Journal of Manufacturing and Materials Processing
Article Process Optimization for 100 W Nanosecond Pulsed Fiber Laser Engraving of 316L Grade Stainless Steel
Stephen D. Dondieu 1,*, Krystian L. Wlodarczyk 1,2 , Paul Harrison 3, Adam Rosowski 3,4, Jack Gabzdyl 3, Robert L. Reuben 2 and Duncan P. Hand 1
1 Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK; [email protected] (K.L.W.); [email protected] (D.P.H.) 2 Institute of Mechanical, Process and Energy Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK; [email protected] 3 SPI Lasers UK Ltd., 6 Wellington Park Tollbar Way, Hedge End, Southampton S030 2QU, UK; [email protected] (P.H.); [email protected] (A.R.); [email protected] (J.G.) 4 Institute for Manufacturing, University of Cambridge, 17 Charles Babbage Road, Cambridge CB3 0FS, UK * Correspondence: [email protected]; Tel.: +44-(0)131-451-3084
Received: 9 November 2020; Accepted: 24 November 2020; Published: 26 November 2020
Abstract: High average power (>50 W) nanosecond pulsed fiber lasers are now routinely available owing to the demand for high throughput laser applications. However, in some applications, scale-up in average power has a detrimental effect on process quality due to laser-induced thermal accumulation in the workpiece. To understand the laser–material interactions in this power regime, and how best to optimize process performance and quality, we investigated the influence of laser parameters such as pulse duration, energy dose (i.e., total energy deposited per unit area), and pulse repetition frequency (PRF) on engraving 316L stainless steel. Two different laser beam scanning strategies, namely, sequential method (SM) and interlacing method (IM), were examined. For each set of parameters, the material removal rate (MRR) and average surface roughness (Sa) were measured using an Alicona 3D surface profilometer. A phenomenological model has been used to help identify the best combination of laser parameters for engraving. Specifically, this study has found that (i) the model serves as a quick way to streamline parameters for area engraving (ii) increasing the pulse duration and energy dose at certain PRF results in a high MRR, albeit with an associated increase in Sa, and (iii) the IM offers 84% reduction in surface roughness at a higher MRR compared to SM. Ultimately, high quality at high throughput engraving is demonstrated using optimized process parameters.
Keywords: nanosecond laser pulses; pulsed fiber lasers; engraving; high average power; surface roughness; material removal rate; interlacing method; sequential method
1. Introduction The use of lasers for rapid prototyping of tools for injection molds, coin dies, stamps, and product identification through engraving has been an established process for some time [1–3]. High flexibility, lack of tool wear, ability to process a wide range of materials, high machining accuracy, and precision are some undeniable advantages in comparison to other engraving processes [4,5]. Both ultrashort pulsed (femtosecond and picosecond) and short-pulsed (nanosecond and microsecond) lasers can be used for engraving metals. These two classes of lasers differ fundamentally in the mechanisms of laser–material interaction and material removal during processing as extensively discussed elsewhere [6,7]. Generally, ultrashort pulsed lasers are noted for high-quality applications and high cost of purchase, maintenance,
J. Manuf. Mater. Process. 2020, 4, 110; doi:10.3390/jmmp4040110 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 110 2 of 15 and operation. Nanosecond lasers, on the other hand, are less costly and are well-established as process tools, making them much more popular and reliable for industrial applications. Industry increasingly demands high manufacturing throughput to minimize costs, and this has propelled the commercial development of pulsed lasers with a higher average power. Master oscillator power amplifier (MOPA)-based nanosecond pulsed fiber lasers have become popular due to their high reliability and effective thermal management [8–10]. However, this scaling of power does not always lead to a concomitant increase in process speed and surface quality due to laser-induced thermal accumulation, the accumulation of structural defects, and particle shielding [11–14]. Over the years, much research effort has been expanded to remediate the effects of thermal build-up in the workpiece during high average power applications to make these lasers industrially acceptable. The research approach is generally to conduct a parametric survey to understand laser–material interactions and also investigate various laser processing strategies to mitigate the issues of thermal load [14–17]. So far, published research has been skewed towards ultrashort pulsed lasers, with relatively little literature on the potential of higher average power nanosecond pulsed lasers. This can be attributed to the general view that using nanosecond pulsed lasers with an average power greater than 50 W leads to an uncontrollable process [11]. Consequently, there is a current gap in the literature on the physical effects of higher average power nanosecond lasers on the material removal rate (MRR) and surface roughness (Sa). Again, although the use of the single-line machining experiments for investigating parametric influence during laser interactions is well established [11,18], the “scaling up” of findings from single lines to the more industrially relevant area engraving has not been reported. This scale-up introduces a further key parameter set, namely, the laser beam scanning strategy, including interlacing, which has been reported as an effective process route for the machining of borosilicate glass [19], and more complex scanning patterns, such as halftone printing angles, that have been shown to provide a good surface polishing effect [20]. This article extends the idea of IM for the engraving of 316 L grade stainless steel using a 100 W nanosecond pulsed fiber laser. The study starts by using single-line machining to study the effect of pulse duration, pulse repetition frequency, and energy dose on the volume of groove and burr formation during engraving. Based on the results from the single line machining, a phenomenological model is proposed to streamline process parameters for area engraving. A correlation is established between the results from the single-line machining and area engraving. The paper concludes by demonstrating that with optimized process parameters, high throughput at high-quality engraving is achievable.
2. Methodology
2.1. Material 316 L grade stainless steel plates with dimensions of 80 mm 50 mm 1.5 mm were used. × × The initial value of average surface roughness (Sa) was measured to be 640 nm using a 3D surface profilometer (Alicona IFM G4) across a field view of 1.43 mm 1.08 mm. Before engraving, the samples × were cleaned with acetone to remove any surface contamination.
