Influence of Ball Burnishing Path Strategy on Surface Integrity and Performance of Laser-Cladded Inconel 718 Alloys

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study evaluated the effect of ball burnishing path on the surface properties of laser-clad Inconel 718 alloy. If you modify the content below, it will become a better quality paper.

1. I think we need a picture comparing the surface of the laser clad specimen with the surface after ball burnishing in the experimental method.

2. What are the full names of the roughness parameters Sa, Sz, Ra, and Rz shown in Figure 6? It doesn't seem to be mentioned in the previous content. And there seems to be a problem with line 257. Please check and fix it.

3. Do the results in Figure 5(b) and Figure 7(b) match? If the results are different, what causes them? Do these results affect corrosion characteristics or mechanical properties?

4. The EBSD-based KAM maps indicate the degree of strain through dislocation density. The results suggest that the highest strain was observed in the BB-Longitudinal specimen. While similar to the results in Figure 7(b), what is the correlation with the results in Figure 5? 

5. What causes the reduced corrosion sensitivity under BB-Longitudinal conditions? Is the increased deformation under BB-Longitudinal conditions related? Generally, corrosion characteristics can be affected by various factors, such as chemical composition, grain size, and precipitate formation. Can you explain the mechanism?

Author Response

Reviewer 1:

This study evaluated the effect of ball burnishing path on the surface properties of laser-clad Inconel 718 alloy. If you modify the content below, it will become a better-quality paper.

1. I think we need a picture comparing the surface of the laser clad specimen with the surface after ball burnishing in the experimental method.

Reply: We thank the reviewer for the helpful suggestion. A schematic figure has been added to the experimental section (Figure 4) to illustrate the typical surface profiles of Inconel cladding before and after ball burnishing, highlighting the morphological changes and toolpath-induced deformation.

2. What are the full names of the roughness parameters Sa, Sz, Ra, and Rz shown in Figure 6? It doesn't seem to be mentioned in the previous content. And there seems to be a problem with line 257. Please check and fix it.

Reply: We thank the reviewer for the comment. The full names of the roughness parameters—Ra (Arithmetic Mean Roughness), Rz (Maximum Height of Profile), Sa (Arithmetic Mean Height), and Sz (Maximum Height of Surface)—were defined in Section 2.3 (lines 189–190). However, we have now clarified this further to ensure visibility and avoid any confusion. Additionally, the issue in line 257 has been corrected in the revised manuscript.

3. Do the results in Figure 5(b) and Figure 7(b) match? If the results are different, what causes them? Do these results affect corrosion characteristics or mechanical properties?

Reply: We thank the reviewer for the insightful observation. Figure 5(b) presents the 3D surface topography of the BB-Longitudinal sample obtained via confocal microscopy, showing a pronounced undulating texture with regularly spaced peaks and valleys aligned with the laser scan and burnishing direction. Figure 7(b), on the other hand, shows the SEM micrograph of the same surface, which appears significantly smoother and free of grinding marks.

This apparent discrepancy is due to the nature of the ball burnishing process. While burnishing smooths out the micro-grooves formed by abrasive grinding, it simultaneously induces broader surface undulations through plastic deformation. These undulations increase roughness parameters (Sa, Sz, Ra, Rz), even though the SEM image suggests a smoother surface. In essence, the burnishing action compacts the ploughed grooves, making the surface appear smoother in SEM, but introduces micro-waves that are captured in the confocal roughness measurements.

Importantly, these undulations are not solely determined by the burnishing toolpath. If that were the case, we would expect the longitudinal and transverse profiles in Figure 5 to be completely opposite. Instead, the surface modulation is strongly influenced by the microstructural features of the laser-cladded layer—particularly the overlap zones between adjacent tracks. With the applied ~50% overlap (~2–2.4 mm), these regions undergo reheating and repeated thermal cycling, resulting in localized softening. When burnishing is aligned with the cladding direction, these softer zones deform more readily, leading to wave-like patterns rather than uniform flattening.

