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Study on processing characteristics and mechanisms of thermally assisted laser materials processing
Surface & Coatings Technology 378 (2019) 124946
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Study on processing characteristics and mechanisms of thermally assisted laser materials processing
T
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Jiandong Yuan, Liang Liang , Guozhi Lin National Engineering Research Center of Near-net-shape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser materials processing Thermally assisted Laser ablation Laser polishing
Nanosecond pulsed lasers have been widely applied to interact with and characterize many different materials, and industrial users are exploring more avenues for various application processes. For the purpose of achieving broader application, the current challenge is to achieve a high processing efficiency for laser materials processing. In this study, thermally assisted laser materials processing (TALMP), which introduces heat from an external source to increase the workpiece material's temperature and make it easier to carry out ablation or modification, was developed and investigated. Moreover, to investigate the materials processing characteristics and mechanisms of TALMP, practical applications using TALMP, including surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V, were carried out. The results showed that TALMP can be successfully applied to various materials by different laser materials processing applications. Due to the workpiece material's high temperature and heat content, the coupling between laser and target materials can be enhanced; thus, physical ablation and modification for laser surface ablation can be achieved effectively. However, with a high offset temperature, a little decrease of ablation accuracy and quality could occur, and a two-step approach is proposed to eliminate this unwanted effect. Moreover, this strategy is proven to be able to achieve the desired surface ablation with high efficiency and high-quality surfaces. For thermally assisted surface polishing, due to higher temperature field with long duration, a smoother and flatter polished surface can be obtained more easily. The experimental results proved the superiority and versatility of TALMP.
1. Introduction Laser materials processing is one of the most common laser applications among the communication, cosmetic medical, military, optical storage, lithography, instrumentation, display, and printing industries [1–3]. Over the past few decades, pulse laser materials processing has been used in a broad range of applications, from material removal to material modification, such as surface ablation [4] and surface polishing [5]. Using pulsed laser surface ablation, Liang et al. prepared desired micro-grooves on WC-Co, and clarified that the occurrence of the WC grain growth in the vicinity of the micro-groove affected the wear resistance of the micro-groove [6]. Ma et al. studied the laser polishing on additive manufactured Ti alloys, and clarified surface hardness and wear resistance were enhanced [7]. By applying laser surface polishing on additive manufactured CoCr components, Yung et al. achieved a high surface quality (Sa) < 1 μm with appropriate choices of the fluence and a certain laser defocusing distance, and found that the surface roughness and contact angles could be improved [8]. Furthermore, various innovative processing methods have explored
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new possibilities for laser materials processing, including laser-induced plasma-assisted ablation (LIPAA) to fabricate microfluidic devices [9], laser-induced molten transfer (LIMT) to fabricate metallic patterns on various target substrates [10], CO2 laser treatment as a clean process for treating denim fabric [11], laser multi-focus separation technology (LMFS) to separate thick KDP crystals [12], and so on. Although there has been a boom in the number of attempts and applications using pulsed lasers in recent years, the main limitation is the low ablation rate (10−6~10−4mm3/pulse) [13,14]. Therefore, the current challenge is to achieve a speedup of the materials processing process. Increasing the on-target laser intensity may be a direct and possible approach, while the use of high-intensity lasers leads to unwanted effects (e.g., saturation, shielding and thermal damage), which further limit the process efficiency and affect the ablation process and ablation quality [15,16]. In the meantime, many other approaches have been applied to realize a strong coupling between the laser and target material, such as optimizing the process parameters [13], improving the absorption coefficient by employing shorter wavelength or duration lasers and changing the machining conditions [17,18], reutilizing a
Corresponding author. E-mail address: [email protected] (L. Liang).
https://doi.org/10.1016/j.surfcoat.2019.124946 Received 20 July 2019; Received in revised form 21 August 2019; Accepted 30 August 2019 Available online 31 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 378 (2019) 124946
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designed offset temperatures. In this study, two different laser materials processing applications (surface ablation on WC-Co and surface polishing on an additive manufactured Ti-6Al-4 V) were selected and tested under each specific machining conditions and parameters. The purpose of selecting significantly different laser processing technologies on different materials and entry conditions was to ensure that the versatility and superiority of the TALMP approach are identified over a number of conditions that may be encountered in the laboratory and in industry.
reflected laser beam [14], and pre-setting the damage on the target surface [19]. Moreover, using ultrafast successions of pulses such that the delay time between two pulses is less than the thermal relaxation time is proven to be an effective way to increase the efficiency of the removal process by an order of magnitude over previously used laser parameters [20]. Recent investigations report that pre-heating workpieces during laser processing should be an alternative approach to enhance the ablation rate [21]. Tavassoli et al. investigated the influence of initial sample temperature on plasma temperatures and ablation depth of copper samples [22]. The results showed that an increase in the initial sample temperature results in an increase in the spectral emission of plasma up to 90% which is attributed to both the plasma temperature and optical absorption. Shi et al. investigated the effect of preheating temperature on microstructure of Fe based alloy coating by laser direct metal deposition, and concluded that preheating treatment is an effective approach to eliminate the cracks in the high thickness Fe60 alloy coating [23]. This approach of increasing processing efficiency by preheating workpieces, called thermally assisted laser materials processing (TALMP) in this study, introduces heat from an external source to increase the workpiece material's temperature and heat content, thereby increasing optical absorption and reducing latent heat to melting and vapor and making the material easer to ablate. This approach is seemingly contrary to common sense. Traditionally, efforts by researchers and industrial engineers have focused on decreasing the workpiece material's temperature by better heat management in the irradiated zone to circumvent heat accumulation around the irradiated zone using various technologies, such as laser ablation under a water layer or ice layer [24], decreasing the laser repetition rate [25,26], and so on. Thus, although there may be a higher processing efficiency for TALMP, it is reasonable to consider that the surface quality obtained by this approach may suffer from thermal degradation. However, until now, there is very little literature available that investigates the processing characteristics of this approach, and explorations that specifically use TALMP as a strategy for realizing a higher processing efficiency and clarify the mechanisms of TALMP appear to be completely lacking. In this study, TALMP was explored to obtain a higher processing efficiency. Moreover, to investigate its materials processing characteristics and obtain a better understand the mechanisms, surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V alloy were carried out using this approach. The results provide detailed information about the processing characteristics of TALMP on the irradiated area. The processing mechanisms of TALMP with different temperatures were analysed. Moreover, the experimental results also validate the versatility and superiority of TALMP.
2.2. Laser surface ablation on WC-Co WC-Co, as a typical structure and tribological material, is widely used in cutting tools and structural components. To further improve its tribological performance, textures (micro/nano-dimples or micro/ nano-grooves) fabricated by surface ablation have been widely applied to WC-Co [6]. Thus, WC-Co was used for the laser surface ablation test. Moreover, based on preliminary tests of surface ablation on WC-Co, four separate temperatures were selected for this test, including 25 °C (room temperature), 200 °C, 400 °C and 600 °C. The samples were mechanically polished to mirror finish and cleaned in an ultrasonic bath with acetone and ethanol for 30 min before and after laser processing. The microstructure morphology after laser ablation was analysed by scanning electron microscopy and energy dispersive spectrometry. The ablation rates were calculated from the cross-sections of the prepared textures, which were analysed by white-light interferometry. There are certain inevitable measuring errors. Because the ablation rates of the TALMP and traditional method were only roughly compared in this study, the measuring errors can be safely ignored. All the tests were performed in the air.
