3.1 Surface morphology
Fig. 3 displays the surface morphology of Ti-6Al-4V alloy at different parameters. Fig. 3 (a) and (b) show the surface morphology of EDM and gas-liquid mixed EDM at small parameters, respectively. It is clear that the EDMed surface has much pores than the gas-liquid mixed EDMed surface, Fig. 3 (e) and (f) show the surface morphology in EDM and gas-liquid mixed EDM process at high discharge parameters, respectively. It can also be seen that the gas-liquid mixed EDMed surface has less pores than the EDMed surface. This may be due to the flow of nitrogen facilitates the molten materials uniformly distributed on the surface, the pores would be covered by the molten materials, thus the number of pores on the gas-liquid mixed dielectric is reduced.
It can be observed on increasing the processing parameters, the EDMed surface becomes worse. This may be due to the single pulse energy is large at high parameters, and more molten materials are ejected from the molten pool, resulting in the deepening of discharge pit and the deterioration of surface quality . However, it can be seen that with the increase of peak current and pulse duration, the gas-liquid mixed EDMed surface has better surface morphology, which is very different from the EDMed surface. This may be due to that the flow of nitrogen disperses the discharge energy, and the energy per unit area is reduced, which leads to less molten materials are thrown from the molten pool, thus the surface morphology is better than the EDMed surface. The results show that the influence of gas-liquid medium is greater than that of process parameters.
Comparing the gas-liquid mixed EDMed surface (Fig. 3 (b) (d) (f)) and the EDMed surface (Fig. 3 (a) (c) (e)), it can be seen that the gas-liquid mixed EDMed surface is smoother than the EDMed surface at the same parameters. This is due to the addition of nitrogen increases the discharge gap and reduces the energy density, so that the spark energy is uniformly distributed on the machined surface . Besides, nitrogen moves randomly in the discharge channel under the flow of spark oil medium, which leads to the distribution of molten materials more even and facilitates the current dispersion in the discharge process, refines the discharge energy of single discharge and thus makes the machined surface smoother. It is also noticed that the size of discharge crater on the gas-liquid mixed EDMed surface is larger as compared with that on the EDMed surface under the same parameter. This may be due to nitrogen is involved in the spark discharge, and the liquid dielectric mixed with nitrogen has a weaker compression effect than the liquid dielectric , thus the discharge channel diameter becomes larger than that of liquid dielectric, which decreasing the energy density. Therefore, the discharge craters are shallower with large diameter. Besides there is additional heat except for the heat generated from the discharge energy. The extra heat may come from exothermic reaction between nitrogen and molten titanium alloy at high temperature, which also lead to larger craters. The similar explanation was also put forward by Singh et al. .
3.2 Cross section morphology
Fig. 4 illustrates the cross section morphology of machined surface at different parameters. Fig. 4 (a) and (b) show the cross section morphology of EDMed and gas-liquid mixed EDMed at small parameters. The results show that some cracks and pores could be observed on the EDMed surface, but not on the gas-liquid EDMed surface, and the recast layer of gas-liquid mixed EDM is more continuous than that of EDM. The reason is that nitrogen enters the discharge channel, which disperse the discharge energy and generates a larger range of spark discharge. Therefore, the discharge point is more uniform, reducing the uneven discharge point in liquid EDM. Accordingly, the recast layer of gas-liquid mixed EDM is more continuous and consistent.
With the increase of machining parameters, the thickness of the recast layer increases regardless of whether the dielectric is liquid (Fig. 4 (c)) or gas-liquid mixture (Fig. 4 (d)). That maybe due to the discharge energy enhances with the peak current and pulse duration increase, which leads to more materials melting. However, the proportion of molten material that can be flushed away by the dielectric is constant . Thus, more molten materials re-solidified on the workpiece surface compared with that of small parameters. Interestingly, no matter the parameters are large or small, the thickness of the gas-liquid mixture is thicker than that of the liquid dielectric, which is contrary to the phenomenon studied by Wang et al. . That may be due to the cooling effect on the multi-hole electrode by nitrogen improves the thermal conductivity of the cooper electrode, so that more heat is transferred into the electrode, and relatively little heat is delivered to the workpiece. Although the heat on the workpiece can melt the workpiece, there is not enough heat to throw the molten material out, thus the thickness is thicker. In addition, the heat convection of nitrogen takes some of the heat away and has a cooling effect on the solidification process . Hence, the cooling rate of gas-liquid mixed dielectric is faster than that of the pure liquid dielectric, and some molten materials re-solidified on the surface before being thrown out. Therefore, the recast layer is thicker than that machined in liquid dielectric.
