*1.4. Laser Impact Welding*

LIW was patented by Daehn and Lippold [23] in 2011. Similar to laser shock peening, a focused laser beam is used to ablate a sacrificial layer, which is placed on the surface of a metal flyer foil. Rapid vaporization of this ablative layer creates a high-pressure plasma. By using a transparent overlay, the plasma is confined to further increase its pressure. The plasma generates shock waves and accelerates the flyer towards the target metal. Upon collision, jetting and interlocking of the foils occur along a weld interface. Since the deformation of the flyer in LIW is a direct consequence of the shock pressure load generated by the intense laser pulse, there likely exist large velocity gradients amongs<sup>t</sup> regions of the flyer foil upon laser incidence, depending on the spatial profiles of the laser beam and associated pressure pulse. Furthermore, the temporal profiles of the laser pulse and corresponding pressure load further determine the nature of and time to impact in LIW. Thus, assuming a single-valued initial flyer velocity in the simulation of LIW neglects the e ffects of spatial and temporal profiles of the laser pulse and its plasma pressure load. On that premise, a review of the experimental and numerical studies of LIW follows.

As reported in the literature, successful welds of similar and dissimilar metal foils have been achieved by LIW. Wang et al. [24] used aluminum flyers to investigate the e ffect of transparent overlay material and laser spot size on the weld quality. They welded nickel flyer (50 μm thick) and base plates using flat and corrugated surfaces on the base. Corrugated base plates provided greater surface area for welding, but large impact angles resulted in no bonding. In a separate experiment using Photonic Doppler Velocimetry (PDV), they revealed that the maximum flyer velocity was achieved within 0.2 μs after impact and in less than 30 μm displacement from the initial position. In addition, they welded 25.4 μm thick aluminum flyer foils to 75 μm thick titanium base foils [25]. It was found that separation of black paint in the ablated area renders it a more suitable ablative layer than black tape. They further studied the e ffects of laser spot size, the gap between the aluminum flyer and titanium target foils, flyer thickness (from 50 to 250 μm), and transparent overlay material on the weld strength and area [26]. The weld strength and area were evaluated using peel tests and voltage drop measurements [27] across the welds, respectively. It was shown that increasing laser fluence increases the impact velocity (measured by PDV) until the ablative layer reaches its energy absorption limit. It was revealed that by increasing the laser spot size, and thus decreasing the laser fluence, achieving a successful weld becomes more di fficult. However, welds achieved by larger spot sizes resulted in greater weld strength and area compared to those achieved by smaller spot sizes. In addition, smaller spot sizes resulted in a greater number of waves with higher amplitude and shorter wavelength on the weld interface. Wang et al. [28] studied the e ffect of laser fluence on weld interface morphology during oblique LIW of 0.1 mm thick aluminum flyer foils to aluminum and copper base foils of the same thickness. While the authors have reported the laser pulse energy values, it is vital to note that the laser-induced e ffects heavily depend on the laser spot size among other parameters such as laser wavelength, pulse duration, and the irradiated material [29]. For instance, using the same laser pulse energy, laser impact might generate ultrasounds or shock waves depending on the laser spot size, pulse duration, etc. Superior results (for laser spot size of 6 mm and impact angle of 20 degrees) were obtained at laser fluences of 13.44, 14.15, and 14.85 J/cm<sup>2</sup> when they welded 0.05 mm thick sheets of aluminum and copper flyers to 0.1 mm thick sheets of aluminum base foils [30]. It was found that doubling and tripling the stando ff distance between the foils increased the weld diameter by 50% and 83%, respectively. Furthermore, they numerically simulated LIW using the SPH technique in a 2D domain. A constant initial velocity and arc shape in the flyer plate was assumed and showed that increasing the stando ff distance resulted in a narrower but taller springback region. The boundary condition applied to the base plate in the numerical model was di fferent from the experimental conditions. In the experiment, the base plate was placed on a back-support while it was left free at the bottom in the numerical simulation. Using the same numerical method and boundary conditions, they simulated LIW of 0.03 mm thick aluminum and titanium flyer plates to 0.1 mm thick copper base plates [31]. Springback of the foils, spallation in the base plate, the jetting, and the wavy interface was simulated and found to be similar to the experimental results. Numerical simulation also revealed that the jetting was mainly produced from a very thin layer of the flyer plate. The numerical simulation was also used to compare LIW of 0.05 mm thick aluminum flyer plates to steel base plates of the same thickness and vice versa [32]. It was shown that using aluminum as the flyer and steel as the target resulted in more successful welds compared to the case

where the roles were interchanged. More importantly, swapping the materials changed the direction of wave formation with reference to the impact weld direction. In performing LIW of 0.03 mm thick aluminum flyer plates to 0.08 mm thick brass target plates [33], the authors showed that by increasing the laser fluence, the melting and thus formation of intermetallic compounds was increased, but so did the amplitude and wavelength of the weld interface waves, and the e ffect of the latter overcame that of the former. Therefore, overall the bond strength was increased with increasing laser fluence. In another experimental work, the authors achieved LIW of 0.03 mm thick crystalline copper flyers to 0.028 mm thick Fe-based metallic glass targets [34]. Copper foil was annealed and attained a finer grain structure prior to LIW. Increased nanoindentation hardness was observed in the copper, the weld interface, and the metallic glass after impact. No crystallization occurred due to LIW and the metallic glass retained an amorphous microstructure after bonding (tested and confirmed only for 1.5 mm laser spot diameter and 47.25 J/cm<sup>2</sup> laser fluence). Liu et al. [35] proposed a preforming technique in LIW of 0.03 mm thick titanium flyers to 0.1 mm thick copper targets. In this method, a preformed local hump replaced the gap between the foils normally used in LIW. The jetting, springback and wavy interface were all observed similar to other LIW experiments (the preforming method was not explained, but the overall dimensions of the flyer after preforming were provided). In addition, they studied the e ffect of adding a 0.02 mm thick aluminum foil as an interlayer between the copper flyer and base foils having 0.03 and 0.05 mm thicknesses, respectively [36]. It was reported that the nanoindentation hardness levels increased more in the flyer compared to the target. Lap shearing tests resulted in failure in the upper weld interface (between the flyer and the interlayer).

In following the above review of prior research into general HVIW methods, and LIW in particular, the remainder of this paper is organized as follows: Section 2 describes in detail the experimental setup and materials and methods used to study the e ffects of incorporating measured spatial and temporal profiles of the laser pulse to better simulate the LIW process. Section 3 contains a comprehensive description of the LIW numerical modeling procedures. The results of modeling and comparisons to the experiments are discussed in Section 4.
