3.1 Single-pulse mode milling
The magnitude of the energy fluence is an essential factor affecting the milling results for laser milling. To determine that the selected energy parameters are within the appropriate range, the ablation threshold of the alumina ceramic specimens used for the experiments was determined before the experiments were carried out. According to the Beer-Lambert law, light absorption leads to an exponential decay of light intensity:
$$I\left(z\right)={Ie}^{-\alpha z}$$
3
where I is the laser energy fluence, Z is the machined depth, and α is the absorption coefficient. Since ablation only occurs when the I exceeds the fluence threshold (Ith) of the material. According to Eq. (3), the machined depth (hα) is related to the incident laser fluence by [33, 34]:
$${h}_{\alpha }=\frac{1}{\alpha }\left[\text{ln}(I)-\text{ln}({I}_{th})\right]$$
4
Figure 3 shows the relationship between machined depth and energy fluence. hα is plotted as a function of ln(I). From Eq. (4), it can be seen that the hα is linearly related to ln(I). After linear fitting, the threshold of alumina for this experiment was obtained as 1.76 J/cm2.
According to the threshold of alumina ceramics in this experiment, energy fluences of 2.19 J/cm2 to 3.35 J/cm2 are selected. The three-dimensional morphologies and cross-sectional profiles of the groove and the surface morphologies of the bottom surface of the groove for different laser parameters in single-pulse mode are presented in Fig. 4, where Fig. 4(A)-(I) show the three-dimensional morphologies of the grooves with different laser parameters, Fig. 4(a1) -(i1), (a2) -(i2) show the corresponding cross-sectional profiles and the three-dimensional morphologies of the bottom surface of the grooves. The three-dimensional morphologies and cross-sectional profiles of typical energy parameters, where the energy fluence is 2.19 J/cm2, 2.68 J/cm2, and 3.35 J/cm2 are selected for description and analysis, as shown in Fig. 4(A)-(C), (a1) -(c1). When the energy fluence is 2.19 J/cm2, as shown in Fig. 4(A), there are some convex structures on the surface that are not fully ablated. As the energy fluence increases to 2.68 J/cm2, the groove surface becomes smooth with only some areas of excessive ablation. The deepest groove is obtained at the energy fluence of 3.35 J/cm2, with ablation linear propagations and voids on the groove bottom surface. The cross-sectional profiles reflect the machined depth and bottom profile. As shown in Fig. 4(a1) -(c1), the machined depth gradually increases with the increasing energy fluence. At high energy fluence, there are concave structures on the surface. However, there are convex areas and fluctuations on the bottom surface profile when the energy fluence is low. The Sover is essential for milling efficiency and surface roughness [3]. The three-dimensional morphologies and cross-sectional profiles of typical Sover of 70%, 80%, and 99% where the scanning spacing is 5 µm are selected for description and analysis, as presented in Fig. 4(D)-(F), (d1) -(f1). When the Sover is 70%, the small Sover reduces the number of pulses deposited in each area of the laser process. The bottom of the groove has a convex area, the depth becomes shallow, as shown in Fig. 4(D) and (d1). As the Sover increases to 80%, as shown in Fig. 4(E) and (e1), the profile of the groove bottom is flat. When the Sover continuously increases to 99%, as presented in Fig. 4(F) and (f1), the deepest groove is obtained. The pulses are deposited at the exact location on the surface during milling. The ablation is severe under the high Sover, and the bottom profile of the groove has a concave area. For laser milling, the scanning spacing is not appropriately set, the three-dimensional structure of the milled substrate will affect the results [6]. The three-dimensional morphologies and cross-sectional profiles of typical scanning spacing of 3 µm, 7 µm and 9 µm where the Sover is 80% are selected for description and analysis, as shown in Fig. 4(G)-(I), (g1) -(i1). When the scanning spacing is 3 µm, as shown in Fig. 4(G) and (g1), there is the generation of ablation linear propagations on the bottom surface of the milled groove due to excessive ablation, and the milling depth is deep. With the scanning spacing increases to 7 µm, as presented in Fig. 4(H) and (h1), the milled groove becomes shallow, and the profile is flat. At the scanning spacing of 9 µm, as shown in Fig. 4(G) and (g1), the shallowest groove is obtained, with the fluctuation bottom profile.
