Research on the Electric-Pulse-Assisted Turning Behavior of TC27 Alloy

Abstract

As a difficult-to-machine material widely used in the aerospace industry, the high-quality and efficient cutting of titanium alloys has been a hot issue in the field of machining. This work performed electric-pulse-assisted turning (EPT) and conventional turning (CT) for Ti5Al4Mo6V2Nb1Fe (TC27) alloy with two kinds of cutters. The results showed that electric pulses significantly improved the machining performance and surface finish quality. With the help of the electric pulse, the cutting force of TiAlN-coated cemented carbide insert (CCI) and uncoated carbide insert (UCI) tools during turning was reduced by more than 20%, and the surface roughness decreased by about 30% with a root mean square of the current density of 0.9 A/mm², because the larger current led to a more obvious electro-plasticity effect. Compared with CT, the surface microhardness of the EPT samples processed with different current densities decreased; this is because the hardened surface was softened by both thermal and athermal effects under the continuous pulse current. However, the microhardness of the EPT samples was still higher than the matrix. At the same time, EPT reduces the wear of the cutting tool, thus helping to extend the tool’s service life. Finally, the electric pulse parameters recommended to assist turning are as follows: an electro-pulsing frequency of 600 Hz, a root mean square of the current density in the cross-section of 0.57 A/mm², and a lasting time of the current per circle of 100 μs.

1. Introduction

Titanium alloys have outstanding advantages in terms of their specific strength (strength/density), thermal strength, low-temperature performance, and corrosion resistance. These advantages have led to titanium alloys being widely used in the aerospace field in the manufacture of jet engine compressors, turbine discs, blades, and magazines [1,2,3,4]. However, titanium alloys are typically difficult-to-machine materials with poor machining performance. Their characteristics are as follows [5,6,7,8,9]: (1) low thermal conductivity but high cutting temperature; (2) high mechanical strength; (3) high tip stress; (4) low modulus of elasticity and workpiece rebound; and (5) high-temperature chemical activity and serious tool wear. Thus, machining titanium alloys with high efficiency and quality is a huge challenge for modern industry.

O.A. Troitskii et al. [10] first reported the electro-plasticity effect of metal materials. It has excellent characteristics in the processing of metal materials, which combines the thermal effect and the athermal effect (electronic wind effect). The thermal effect is due to the scattering effect of ion solids inside the conductor on the drifting electrons. When the electric field is applied to the metal material, a large number of the drifting electrons undergo inelastic collision with ion solids, which accelerates the vibration of the ion solids inside the metal and converts the kinetic energy of the drifting electrons into the internal energy of the metal. This macroscopic phenomenon is manifested in the rise in the temperature of the metal materials. For athermal effects, it is believed that under the action of the current, the internal defects of the metal (atoms and dislocation, etc.) collide with drifting electrons, and part of the kinetic energy of the electrons is transferred to the defects, thus reducing the deformation resistance of the metal. Under the coupling effect of the thermal effect and the non-thermal effect, the deformation resistance of metal materials decreases significantly. This phenomenon is called the electro-plasticity effect. Conrad [11,12,13] confirmed the existence of thermal and non-thermal effects. Hameed et al. [14] explored the effect of electro-plastic cutting on energy consumption. They found that electric pulses reduced the cutting energy of 7075 aluminum alloy and 1045 steel by 27% and 17%, respectively. Egea et al. [15] used electric pulses to assist in cutting 1020 carbon steel, 1045 carbon steel, and 4140 carbon steel. The results showed that a high-frequency pulse current can reduce the surface roughness and the surface-hardening rate of the workpiece. Wang [16] performed electrical-pulse-assisted turning of stainless steel AISI304. Their study found that the use of electrical pulses in the turning process can improve the plastic deformation ability of the cutting area and the lubricity between the workpiece and the cutting tool. When the pulse current density was 1.6 A/mm, the decrease in microhardness was 61.7% compared to that of ordinary cutting.

The literature shows that the application of a pulse current can improve the plasticity of metal materials and then reduce the resistance of processing and obtain the low friction coefficient of the material [11,12,13,14,16]. The electro-plastic effect not only accelerates the microstructure transformation of the second shear zone but also increases the plastic deformation of the particles and the deformation coefficient of the chips [17,18,19]. Finally, one study improved the machining performance [15]. In summary, the electro-plastic effect provides an effective method to improve the machining performance of titanium alloy materials.

