3.1. Laser Texturing
From the measurements with the confocal profilometer, the 3D geometry reconstruction was carried out. For a more complete analysis, the geometries were exported as a point cloud to the Matlab
© software for subsequent analysis. Using an own developed algorithm, the profiles of the generated grooves were extracted in each measurement and the average profile with the corresponding deviations was calculated.
Figure 8 shows 3D topographies and extracted profiles using Matlab
© for different pulse frequencies using the same feed rate of 800 mm/s and 3 overlapped tracks.
Figure 8a shows the topography reconstruction for 30 kHz and
Figure 8b shows the mean profile, where the band containing all extracted profile measurements can also be seen. In
Figure 8c,d, the topography and profile measurements for 50 kHz are shown. The extracted profiles show a variability higher than 50 µm in the burr height, causing a non-uniform behavior in shear strength. On the other hand, when the frequency is 70 kHz,
Figure 8e,f show better results in terms of burr height homogeneity.
The result was evaluated by measuring the upper width of the grooves, the depth, and the generated slot burr height. In
Figure 9, the resulting width and depth for each of the grooves are shown, as well as the aspect ratio (AR) as a function of the energy density (ED). The aspect ratio AR is defined as the ratio between the total depth and the width of the grooves, considering the total depth as the sum of the depth of the slot and the height of the generated burr.
As the number of overlapped tracks increases, the groove depth also increases linearly; however, the evolution of the width is different and as a deeper groove is generated, the upper width of the groove is progressively reduced, as can be appreciated in
Figure 9c for 50 kHz and
Figure 9e for 70 kHz. This phenomenon is due to the material melting and the vaporization mechanism in the texturing, so during the process, part of the material is melted and is expelled from the cavity, generating an accumulation of material in the surface area that results in a kind of burr. As the number of overlapped tracks increases, the cavity becomes deeper and some of the melted material does not reach the top, leading to a narrowing of the cavity in the upper area. Since the groove depth is lower with the 30 kHz pulse frequency, this effect is less noticeable (
Figure 9a), but also after 10 overlapped tracks the groove width is below the actual laser spot diameter. Thus, when more than 5 or 7 tracks are overlapped, the width of the grooves reach values below the diameter of the laser beam, which is 50 µm at the focal point.
On the other hand, the linear behavior in the evolution of depth for the different pulse frequencies and feed rates is remarkable, as can be appreciated in
Figure 9a,c,e for different frequencies and feed rates. The slope of the linear approach has little sensitivity to speed variation and increases significantly with an increase of pulse rate. Also, as the frequency increases, a higher material removal rate is achieved, which translates into a greater depth for the same number of overlapped tracks. After 5 overlapping tracks, the slot width decreases to values below the diameter of the laser beam itself, indicating an accumulation of re-solidified material in the upper area of the cavity. If the slot width is below 40 µm, the thermoplastic material cannot flow correctly at the junction. Thus, for 50 kHz the limit would be 5 passes and for 70 kHz the limit would be 3 passes, while at 30 kHz the material removal rate is insufficient and has significant variability. This effect is also noticeable when the groove Aspect Ratio (AR) is evaluated based on the energy density (ED). In
Figure 9b,d,f, instead of showing a linear evolution, for ED over 200 J/cm
2 the groove exhibits the above-mentioned effect—the width is reduced while the depth is increased, resulting in an AR higher than expected. Taking into account a minimum width of 60 µm, a maximum removal rate, and a minimum deviation for the tests, the parameters that provided better results included a pulse frequency of 70 kHz, a feed rate of 800 mm/s, and 3 overlapped tracks.
With these parameters as a reference, a second study was carried out for the generation of grooves with different AR, but while maintaining a minimum surface width of 60 µm. For this purpose, parameters of 70 kHz, 800 mm/s, and 3 overlapped tracks were used to generate reference grooves. Combining these reference grooves with an overlap ratio of 50%, different AR slots were obtained.
Table 6 shows the lap shear results of the slots, with an AR between 0.94 and 4.15 and a minimum width of 60 µm in all cases.
The first joining tests were carried out by taking the joining parameters used in previous work as a reference [
16]. These joining tests were carried out to evaluate the influence of texture on the final bond strength. In all the tests carried out, shear strengths above 19 kN were obtained for a joint area of 35 × 45 mm, which is equivalent to 12 MPa. The maximum value was obtained for a slot with two passes in width, where the average value reached was 23.55 kN (15 Mpa).
