Underwater Laser Welding of Pure Ti: Oxidation and Hardening Behaviors

Manlelan Luo,Renzhi Hu,Anguo Huang,Pengyu Wei,Shengyong Pang,Quanhong Li

doi:10.3390/met11040610

Manlelan Luo, Renzhi Hu + Show 4 more

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https://doi.org/10.3390/met11040610

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Abstract

The local dry underwater laser welding of cp-Ti, with air as an assisting gas, and in a simulated underwater facility was researched, aiming to find a viable and economical method for repairing titanium alloy underwater vehicles in situ in the future. Macro-morphology, microstructure, and microhardness of the cp-Ti laser welds, as a function of welding parameters, were experimentally characterized. The oxidation and hardening behaviors of the welds were also studied in detail. It was found that local dry underwater laser welding with air assisted blowing is feasible for obtaining a complete and glossy weld. Compared with a weld in atmosphere, the cross-section morphology of the weld was almost unaffected by the special underwater welding environment. The weld presented a three-layer structure. High temperature and high pressure water vapor and local blowing are the direct causes of weld oxidation, and porosity defects further aggravate the oxidation behavior. The oxygen-enriched areas were mostly concentrated in the top area of the weld center and near the fusion zone, because of the higher number of grain boundaries and phase boundaries. In addition, the partial oxidation caused by local blowing and water vapor atmosphere, and also the higher strength acicular martensite caused by the rapid cooling effect of water, will lead to weld hardening. However, adjusting the welding process parameters, such as increasing the welding speed, can effectively reduce the microhardness of the weld. Our findings can provide an understanding of the influence of water environment on underwater laser welding, and verify the feasibility of a more economical method for the in situ repair of large underwater facilities.

Highlights

Underwater laser welding can realize in situ joining and repair of underwater operating facilities
The results showed that the effect of water on the heat affected zone (HAZ) is similar to tempering
The above phenomenon may have been the combined result of a variation of laser focus caused by the disturbance of water and the accelerated oxidation effect caused by water dissociation

Summary

Introduction

Underwater laser welding can realize in situ joining and repair of underwater operating facilities. Local dry underwater welding, which uses gas to drain the water in the local area around the weld of the workpiece, so as to reduce the shielding effect of water on the laser beam, is considered as an effective solution to realize underwater joining and repair [5,6,7]. In order to reduce the cost of expensive inert gas and the complex operation of replacing protective gas cylinders, in coordination with the long-distance flexible transmission of laser fiber, cheaper air is considered as an assisting gas. In this way, air can be continuously transported into the underwater environment through a pipe together with the optical fiber to realize economical and feasible underwater laser welding

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Conclusion