In order to study the influence of the frequency, duty cycle and flushing pressure on the surface morphology and cross-sectional re-solidified layer (RSL), heat-affected zone (HAZ), micro-hardness and chemical composition of Ti6Al4V, other parameters are kept constant in the SEAM process. Table 4 shows that six different levels of the three parameters are named E1, E2, E3 and E4, respectively.
4.1 Surface morphology analysis of the electrode
In the experiment, when the electrode feed rate is set to 6 mm/min, the corresponding REWR is -4%. This group of electrodes is selected to observe and detect the micro-morphology and chemical composition of the graphite electrode surface by SEM and EDS, respectively.
Figure 10 shows the SEM image and EDS spectrum of the graphite electrode. Figure 10(a) illustrates the SEM image of the processed surface of the graphite electrode. It is found that a large number of pits are generated on the surface due to the influence of electric discharge, and some eroded particles are attached to the electrode surface. Moreover, Figs. 10(a) and 10(b) show that a large amount of Ti, O, Ca and Al are distributed on the eroded particles. Figure 10(c) illustrates that characteristic elements of Ti6Al4V workpieces such as Ti, Fe and Al appear on the surface of the graphite electrode, as well as typical elements of tap water such as O, Ca, Na, and Cl. This is because processing under certain pressure of water and gas medium is affected by the multi-field coupling when the electrodes are discharged, and electrochemical reactions will occur to produce titanium, aluminum, oxygen, calcium, chlorine and other ions. The above-mentioned ions will form TiO2, Al2O3, CaO and other compounds under the rapid cooling of the water-gas flushing liquid, and these oxides adhere to the surface of the electrode and workpiece to form an oxide layer similar to an oxide film . The melting points of TiO2, Al2O3 and CaO are 1842℃, 2054℃, and 2572℃, respectively, which are all higher than the melting point of the Ti6Al4V material (1604–1660℃). Therefore, to a certain extent, it reduces the further wear of the electrode and affects the removal efficiency of workpiece material. When these oxides adhere to the surface of the electrode, the weight of the electrode increases, which reduces REWR and even compensate the electrode wear. This is one of the reasons that the electrode wear is -4%.
4.2 Surface morphology analysis of the Ti6Al4V
Figure 11 shows the SEM image of the surface after pulsed SEAM Ti6Al4V under different processing parameters. Erosion holes, micro-cracks, RSL, pits, droplets and spherical particles are observed on the Ti6Al4V surface processed with different parameters. Erosion holes are formed due to the rapid flushing of water-gas mixed medium and high-speed rotation of electrode. As a result, the erosion debris hits the molten pool on the surface of workpiece at a high speed. Since the molten material is rapidly cooled and solidified under the high-pressure flushing liquid between poles, the internal thermal stress of the material cannot be released in time, thereby forming micro-cracks. The RSL is formed because the molten workpiece material is rapidly solidified and attached to the surface of workpiece under the cooling of the rapid flushing medium. Due to the different processing parameters, different features will appear on the surface of the workpiece after processing.
Comparison of Figs. 11(a) and 11(b) show that the SEM images are significantly different. It is found that when the frequency is reduced from 1000 Hz to 600 Hz, large and deep pits appear on the surface of the workpiece and the bottom is relatively smooth without RSL. Figure 11b shows that the number of micro-cracks on the surface of workpiece is significantly reduced when the frequency is 600 Hz. When the frequency is reduced, the single arc discharge time is prolonged. Therefore, Fig. 11b shows that large and deep pits occur, which is consistent with the analysis results in Sect. 3.2. Comparing Figs. 11(a) and 11(c) shows that when the duty cycle drops from 40–30%, the surface integrity of the workpiece is significantly improved, the microcracks are reduced, and almost no erosion holes are observed. Moreover, laminated droplets are observed. This is because the pulse width in a single discharge cycle is narrowed, resulting in a shorter arc discharge energy duration. Therefore, the newly melted material cannot be sufficiently cooled and the adhesion between the droplets occurs.
Comparing Figs. 11(a) and 11(d) illustrate that when the flushing pressure increases from 0.3 MPa to 0.5 MPa, larger diameter pits, smaller erosion holes, fewer microcracks and irregular edges RSL are observed on the surface of the workpiece. Figure 11d shows that when the flushing pressure increases to 0.5 MPa, due to the high-speed flushing of inner and outer flushing fluid, the inter-electrode discharge channel is destroyed and a large explosive force is produce . Therefore, the molten workpiece material does not have enough time to cool and adhere to the surface of the workpiece, which leads to larger pits, smaller erosion holes and fewer micro-cracks.