The above research progress shows that EPT has been widely used in ceramic and metallic materials, and excellent results have been achieved. However, there are few reports on EPT for titanium alloys, especially the high-strength TC27 alloy, and its turning mechanism is still unclear. EPT is expected to improve the turning performance of TC27 alloy and significantly improve the quality of the turned surface. In this study, we explored the mechanism of EPT and its advantages over CT. So, the effect of EPT can be judged without lubrication and the turning process is set to dry turning. To investigate the effect of different EPT parameters on the cutting performance of TC27, the cutting force, cutting temperature, surface roughness, and surface microhardness were measured. Additionally, we also compared the surface topography, workpiece cross-section, chip surface, and the wear of the cutting tool of both CT and EPT.

2. Experimental Procedure

2.1. Materials and Cutting Tool

In this study, the original material was a forged commercial TC27 alloy that had not undergone any heat treatment processes, and the microhardness was measured as 370 HV_0.2. After machining, a 16 mm-diameter bar with a length of 180 mm was used for the later investigation of the turning properties. Its chemical composition and mechanical properties are shown in

Table 1. Chemical compositions of the titanium alloy TC27.

Element	Al	Mo	V	Nb	Fe	Si	C	H	O	Ti
wt %	5.59	4.10	5.92	1.94	1.02	0.054	0.0064	0.0016	0.074	81.294

and

Table 2. Mechanical properties of TC27.

Yield Strength (Mpa)	Ultimate Tensile Strength (MPa)	Reduction in Cross-Section Area (%)	Elongation (%)
1047	1111	70.3	15.2

, respectively.

In order to investigate the microstructure of TC27, a cube with a side length of 10 mm was cut from the workpiece by electrical discharge machining (EDM). The cube was ground with SiC abrasive paper from 360 # to 3000 # grit and polished. Then, the specimen was etched with Kroll’s reagent (6.5% HF + 15.6% HNO₃ + 77.9% H₂O) for 10 s to observe the microstructure. Finally, a LEICA CTR4000 optical microscope (Leica, Weitzlal, Germany) was used to observe the microstructure image, as shown in Figure 1.

The specification of the tool holder was MCLNR2020K12C (ZCCCT Co., Ltd., Zhuzhou, China). After machining with different turning tools, the turning surface quality was different. In order to investigate whether the improvement of EPT on the turning surface quality is universal, two different tools were selected for turning experiments. A TiAlN-coated cemented carbide insert (CCI) with a chip-groove structure (CNMG120404-MA VP 15TF, Mitsubishi Materials, Tokyo, Japan) was used. The substrate material of the insert was ultra-fine grain cemented carbide. Another one was an uncoated carbide insert (UCI) (CNMG120404 (K20), Mitsubishi Materials, Tokyo, Japan) with the material of WC-Co (UTi20T). Two kinds of inserts had the same geometrical parameters when fixed in the tool holder, as shown in

Table 3. Geometry parameters of the tool holder with the insert.

Relief Angle	Cutting Edge Angle	Minor Cutting Edge Angle
5°	95°	5°

.

2.2. Machining Experiment

During the experiment, a dry experiment was conducted on a CNC CA6150i lathe (DMTG Co., Dalian, China) without lubricant. The workpiece was clamped on the lathe, as shown in Figure 2. The cutting tool and insulating parts were also clamped. The lathe was turned on and it ran at a low spindle speed. Then, we supplied the pulse power and set the frequency, current, and other parameters of the pulse power supply for each sample. We used a thermocouple thermometer to measure the surface temperature of the workpiece until it reached equilibrium. We adjusted the spindle speed and feed rate of the lathe to perform electrical pulse auxiliary machining on the workpiece. In order to measure the cutting force, a YDC-III89A piezoelectric quartz dynamometer (Dalian University of Technology, Dalian, China ) was integrated with the turning tool using a work fixture, and then the integrated device was installed on the lathe. All of the parameters were measured five times and the average value was taken. According to the results of previous research, the optimized cutting parameters were adopted, that is, a linear speed of 40 m/min, a cutting depth of 1.0 mm, and a feed rate of 0.1 mm/r [20]. In CT, we turned off the pulse power, but the experiment was still performed according to the established lathe motion parameters.