2.2. Laser Set-Up
The laser used was an SPI pulsed fiber laser EP-Z G4 with measured average power (Pav) of 108 W, a wavelength of 1062 nm, maximum pulse energy of 1.1 mJ, pulse repetition frequency (PRF) up to 1 MHz, a near-Gaussian spatial profile with beam quality (M2) 1.6, and pulse duration (τ) in ≤ the range 17 to 500 ns termed as waveforms (WFM). Each WFM has a unique PRF (called PRF0) at which the maximum pulse energy (Ep) and Pav are obtained. The measured temporal profile of these waveforms is characterized by a fast rise time peak and a tail portion as shown in Figure1. The peak offers high peak power which overcomes surface reflectivity and facilitates laser absorption. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 16
J.J. Manuf.Manuf. Mater. Process. 2020,, 4,, 110x FOR PEER REVIEW 33 of 15 16 (a) (b) WFM 0 AT VARYING PRF 1.0 1.0 WFM 23 100 kHz 0.9 WFM 14 (a) 0.9 (b) WFM 0 AT VARYING PRF 150 kHz 1.0 WFM 5 1.0 200 kHz 0.8 WFM 23 WFM 0 0.8 300100 kHzkHz 0.9 WFM 14 WFM 28 0.9 500150 kHzkHz 0.7 WFM 5 0.7 WFM 31 1200 MHz kHz 0.8 WFM 0 0.8 0.6 300 kHz WFM 28 0.6 0.7 500 kHz WFM 31 0.7 0.5 0.5 1 MHz 0.6 0.6 0.4 0.4 0.5 0.5 Normalised Intensity Normalised 0.3 Normalised Intensity 0.3 0.4 0.4 0.2 0.2 Normalised Intensity Normalised 0.3 Normalised Intensity 0.3 0.1 0.1 0.2 0.2 0.0 0.0 0.1-50 0 50 100 150 200 250 300 350 400 450 500 550 0.1 0 50 100 150 200 250 300 0.0 Pulse duration (ns) 0.0 Pulse duration (ns) -50 0 50 100 150 200 250 300 350 400 450 500 550 0 50 100 150 200 250 300 Figure 1. (a) MeasuredPulse temporal duration (ns) profiles of some selected waveforms shortlistedPulse duration for (ns) the experimental work. WFM 23 (17 ns), WFM 14 (60 ns), WFM 5 (150 ns), WFM 0 (280 ns), WFM 28 (380 ns), and WFM FigureFigure 1.1. (a) Measured temporal profilesprofiles ofof some selected waveforms shortlistedshortlisted forfor thethe experimentalexperimental 31 (500 ns). (b) Temporal profile as a function of pulse repetition frequency (PRF) for WFM 0. work.work. WFM 23 (17 ns), WFM 14 (60 ns), WFM 5 (150 ns), WFM 0 (280 ns), WFM 28 (380 ns), and WFM 31 (500 ns). (b) Temporal profile as a function of pulse repetition frequency (PRF) for WFM 0. For31 (500 the ns). laser (b )machining Temporal profile set-up, as an a function optical fiberof puls dee liversrepetition the laserfrequency beam (PRF) to a forcollimator, WFM 0. a Raylase® galvanometer scan head (RLA-1504 [Y] D2), and then a 160 mm focal length fused silica f-theta lens® ForFor thethe laser machining set-up,set-up, anan opticaloptical fiberfiber deliversdelivers thethe laserlaser beambeam toto aa collimator,collimator, aa RaylaseRaylase® that focused the laser beam onto the workpiece. The laser and the galvoscanner are synchronized galvanometergalvanometer scan head (RLA-1504 [Y] D2), and thenthen aa 160160 mmmm focalfocal lengthlength fusedfused silicasilica f-thetaf-theta lenslens using a SCAPS® hardware controller and SAMLight 2D® software. The diameter of the collimated thatthat focusedfocused the the laser laser beam beam onto onto the the workpiece. workpiece. The The laser laser and and thegalvoscanner the galvoscanner are synchronized are synchronized using beam is ®8 mm, providing a focused spot diameter® (2ωo) of 38 μm (calculated at 1/e2 of its peak ausing SCAPS a SCAPShardware® hardware controller controller and SAMLight and SAMLight 2D software. 2D® software. The diameter The diameter of the collimated of the collimated beam is intensity) on the workpiece. 2 8 mm, providing a focused spot diameter (2ωo) of 38 µm (calculated at 1/e of its peak intensity)2 on beam is 8 mm, providing a focused spot diameter (2ωo) of 38 μm (calculated at 1/e of its peak the workpiece. 2.3.intensity) Experimental on the Protocolworkpiece. 2.3. Experimental Protocol 2.3. ExperimentalThe experimental Protocol approach is summarized in Figure 2, in which a single line machining experimentThe experimental is used to approachdown-select is summarized a suitable range in Figure of 2laser, in which parameters a single for line testing machining a square-shaped experiment The experimental approach is summarized in Figure 2, in which a single line machining isarea used engraving. to down-select The temporal a suitable profiles range ofof laserselected parameters pulse durations for testing (waveforms) a square-shaped are shown area engraving. in Figure experiment is used to down-select a suitable range of laser parameters for testing a square-shaped The1a. temporal profiles of selected pulse durations (waveforms) are shown in Figure1a. area engraving. The temporal profiles of selected pulse durations (waveforms) are shown in Figure 1a. Investigating the influence of pulse duration, Investigating the effect of pulse repetition rate and energy dose scanning strategies Investigating the influence of pulse duration, Investigating the effect of pulse repetition rate and energy dose scanning strategies Single line Down-selecting Area Parametric machining parameters engraving summary Single line Down-selecting Area Parametric machiningFigure 2. Flowchartparameters of the experimental protocol.engraving summary Figure 2. Flowchart of the experimental protocol. 2.3.1. Single Line Machining 2.3.1. Single Line MachiningFigure 2. Flowchart of the experimental protocol. Parallel grooves of length 10 mm were machined into the metal workpiece surface, as illustrated in 2.3.1.Parallel Single Linegrooves Machining of length 10 mm were machined into the metal workpiece surface, as illustrated Figure3a, using the laser process parameters listed in Table1. The line energy dose ( EDL) is calculated in Figure 3a, using the laser process parameters listed in Table 1. The line energy dose (𝐸𝐷 ) is usingParallel the formula grooves of length 10 mm were machined into the metal workpiece surface, as illustrated calculated using the formula EDL = (1mm/∆S) Ep, (1) in Figure 3a, using the laser process parameters listed ×in Table 1. The line energy dose (𝐸𝐷 ) is calculated using the formula 𝐸𝐷 =(1𝑚𝑚/𝛥𝑆)×𝐸 , (1) where Ep is the pulse energy and ∆S is the pulse to pulse distance in the scan direction (see Figure4). where 𝐸 is the pulse energy and 𝛥𝑆 is𝐸𝐷 the =(1𝑚𝑚/𝛥𝑆)×𝐸pulse to pulse distance , in the scan direction (see Figure(1) 4). where 𝐸 is the pulse energy and 𝛥𝑆 is the pulse to pulse distance in the scan direction (see Figure 4).
J. Manuf. Mater. Process. 2020, 4, 110 4 of 15 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 4 of 16
(a)
0.28 0.14 0.09 0.07 0.06 0.05 0.04 0.03 (b) EDL (J/mm) (c) (d)