2.3. Laser surface polishing on additive manufactured Ti-6Al-4V For laser surface polishing, a schematic description of the laser scanning path is shown in Fig. 1(c). There are two types of overlap on the scanning path: laser scan line overlap L and laser spot overlap H, which depend on the laser spot diameter, pulse repetition rate, scanning speed, and line-scan spacing. With larger L or H, more laser pulses per unit area will irradiate on the target surface. However, the extremely large L or H can result in an excessive incoming laser energy per unit area, which will trigger severe plasma shielding of the incoming laser energy. However, a low L or H (or even no overlap) will only slightly modify the target surface. Thus, multiple scanning passes N and a long time may be needed to obtain the desired modified surface. In this study, the laser surface polishing test was applied on the pristine surface of additive manufactured Ti-6Al-4V. When the laser was turned on, the specimen was placed under N2 gas protection. Based on our preliminary tests of surface polishing on additive manufactured Ti-6Al-4V, four separate temperatures were selected for the thermally assisted surface polishing test: 25 °C (room temperature), 150 °C, 300 °C and 450 °C. In this temperature range, microstructural changes occur in additive manufactured Ti-6Al-4V. Moreover, the focal offset was fixed at the off-focus distance, and the laser beam diameter D was determined to be 90 ± 1 μm. Furthermore, in this surface polishing test, the laser scanning path parameters were L = 30 μm and N = 2. The scanning speed v was set to 300–600mm/s; thus, H = 30–60 μm. Surfaces after laser polishing as well as the cross-section were analysed by confocal microscopy, scanning electron microscopy, and energy dispersive spectrometry. Moreover, the Vickers microhardness was measured using a Vickers microhardness tester with a load of 100 N and a dwelling time of 15 s.
2. Experimental setup, materials and methods 2.1. Experimental setup The schematic of the TALMP system is schematically shown in Fig. 1(a). The system consisted of a laser processing system, heating module and temperature measurement module. The laser processing system consisted of a pulsed fibre laser (waist diameter w0 of 31.5 μm, wavelength λ of 1064 nm, pulse repetition rate f of 20 kHz and pulse duration of 5–7 ns). The heating module controls the temperature of the workpiece, and the heat source used in the heating module is an electric furnace iron, which is installed below the workpiece. This method of heating is used in this study because it is easily controllable and provides uniform heat along the entire workpiece. The measurement temperature module is used to check the temperature of the workpiece to adjust the heating module. The temperature of the workpiece was measured by three K-type contact thermocouples, which are located at different sites on the workpiece surface to measure and ensure a uniform temperature along the entire target surface, as shown in Fig. 1(b). This system is used for the laser machining process under various 2
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Fig. 1. (a) Test setup of TALMP, (b) temperature extraction point, and (c) schematic description of laser scanning path for laser surface polishing.
3. Results and discussion
of the ablation spot. This suggests that the ablation threshold of WC-Co with thermally assisted laser processing can be reduced, which may be due to the high heat content around the irradiated area, thereby reducing the latent heat to melting and vapor and making the material easier to ablate. This can also account for the decline in ablation accuracy of TALMP with a high offset temperature. To further explore the details of the ablation spot, the ablation characteristics of the resolidification regions of Zone A, Zone B, Zone C and Zone D in Fig. 2 at higher magnification are shown in Fig. 3. It can be observed that with E0 = 0.2 mJ, a significant bulge is distributed on the edge of the ablated spot in Fig. 3(a), and there are some microcracks at the centre of the ablated spot in Fig. 3(b). With E0 = 0.4 mJ, in addition to the bulge structure, many micron-sized finger-shaped microstructures are present around micro-spots, as shown in Fig. 3(c). Moreover, it should be noted that a microporous structure formed on the ablation spot. It is reported that when a laser pulse with sufficiently high intensity irradiates a surface, melting, vaporization, phase explosion and particle ejection occur in the irradiated zone, thus realizing the ablation of the target material [27]. The main ablation mechanisms may depend on the extent of the laser-matter interaction and the optical and physical properties of the target material [28]. Thus, based on the characteristics of ablation spots, it is reasonable to infer that ablation mechanisms with different temperatures T may be different. 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
3.1. Laser surface ablation The SEM images of the prepared micro-spots on the WC-Co surface with mono-pulse energy E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T are shown in Fig. 2. It can be observed that under the same mono-pulse energy, the area of ablation spots increases with increasing temperature T, which implies that, in addition to the higher ablation rate, the ablation accuracy of TALMP with a high offset temperature can be reduced to a certain extent compared to that without an offset temperature. Under the same temperature T, the ablation spot with E0 = 0.4 mJ is much larger than that with E0 = 0.2 mJ. Moreover, the recast debris around the edge of the ablation spot with increasing temperature T becomes more severe. Under the same mono-pulse energy, a transition of the ablated surface quality from relatively smooth [Fig. 2(a), (b), (c) and (e)] to bumpy [Fig. 2(d), (f), (g) and (h)] is observed with increasing temperature. Furthermore, it can be seen from Fig. 2(a) that there exists a circular ring-shaped ablated area with a diameter of approximately 20 μm (< w0) on the WC-Co surface. The whole ablated area shows few melting traces, which can imply that the pulse-energy-formed surface circular ring is relatively close to the ablation threshold of the material. Under the same pulse energy with thermally assisted temperature T = 600 °C, Fig. 2(d) shows that the diameter of the ablated area is approximately 40 μm, and there is severe recast debris around the edge
Fig. 2. SEM images of the prepared micro-spots on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T. 3
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Fig. 3. SEM morphology of Zone A, Zone B, Zone C and Zone D in Fig. 2 at higher magnification (mono-pulse energy E0 = 0.4 mJ).
successive pulses. The original microstructure in the non-overlapping area still exists and forms the microripples. This is the formation mechanism of the morphology shown in Fig. 4. Thus, the overlapping area decreases with increasing scanning speed. Therefore, the distance between the microripples can be designed and realized by changing the scanning speed. Moreover, the resolidification regions of the micro-grooves (Zones E and F in Fig. 4) show different characteristics. To further study the resolidification regions, detailed observations at higher magnification are shown in Fig. 5. Fig. 5(a) shows a small, droplet-like edge with a diameter on the order of 1 μm. Based on the shape and size of the edge of the ripple, we postulate that these types of edges were generated because melting materials in the irradiated region were pushed outside by the shockwave and resolidified into droplet-like edges due to surface tension while the ejected materials were still in the liquid phase. In contrast, these edges presented in Fig. 5(b) contain droplet-like edges but also fractured microstructures. In addition to melting and resolidification, the shockwave generated after laser irradiation can cause mechanical failure, which means that the stress caused by the shockwave exceeds the tensile strength of the material. This may be because the laser-matter interaction with thermally assisted laser processing is stronger. In addition, because all the tests were performed in the air, the treated surfaces are oxidized during the laser ablation. However, the quantitative measurement of O element by EDX is not accurate. So instead, the concentrations of Co element around the irradiated zone for different scanning speeds are summarized and shown in Fig. 6. It is obvious that under the same scanning speed, a higher offset temperature induces a lower concentration of Co element, which means that the occurrence of oxidation reaction gets stronger with a higher offset temperature. With the increase in temperature, the concentration of Co element for all scanning speeds has a certain decrease, which means the increase of the concentration of O element. It was reported that a higher temperature field and longer duration
between the ambient and dense plumes [18]. Melting materials in the irradiated zone are pushed outside by the shock wave, resulting in more materials being redeposited in the periphery, which is the formation mechanism of the bulge structure on the ablation spot in Fig. 2(a). Moreover, if the energy transfer in the irradiated zone is extremely intense, the melting layer may become thermodynamically unstable, resulting in the formation of vapor bubbles inside the liquid, which combine homogeneously, leading to phase explosion [27]. After the explosion of vapor bubbles inside the melting layer, a microporous structure was left. In the meantime, melting materials were ejected out severely, resolidified around the ablation spots and generated the fingering phenomenon, as shown in Fig. 2(e), (f), (g) and (h). This characteristic suggests that the laser-matter interaction with thermally assisted laser processing is more severe. Moreover, the high temperature gradient results in the dilatation of materials without thermal assistance, hence modifying the surface tension, and after the termination of laser ablation, the liquid interface is rapidly solidified at a high cooling rate, which is certain to induce surface tensile stresses. Consequently, microcracks are generated on the ablation spot, as shown in Fig. 3(b). Fig. 4 shows SEM images of the micro-grooves on the WC-Co surface with mono-pulse energy E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T. It is obvious that under the same mono-pulse energy, the width of the micro-groove increases with increasing temperature T. Under the same temperature T, the width of the micro-groove with E0 = 0.4 mJ is larger than that with E0 = 0.2 mJ, which suggests that there is a more severe laser-matter interaction with thermally assisted laser processing. In addition, these microstructures feature microripples. As mentioned above, when a pulse irradiates the target surface, a melting area is formed in the irradiated area, followed by a shockwave. The melting materials at the centre are pushed outside by the shockwave, resulting in more materials being redeposited in the periphery. With the irradiation of a train of pulses, the thermal melting and material deposition repeat. In the overlapping area, the microstructure formed by the former pulse can be erased and reformed again by the 4
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Fig. 4. SEM images of the prepared micro-grooves on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T (scanning speed v = 200mm/s).
