Process Optimization for 100 W Nanosecond Pulsed Fiber Laser Engraving of 316L Grade Stainless Steel

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.


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][2][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, For the laser machining set-up, an optical fiber delivers the laser beam to a collimator, a Raylase ® galvanometer scan head (RLA-1504 [Y] D2), and then a 160 mm focal length fused silica f-theta lens that focused the laser beam onto the workpiece. The laser and the galvoscanner are synchronized using a SCAPS ® hardware controller and SAMLight 2D ® software. The diameter of the collimated beam is 8 mm, providing a focused spot diameter (2ωo) of 38 μm (calculated at 1/e 2 of its peak intensity) on the workpiece.

Experimental Protocol
The experimental approach is summarized in Figure 2, in which a single line machining experiment is used to down-select a suitable range of laser parameters for testing a square-shaped area engraving. The temporal profiles of selected pulse durations (waveforms) are shown in Figure  1a.

Single Line Machining
Parallel grooves of length 10 mm were machined into the metal workpiece surface, as illustrated in Figure 3a, using the laser process parameters listed in Table 1. The line energy dose ( ) is calculated using the formula where is the pulse energy and is the pulse to pulse distance in the scan direction (see Figure  4). For the laser machining set-up, an optical fiber delivers the laser beam to a collimator, a Raylase ® galvanometer scan head (RLA-1504 [Y] D2), and then a 160 mm focal length fused silica f-theta lens that focused the laser beam onto the workpiece. The laser and the galvoscanner are synchronized using a SCAPS ® hardware controller and SAMLight 2D ® software. The diameter of the collimated beam is 8 mm, providing a focused spot diameter (2ω o ) of 38 µm (calculated at 1/e 2 of its peak intensity) on the workpiece.

Experimental Protocol
The experimental approach is summarized in Figure 2, in which a single line machining experiment is used to down-select a suitable range of laser parameters for testing a square-shaped area engraving. The temporal profiles of selected pulse durations (waveforms) are shown in Figure 1a. For the laser machining set-up, an optical fiber delivers the laser beam to a collimator, a Raylase ® galvanometer scan head (RLA-1504 [Y] D2), and then a 160 mm focal length fused silica f-theta lens that focused the laser beam onto the workpiece. The laser and the galvoscanner are synchronized using a SCAPS ® hardware controller and SAMLight 2D ® software. The diameter of the collimated beam is 8 mm, providing a focused spot diameter (2ωo) of 38 μm (calculated at 1/e 2 of its peak intensity) on the workpiece.

Experimental Protocol
The experimental approach is summarized in Figure 2, in which a single line machining experiment is used to down-select a suitable range of laser parameters for testing a square-shaped area engraving. The temporal profiles of selected pulse durations (waveforms) are shown in Figure  1a.

Single Line Machining
Parallel grooves of length 10 mm were machined into the metal workpiece surface, as illustrated in Figure 3a, using the laser process parameters listed in Table 1. The line energy dose ( ) is calculated using the formula where is the pulse energy and is the pulse to pulse distance in the scan direction (see Figure  4).

Single Line Machining
Parallel grooves of length 10 mm were machined into the metal workpiece surface, as illustrated in Figure 3a, using the laser process parameters listed in Table 1. The line energy dose (ED L ) is calculated using the formula where E p is the pulse energy and ∆S is the pulse to pulse distance in the scan direction (see Figure 4).  Table 1. List of parameters for single-line machining study. The pulse to pulse distance (Δ ) and the pulse overlap factor (Po), which is the fraction of the laser spot diameter that overlaps with the previous pulse on the surface of the material, at given energy doses ( ) for different pulse durations (τ) are provided. All pulse repetition frequencies (PRF) indicated correspond to the PRF0 condition. For this study, the effect of 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 was examined. Each parameter combination was replicated three times and the mean value presented.   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 presented in our previous work [19]. ). In all these scanning modes, the total process time is identical. For image clarity, the hatch distance was chosen at half the laser spot diameter in this illustration. . In all these scanning modes, the total process time is identical. For image clarity, the hatch distance was chosen at half the laser spot diameter in this illustration.
For this study, the effect of ED L and τ on the volume of groove and burr formation was investigated. Using the parameters listed in Table 2, the influence of changing PRF at a constant ED L was examined. Each parameter combination was replicated three times and the mean value presented.