To better characterize the quality of the milled groove, the effect of different laser parameters in single-pulse mode on the quality of milled grooves is analyzed, and the results are discussed by analyzing the three-dimensional morphologies of the bottom surface of the grooves. Figure 4(a2) -(c2) reflect the three-dimensional morphologies of the bottom surface of the groove for the above typical energy fluence. For the energy fluence of 2.19 J/cm2, since the alumina ceramics had an uneven local structure during the manufacturing, the fragments produced in the process weakens the propagation of the laser [21], resulting in the generation of convex areas and in the local range of the machined surface. When the energy fluence is 2.68 J/cm2, the surface is flatter, and the surface roughness is minor, but there is still a small area of convex areas and voids. When the energy fluence is 3.35 J/cm2, the high pulse energy produces thermal stress. The generation of thermal stress leads to brittle fracture of the alumina ceramics, resulting in the phenomenon of flaking and the generation of regional voids [6]. The generation of voids is big, affecting the quality of the surface. Figure 4(d2) -(f2) reflect the three-dimensional morphologies of the bottom surface of the milled groove for the corresponding typical Sover described above. As shown in Fig. 4(d2), when the Sover is 70%, due to the low Sover, the small total energy of the pulses deposited. The energy of the laser irradiation on the surface is uneven, resulting in the tendency for brittle fractures to occur during the process. The fluctuation on the surface reduces when the Sover increases to 80%, as presented in Fig. 4(e2). The mechanism of laser-excited nanoparticle generation changes, the particle aggregation is evident, resulting in the number and size of voids significantly reduces. However, more deep voids exist when the Sover is 99%, as presented in Fig. 4(f2). Due to the high Sover, the previous pulses influence the interaction of the subsequent pulse and the material, further affecting the laser propagation. The groove bottom surface has deep voids [13]. Simultaneously, the thermal incubation effect at high Sover made the process of ceramics with thermal effects. There is fracture and peeling of the surface, leading to the appearance of deeper grooves and voids [3, 13]. Figure 4(g2) -(i2) show the three-dimensional morphologies of the groove bottom surface for the above typical scanning spacing. As shown in Fig. 4(g2), when scanning spacing is small. the surface is severely ablated, producing more voids on the surface and poorer surface roughness [6]. As the scanning spacing increases, the surface voids area decreases, and when the scanning spacing is 7 µm, as presented in Fig. 4(e2), no significant processing defects exist. As shown in Fig. 4(i2), when the scanning spacing is large, the periodic structure parallel to the direction of the laser scanning path appears on the machined surface, which significantly affects the processing quality. With the larger scanning spacing, the surface of the alumina ceramics cannot be ablated due to the weak energy at the edge of the Gaussian laser spot. Periodic structure forms parallel to the direction of the laser scanning path where the material was not effectively removed.
A further demonstration of the effect of different laser parameters on groove milling results in single-pulse mode is discussed via the surface roughness. Figure 5(a) shows the groove bottom surface roughness under different energies. When the energy fluence is 2.19 J/cm2, surface roughness is 0.677 µm. With the increasing energy fluence, surface roughness decreases. At the energy fluence of 2.68 J/cm2, the surface roughness is the lowest at 0.520 µm. As the energy fluence is above 2.68 J/cm2, the surface roughness of the surface increases with the increase of energy. When the energy fluence is 3.35 J/cm2, the surface roughness is 0.728 µm. It is important to note that the energy fluence of 2.68 J/cm2 is the optimal energy fluence. Therefore, for all the milling experimental studies about the Sover and scanning spacing, the energy fluence is 2.68 J/cm2. Figure 5(b) shows a graph of the surface roughness for a combination of parameters corresponding to Sover and scanning spacing above. When the scanning spacing increases from 3 µm to 7 µm, the surface roughness decreases with the same Sover. It is easy to find that when the scanning spacing is 7 µm, the surface roughness of each experimental group reaches the minimum value. At the Sover of 70%, the surface roughness is relatively high, from 0.580 µm to 0.671 µm. When the Sover increases to 80%, the surface roughness for the bottom surface of the groove is better, from 0.502 µm to 0.516 µm. At the Sover of 80% and the scanning spacing of 7 µm, the surface roughness is optimal at 0.50 µm. As the Sover increases, the surface roughness increases. As the Sover increases to 99%, the surface roughness is generally above 1 µm, as the redeposition of some of the material ejected by the thermal effect on the machined surface significantly increases the surface roughness. At the Sover of 99% and the scanning spacing of 3 µm, the process is poor with the surface roughness of 1.115 µm.
