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Abstract
Laser micromachining technology is a method of precision manufacturing that is experiencing growth with wide applications. Due to the increasingly tense energy situation, it is an appropriate time to consider the efficiency and economic issues of growth in precision manufacturing. The reutilization of a reflected laser beam (RRLB) for modifying metallic materials with low laser absorption is proposed in this article to reduce the energy and time consumption of the laser micromachining process. The novel laser machining approach, using an RRLB, which combined a nanosecond laser with an RRLB optical system of fiber laser, was applied on 6061 aluminum to validate its superior characteristics. The characteristics of the RRLB were clarified by the experiment on 6061 aluminum. Compared with normal laser machining (NLM), the processing efficiency of the RRLB can be greatly improved. Owing to the reutilization of the reflected laser beam energy without a thermal relaxation time, a higher-intensity laser-metal interaction can be realized for the RRLB; thus, the incoming energy can be utilized more effectively by ablating more material instead of diffusing it into the sample. Moreover, practical examples by using the RRLB, such as surface darkening on 6061 aluminum, surface polishing on additive manufactured Al alloy, and surface colorization on titanium and stainless steel, also demonstrated the excellent versatility and superiority of the RRLB approach.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As a major area of the global economy, manufacturing is responsible for over one third of the global energy consumption and CO2 emissions [1,2]. Reducing energy consumption and increasing energy usage are significant for alleviating the energy demand and environmental impact [3]. In the past decade, substantial effort has been made to reduce the overall energy consumption in industry [4]. Green technologies, e.g., laser micromachining technology, have been used in a broad range of applications from material removal [5] to additive manufacturing [6], owing to the fact that lasers have been become less costly every year [7].
Materials processing is one of the most common laser applications among the communication, cosmetic medical, military, optical storage, lithography, instrumentation, display, and printing industries. Compared with traditional subtractive machining methods, laser materials processing is suitable for precision microscale processing [8] and can be applied to almost all kinds of materials, such as metals [9], ceramics [10,11] and superhard materials [12,13]. However, the energy utilization of laser material processing has always been overlooked, and many issues remain to be solved, especially the energy efficiency for metallic materials with low laser absorptivity.
For metallic materials, laser processing has the disadvantage of being low in energy efficiency in terms of processing rate due to high reflectivity and low laser absorption [14,15]. For instance, aluminum has reflectivity of 91% at the 1064 nm wavelength, and that of nickel and steel is about 70%, which means low percentage of laser energy left for absorption [15]. Metals such as gold, silver, copper and aluminum have a very low laser absorption [16]. Thus, the subject of the laser-metal interaction has always been one of the most concerned issues in laser materials processing [17,18]. To solve this problem of low laser absorption for these materials, many studies have been performed. Basov et al. investigated the absorption of Nd-laser pulses for Al, Sn, and Cu with different laser intensities and realized a reduction of the reflection coefficient to ~0.1 for ablation in vacuum [19]. The time-integrated absorptivity of copper surfaces with laser energy densities above 15 J/cm2 was obtained to be approximately 0.7 and 0.8 in vacuum and air, respectively [20]. Wang et al. employed a blue diode laser with a wavelength of 445 nm to achieve a higher absorption coefficient than a traditional laser [21]. In addition to short-wavelength lasers [22], Harzic et al. utilized short-duration pulsed lasers, with ps and fs pulses, realizing a high-intensity laser-metal interaction [23]. Moreover, Mannion et al. investigated the effect of defects randomly distributed in a material [24]. The results showed that the preset damage on the surface can easily trigger a strong laser-matter interaction in nanosecond laser machining. Although this issue has attracted much attention and many approaches have been applied, the strategy of realizing a high-intensity laser-metal interaction via reutilization of a laser beam reflected from the workpiece based on the principle of light reflection has not yet been used or studied.
Resonator designs with variable mirrors were investigated widely to realize different output powers and beam parameters according to the principle of light reflection [25,26]. However, there has been hardly any research about improving energy utilization by reutilizing a reflected nanosecond laser beam. How to realize this has become an important and urgent subject for low machining efficiency and energy utilization in the industrial application of laser surface modification technology.
