In-situ formation of laser-cladded layer on Ti-6Al-4 V titanium alloy in underwater environment

Titanium alloys such as Ti-6Al-4 V have been extensively researched for potential application in marine engineering and nuclear power production owing to their excellent properties. Unfortunately, the marine industrial components exposed to the underwater environment are highly susceptible to water erosion and corrosion-related damages than those exposed to the in-air environment. As such, it is necessary to develop advanced in-situ restoration techniques to repair the damaged parts and prolong the lifetime of the components exposed to the underwater environment. Recently, underwater laser cladding technology was proposed to overcome the challenges of conventional methods used in the underwater repair. However, only a few studies on underwater laser cladding technology have been reported. Consequently, the few studies on underwater laser cladding have majorly concentrated on stainless steels and nickel aluminum bronze alloys.

To this note, a team of researchers at the Harbin Institute of Technology: Dr. Yunlong Fu, Professor Ning Guo, Dr. Qi Cheng, Dr. Di Zhang, and Professor Jicai Feng investigated the in-situ formation of laser-cladding layers on Ti-6Al-4 V titanium alloys in the underwater environment. Their main objective was to fabricate high-quality underwater cladding layers of Ti-6Al-4 V titanium alloys for repairing damaged marine industrial and nuclear power components exposed to the underwater environment. Their work is currently published in the journal, Optics and Lasers in Engineering.

In their approach, the repairing layer of Ti-6Al-4 V alloy substrate in the underwater environment was prepared by in-situ underwater laser cladding technology. A laser cladding nozzle, located under the laser cladding head, was used to protect the molten pool from oxidation by preventing the possible reaction of the high-temperature Ti-6Al-4 V alloy with water. The research team also determined the effects of the water environment on the formation characteristics, metallographic structure, and the corrosion behavior and performance of the prepared underwater laser cladding layers. Additionally, the crack formation and laser propagation mechanisms were analyzed with the help of direct characterization techniques such as X-ray diffraction and scanning electron microscopy.

The authors found spatters on the cladding layer surface, attributed to the slight oxidation of the underwater layer and the aerosol particle’s disturbance on laser propagation. Nevertheless, the underwater layer characterized by a conduction mode pool appeared continuous and uniform. A decrease in the laser power density acting on the molten pool and a corresponding increase in the cooling rate caused the molten pool to change from keyhole mode to conduction mode, and a corresponding decrease in the grain size and thickness in of the plate. Compared to the in-air layer, the underwater layer exhibited slightly higher microhardness behaviors and almost the same corrosion resistance. Furthermore, the macroscopic cold cracks exhibited trans-granular fracture behavior due to the high residual stress and low toughness of the deposited metal.

In a nutshell, the study investigated the effects of water environment on the formation, microstructure, and performance of Ti-6Al-4 V titanium alloy based cladding layer. Based on the results, the performance of the prepared cladding layers was comparable to those of in-air layers. In a statement to Advances in Engineering, the authors noted that the fabricated cladding layers demonstrated outstanding potential for use in repairing and prolonging the life of marine structural and nuclear power plant components exposed to underwater environment.