Efficiency is also an essential indicator for laser milling, characterized by the MRR. Figure 6(a) and (b) show the groove milling with above energy fluences in single-pulse mode. Since the other processing parameters were the same, and the volume of material removed is approximately positively correlated with the machined depth, the depth and MRR with energy fluence follows the same trend. As the energy fluence is 2.19 J/cm2, the machined depth is 24.785 µm with the minimum MRR of 4.573 × 106 µm3/s. The depth and MRR increases with the increasing energy fluence. As the energy fluence increases to 3.35 J/cm2, deepest groove is obtained, the depth is 40.771 µm with the maximum MRR of 7.646 × 106 µm3/s.
Figure 7 shows the trend of machined depth and MRR for the above Sover and scanning spacing parameters. As shown in Fig. 7, as the scanning spacing increases, the depth of the groove decreases at different Sover, but the MRR increases. Since the scanning spacing increases, the time spent processing the material decreases, which leads to the depth decreases. Moreover, when the scanning spacing is small, the surface undergoes reheating and evaporation during the scanning. A large amount of debris from the ablation of the ceramic forms a redeposition on the groove bottom surface, which resulting in a decrease in MRR. With the increasing Sover, the machined depth increases at different scanning spacing. For the MRR, when the Sover is below 80%, with the increasing Sover from 70–80%, the MRR increases. When the Sover is low, it is not able to produce effective volume removal of the material, resulting in a decrease in MRR. At the Sover of 70% and scanning spacing of 9 µm, the minimum machined depth is obtained at 10.263 µm, with the MRR of 10.156 × 106 µm3/s. As the Sover increases, the depth increases due to the increased regional pulse deposition. However, when the Sover is above 80%, as the Sover increases, the MRR decreases. When the Sover is high, the scanning speed is low. The debris generated by material ablation cannot be effectively discharged and deposited on the groove bottom surface, decreasing the MRR [35]. At the Sover of 99% and scanning spacing of 3 µm, the deepest groove is 54.443 µm, with the minimum MRR of 0.597 × 106 µm3/s. It is easy to find that Sover of 80% can milling the material with the highest efficiency. At the Sover of 80% and scanning spacing of 9 µm, the maximum MRR is 10.940 × 106 µm3/s.
In summary, the milling quality and efficiency of single-pulse mode milling with different energy fluences, spot overlap ratios, and scanning spacing is investigated. The optimal parameters is energy fluence of 2.68 J/cm2, Sover of 80%, and the scanning spacing of 7 µm, resulting in the groove bottom surface roughness of 0.502 µm. Even with the optimal parameters, the machined surface still has small convex areas and small voids, surface roughness is high. The processing quality needs to be improved. Also, the milling MRR is 10.790 × 106 µm3/s, which is lower than the milling processing efficiency at high energy fluence. The milling efficiency needs to be improved while optimizing the milling surface quality.
3.2 Optimization of the surface quality via the burst-mode
Given that other scholars have studied the optimization of the surface quality via the burst-mode, a series of milling experiments are carried out using different burst-mode processing parameters. The processing quality is measured by the surface roughness of the bottom surface of the milled groove. It is important to note that before optimization using the burst-mode, the alumina ceramics surface is machined separately using two sets of parameters to obtain different rough machined surfaces. For the RP1, it is the optimal processing parameters in the single-pulse mode. By optimizing this machined surface, it can investigate whether even for the best quality surface in single-pulse mode, the milled surfaces optimized via the burst-mode give better results. For the RP2, a high milling efficiency is obtained for the parameters where the Sover is 80%, and scanning spacing is 7 µm. The energy fluence is increased to 3.35 J/cm2 to maximize the processing efficiency. With the RP2, the milling efficiency is maximized without generating extensive processing defects. By optimizing this machined surface, it is possible to investigate whether the burst-mode can optimize the surface with poor surface roughness obtained by the process with the large energy fluence. Also, since the maximum MRR is obtained under the RP2, it provides the basis for the study in the next subsection.