4.3 Cross-section analysis of the Ti6Al4V
1) Cross-sectional topography analysis
Figure 12 shows the SEM images of the cross-sectional morphology of Ti6Al4V after SEAM under different processing parameters, including partial enlarged images of the RSL and HAZ. Moreover, Fig. 13a shows the average resolidified layer thickness (ARSLT) and average heat-affected zone thickness (AHAZT) under different processing parameters. Figure 12 presents that the RSL, HAZ and base material can be observed in all cross-sectional SEM images . Figure 13a shows that when the frequency is reduced from 1000 Hz to 600 Hz, the ARSLT increases from 41.4 µm to 49.6 µm. Moreover, it is found that the AHAZT is reduced from 14.1 µm to 5.1 µm. Figures 12a and b show that the microstructure of the RSL changes from granular to larger snowflake-like shape, and columnar grains appear in the microstructure of the HAZ. This is because when the frequency decreases, the discharge time of a single pulse is prolonged. Therefore, the energy in a single pulse increases and the ARSLT increases. The decrease in the AHAZT may be explained by the poor thermal conductivity of Ti6Al4V. When other parameters are constant, reducing the frequency will increase the energy of a single pulse, resulting in a snowflake-like microstructure with larger particle size. Due to the poor thermal conductivity of Ti6Al4V, the heat in the cross-section is reduced in a very short time. Therefore, grain refinement appears in the HAZ.
Figure 13a shows that when the duty cycle is reduced from 40–30%, the ARSLT decreases from 41.4 µm to 39.6 µm, and the AHAZT is reduced from 14.1 µm to 7.4 µm. Moreover, Figs. 12a and c show that the microstructure of the RSL changes from larger particles to smaller particles. The microstructure of the HAZ changes from refined columnar crystals to coarse α + β phases. This is because when the duty cycle decreases, the pulse width becomes narrower. Therefore, the energy in a single pulse decreases, which will lead to a decrease in ARSLT and AHAZT. When other parameters are constant, the energy of a single pulse is reduced after the duty cycle is reduced, which leads to a smaller size of the microstructure particles of RSL when the duty cycle is 30%. When the heat transferred from the Ti6Al4V machining surface to the HAZ reduces, the degree of the grain refinement reduces too.
Figure 13a shows that when the flushing pressure increases from 0.3 MPa to 0.5 MPa, ARSLT decreases from 41.4 µm to 26.0 µm, and AHAZT decreases from 14.1 µm to 9.3 µm. Moreover, Figs. 12a and 12d show that the microstructure of RSL changes from a granular shape to a snowflake-like shape with larger gaps, while the microstructure of HAZ has fewer columnar crystals. This is because when the flushing pressure increases, the cooling effect between the machining electrodes is better, which leads to the reduction of ARSLT and AHAZT. When other parameters are constant, increasing the flushing pressure will enhance the ability of the inter-electrode to discharge particles, resulting in a larger gap between the snowflake-like particles in the RSL microstructure when the flushing pressure is 0.5 MPa. Due to the enhancement of the cooling capacity, most of the heat is difficult to transfer to the HAZ. Therefore, the microstructure of HAZ does not change significantly.
2) Micro-hardness and EDS analysis
Figures 13b E1 and E2 show the micro-hardness of RSL, HAZ and base material. It is observed that when the frequency is reduced from 1000 Hz to 600 Hz, the micro-hardness of RSL increases from 464HV to 525HV, and the micro-hardness of HAZ increases from 364.5HV to 482HV. This is because reducing the frequency will increase the duration of a single pulse discharge, so it will increase the energy of a single pulse, so the micro-hardness values of RSL and HAZ increase. According to reports, in the HAZ, because some α-Ti transforms into β-Ti under high temperature and rapid cooling, the micro-hardness of HAZ is lower than that of RSL . Figures 13c and d show the EDS spectra of RSL, HAZ and base material. It is observed that when the frequency is reduced to 600 Hz, the content of Al in the RSL is reduced from 2.10 % to 0.28%, the C content in RSL is more than 1.8 times that of 1000 Hz, and the C content in HAZ is more than 2.6 times that of 1000 Hz. This is due to the lower melting point of Al. When the frequency decreases, the duration of discharge energy increases, and Al is easily lost during processing. In addition, the increase in the duration of energy will lead to an increase in the carbon deposit content.
Comparing Figs. 13b E1 and E3 shows that when the duty cycle reduces from 40–30%, the micro-hardness of HAZ is reduced from 364.5HV to 352.5HV, the micro-hardness of RSL increases from 464HV to 525HV. As the pulse width decreases, the discharge energy decreases, and the micro-hardness of the HAZ decreases. It is interesting that the micro-hardness in RSL has increased, which may be due to the rapid cooling effect of the working medium. Figures 13c and e show that the C content in the RSL at a duty cycle of 40% is lower than at a duty cycle of 30%. This is because when the discharge energy decreases, the electrode wear will increase, and therefore the C content will increase.
Comparing Figs. 13b E1 and E4 shows that when the flushing pressure increases from 0.3 MPa to 0.5 MPa, the rapid cooling between the processing electrodes and the ability to discharge ablated particles are greatly enhanced. The process is similar to quenching. Therefore, the RSL micro-hardness increases from 464HV to 583HV, and the HAZ micro-hardness increases from 364.5HV to 491HV. Combined with the changes in the content of C and O elements in RSL and HAZ, it is found that after increasing the flushing pressure, the discharge channel will be destroyed by working medium fluid. As a result, Figs. 13c and f show that a greater explosive force is generated to accelerate the interpolar element infiltration, which will lead to an increase in the content of C and O elements in RSL at 0.5 MPa. In addition, due to the better cooling effect of rapid flushing, the loss of Al content can be significantly reduced.