. 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 mm 2 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 S a . The corresponding area energy dose (ED A ) from the down-selected parameters was calculated as follows.
An initial study was conducted to investigate the influence of laser beam scanning strategies on MRR and S a 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 presented in our previous work [19].
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 A was investigated.

Sample Postprocessing and Analysis
The processed samples were cleaned with deionized water in an ultrasonic bath at 25 • C for 10 min to remove loose particles. To avoid possible end effects, only the central section of the grooves at a sampling length (L) of 2.7 mm was measured. A 3D-profile of each groove was obtained using the Alicona surface profilometer. Using a MATLAB™ script, the profile of each machined line was processed (see Figure 3b) and separated into the groove (material removed) (see Figure 3c) and the burr (redeposited material) (see Figure 3d). During the groove analysis, the sample was levelled so that the data above and below the levelled surface were used to calculate the area of burr and groove respectively using Equation (3). The volume of groove and burr are calculated using Equation (4).
x,y z(x, y).dx.dy, After the 3D profiling, each groove was sectioned in the middle, cold mounted, polished, and then examined using a Leica optical microscope (DM6000M) at × 20 magnification (see Figure 5b). For the area machining experiment, a 3D profile was obtained for each parameter combination and, from this, the depth was determined and subsequently used to calculate an overall MRR value (the product of the depth and area of engraving per unit processing time). The arithmetic average of absolute surface roughness (S a ) was obtained using a cut-off wavelength of 284 µm.
"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 was investigated.

Sample Postprocessing and Analysis
The processed samples were cleaned with deionized water in an ultrasonic bath at 25°C for 10 min to remove loose particles. To avoid possible end effects, only the central section of the grooves at a sampling length ( ) of 2.7 mm was measured. A 3D-profile of each groove was obtained using the Alicona surface profilometer. Using a MATLAB script, the profile of each machined line was processed (see Figure 3b) and separated into the groove (material removed) (see Figure 3c) and the burr (redeposited material) (see Figure 3d). During the groove analysis, the sample was levelled so that the data above and below the levelled surface were used to calculate the area of burr and groove respectively using Equation (3). The volume of groove and burr are calculated using Equation (4).
After the 3D profiling, each groove was sectioned in the middle, cold mounted, polished, and then examined using a Leica optical microscope (DM6000M) at × 20 magnification (see Figure 5b). For the area machining experiment, a 3D profile was obtained for each parameter combination and, from this, the depth was determined and subsequently used to calculate an overall MRR value (the product of the depth and area of engraving per unit processing time). The arithmetic average of absolute surface roughness (Sa) was obtained using a cut-off wavelength of 284 μm.

Groove Analysis
In general, four classes of grooves were observed as shown in Figure 5. Class I is representative of closed-up grooves which result from a combination of high (0.28 J/mm) and high Po (99%). Class II is representative of shallow grooves resulting from lower and τ, in this case, 0.05 J/mm at 17 ns. In Class III, there is a pronounced burr formation at the edges of the groove, which suggests an inefficient removal of molten material. In extreme conditions, this can result in a Class I groove. The nature of this type of groove obstructs proper surface profiling (see Class III Figure 5a).

Groove Analysis
In general, four classes of grooves were observed as shown in Figure 5. Class I is representative of closed-up grooves which result from a combination of high ED L (0.28 J/mm) and high Po (99%). Class II is representative of shallow grooves resulting from lower ED L and τ, in this case, 0.05 J/mm at 17 ns. In Class III, there is a pronounced burr formation at the edges of the groove, which suggests an inefficient removal of molten material. In extreme conditions, this can result in a Class I groove. The nature of this type of groove obstructs proper surface profiling (see Class III Figure 5a).
Class IV represents a stable groove with a well-defined groove and burr which represents a condition suitable for analysis. Based on the above classifications, the 17 ns pulses with an ED L of 0.28 J/mm and any ED L below 0.05 J/mm were eliminated from the parametric analysis in the section below. The general classification of grooves as elaborated above corroborates results presented by (Leone et al., 2018). Their study investigated parametric influence during laser marking application of Inconel 718 using a 30 W Q-switched Yb: YAG fiber laser.

