Pore Formation Mechanism in W–C Hard Coatings Using Directed Energy Deposition on Tungsten Alloy

Porosity is a common phenomenon and can significantly hinder the quality of the coating. Here, the pore formation mechanism and the characteristics of the single tracks of the W–C coating using directed energy deposition (DED) are systematically investigated. The forming quality of the tracks, the distribution of the pores, and the elemental distribution near the pores are analyzed by the observations of the cross-sections of the tracks. The temperature field of the melt pool is discussed comprehensively to reveal the pore formation mechanism. The results confirm that Ni and Co evaporated during the DED process due to the high temperature of the melt pool. Pores were continuously produced adjacent to the fusion line when the melt pool was about to solidify since the temperature at the solidification front was higher than the boiling point of Ni. The vaporization area at the fusion line was proposed, where Ni could also evaporate at the time the melt pool started to solidify. The relationship between the solidification rate, the size of the vaporization area and the DED parameters (laser power and scanning speed) was established to discuss the causes of severe pores above the fusion line. This work contains a practical guide to reduce or eliminate the porosity in the coating preparation process on the surface of the tungsten alloy.