FigureFigure 3.3. ((aa)) SurfaceSurface heightheight mapmap ofof lineslines machinedmachined withwith 280280 nsns pulsespulses atat didiffefferentrent lineline energyenergy dosesdoses (ED ). (b–d) Extracted 3D profiles of the highlighted region: (b) full profile, (c) extracted groove profile (𝐸𝐷L ). (b–d) Extracted 3D profiles of the highlighted region: (b) full profile, (c) extracted groove profile (material(material removed),removed), andand ((dd)) extractedextracted burr burr profile profile (redeposited (redeposited material) material) [ µ[μm].m]. Table 1. List of parameters for single-line machining study. The pulse to pulse distance (∆S) and the Δ𝑆 pulseTable overlap 1. List of factor parameters (Po), which for issingle-line the fraction machining of the laser study. spot The diameter pulse that to pulse overlaps distance with the( previous) and the pulse overlap factor (Po), which is the fraction of the laser spot diameter that overlaps with the pulse on the surface of the material, at given energy doses (EDL) for different pulse durations (τ) are provided.previous pulse All pulse on the repetition surface of frequencies the material, (PRF) at given indicated energy correspond doses (𝐸𝐷 to ) thefor different PRF0 condition. pulse durations (τ) are provided. All pulse repetition frequencies (PRF) indicated correspond to the PRF0 condition. Scan τ, PRF0, Ep Speed EDL τ, PRF0, 𝑬𝒑 Scan Speed 𝑬𝑫17𝑳 ns 1000 kHz 60 ns 340 kHz 150 ns 175 kHz 280 ns 100 kHz 380 ns 100 kHz 500 ns 100 kHz (mm/s) (J/mm) 17 ns 1000 60 ns 340 150 ns 175 280 ns 100 kHz 380 ns 100 500 ns 100 (mm/s) (J/mm) 0.11 mJ 0.32 mJ 0.62 mJ 1.1 mJ 1.1 mJ 1.1 mJ kHz 0.11 mJ kHz 0.32 mJ kHz 0.62 mJ 1.1 mJ kHz 1.1 mJ kHz 1.1 mJ ∆S = 0.4 µm ∆S = 1.1 µm ∆S = 2.2 µm ∆S = 3.8 µm ∆S = 3.8 µm ∆S = 3.8 µm 379 0.28 𝛥𝑆 = 0.4 μm 𝛥𝑆 = 1.1 μm 𝛥𝑆 = 2.2 μm 𝛥𝑆 = 3.8 μm 𝛥𝑆 = 3.8 μm 𝛥𝑆 = 3.8 μm 379 0.28 Po = 99% Po = 97% Po = 94% Po = 90% Po = 90% Po = 90% Po = 99% Po = 97% Po = 94% Po = 90% Po = 90% Po = 90% ∆S = 0.8 µm ∆S = 2.2 µm ∆S = 4.3 µm ∆S = 7.6 µm ∆S = 7.6 µm ∆S = 7.6 µm 759 0.14 𝛥𝑆 = 0.8 μm 𝛥𝑆 = 2.2 μm 𝛥𝑆 = 4.3 μm 𝛥𝑆 = 7.6 μm 𝛥𝑆 = 7.6 μm 𝛥𝑆 = 7.6 μm 759 0.14 Po = 98% Po = 94% Po = 89% Po = 80% Po = 80% Po = 80% ∆S = 1.1Poµm = 98% ∆S = 3.4Poµm = 94% ∆S = 6.5Poµ m= 89% ∆S = 11.4Poµ =m 80% ∆S = 11.4Po µ=m 80% ∆S =Po11.4 = µ80%m 1140 0.09 𝛥𝑆 = 11.4 𝛥𝑆 = 11.4 Po =𝛥𝑆97% = 1.1 μm Po =𝛥𝑆91% = 3.4 μm Po =𝛥𝑆82% = 6.5 μm Po =𝛥𝑆70% = 11.4 μm Po = 70% Po = 70% 1140 0.09 ∆S = 1.5 µm ∆S = 4.5 µm ∆S = 8.7 µm ∆S = 15.2 µm ∆S = 15.2 µμmm ∆S = 15.2μmµm 1520 0.07 Po = 97% Po = 91% Po = 82% Po = 70% Po = 96% Po = 88% Po = 77% Po = 60% Po = Po60% = 70% Po =Po60% = 70% 𝛥𝑆 = 15.2 𝛥𝑆 = 15.2 ∆S = 1.9𝛥𝑆µ =m 1.5 μm∆ S = 5.6𝛥𝑆µ =m 4.5 μm∆ S = 10.9𝛥𝑆 µ= m8.7 μm ∆S =𝛥𝑆19 µ= m15.2 μm ∆S = 19 µm ∆S = 19 µm 19001520 0.06 0.07 μm μm Po = 95%Po = 96% Po = 85%Po = 88% Po = 71%Po = 77% Po = 50%Po = 60% Po = 50% Po = 50% ∆S = 2.3 µm ∆S = 6.7 µm ∆S = 13.0 µm ∆S = 22.8 µm ∆S = 22.8Po µ=m 60% ∆S =Po22.8 = µ60%m 2280 0.05 𝛥𝑆 = 1.9 μm 𝛥𝑆 = 5.6 μm 𝛥𝑆 = 10.9 μm 𝛥𝑆 = 19 μm 𝛥𝑆 = 19 μm 𝛥𝑆 = 19 μm 1900 0.06 Po = 94% Po = 82% Po = 66% Po = 40% Po = 40% Po = 40% ∆S = 3.0Poµm = 95% ∆S = 8.8Poµm = 85% ∆S = 17.1Poµ =m 71% ∆S = 30Poµ m= 50% ∆S = 30Poµ m= 50% ∆S =Po30 =µ 50%m 3000 0.04 𝛥𝑆 = 22.8 𝛥𝑆 = 22.8 Po =𝛥𝑆92% = 2.3 μm Po =𝛥𝑆77% = 6.7 μm Po =𝛥𝑆55% = 13.0 μm Po =𝛥𝑆21% = 22.8 μm Po = 21% Po = 21% 2280 0.05 ∆S = 3.4 µm ∆S = 10.1 µm ∆S = 19.5 µm ∆S = 34.2 µm ∆S = 34.2 µμmm ∆S = 34.2μmµm 3420 0.03 Po = 94% Po = 82% Po = 66% Po = 40% Po = 91% Po = 74% Po = 49% Po = 10% Po = Po10% = 40% Po =Po10% = 40% 𝛥𝑆 = 3.0 μm, 𝛥𝑆 = 8.8 μm 𝛥𝑆 = 17.1 μm 𝛥𝑆 = 30 μm 𝛥𝑆 = 30 μm 𝛥𝑆 = 30 μm 3000 0.04 Po = 92% Po = 77% Po = 55% Po = 21% Po = 21% Po = 21% 𝛥𝑆 = 10.1 𝛥𝑆 = 34.2 𝛥𝑆 = 34.2 𝛥𝑆 = 3.4 μm 𝛥𝑆 = 19.5 μm 𝛥𝑆 = 34.2 μm 3420 0.03 μm μm μm Po = 91% Po = 49% Po = 10% Po = 74% Po = 10% Po = 10%
For this study, the effect of 𝐸𝐷 and τ on the volume of groove and burr formation was investigated. Using the parameters listed in Table , the influence of changing PRF at a constant 𝐸𝐷 was examined. Each parameter combination was replicated three times and the mean value presented.
J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 16
Table 2. List of parameters for investigating the influence of PRF on single-line machining at a scan
speed of 1140 mm/s (𝐸𝐷 = 0.09 J/mm).
PRF (kHz) Ep (mJ) Po (%) at 0.09 J/mm 100 1.08 70 150 0.72 80 200 0.54 85 250 0.43 88 300 0.36 90 350 0.31 91 400 0.25 93
2.3.2. Area Machining Experiment Following the single-line machining study, square-shaped area machining was investigated, in each case based on the machining of a 5 × 5 mm2 flat-bottomed area using a parallel hatch distance (𝛥𝐻) of 13.3 μm, which was chosen out of several hatch distances to give the highest MRR and better
Sa. The corresponding area energy dose (𝐸𝐷 ) from the down-selected parameters was calculated as follows.
𝐸𝐷 =(1𝑚𝑚/𝛥𝐻)×𝐸𝐷 , (2) An initial study was conducted to investigate the influence of laser beam scanning strategies on MRR and Sa using SM and IM, as shown schematically in Figure 4. In these experiments, the interlacing distance (∆IL) was varied at 13.3 μm, 26.6 μm, 39.9 μm, 53.2 μm, 66.5 μm. and 79.8 μm to identify an optimized value for further studies. More details of this method of machining are presentedJ. Manuf. Mater. in our Process. previous2020, 4 ,work 110 [19]. 5 of 15
ΔIL
(a) (b)
ΔH ΔS
12367 45 89 10 11 12 172839410511 6 12 (c) (d)
159261037114812 147102581136129 (e) (f)
1479112581012 3 6 135791124681210
Figure 4. Schematic representation of (a) bidirectional sequential method (SM) and (b–f) bidirectional Figureinterlacing 4. Schematic method representation (IM). (b) IM with of (a one) bidirectional line skip (∆ sequentiIL = 2 ∆Hal), method (c) IM with(SM) 2 and line (b skips–f) bidirectional (∆IL = 3 ∆H ), interlacing(d) IM with method 3 line (IM). skips (b (∆) ILIM= with4 ∆ Hone), ( eline) IM skip with (∆ 4IL line = 2 skips𝛥𝐻), (c (∆) IMIL = with4 ∆ 2H line), and skips (f) IM(∆IL with = 3 𝛥𝐻 5 line), (dskips) IM with (∆IL 3= line5 ∆ Hskips). In (∆ allIL these = 4 𝛥𝐻 scanning), (e) IM modes,with 4 line the totalskips process (∆IL = 4 time 𝛥𝐻), isand identical. (f) IM with For image5 line skips clarity, (∆theIL = hatch 5 𝛥𝐻 distance). In all these was chosen scanning at halfmodes, the laserthe total spot proces diameters time in this is identical. illustration. For image clarity, the hatch distance was chosen at half the laser spot diameter in this illustration. For this study, the effect of ED and τ on the volume of groove and burr formation was investigated. L Using the parameters listed in Table2, the influence of changing PRF at a constant EDL was examined. Each parameter combination was replicated three times and the mean value presented.