Influence of Change in ED L and τ on Groove and Burr Formation
As expected, an increase in ED L increases both the volume of the groove and that of the burr, as shown in Figure 6. When a keyhole is produced as a result of high ED L , the depth increases such that subsequent ejection due to the recoil pressure is compromised. This leads to an ineffective ejection and excessive burr formation, as shown in the highlighted region in Figure 6c, in some cases collapsing back into the machined region (see the results for τ = 60 ns at ED L = 0.28 J/mm). The evidence presented does not suggest that material removal is solely through melt ejection, in fact, some material is removed through vaporization; however, the method used cannot quantify this effect. Class IV represents a stable groove with a well-defined groove and burr which represents a condition suitable for analysis. Based on the above classifications, the 17 ns pulses with an of 0.28 J/mm and any below 0.05 J/mm were eliminated from the parametric analysis in the section below. The general classification of grooves as elaborated above corroborates results presented by (Leone et al., 2018). Their study investigated parametric influence during laser marking application of Inconel 718 using a 30 W Q-switched Yb: YAG fiber laser.

Influence of Change in and τ on Groove and Burr Formation
As expected, an increase in increases both the volume of the groove and that of the burr, as shown in Figure 6. When a keyhole is produced as a result of high , the depth increases such that subsequent ejection due to the recoil pressure is compromised. This leads to an ineffective ejection and excessive burr formation, as shown in the highlighted region in Figure 6c, in some cases collapsing back into the machined region (see the results for τ = 60 ns at = 0.28 J/mm). The evidence presented does not suggest that material removal is solely through melt ejection, in fact, some material is removed through vaporization; however, the method used cannot quantify this effect. As shown in Figure 6a, longer τ produces larger grooves, as is particularly evident at 380 ns and 500 ns for the highest . 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 is attributable to the high pulse overlap as detailed in Table 1. As shown in Figure 6a, longer τ produces larger grooves, as is particularly evident at 380 ns and 500 ns for the highest ED L . 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 L is attributable to the high pulse overlap as detailed in Table 1. From Figure 6b,c, although longer τ typically leads to a higher burr volume, a shorter τ with high pulse overlap inhibits rapid heat dissipation resulting in heat accumulation, which increases the burr volume. From the above observations, it seems likely that a longer τ will give a high MRR with higher S a 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 L may as well result in higher MRR, but this might be at the expense of surface quality.

Effect of Change in PRF and τ on the Volume of Groove and Burr Formation
Given the requirement for high MRR, τs of 280 ns, 380 ns, and 500 ns were selected for further investigation, to evaluate the impact of PRF. Here, the ED L was kept constant at 0.09 J/mm as this provided a good combination of high MRR and machining quality. Results in Figure 7 indicate that an increase in PRF increases the volume of both groove and burr. However, a critical point is reached where the groove collapses. From Figure 6b,c, although longer τ typically leads to a higher burr volume, a shorter τ with high pulse overlap inhibits rapid heat dissipation resulting in heat accumulation, which increases the burr volume. From the above observations, it seems likely that a longer τ will give a high MRR with higher Sa at higher during area engraving due to the high melt and burr formation. Shorter τ, such as 60 ns and 150 ns, with high may as well result in higher MRR, but this might be at the expense of surface quality.

Effect of Change in PRF and τ on the Volume of Groove and Burr Formation
Given the requirement for high MRR, τs of 280 ns, 380 ns, and 500 ns were selected for further investigation, to evaluate the impact of PRF. Here, the was kept constant at 0.09 J/mm as this provided a good combination of high MRR and machining quality. Results in Figure 7 indicate that an increase in PRF increases the volume of both groove and burr. However, a critical point is reached where the groove collapses. This happens at a PRF above 250 kHz at 500 ns, 300 kHz at 380 ns, and 350 kHz at 280 ns (see 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 or higher PRFs is assumed to result from the thermal accumulation and the consequent larger melt zone. However, the reduction in 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  This happens at a PRF above 250 kHz at 500 ns, 300 kHz at 380 ns, and 350 kHz at 280 ns (see 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 ED L or higher PRFs is assumed to result from the thermal accumulation and the consequent larger melt zone. However, the reduction in E p 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 longer τ means more melt, however, due to the reduction in E p at high PRF (less energetic pulse), the melt collapses (see Figure 7c-500 ns at 300 kHz) and therefore lowers the critical PRF.
In general, it can be hypothesized that higher PRF and higher ED L would cause high MRR, but the increase in the volume of burr could imply a higher S a 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 Tables 1 and 2 were down-selected by excluding pulse duration 17 ns, ED L > 0.09 J/mm, ED L < 0.04 J/mm, and PRF of 400 kHz. The corresponding ED A 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.

