In-situ observation of CO gas bubble behavior in molten pool during direct laser deposition of WC-Co cermet material

Laser cladding of WC-Co cermet material is a promising means of improving the wear resistance of metal products. Layer-by-layer processing of laser cladding, also designated as laser directed energy deposition and laser metal deposition, is attracting attention as an alternative manufacturing process of cermet materials for conventional powder metallurgy processing. Laser cladding of WC-Co has constraints on its practical use because a clad bead of WC-Co often includes numerous defects such as cracks and pores. Various factors cause pores in clad beads. Therefore, establishing a method for restraining them is difficult. Obtaining dense clad beads having few pores requires the elucidation of behaviors of gas bubbles: the precursors of pores. Nevertheless, observations inside molten pools are difficult, thereby complicating the characterization of gas bubble behavior. Our group developed a method for direct observation inside a molten pool during laser cladding. This study specifically examines pore formation mechanisms in clad beads of WC-Co. Gases emitted from the molten pool were analyzed and observations were made of gas bubble behavior inside the molten pool during laser cladding of WC-Co. Results revealed that CO gas bubbles are emitted from the molten pool during laser cladding of WC-Co. After the CO gas bubbles grow rapidly, some coalesce to form large irregularly shaped pores. When using WC-Co powder particles with adherent aluminum, the gas bubble growth rate is much lower. The aluminum addition effects on gas bubble behavior and the role of aluminum in pore reduction are discussed.