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Abstract
A compact wedge prism scanner for laser surface cleaning is proposed, wherein the concept of system was studied based on geometric analysis. The final equations for the design express the transposition of the laser beam focal point and scanning radius. The results were verified through comparisons to both Zemax simulation and experiments. There was satisfactory agreement between the equations and Zemax simulation, but slight disagreement with the experiments. Additionally, two main factors of wedge prism scanner in commercial laser removal applications (circumferential overlap and spot overlap) was also discussed and the completely corrosion removal experiment indicated the potential use of our scanner.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, laser surface cleaning has attracted significant as a promising procedure for surface preparation. Unlike traditional cleaning methods that rely on wet chemical reaction solvents or mechanical forces involving air abrasive blasting, which struggle with environmental concerns, contaminants, and loss of durability, lasers offer many advantages. A remarkable feature of laser surface cleaning is that it is a non-mechanical contact and dry process that is highly efficient and environmentally friendly. Furthermore, based on the typical mechanisms of laser cleaning, spallation/evaporation/ablation/shockwave generations result in the removal of contaminants without damaging substrates. These characteristics have brought lasers closer to enterprise implementation in various fields. Significant efforts have been devoted to numerous substrate materials, including Cu-based alloys [1], aluminum alloys [2], bronze [3], Ti–3Al–2.5V alloy [4] stainless steel [5], cast steel [6], and polycarbonate [7] and diverse types of contaminants ranging from native surface oxides, organic or inorganic particles, aged preservation resins, fire damage, hydraulic oils, and compounds of metal powder and epoxy [8–13]. These works have not only successfully removed contaminants, but also created a barrier between the environment and substrate, thereby improving corrosion resistance [14–16].
Over the past three decades, the investigation of laser scanning systems has been considered a vital factor to meet market demands. Approximately 40 distinct designs have been introduced for optical and laser scanners [17].Various configurations have been discussed and developed, including monogonal (single mirror facet) and polygonal scanning system designs, and oscillatory (galvanometric and resonant scanning systems), holographic, refractive (with prisms or lenses), acousto-optical, electro-optical, and piezoelectric scanners. Each concept has its own advantages and disadvantages, and it is necessary to consider the target application to determine a suitable design.
Manufacturing a compact scanner head is necessary within the scope of the laser cleaning process. A portable handheld scanner must not only have a simple design, low weight, and few elements, but must also be as large as the scanning field and as fast as possible. To clarify which characteristics are the most important in industry, Fig. 1 presents a comparison of three most commonly used industrial designs (Galvano, Polygon, Prism) with the simplest concepts (one objective lens and one deflective element). In the case of one galvano mirror, the velocity of the scanning beam reaches zero at the start and end of the scanned lines, producing a higher power than intended (called the “dead zone”). An additional reason that galvano-based scanners are incomparable to polygon- and prism-based scanners is their scanning speed, which is recognized as one of the most important characteristics in commercial applications. Polygon-based methods have received much more interest because they can be considered as a hardware implementation of the raster scanning mode, allowing them to deliver much higher scanning speeds [18]. However, back-reflection [19] and power loss can occur if the element position and dimensions are not calculated carefully. In contrast, prisms meet all the requirements of a surface cleaning handheld scanner. Essentially, prisms enable the minimization of dimensions, leading to superior compactness compared to galvano mirrors or polygons. Furthermore, rotating prisms can realize 2D scans, whereas single-galvanometer-based and polygon scanners only provide 1D (unidimensional) scans. Additionally, rotating prisms can perform continuous scanning at high speeds.
Prism-based scanners have several advantages as potential candidates for surface cleaning. However, thus far, there have been few reports on prisms applied to laser surface cleaning processes. Most previous studies have focused on analyzing the complicated patterns generated by double-s [20–23], tri [24], tilting [25], or more compact constructions [26,27]. Based on the fundamental approach by Marshall that only determines approximation patterns [20], exact scan patterns were derived by Duma based on ray tracing using a 3D mechanical design program called CATIA V5R20. Duma et al. also pointed out the particular strengths and weaknesses of prism scanners [28]. Although these studies are interesting for prism scanners, they are not applicable in real industrial applications. Therefore, this study exploited the feasibility of wedge prisms for industrial surface cleaning. There are two major aspects to be considered. We first present equations for determining the exact scanning radius and transposition of the focal point. These equations facilitate the selection of the desired scanning size and placement of the target at the correct focusing position. We then investigate the use of our scanner for corrosion removal. The main parameters (spinning speed and state movement velocity) are discussed and optimized. Additionally, the surface performance before and after laser cleaning is systematically studied.
