1：Experimental Design and Method Selection

UWLD experiments were conducted using an underwater LD system comprised of a semiconductor laser with a maximum output power of 5 kW, a YASKAWA 6-axis robot, a wire delivery system, and a LD nozzle. A gas-shielding LD nozzle was designed to protect the deposited metal from the water environment.

2：Sample Selection and Data Sources

A Ti-6Al-4V solid wire with a diameter of 1.0 mm was deposited onto the Ti-6Al-4V substrate with a thickness of 10 mm.

3：List of Experimental Equipment and Materials

Semiconductor laser (WFD5000), YASKAWA 6-axis robot, wire delivery system, LD nozzle, Ti-6Al-4V solid wire, Ti-6Al-4V substrate.

4：Experimental Procedures and Operational Workflow

The water depth was 60 mm, and the gas-shielding LD nozzle was installed below the laser head. Air was induced in the outer layer of the nozzle to drain the water environment and generate a local dry cavity, while the inner layer was induced in argon to prevent oxidation of the molten pool and reduce laser plasma and fumes.

5：Data Analysis Methods

Metallographic specimens were prepared through grinding, mechanical polishing, and etched using a Kroll etching agent. The macrostructure was captured by an optical digital microscope, and the microstructure was investigated using a scanning electron microscope (SEM). The compositions of the deposition metal on the cross sections were explored using an energy dispersive spectrometer (EDS). The Vickers microhardness was measured by a Vickers microhardness indentation machine.