of the high temperature field can accelerate the oxidation reaction in the irradiation zone, resulting in a higher concentration of O element [18]. The longer the laser irradiation time per unit area is, the greater the amount of O element deposited. Thus, in turn, the variations in temperature of the irradiation zones for the same laser irradiation time per unit area (constant pulse duration) can be inferred on the basis of the O element concentration. Thus, based on the results in Fig. 6, it can be inferred that with increasing offset temperature, a higher temperature field with longer duration of the high temperature field is left on ablation zones after laser beam moves away. This suggests that by introducing heat from an external source, this approach can increase the workpiece material's heat content, thereby reducing latent heat to melting and vapor and making the material easier to ablate. So less of the absorbed energy and residual heat deposited diffuse away from the irradiated zone during laser processing. Thus, the incoming laser energy can be utilized more effectively by ablating more material instead of diffusing it into the sample. Fig. 7 shows the overhead SEM micrographs of the micro-grooves fabricated on WC-Co with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T after scanning passes of 50. It can be observed that under the same mono-pulse energy, with increasing number of scanning passes, the width and depth of the micro-grooves increase. Under the same temperature T, the depth and width of the micro-grooves with E0 = 0.4 mJ are much larger than those with E0 = 0.2 mJ. Moreover,
the recast debris around the edge of the micro-grooves become more severe with increasing temperature T. For a more quantitative comparison of ablation efficiency under different temperatures, the dependence of the ablation width and depth of the micro-grooves and the ablation rate per pulse on temperature are presented with E0 = 0.2 mJ and E0 = 0.4 mJ in Fig. 8, respectively. It can be observed that the ablation width and depth and the ablation rate per pulse under E0 = 0.2 mJ and E0 = 0.4 mJ are much higher than the results without thermal assistance. For both mono-pulse energies, as the temperature increases, the ablation width and depth and the ablation rate per pulse increase. Moreover, with E0 = 0.2 mJ, a sudden increase in ablation width and depth and ablation rate per pulse is observed at temperature T = 400 °C, which may be due to the occurrence of phase explosion, resulting in severe particle ejection. This characteristic coincides with the occurrence of finger-shaped microstructures in Fig. 2(d). For E0 = 0.4 mJ, there is no obvious sudden increase, which is probably attributed to the fact that 0.4 mJ is close to the threshold of phase explosion even without thermal assistance, which coincides with the trend shown in Fig. 2(e) and (f). Moreover, when the temperature T is 600 °C, the maximal relative ablation rate obtained is approximately 7.94 and 4.65 times higher than the result without thermal assistance with E0 = 0.2 mJ and E0 = 0.4 mJ, respectively. Based on the results and analysis above, it can be summarized that laser surface ablation with a high offset temperature can achieve
Fig. 5. SEM images of the prepared micro-grooves on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T (scanning speed v = 200mm/s). 5
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microns. While the ablation accuracy and quality can be reduced to a certain extent for the micro-texture fabricated by the TALMP directly. Moreover, based on the scanning pass n, it can be inferred that the processing efficiency of the two-step approach is approximately 2.3 times faster than that of the traditional method without thermal assistance.
3.2. Laser surface polishing Fig. 10 shows sample surfaces polished with various laser energy densities E and different temperatures. It can be observed that with different temperatures, pristine rough surfaces can be well polished, and a sub-micron resolution can be achieved for all temperatures. Moreover, laser melting tracks with peaks and valleys can be observed on polished surfaces in Fig. 10(a), (b) and (c). With increasing temperature up to T = 450 °C, the polished surface in Fig. 10(d) is smooth and flat. Moreover, it is clear that at a higher temperature, a lower energy density is required for a similar surface roughness design. This successfully suggests that to obtain a similar design, laser surface polishing with higher offset temperatures can reduce the laser energy compared with that with a low temperature, which also means that with the same laser output power, TALMP with higher offset temperatures can achieve a higher process efficiency by using a higher scanning speed and is more suitable for realizing quick and large-lot production. Furthermore, the cross-section profiles of the polished surfaces with different temperature are shown in Fig. 11. It is shown that under all the temperatures, there lies a resolidified layer under the polished surface. And with the increase of the offset temperature, the thickness of the resolidified layer gets larger. To further observe the resolidified layer, the SEM morphology of the edge of the micro-groove sidewall at higher magnification is presented in Fig. 12. As can easily be seen from Fig. 12(a), for temperature T = 25 °C, there are columnar grains of approximately 7.5 μm at the outermost edge and then a thin layer of fine equiaxed grains and substrate materials next to them. For temperature T = 450 °C, a thicker layer of regular grain structure (approximately 21.8 μm) is observed, followed by an obvious fine equiaxed grain layer. It is reported that the thermal gradient drives heat away from the molten layer and initiates a fast-moving resolidification front, which advances from the substrate towards the top surface. A slower resolidification front will cause a longer-lasting molten phase and
Fig. 6. EDX analyses of the ablated spot for various scanning speeds and different temperatures.
material removal with high efficiency, but its ablation accuracy and quality can be reduced to a certain extent. Therefore, using TALMP with a high offset temperature to fabricate the desired micro-textures directly may be an efficient method but not the best method. A strategy to achieve the desired surface ablation with high efficiency and highquality surfaces is proposed. The sample to be machined first is irradiated with thermal assistance to realize the mean material removal with enough machining allowance remaining (Step 1). Then, the irradiation without thermal assistance removes the machining allowance and realizes a high-quality texture (Step 2). Fig. 9 shows micro-textures and corresponding cross section profile prepared on the WC-Co substrate fabricated by the TALMP approach with T = 400 °C, the traditional method without thermal assistance and a two-step approach. It is obvious that for both traditional method and the two-step approach, the micro-textures are clear, and the resolution can be achieved to several
Fig. 7. SEM images of the prepared micro-grooves on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T (scanning speed v = 500 mm/s and scanning pass n = 50). 6
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Fig. 8. Dependence of the ablation depth and width and the ablation rate per pulse on temperature with (a) E0 = 0.2 mJ, and (b) E0 = 0.4 mJ.