The electrical pulse parameters and equilibrium temperature of the workpiece are shown in

Table 4. Cutting parameters of CT and EPT.

Specimen Number	Electro-Pulsing Frequency (Hz)	Current Density of the Amplitude in the Cross-Section (A/mm2)	Root Mean Square of the Current Density in the Cross-Section (A/mm2)	Lasting Time of the Current per Circle (μs)	Equilibrium Temperature of the Surface (°C)
1	0	0	0	0	26
2	500	0.67	0.42	100	103
3	600	0.89	0.57	100	154
4	700	1.42	0.90	100	197

; each parameter was measured three times, and the average was taken. The equilibrium temperature refers to the stable surface temperature of the EPT sample before turning. The equilibrium temperature of the workpiece surface increased with the current density. It reached about 200 °C when the root mean square of the current density in the cross-section was 0.90 A/mm². The root mean square current density in the cross-section J_r was computed by Equation (1), as follows:

$$J_{\mathrm{r}}=\frac{\sqrt{(t_1\times I_A{}^2+t_2\times {(60\%I_A)}^2)/T}}{S}$$

(1)

where t₁ is the pulse on time (pulse width), t₂ is the pulse off time (pulse interval), T is the pulse period, I_A is the peak current, and S is the cross-sectional area of the workpiece. A screw thread micrometer was applied to measure the diameter of the sample so that the cross-sectional area of the sample could easily be calculated. Figure 3 shows the schematic diagram of the applied ideal pulse current waveform.

The plasticity of the workpiece increased with the equilibrium temperature, which can reduce the deformation resistance effectively. Therefore, the current can be appropriately increased in the cutting process to improve the processing performance. However, when the loading current is heavy and the equilibrium temperature is high, it will cause oxidation of the surface and reduce the processing quality [21]. In addition, excessive current will penetrate the workpiece and cause its failure [22]. Experiments and finite element simulation have proven that a high temperature of nearly 500 °C can be reached in the traditional turning process [23,24]. If the surface equilibrium temperature is too high, it will generate a higher temperature during the turning process and deteriorate the turning surface. Therefore, the maximum equilibrium temperature of the workpiece in this study was controlled below 200 °C during the experiment.

2.3. Characterization of the Experiment

A Vickers hardness tester (MVS 1000D1; Guangzhou, China) was used to measure the microhardness along the normal direction at a load of 200 N and a dwelling time of 15 s. For every parameter, a microhardness of five points was measured and averaged for a set. The axial surface roughness of the turning workpiece was tested by MarSurf M300C (Mahr GmbH, Göttingen, Germany). For the set of turning parameters, six groups of roughness were measured and averaged. The surface topography of the machined surface was obtained using a three-dimensional optical profilometer (RTEC UP Dual, RTEC Co., San Jose, CA, USA). The surface deformation layer was prepared and observed as described in Section 2.1. For the cutting tool, the microstructure images of both the flank face and the rake face were obtained using Quanta 200 FEG SEM (FEI Co., Hillsboro, OR, USA).

3. Results and Discussion

3.1. Effect of Electric Pulses on the Cutting Performance of the Titanium Alloy

3.1.1. Cutting Force

The tangential force (F_t) is the component force along the cutting speed, which consumes most of the cutting power and has a greater impact on the machining process, tool wear, and machining quality in the cutting process. Therefore, taking the F_t as the main research content in this section.

The variation curve of F_t with current strength is shown in Figure 4. It can be seen that the F_t of both of the turning tools during EPT was less than that of CT. In addition, the cutting force showed a downward trend with the increase in the current density, with it achieving a maximum drop rate of up to 30% at 0.6 A /mm² root mean square of the current density. Research shows that the higher the heating temperature of the workpiece, the more fully the material surface softens and the lower the yield strength of the surface layer. This is the reason for the significant reduction in turning forces [25]. In this study, the plasticity of the workpiece was increased during EPT, while the deformation resistance and yield strength were reduced. These reasons caused the improvement in the cutting performance. Under the action of heat and electronic wind, the generation and migration speed of dislocation inside the workpieces were accelerated. Thus, it was difficult for dislocation entanglement and the dislocation wall to be formed. Finally, the main cutting force in EPT was reduced.

