Dynamic Electropulsing Induced Phase Transformations and Their Effects on Single Point Diamond Turning of AZ 91 Alloy

The effects of dynamic electropulsing on microstructure changes and phase transformations of a rolled Mg-9Al-1Zn alloy were studied by using optical microscopy, X-ray diffraction, back-scattered scanning microscopy and transmission electron microscopy techniques. The decomposition of β phase was accelerated under dynamic electropulsing, compared with the conventional thermal processes. Dynamic electropulsing was less effective in affecting the phase transformations, but more effective in reducing residual stress than the static electropulsing. Dynamic electropulsing improved machinability of single point diamond turning, the mechanism of which is discussed from the point of view of dislocation dynamics.


Introduction
Alloy materials suffer various external stresses during their manufacturing and in subsequent service [1].It is of significant practical importance to reduce these residual stresses.It was reported that under electropulsing treatment (EPT), the residual stress was reduced significantly [2][3][4][5][6][7][8][9][10].There are two practical types of electropulsing: static electropulsing and dynamic electropulsing.The former combines a thermal process with electropulsing.The latter is a complex thermal process, which combines simultaneously both electropulsing and plastic deformation.
In the present work, the dynamic electropulsing induced phase transformations and microstructural changes, and their effects on Single Point Diamond Turning (SPDT) of the AZ91 alloy are studied from the point view of dislocation dynamics.

Experimental Procedures
A commercial magnesium alloy AZ91 (9.1wt% Al, 0.9wt% Zn, 0.2wt% Mn, balance Mg) was used.The ingot was homogenized at 693 K for 16 hrs and subsequently extruded into strip of 2.90 mm wide and 1.45 mm thick.The extruded strip was then rolled under electropulsing.During the rolling, the strip was moving at a speed of 2 m/min between two electrodes set at a distance of 225 mm.The thicknesses of the strip specimens before and after rolling were 1.45 mm and 1.20 mm thick, respectively.A self-made electropulsing generator was continuously applied to discharge multiple positive pulses with various current parameters on the rolling material, and it took about 10s for the strip to pass through the electrodes.During the dynamic electropulsing, the surface temperature was measured by a contact thermocouple for each test, and the current parameters including frequency, root-mean-square current (RMS), amplitude current and duration of multiple pulses were monitored by an oscilloscope connected with a Hall effect sensor.These are listed in Table 1.A schematic illustration of the dynamic electropulsing system is shown in Figure 1.
Longitudinal cross-sections of the specimens with various frequencies of EPT were polished before being examined under an electron microscope.A Hitachi-S4800 ultra-high resolution field emission scanning electron microscope was used in a back-scattered electron mode (BSEM) for examination of the microstructure of the specimens.The polished specimens were etched with a commercial solution [9] before the metallographic observation using an Olympus GX51 Inverted Microscope.An X-ray diffraction examination was performed with an X-ray diffractometer (Bruker D8 Advance), using nickel-  filtered Cu K α radiation.The range of diffraction was selected from 25˚ to 65˚ (2θ).The scanning speed was 1 degree/min.An examination of TEM was carried out using a JEOL 2010 transmission electron microscope.A Gatan 691 Precision Ion Polishing System (PIPS) was used in the preparation of a thin film of the alloy for TEM examination.After EPT, SPDT was performed on a two-axis computer numerically controlled (CNC) precision single point diamond turning machine (OptoForm 30 SPDT machine), and a face turning model of SPDT was used.The machining parameters are listed in Table 2.A force transducer Kistler 9252A was mounted with a pre-loaded force under the tool shank.A 14-bit multifunctional data acquisition (DAQ) card PCI-6132 (National Instrument) was configured on a PC workstation to record the data of the cutting force.

Electropulsing Induced Phase Transformations and Microstructure Changes
Figure 2 shows the X-ray diffractograms of the as-rolled (non-EPT) specimen and the dynamic EPT specimens.The non-EPT specimen consisted of mainly two phases,  static 105 Hz-EPT for 10 s, while that completed after dynamic 309 Hz-EPT for 10 s.A higher frequency of electropulsing was needed in order to complete the decomposition of β phase in the same duration of electropulsing in the dynamic EPT alloy specimens.
It has been reported that for the decomposition to complete under conventional thermal process of the AZ91 strips, 10 hours of ageing in the range of 663 K was required [11].This implies that electropulsing tremendously accelerated the phase transformations of the The optical micrographs of the non-EPT and the dynamic EPT specimens with various frequencies are shown in Figure 6.For the non-EPT specimen, plenty of deformation twins were observed inside the grains, as shown in Figure 6(a).When the frequency of dynamic electrouplsing increased to 126 Hz, the twins inside the grains were rarely observed, as shown in Figure 6(b).Upon further increasing the frequency to 309 Hz, a relatively homogeneous microstructure of equal-axed grains was obtained, as shown in Figures 6(c)-(e).This was because under the dynamic electropulsing the external stress that resulted from the plastic deformation was reduced simultaneously, and the twins quickly vanished under the dynamic electropulsing.
In comparison, the twins microstructure decreased gradually in the static EPT specimens [12], when the frequency increased from non-EPT to 209 Hz and 253 Hz, and disappeared after the 294 Hz-EPT.