Fig. 9. Micro-textures and corresponding stereoscopic profile and cross section profile on the WC-Co substrate fabricated via (a) the TALMP approach with T = 400 °C, (b) the traditional method with T = 25 °C, (c1) the first step of the two step approach with T = 400 °C and (c2) the second step of the two step approach with T = 25 °C (scanning speed v = 500 mm/s and mono-pulse energy E0 = 0.2 mJ).
T = 450 °C, melting is typically followed by growth of the regular crystalline structures, as shown in Fig. 12(b). Moreover, the thickness of the resolidified layer and surface roughness of Ti-6Al-4 V after laser polishing are summarized in Fig. 13. It can be seen that the hardness of all polished surfaces is higher than that of the initial substrates (396 ± 17 HV). The enhancement of
epitaxial regrowth with a crystalline resolidified layer [29]. The resolidification-front speed determines the chemistry, crystallinity, and morphology of the resolidified layer, which is affected by the material's heat content and cooling process around the irradiated zone. A higher heat content and slower cooling process can result in a longer melt duration and a slower resolidification front. Thus, with temperature 7
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Fig. 10. Surface morphology of polished samples for various energy densities and different temperatures. (a) T = 25 °C, (b) T = 150 °C, (c) T = 300 °C, and (d) T = 450 °C.
Fig. 11. Cross-section SEM micrographs of the polished surfaces with (a) 25 °C, (b) 150 °C, (c) 300 °C and (d) 450 °C.
Fig. 12. SEM morphology of Zone G and Zone H in Fig. 11 at higher magnification.
increasing temperature. These results suggest that there may be certain controllable changes in grain structure and microstructure for the polished surfaces that can be achieved by controlling the resolidification process of the resolidified layer. Thus, it is possible to change the physical properties of the polished surfaces, such as hardness, toughness and even surface wettability, by adjusting the laser processing parameters or scanning path parameters or by controlling the offset temperature. During the laser polishing process, a molten layer remains after the laser beam moves forward. The thickness of the molten layer depends on the laser parameters, scanning path parameters, physical properties of the target material, and experimental conditions, including different temperature fields. Due to the surface tension, the molten peak material flows into valleys. During this reallocation process of protruding materials, there is hardly any material removal. As the laser beam then moves away, the reallocated molten layer resolidifies, and a polished surface is obtained. Based on the above analysis, it is suggested that with longer duration of the high temperature field and a thicker molten layer after the arrival of the pulse, more significant fluid can flow with enough time during thermally assisted laser surface polishing
Fig. 13. Resolidified layer thickness and micro-hardness of additive manufactured Ti-6Al-4V of polished surfaces.
surface hardness may be due to the formation of the resolidified layer on surfaces. The thickness of the resolidified layer increases with 8
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Fig. 14. Schematic illustration of the laser polishing process of TALMP with a (a) high offset temperature and (b) low offset temperature.
processing. The corresponding polishing processing for different offset temperatures is schematically illustrated in Fig. 14.
processing, Light-Sci. Appl. 3 (4) (2014) 149. [3] A.I. Aguilar-Morales, S. Alamri, A.F. Lasagni, Micro-fabrication of high aspect ratio periodic structures on stainless steel by picosecond direct laser interference patterning, J. Mater. Process. Technol. 252 (2018) 313–321. [4] A.K. Dubey, V. Yadava, Laser beam machining—a review, Int J Mach Tool Manu 48 (2008) 609–628. [5] T.M. Shao, M. Hua, H.Y. Tam, E.H.M. Cheung, An approach to modelling of laser polishing of metals, Surf. Coat. Technol. 197 (1) (2005) 77–84. [6] L. Liang, J.D. Yuan, X.Q. Li, F. Yang, L.L. Jiang, Wear behavior of the micro-grooved texture on WC-Ni3Al cermet prepared by laser surface texturing, Int. J. Refract. Met. Hard Mater. 72 (2018) 211–222. [7] C.P. Ma, Y.C. Guan, W. Zhou, Laser polishing of additive manufactured Ti alloys, Opt. Lasers Eng. 93 (2017) 171–177. [8] K.C. Yung, W.J. Wang, T.Y. Xiao, H.S. Choy, X.Y. Mo, S.S. Zhang, Z.X. Cai, Laser polishing of additive manufactured CoCr components for controlling their wettability characteristics, Surf. Coat. Technol. 351 (2018) 89–98. [9] C. Pan, K. Chen, B. Liu, L. Ren, J.R. Wang, Q.K. Hu, L. Liang, J.H. Zhou, L.L. Jiang, Fabrication of micro-texture channel on glass by laser-induced plasma-assisted ablation and chemical corrosion for microfluidic devices, J. Mater. Process. Technol. 240 (2007) 314–323. [10] S. Xu, L. Ren, B. Liu, J.R. Wang, B. Tang, W. Zhou, L.L. Jiang, Single-step selective metallization on insulating substrates by laser-induced molten transfer, Appl. Surf. Sci. 454 (2018) 16–22. [11] Kan, and Chi-wai, CO2 laser treatment as a clean process for treating denim fabric, J. Clean. Prod. 66 (2014) 624–631. [12] P. Liu, L. Deng, J. Duan, B.Y. Wu, X.Y. Zeng, S.G. Ying, X.Z. Wang, A study on laser multi-focus separation technology of thick KDP crystal, Int J Mach Tool Manu 118 (2017) 11926–11936. [13] A. Žemaitis, M. Gaidys, M. Brikas, P. Gečys, G. Račiukaitis, Advanced laser scanning for highly- efficient ablation and ultrafast surface structuring: experiment and model, Sci. Rep. 8 (17376–17389) (2018). [14] J.D. Yuan, L. Liang, G.Z. Lin, X. Li, Reutilization of a reflected laser beam as an effective approach for machining metallic materials with low laser absorptivity, Opt. Express 27 (2019) 12048–12060. [15] B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, M.D. Perry, Laser-induced damage in dielectrics with nanosecond to subpicosecond pulses, Phys. Rev. Lett. 74 (1995) 2248–2251. [16] J.D. Yuan, L. Liang, L.L. Jiang, X. Liu, Influence of the shielding effect on the formation of a micro-texture on the cermet with nanosecond pulsed laser ablation, Opt. Lett. 43 (7) (2018) 1451–1455. [17] H. Wang, Y. Kawahito, R. Yoshida, Y. Nakashima, K. Shiokawa, Development of a high-power blue laser (445 nm) for material processing, Opt. Lett. 42 (2017) 2251–2254. [18] R.L. Harzic, D. Breitling, M. Weikert, S. Sommer, C. Föhl, S. Valette, C. Donnet, E. Audouard, F. Dausinger, Pulse width and energy influence on laser micromachining of metals in a range of 100 fs to 5 ps, Appl. Surf. Sci. 249 (2005) 322–331. [19] P. Mannion, J. Magee, E. Coyne, G. O'Connor, T. Glynn, The effect of damage accumulation behavior on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air, Appl. Surf. Sci. 233 (2004) 275–287. [20] C. Kerse, H. Kalaycıoğlu, P. Elahi, B. Çetin, D.K. Kesim, Ö. Akçaalan, S. Yavaş, M.D. Aşık, B. Öktem, H. Hoogland, R. Holzwarth, F.Ö. Ilday, Ablation-cooled material removal with ultrafast bursts of pulses, Nature 537 (2016) 84–88. [21] F. Bauer, A. Michalowski, T. Kiedrowski, S. Nolte, Heat accumulation in ultra-short pulsed scanning laser ablation of metals, Opt. Express 23 (2015) 1035–1043. [22] S.H. Tavassoli, M. Khalaji, Laser ablation of preheated copper samples, J. Appl. Phys. 103 (8) (2008) 083118. [23] Effect of preheating temperature on microstructure of Fe based alloy coating by laser direct metal deposition [24] L. Jiang, C. Pan, K. Chen, J.T. Ling, W. Zhou, J.H. Zhou, L. Liang, Fiber laser carving under ice layer without laser energy attenuation, J. Mater. Process. Technol. 216 (2015) 278–286. [25] S.M. Eaton, H. Zhang, P.R. Herman, F. Yoshino, A.Y. Arai, Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate, Opt.