3.1.2. Surface Roughness

The axial roughness of the machined surface in different electric pulse parameters is shown in Figure 5. The surface roughness values of CT using CCI and UCI were Ra 1.25 μm and Ra 1.2 μm, respectively. In EPT, the surface roughness decreased significantly with the increase in the current density. When the root mean square of the current density was 0.9 A/mm², the roughness decreased to Ra 0.9 μm and Ra 0.8 μm, with a decrease of 28% and 33%, respectively. Moreover, the surface roughness of the workpiece processed by the (TiAl)N-coated tool was better than that of the uncoated tool.

As shown in Figure 6 and Figure 7, the surface topography of EPT was also improved, compared to that of CT. The regular concave and convex morphology formed on the workpiece surface during the turning process is called the feed mark. The convex shape is called “wave peaks” and the concave shape is called “wave valleys”. It can be seen that the “wave peaks” and “wave valleys” of CT were obvious. However, the “wave peaks” and “wave valleys” of EPT were generally shallow, which was even smoother with the increase in the root mean square of the current density. When the root mean square of the current density was 0.9 A/mm², we obtained the flattest surface topography. This is because the electro-plastic effect of the pulse current increased the plasticity and simultaneously reduced the deformation resistance of the workpiece surface. The processed material was squeezed by the tool to produce plastic flow, which optimized the surface topography and improved the surface quality of the workpiece.

3.1.3. Surface Work Hardening

The microhardness of the machined surface for each parameter of the electric pulse is shown in Figure 8. After conventional turning with UCI and CCI, the surface of the workpiece underwent severe work hardening. The surface microhardness value increased from 370 HV_0.2 to 426 HV_0.2 and 422 HV_0.2.

Compared with conventional turning, electric-pulse-assisted cutting obviously reduced the microhardness of the machined surface. When the root mean square of the current density was 0.90 A/mm², the surface microhardness after machining with UCI was 396 HV_0.2, which was 7.0% lower than that of 426 HV_0.2 in CT, while the surface microhardness after machining with CCI was 387 HV_0.2, and this was 8.3% lower than that of 422 HV_0.2. The surface microhardness value of the workpiece decreased with the increase in the root mean square of the current density value. The combined effect of the thermal and athermal effects after applying the electric pulse caused a softening of the workpiece surface, which weakened the work hardening and made cutting easier.

When the root mean square of the current density was 0.90 A/mm², the surface microhardness value of the workpiece decreased with the increase in the current density. The combined effect of thermal and athermal effects after applying the electric pulse caused a softening of the workpiece surface, which weakened the work hardening and made cutting easier.

In addition, the work hardening of CCI was lower than that of UCI.

3.2. Cross-Section Microstructure of the Workpiece

The influence of different processing conditions on the deformed layer is shown in Figure 9 and Figure 10, the thickness of the deformation layer was measured by Image J software and averaged. Figure 9 shows that the deformation layer is about 11 µm thick after CCI turning and up to 18 µm after electric-pulse-assisted turning. Figure 10 shows that the deformation layer is about 7 µm thick after UCI turning, and there is almost no change in the thickness of the deformation layer of about 8 µm after electric-pulse-assisted UCI turning. Comparing the two figures, it was found that pulsed-current-assisted CCI processing can significantly increase the thickness of the deformation layer, while pulsed-current-assisted UCI processing does not produce much of an effect.

There are two main reasons as to why the surface deformed layer was thicker after processing by CCI. First, the pulse current can promote the movement of dislocations, thus resulting in the plastic deformation resistance of the material being reduced [26]; second, the coating had good thermal insulation properties that prevented the heat from being able to enter the tool [27] and it gathered around the blade and workpiece, which enhanced the plasticity of the material.

