Underwater additive manufacturing of Ti-6Al-4V alloy by laser metal deposition: Formability, gran growth and microstructure evolution

Highlights: • The underwater thin-walled Ti-6Al-4V part was manufactured by underwater laser metal deposition. • The stable local dry cavity formed during underwater laser metal deposition process. • Grain distribution and microstructure evolution of the underwater thin-walled part was analyzed. • The effect mechanism of water environment on the deposited metal was investigated. No underwater laser additive manufacturing technology has been reported. In this paper, the underwater thin-walled Ti-6Al-4V part was built firstly by underwater laser metal deposition (ULMD), and the formability, grain growth and microstructure transformation were investigated. The stable local dry cavity and the shielding-gas flow layer surrounding the deposited layer formed during the ULMD process utilizing an improved ULMD nozzle, generating the uniform thin-walled part without cracks and pores. Columnar β-grains, growing epitaxially opposite to the heat flow direction, were observed in the whole ULMD part, and the grain size increased firstly and then reduced with increasing the deposition layer, attributed to the comparative growth and decreasing cooling rate. The water cooling effect suppressed the formation of equiaxed β grains that only formed in the top region of ULMD part. For the microstructures of ULMD sample, only acicular martensites α' with different size scales formed in the bottom region, and martensites α' were replaced gradually by α phase with lamellar, lathy and blocky shapes with increasing the building height, except the top region because the absence of multiple thermal cycles. The larger cooling rate promoted the formation of α' martensite and decreased the width of α lath of ULMD part.