*1.1. Explosive Welding*

In EXW, a controlled detonation provides the source of the high-velocity impact. The explosion accelerates a flyer plate towards a base (target) plate, leading to a weld due to high-velocity collision. Since its discovery in 1957 [1], EXW has been studied by many researchers. Szecket and Mayseless [2] reported on methods of introducing an 'artificial disturbance' to stabilize the wave formation at the weld interface when joining similar metals (copper to copper, and mild steel to mild steel). Mousavi and Al-Hassani [3] designed a physical experiment to mimic the conditions of explosive welding using a pneumatic gun. They performed oblique welding of 3 mm thick flyers of copper, stainless steel, titanium, and zirconium to 30 mm thick base plates of mild steel. Assuming a wide range of constant initial impact angles and flyer velocities, they performed 2D Eulerian simulations of the process to show spallation and jetting of material at the collision interface. Aside from Eulerian simulation methods, meshfree methods such as the Material Point Method (MPM) and Smooth Particle Hydrodynamics (SPH) have also been used to simulate EXW. For example, Wang et al. [4] utilized MPM to model the welding of a 10 mm thick copper flyer plate to a 50 mm thick steel base plate driven by the explosion of ammonium nitrate. Their method did not simulate the formation of a wavy interface but did capture the 'flight' shape of the flyer. Wang et al. [5] simulated EXW of aluminum, steel, and copper flyers against titanium base plates using SPH, but with constant initial flyer velocities assumed. They observed waves having larger wavelengths and greater amplitudes in the collision of stainless steel and titanium compared to aluminum and titanium. Zhang et al. [6] implemented a density-adaptive SPH technique to model EXW, including the detonation of the explosive. They reported that the attainment of requisite impact angles in a parallel flyer/target setup is more challenging compared to oblique orientations.
