The raw 420 stainless steel plate was formed by ferrite and spheroidised chromium carbides (Fig. 2), characteristic of an annealed state.
Figure 3 shows the SEM images of the textured 420 stainless steel produced with different processing parameters. The quality of the textures was evaluated considering two factors: continuity of the machined grooves and dimension of cavities produced. Fourteen different combinations of processing parameters were found to give rise to continuous textures (marked in red in Fig. 3). Another fourteen conditions (marked in yellow) corresponding to low laser powers and high scanning speed did not reveal any visible cavities using SEM cross-section analysis, probably due to the accumulation of material at the site where the supposed laser ablation occurs or lack of removal capacity (low laser energy fluence) [16, 17].
The definition and quality of the grooves depends on the processing parameters. For low laser powers, low scanning speeds and a high number of passes and line spacings are required for a suitable ablation process. Low laser powers (1 and 16%) combined with high scan speeds (2,000 and 5,000 mm/s), and line spacings of 40 and 50 µm, led to low-quality and non-uniform textures, characterised by discrete machined areas (Fig. 4a). This phenomenon is due to the low laser energy fluence obtained for these parameter combinations. Furthermore, for low laser power (1%) and mainly for a low number of passes, it is possible to observe a continuous action of the laser, but non-machined areas are also observed (Fig. 4b). Average power laser values of 16 and 64%, scanning speeds up to 2,000 mm/s, 8 passes or more and line spacings of 40 and 50 µm gave rise to the largest number of good quality grooves (Fig. 4c). In fact, the definition of the machined lines increased as the line spacing increased, because of the reduced interaction between successive passes of the laser.
For the maximum laser power (100%), high scanning speeds and greater line spacings are required to produce suitable textures. The number of passes plays an important role in this case: for n = 1, the tracks are shallow whilst for n > 8 an excessive remelting of the base material occurs. The high energy leads to an exaggerated increase in the surface temperature, which causes the material around the area being irradiated by the laser to melt, therefore impeding its ablation. This phenomenon was also detected for high laser powers and low scan speeds and leads to some material accumulating inside the grooves. This is particularly true for a high number of passes (Fig. 5).
In fact, the amount of laser energy absorved by the material decreases significantly throughout the thickness and has a great influence on the surface outcome. Therefore, if the energy is high enough the material is ablated, and it creates a textured surface. This may occur when combining medium to high laser power with a medium scan speed. On the other hand, a remelted surface occurs when the amount of energy is insufficient for ablation or is too high and causes the material to overheat, which occurs when high laser powers and low scan speeds are used.
The morphology of the abovementioned fourteen promising textures was analysed by 3D profilometry (Fig. 6). The corresponding width and depth values were determined in the middle of the textured grooves and are presented in Figure 7.
From Figures 6 and 7 it is possible to conclude that: (i) the groove width grows with the increase in the number of passes for medium and high laser powers and scanning speeds (16-64% and 500-2,000 mm/s, respectively). For low laser powers (1%) and scanning speeds (100 mm/s), increasing the number of passes is no longer beneficial to increase the width. Regarding the depth, as expected, it tends to increase with the number of passes; (ii) the increase in laser power has a significant impact on width and depth (cases analysed: P64 s2,000 n8 l50 and P100 s2,000 n8 l50). The percentage difference verified in the case of the width is 20% while in the case of the depth it is 16%; (iii) in the case of scanning speed, two cases could be analysed: the use of low scanning speeds (100 and 500 mm/s) and the use of high scanning speeds (2,000 and 5,000 mm/s). In the former, the increase in scanning speed led to a sharp decrease in width, while in the latter the variation in width is inverse. On the other hand, in the case of depth, a low scanning speed has a no significant impact while a high scanning speed increases this value. Therefore, for low laser powers (16%) and scanning speeds (100 and 500 mm/s), increasing the scanning speed has a strong negative impact on width, whereas for high laser powers (100%) and scanning speeds (2,000 and 5,000 mm/s), increasing the scanning speed has a strong positive impact on width; (iv) The line spacing proved not to be a determinant factor regarding the width and depth of the textured grooves for the parameters studied.
(P = laser power in %, s = scanning speed in mm/s, n = number of passes, l = line spacing in µm)
Figure 8 shows the SEM images of the cross-section of the grooves. For low laser powers (1 and 16%), the grooves produced present a waveform, with no noticeable differences when the other parameters are changed (scan speed, number of passes, and line spacing). On the other hand, when the laser power is increased (64 and 100%) the cavity has a more defined shape. In this case, the increase in the number of passes led to the formation of a cavity with a more funneled shape. Therefore, it is possible to conclude that the width, depth, and shape of the cavities are influenced not only by the laser energy fluence but also by each individual process parameter. For the same laser energy fluence values, the change in laser power and scanning speed was shown to affect these cavities’ characteristics significantly (width, depth, and shape). Comparing the results obtained by 3D profilometry and SEM (Figs. 7 and 8, respectively), a discrepancy in width and, mainly, depth of the grooves is observed. This can be explained by Figure 9 which shows that the depth of the cavity along the groove is not constant, it is greater at the beginning of the ablation process. This is due to the excessive increase in the surface temperature and with the formation of a great amount of the liquid phase that impedes the penetration of the laser beam and consequent ablation of the material.
The influence of the processing parameters on the microstructure of the different textured samples was assessed by optical metallography from their cross-section. Figure 10 shows the two typical examples of textures obtained from different processing parameters.
For low laser powers, the scanning speed did not influence the microstructure of the base material (Fig. 10a), formed by ferrite + chromium carbides. However, for higher laser power values, and number of passes higher than 1, white regions with no carbides could be detected on the top and the edges of the unmachined areas, corresponding to remelted base material (Fig. 10b). This means that high laser powers are responsible for the dissolution of the chromium carbides. Microhardness evaluation was performed in these regions and far from the machined tracks. Values of 320 ± 11 HV and 255 ± 5 HV were obtained, respectively. The dissolution of the chromium carbides led to the incorporation of chromium in the austenite at a high temperature with the corresponding hardening of the microstructure after cooling.