4. Conclusions and outlook Thermally assisted laser materials processing technology (TALMP) has been explored for a higher processing efficiency in this study. The processing mechanism was expounded, and the successful applications of surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V exhibited the excellent versatility and superiority of the TALMP approach, which in turn provides a more comprehensive understanding of the mechanisms of TALMP. For surface ablation, comparing to the results without thermal assistance, although there is a little amount of thermal degradation and a minor decrease in precision, the ablation efficiency of WC-Co with a temperature of 600 °C can be up to 7.94 times higher than that with a temperature of 25 °C. Moreover, a two-step approach was proposed to eliminate the minor decrease in precision efficiently. For surface polishing, with increasing temperature, the molten layer becomes thicker, which is beneficial to the formation of a smoother surface. The hardness of the polished surfaces increases, which suggests that the properties of the polished surface can be modified. This study demonstrated significant improvements in the processing efficiency, and revealed there were certain direct correlation between the offset temperature and the physical properties and functional performance of these structures. But more studies are needed to provide insight into real-time laser-induced enhancements in the physical properties and the functional performance of materials. In the meantime, for a better understanding of the dynamic of materials melting, vapouring and crystallization, more attention is worthwhile to be paid to the real-time temperature evolution around the irradiated zone. Funding National Natural Science Foundation of China (NSFC) (51575193 and 51474108); Guangzhou Municipal Science and Technology Project (201904010239); Guangzhou Science and Technology Program key projects (2018B030311051). Acknowledgments The authors thank the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials for the use of their equipment. References [1] A.N. Samant, N.B. Dahotre, Laser machining of structural ceramics—a review, J. Eur. Ceram. Soc. 29 (6) (2009) 969–993. [2] K. Sugioka, Y. Cheng, Ultrafast lasers—reliable tools for advanced materials
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[28] J.D. Yuan, L. Liang, G.Z. Lin, X.Q. Li, J. Ming, Experimental study on the lasermatter-plume interaction and its effects on ablation characteristics during nanosecond pulsed laser scanning ablation process, Opt. Express 27 (16) (2019) 23204–23217. [29] J.A. Kittl, P.G. Sanders, M.J. Aziz, D.P. Brunco, M.O. Thompson, Complete experimental test of kinetic models for rapid alloy solidification, Acta Mater. 48 (2000) 4797–4811.
Express 13 (2005) 4708–4716. [26] R. Weber, T. Graf, P. Berger, V. Onuseit, M. Wiedenmann, C. Freitag, A. Feuer, Heat accumulation during pulsed laser materials processing: erratum, Opt. Express 22 (2014) 28232–28242. [27] D. Marla, U.V. Bhandarkar, S.S. Joshi, A model of laser ablation with temperaturedependent material properties, vaporization, phase explosion and plasma shielding, Appl. Phys. A Mater. Sci. Process. 116 (1) (2014) 273–285.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Study on processing characteristics and mechanisms of thermally assisted laser materials processing
T
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Jiandong Yuan, Liang Liang , Guozhi Lin National Engineering Research Center of Near-net-shape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser materials processing Thermally assisted Laser ablation Laser polishing
Nanosecond pulsed lasers have been widely applied to interact with and characterize many different materials, and industrial users are exploring more avenues for various application processes. For the purpose of achieving broader application, the current challenge is to achieve a high processing efficiency for laser materials processing. In this study, thermally assisted laser materials processing (TALMP), which introduces heat from an external source to increase the workpiece material's temperature and make it easier to carry out ablation or modification, was developed and investigated. Moreover, to investigate the materials processing characteristics and mechanisms of TALMP, practical applications using TALMP, including surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V, were carried out. The results showed that TALMP can be successfully applied to various materials by different laser materials processing applications. Due to the workpiece material's high temperature and heat content, the coupling between laser and target materials can be enhanced; thus, physical ablation and modification for laser surface ablation can be achieved effectively. However, with a high offset temperature, a little decrease of ablation accuracy and quality could occur, and a two-step approach is proposed to eliminate this unwanted effect. Moreover, this strategy is proven to be able to achieve the desired surface ablation with high efficiency and high-quality surfaces. For thermally assisted surface polishing, due to higher temperature field with long duration, a smoother and flatter polished surface can be obtained more easily. The experimental results proved the superiority and versatility of TALMP.
1. Introduction Laser materials processing is one of the most common laser applications among the communication, cosmetic medical, military, optical storage, lithography, instrumentation, display, and printing industries [1–3]. Over the past few decades, pulse laser materials processing has been used in a broad range of applications, from material removal to material modification, such as surface ablation [4] and surface polishing [5]. Using pulsed laser surface ablation, Liang et al. prepared desired micro-grooves on WC-Co, and clarified that the occurrence of the WC grain growth in the vicinity of the micro-groove affected the wear resistance of the micro-groove [6]. Ma et al. studied the laser polishing on additive manufactured Ti alloys, and clarified surface hardness and wear resistance were enhanced [7]. By applying laser surface polishing on additive manufactured CoCr components, Yung et al. achieved a high surface quality (Sa) < 1 μm with appropriate choices of the fluence and a certain laser defocusing distance, and found that the surface roughness and contact angles could be improved [8]. Furthermore, various innovative processing methods have explored
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new possibilities for laser materials processing, including laser-induced plasma-assisted ablation (LIPAA) to fabricate microfluidic devices [9], laser-induced molten transfer (LIMT) to fabricate metallic patterns on various target substrates [10], CO2 laser treatment as a clean process for treating denim fabric [11], laser multi-focus separation technology (LMFS) to separate thick KDP crystals [12], and so on. Although there has been a boom in the number of attempts and applications using pulsed lasers in recent years, the main limitation is the low ablation rate (10−6~10−4mm3/pulse) [13,14]. Therefore, the current challenge is to achieve a speedup of the materials processing process. Increasing the on-target laser intensity may be a direct and possible approach, while the use of high-intensity lasers leads to unwanted effects (e.g., saturation, shielding and thermal damage), which further limit the process efficiency and affect the ablation process and ablation quality [15,16]. In the meantime, many other approaches have been applied to realize a strong coupling between the laser and target material, such as optimizing the process parameters [13], improving the absorption coefficient by employing shorter wavelength or duration lasers and changing the machining conditions [17,18], reutilizing a
Corresponding author. E-mail address: [email protected] (L. Liang).
https://doi.org/10.1016/j.surfcoat.2019.124946 Received 20 July 2019; Received in revised form 21 August 2019; Accepted 30 August 2019 Available online 31 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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designed offset temperatures. In this study, two different laser materials processing applications (surface ablation on WC-Co and surface polishing on an additive manufactured Ti-6Al-4 V) were selected and tested under each specific machining conditions and parameters. The purpose of selecting significantly different laser processing technologies on different materials and entry conditions was to ensure that the versatility and superiority of the TALMP approach are identified over a number of conditions that may be encountered in the laboratory and in industry.