Table 2. List of parameters for investigating the influence of PRF on single-line machining at a scan speed of 1140 mm/s (EDL = 0.09 J/mm).
PRF (kHz) Ep (mJ) Po (%) at 0.09 J/mm 100 1.08 70 150 0.72 80 200 0.54 85 250 0.43 88 300 0.36 90 350 0.31 91 400 0.25 93
2.3.2. Area Machining Experiment Following the single-line machining study, square-shaped area machining was investigated, in each case based on the machining of a 5 5 mm2 flat-bottomed area using a parallel hatch distance × (∆H) of 13.3 µm, which was chosen out of several hatch distances to give the highest MRR and better Sa. The corresponding area energy dose (EDA) from the down-selected parameters was calculated as follows. ED = (1mm/∆H) EDL, (2) A × An initial study was conducted to investigate the influence of laser beam scanning strategies on MRR and Sa using SM and IM, as shown schematically in Figure4. In these experiments, the interlacing J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 16
To prevent a possible Moiré effect, the laser scanning angle was changed after each full machining pass, to give four different angles (0°, 45°, 18.43°, and 71.58°), popularly known as “halftone angles”. Gora et al. studied the effect of scanning patterns during laser polishing [21]. They found that a better surface finish was achievable by using the halftone angles. The total number of passes during the area engraving test was 20. Finally, in a similar way as the single-line machine experiment, the influence of τ and PRF at constant 𝐸𝐷 was investigated.
2.4.J. Manuf. Sample Mater. Postprocessing Process. 2020, 4 and, 110 Analysis 6 of 15 The processed samples were cleaned with deionized water in an ultrasonic bath at 25°C for 10 mindistance to remove (∆IL) wasloose varied particles. at 13.3 To µavoidm, 26.6 possibleµm, 39.9 endµ m,effects, 53.2 µonlym, 66.5 the µcentralm. and section 79.8 µ mof tothe identify grooves an atoptimized a sampling value length for ( further𝐿) of 2.7 studies. mm was More measured. details ofA this3D-profile method of of each machining groove was are presentedobtained using in our theprevious Alicona work surface [19]. profilometer. Using a MATLAB script, the profile of each machined line was processedTo prevent (see Figure a possible 3b) and Moir separatedé effect, the into laser the scanning groove (material angle was removed) changed after(see eachFigure full 3c) machining and the burrpass, (redeposited to give four material) different angles(see Figure (0◦, 45 3d).◦, 18.43 During◦, and the 71.58 groove◦), popularly analysis, the known sample as “halftone was levelled angles”. so thatGora the et data al. studied above and the ebelowffect of the scanning levelled patterns surface duringwere used laser to polishing calculate [ 21the]. area They of found burr and that groove a better respectivelysurface finish using was Equation achievable (3). by The using volume the halftone of groove angles. and Theburr total are calculated number of using passes Equation during the (4). area engraving test was 20. Finally, in a similar way as( the) single-line machine experiment, the influence of ∬ , 𝑧 𝑥, 𝑦 .𝑑𝑥.𝑑𝑦, (3) τ and PRF at constant EDA was investigated.
2.4. Sample Postprocessing and Analysis 𝑉 =𝐴 ×𝐿, (4) After the 3D profiling, each groove was sectioned in the middle, cold mounted, polished, and The processed samples were cleaned with deionized water in an ultrasonic bath at 25 ◦C for then10 min examined to remove using loose a Leica particles. optical To avoidmicroscope possible (DM6000M) end effects, at only × 20 the magnification central section (see of Figure the grooves 5b). Forat athe sampling area machining length (L experiment,) of 2.7 mm wasa 3D measured.profile was A obtained 3D-profile for ofeach each parameter groove was combination obtained usingand, fromthe Alicona this, the surface depth was profilometer. determined Using and asubsequent MATLAB™ly usedscript, to the calculate profile an of overall each machined MRR value line (the was productprocessed of (seethe depth Figure and3b) andarea separated of engraving into theper grooveunit processing (material time). removed) The (seearithmetic Figure 3averagec) and the of absoluteburr (redeposited surface roughness material) (S (seea) was Figure obtained3d).During using a thecut-off groove wavelength analysis, of the 284 sample μm. was levelled so that the data above and below the levelled surface were used to calculate the area of burr and groove 3. Results and Discussion respectively using Equation (3). The volume of groove and burr are calculated using Equation (4).
3.1. Single Line Machining Experiment A = z(x, y).dx.dy, (3) x x,y 3.1.1. Groove Analysis V = A L, (4) In general, four classes of grooves were observed× as shown in Figure 5. Class I is representative After the 3D profiling, each groove was sectioned in the middle, cold mounted, polished, and then of closed-up grooves which result from a combination of high 𝐸𝐷 (0.28 J/mm) and high Po (99%). examined using a Leica optical microscope (DM6000M) at 20 magnification (see Figure5b). For the Class II is representative of shallow grooves resulting from× lower 𝐸𝐷 and τ, in this case, 0.05 J/mm area machining experiment, a 3D profile was obtained for each parameter combination and, from this, at 17 ns. In Class III, there is a pronounced burr formation at the edges of the groove, which suggests the depth was determined and subsequently used to calculate an overall MRR value (the product of an inefficient removal of molten material. In extreme conditions, this can result in a Class I groove. the depth and area of engraving per unit processing time). The arithmetic average of absolute surface The nature of this type of groove obstructs proper surface profiling (see Class III Figure 5a). roughness (Sa) was obtained using a cut-off wavelength of 284 µm.
Class I Class II Class III Class IV
(a)
(b)
100μm 1100μm00μm 100μm 100μm 17 ns-0.28 J/mm 17 ns-0.05 J/mm 500 ns-0.28 J/mm 150 ns-0.28 J/mm
Figure 5. (a) 3D profiles of various classes of groove produced during the single-line machining and the (b) corresponding cross-sectional profiles. J. Manuf. Mater. Process. 2020, 4, 110 7 of 15
3. Results and Discussion
3.1. Single Line Machining Experiment
3.1.1. Groove Analysis In general, four classes of grooves were observed as shown in Figure5. Class I is representative of closed-up grooves which result from a combination of high EDL (0.28 J/mm) and high Po (99%). Class II is representative of shallow grooves resulting from lower EDL and τ, in this case, 0.05 J/mm at
17 ns.J. InManuf. Class Mater. III, Process. there 2020 is, 4,a x FOR pronounced PEER REVIEW burr formation at the edges of the groove, which 7 of 16 suggests an inefficient removal of molten material. In extreme conditions, this can result in a Class I groove. The natureFigure of this 5. (a type) 3D profiles of groove of various obstructs classes of proper groove pr surfaceoduced profilingduring the single-line (see Class machining III Figure and 5a). Classthe IV (b) represents corresponding a cross-sectional stable groove profiles. with a well-defined groove and burr which represents a conditionClass suitable IV represents for analysis. a stable Based groove on with the a above well-defined classifications, groove and the burr 17 which ns pulses represents with a an EDL of
0.28 J/conditionmm and suitable any ED forL below analysis. 0.05 Based J/mm on the were above eliminated classifications, from the the 17 parametric ns pulses with analysis an 𝐸𝐷 in of the section below.0.28 The J/mm general and any classification 𝐸𝐷 below 0.05 of J/mm grooves were eliminated as elaborated from the above parametric corroborates analysis in results the section presented by (Leonebelow. et al., The 2018). general Their classification study investigated of grooves as parametric elaborated influenceabove corroborates during laserresults marking presented application by of (Leone et al., 2018). Their study investigated parametric influence during laser marking application Inconel 718 using a 30 W Q-switched Yb: YAG fiber laser. of Inconel 718 using a 30 W Q-switched Yb: YAG fiber laser.