2. Experimental apparatus and methods
2.1 Geometrical optics analysis
The main purpose of this analysis was to select a suitable focusing lens and prism for the desired scanning diameter. In this study, a focusing lens and prism were used as the basic structures. Figure 2 presents the configuration of the scanner head in a 2D inertial reference frame ${O_{yz}}$ and relative motion frame $O{^{\prime}_{x^{\prime}y^{\prime}}}$. The laser beam is described by two marginal rays on the top ${i_1}$ and bottom ${i_2}$, and the collimated beam diameter is D. The principle ray ${i_0}$ impinges on the focusing lens and then travels through having been deflected from the original direction by an angle $\delta $ called the deviation angle. The total deviation angle $\delta $ is defined as the sum of the first refraction angle ($\theta {}_{i1} - {\theta _{t1}}$) and second reflection angle $({\theta _{i2}} - {\theta _{t2}})$ as follows:
The apex angle $\alpha = {\theta _{t1}} + {\theta _{i2}}$ and system is immersed in air (${n_0} \approx 1$).
Fig. 2. (a) Schematic of the optical path of a laser beam through the scanner and (b) an enlarged view.
Following Snell’s law, we have
We then replace $\cos {\theta _{t1}}$ with ${(1 - {\sin ^2}{\theta _{t1}})^{1/2}}$ as follows:
This yields the deviation
Let us assume that the objective lens is a perfect lens without aberration. Therefore, all rays converge at one focusing point F. Both the top and bottom marginal rays are bent at the same angle $\varphi $, but are inverted relative to each other.
Therefore, the incident angle ${\theta _{i1}}$ is in the range $\alpha - \varphi < {\theta _{i1}} < \alpha + \varphi $
In the relative motion frame $O{^{\prime}_{z^{\prime}y^{\prime}}}$.
The equation of the emergent ray of ${i_1}$ is defined as
The equation of the emergent ray of ${i_2}$ is defined as
Therefore, the coordinates of the intersection point H’ of two rays are calculated as
Equations (7) and (8) represent the position of the focused point H’ as a function of the prism angle α. Equation (7) determines the translocation of the focal point after traveling through the prism and Equation (8) approximately defines the scanning radius R.
2.2 Experimental methodology
In this study, the scanner head, including one objective lens and one prism, was fixed at a position perpendicular to the specimen and then the state was moved to a more convenient location to investigate the scanner speed (Fig. 3(a)). The two chief factors of wedge prism scanner affecting the efficiency of the laser cleaning process are the circumferential overlap and spot overlap. Figure 3(b) presents a schematic of the scan pattern over the specimen surface. Circumferential overlap and spot overlap are calculated using the following equations.
Fig. 3. (a) Schematic illustration of a prism scanner for surface treatment, and (b) circumferential and spot overlap.
Circumferential overlap:
Spot overlap:
The system parameters are listed in Table 1.
Table 1. System parameters.
2.3 Experimental materials
To demonstrate the potential use of our scanner in commercial laser cleaning applications, this study employed an SS 304L stainless steel sample from POSCO C&C, Korea. The sample dimensions were 100 × 100 × 10 mm (length × width × thickness) and the detailed element contents are listed in Table 2. We used a reliable procedure based on the recommendations of Sara Kheiri [29]. The SS 304L samples were annealed at 1050 °C for 1 h and then gradually cooled to 25 °C via furnace cooling (FP-03, WiseTherm). The final corroded SS 304L specimen was artificially formed using a 20% NaCl (99.9%, Sigma–Aldrich) solution. The SS 304L specimen was heated to 500 °C in a muffle furnace for 72 h and the NaCl solution was sprayed on the SS 304L specimen every 12 h. Laser surface treatment was conducted using a 300 W YTTERBIUM PULSED FIBER laser (YLPN-2-20×500-300, IPG Photonics) with a central wavelength of 1,064 nm, pulse duration of 500 ns, and repetition rate of 150 kHz. The 4.2-mm-diameter laser beam had a super-Gaussian energy distribution with a laser beam quality factor (M2) of 1.2. The maximum average power on the workpiece surface was 298 W when all losses in the optical path were considered. During laser treatment, the steel plate was fixed on an XY-axis motorized stage and moved in a particular direction. The speed was varied to obtain the desired overlap rate.