Effect of Different Laser Beam Scanning Strategies
In Figure 8, the SM Figure 8b is compared with different IMs (Figure 8c-g) for a mid-range ED A of 4.2 J/mm 2 (ED L = 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 Figure 8a). longer τ means more melt, however, due to the reduction in at high PRF (less energetic pulse), the melt collapses (see Figure 7c-500 ns at 300 kHz) and therefore lowers the critical PRF.
In general, it can be hypothesized that higher PRF and higher 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 Tables 1 and 2 were down-selected by excluding pulse duration 17 ns, > 0.09 J/mm, < 0.04 J/mm, and PRF of 400 kHz. The corresponding 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.

Effect of Different Laser Beam Scanning Strategies
In Figure 8, the SM Figure 8b is compared with different IMs (Figure 8c-g) for a mid-range of 4.2 J/mm 2 ( = 0.06 J/mm and = 13.3 μm) processed at 280 ns. A blackened surface and higher burr formation at the edges were observed with SM (see Figure 8a). The Sa is reduced by a factor of 3 and the MRR is increased by 20% with IM (see Figure 8c-f). Figure 9 presents the cross-sectional differences between IM and SM at = 5.3 J/mm 2 . At higher , IM significantly reduces the Sa by 84% at a slightly higher MRR. The time delay between adjacent scan for IM helps reduce undue heat accumulation in the workpiece. The S a is reduced by a factor of 3 and the MRR is increased by 20% with IM (see Figure 8c-f). Figure 9 presents the cross-sectional differences between IM and SM at ED A = 5.3 J/mm 2 . At higher ED A , IM significantly reduces the S a by 84% at a slightly higher MRR. The time delay between adjacent scan for IM helps reduce undue heat accumulation in the workpiece. This implies a uniform thermal gradient around the processed area which could cause a more efficient cooling. With SM, the heat is built up to produce more melt. The ineffective removal of this melt ends up compromising MRR and Sa. Figure 9 also illustrates the presence of undercut at the corners for both SM and IM, which is due to the lack of synchronization between the laser and the scanner system. This feature compromises the engraving quality; however, with appropriate scan delays this effect can be rectified.
For glass, the authors of [19,22] suggest that the IM increases laser coupling efficiency as a result of the smaller angle of incidence of the laser beam. This results from the perpendicular beam interaction to the surface in the case of IM as opposed to the SM, where the beam is incident on the wall of the previously machined groove. This could as well contribute to the differences observed for both scanning strategies. Although a change in interlacing distance increases MRR, at ∆IL = 79.8 μm (see Figure 8g) the surface quality is deteriorated and characterized by massive structures (solidified melt redeposition) resulting from ineffective melt ejection. Obviously, IM offers a better surface quality in comparison to SM, hence all results presented below in the next sections were conducted using IM at an optimized ∆IL of 39.9 μm (see Figure 8d) due to its high MRR and low Sa.