Figure 8(a), (b), (d) and (e) show the surface roughness in different burst-modes at the Sover of 70% and 80%. At the low Sover, the roughness of the groove surface optimized via the burst-mode 5 is generally better than that of the burst-mode 3. Unlike the “cold processing” of the single-pulse mode femtosecond laser, there is a particular thermal effect while using the burst-mode due to the time interval between burst sub-pulses being only 25 ns. The thermal accumulation generates between the sub-pulse intervals. Also, due to the lower energy of the burst-mode sub-pulses, the laser pulses acting on the ablation of the material are different than the single-pulse mode. When the Sover is low, the number of regional pulses deposited is low. Since the lower energy of the burst sub-pulses, gentle melting of the material occurs under the action of each sub-pulse. The higher the number of sub-pulses, the more heat accumulation, the better the melting via the action of multiple pulses. Meantime, during the melting, the voids and unmelted particles on the machined surface can be healed by the liquid phase sintering. Burst-mode 5 is more potent than burst-mode 3 in healing the existing voids on the rough surface, and the optimized surface of the groove is smooth. However, when the Sover is 90%, as shown in Fig. 8(c) and (f), the processing results are generally better for burst-mode 3 than burst-mode 5. As the changing scanning spacing. When the Sover is high, the number of pulses deposited in the area becomes larger. For the same pulse deposition area, the material melts and recasts under the action of the previous pulses. As the subsequent burst sub-pulses continues to act on the already formed local liquid phase layer and recast layer, the cumulative effect of the sub-pulses would lead to the splashing of the surface material in the form of vapor droplets, which affects the processing quality. The larger the number of burst sub-pulses, the greater the effect. Also, when the number of pulses deposited is large, the area suffers more thermal accumulation of sub-pulses, which increases with the number of sub-pulses. Excessive thermal accumulation leads to the serve thermal effect, resulting the increase of the surface roughness.
It is noticeable that for all the burst-mode milling, the surface roughness of the groove bottom is decreased as the scanning spacing increases from 5 µm to 7 µm. The surface suffers increasing thermal accumulation when the scanning spacing is low. The serve thermal effect affects the surface roughness. As the scanning spacing increases to the range of 7–11 µm, it is easy to find that the surface roughness at this range is the lowest. The particles and debris on the rough machined surface are finished by the effect of the burst pulse and the thermal accumulation between the sub-pulses. For the rough machined surface obtained by the process with the RP1, the lowest roughness optimizes was 0.447 µm when the Sover is 80%, burst-mode 5, and the scanning spacing is 9 µm. And for the rough machined surface obtained by the process with the RP2, the lowest roughness optimizes is 0.454 µm when the Sover is 80%, and the scanning spacing is 9 µm for burst-mode 5. However, as the scanning spacing continues to increase, the surface roughness increases. Since the low energy of sub-pulses, the energy at the edge of the spot is even below the ablation threshold. There is still incomplete processing between the two parallel laser scanning paths when the scanning spacing is large, ultimately failing to finish the surface. Comparing the surface roughness obtained by optimizing different rough machined surfaces, it can be seen that there is only a tiny difference in the optimization effect of the burst-mode for different rough machined surfaces, and the surface roughness varies in the same trend. However, for the rough machined surface obtained with the RP1, the optimization result is better than that of the rough machined surface obtained with the RP2. This is because the rough machined surface obtained with the RP1 leaves fewer defects on the surface after processing, and the surface is more regular. Additionally, as shown in Fig. 8, for the two different rough machined surfaces, it can be seen from the optimized surface roughness that the burst-mode can reduce the surface roughness compared to the optimal result of 0.502 µm obtained with the single-pulse mode. By selecting the appropriate parameters, the roughness of the groove bottom is lower when optimally milled via the burst-mode. The processing quality is superior compared to the single-pulse mode.
