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Abstract
The efficiency of pulsed laser ablation has always been the focus point of research. A novel high-frequency electromagnetic induction heating-assisted laser ablation scheme is proposed and investigated to enhance the efficiency and improve the surface processing quality during the nanosecond laser ablation of metal substrates. To reduce laser energy required to reach the ablation threshold of metal, this method utilizes the electromagnetic induction to rapidly elevate substrate temperature, making the metal easier to be ablated. The results show that ablation width increases 16% and ablation depth increases 31% with the assistance of electromagnetic induction heating at a laser fluence of 1.32 J/cm2, which increases 90% of the laser-ablated volume. Meanwhile, the surface ablation quality is significantly improved due to the smaller temperature gradient around the ablation region. This new method has great potentials in the laser micromachining at a higher processing efficiency and better laser-processed surface quality.
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1. Introduction
Pulsed laser ablation is based on strong interaction between high-intensity laser beam and material, leading to the formation of a laser-ablated crater on material surface [1,2]. Laser ablation has the advantages of high precision, excellent repeatability, and high flexibility compared to traditional machining methods [3]. It holds significant potentials in precision machining, cutting, microfabrication, and various other fields [4–8]. However, due to strong thermal diffusion in laser ablation, the high ablation threshold significantly constrains the efficiency of laser ablation [9]. The physical properties of the material, such as absorption coefficient, thermal diffusivity, melting, and boiling behaviors, determine its ablation threshold [10]. Most of the energy in the laser ablation is used to increase the substrate temperature from room temperature to its melting temperature and then vaporization temperature, which limits the laser ablation efficiency. At the same time, the processed surface quality is degraded by thermal effects during the laser ablation.
Many studies have developed feasible methods to improve the ablation efficiency. Using high-power lasers can provide a higher energy density [11]. However, it is challenging to avoid adverse effects such as thermal damage, which can impact the ablation process and surface quality. Ultrafast laser ablation has advantages in high precision and extremely low thermal effects, which meet the requirements of high-quality micro/nano-structure fabrication [12–14]. However, ultrafast ablation is expensive with much lower power and has a lower ablation efficiency than nanosecond laser ablation [15,16]. A commonly used method is the surface pretreatment of materials, such as surface coating [17]. Applying a layer on the surface of target material with absorptive material (e.g., graphite powder, mica, and black paint) can convert more optical energy into thermal energy. This increases the laser absorption, making the interaction between laser and material more effective, thus enhancing the ablation efficiency. Double-pulse laser ablation is also an effective method [18]. It involves two laser pulses at different time scales, where these two pulses are sequentially irradiated onto the material surface at a time delay. The first pulse is used for pre-heating the substrate, while the second pulse is for ablation and material removal, thus enhancing the efficiency of laser processing. A spatial double-pulse laser ablation approach is developed as well [19]. Two split laser beams are simultaneously irradiated on the material surface at a tunable gap. By optimizing the gap distance, the significant enhancement of the laser ablation can be achieved. Steam-assisted laser ablation is a creative method, due to stronger shock wave confinement, greater heat dissipation, and weaker laser-plasma interactions, resulting in more material removal [20]. Plasma-assisted laser ablation is also an effective way to reduce the optical reflectivity and change the thermo-physical properties of material by the formation of micro-bubbles, lowering the ablation threshold [21]. However, the methods require complex system setups, which are disadvantageous in practical applications.
In this paper, a high-frequency electromagnetic induction heating-assisted laser ablation scheme is proposed to enhance the efficiency and improve the surface quality of laser ablation. By rapidly increasing the surface temperature of metal, this method reduces the ablation threshold, thereby improving the efficiency of laser ablation. Simultaneously, the smaller temperature gradient around the ablation region by the high-temperature pre-heating reduces the thermal impact, thus improving the quality of the laser-ablated surface.