3.1.2. Influence of Change in EDL and τ on Groove and Burr Formation 3.1.2. Influence of Change in 𝐸𝐷 and τ on Groove and Burr Formation
As expected,As expected, an an increase increase in 𝐸𝐷ED L increasesincreases both both the volume the volume of the groove of the and groove that of and the burr, that as of the burr, as shownshown in in Figure Figure6 .6. When When a keyhole keyhole is isproduced produced as a as result a result of high of high𝐸𝐷 , theED depthL, the increases depth increases such that such that subsequentsubsequent ejection ejection due due to theto the recoil recoil pressure pressure isis compromised.compromised. This This leads leads to an to ineffective an ineffective ejection ejection and excessiveand burrexcessive formation, burr formation, as shown as inshown the highlighted in the highlighted region inregion Figure in 6Figurec, in some 6c, in cases some collapsing cases back collapsing back into the machined region (see the results for τ = 60 ns at 𝐸𝐷 = 0.28 J/mm). The into the machined region (see the results for τ = 60 ns at ED = 0.28 J/mm). The evidence presented evidence presented does not suggest that material removal is solelyL through melt ejection, in fact, does notsome suggest material that is removed material throug removalh vaporization; is solely through however, melt the ejection, method inused fact, cannot some quantify material this is removed througheffect. vaporization; however, the method used cannot quantify this effect.
(a) (b)
60 ns 5. 0x10 -3 5.0x10-3 60 ns 150 ns 150 ns 280 ns 280 ns
) 380 ns
3 380 ns
-3 ) -3 4. 0x10 500 ns 3 4.0x10 500 ns
3. 0x10 -3 3.0x10-3
2. 0x10 -3 2.0x10-3 Volume of burr (mm burr of Volume Volume of groove (mm of groove Volume
1. 0x10 -3 1.0x10-3
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.04 0.06 0.08 0.10 0.12 0.14 0.16 ED (J/mm) ED ( J/m m) L (c) L
0.05 J/mm 0.06 J/mm 0.07 J/mm 0.09 J/mm 0.14 J/mm 0.28 J/mm 60 ns 100μm
500 ns 100μm
FigureFigure 6. (a 6.) Extracted(a) Extracted volume volume ofof groove and and (b ()b extracted) extracted volume volume of burr of produced burr produced as a function as a of function of line energy dose (𝐸𝐷 ) for different pulse durations at PRF0. (c) Cross section of grooves produced at line energy dose (EDL) for different pulse durations at PRF0.(c) Cross section of grooves produced at 60 ns and 500 ns pulse durations for different energy doses. The parameter combinations highlighted 60 ns and 500 ns pulse durations for different energy doses. The parameter combinations highlighted in red have been excluded from further experiments as the quality of machining is poor. in red have been excluded from further experiments as the quality of machining is poor. As shown in Figure 6a, longer τ produces larger grooves, as is particularly evident at 380 ns and
500 ns for the highest 𝐸𝐷 . This is likely due to more laser interaction time and deeper thermal penetration at longer τ. This is influenced by the extended intensity distribution profile as presented in Figure 1, which helps retain heat before successive pulses. The similar volume of the groove at 60
ns, 150 ns, and 280 ns τ at lower 𝐸𝐷 is attributable to the high pulse overlap as detailed in Table 1.
J. Manuf. Mater. Process. 2020, 4, 110 8 of 15
As shown in Figure6a, longer τ produces larger grooves, as is particularly evident at 380 ns and 500 ns for the highest EDL. This is likely due to more laser interaction time and deeper thermal penetration at longer τ. This is influenced by the extended intensity distribution profile as presented J. Manuf.in Figure Mater.1 Process., which 2020 helps, 4, x FOR retain PEER heat REVIEW before successive pulses. The similar volume of the groove 8 of 16 at 60 ns, 150 ns, and 280 ns τ at lower EDL is attributable to the high pulse overlap as detailed in Table1. FromFrom Figure Figure 6b,c,6b,c, although although longer longer τ typicallyτ typically leads leads to toa higher a higher burr burr volume, volume, a shorter a shorter τ withτ with high high pulsepulse overlap overlap inhibits inhibits rapid rapid heat heat dissipation dissipation resulting resulting in heat in heat accumulation, accumulation, which which increases increases the the burr burr volume.volume. From From the the above above observations observations,, it seems it seems likely likely that that a longer a longer τ willτ will give give a high a high MRR MRR with with higher higher Sa at higher 𝐸𝐷 during area engraving due to the high melt and burr formation. Shorter τ, such as Sa at higher ED L during area engraving due to the high melt and burr formation. Shorter τ, such as 60 ns and 150 ns, with high 𝐸𝐷 may as well result in higher MRR, but this might be at the expense 60 ns and 150 ns, with high ED L may as well result in higher MRR, but this might be at the expense of of surface quality.
3.1.3.3.1.3. Effect Effect of ofChange Change in inPRF PRF and and τ onτ on the the Volume Volume of ofGroove Groove and and Burr Burr Formation Formation GivenGiven the the requirement requirement for for high high MRR, MRR, τs τofs of280 280 ns, ns, 380 380 ns, ns, and and 500 500 ns nswere were selected selected for for further further investigation,investigation, to toevaluate evaluate the the impact impact of ofPRF. PRF. Here, Here, the the 𝐸𝐷ED wasL was kept kept constant constant at at0.09 0.09 J/mm J/mm as asthis this providedprovided a good a good combination combination of of high high MRR MRR and and machin machininging quality. quality. Results Results in in Figure 7 7 indicate indicate that that an an increaseincrease inin PRF increases the the volume volume of of both both groove groove and and burr. burr. However, However, a critical a critical point point is reached is reached wherewhere the the groove groove collapses. collapses.