Figure 9 illustrates the mechanism of the surface processed by the burst-mode compared to the single-pulse mode. The reasons for the optimization of the material surface in the burst-mode compared to single-pulse mode under appropriate parameters can be inferred from the following: (1) Reduction of ablation particles redeposition. The deposition of ablated particles during laser ablation is a typical phase explosion feature. The slowing down or suppression of the phase explosion via the burst-mode can be explained on both the time and temperature scales. The characteristic time of the phase explosion after the laser reached the material surface is between picoseconds and nanoseconds [36, 37]. According to the fact that the ablation particles in the non-processed area could not be covered by the second pulse, the time for the lattice to reach the critical temperature for phase explosion becomes longer due to the smaller energy of the sub-pulse in the burst-mode. Since the interval between sub-pulses is on the nanosecond scale, it can be inferred that the physical process of phase explosion caused by the previous sub-pulse has partially occurred or is occurring when the subsequent sub-pulse reaches the material surface. Therefore, the burst sub-pulse is able to suppress part of the phase explosion process of the material under laser irradiation. On the temperature scale, the burst-mode has lower energy of individual sub-pulses due to the same total laser power. The lower pulse energy could not bring the material lattice temperature to the critical phase explosion temperature, which inhibits the occurrence of phase explosion and results in fewer particles being obtained by ablation. The redeposition on the machined surface is reduced. (2) Transformation of the form of material removal. There are two material removal mechanisms in the strong ablation zone of ultrafast laser: phase explosion and spallation. For the phase explosion, vapor and small agglomerates are ejected from the surface violently, and melting and flow are unlikely to occur. The burst-mode mentioned above partially inhibits the generation and conduct of phase explosion, transforming the material removal process into the spallation of the liquid phase layer. For the spallation, material removal occurred gently and stably, the deposition of debris on the surface from the serve ablation is significantly less, and the surface of the machined material is smooth [36]. (3) Removal of roughing defects. Surface debris and high surface energy nanoparticles left behind by rough process has a lower melting point than the material itself [6]. They are remelted and resolidified by thermal accumulation in the burst-mode. The hydrodynamic effect causes the material deposits to become smooth and gentle, forming a dense and homogeneous melting layer. In addition, since the low energy of burst sub-pulse, laser energy could not vaporize most of the material, the voids and the unmelted particles of the surface would have the liquid phase sintering under the action of the laser irradiation, resulting in the healing of the voids [12].
For the burst-mode milling optimization experiments under different rough machined surfaces, the machined depth and MRR follow the same trend, while the relationship between machined depth and MRR has been described in the previous discussion. Therefore, to reflect the milling efficiency of the burst-mode, the MRR for the burst-mode milling optimization experiments for the rough machined surface obtained with the RP1 is selected for discussion.
Figure 10 shows the MRR for the different burst-mode optimization experiments. For the different burst-modes, When the scanning spacing is below 7 µm, as the scanning spacing increases, the MRR increases at different Sover. When the scanning spacing is low, the surface suffers more vaporization and thermal effects, resulting in a large amount of vaporization and melting of the material, leading to redeposition and recasting of debris on the groove bottom surface. The MRR reduces. As the scanning spacing increases to 7–9 µm, the MRR is high at this range. For the burst-mode, the maximum MRR is 5.744 × 106 µm3/s when the Sover is 80%, and the scanning spacing is 7 µm. However, as the scanning spacing continues to increase, the MRR decreases. Due to the low energy of the Gaussian spot edge, the material could not be fully processed, resulting in a decrease in MRR when the scanning spacing is above 9 µm. Additionally, as shown in Fig. 10, With the same Sover and scanning spacing, the MRR of burst-mode 3 and 5 is lower than the single-pulse mode. As the number of sub-pulses increases, the MRR decreases. Since the larger number of sub-pulses, the lower the energy averaged into individual pulses while maintaining the same total laser power. The lower energy of the sub-pulses could not allow for effective material removal. Also, due to the nanosecond time scale between burst sub-pulses, the ablation cloud generated by the previous laser pulse interacted with another laser pulse during the laser process. The laser pulse is partially shielded, and the material generated by the ablation cloud is redeposited, resulting in a low MRR in the burst-mode process [38].
In summary, for the burst-mode, with appropriate parameters, it is possible to optimize the surface roughness of the milled surface better than in the single-pulse mode. The minor surface roughness of 0.447 µm is obtained by the rough machined surface with RP1 when the Sover is 80%, burst-mode 5, and the scanning spacing is 9 µm. However, the MRR is 4.194 × 106 µm3/s. Though the appropriate burst-mode parameters can improve the milling quality, the efficiency of milling is relatively low. A reasonable processing strategy is needed for high-quality and high-efficiency milling.
