Direct observation of bubble generation processes inside a molten pool during laser cladding

A direct observation technique using quartz glass enabled visualization of the bubble generation processes occurring inside a molten pool during laser cladding. This technique revealed that bubble generation processes during laser cladding of Inconel 625 and WC-12 wt% Co powder differed. For the Inconel 625 powder, the powder particles rushed into the molten pool. A cavity formed behind the particles. Subsequently, the particles disappeared inside the molten pool, and were replaced with bubbles. Bubbles were regarded as cavities that separated and remained behind the particle. To reduce bubble generation during laser cladding of Inconel 625, cavity formation suppression is best achieved by reducing the kinetic energy of the powder particles. However, the bubbles in WC-12 wt% Co powder were generated mainly from inside of the molten pool. Results suggest that gas was generated inside of the molten pool. The bubbles expanded because of growth and coalescence as time passed. Some bubbles opened the molten pool surface; others were trapped in the solidified molten pool as a porosity or a pit. Results indicate that gas-formation suppressing conditions must exist in the molten pool to reduce the WC-12 wt% Co porosity. Applying the direct observation technique to widely various laser cladding processes is expected to enable the fabrication of porosity-free clad beads in the near future.