(b) 6. 0x10 -3 (a) 6.0x10-3 280ns 280ns 380ns 380ns -3 -3 5. 0x10 500ns 5.0x10 500ns ) 3 ) 3 4. 0x10 -3 4.0x10-3
3. 0x10 -3 3.0x10-3
2. 0x10 -3 2.0x10-3 Volume of burrVolume of (mm Volume of (mm groove
1. 0x10 -3 1.0x10-3
0. 0 0.0 50 100 150 200 250 300 350 400 50 100 150 200 250 300 350 400 Frequency (kHz) (c) Frequency (kHz)
100 kHz 150 kHz 200 kHz 250 kHz 300 kHz 350 kHz 400 kHz 60 ns
100μm
100 kHz 150 kHz 200 kHz 250 kHz 300 kHz 380 ns 100μm
100 kHz 150 kHz 200 kHz 250 kHz 300 kHz 500 ns
100μm
Figure 7. (a) Volume of groove; (b) volume of the burr as a function of PRF at a constant EDL of Figure 7. (a) Volume of groove; (b) volume of the burr as a function of PRF at a constant 𝐸𝐷 of 0.09 0.09 J/mm; and (c) cross-sectional view for the influence of change in PRFs at 280 ns, 380 ns, and 500 ns. J/mm; and (c) cross-sectional view for the influence of change in PRFs at 280 ns, 380 ns, and 500 ns. This happens at a PRF above 250 kHz at 500 ns, 300 kHz at 380 ns, and 350 kHz at 280 ns (see arrowedThis happens points onat Figurea PRF 7abovea). An 250 increase kHz at in 500τ increases ns, 300 kHz the volume at 380 ns, of grooveand 350 and kHz burr at 280 formation ns (see for arrowed points on Figure 7a). An increase in τ increases the volume of groove and burr formation for similar reasons to those earlier discussed. The increased volume of groove and burr at higher EDL or similar reasons to those earlier discussed. The increased volume of groove and burr at higher 𝐸𝐷 or higher PRFs is assumed to result from the thermal accumulation and the consequent larger melt zone.
However, the reduction in 𝐸 and the change in temporal profile (Figure 1b) at high PRF decreases the pulse intensity and this lowers the vapor pressure needed for effective removal, as seen in the highlighted areas of Figure 7c. The critical PRF (PRF beyond which groove is unstable) is dependent on τ due to the differences in laser interaction time. The higher laser-material interaction time for
J. Manuf. Mater. Process. 2020, 4, 110 9 of 15 higher PRFs is assumed to result from the thermal accumulation and the consequent larger melt zone. However, the reduction in Ep and the change in temporal profile (Figure1b) at high PRF decreases the pulse intensity and this lowers the vapor pressure needed for effective removal, as seen in the highlighted areas of Figure7c. The critical PRF (PRF beyond which groove is unstable) is dependent on τ due to the differences in laser interaction time. The higher laser-material interaction time for longer τ means more melt, however, due to the reduction in Ep at high PRF (less energetic pulse), the melt collapses (see Figure7c—500 ns at 300 kHz) and therefore lowers the critical PRF. In general, it can be hypothesized that higher PRF and higher EDL would cause high MRR, but the increase in the volume of burr could imply a higher Sa during area engraving. Finally, it can be suggested that the closing of shallow grooves at high PRFs could provide a polishing effect. Based on the results above, a phenomenological model is proposed such that the full parametric range indicated in Tables1 and2 were down-selected by excluding pulse duration 17 ns, EDL > 0.09 J/mm, EDL < 0.04 J/mm, and PRF of 400 kHz. The corresponding EDA values from these down-selected parameters were used to investigate the transferability of results from the single-line machining to a square-shaped area engraving.
3.2. Area Machining Experiment
3.2.1. Effect of Different Laser Beam Scanning Strategies
In Figure8, the SM Figure8b is compared with di fferent IMs (Figure8c–g) for a mid-range EDA of 2 4.2 J/mm (EDL = 0.06 J/mm and ∆H = 13.3 µm) processed at 280 ns. A blackened surface and higher burr formation at the edges were observed with SM (see Figure8a).
µ µ SEQUENTIAL INTERLACING
500 µm 500 µm 500 µm ∆IL = 13.3 µm ∆IL = 26.6 µm ∆IL = 39.9 µm
500 µm 500 µm 500 µm ∆IL = 53.3 µm ∆IL = 66.5 µm ∆IL = 79.8 µm
Figure 8. (a) Photograph comparing sequential method (SM) to interlacing method (IM). (b) Surface morphology for SM. (c–g) Surface morphology at different interlacing distances (∆IL), produced at a 2 scan speed of 1900 mm/s, 280 ns and EDA= 4.2 J/mm . J. Manuf. Mater. Process. 2020, 4, 110 10 of 15
The Sa is reduced by a factor of 3 and the MRR is increased by 20% with IM (see Figure8c–f). 2 Figure9 presents the cross-sectional di fferences between IM and SM at EDA = 5.3 J/mm . At higher EDA, IM significantly reduces the Sa by 84% at a slightly higher MRR. The time delay between adjacent J. scanManuf. for Mater. IM Process. helps reduce2020, 4, x undueFOR PEER heat REVIEW accumulation in the workpiece. 10 of 16
MRR = 14 mm3/min Sequential Method (a) Sa = 32 µm
500μm
3 MRR = 15 mm /min Interlaced Method S = 5 µm (b) a
500μm
FigureFigure 9. 9.Cross-sectionalCross-sectional profile profile highlighting highlighting the the difference difference between between (a ()a SM) SM at at material material removal removal rate rate 3 3 2 (MRR)(MRR) = =1414 mm mm3/min,/min, Sa S =a =3232 μmµm and and (b ()b IM) IM at at MRR MRR = =1515 mm mm3/min,/min, Sa S =a 5= μ5mµ mprocessed processed at at 5.3 5.3 J/mm J/mm2 atat 280 280 ns. ns.
ThisThis implies implies a auniform uniform thermal thermal gradient gradient around around the the processed processed area area which which could could cause cause a amore more efficientefficient cooling. cooling. With With SM, SM, the the heat heat is is built built up up to to produce produce more more melt. melt. The The ineffective ineffective removal removal of of this this melt ends up compromising MRR and S . Figure9 also illustrates the presence of undercut at the melt ends up compromising MRR and Sa.a Figure 9 also illustrates the presence of undercut at the cornerscorners for for both both SM SM and and IM, IM, which which is is due due to to the the lack lack of of synchronization synchronization between between the the laser laser and and the the scannerscanner system. system. This This feature feature compromises compromises the the engr engravingaving quality; quality; however, however, with with appropriate appropriate scan scan delaysdelays this this effect effect can can be be rectified. rectified. ForFor glass, glass, the the authors authors of of [19,22] [19,22] suggest suggest that that the the IM IM increases increases laser couplingcoupling eefficiencyfficiency as as a a result result of ofthe the smaller smaller angle angle of incidenceof incidence of the of laser the beam.laser beam. This results This fromresults the from perpendicular the perpendicular beam interaction beam interactionto the surface to the in surface the case in ofthe IM case as of opposed IM as opposed to the SM, to the where SM, thewhere beam the is beam incident is incident on the on wall the of wallthe of previously the previously machined machined groove. groove. This couldThis could as well as well contribute contribute to the to dithefferences differences observed observed for bothfor bothscanning scanning strategies. strategies. Although Although a changea change in in interlacing interlacing distance distance increasesincreases MRR,MRR, at ∆ILIL == 79.879.8 μµmm (see(see Figure Figure 8g)8g) the the surface surface quality quality is is deteriorated deteriorated and and characterized characterized by by massive massive structures structures (solidified (solidified meltmelt redeposition) redeposition) resulting fromfrom ineineffectiveffective melt melt ejection. ejection. Obviously, Obviously, IM oIMffers offers a better a better surface surface quality qualityin comparison in comparison to SM, to hence SM, allhence results all results presented presented below inbelow the next in the sections next sections were conducted were conducted using IM at an optimized ∆IL of 39.9 µm (see Figure8d) due to its high MRR and low S . using IM at an optimized ∆IL of 39.9 μm (see Figure 8d) due to its high MRR anda low Sa.