3.3. Morphology of the Chips

The cross-sections of the chips processed by UCI and CCI are shown in Figure 11 and Figure 12. Black arrows point to the chip scratches. The width of the adiabatic shear band of the chips produced by both of the two kinds of cutters widened with the increase in current density. According to the adiabatic shear theory, the cutting force and heat are coupled during the formation of the chip [28]. The material undergoes shear deformation in the first deformation zone, which results in the work-hardening effect. In addition, the heat processed by shear deformation is concentrated in the shear zone. This makes the deformation process adiabatic, which causes a rise in the temperature in the shear zone and softens the material. When the thermal-softening effect exceeds the work-hardening effect, thermoplastic instability occurs in the shear band, resulting in the formation of sawtooth chips. The higher the current density, the higher the temperature and the more obvious the adiabatic shear band.

Moreover, the adiabatic shear band produced by CCI was wider and more obvious, for the (TiAl)N coating had higher microhardness, excellent high-temperature oxidation, and high-temperature wear resistance. In particular, its low friction coefficient can make the “shear slip” in chip formation smoother [29,30,31], thus the formation of the adiabatic shear band was an easier process than that of UCI. The “shear slip” in this study means that: due to the enhancement of plasticity by electric pulse, when the shear deformation of the material is not complete, the deformation force generated by the tool has exceeded the binding strength of the material here, resulting in the peeling and tearing phenomenon, resulting in the reduction of the feed mark height. The more concentrated heat also made the adiabatic shear band wider and more obvious.

3.4. Wear Mechanisms of the Tool

3.4.1. Flank Face Wear

The side wear morphology of the two kinds of cutting tools under different process parameters is shown in Figure 13 and Figure 14. Both UCI and CCI caused adhesive wear under different cutting conditions.

Table 5. EDS analysis of A, B, C, and D.

Element (wt%)	Ti	Al	Mo	V	Nb	Fe	Co	N
A	75.66	4.68	3.93	6.11	1.74	1.07	6.81	/
B	84.90	4.60	3.13	6.40	/	0.96	/	/
C	80.88	5.69	4.28	6.16	2.07	0.90	/	/
D	79.94	5.05	4.76	6.74	2.27	1.24	/	/
Coating	35.83	30.75	/	0.13	/	/	0.1	33.19

shows the chemical components of points A, B, C, and D and the coating. The material of the workpiece adhered to the flank face of the blade. The reason for this was that the high temperature and pressure acted on the titanium alloy and inspired strong chemical adhesion of it, which generally reached 750–800 °C and 1–1.5 GPa at the cutting area [32,33]. Therefore, the close contact and strong friction between the workpiece and the tool made the workpiece material adhere to the flank surface during machining [34]. With the increase in the current density, the adhesion of the workpiece material on the flank face also increased. This phenomenon was observed on both the UCI- and CCI-processed surfaces. The higher the current density, the greater the Joule heat effect and the higher the surface temperature during cutting, which is the reason for the more serious adhesion of the chips after EPT.

3.4.2. Rake Face Wear

Figure 15 and Figure 16 show the wear morphology of the rake face. The built-up edge and even surface fracture occurred during the CT process. When applying the pulse current, however, no further surface chipping occurred. However, the volume of the built-up edge on the UCI increased with the current density, while that of CCI decreased. After the analysis of the chemical composition, the main component of the built-up edge was found to be titanium. It can be determined that the built-up edge consisted of the adhered workpiece material or the cutting chips.

For the UCI, there were fewer built-up edges on the rake face during CT, while there were generally more and larger built-up edges in EPT. In Figure 15d, it can be seen that the chips stuck to the rake face. Different from the UCI, the rake face of the CCI had a larger built-up edge during CT. However, after applying the electric pulse, the built-up edge sticking to the rake face became smaller and more unstable. Figure 16c shows the decrease in the built-up edge. This is because the coating can prevent the built-up edge from becoming excessively large due to its good high-temperature oxidation resistance and low friction coefficient. In addition, EPT improved the cutting conditions of the CCI, making it hard to generate a built-up edge. Overall, when assisted by an electric pulse, the CCI had better properties to prevent the built-up edge from becoming larger.