Regarding corrosion characteristics, increased surface undulation could theoretically raise the effective surface area, potentially influencing the measured corrosion current density (icorr), especially since the exposed area in the electrochemical cell is assumed to be flat for calculation purposes. Our results showed only a comparable slightly higher icorr for the BB-Longitudinal sample, which may indicate a minor overestimation due to this effect. Nevertheless, the most notable difference observed in the corrosion tests for the BB-Longitudinal sample was a significantly higher corrosion potential (Ecorr), suggesting improved corrosion resistance. This enhancement may be attributed to the greater degree of plastic deformation and the associated compressive residual stresses induced by the alignment of the burnishing path with the laser cladding direction.

That said, supporting data from KAM analysis and hardness mapping are not fully conclusive. While BB-Longitudinal showed the highest Ecorr, it also exhibited a relatively high standard deviation, indicating non-uniform deformation—likely due to the alternating hard and soft zones created by the cladding overlap. This reinforces the need for further investigation to fully understand the relationship between surface modulation, deformation uniformity, and corrosion behavior.

We thank again the reviewer for the comment and have incorporated these discussions into the Results and Conclusions sections of the manuscript.

4. The EBSD-based KAM maps indicate the degree of strain through dislocation density. The results suggest that the highest strain was observed in the BB-Longitudinal specimen. While similar to the results in Figure 7(b), what is the correlation with the results in Figure 5?

Reply: We thank the reviewer for the valuable question. The correlation between the KAM maps (Figure 8) and the surface topography (Figure 5) lies in the deformation pattern induced by the ball burnishing process. The BB-Longitudinal specimen, which exhibited the highest KAM intensity, also presented the most pronounced undulated topography in Figure 5(b). This indicates that the regions with higher dislocation density correspond to the areas where plastic deformation was concentrated, forming periodic surface waves along the burnishing path. The topographic undulations observed by confocal microscopy are thus the macroscopic manifestation of the localized plastic strain revealed in the KAM maps. Furthermore, these deformation bands coincide with the overlap zones of the laser-cladded layer, which, due to repeated thermal cycles, display slightly reduced hardness and are more prone to plastic flow under the applied burnishing pressure. Consequently, the high strain regions detected by EBSD are directly related to these softened tracks. This correlation reinforces the interpretation that the longitudinal burnishing strategy promotes more effective sub-surface strain accumulation and residual stress generation along the direction of laser solidification.

5. What causes the reduced corrosion sensitivity under BB-Longitudinal conditions? Is the increased deformation under BB-Longitudinal conditions related? Generally, corrosion characteristics can be affected by various factors, such as chemical composition, grain size, and precipitate formation. Can you explain the mechanism?

Reply: We thank the reviewer for this important question. The improved corrosion potential (Ecorr) observed under BB-Longitudinal conditions is primarily attributed to the compressive residual stresses and localized plastic deformation induced by the alignment of the burnishing path with the laser cladding direction. This alignment enhances strain accumulation in the softened overlap zones of the cladding, promoting deeper and more uniform compressive stress fields.

Compressive residual stresses are known to reduce the thermodynamic driving force for corrosion initiation by stabilizing the passive oxide layer and shifting the open circuit potential toward more noble values, as supported by McMahon (2021), Sequera et al. (2014) and López de Lacalle et al. (2007). However, this effect is primarily thermodynamic in nature and does not necessarily influence the kinetics of corrosion processes. This explains why no significant improvements were observed in corrosion current density (icorr) or pitting potential (Epit), which are more closely related to passivation behavior and localized corrosion resistance.