In this study, the reutilization of a reflected laser beam (RRLB) was proposed to machine the surface of metallic materials with low laser absorption. Based on the principle of reflection, a concave reflector with a small pore was applied for collection and delivering the reflected laser beam to the workpiece again, equivalent to realizing an energy utilization ratio for metal samples with low laser absorptivity. Optical analysis clarified the mechanism of the reutilization of the reflected nanosecond laser beam, and the optical system was optimized and designed. The ablation process, morphologies and ablation characteristics involved in the formation of the modified microgroove on 6061 aluminum were investigated and analyzed. Moreover, some practical examples of surface darkening on 6061 aluminum, surface polishing on additive manufactured Al alloy and surface colorization on titanium and stainless steel were applied to prove the versatility and superiority of the RRLB approach.
2. Experimental setup
2.1 Optical mechanism and parameter optimization of the RRLB system
A schematic of the optical system is schematically shown in Fig. 1. The main components of the RRLB optical system are an optical isolator, a convex lens and a circular cross-section concave reflector with a small pore in the center. The sample to be machined is set right below the concave reflector on a microdistance worktable. The concave reflector is set above the sample with the laser beam and can pass through the small pore. The circular focus is adjusted on the sample surface. After focusing, the laser beam passes through the pore, and the focus will be generated on the sample surface. Due to the high laser reflection of the sample, most of the laser beam is reflected, which will irradiate the concave reflector. Then, this part of the laser beam will be reflected back to the focus again by the concave reflector due to the optical reflection of the circular cross-section concave reflector. In this way, in addition to part of the laser energy being absorbed by the sample, the surplus scattered light reaches the concave reflector again and is once again reflected back into the focus. This way the laser beam can travel back and forth between the sample and concave reflector until the laser energy is completely absorbed by the sample or escapes from the RRLB optical system due to the continuous divergence of the diameter of the laser beam in the reflection and scattering processes, which would exceed the diameter of the concave reflector. Therefore, the reutilization of the reflected laser beam can be realized.
The parameters of the RRLB optical system can be optimized by establishing a nonsequential ray tracing model in ZEMAX software. The optimization objective of the evaluation function is the diameter of the light spot. A simulation image of the optical system is shown in Fig. 2. Table 1 shows the parameters of the RRLB optical system after optimization by ZEMAX software.
Fig. 2 Laser ray tracing simulations of the RRLB optical system by ZEMAX software: nonsequential ray tracing model.
Table 1. Parameters of the RRLB optical system (unit: mm)
2.2 Test setup of the RRLB
The basic test setup is schematically shown in Fig. 3. In the RRLB system, a fiber laser with a waist diameter w0 of 31.5 μm, wavelength of λ = 1064 nm, pulse repetition rate of f = 200 kHz and pulse duration of 5-7 ns is used. The cross-section of concave reflector Q1 is circular. The aperture of the small pore is as small as possible to increase the effect of reflecting the laser beam but cannot impede the propagation of the incident laser beam. The optical isolator can protect the fiber laser from the reflected laser beam. In addition, the convex lens used in this study has a depth of focus of approximately 6 mm (i.e., two times the Rayleigh range) given by 2πw02/λ, which ensures that the RRLB works well even if the surface of the workpiece is continually being updated. Moreover, based on the optical mechanism of the RRLB system, the 6061 aluminum alloy with a low laser absorptivity and high laser reflectivity was selected as the workpiece material. And the thermophysical properties of the 6061 aluminum are listed in Table 2. Prior to testing, the 6061 aluminum workpiece, used in this experiment with a size of 20 mm × 20 mm × 2 mm, was polished and ultrasonically cleaned in fresh dehydrated alcohol.
Table 2. Properties of 6061 aluminum [27]
In this study, the ablation threshold of the 6061 aluminum was also evaluated using the J.M Liu method [28]. This method involves the use of relationship between ablation area and laser pulse energy density, whereby the ablation area is extrapolated to the value of zero, and the corresponding energy density is regarded as the ablation threshold. By using this method, the ablation threshold of the processed material Fth can be calculated as following:
where w0 is the focused laser spot radius, F0 is the incident laser fluence, and D is the measured diameter of the ablation spot with respect to the given F0.In order to reduce the deviation errors and the measured errors of the ablation crater area, the values of Fth under various pulse energy were calculated based on Eq. (1), then the average calculated values were adopted as the ablation threshold. The calculated results of the ablation threshold of the 6061 Aluminum alloy are listed in Table 3, together with the data of Gedvilas al [29]. for comparison. It should be mentioned that with the laser fluence increasing above the plasma formation threshold, the reflectivity of metals drops and remains unchanged with further increasing laser fluence, and the plasma generated will affect the propagate of laser beam. In the pre-experiment of machining on 6061 aluminum, it is found that with the laser fluence below 5Fth, the formation of plasma is mild, and the plasma shielding effect can be ignored. Based on this consideration, the selected laser fluence in this study is below 5Fth, i. e., 2.525 J/cm2, which also guarantees that the concave reflector surface does not get contaminated or damaged by plasma during laser ablation process.