Observing the cross section morphology of the sample obtained in the gas-liquid mixed dielectric (Fig. 4 (b) and (d)), it is noticed that the bond between recast layer and the matrix is dense. Besides, the microstructure of the recast layer is mainly granular and dendritic, moreover, the particles gradually become fine and dense from the matrix to the machined surface, and transit from dendritic to granular. The similar results have been found by Morton et al. . Different solidification rates at different depth lead to different microstructure. During the gas-liquid mixed EDM process, nitrogen is blown into the machined surface, and the machined surface cools rapidly, forming a dense recast layer. However, with the increase of the depth, the internal temperature of the workpiece is higher, the temperature difference increases, thus the cooling rate is slower than the machined surface. Due to the difference of thermal expansion coefficient of titanium alloy, the volume of the internal workpiece is contracted, hence the molecular density changes, forming the dendritic structure.
3.3 Surface element analysis
Different microstructures are usually related to performance. To further explore the cause of different microstructure, two typical samples with different microstructures were chosen for surface element analysis. Fig. 5 shows the surface element analysis of EDM and gas-liquid mixed EDM process at the same parameter, respectively. The analysis region is shown in the red box. Different surface morphology has distinct element content. It can be observed the peak of N element is visible on the gas-liquid mixed EDMed surface, but not on the EDMed surface. The presence of N element on the gas-liquid mixed machined surface confirmed that the migration of N element from nitrogen to the machined surface. That is because nitrogen ionizes under high temperature and high pressure produced by spark discharge, resulting in the deposition of ionized nitrogen on titanium alloy surface. Therefore, the existence of N can be obtained on the gas-liquid mixed EDMed surface, which lead a different microstructure on the machined surface.
3.3 XRD analysis
To further explore the phase composition of the machined surface, the gas-liquid EDM surface obtained under large parameters was analyzed by XRD, as shown in Fig. 6. Results show that TiN, Ti and Al phase are present on the workpiece surface. The formation of TiN hard phase is owing to the instantaneous energy of spark discharge melts the titanium alloy and forms a molten pool, at the same time, nitrogen decomposes into nitrogen atoms at high temperature, and even ionizes into nitrogen ions. The molten titanium reacts with nitrogen to form TiN hard phase. The formation of TiN proves that the element N in section 3.2 exists in the form of compound. The Ti and Al phases come from the remaining molten titanium after nitriding reaction.
3.4 Micro Hardness
The micro hardness of the recast layer plays an important part in wear resistance. Fig. 7 depicts the distribution curve of micro hardness on the cross section of surfaces machined by EDM and gas-liquid mixed EDM process at high parameter, respectively. Results shows that the micro hardness of gas-liquid mixed EDM process varies between 1329.5 HV and 314 HV, while that of EDM ranges between 801.6 HV and 314 HV. This shows that the micro hardness of gas-liquid mixed EDM is almost 65.9% higher than that of EDM, which is more than three times of that of the matrix material. The improvement of micro hardness is due to the formation of TiN hard phase on the sample surface. Because the research shows that TiN ceramic layer has high hardness [27, 28]. The micro hardness of the machined surface is the highest (1329.5 HV). With the increase of the depth, the micro hardness decreases gradually until it reaches the matrix hardness (315 HV). The micro hardness of the location below the surface 20 μm is 1001.9 HV, and the micro hardness at 60um away from the surface is still higher than 679HV, while at the same distance the micro hardness of EDM process is 314 HV, which is the same as the matrix hardness. This variation of micro hardness corresponds to the analysis of cross section morphology as mentioned in Section 3.2 that the thickness of TiN recast layer of gas-liquid EDM is thicker.
3.5 Wear resistance
Fig. 8 illustrates the surface morphology of high parameter machined surfaces obtained by gas-liquid mixed EDM and EDM after wear test. The wear test shows that there are obvious marks and grooves on the EDMed surface, while the marks on the surface of gas-liquid EDM are not obvious under the same parameters, even if the surface features are resemble to the original features of EDM. Thus, the wear resistance of gas-liquid mixed EDMed surface is better than the EDMed surface. That is due to the formation of TiN hard layer on the sample surface, which has better wear resistance [29, 30]. This is consistent with the improvement of micro hardness discussed in Section 3.4.