Figure 10 shows the filling of the grooves and the arrangement of the glass fibers in the composite material. A complete filling is observed in all the grooves, which indicates that the joint parameters used ensure the proper flow of the material. On the other hand, an accumulation of passes also ends up generating an area of resolved material that narrows the upper area of the groove, as can be appreciated in
Figure 10c,e. In
Figure 10a, two reference grooves were overlapped to reach a depth of 102 µm, keeping a width of almost 70 µm in the generated slot. In
Figure 10b, as a result of overlapping height and width, the groove generated with the reference parameters is shown. In this way, the depth is almost the same as in
Figure 10a, while the slot width is 85 µm. In
Figure 10c, two reference grooves are overlapped in depth and three are overlapped in width, with an overlap ratio of 50%. With this strategy, slots of more than 100 µm in width are achieved, also keeping the depth constant at 100 µm. However, in this case the result is much more variable and some slots contain re-solidified material, decreasing the effective width of the slot. In
Figure 10d–f, more reference grooves are overlapped in depth while the width is kept constant. In this case, the problem of re-solidified material is more noticeable, giving a higher variability in the results, as can be appreciated in
Table 6 and
Figure 11, where the mean values of shear strength achieved are summarized for different AR. The results in
Figure 11 show that the highest shear strength with the least deviation is achieved with an AR of 1.24, corresponding to the groove shown in
Figure 10b.
3.2. Laser Direct Joining
All the specimens were tested by measuring the deformation and force according to standard ASTM-D3528 of American Society for Testing and Materials. Joining was carried out following a zig-zag strategy parallel to the shortest dimension with a 5 mm radial step, which is equivalent to a 50% overlap for a 10 mm spot. The process was controlled through a pyrometer, which recorded the temperature of the central point of the 35 × 45 mm2 junction area.
The results show that the temperature is the most influential parameter, followed by the feed rate.
Figure 12a shows the results obtained for a bonding pressure of 3.5 bar and with two zig-zag sweeps. It can be seen that the set point temperature of the pyrometer is the most decisive factor, with the optimum value being 370 °C. Lower values provide insufficient melting, while higher values cause excessive melting. Similarly, a lower speed also causes higher heat accumulation, so for the reference temperature of 370 °C, when the feed rate is 30 mm/s, variable results are produced with a significant deviation, as shown in
Figure 12a. On the other hand, variables such as tool pressure or even the number of passes have less influence on the result.
Figure 12b shows the resistance obtained at a feed rate of 50 mm/s and with two passes for different reference temperatures and tool closing pressures of 3.5 and 5 bar. In this case, the influence of closing pressure is relatively low, with the results being slightly better at 3.5 bar, although in both cases the temperature has a greater influence. Something similar happens when the influence of the number of passes on the process is analyzed.
Figure 12c shows the resistance values achieved for a feed rate of 50 mm/s and 3.5 bar pressure when the process is carried out with one and two passes at different set temperatures. For temperatures of 340 °C and 370 °C, better results are obtained with two passes, while for reference temperatures of 400 °C and 430 °C, the result is better with a single pass. This behavior is consistent with the previously identified evolution, in which excessive accumulation of heat was undesirable.
Figure 12d shows the results confirming this theory, where a combination of a speed of 30 mm/s and two passes at a pressure of 5 bar gives worse results as the reference temperature increases. The fact that it is more convenient to use a two-pass versus a one-pass sweep strategy doubles the cycle time, however, it should be noted that even if the maximum of 26.7 kN is not reached, it is possible to achieve resistance values above 22 kN by increasing the reference temperature or reducing the feed rate to compensate for the overall energy input.
In relation to the evolution of the deformation in the shear test, when the joint is adequate, the result obtained is in line with the expected behavior for PA6. Thus, in
Figure 13, different curves are observed for joints that reach values ranging from 8 kN to more than 26 kN. Although the evolution of the deformation is different depending on the quality of the joint, in practically all cases a linear evolution can be seen. Taking into account that the total length of the specimen once joined is 175 mm (corresponding to an aluminum and a composite specimen of 105 mm in length minus the overlap area), the maximum deformation achieved for an optimum joint is 6 mm. It should be noted, however, that since the specimen is a multi-material one, it is not possible to define a unitary deformation of the specimen, since in this case the composite’s PA6 material matrix inserted in the textured grooves is the one that suffers the deformation.