The relatively high standard deviation in Ecorr for the BB-Longitudinal sample suggests non-uniform deformation, likely due to the heterogeneous response of the laser-cladded layer—particularly in the overlap zones that undergo repeated thermal cycling and exhibit lower hardness. These regions are more susceptible to plastic flow, leading to localized variations in residual stress and strain distribution. This non-uniformity may partially offset the beneficial effects of compressive stress in some areas, reinforcing the need for further investigation into the spatial distribution of deformation and its electrochemical implications.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper the authors studied the effect of ball burnishing on surface roughness, hardness, wear, and corrosion resistance of Inconel 718 laser claddings. The work is comprehensive as several different ball burnishing paths are studied. The materials are characterized by several different experimental methods.  The results obtained are interesting and publishable. The paper is acceptable subject to revision. The following comments should be considered:

1.You should include either a visible scale bar or specify the sample dimensions in Figs. 1b and 3.

2.The laser claddings were ground before the ball burnishing process; see Fig. 3a. However, the grinding details are not provided. They should be included.

3.The scale bars in Figs. 8a-d are illegible. Increase the font size.

4.Fig. 11 can be removed. Identical information is provided in Fig. 12.

5.The standard deviations of the corrosion potentials are very high, up to 88 mV. See Table 2. As such, it is very difficult to compare the specimens. Have you measured an open-circuit potential before the polarization? If so, it should be included in Table 2 for the sake of comparison.

6.The authors conclude that pitting corrosion was occurring as the pitting potentials have been determined. See Table 2. However, the surface images of the post-corroded specimens have not been provided. You should provide some representative SEM images of the post-corroded laser claddings to confirm the pit formation.

7.It is mentioned that polarization resistance was also measured. See lines 226-228. However, the results are not presented. They should be included in Table 2 for the sake of comparison.

Author Response

Reviewer 2:

In this paper the authors studied the effect of ball burnishing on surface roughness, hardness, wear, and corrosion resistance of Inconel 718 laser claddings. The work is comprehensive as several different ball burnishing paths are studied. The materials are characterized by several different experimental methods.  The results obtained are interesting and publishable. The paper is acceptable subject to revision. The following comments should be considered:

1.You should include either a visible scale bar or specify the sample dimensions in Figs. 1b and 3.

Reply: We thank the reviewer for the helpful suggestion. The sample dimensions (25 × 15 mm) have now been added to Figure 1(b) and dimensions (25 × 15 × 3 clad./4.5 subst. mm) to Figure 3(a) to clarify the scale of the cladded coupons.

2.The laser claddings were ground before the ball burnishing process; see Fig. 3a. However, the grinding details are not provided. They should be included.

Reply: We thank the reviewer for the observation. The grinding procedure is described in detail in the manuscript (lines 129–134), including the material removal depth, grinding equipment, wheel type, speed, and final finishing steps. We hope this clarifies the process.

3.The scale bars in Figs. 8a-d are illegible. Increase the font size.

Reply: We thank the reviewer for the observation. The scale bars in Figures 8a–d have been updated with increased font size for improved legibility.

4. Fig. 11 can be removed. Identical information is provided in Fig. 12.

Reply: We thank the reviewer for this helpful observation. After reevaluating the manuscript, we agree that Figures 12 and 13 convey overlapping information regarding the cross-sectional hardness distribution. Therefore, Figure 12 has been removed to streamline the presentation and avoid redundancy. The remaining figure (now renumbered as Figure 12) clearly illustrates the comparative hardness profiles between the BB-Longitudinal and BB-Transverse samples, which sufficiently support the discussion in Section 3.4. Corresponding text references have been updated accordingly.

5. The standard deviations of the corrosion potentials are very high, up to 88 mV. See Table 2. As such, it is very difficult to compare the specimens. Have you measured an open-circuit potential before the polarization? If so, it should be included in Table 2 for the sake of comparison.

Reply: We appreciate the reviewer’s observation regarding the high standard deviations in corrosion potential (Ecorr), particularly for the BB-Longitudinal condition. Prior to each polarization test, a 45-minute stabilization period was implemented to ensure a steady open-circuit potential (OCP) for all specimens. These OCP values were consistent across repeated trials and confirmed the electrochemical stability of the samples at the start of each test.