2. Experimental setup, materials and procedure
2.1 Experimental setup
The schematic of the high-frequency electromagnetic induction heating-assisted laser ablation system is schematically shown in Fig. 1. The system consists of laser ablation, electromagnetic induction heating, water cooling, and real-time temperature measurement. The laser ablation system is composed of a nanosecond laser at a wavelength of 1064 nm, and the spot diameter of the laser beam after focusing through a field lens is ∼50 µm. The repetition rate of the pulsed laser is 250 kHz and the pulse duration is 45 ns. The scanning speed is set to 50 mm/s and the laser fluences range from 0.85 - 2.40 J/cm2. High-frequency alternating current (60 kHz, 800 A) is employed to power the electromagnetic induction heating module, which induces a rapid alternating magnetic field within the induction coil. Under the influence of the high-frequency magnetic field, an eddy current is induced on the surface of the metal, resulting in a rapid increase of temperature. This heating method is environmentally friendly, contactless, and rapidly provides uniform heat across the sample surface [22]. The real-time temperature measurement module is used to detect the sample surface temperature. The water cooling system utilizes a water pump to circulate the coolant inside the induction coil.
Fig. 1. Schematic of high-frequency electromagnetic induction heating-assisted laser ablation system.
2.2 Materials and procedure
Titanium (Ti) has the advantages of high toughness, high specific strength, excellent high-temperature resistance, and good fatigue resistance. It is widely used in the industrial manufacturing, aerospace, military engineering, and other fields. A 20 × 20 × 2 mm Ti plate is used as the sample in this study, and several ablation experiments are conducted at different laser fluences. Considering the melting temperature of Ti at 1933K and the phase transition temperature at 1155 K, the pre-heating temperature chosen for this experiment is 1200 K (62% of the melting temperature). At the pre-heating temperature of 1200 K, the grain size of Ti is slightly increased, the hexagonal close packed (HCP) grain structure is transformed into body centered cubic (BCC), and the alpha phase is transformed into the beta phase [23,24]. The focus of this work is on the rate of material removal. Before the laser ablation, the sample is mechanically polished and cleaned in an ultrasonic bath with acetone and ethanol for 15 minutes. After heating the sample using the high-frequency electromagnetic induction heating, laser ablation along a linear path is performed across the Ti surface. At the same time, unheated sample is also ablated under the same laser processing parameters for comparison. After the laser processing, the sample is cleaned in an ultrasonic bath with acetone and ethanol for 15 minutes. Characterization of the ablated samples is conducted to analyze the enhancement of laser ablation and the quality of the ablated surfaces. A confocal laser scanning microscope (Model: VK-X1000, KEYENCE) is used to characterize the surface morphology, cross-sectional profile and 3D reconstruction. Scanning electron microscope (SEM) (Model: SUPRA55 SAPPHIRE, ZEISS) is used to characterize the microscopic morphologies of the ablated surfaces.
3. Results and discussion
Figure 2 shows the results of micro-grooves formed by the laser ablation of the Ti surfaces after the scanning passes of 30 at a laser fluence of 1.32 J/cm2, with and without the high-frequency electromagnetic induction heating. It is clear that the laser ablation assisted by the high-frequency electromagnetic induction heating results in a wider and deeper ablation micro-groove, significantly increasing the volume of removed materials and greatly enhancing the ablation efficiency. Specifically, as observed from the cross-sectional profiles of Fig. 2, the micro-groove width obtained by the laser ablation without the high-frequency electromagnetic induction heating is approximately 58.5 µm. With the assistance of the high-frequency electromagnetic induction heating, the width of the ablation micro-groove reaches 68.4 µm. It increases over 16%. The depth of laser ablation micro-grooves increases from 8.7 µm to 11.4 µm, with an increase over 31%. The analyses of Fig. 2 reveal that microfeatures produced on the Ti surfaces appear to be trapezoidal in the first case and near parabolic in the second. The ablation volume of the micro-groove is estimated by the cross-sectional area times micro-groove length. The comparison of laser-ablated volume at the same length is estimated based on the cross-sectional area. It is calculated that the cross-sectional area with the high-frequency electromagnetic induction heating is 603 µm2, while the area without the high-frequency electromagnetic induction heating is 315 µm2. It increases over 90%, which represents a significant increase in the volumetric removal. The higher heat content around the ablation region by the pre-heating, which makes the material be easier to be ablated, resulting in the removal of a greater amount of materials [25]. From the 3D reconstruction in Fig. 2(b), the edge protrusion with the high-frequency electromagnetic heating can be observed. During the laser ablation, the substrate material undergoes heating, melting, evaporation, and explosive removal [26]. When a pulsed laser irradiates the substrate surface, a high-temperature melting region is formed due to strong energy transfer around the irradiation region. Meanwhile, a shock wave is formed due to the pressure difference between ambient and dense plume [27]. The molten material inside the ablation zone is pushed out by the shock wave and redeposited around. On this basis, laser ablation is exacerbated by the high-frequency electromagnetic induction heating. After the laser pulse turns OFF, the molten materials re-solidify on both sides, which forms the edge protrusion.