Table 2. Chemical properties of sample material.
2.4 Characterization
To determine the optimal conditions for laser surface cleaning, X-ray diffraction (XRD, SmartLab, Rigaku), energy dispersive spectroscopy (EDS), and field-emission scanning electron microscopy (FESEM, JSM-7000F, JEOL) were applied. The surface crystalline phases were analyzed using XRD with Cu-Kα radiation (λ = 1.5418 Å). The X-ray source was operated under generator settings of 40 kV and 200 mA and the two-angle sin2Ψ technique was adopted for the diffractive plane (2 2 0), where the diffractive angle was 74.62°. EDS was used to identify and quantify the elemental compositions of the sample areas. Finally, cross sections of the specimens were prepared for microstructural observation using FESEM.
3. RESULTS AND DISCUSSION
3.1 Geometrical optics analysis
The accuracy of Eqs. (7) and (8) was verified and the results were compared to the Zemax simulation and experimental results. Two lenses of different focal lengths of 145 and 190 mm, and a wedge prism (angle $\alpha $ range of 5° to 30°) were used. The geometrical optical properties are detailed in Table 3 and the results are presented in Fig. 4. As expected, the Zemax simulation and our equations yield almost the same results. This demonstrates the validity of Eqs. (7) and (8). Figure 4(a) also reveals that the wedge prism leads to a shift in the focal point far away from the original position. Specifically, the shift is slight at a small prism angle $\alpha $, but significantly greater at a large angle. In the case of $\alpha $ = 30° and f = 190 mm, the transitions are up to 16, 15, and 10 mm in the Zemax simulations, equations, and experiments, respectively. Additionally, lenses with shorter focal lengths exhibit smaller shifts. The transposition is smaller in the case of a 145 mm focal length and is approximately 10 mm at a wedge prism angle of 30°. Regarding the scanning radius in Fig. 4(b), there is no inequality between the equation and Zemax simulation results, but they differ slightly from the experimental results. Overall, a larger prism angle and longer focal length broaden the gaps between the simulations and experiments. The largest difference of 4mm can be observed at $\alpha $ = 30° and f = 190. However, this is not particularly surprising when considering the fact that the equation calculations and Zemax simulations represent ideal conditions without aberration and measurement errors.
Fig. 4. (a) Focal position and (b) scanning radius versus angle for Eqs. (7) and (8), Zemax simulations, and experiments.
Table 3. Geometrical optics parameters.
3.2 Laser corrosion removal
In this section, the influence of two distinctive factors (spot overlap rso and circumferential overlap rco) of wedge prism scanner on surface cleaning efficiency is discussed. An objective lens with a focal length of 145 mm and wedge prism angle of 5° was selected (Fig. 5(a)). The beam diameter both in horizontal x and vertical y along z direction of system with and without wedge prism was examined and graphed in Fig. 5(b). The data did not show any significant difference between beam diameter in horizontal and vertical in case of only lens inside the scanner. The focus beam spot size measured is 25 $\mu $m. However, beam size in horizontal and vertical with wedge prism included system is disagreement. Focused position was shifted 0.5 mm and 1 mm in horizontal and vertical, respectively. This result not only confirmed the transposition that was mentioned in formula above but also indicated the beam aberration. The focus spot size was enlarged to 28 $\mu $m due to the distortion of beam.
Fig. 5. experiment setup (a); beam diameter versus z direction of system with and without wedge prism (b)
Maximum laser power was set at 298 W, corresponding to a laser energy density of 0.7 J/cm2 was calculated as follows:
Now, according to Eqs. (9) and (10), the spot overlap and the circumferential overlap only depend on the rotation speed and the state movement velocity. Therefore, this section discusses the rotation speed and state movement velocity instead of rso and rco.
Figure 6 presents the (a) cross-sectional morphology and (b, c) surface morphology of the corrosion specimen. The cross section indicates that the thickness of the corrosion layer is approximately 186 ${\pm} $ 17 µm. The surface is uneven with multiple voids were created during the formation of corrosion products. The rotation speed was investigated in the range of 1000 to 5000 rpm at a fixed state movement velocity of 2 mm/s. The state movement velocity was then studied at the optimal rotation speed in a range of 1 to 3 mm/s.