reflected laser beam [14], and pre-setting the damage on the target surface [19]. Moreover, using ultrafast successions of pulses such that the delay time between two pulses is less than the thermal relaxation time is proven to be an effective way to increase the efficiency of the removal process by an order of magnitude over previously used laser parameters [20]. Recent investigations report that pre-heating workpieces during laser processing should be an alternative approach to enhance the ablation rate [21]. Tavassoli et al. investigated the influence of initial sample temperature on plasma temperatures and ablation depth of copper samples [22]. The results showed that an increase in the initial sample temperature results in an increase in the spectral emission of plasma up to 90% which is attributed to both the plasma temperature and optical absorption. Shi et al. investigated the effect of preheating temperature on microstructure of Fe based alloy coating by laser direct metal deposition, and concluded that preheating treatment is an effective approach to eliminate the cracks in the high thickness Fe60 alloy coating [23]. This approach of increasing processing efficiency by preheating workpieces, called thermally assisted laser materials processing (TALMP) in this study, introduces heat from an external source to increase the workpiece material's temperature and heat content, thereby increasing optical absorption and reducing latent heat to melting and vapor and making the material easer to ablate. This approach is seemingly contrary to common sense. Traditionally, efforts by researchers and industrial engineers have focused on decreasing the workpiece material's temperature by better heat management in the irradiated zone to circumvent heat accumulation around the irradiated zone using various technologies, such as laser ablation under a water layer or ice layer [24], decreasing the laser repetition rate [25,26], and so on. Thus, although there may be a higher processing efficiency for TALMP, it is reasonable to consider that the surface quality obtained by this approach may suffer from thermal degradation. However, until now, there is very little literature available that investigates the processing characteristics of this approach, and explorations that specifically use TALMP as a strategy for realizing a higher processing efficiency and clarify the mechanisms of TALMP appear to be completely lacking. In this study, TALMP was explored to obtain a higher processing efficiency. Moreover, to investigate its materials processing characteristics and obtain a better understand the mechanisms, surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V alloy were carried out using this approach. The results provide detailed information about the processing characteristics of TALMP on the irradiated area. The processing mechanisms of TALMP with different temperatures were analysed. Moreover, the experimental results also validate the versatility and superiority of TALMP.
2.2. Laser surface ablation on WC-Co WC-Co, as a typical structure and tribological material, is widely used in cutting tools and structural components. To further improve its tribological performance, textures (micro/nano-dimples or micro/ nano-grooves) fabricated by surface ablation have been widely applied to WC-Co [6]. Thus, WC-Co was used for the laser surface ablation test. Moreover, based on preliminary tests of surface ablation on WC-Co, four separate temperatures were selected for this test, including 25 °C (room temperature), 200 °C, 400 °C and 600 °C. The samples were mechanically polished to mirror finish and cleaned in an ultrasonic bath with acetone and ethanol for 30 min before and after laser processing. The microstructure morphology after laser ablation was analysed by scanning electron microscopy and energy dispersive spectrometry. The ablation rates were calculated from the cross-sections of the prepared textures, which were analysed by white-light interferometry. There are certain inevitable measuring errors. Because the ablation rates of the TALMP and traditional method were only roughly compared in this study, the measuring errors can be safely ignored. All the tests were performed in the air.
2.3. Laser surface polishing on additive manufactured Ti-6Al-4V For laser surface polishing, a schematic description of the laser scanning path is shown in Fig. 1(c). There are two types of overlap on the scanning path: laser scan line overlap L and laser spot overlap H, which depend on the laser spot diameter, pulse repetition rate, scanning speed, and line-scan spacing. With larger L or H, more laser pulses per unit area will irradiate on the target surface. However, the extremely large L or H can result in an excessive incoming laser energy per unit area, which will trigger severe plasma shielding of the incoming laser energy. However, a low L or H (or even no overlap) will only slightly modify the target surface. Thus, multiple scanning passes N and a long time may be needed to obtain the desired modified surface. In this study, the laser surface polishing test was applied on the pristine surface of additive manufactured Ti-6Al-4V. When the laser was turned on, the specimen was placed under N2 gas protection. Based on our preliminary tests of surface polishing on additive manufactured Ti-6Al-4V, four separate temperatures were selected for the thermally assisted surface polishing test: 25 °C (room temperature), 150 °C, 300 °C and 450 °C. In this temperature range, microstructural changes occur in additive manufactured Ti-6Al-4V. Moreover, the focal offset was fixed at the off-focus distance, and the laser beam diameter D was determined to be 90 ± 1 μm. Furthermore, in this surface polishing test, the laser scanning path parameters were L = 30 μm and N = 2. The scanning speed v was set to 300–600mm/s; thus, H = 30–60 μm. Surfaces after laser polishing as well as the cross-section were analysed by confocal microscopy, scanning electron microscopy, and energy dispersive spectrometry. Moreover, the Vickers microhardness was measured using a Vickers microhardness tester with a load of 100 N and a dwelling time of 15 s.
2. Experimental setup, materials and methods 2.1. Experimental setup The schematic of the TALMP system is schematically shown in Fig. 1(a). The system consisted of a laser processing system, heating module and temperature measurement module. The laser processing system consisted of a pulsed fibre laser (waist diameter w0 of 31.5 μm, wavelength λ of 1064 nm, pulse repetition rate f of 20 kHz and pulse duration of 5–7 ns). The heating module controls the temperature of the workpiece, and the heat source used in the heating module is an electric furnace iron, which is installed below the workpiece. This method of heating is used in this study because it is easily controllable and provides uniform heat along the entire workpiece. The measurement temperature module is used to check the temperature of the workpiece to adjust the heating module. The temperature of the workpiece was measured by three K-type contact thermocouples, which are located at different sites on the workpiece surface to measure and ensure a uniform temperature along the entire target surface, as shown in Fig. 1(b). This system is used for the laser machining process under various 2
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Fig. 1. (a) Test setup of TALMP, (b) temperature extraction point, and (c) schematic description of laser scanning path for laser surface polishing.
3. Results and discussion
of the ablation spot. This suggests that the ablation threshold of WC-Co with thermally assisted laser processing can be reduced, which may be due to the high heat content around the irradiated area, thereby reducing the latent heat to melting and vapor and making the material easier to ablate. This can also account for the decline in ablation accuracy of TALMP with a high offset temperature. To further explore the details of the ablation spot, the ablation characteristics of the resolidification regions of Zone A, Zone B, Zone C and Zone D in Fig. 2 at higher magnification are shown in Fig. 3. It can be observed that with E0 = 0.2 mJ, a significant bulge is distributed on the edge of the ablated spot in Fig. 3(a), and there are some microcracks at the centre of the ablated spot in Fig. 3(b). With E0 = 0.4 mJ, in addition to the bulge structure, many micron-sized finger-shaped microstructures are present around micro-spots, as shown in Fig. 3(c). Moreover, it should be noted that a microporous structure formed on the ablation spot. It is reported that when a laser pulse with sufficiently high intensity irradiates a surface, melting, vaporization, phase explosion and particle ejection occur in the irradiated zone, thus realizing the ablation of the target material [27]. The main ablation mechanisms may depend on the extent of the laser-matter interaction and the optical and physical properties of the target material [28]. Thus, based on the characteristics of ablation spots, it is reasonable to infer that ablation mechanisms with different temperatures T may be different. 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
3.1. Laser surface ablation The SEM images of the prepared micro-spots on the WC-Co surface with mono-pulse energy E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T are shown in Fig. 2. It can be observed that under the same mono-pulse energy, the area of ablation spots increases with increasing temperature T, which implies that, in addition to the higher ablation rate, the ablation accuracy of TALMP with a high offset temperature can be reduced to a certain extent compared to that without an offset temperature. Under the same temperature T, the ablation spot with E0 = 0.4 mJ is much larger than that with E0 = 0.2 mJ. Moreover, the recast debris around the edge of the ablation spot with increasing temperature T becomes more severe. Under the same mono-pulse energy, a transition of the ablated surface quality from relatively smooth [Fig. 2(a), (b), (c) and (e)] to bumpy [Fig. 2(d), (f), (g) and (h)] is observed with increasing temperature. Furthermore, it can be seen from Fig. 2(a) that there exists a circular ring-shaped ablated area with a diameter of approximately 20 μm (< w0) on the WC-Co surface. The whole ablated area shows few melting traces, which can imply that the pulse-energy-formed surface circular ring is relatively close to the ablation threshold of the material. Under the same pulse energy with thermally assisted temperature T = 600 °C, Fig. 2(d) shows that the diameter of the ablated area is approximately 40 μm, and there is severe recast debris around the edge
Fig. 2. SEM images of the prepared micro-spots on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T. 3
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Fig. 3. SEM morphology of Zone A, Zone B, Zone C and Zone D in Fig. 2 at higher magnification (mono-pulse energy E0 = 0.4 mJ).