Fig. 2. Micro-textures, 3D reconstructions, and cross-sectional profiles of micro-grooves on Ti substrates fabricated via laser ablation (a) without and (b) with the high-frequency electromagnetic heating assistance.
The SEM images of the laser-ablated micro-grooves surface are shown in Fig. 3. The overall surface images of the micro-grooves fabricated by the laser ablation without and with the high-frequency electromagnetic induction heating assistance are depicted in Figs. 3(a) and 3(b), respectively. It can be observed that the re-solidification regions of the micro-grooves show different characteristics. To further explore the details, the ablation characteristics of Zones A and B in Fig. 3(a) and Zones C and D in Fig. 3(b) at higher magnification are shown in Fig. 3. The results indicate that the laser-ablated micro-groove with better surface quality can be achieved with the high-frequency electromagnetic induction heating. As shown in Zone A, the bottom of the micro-groove without the high-frequency electromagnetic heating is covered with many microcracks. In contrast, no microcracks are detected at the bottom of the micro-groove with the assistance of the high-frequency electromagnetic induction heating. When a laser ablates metal in ambient air, significant temperature gradient is generated in the ablation region due to local heating and rapid cooling. The high temperature gradient causes material expansion, altering the surface tension. Following the termination of laser ablation, the ablated region solidifies rapidly at a high cooling rate, inevitably inducing surface tensile stress. As a result, microcracks are formed at the bottom of the ablation micro-groove. In contrast, the metal substrate being pre-heated to 1200 K through the electromagnetic induction exhibits a much smaller temperature gradient, which reduces the thermal stress and effectively reduces the formation of microcracks. Similarly, from Zone B, it can be observed that the micro-groove without the high-frequency electromagnetic heating has severe recast debris accumulation and larger microcracks at the edges compared to Zone D. The molten material is ejected out severely under the action of the shock wave. At the same time, it becomes thermodynamically unstable and deformation under the high temperature gradient, leading to random and disordered re-solidification [28]. Consequently, the debris and microcracks are formed at the edges, as shown in Zone B. Whereas the micro-groove edges under the high-frequency electromagnetic induction heating-assisted laser ablation are shown in Zone D, instead of fragmented recast layer and microcracks, a relatively regular and more integrated surface is found. The temperature gradient drives heat away from the molten layer and triggers a rapidly moving re-solidification front by the shock waves, which advances from the ablation region towards both sides. A slower re-solidification front speed causes a longer melt state time, which can improve the surface quality of the re-solidified layer. The re-solidification-front speed is affected by the material's heat content and cooling process around the ablation region. The higher heat content by the pre-heating and slower cooling process result in a longer melt duration and a slower re-solidification front speed [29]. Thus, with the pre-heating temperature of 1200 K, melting is typically followed by growth of the integrated and dense surface, as shown in Zone D.
Fig. 3. SEM images of micro-grooves on Ti substrates ablated by laser (a) without and (b) with the high-frequency electromagnetic induction heating assistance.
A series of laser ablation experiments are carried out to investigate the enhancement of laser ablation at different laser fluences. The Ti substrate is pre-heated to a temperature of 1200 K by the high-frequency electromagnetic induction heating and then laser ablated at laser fluences ranging from 0.85 - 2.40 J/cm2. The ablation experiments are carried out without the high-frequency electromagnetic induction heating at the same laser fluence for comparative analyses. The cross-sectional profiles of these ablative micro-grooves are shown in Fig. 4. It is evident that the width and depth of the ablation micro-grooves obtained after the pre-heating are increased. Specifically, the width increases by approximately 20% and the depth increases by over 32% at a laser fluence of 0.85 J/cm2, representing the most significant enhancement of laser ablation. This enhancement trend slows down at higher laser fluences. In particular, the increase in the width of the ablation micro-grooves decreases as the increase of laser fluence. An analysis of the cross-sectional profile obtained at a laser fluence of 2.40 J/cm2 shows the contour appears undulating and the recast layer on both sides of the micro-grooves has a significant bulge. This may be attributed to the strong reaction between the pre-heated sample and laser, resulting in the severe ablation of the material. This results in the degradation of ablation quality. In practical machining applications, it is common to utilize a appropriate laser fluence for the laser processing to achieve the best ablation.
