Laser-induced recoil pressure on metal drops and powder particles
Abstract: Additive Manufacturing became a major research topic and part of industrial production in the past years. Numerous techniques now allow to build 3D structures with a wide choice of materials. When it comes to processing of metals, a laser beam is often used as a heat source to melt either a wire or powder, where the trajectory of spatters and powder particles can be affected by the laser beam radiation. Laser beam irradiation is partly absorbed by the material, and is then converted to heat, which can cause melting and even vaporization. The vaporization of material induces a recoil pressure on the melt pool, which affects its geometry and dynamics. However, the effects of the recoil pressure on airborne objects such as drops and powder particles are still relatively unknown. Their different sizes and boundary conditions compared to a melt pool might affect their behaviour under high laser beam radiation. Therefore, this thesis aims at better understanding the effects of the recoil pressure on metal drops and powder particles, as well as their impacts on Additive Manufacturing processes, especially Directed Energy Deposition and Laser Metal Wire Deposition. In the three adjoined papers, high-speed imaging was used to observe (i) powder blown through a laser beam, (ii) drops falling in a laser beam, and (iii) drops detaching from a wire in a laser beam. The videos enabled to calculate the acceleration of powder particles and drops of different sizes, the density map of the powder stream, and the detachment direction of the drops. The experimental results were completed with theoretical calculations of thermodynamics, recoil pressure and surface tension. These studies allowed to conclude that the acceleration induced by the recoil pressure on a drop or a powder particle increases with decreased size. Moreover, the recoil pressure causes a slight deviation of the powder stream in Directed Energy Deposition that can induce a better powder focusing. The recoil pressure can also cause the disintegration of powder particles in the laser beam. Finally, it was shown that the recoil pressure can be used to detach drops on demand from a wire and accelerate them towards the substrate where they can be strategically deposited for building additive structures.
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