The observed variability in Ecorr is attributed to the non-uniform plastic deformation introduced by the ball burnishing process, especially in single-directional strategies such as BB-Longitudinal and BB-Transverse. These strategies interact differently with the underlying laser-cladded microstructure, which includes overlapping tracks and reheated zones with varying hardness and grain characteristics. Such heterogeneity can lead to localized differences in residual stress and strain, thereby influencing the electrochemical response (also refer to Reviewer 1, comment 5).

Notably, the BB-Crosshatch condition exhibited a standard deviation similar to the ground sample, suggesting that multidirectional burnishing may help neutralize deformation anisotropy and reduce variability. We will clarify this interpretation in the revised manuscript and expand the discussion to highlight the influence of cladding-induced microstructural heterogeneity on corrosion behavior.

6. The authors conclude that pitting corrosion was occurring as the pitting potentials have been determined. See Table 2. However, the surface images of the post-corroded specimens have not been provided. You should provide some representative SEM images of the post-corroded laser claddings to confirm the pit formation.

Reply: We thank the reviewer for raising the point. We have taken the optical microscopic images of the corroded surfaces for all four-surface treatment conditions and included Fig.17 in the revised draft. The evidence of passivation layer and pitting observed was discussed as well (see Line 621 – 628, page 22).

7.It is mentioned that polarization resistance was also measured. See lines 226-228. However, the results are not presented. They should be included in Table 2 for the sake of comparison.

Reply: We thank the reviewer for highlighting the omission of polarization resistance (Rp) values. Rp was measured as part of our standard electrochemical testing protocol and has now been retrieved from our records. These values will be included in the revised Table 2 for completeness and comparison.

While Rp is a useful indicator of general corrosion resistance, we note that in materials with complex surface morphologies—such as laser-cladded and ball-burnished Inconel 718—Rp can be influenced by localized variations in surface roughness and passive film stability. Therefore, we have prioritized Tafel parameters (Ecorr, icorr, and Epit) for interpreting corrosion behavior. Nonetheless, the Rp values are consistent with expectations for passivated nickel-based alloys and support the trends observed in the Tafel analysis.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors answered most of my previous comments. The paper has been improved. It is acceptable subject to minor revision. The following comments should be considered:

1.I advise you to include the stabilized OCP values measured over the 45-minute stabilization period in Table 2. They should be included for the sake of comparison with E_corr and E_pitt.

2.The dimension of the polarization resistance should be MΩ.cm². Correct it in Table 2.

Author Response

Review round 2:

The authors answered most of my previous comments. The paper has been improved. It is acceptable subject to minor revision. The following comments should be considered:

1.I advise you to include the stabilized OCP values measured over the 45-minute stabilization period in Table They should be included for the sake of comparison with E_corr and E_pitt.

Reply: We thank the reviewer for raising this point. We have updated Table 2 by including OCP values for four surface treatment conditions. Further, we have added the following comments in the revised manuscript to discuss the relationship between OCP and E_corr and I_corr values (see last para of Section 3.4.3).

“It is also worth noting the stabilized OCP values (after 45 minutes of exposure) provide insight into the initial electrochemical stability of each surface condition. The BB-Longitudinal sample exhibited the most positive OCP (+14 mV vs. SCE), suggesting a relatively stable passive surface prior to external perturbation. In contrast, the ground and BB-Transverse samples showed more negative OCP values (-61 mV and -77 mV vs. SCE, respectively), indicating a higher initial tendency toward corrosion. These OCP trends are generally consistent with the E_corr values obtained from Tafel analysis, although slight deviations may arise due to surface changes during the polarization sweep.”

2. The dimension of the polarization resistance should be MΩ.cm². Correct it in Table 2.

Reply: We have corrected the unit of polarization resistance as MΩ.cm² in Table 2.