Fig. 4. Cross-sectional profiles of micro-grooves on Ti surfaces ablated by laser without and with the high-frequency electromagnetic heating assistance at different laser fluences.
Above studies indicate that the laser ablation of metals is significantly influenced with the assistance of high-frequency electromagnetic induction heating. It has been studied that when a pulsed laser with sufficiently high intensity irradiates the material surface, the irradiated zone undergoes melting, vaporization, phase explosion, and particles’ ejection, resulting in materials’ removed [30]. The main ablation mechanism depends on the laser-material interaction and the optical and physical properties of the material [31]. The magnetic flux is generated by the high-frequency alternating current flowing in the induction coil and is continuously changed its direction. In the substrate surface, electromotive force corresponding to the change rate of the magnetic flux inside of the substrate is generated. As a result, an eddy current is generated in the substrate surface with closed circuit, which induce rapid frictional movement of electrons inside the metal. Then, a lot of heat is rapidly generated. Metal materials are pre-heated with the high-frequency electromagnetic induction heating, which enables the metal surface to rapidly attain a uniform temperature. In this study, the Ti surface is rapidly pre-heated to a temperature of 1200 K as shown in region A of Fig. 5. When a pulsed laser irradiates on the material surface, the temperature of the ablated area with the pre-heating increases much faster, resulting in a shorter time for the material to reach its melting point. Consequently, the ablation time is extended as shown in region B of Fig. 5. More incident pulsed laser energy can be used to melt and vaporize the material, leading to more materials to be removed. Meanwhile, the high-temperature pre-heating by the high-frequency electromagnetic induction heating provides extra absorbed energy to the material during the laser ablation. The energy used in laser ablation to increase the substrate temperature from room temperature to the pre-heating temperature is not required. Therefore, more laser energy is available to have the stronger laser ablation with more materials removal to achieve the enhancement of laser ablation. The evolution of the cooling temperature in the ablation region is shown in region C of Fig. 5, which can be used to explain the improvement of the ablated surface quality. Laser ablation leads to local high temperature and rapid cooling, which causes the thermal stress. The uniform high-temperature pre-heating of the material reduces the temperature difference around the ablated region, resulting in a smaller temperature gradient. As shown in region C of Fig. 5, the temperature of the ablation zone first drops from the vaporization temperature to the melting temperature at the same rate after the laser pulse is OFF. The high-temperature pre-heating of the material leads to a longer duration of high temperature in the ablation region, accompanied by a slower cooling rate. High-frequency electromagnetic induction heating can make the temperature of the whole material surface more uniform and the temperature gradient smaller. This avoids the rapid expansion and solidification of the material, which helps to reduce the thermal stress caused by large temperature difference. As the thermal stress is reduced, the microcracks formation is also greatly reduced.
Fig. 5. Schematic of temperature evolution in laser ablation region without and with the high-frequency electromagnetic induction heating assistance.
Based on the above discussion, this method can effectively improve the laser ablation efficiency and the processed surface quality of laser ablation.
4. Conclusions
In this work, a high-frequency electromagnetic induction heating-assisted laser ablation is proposed and investigated to realize the enhancement of laser ablation of metal substrates. To demonstrate the unique ablation effect, the laser ablation experiments of Ti surfaces are carried out without and with the high-frequency electromagnetic heating assistance. During the process of the high-frequency electromagnetic induction-assisted laser ablation, the metal sample is pre-heated to a high temperature that is close to its melting point to increase the laser ablation efficiency. Experimental results reveal that the width of the ablation micro-groove increases by 16% and the depth increases by 31% at a laser fluence of 1.32 J/cm2, which increases 90% of the laser-ablated volume. The metal pre-heated at a high temperature makes more pulse laser energy being used to heat the material surface to the melting point and then the vaporization temperature for laser ablation in a shorter time, which greatly reduces the ablation threshold. Thus, a stronger ablation is achieved and the efficiency of laser ablation is improved. In addition, the experimental results show a significant improvement in the surface ablation quality. The high-temperature pre-heating generates a smaller temperature gradient around the ablation region, which reduces the thermal influence on the ablated surface. The reduction of thermal stresses greatly reduces the formation of microcracks on the ablated surfaces. This new method has a wide range of promotion and application prospects in the field of laser processing.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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