Fig. 6. Stainless steel SS 304L samples with rust layer: (a) cross-sectional SEM image, (b) optical image, and (c) SEM image of enlarged region.
EDS micrographs of the laser-cleaned surface were captured to obtain a clear picture of the effects of varying the laser cleaning process parameters. Figure 7 presents the elemental distribution maps of Fe, Cr, Ni, Mn, Na, Cl, and C at different rotation speeds and state movement velocities. The chemical compositions of surface specimens were summarized in Table 4. For the corrosion specimen, nonhomogeneous elemental distributions can be observed as a result of the formation of corrosion products on the surface. Furthermore, the appearance of Na and Cl can be detected, as listed in Table 4 and graphed in Fig. 8, because a NaCl solution was used to corrode the surface of the SS 304L artificially. EDS analysis did not reveal any significant differences related to the rotation speed, as shown in Fig. 7(a). Neither Na nor Cl can be observed on the surface at any of the three rotation speeds in either EDS mapping or weight percentage detection. However, the 1000 rpm case implies a high segregation of C in the elemental distribution map, suggesting the formation of carbides. Table 4 also confirmed a higher C element weight percentage on surface at 1000rpm. Beside, the distribution of Fe was identified as nonhomogeneous at 5000 rpm. In contrast, the 2500 rpm case yielded homogeneous elemental distribution maps with no indication of corrosion products or carbides at the surface. This suggest that the corrosion layer was best removed at 2500 rpm. The following investigation of the state movement velocity will be conducted at 2500 rpm.
Fig. 7. Elemental distribution maps obtained from EDS analysis for sample cleaning at 2 mm/s with different rotation speeds of (a) 1000, 2500, and 5000 rpm, and (b) at 2500 rpm with different state movement velocities of 1, 2, and 3 mm/s.
Table 4. Chemical compositions obtained in Fig. 6 (units: wt%)
Figure 7(b), Table 4 and Fig. 8 highlight Na and Cl on the cleaned surface under a state movement velocity of 3 mm/s. This indicates that the corrosion products were not completely removed. Regardless, this analysis did not identify any significant differences between the velocities of 1 and 2 mm/s. The elemental distribution was even, and Na and Cl were cleaned perfectly. Therefore, the XRD profile was examined, as shown in Fig. 9.
Fig. 9. XRD spectra of the cleaned substrate with (a) different rotation speeds of 1000, 2500, and 5000 rpm, and (b) different state movement velocities of 1, 2, 3 mm/s.
The XRD profiles summarize the test results for rotation speed in Fig. 9(a) and state movement velocity in Fig. 9(b). $\gamma $-Fe peaks can be observed for the bare sample and all samples were cleaned at different rotation speeds. Overall, the results indicate that the rotation speed does not significantly affect phase transformation. Regardless, the $\delta $-Fe peaks were pinpointed in the case of a state movement velocity of 1 mm/s. This may be a result of the fact that slow motion causes overheating on the surface. Based on these data, scanning at 2 mm/s was found to be the best for corrosion removal.
The removed thickness was measured by examining the thickness of the specimen before and after the cleaning process (Fig. 10). It was found to be 186 µm for laser surface cleaning at 1 mm/s and 2 mm/s. At a state movement velocity of 3 mm/s, a corrosion layer of 9 µm in thickness remained on the surface.
4. Conclusions
This paper introduced an exceptionally simple and compact wedge prism scanner approach for laser cleaning scanners. By analyzing geometrical optics, equations for the scanning radius and transposition of the focal point were derived. The results calculated by these equations were verified through comparisons to both Zemax simulation and experimental results. There was satisfactory agreement between the equations and Zemax simulation results, but slight disagreement with the experimental results because the equations represented ideal conditions and lack aberration and measurement errors. Furthermore, corrosion removal on SS 304L were conducted using our scanner to demonstrate the practicality of laser surface cleaning. A process for determining the best conditions with different rotation speeds and state movement velocities was implemented. The primary target was minimal alteration of the microstructure and mechanical properties. The data indicated that a rotation speed of 2500 rpm and state movement velocity of 2 mm/s were the best for completely removing approximately 186 µm of corrosion.
Funding
Korea Evaluation Institute of Industrial Technology (20010323).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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