successive pulses. The original microstructure in the non-overlapping area still exists and forms the microripples. This is the formation mechanism of the morphology shown in Fig. 4. Thus, the overlapping area decreases with increasing scanning speed. Therefore, the distance between the microripples can be designed and realized by changing the scanning speed. Moreover, the resolidification regions of the micro-grooves (Zones E and F in Fig. 4) show different characteristics. To further study the resolidification regions, detailed observations at higher magnification are shown in Fig. 5. Fig. 5(a) shows a small, droplet-like edge with a diameter on the order of 1 μm. Based on the shape and size of the edge of the ripple, we postulate that these types of edges were generated because melting materials in the irradiated region were pushed outside by the shockwave and resolidified into droplet-like edges due to surface tension while the ejected materials were still in the liquid phase. In contrast, these edges presented in Fig. 5(b) contain droplet-like edges but also fractured microstructures. In addition to melting and resolidification, the shockwave generated after laser irradiation can cause mechanical failure, which means that the stress caused by the shockwave exceeds the tensile strength of the material. This may be because the laser-matter interaction with thermally assisted laser processing is stronger. In addition, because all the tests were performed in the air, the treated surfaces are oxidized during the laser ablation. However, the quantitative measurement of O element by EDX is not accurate. So instead, the concentrations of Co element around the irradiated zone for different scanning speeds are summarized and shown in Fig. 6. It is obvious that under the same scanning speed, a higher offset temperature induces a lower concentration of Co element, which means that the occurrence of oxidation reaction gets stronger with a higher offset temperature. With the increase in temperature, the concentration of Co element for all scanning speeds has a certain decrease, which means the increase of the concentration of O element. It was reported that a higher temperature field and longer duration
between the ambient and dense plumes [18]. Melting materials in the irradiated zone are pushed outside by the shock wave, resulting in more materials being redeposited in the periphery, which is the formation mechanism of the bulge structure on the ablation spot in Fig. 2(a). Moreover, if the energy transfer in the irradiated zone is extremely intense, the melting layer may become thermodynamically unstable, resulting in the formation of vapor bubbles inside the liquid, which combine homogeneously, leading to phase explosion [27]. After the explosion of vapor bubbles inside the melting layer, a microporous structure was left. In the meantime, melting materials were ejected out severely, resolidified around the ablation spots and generated the fingering phenomenon, as shown in Fig. 2(e), (f), (g) and (h). This characteristic suggests that the laser-matter interaction with thermally assisted laser processing is more severe. Moreover, the high temperature gradient results in the dilatation of materials without thermal assistance, hence modifying the surface tension, and after the termination of laser ablation, the liquid interface is rapidly solidified at a high cooling rate, which is certain to induce surface tensile stresses. Consequently, microcracks are generated on the ablation spot, as shown in Fig. 3(b). Fig. 4 shows SEM images of the micro-grooves on the WC-Co surface with mono-pulse energy E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T. It is obvious that under the same mono-pulse energy, the width of the micro-groove increases with increasing temperature T. Under the same temperature T, the width of the micro-groove with E0 = 0.4 mJ is larger than that with E0 = 0.2 mJ, which suggests that there is a more severe laser-matter interaction with thermally assisted laser processing. In addition, these microstructures feature microripples. As mentioned above, when a pulse irradiates the target surface, a melting area is formed in the irradiated area, followed by a shockwave. The melting materials at the centre are pushed outside by the shockwave, resulting in more materials being redeposited in the periphery. With the irradiation of a train of pulses, the thermal melting and material deposition repeat. In the overlapping area, the microstructure formed by the former pulse can be erased and reformed again by the 4
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Fig. 4. SEM images of the prepared micro-grooves on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T (scanning speed v = 200mm/s).
of the high temperature field can accelerate the oxidation reaction in the irradiation zone, resulting in a higher concentration of O element [18]. The longer the laser irradiation time per unit area is, the greater the amount of O element deposited. Thus, in turn, the variations in temperature of the irradiation zones for the same laser irradiation time per unit area (constant pulse duration) can be inferred on the basis of the O element concentration. Thus, based on the results in Fig. 6, it can be inferred that with increasing offset temperature, a higher temperature field with longer duration of the high temperature field is left on ablation zones after laser beam moves away. This suggests that by introducing heat from an external source, this approach can increase the workpiece material's heat content, thereby reducing latent heat to melting and vapor and making the material easier to ablate. So less of the absorbed energy and residual heat deposited diffuse away from the irradiated zone during laser processing. Thus, the incoming laser energy can be utilized more effectively by ablating more material instead of diffusing it into the sample. Fig. 7 shows the overhead SEM micrographs of the micro-grooves fabricated on WC-Co with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T after scanning passes of 50. It can be observed that under the same mono-pulse energy, with increasing number of scanning passes, the width and depth of the micro-grooves increase. Under the same temperature T, the depth and width of the micro-grooves with E0 = 0.4 mJ are much larger than those with E0 = 0.2 mJ. Moreover,
the recast debris around the edge of the micro-grooves become more severe with increasing temperature T. For a more quantitative comparison of ablation efficiency under different temperatures, the dependence of the ablation width and depth of the micro-grooves and the ablation rate per pulse on temperature are presented with E0 = 0.2 mJ and E0 = 0.4 mJ in Fig. 8, respectively. It can be observed that the ablation width and depth and the ablation rate per pulse under E0 = 0.2 mJ and E0 = 0.4 mJ are much higher than the results without thermal assistance. For both mono-pulse energies, as the temperature increases, the ablation width and depth and the ablation rate per pulse increase. Moreover, with E0 = 0.2 mJ, a sudden increase in ablation width and depth and ablation rate per pulse is observed at temperature T = 400 °C, which may be due to the occurrence of phase explosion, resulting in severe particle ejection. This characteristic coincides with the occurrence of finger-shaped microstructures in Fig. 2(d). For E0 = 0.4 mJ, there is no obvious sudden increase, which is probably attributed to the fact that 0.4 mJ is close to the threshold of phase explosion even without thermal assistance, which coincides with the trend shown in Fig. 2(e) and (f). Moreover, when the temperature T is 600 °C, the maximal relative ablation rate obtained is approximately 7.94 and 4.65 times higher than the result without thermal assistance with E0 = 0.2 mJ and E0 = 0.4 mJ, respectively. Based on the results and analysis above, it can be summarized that laser surface ablation with a high offset temperature can achieve
Fig. 5. SEM images of the prepared micro-grooves on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T (scanning speed v = 200mm/s). 5
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microns. While the ablation accuracy and quality can be reduced to a certain extent for the micro-texture fabricated by the TALMP directly. Moreover, based on the scanning pass n, it can be inferred that the processing efficiency of the two-step approach is approximately 2.3 times faster than that of the traditional method without thermal assistance.
