A novel directed energy deposition-arc method of deposition layer without focused thermal energy heating: droplet transfer, morphology and microstructure

Abstract

Additive manufacturing (AM) of metal usually forms columnar grains along the building direction due to the focused thermal energy directly heating the deposition layer to easily obtain an obvious temperature gradient. The columnar grains are not conducive to AM-sample application, which results in the property anisotropy of the AM-sample. In this work, a novel directed energy deposition-arc (DED-Arc) method for depositing layers without direct heating by focused thermal energy is proposed. The arc burns between the tungsten electrode and a water-cooled electrode, were only used to melt the wire during DED-Arc process. Using stainless steel wire as a model alloy, by studying droplet transfer, morphology of the deposition layer and microstructure evolution, the feasibility of the method was fully verified. Two kinds of droplet transfer modes were observed, and the difference between them was whether the droplet contacted the molten pool before falling off. The focused thermal energy is only used to melt the wire, and the droplet transfer cycle is mainly affected by gravity, greatly decreasing the heat accumulation compared with existing methods. The heat-affected zone (HAZ) was obviously inhibited. The grain morphology of the deposition layer mainly consists of equiaxed crystals (∼300 μm) without an obvious preferred orientation. The deposition layer without direct heating by focused thermal energy could reduce the heat accumulation, resulting in a decrease in the temperature gradient and the morphology of equiaxed grains. The microstructure of a cylindrical sample was produced and has the same characteristics as with the deposition layer. The average microhardness of 172.63 HV, 167.95 HV and 169.26 HV are obtained in the top, middle and bottom regions, respectively. In addition, the tensile strengths are 500 MPa and 490 MPa, respectively.

Keywords

• Directed energy deposition-arc
• Unfocused thermal energy heating deposition layer
• Droplet transfer
• Morphology
• Microstructure