ-TiAl alloy with DOC=3 nm and different surface roughness: (a) RMS=0, (b) RMS=6.17, and (c) RMS=9.59

Fig.9a shows the change curve of the number of defective atoms with time, while the number of defective atoms in the chip and on subsurface of the equilibrated workpieces is demonstrated in Fig.9b. It can be seen from Fig.9a that the curves under different roughness conditions show the similar trends compared with those in

Fig.10 Residual hydrostatic stress distributions on different rough surfaces before and after equilibration: (a) RMS=0, t=250 ps; (b) RMS=0, t=300 ps; (c) RMS=9.59, t=250 ps; (d) RMS=9.59, t=300 ps

Fig.11 shows the evolution of cutting force with RMS=0, 9.59 and different DOCs. Three phenomena need to be noticed: (1) with increasing the DOC, the cutting force also tends to increase; (2) when DOC=1 nm, the roughness can significantly reduce the normal cutting force rather than the tangential force; (3) as DOC increases, the effect of roughness on the normal cutting force is negligible. In brief, the effect of roughness becomes less obvious with increasing the cutting depth.

The average cutting forces under different conditions are shown in Fig.12. As shown in Fig.12, when DOC=1 nm, the difference between the cutting force along x-direction Fx and the cutting force along z-direction Fz is augmented with increasing the surface roughness. Fz is decreased obviously with increasing the roughness. However, when DOC=3 nm, the difference between Fx and Fz under different roughness conditions is negligible, because with increasing the DOC, the stable stacking faults begin to form, thereby increasing the cutting force. Compared with the effects caused by stacking faults, the geometry effect is slight. In addition, when DOC=3 nm, the change trends of Fx and Fz are opposite. Because the surface roughness originates from the statistical data and the variation trend of Fx depends on the specific fractal condition of the surface, the specific change trend of cutting force is meaningless. However, the opposite trends of cutting forces at different cutting depths can reveal the qualitative conclusions. When DOC=1 nm, no stable stacking faults can be formed, as shown in

Fig.13 shows the average temperature of the cutting process under different cutting conditions. With increasing the DOC, the average temperature is increased almost linearly regardless of the roughness. However, due to the random surface fractals, the average temperature also changes randomly. Hence, the roughness cannot be used as a basis for temperature changes. Therefore, the surface fractal can influence the temperature of the cutting process, but the surface roughness will not have a qualitative influence on cutting temperature when DOC is increased from 1 nm to 3 nm.
The rake angle is also considered during the simulations.

Fig.14 Residual defect distributions under the conditions of rake angle of -30° with RMS=0 (a) and RMS=9.59 (b)

The number of subsurface defects under the condition of RMS=9.59 is more than that under RMS=0. The fewer the chips, the larger the SIP region; the larger the SIP region, the more defective atoms in subsurface. Meanwhile, the number of defective atoms in chips is not sensitive to roughness, as shown in Fig.15, because the chip is always pressed into the surface and can never accumulate. Hence, when the tool stops moving and the workpiece begins to restore the equilibration, there is no difference in chip size of different workpieces.
1) When the cutting depth is 1 nm, only the effects caused by contact-induced amorphization (CIA) region are sensitive to the surface roughness. The effects caused by strain-induced phase (SIP) region in the cutting process are quickly released due to the thermal activation. Meanwhile, the scale of CIA region in the cutting process is decreased with increasing the surface roughness.
2) When the cutting depth is 3 nm, both CIA and SIP regions are important. However, with increasing the surface roughness, the number of defective atoms is decreased in CIA region, whereas that in SIP region is slowly increased. CIA region shows more obvious influence when the cutting depth is 3 nm.
3) The cutting force is decreased with increasing the cutting depth. Under the condition of shallow cutting depth, the surface shape has a great influence on the cutting force. However, with increasing the cutting depth, SIP region begins to dominate the influence on the cutting force. Besides, the surface fractals can influence the average temperature in the cutting process regardless of roughness with increasing the cutting depth from 1 nm to 3 nm.
4) Compared with that with a positive rake angle, the cutter with a negative rake angle can produce more defects in the cutting process. The increase in roughness can also increase the number of defective atoms on the subsurface of workpieces. The effects are more obvious when the cutter has a negative rake angle.
5) The surface roughness can affect the number of defects on the subsurface and in the whole alloy system. The increase in roughness can lead to the growth of defective atoms.
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