Influence of Change in and τ on the MRR and Sa
As shown in Figure 10, an increase in and τ leads to a general increase in both MRR and Sa, as suggested in Section 3.1.2. From Figure 10b, there is a critical at 5.3 J/mm 2 for 60 ns, 150 ns, and 500 ns (see the dashed line) where the Sa starts to reduce for further increase in , which is 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 high Sa values mainly due to the high pulse overlaps employed in this study (see Table 1). An increase in scan speed could offset the issues of high overlap, but this would mean a lower MRR which is not consistent with the overall requirement for high throughput engraving. This highlights the trade-off between high-throughput and high-quality engraving, and, therefore, to maintain a balance, the area highlighted in green in Figure 10 with MRR in the range of 5 to 16 mm 3 /min and corresponding Sa of 2.6-13 μm is an area of focus for high-quality engraving. This implies a uniform thermal gradient around the processed area which could cause a more efficient cooling. With SM, the heat is built up to produce more melt. The ineffective removal of this melt ends up compromising MRR and S a . Figure 9 also illustrates the presence of undercut at the corners for both SM and IM, which is due to the lack of synchronization between the laser and the scanner system. This feature compromises the engraving quality; however, with appropriate scan delays this effect can be rectified.
For glass, the authors of [19,22] suggest that the IM increases laser coupling efficiency as a result of the smaller angle of incidence of the laser beam. This results from the perpendicular beam interaction to the surface in the case of IM as opposed to the SM, where the beam is incident on the wall of the previously machined groove. This could as well contribute to the differences observed for both scanning strategies. Although a change in interlacing distance increases MRR, at ∆IL = 79.8 µm (see Figure 8g) the surface quality is deteriorated and characterized by massive structures (solidified melt redeposition) resulting from ineffective melt ejection. Obviously, IM offers a better surface quality in comparison to SM, hence all results presented below in the next sections were conducted using IM at an optimized ∆IL of 39.9 µm (see Figure 8d) due to its high MRR and low S a .

Influence of Change in ED A and τ on the MRR and S a
As shown in Figure 10, an increase in ED A and τ leads to a general increase in both MRR and S a , as suggested in Section 3.1.2. From Figure 10b, there is a critical ED A at 5.3 J/mm 2 for 60 ns, 150 ns, and 500 ns (see the dashed line) where the S a starts to reduce for further increase in ED A , which is 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 high S a values mainly due to the high pulse overlaps employed in this study (see Table 1). An increase in scan speed could offset the issues of high overlap, but this would mean a lower MRR which is not consistent with the overall requirement for high throughput engraving. This highlights the trade-off between high-throughput and high-quality engraving, and, therefore, to maintain a balance, the area highlighted in green in Figure 10 with MRR in the range of 5 to 16 mm 3 /min and corresponding S a of 2.6-13 µm is an area of focus for high-quality engraving. Clearly, to achieve Sa below 2.6 μm at high MRR, it is recommended to use a sequential combination of deep engraving with longer pulse durations (>280 ns) and periodic cleaning with shorter pulses (<280 ns) at lower (<2.7 J/mm 2 ). The increase in MRR as a function of τ agrees with previous work on the use of 20 W nanosecond pulsed fiber laser engraving of stainless steel by Manninen et al [23].

Influence of Change in PRF and τ on the MRR and Sa
Considering at 4.2 J/mm 2 for longer τ (>150 ns), the influence of PRF was investigated. As demonstrated in Figure 11a, increasing PRF results in an initial increase in MRR up to a critical PRF value (200 kHz for 380 ns and 280 ns) after which the MRR starts to decline. As discussed earlier, thermal accumulation plays a key role in the increase in MRR at an increased PRF, but, as the MRR increases, so does the surface roughness, as seen in Figure 11b.  Clearly, to achieve S a below 2.6 µm at high MRR, it is recommended to use a sequential combination of deep engraving with longer pulse durations (>280 ns) and periodic cleaning with shorter pulses (<280 ns) at lower ED A (<2.7 J/mm 2 ). The increase in MRR as a function of τ agrees with previous work on the use of 20 W nanosecond pulsed fiber laser engraving of stainless steel by Manninen et al [23].

Influence of Change in PRF and τ on the MRR and S a
Considering ED A at 4.2 J/mm 2 for longer τ (>150 ns), the influence of PRF was investigated. As demonstrated in Figure 11a, increasing PRF results in an initial increase in MRR up to a critical PRF value (200 kHz for 380 ns and 280 ns) after which the MRR starts to decline. As discussed earlier, thermal accumulation plays a key role in the increase in MRR at an increased PRF, but, as the MRR increases, so does the surface roughness, as seen in Figure 11b. Clearly, to achieve Sa below 2.6 μm at high MRR, it is recommended to use a sequential combination of deep engraving with longer pulse durations (>280 ns) and periodic cleaning with shorter pulses (<280 ns) at lower (<2.7 J/mm 2 ). The increase in MRR as a function of τ agrees with previous work on the use of 20 W nanosecond pulsed fiber laser engraving of stainless steel by Manninen et al [23].