3.3 Comparison experiment between the strategies of composite process and single process
To mill the alumina ceramic with high-quality and high-efficiency, experiments are conducted to investigate the feasibility of milling in a composite process and compare the quality and efficiency with the single process with optimal parameters in the single-pulse mode. The composite process is conducted for roughing and then finishing. For the roughing, from above subsection, for the rough machined surface with RP2, the MRR is maximized. The milling efficiency is maximized without generating extensive processing defects. Therefore, the RP2 is used as roughing parameters. The optimization of the rough machined surface with RP2 via the burst-mode is discussed from above subsection. The minimum surface roughness of 0.460 µm and 0.454 µm is obtained when the Sover is 90%, the scanning spacing is 7 µm for burst-mode 3, and the Sover was 80%, the scanning spacing is 9 µm for burst-mode 5, respectively. Therefore, two sets of parameters are used as finishing parameters. A comparison experiment is carried out between milling the same groove structure in the composite process and the single process with different processing strategies to investigate the feasibility of the composite process for high-quality and high-efficiency milling. The quality and efficiency of the milling are characterized by the surface roughness and MRR, respectively. Three processing strategies are selected for comparison: processing strategies 1 and 2 (PS1, PS2) are for milling in a composite process, and processing strategy 3 (PS3) is for milling in a single process with optimal parameters. The specific parameters of the processing strategies are listed in Table 1.
Table 1
Experimental parameters for different processing strategies.
Serial number
|
Processing
strategy
|
Fluence
(J/cm2)
|
Spot overlap ratio (%)
|
Scanning spacing(µm)
|
Burst-mode
|
Scanning number
|
PS1
|
Roughing
|
3.35
|
80
|
7
|
1
|
7
|
Finishing
|
2.68
|
90
|
7
|
3
|
1
|
PS2
|
Roughing
|
3.35
|
80
|
7
|
1
|
7
|
Finishing
|
2.68
|
80
|
9
|
5
|
2
|
PS3
|
Single process
|
2.68
|
80
|
7
|
1
|
12
|
Figure 11 shows the three-dimensional morphologies and cross-sectional profiles of the groove and the surface morphologies of the bottom surface of the groove for different processing strategies. Comparing Fig. 11(A)-(C) (PS1-PS3), it could be seen that in contrast to the linear propagations and concave areas due to excessive ablation that exist on the surface of the single process milling, the surface obtained by the composite process has fewer processing defects. Figure 11(a1) -(c1) show the cross-sectional profiles of the corresponding groove bottoms. As shown in Fig. 11(a1) -(c1), the machined depths for the three different strategies are 39.882 µm, 40.254 µm, and 40.041 µm, respectively, with the error controlled to within 0.500 µm. Compared to Fig. 11(c1), the machined surfaces of Fig. 11(a1) and (b1) are more regular and the profiles do not have large convex or concave areas, resulting in better groove profiles. Figure 11(a2) -(c2) show the surface morphologies of the bottom surface of the groove under the corresponding processing strategies described above. As shown in Fig. 11(c2), under single process milling, there are voids and convex areas on the bottom surface of the groove, the surface roughness is relatively poor. The processing quality needs to be improved. For the composite process, the rough machined surface has voids with excessive ablation due to the high energy fluence. After the burst-mode finishing, the rough surface particles, debris remelting and redistribution by the heat accumulation between burst sub-pulses and sintering of voids in the surface, resulting in a certain degree of healing of voids on the rough machined surface. The deposition of debris on the machined surface reduces. As shown in Fig. 11(a2), the surface concave areas are small. The surface is smooth, with the ablation particles and debris deposition reduced due to the slowing down or suppression of phase explosion in the burst-mode. As shown in Fig. 11(b2), compared to the single process, the surface convex structures reduces, and the distribution is less concentrated. There is no significant defect, and the surface roughness and processing quality significantly improve compared to the single process.
A comparison of processing quality and efficiency is carried out with different strategies. Figure 12 reflects the comparison of the surface roughness and MRR of the milled grooves under different processing strategies. Figure 12(a) shows that the bottom surface roughness of the grooves obtained from the composite process is 0.463 µm and 0.451 µm, respectively, which is significantly reduced compared to the roughness of 0.505 µm obtained from the single process, with a reduction in the roughness of up to 10.69%. Figure 12(b) illustrates the MRR of the milling. The composite process significantly increases the average processing efficiency due to the higher energy fluence used in the roughing, with the MRR of 12.954 × 106 µm3/s and 12.228 × 106 µm3/s for the composite process, respectively, compared to the single process MRR of 8.641 × 106 µm3/s. The MRR increases by 49.91%, significantly improves the processing efficiency. The results show that the composite strategy via the femtosecond laser burst-mode has higher milling quality results and a more efficient milling than the single process strategy. The composite strategy is reasonable and feasible.