Table 3. Calculated ablation threshold of 6061 aluminum
Moreover, the optical photographs during laser ablation process were taken by a high speed intensified CCD camera (4 Quick And Stanford Computer Optics). After the ablation test, the micro-morphology of microgrooves was inspected by SEM (Nova Nano430, FEI, USA). The stereoscopic profile of microstructure was analyzed by software Image J 1.43u (issued by National Institutes of Health). The diameter and the depth of the ablated microgrooves were measured by the white light interferometric microscope. And the composition of the recast layer was determined by EDS (Inca300, Oxford, UK).
3. Results and discussion
3.1 Ablation process
Optical images around the irradiated zone at 5 ms delay following the irradiation of first pulse during the laser scanning ablation process with the RRLB and NLM for F0 = 0.8 J/cm2, 1.6 J/cm2 and 2.4 J/cm2 are presented in Fig. 4. It can be observed that scattering effect, a phenomenon of laser beam scattering from the nanoscale ejected particles, occurs during the laser ablation process for both the RRLB and NLM. This phenomenon can be used as a reference of the temporal and spatial distribution of the ejected particles [30]. Thus, it can be inferred that the particle ejection in Figs. 4(e) and 4(f) can spatter severely using the RRLB, which is much stronger than the results generated by NLM shown in Figs. 4(b) and 4(c) for the same laser parameters. Therefore, it can be inferred from Fig. 4 that more severe ablation and ejection processes occur, and a larger ablation rate can be realized using the RRLB.
Fig. 4 Optical images around the irradiated zone at 5 ms delay following the irradiation of first pulse during the laser ablation process with the RRLB and NLM. (Scanning speed vs = 200 mm/s).
3.2 Ablation trace morphology
Figure 5 shows the SEM morphologies of the ablation spots by the irradiation of mono-pulse with respect to RRLB and the NLM. The corresponding stereoscopic profile of the ablation spot is shown in the bottom of Fig. 5. Under the same laser fluence, the area of ablation spot for the NLM is smaller than that for RRLB. It should be mentioned that for both test setup, the fiber laser is same, which implies the ablation accuracy of the RRLB can be reduced to a certain extent compared to the NLM. This may be due to the continuous divergence of the beam diameter during reflection and the manufacturing and installation errors of concave mirror. Moreover, the recast debris around the edge of the ablation spot for the RRLB is severer than that for the NLM. This signifies that the extent of the spatter process for the RRLB is more intense compared to the NLM.
The morphologies of microgrooves generated by the RRLB and NLM with different laser fluence are shown in Fig. 6. The corresponding stereoscopic profile of microgrooves is shown in the bottom of Fig. 6. The width d and depth h of these microgrooves have measured and added next to microgrooves. Under all the used laser fluence, microripples present on the bottom of the microgrooves. Those structures may be attributed to the movement of the molten metal from hot areas to colder ones, adding the material to the ripple lines, which is referred as the Marangoni effect [29,31]. Moreover, both the width and depth of microgrooves fabricated by both approaches increase with increasing laser fluence. The microgrooves fabricated by the RRLB are wider and deeper than those fabricated by NLM for the same laser fluence. In addition, many micron-sized droplets are present around the microgroove in Figs. 6(e) and 6(f), which are the manifestations of the occurrence of severe particle ejection in accordance with the severe scattering effect shown in Fig. 4(f). When a pulse irradiates the surface, a high-temperature melting zone is formed due to intense energy transfer in the irradiated zone. In the meantime, a shock wave is formed due to the pressure difference between the ambient and dense plumes. Melting materials in the irradiated zone are pushed outside by the shock wave, resulting in more materials being redeposited in the periphery. Moreover, the shock wave can be strong enough to blast the melting material away with a sufficiently high intensity laser. This is the formation mechanism of severe particle ejection in Figs. 4(e) and 4(f), and micron-sized droplets around the microgroove in Figs. 6(e) and 6(f). Furthermore, when a train of pulses is used in the scanning process, thermal melting and material resolidification repeats. In the overlapping area, a microstructure formed by a former pulse can be erased and reformed again by the next pulse. The original microstructure in the nonoverlapping area still exists and forms microripples.