3.2. Laser surface polishing Fig. 10 shows sample surfaces polished with various laser energy densities E and different temperatures. It can be observed that with different temperatures, pristine rough surfaces can be well polished, and a sub-micron resolution can be achieved for all temperatures. Moreover, laser melting tracks with peaks and valleys can be observed on polished surfaces in Fig. 10(a), (b) and (c). With increasing temperature up to T = 450 °C, the polished surface in Fig. 10(d) is smooth and flat. Moreover, it is clear that at a higher temperature, a lower energy density is required for a similar surface roughness design. This successfully suggests that to obtain a similar design, laser surface polishing with higher offset temperatures can reduce the laser energy compared with that with a low temperature, which also means that with the same laser output power, TALMP with higher offset temperatures can achieve a higher process efficiency by using a higher scanning speed and is more suitable for realizing quick and large-lot production. Furthermore, the cross-section profiles of the polished surfaces with different temperature are shown in Fig. 11. It is shown that under all the temperatures, there lies a resolidified layer under the polished surface. And with the increase of the offset temperature, the thickness of the resolidified layer gets larger. To further observe the resolidified layer, the SEM morphology of the edge of the micro-groove sidewall at higher magnification is presented in Fig. 12. As can easily be seen from Fig. 12(a), for temperature T = 25 °C, there are columnar grains of approximately 7.5 μm at the outermost edge and then a thin layer of fine equiaxed grains and substrate materials next to them. For temperature T = 450 °C, a thicker layer of regular grain structure (approximately 21.8 μm) is observed, followed by an obvious fine equiaxed grain layer. It is reported that the thermal gradient drives heat away from the molten layer and initiates a fast-moving resolidification front, which advances from the substrate towards the top surface. A slower resolidification front will cause a longer-lasting molten phase and
Fig. 6. EDX analyses of the ablated spot for various scanning speeds and different temperatures.
material removal with high efficiency, but its ablation accuracy and quality can be reduced to a certain extent. Therefore, using TALMP with a high offset temperature to fabricate the desired micro-textures directly may be an efficient method but not the best method. A strategy to achieve the desired surface ablation with high efficiency and highquality surfaces is proposed. The sample to be machined first is irradiated with thermal assistance to realize the mean material removal with enough machining allowance remaining (Step 1). Then, the irradiation without thermal assistance removes the machining allowance and realizes a high-quality texture (Step 2). Fig. 9 shows micro-textures and corresponding cross section profile prepared on the WC-Co substrate fabricated by the TALMP approach with T = 400 °C, the traditional method without thermal assistance and a two-step approach. It is obvious that for both traditional method and the two-step approach, the micro-textures are clear, and the resolution can be achieved to several
Fig. 7. SEM images of the prepared micro-grooves on a WC-Co surface with E0 = 0.2 mJ and E0 = 0.4 mJ under different temperatures T (scanning speed v = 500 mm/s and scanning pass n = 50). 6
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Fig. 8. Dependence of the ablation depth and width and the ablation rate per pulse on temperature with (a) E0 = 0.2 mJ, and (b) E0 = 0.4 mJ.
Fig. 9. Micro-textures and corresponding stereoscopic profile and cross section profile on the WC-Co substrate fabricated via (a) the TALMP approach with T = 400 °C, (b) the traditional method with T = 25 °C, (c1) the first step of the two step approach with T = 400 °C and (c2) the second step of the two step approach with T = 25 °C (scanning speed v = 500 mm/s and mono-pulse energy E0 = 0.2 mJ).
T = 450 °C, melting is typically followed by growth of the regular crystalline structures, as shown in Fig. 12(b). Moreover, the thickness of the resolidified layer and surface roughness of Ti-6Al-4 V after laser polishing are summarized in Fig. 13. It can be seen that the hardness of all polished surfaces is higher than that of the initial substrates (396 ± 17 HV). The enhancement of
epitaxial regrowth with a crystalline resolidified layer [29]. The resolidification-front speed determines the chemistry, crystallinity, and morphology of the resolidified layer, which is affected by the material's heat content and cooling process around the irradiated zone. A higher heat content and slower cooling process can result in a longer melt duration and a slower resolidification front. Thus, with temperature 7
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Fig. 10. Surface morphology of polished samples for various energy densities and different temperatures. (a) T = 25 °C, (b) T = 150 °C, (c) T = 300 °C, and (d) T = 450 °C.
Fig. 11. Cross-section SEM micrographs of the polished surfaces with (a) 25 °C, (b) 150 °C, (c) 300 °C and (d) 450 °C.
Fig. 12. SEM morphology of Zone G and Zone H in Fig. 11 at higher magnification.
increasing temperature. These results suggest that there may be certain controllable changes in grain structure and microstructure for the polished surfaces that can be achieved by controlling the resolidification process of the resolidified layer. Thus, it is possible to change the physical properties of the polished surfaces, such as hardness, toughness and even surface wettability, by adjusting the laser processing parameters or scanning path parameters or by controlling the offset temperature. During the laser polishing process, a molten layer remains after the laser beam moves forward. The thickness of the molten layer depends on the laser parameters, scanning path parameters, physical properties of the target material, and experimental conditions, including different temperature fields. Due to the surface tension, the molten peak material flows into valleys. During this reallocation process of protruding materials, there is hardly any material removal. As the laser beam then moves away, the reallocated molten layer resolidifies, and a polished surface is obtained. Based on the above analysis, it is suggested that with longer duration of the high temperature field and a thicker molten layer after the arrival of the pulse, more significant fluid can flow with enough time during thermally assisted laser surface polishing
Fig. 13. Resolidified layer thickness and micro-hardness of additive manufactured Ti-6Al-4V of polished surfaces.
surface hardness may be due to the formation of the resolidified layer on surfaces. The thickness of the resolidified layer increases with 8
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Fig. 14. Schematic illustration of the laser polishing process of TALMP with a (a) high offset temperature and (b) low offset temperature.
processing. The corresponding polishing processing for different offset temperatures is schematically illustrated in Fig. 14.
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4. Conclusions and outlook Thermally assisted laser materials processing technology (TALMP) has been explored for a higher processing efficiency in this study. The processing mechanism was expounded, and the successful applications of surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V exhibited the excellent versatility and superiority of the TALMP approach, which in turn provides a more comprehensive understanding of the mechanisms of TALMP. For surface ablation, comparing to the results without thermal assistance, although there is a little amount of thermal degradation and a minor decrease in precision, the ablation efficiency of WC-Co with a temperature of 600 °C can be up to 7.94 times higher than that with a temperature of 25 °C. Moreover, a two-step approach was proposed to eliminate the minor decrease in precision efficiently. For surface polishing, with increasing temperature, the molten layer becomes thicker, which is beneficial to the formation of a smoother surface. The hardness of the polished surfaces increases, which suggests that the properties of the polished surface can be modified. This study demonstrated significant improvements in the processing efficiency, and revealed there were certain direct correlation between the offset temperature and the physical properties and functional performance of these structures. But more studies are needed to provide insight into real-time laser-induced enhancements in the physical properties and the functional performance of materials. In the meantime, for a better understanding of the dynamic of materials melting, vapouring and crystallization, more attention is worthwhile to be paid to the real-time temperature evolution around the irradiated zone. Funding National Natural Science Foundation of China (NSFC) (51575193 and 51474108); Guangzhou Municipal Science and Technology Project (201904010239); Guangzhou Science and Technology Program key projects (2018B030311051). Acknowledgments The authors thank the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials for the use of their equipment. References [1] A.N. Samant, N.B. Dahotre, Laser machining of structural ceramics—a review, J. Eur. Ceram. Soc. 29 (6) (2009) 969–993. [2] K. Sugioka, Y. Cheng, Ultrafast lasers—reliable tools for advanced materials
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