3.2.2. Influence of Change in EDA and τ on the MRR and Sa 3.2.2. Influence of Change in 𝐸𝐷 and τ on the MRR and Sa As shown in Figure 10, an increase in EDA and τ leads to a general increase in both MRR and Sa, As shown in Figure 10, an increase in 𝐸𝐷 and τ leads to a general increase in both2 MRR and Sa, as suggested in Section 3.1.2. From Figure 10b, there is a critical EDA at 5.3 J/mm2 for 60 ns, 150 ns, as suggested in Section 3.1.2. From Figure 10b, there is a critical 𝐸𝐷 at 5.3 J/mm for 60 ns, 150 ns, and 500 ns (see the dashed line) where the Sa starts to reduce for further increase in EDA, which is and 500 ns (see the dashed line) where the Sa starts to reduce for further increase in 𝐸𝐷 , which is most likely attributable to the “closing-up” effect observed in the single-line machining experiments, most likely attributable to the “closing-up” effect observed in the single-line machining experiments, rather than any inherent machining quality improvement. The shorter τ gives lower MRR coupled with rather than any inherent machining quality improvement. The shorter τ gives lower MRR coupled high Sa values mainly due to the high pulse overlaps employed in this study (see Table1). An increase with high Sa values mainly due to the high pulse overlaps employed in this study (see Table 1). An in scan speed could offset the issues of high overlap, but this would mean a lower MRR which is not increase in scan speed could offset the issues of high overlap, but this would mean a lower MRR consistent with the overall requirement for high throughput engraving. This highlights the trade-off which is not consistent with the overall requirement for high throughput engraving. This highlights between high-throughput and high-quality engraving, and, therefore, to maintain a balance, the area the trade-off between high-throughput and high-quality engraving, and, therefore, to maintain a highlighted in green in Figure 10 with MRR in the range of 5 to 16 mm3/min and corresponding S of balance, the area highlighted in green in Figure 10 with MRR in the range of 5 to 16 mm3/min anda 2.6–13 µm is an area of focus for high-quality engraving. corresponding Sa of 2.6–13 μm is an area of focus for high-quality engraving.
J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 16
J. Manuf. Mater. Process. 2020, 4, 110 11 of 15 30 (a)30 (b)
25 25 /min) 3 20 20
15 15
10 60 ns 10 60 ns 150 ns 150 ns Surface roughness (µm) 280 ns 280 ns 5 5 Materialremoval rate (mm 380 ns 380 ns 500 ns 500 ns 0 0 234567 234567 ED (J/mm2) ED (J/mm2) A A
Figure 10. (a) MRR and (b) Sa as a function of area energy dose (𝐸𝐷 ) for different pulse durations. The highlighted region in panel (a) excludes 60 ns (>2.7 J/mm2), and the dashed line represents a
critical 𝐸𝐷 beyond which the surface is blackened and surface quality deteriorates significantly. Figure 10. (a) MRR and (b)Sa as a function of area energy dose (EDA) for different pulse durations. The highlighted region in panel (a) excludes 60 ns (>2.7 J/mm2), and the dashed line represents a critical Clearly, to achieve Sa below 2.6 μm at high MRR, it is recommended to use a sequential ED beyond which the surface is blackened and surface quality deteriorates significantly. combinationA of deep engraving with longer pulse durations (>280 ns) and periodic cleaning with 2 shorterClearly, pulses to achieve (<280 ns) Sa below at lower 2.6 µ𝐸𝐷m at (<2.7 high J/mm MRR,). it The is recommended increase in MRR to use as aa sequentialfunction of combination τ agrees with ofprevious deep engraving work on with the longer use of pulse 20 W durations nanosecond (>280 pu ns)lsed and fiber periodic laser engraving cleaning with of stainless shorter pulsessteel by 2 (
20 (a)20 (b) 280ns 280ns 380ns 380ns 500ns 500ns 16 16 /min) 3
12 12
8 8 surface roughness (µm) roughness surface 4 4 Material removal rate (mm
0 0 50 100 150 200 250 300 350 400 50 100 150 200 250 300 350 400 Frequency (kHz) Freq uency (kHz)
2 Figure 11. (a) MRR and (b)Sa as a function of PRF and τ at a constant EDA of 4.2 J/mm . 2 Figure 11. (a) MRR and (b) Sa as a function of PRF and τ at a constant 𝐸𝐷 of 4.2 J/mm . As more melt is formed, the impact from subsequent pulses ejects melt across the surface, and rapid solidification of this splashed melt results in high Sa values. Beyond the critical PRF, the Sa drops due to a reduction in pulse energy at higher PRFs. Interestingly,1 500 ns show a general reduction in MRR and Sa as PRF increases. This phenomenon was partially observed during the single-line machining J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 12 of 16 J. Manuf. Mater. Process. 2020, 4, 110 12 of 15 As more melt is formed, the impact from subsequent pulses ejects melt across the surface, and rapid solidification of this splashed melt results in high Sa values. Beyond the critical PRF, the Sa drops experimentdue to where a reduction the groove in pulse close-up energy at ehigherffect occurredPRFs. Interestingly, earlier for 500 500 ns show ns which a general could reduction be as ain result of the longerMRR laser and interactionSa as PRF increases. time and This lower phenomenon laser intensity was partially at higher observed PRFs. during the single-line Figuresmachining 12 and experiment 13 show where scanning the groove electron close-up microscope effect occurred (SEM) earlier micrographs for 500 ns of which the surfaces could be processedas a result of the longer laser interaction time and lower laser intensity at higher PRFs. at 4.2 J/mm2 with 280 ns and 500 ns pulse durations, respectively, distinguishing between the surface Figures 12 and 13 show scanning electron microscope (SEM) micrographs of the surfaces morphologyprocessed for 280at 4.2 ns J/mm and2 with 500 ns280 at ns various and 500 PRFs.ns pulse Typical durations, surface respectively, features distinguishing are highlighted, between created by the intensitythe surface of the morphology laser pulse for and280 ns the and mechanisms 500 ns at various of melt PRFs. flow Typical and surface surface tension.features are Wu et al. conductedhighlighted, a similar created experiment by the intensity to investigate of the laser the pulse impact and the of amechanisms change in of PRF melt on flow surface and surface morphology using atension. picosecond Wu et laser al. conducted for machining a similar Cr12MoV experiment cold to investigate work mold the steel impact [5]. of The a change study in highlighted PRF on the impactsurface of heat morphology accumulation using at a higherpicosecond PRFs laser and for alsomachining reported Cr12MoV an improved cold work surface mold steel quality [5]. The at higher study highlighted the impact of heat accumulation at higher PRFs and also reported an improved PRFs whichsurface agrees quality with at higher the PRFs results which presented agrees with here. the results The paper, presented however, here. The did paper, not providehowever, did the typical MRR achievednot provide at suchthe typical high MRR PRFs. achieved at such high PRFs.
100 kHz 150 kHz 200 kHz (a) MRR=11mm3/min (b) MRR=13mm3/min (c) MRR=15mm3/min
Globules
Splash ( 280 ns 200 μm 200 μm 200 μm
(d) MRR=13mm3/min (e) MRR=11mm3/min (f) MRR= 8mm3/min
Coalescence Polishing
200 μm 200 μm 200 μm 250 kHz 300 kHz 350 kHz
2 2 Figure 12.FigureSEM 12. micrographsSEM micrographs of the of the surface surface topology topology atat didifferentfferent PRFs PRFs processed processed at 𝐸𝐷 at ED of A4.2of J/mm 4.2 J/ mm at 280J. ns.Manuf.at 280 Mater. ns. Process. 2020, 4, x FOR PEER REVIEW 13 of 16
Figure 13. ED 2 FigureSEM 13. micrographs SEM micrographs of the of surfacethe surface morphology morphology processed processed at at di differentfferent PRFsPRFs at 𝐸𝐷 A ofof 4.2 4.2 J/mm at 500 ns.J/mm2 at 500 ns.