Dislocation Dynamics
The dynamic electropulsing induced dislocation evolution was detected by TEM examination.In the non-EPT specimen, there were plenty of dislocation arrays and nodes, which were formed during the previous rolling, as shown in Figure 3 It is suggested that under electropulsing, electron wind formed and pushed the defects in the specimen, such as dislocations and atomic vacancies towards the grain boundaries, where accumulation and annihilation of dislocation occurred at the same time [5][6][7][8].
With increasing frequency of electropulsing, both the accumulation and the annihilation of dislocation increased and were in an adequate balance at the grain boundaries.Thus, the dislocation density decreased [10].

Driving Force for Dynamic Electropulsing Induced Phase Transformation
The driving force for phase transformation in the dynamic EPT alloy specimens consists mainly of three parts: where G chem is the chemical Gibbs free energy, G ep the electrpulsing induced Gibbs free energy and G stress the stress induced Gibbs free energy.In the present study, the electropulsing considerably accelerated the decom- During dynamic electropulsing new structural distortions, such as dislocations and vacancies were created.When electron wind passed through the specimens, part of the electropulsing induced Gibbs free energy (G ep ) was exhausted through interaction between electrons and dislocation and vacancies which were created during mechanical deformation.Accordingly, the electropulsng induced G ep for the reverse phase transformation was reduced.In addition, during dynamic electropulsing, the rolling induced stress reduced instantaneously, and the residual stress induced G stress in the dynamic EPT specimen was less than that in the static rolled specimen.
Therefore, the total G in the dynamic EPT specimen was smaller than that in the static EPT specimen, i.e. the dynamic electropulsing was less effective than the static electropulsing in accelerating decomposition of  phase.

Changes of Machining Properties Induced by EPT
Shown in Figure 7 and

Conclusions
In summary, it is concluded as follows:  1) The β phase decomposition was considerably accelerated by a way of up-quenching under dynamic electropulsing, compared with the conventional thermal processing.The dynamic electropulsing was less effective than the static electropulsing in accelerating decomposition of  phase.
2) Under dynamic electropulsing, the twins microstructure disappeared in the rolled alloyAZ91, and a homogeneous fine grain structure was achieved.Dynamic electropulsing was more effective in reducing residual stress than the static electropulsing.
3) Under dynamic electropulsing, the SPDT cutting force decreased and surface roughness improved.The machinability of AZ91 alloy was improved.

Acknowledgements
The work described in this paper was partially supported

Figure 1 .
Figure 1.A schematic view of dynamic electropulsing process.

Figure 2 .Figure 3 .Figure 4 .Figure 3 .
Figure 2. XRD patterns for specimens under various frequencies of electropulsing.α-Mgphase and a β-phase (Mg 17Al 12  ).When the frequency of electropulsing increased to 204 Hz, the XRD intensity of β phase reduced.Upon further increasing the frequency to 309 Hz, the XRD intensity of β phase further reduced.That implies the decomposition of β phase was accelerated when the frequency of electropulsing increased during dynamic electropulsing.The BSEM images and TEM bright field images of specimens under various frequencies of dynamic EPT are shown in Figure3.The matrix was α-Mg phase, while the precipitates were the β-phase (Mg 17 Al 12 ).For the as rolled specimen, numerous precipitates of β-phase can be found in Figure3(a1); as the frequency of dynamic electropulsing increased to 204 Hz, the amounts of β-phase precipitates decreased as shown in Figure3(a2).When the frequency of electropulsing was increased to 309 Hz, even fewer precipitates could be observed, as shown in Figure 3(a3).TEM bright field images of these specimens confirmed the same microstructure changes as shown in Figures 3(b1), (b2) and (b3).As the frequency of electropulsing increased, the amounts of precipitates marked by the white arrows decreased significantly, meaning that the decomposition of β phase occurred simultaneously.Both the SEM results and TEM observations were in good agreement with the XRD patterns, which are shown in Figure 2. The TEM examination of β phase is shown in Figure 4.The bright field of the 204 Hz-EPT specimen is shown
(b1).After 204 Hz-EPT of dynamic electropulsing for about 10s, dislocation arrays and nodes decreased, as shown in Figure 3(b2).When the frequency of electropulsing increased to 309 Hz, the amount of dislocation was further reduced, as shown in Figure 3(b3).

Figure 8
are the SPDT cutting force and surface roughness of the non-EPT specimen and four dynamic EPT specimens, with various frequencies of electropulsing (126 Hz, 204 Hz, 265 Hz and 309 Hz), respectively.It can be seen that the mean cutting force decreased significantly when the frequency of electropulsing increased.For the non-EPT specimen, the mean cutting force was 67.4 mN, with the surface roughness of 16.2 nm.When the frequency of electropulsing increased to 126 Hz, the mean cutting force decreased to 46.2 mN, with the surface roughness of 15.8 nm; upon the frequency being further increased to 204 Hz, 26 Hz and 309 Hz, the mean cutting force decreased to 44.7 mN, 49.8 mN, and 50.7 mn, with the surface roughness of 12.3 nm, 10.6 nm and 12.1 nm, respectively.From Figure5(b)and Figure6, it can be seen that under dynamic electropulsing, dislocation density decreased, and large amounts of deformation twins diminished with a homogenous fine grain structure.As a result, the cutting force for SPDT processing decreased and surface roughness improved when the frequencies increased.The machinability of AZ91 alloy in SPDT was improved under dynamic electropulsing.

Figure 7 .
Figure 7. Mean cutting force under various frequencies of electropulsing with depth of cut of 20 µm.

Figure 8 .
Figure 8. Surface roughness changes under different frequency of EPT, with depth of cut of 20 µm.