Investigation of Causes of Pore Defects and Development of Suppression Technology in Laser Metal Deposition of WC-Co Cermet Material

Laser metal deposition of WC-Co cermet material is a promisin g means of improving the wear resistance of metal products.Layer-by-layer processing of laser metal deposition, also designated as laser directed energy deposition, is attracting attention as an alternative manufacturing process of cermet materials for conventional powder metallurgy processing.Laser metal deposition of WC-Co has constraints on its practical use because a clad bead of WC-Co often includes numerous pores.Earlier studies have suggested gas generation in a molten pool by reaction of carbon and oxygen to be the main cause of gas pores in a WC-Co clad bead.However, the gas bubble behavior in the molten pool and the gas composition remain unclear.The method for suppressing these gas pores has not been established.Our group developed a method for direct observation inside a molten pool during laser metal deposition.Gases emitted from the molten pool were analyzed and observations were made of gas bubble behavior inside the molten pool during laser metal deposition of WC-Co.Results revealed that CO gas bubbles were generated inside the molten pool during laser metal deposition of WC-Co.After the CO gas bubbles grew rapidly, some coalesced to form large irregularly shaped pores.Laser metal deposition in low oxygen atmosphere was found to be effective to suppress the gas generation in the molten pool.Addition of elements with strong affinity for oxygen, such as aluminum, was also found to be effective for suppressing the gas generation in the molten pool.Consequently, clad beads having low porosity and fine microstructure were obtained.