Fig. 6 Morphology of microgrooves using the NLM and RRLB with different laser fluence. (Scanning speed vs = 200 mm/s).
For a more quantitative comparison for the RRLB and NLM, the dependence of ablation depth of microgrooves and ablation rate per pulse on laser fluence is presented in Fig. 7 and Fig. 8, respectively. It can be observed that the ablation depth and ablation rate per pulse of the RRLB under all laser fluence are both much higher than the results of the NLM. For both approaches, as the laser fluence increases, the ablation depth and ablation rate per pulse increases. Moreover, a sudden rise in the ablation depth was observed at a laser fluence of F0 = 1.2~1.6 J/cm2 for the RRLB. It is obvious that this sudden rise is the result of severe particle ejection. In such a case, a large part of the melting materials is ejected, which coincides with the scattering effect in Figs. 4(e) and 4(f). When the laser fluence F0 exceeds 1.6 J/cm2, the relative ablation rate of the RRLB is about 2.42~2.99 times higher than that of the NLM.
3.3 Resolidification characteristics
To further explore the removal details in the scanning ablation process for NLM and the RRLB, the ablation characteristics of the resolidification regions of Zone A and Zone B in Fig. 6 at higher magnification are shown in Fig. 9. In addition, EDS patterns of Zone C and Zone D in the ablation zone were measured; the results are shown in the bottom of Figs. 9(b) and 9(d), respectively. Figure 9(a) shows small, droplet-like edges of microripples with diameters on the order of 1 μm for NLM. Based on the shape and size of the edges, it can be inferred that this type of edge was generated because melting materials in the irradiated region were pushed outside by the shock wave, and resolidification occurred while the ejected materials were still in the liquid phase. In contrast, for the RRLB, the edges of microripples presented in Fig. 9(d) contain material melted but also part of fractured material, which means that in addition to melting and resolidification, this type of edge is produced by mechanical failure. It can be inferred that this region was exposed to higher pressures, generated by shockwave, above the mechanical failure threshold of the material. This implies that the shockwave in the irradiated zone of the RRLB is higher than that of NLM.
Fig. 9 Morphologies of Zone A and Zone B in Fig. 6 at higher magnification. (Laser fluence F0 = 2.4 J/cm2).
In addition, elemental analyses of the modified surface indicate that there is a higher component of O around the irradiated zone using the RRLB compared with NLM. This means that there are different degrees of oxidation of Al during laser ablation. As the oxidation of metals is a temperature-dependent effect, the larger amount of oxides suggests that the surface modification is caused by a higher temperature for the RRLB. To further explore the influence of different laser fluence on the O element concentration of the RRLB, the evolution of O at different laser fluence of the RRLB is summarized in Fig. 10. The results for NLM with corresponding laser fluence are also presented in Fig. 10.
It is obvious that from Fig. 10 for the RRLB, with an increase in laser fluence, the concentration of O exhibits a pronounced increase. While for NLM, the concentration of O increases with increasing laser fluence slowly, which is much lower than that for the RRLB in the same laser fluence. It was reported that oxidation in the irradiation zone can be accelerated by higher temperature due to higher laser fluence [32]. In turn, variations in the temperature in the irradiation zone for different laser fluence can be inferred by the degree of the O concentration. Thus, the larger amount of O content in the irradiation zone for the RRLB in Fig. 10 suggests that the temperature in the irradiated zone for the RRLB is higher than that for NLM during processing.
From the above results, two major superior characteristics for the RRLB can be summarized compared with NLM. The first superior characteristic for the RRLB is increasing the energy utilization ratio via reutilization of the reflected nanosecond laser beam. The second superior characteristic is due to the high-temperature profile in the laser irradiation zone, which more easily heats the material to the boiling point or even above. In this way, material removal may occur by a severe ejection process in the irradiation region for the RRLB. Thus, an efficient ablation process for the RRLB occurs, characterized by strong scattering effect and a larger ablation zone with ejected debris. A large part of the absorbed energy of the sample is carried out through ablation, which means that less of the incident energy is utilized ineffectively by diffusing into the sample; instead, the energy contributes to ablation. Therefore, compared to NLM, the RRLB is able to realize a higher processing efficiency for the same laser parameters.