3.4.3. The Pulse Current Improves the Friction State between the Chip and the Tool

Studies have shown that the frictional contact between chips and the rake face can be divided into two areas: the sticking zone and the sliding zone [35], as shown in Figure 17. The sticking zone is close to the tip of the tool while the sliding zone is far away. In the sticking zone, a large amount of heat is caused by the strong mutual extrusion between the rake face and chips, and the chips stick to the rake face. The friction in this area belongs to internal friction and relates to the critical shear stress of the material, which is equal to the critical shear stress of the material. The sliding zone is far from the tool tip, thus heat dissipation is faster. The chips undergo cold work hardening and slide with an approximately constant friction coefficient. The friction in the sliding zone belongs to external friction and obeys Coulomb’s law, while the tangential friction force is proportional to the normal stress of the rake face.

The friction stress of the entire cutting contact area can be expressed by Equation group (1) [36]:

$$\left\{\begin{array}{cc}{\tau }_f={\tau }_{\mathrm{s}},& (\mu {\sigma }_{\mathrm{n}} \ge {\tau }_{\mathrm{s}}, \mathrm{Sticking\; zone})\\ {\tau }_f = \mu {\sigma }_{\mathrm{n}},& (\mu {\sigma }_{\mathrm{n}} \le {\tau }_{\mathrm{s}}, \mathrm{Sliding\; zone})\end{array}\right.$$

(2)

Due to the athermal effect, the movement speed of dislocation was accelerated, thus the dislocation entanglement and accumulation were dissipated. In addition, the plasticity of the material was simultaneously increased by the thermal effect. Therefore, the critical shear stress of the material in the sticking zone was reduced, which made shear slip deformation easier to achieve and reduced the wear of the tool; the friction was also reduced in the slip zone for the drop in normal stress. The heat in the cutting area was effectively reduced with the reduction in friction, and that is why the flow of chips along the rake surface was smoother. In addition, it avoided excessively large built-up edges in the rake face.

4. Conclusions

In this study, experiments on the machining performance of a TC27 titanium alloy were carried out with the assistance of an electric pulse. Moreover, the properties of UCI and CCI in different conditions were compared. Finally, we explained the mechanism of the electric-pulse-assisted turning of a titanium alloy. The main conclusions are as follows:

(1) EPT improved the cutting performance of the TC27 titanium alloy. It reduced the cutting force by more than 20% for the CCI and the UCI and the surface roughness by 28% and 33%, respectively. Compared with the matrix, the surface microhardness of the turning samples was improved, and a deeper hardening layer can be obtained by electric-pulse-assisted turning.

(2) EPT improved the surface topography of the workpiece and reduced the height of the “wave peak” and the depth of the “wave valley”. Lower cutting force and surface roughness were obtained by a root mean square current density of 0.9 A/mm². However, the surface microhardness of the sample turned was significantly lower than that of other samples and was still higher than that of the matrix. It is considered that the current density of 0.57 A/mm² not only improves the quality of the turning surface but also obtains relatively high surface microhardness. Therefore, the optimized electrical pulse parameters in this paper were a pulse frequency of 600 Hz, an amplitude current density of 0.57 A/mm², and a pulse length of 100 μs.

(3) EPT reduced the adhesive wear of the tool and significantly reduced the volume of the built-up edge at the rake face when using the CCI. Furthermore, (TiAl)N-coated carbide inserts were superior to uncoated carbide inserts in terms of the cutting force, the surface quality of the machined workpieces, work hardening, and resistance to wear.

(4) This study explained the mechanism of EPT from two aspects. On one hand, the electric pulse improved the plasticity of the workpiece material. The thermal effect softened the material and reduced the deformation resistance; the electronic wind effect (the athermal effect) accelerated the movement of dislocations to reduce their density and then reduced the microhardness and the work hardening of the surface layer. On the other hand, the electric pulse improved the friction state of the cutting area at the rake face, reduced the friction of the sticking zone and slipping zone, and finally reduced the wear of the tool. In addition, the built-up edge in the rake face can be effectively avoided.

(5) Electric-pulse-assisted turning can reduce surface roughness and turning resistance, increase the depth of the surface deformation layer, and reduce the wear of the turning tool. However, the surface microhardness was reduced compared with traditional turning, which is an inevitable trade-off relation. Compared with traditional turning, ultrasonic turning has a better surface-hardening effect and can obtain a better turning surface. The application of an electric pulse in the ultrasonic turning process may solve the trade-off relation mentioned above. Next, we will carry out a study on electric-pulse-assisted ultrasonic rolling.