For the 280 ns sequence (see Figure 12), the good surface quality at low PRF (100 kHz) is due to efficient ejection, whereas the middle frequencies show evidence of unstable and partial ejection. At the highest PRF, the Sa begins to improve again, but this is associated with a less energetic melt resulting from a reduction in pulse energy, as further evidenced by the associated reduction in MRR (see Figure 12f). The coalescence of melt which represents a smoothing effect is more pronounced for 500 ns (see Figure 13f) and therefore would serve as a better parameter for laser polishing. Vadali et al. reported a similar result where longer τ were deemed capable of attenuating longer wavelengths and hence results in better Sa [24]. This was presented in a study they conducted to investigate the influence of τ on the laser polishing of Ti6Al4V. Even though the single-line machining approach did not take into consideration multiple passes, it predicts the parametric influence on MRR and Sa during area engraving. It, therefore, serves as a quick model to shortlist parameters for a multi-pass area engraving.
4. High-Quality Engraving As suggested in Section 3.2.2, an engraving strategy has been adapted using three different
parametric combinations: (i) deep engraving with longer τ at high 𝐸𝐷 , (ii) periodic cleaning passes using shorter τ at low 𝐸𝐷 , and (iii) final polishing using longer τ at higher PRF. Figure 14 depicts two engraved structures: (a) a square of constant depth with MRR of 13 mm3/min providing Sa ~ 1.4 μm and (b) an inverted stepped pyramidal structure using similar engraving parameters in (a) while compensating for the change in depth at each stage during the engraving.
J. Manuf. Mater. Process. 2020, 4, 110 13 of 15
For the 280 ns sequence (see Figure 12), the good surface quality at low PRF (100 kHz) is due to efficient ejection, whereas the middle frequencies show evidence of unstable and partial ejection. At the highest PRF, the Sa begins to improve again, but this is associated with a less energetic melt resulting from a reduction in pulse energy, as further evidenced by the associated reduction in MRR (see Figure 12f). The coalescence of melt which represents a smoothing effect is more pronounced for 500 ns (see Figure 13f) and therefore would serve as a better parameter for laser polishing. Vadali et al. reported a similar result where longer τ were deemed capable of attenuating longer wavelengths and hence results in better Sa [24]. This was presented in a study they conducted to investigate the influence of τ on the laser polishing of Ti6Al4V. Even though the single-line machining approach did not take into consideration multiple passes, it predicts the parametric influence on MRR and Sa during area engraving. It, therefore, serves as a quick model to shortlist parameters for a multi-pass area engraving.
4. High-Quality Engraving As suggested in Section 3.2.2, an engraving strategy has been adapted using three different parametric combinations: (i) deep engraving with longer τ at high EDA, (ii) periodic cleaning passes using shorter τ at low EDA, and (iii) final polishing using longer τ at higher PRF. Figure 14 depicts two 3 engraved structures: (a) a square of constant depth with MRR of 13 mm /min providing Sa ~1.4 µm and (b) an inverted stepped pyramidal structure using similar engraving parameters in (a) while compensatingJ. Manuf. Mater. Process. for the 2020 change, 4, x FOR in PEER depth REVIEW at each stage during the engraving. 14 of 16
1mm
1mm
Figure 14. Morphology of high-quality constant depth area engraving (a) 13 mm3/min MRR at 1.4 µm 2 2 Sa using: Engraving pass at 4.2 J/mm , 380 ns, 175 kHz; Cleaning pass at 150 ns, 350 kHz, 3.5 J/mm and polishing pass at 1.82 J/mm2, 500 ns, 500 kHz using IM and halftone scanning. (b) Inverted stepped Figure 14. Morphology of high-quality constant depth area engraving (a) 13 mm3/min MRR at 1.4 μm pyramid-shaped engraving. Sa using: Engraving pass at 4.2 J/mm2, 380 ns, 175 kHz; Cleaning pass at 150 ns, 350 kHz, 3.5 J/mm2 5. Conclusionsand polishing pass at 1.82 J/mm2, 500 ns, 500 kHz using IM and halftone scanning. (b) Inverted stepped pyramid-shaped engraving. A systematic experimental approach has been used to investigate 100 W nanosecond pulsed fiber laser engraving of 316L grade stainless steel. The study sought to optimize process parameters for 5. Conclusions high-quality area engraving using a simple single-line machining technique while investigating the influenceA systematic of laser beam experimental scanning approach strategies. has A significant been used finding to investigate to emerge 100 from W nanosecond this study is thatpulsed an IMfiber laser laser beam engraving scanning of strategy316L grade reduces stainless undue steel. heat The accumulation study sought and to serves optimize as a process better option parameters to the traditionalfor high-quality SM. We area report engraving that IM using not only a simple provide single-line a lower S amachining, but also increases technique MRR. while Again, investigating a change inthe the influence interlacing of laser distance beam increases scanning MRRstrategies. at considerably A significant lower finding Sa; however, to emerge beyond from this a critical study is value, that thean IM Sa deteriorates.laser beam scanning The study strategy also established reduces undue that aheat combination accumulation of longer and servesτ, higher as aED betterA, and option PRF to the traditional SM. We report that IM not only provide a lower Sa, but also increases MRR. Again, a change in the interlacing distance increases MRR at considerably lower Sa; however, beyond a critical value, the Sa deteriorates. The study also established that a combination of longer τ, higher
𝐸𝐷 , and PRF slightly above the PRF0 produces a high MRR at high Sa. However, at higher PRF, there is a change in the engraving mechanism which initiates a polishing effect. For low Sa and MRR, a
shorter τ at low 𝐸𝐷 is required. Overall, we have shown that high-quality engraving is possible for high power engraving only when IM is coupled with careful parametric combinations. A further study would be a thermal analysis and surface elemental compositional change for both SM and IM. It will also be interesting to investigate the impact of IM on other metals to explore its full potential and finally, consolidate the findings to produce high-quality engraving.
Author Contributions: S.D.D.: Conceptualisation, Methodology, Investigation, Visualisation, Writing-original draft. K.L.W.: Supervision, Writing—review and editing. P.H.: Supervision, Writing—review and editing. A.R.: Supervision, Writing—review and editing. J.G.: Funding acquisition, Resources. R.L.R.: Supervision, Writing— review and editing. D.P.H.: Resources, Supervision, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflicts of interest.
References
1. Lopez, J.; Zaouter, Y.; Torres, R.; Faucon, M.; Hönninger, C.; Georges, P.; Kling, R. Parameters of influence
J. Manuf. Mater. Process. 2020, 4, 110 14 of 15
slightly above the PRF0 produces a high MRR at high Sa. However, at higher PRF, there is a change in the engraving mechanism which initiates a polishing effect. For low Sa and MRR, a shorter τ at low EDA is required. Overall, we have shown that high-quality engraving is possible for high power engraving only when IM is coupled with careful parametric combinations. A further study would be a thermal analysis and surface elemental compositional change for both SM and IM. It will also be interesting to investigate the impact of IM on other metals to explore its full potential and finally, consolidate the findings to produce high-quality engraving.
Author Contributions: S.D.D.: Conceptualisation, Methodology, Investigation, Visualisation, Writing-original draft. K.L.W.: Supervision, Writing—review and editing. P.H.: Supervision, Writing—review and editing. A.R.: Supervision, Writing—review and editing. J.G.: Funding acquisition, Resources. R.L.R.: Supervision, Writing—review and editing. D.P.H.: Resources, Supervision, Writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.
References
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