4. Practical examples
Figure 11 compares practical applications by the RRLB and NLM, including (a) surface darkening, (b), (c) and (d) surface polishing, and (e) and (f) surface colorization. For surface darkening on 6061 aluminum surface, the average irradiation time was nearly 10 s for each area, and the values of the laser fluence were varied from 0.8 J/cm2 to 2.4 J/cm2 for lines No. 1 to No. 10, as shown in Fig. 11(a). The irradiated zones at high laser fluence were observed to be very black with the naked eye, as indicated by No. 8, No. 9 and No. 10 in the figure. Additionally, the RRLB can realize a darker surface than NLM for the same laser fluence. The same design could be realized by the RRLB with a lower laser fluence. Therefore, the laser surface darkening treatment using the RRLB can save power during the laser process.
Fig. 11 Practical application examples for the RRLB compared with NLM. (a) Photographic images of the irradiated zones with increasing laser fluence on 6061 aluminum surfaces modified in air for NLM and the RRLB; (b) the surface of additive manufactured Al alloys after being polished by NLM and the RRLB, (c) and (d) macro-scale photographs of surfaces of additive manufactured Al alloy after being polished by NLM and the RRLB; (e) and (f) photographic images of titanium and stainless steel surfaces processed with various different scanning speeds by NLM and the RRLB. (Laser fluence F0 = 2.4 J/cm2).
For the surface polishing of additive manufactured Al alloys, a hatched scanning mode with 50% overlap, i. e., the scanning speed of 30mm/s, was used on the irradiated zone under Ar gas protection during the laser scanning process. After preliminary experiments, we determined that for NLM, the optimized power density was 6.80 × 106 W/cm2. In contrast, for the RRLB, the optimized power density was 2.70 × 106 W/cm2. Figure 11(b) shows surfaces polished by both approaches, and Figs. 11(c) and 11(d) show macro-scale photographs of sample surfaces fabricated by the NLM and RRLB approaches. For both approaches, rough surfaces were well polished, and laser melting tracks were observed on the sample surface. The resolution can be achieved to several microns. It successfully demonstrated that the RRLB can save energy and time when compared with NLM in fabricating the same design.
Furthermore, the RRLB approach can be employed to modify the surface color of other metals and alloys. Figures 11(e) and 11(f) shows photographic images of stainless steel and titanium surfaces processed at various scanning speeds by NLM and the RRLB, respectively. It can be observed that for both methods, various colors in the irradiated zones can be obtained by using different scanning speed. The lower scanning speed will induce the blacker color on irradiated zones with the naked eye. Moreover, in the same scanning speed, the RRLB can realize a darker color than NLM. In other words, the similar color can be realized by the RRLB with a higher scanning speed, e. g., the similar color in Figs. 11(e3) and 11(e14) for stainless steel was obtained by the RRLB with vs = 120 mm/s and the NLM with vs = 80 mm/s, respectively. Therefore, the laser surface colorization using the RRLB can significantly improve processing efficiency. Both examples demonstrate the superiority of the RRLB: high processing efficiency and high energy utilization. Therefore, this approach is expected to contribute to rapid fabrication and large manufacturing of metallic materials. Above all, the successful fabrication of the above examples exhibited the excellent versatility and superiority of the RRLB approach.
5. Conclusion
In conclusion, an innovative laser machining method based on the reutilization of a reflected laser beam (RRLB) was developed for metallic materials with low laser absorption. The mechanism of the reutilization of the reflected nanosecond laser beam was expounded, and an RRLB optical system was optimized and designed. The ablation characteristics of the RRLB were investigated using NLM as a reference to validate the superiority of the RRLB approach. The experimental results indicate that the RRLB has advantages of not only obtaining a higher energy utilization ratio but also realizing a higher material removal efficiency. Comparing to the NLM, the relative ablation rate of the RRLB can be enhanced 2.42~2.99 times. Moreover, several practical examples of the RRLB on Al alloy, titanium and stainless steel were carried out, which prove the versatility of the RRLB approach. Based on the results of this study, the RRLB makes it possible for nanosecond pulsed lasers to realize wider industrial applications.
Funding
National Natural Science Foundation of China (51575193, 51474108); Fundamental Research Funds for the Central University (2017ZD028, 2015ZP029); Guangzhou Science and Technology Project (201604020139).
Acknowledgments
The authors thank the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials for the use of their equipment.
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