Influence of Change in PRF and τ on the MRR and Sa
Considering at 4.2 J/mm 2 for longer τ (>150 ns), the influence of PRF was investigated. As demonstrated in Figure 11a, increasing PRF results in an initial increase in MRR up to a critical PRF value (200 kHz for 380 ns and 280 ns) after which the MRR starts to decline. As discussed earlier, thermal accumulation plays a key role in the increase in MRR at an increased PRF, but, as the MRR increases, so does the surface roughness, as seen in Figure 11b.  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 S a values. Beyond the critical PRF, the S a drops due to a reduction in pulse energy at higher PRFs. Interestingly, 500 ns show a general reduction in MRR and S a as PRF increases. This phenomenon was partially observed during the single-line machining experiment where the groove close-up effect occurred earlier for 500 ns which could be as a result of the longer laser interaction time and lower laser intensity at higher PRFs. Figures 12 and 13 show scanning electron microscope (SEM) micrographs of the surfaces processed at 4.2 J/mm 2 with 280 ns and 500 ns pulse durations, respectively, distinguishing between the surface morphology for 280 ns and 500 ns at various PRFs. Typical surface features are highlighted, created by the intensity of the laser pulse and the mechanisms of melt flow and surface tension. Wu et al. conducted a similar experiment to investigate the impact of a change in PRF on surface morphology using a picosecond laser for machining Cr12MoV cold work mold steel [5]. The study highlighted the impact of heat accumulation at higher PRFs and also reported an improved surface quality at higher PRFs which agrees with the results presented here. The paper, however, did not provide the typical MRR achieved at such high PRFs. 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, 500 ns show a general reduction in MRR and Sa as PRF increases. This phenomenon was partially observed during the single-line machining experiment where the groove close-up effect occurred earlier for 500 ns which could be as a result of the longer laser interaction time and lower laser intensity at higher PRFs. Figures 12 and 13 show scanning electron microscope (SEM) micrographs of the surfaces processed at 4.2 J/mm 2 with 280 ns and 500 ns pulse durations, respectively, distinguishing between the surface morphology for 280 ns and 500 ns at various PRFs. Typical surface features are highlighted, created by the intensity of the laser pulse and the mechanisms of melt flow and surface tension. Wu et al. conducted a similar experiment to investigate the impact of a change in PRF on surface morphology using a picosecond laser for machining Cr12MoV cold work mold steel [5]. The study highlighted the impact of heat accumulation at higher PRFs and also reported an improved surface quality at higher PRFs which agrees with the results presented here. The paper, however, did not provide the typical MRR achieved at such high PRFs.  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 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 S a 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 S a [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 S a during area engraving. It, therefore, serves as a quick model to shortlist parameters for a multi-pass area engraving.

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 A , (ii) periodic cleaning passes using shorter τ at low ED A , and (iii) final polishing using longer τ at higher PRF. Figure 14

Conclusions
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 high-quality area engraving using a simple single-line machining technique while investigating the influence of laser beam scanning strategies. A significant finding to emerge from this study is that an IM laser beam scanning strategy reduces undue heat accumulation and serves as a better option 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 , 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

Conclusions
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 high-quality area engraving using a simple single-line machining technique while investigating the influence of laser beam scanning strategies. A significant finding to emerge from this study is that an IM laser beam scanning strategy reduces undue heat accumulation and serves as a better option to the traditional SM. We report that IM not only provide a lower S a , but also increases MRR. Again, a change in the interlacing distance increases MRR at considerably lower S a ; however, beyond a critical value, the S a deteriorates. The study also established that a combination of longer τ, higher ED A , and PRF slightly above the PRF 0 produces a high MRR at high S a . However, at higher PRF, there is a change in the engraving mechanism which initiates a polishing effect. For low S a and MRR, a shorter τ at low ED A 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.