*4.1. Experimental Results*

Di fferent stando ff distances and laser fluence values were used in experiments to acquire the LIW process window. These parameters are listed in Table 5. It was seen that regardless of the tested laser fluence values, stando ff distances of 0.12 and 0.54 mm were too small and too large respectively. Successful welds were obtained using stando ff distances of 0.26 and 0.40 mm, irrespective of the laser fluence values tested in experiments.


**Table 5.** List of experimental parameters.

Using Leica microscopes, optical images of the weld cross-sections were obtained. Sample images for the case with a laser fluence of 31.08 J/cm<sup>2</sup> and a stando ff distance of 0.26 mm are shown in Figures 10 and 11. As mentioned in Section 2, in most LIW experimental results reported in the literature, due to very high flyer impact velocities and very low impact angles, springback occurs and welding is not achieved in the center of the laser-ablated region [24,30–36]. This is an unwanted event which decreases the strength and integrity of the weld. Interestingly, in the experimental results here, the springback phenomenon was not observed in the center of the weld region. As seen in Figure 10, the foils are indeed welded together in the center of impact region, while gaps are observed on the outside of the weld. Figure 11 shows the same weld at two higher magnifications. A combination of flat and wavy interface patterns is observed along the weld interface. Elimination of the springback phenomenon in these experiments could be attributed to the polymeric black tape placed between the target foil and the fixed metal specimen. This tape was used to prevent the target foil from bonding with the metal specimen after laser impact. It is hypothesized that direct placement of the target foil on the metal specimen leads to metal foils (flyer in particular) bouncing o ff of the specimen after impact resulting in springback at the center of the impact region. It is possible that the addition of a polymer interlayer between the target foil and metal specimen elongates the collision time of the foils, preventing the flyer foil from rebounding (which could lead to springback). The validity of this hypothesis needs to be investigated through further experiments as part of future research.

**Figure 10.** Optical microscope images of different regions of a LIW sample.

**Figure 11.** SEM images of a LIW sample at two different magnifications.

Another important observation in these experimental results relates to the overall shape of the welded foils. As seen in Figure 10, while the foils are welded in the central region, there is a gap present on each side of the weld. Since the loading conditions are axisymmetric, this means that the gap has an annular shape. Considering that the experiments include an initial standoff distance between the two foils, and because of the 3D Gaussian profile of the ablation pulse pressure, the generation of a gap that gradually increases with position away from the center of the weld might be expected. However, as seen in Figure 10, starting from the center of the weld, and moving away in the radial direction, the gap gradually increases to a maximum and then starts decreasing, resulting in a shape very similar to the springback region reported in the literature [24,30–36]. Although similar in shape, these are two di fferent phenomena and they happen for di fferent reasons; the annular gap observed in these experiments is attributed to the Gaussian profile of the laser-induced plasma pulse pressure, and the size of the double-sided tape attached to the flyer's laser-ablated area. The double-sided tape is used to enhance the plasma confinement in the glass overlay by eliminating any gap between the flyer foil and the transparent overlay. Due to the Gaussian profile of the pressure load, moving away from the center of the laser-ablated spot in the radial direction, the magnitude of the pressure load decreases significantly. Therefore, if the size of the double-sided tape is considerably larger than that of the laser spot, the laser impact cannot separate the flyer foil from the transparent overlay. In the experiments conducted here, after some trial and error, it was found that a double-sided tape slightly larger than the laser spot resulted in successful plasma confinement and separation of the flyer from the transparent overlay. In the center of the impact area, the flyer detaches from the transparent overlay and is launched towards the target. Away from the center, the resistance of the double-sided tape against separation becomes gradually more significant, together with the decrease in the magnitude of the laser beam pulse pressure. As a result, the size of the gap starts increasing until the edge of the double-sided tape is reached, the flyer is completely detached, and the resistance is no longer present. Subsequently, the gap starts decreasing and then increasing again as the flyer accelerates towards the target.

Lap shear tests were performed on samples with di fferent laser fluences and stando ff distances. These results are shown in Figure 12. It was observed that the force linearly increased with displacement until it reached a maximum. Then the weld failed (on the aluminum flyer side) resulting in a sudden drop in force values. Further increasing the displacement resulted in increased tearing of the flyer and thus sliding of the foils. Therefore, the force values gradually decreased until they reached an almost constant value with little change with respect to displacement. Higher fluences resulted in higher maximum force values while increasing the stando ff distance decreased the maximum forces.

Numerical results are presented next. Prior to the viewing of numerical results, it is important to note that all experiments were conducted in air at atmospheric pressure. Since the flyer has a supersonic flight, shock waves are generated upon collision of the foils resulting in a strong air-cushion e ffect. However, the air and its e ffect on the collision were neglected in the numerical simulations. Therefore, an investigation into the air versus vacuum conditions in the LIW process is a potential area of research as part of future work.

**Figure 12.** Force-displacement curves obtained from lap shear tests.
