A novel underwater directed energy deposition (UDED) technique was developed for the on-site repair of marine equipment in an underwater environment. A special drainage nozzle was integrated with a laser cladding head to form the underwater manufacturing tool which ensured the successful repair of the damage zone on an HSLA-100 steel plate. The influence mechanisms of inherently complex heat-treatment cycles and kinetics involved in the UDED process on the microstructural formation/evolution process were systematically investigated using both experimental and numerical approaches. The mechanical properties, including microhardness, nanohardness, tensile strength and Charpy impact toughness of the UDED repaired samples were investigated and the results were compared with those repaired by in-air directed energy deposition (DED). The experimental results show that the rapid cooling rate accelerated by the water quenching effect promoted the formation of columnar austenite and the subsequent fine martensite. Compared with the DED samples, the activation energy values for the nucleation and growth of the precipitates in UDED samples were much lower due to the existence of high-density crystal defects and large internal residual stresses. The numerical results indicate that compared with DED, the cooling rate significantly increased and the peak temperature decreased during UDED. The average microhardness of the samples prepared by UDED (315–334 HV) was higher than that prepared by DED (307 HV). The tensile properties and impact toughness of the as-repaired samples by UDED were equal or even better than those obtained by DED owing to the formation of fine martensite and high dislocation densities in the UDED samples. This work clarifies the difference in metallurgical processes in air and underwater environments, which can provide guidance for future structural manipulation of materials additively manufactured in underwater environments.

Investigation of the underwater laser directed energy deposition technique for the on-site repair of HSLA-100 steel with excellent performance

A kind of integrated process of extreme high-speed laser cladding (EHLC) and laser remelting (LR) was firstly proposed to comprehensively improve surface quality and internal interlayer bonding strength of pure EHLCed coatings. The influence mechanism of integrated process on microstructure and corrosion resistance was carefully studied. The integrated process effectively optimized the surface roughness (reduced >60%), and eliminated interlayer defects, especially for the multi-layer coating. Results indicated that both of the coatings manufactured by EHLC and EHLC-LR comprised dominant ultrafine grains. However, electrochemical measurements showed that the EHLC-LRed coating possessed better corrosion resistance and lower corrosion rate compared to EHLCed counterpart. It was proposed that this behavior was due to a high density of ultra-fine grains with numerous grain boundaries uniformly distributed on the coating surface forming a more stable oxide film.

Microstructural features and corrosion behavior of Fe-based coatings prepared by an integrated process of extreme high-speed laser additive manufacturing

The effect and mechanism of ultrasonic vibration on the microstructure and performance of laser cladding 316L coating has been studied, providing a theoretical basis and technical support for the application of the ultrasonic-assisted laser cladding process. The results show that ultrasonic vibrations with different amplitudes can effectively improve the macroscopic forming quality of the cladding layer, among which the ultrasonic vibration with 17.5 μm amplitude is the best. Ultrasonic vibration can also significantly improve the micro-forming quality of the cladding layer, such as homogenizing the structure, refining grains and reducing porosity. The wear mechanism of the cladding layer with an ultrasonic assistance of 17.5 μm is mainly abrasive wear, so the wear resistance has been significantly improved. The ultrasonic-assisted cladding layer exhibits significant passivation behavior in 3.5 wt% NaCl solution. It may be that the surface roughness reduction and grain refinement caused by ultrasonic assistance can improve the integrity and compactness of the cladding layer, so the dynamic equilibrium process of the dissolution and reformation of the passive film is changed, which can improve the corrosion resistance of the surface. There are some equiaxed grains in the columnar-grains area of the ultrasonic-assisted cladding layer, mainly because the inside of some primary columnar grains has gone through a process of lattice distortion → dislocation multiplication → substructure rotation → formation of small-angle grain boundary → misorientation accumulation → formation of new grains in the process of ultrasonic-assisted laser cladding of 316L stainless steel. 

Effect and action mechanism of ultrasonic assistance on microstructure and mechanical performance of laser cladding 316L stainless steel coating

In this paper, underwater dissimilar aluminum alloy laser cladding has been investigated for the first time through a laser nozzle. The influences of water environment on laser cladding of dissimilar aluminum alloy were researched under optimized process parameters. With the transfer of cladding environment from air to water, the height of cladding zone (CZ) and the cladding angle increased, but the width of CZ and the depth of melting zone (MZ) decreased. The microstructure of MZ and CZ were columnar dendrites and equiaxial dendrites, but the underwater microstructure was much smaller than that of the in-air. Both underwater and in-air CZ were composed of Aluminum phase and Al0.5Fe3Si0.5 phase, while the formation of intermetallic compound increased in the in-air CZ. The water environment was beneficial to reduce the magnesium burning loss in the CZ. In the underwater CZ, although the decrease of hard phase compound (Al0.5Fe3Si0.5) formation made microhardness value go down, the microhardness value went up with the increase of magnesium content and the refinement of microstructure. Finally, under the combined action of these three factors, the average microhardness value of underwater cladding layer was higher than that of the in-air cladding layer.

Investigation on in-situ laser cladding 5356 aluminum alloy coating on 5052 aluminum alloy substrate in water environment

Underwater directed energy deposition (UDED) is a promising technology for on-site maintenance and repair of underwater structures. In this study, the damage zone on a Ti–6Al–4V plate was successfully repaired by UDED and no obvious internal defects were observed. The mechanisms of the special underwater thermal behaviors influencing the microstructural formation/evolution process and the resultant mechanical properties of Ti–6Al–4V repaired by UDED were systematically investigated by experimental and simulation methods. Compared with that prepared by in-air directed energy deposition (DED), the sample repaired by UDED presented the following differences: (1) The cooling rate was large and the heat accumulation of the sample was low during UDED due to the rapid heat dissipation by water and gas curtain gas. (2) The sample repaired by UDED was dominated by fine acicular martensite α′ with a high density of dislocations due to the weak intrinsic heat treatment (IHT), while the intensified IHT involved in DED provoked the significant decomposition of α′ into α and β film. (3) The weak IHT during UDED resulted in the formation of thin β films with a low vanadium content. The intensified IHT during DED promoted the diffusion of vanadium atoms and the coarsening of β films. (4) The average microhardness of the sample repaired by UDED (379 ± 15 HV) was higher than that repaired by DED (359 ± 11 HV). The sample repaired by UDED exhibited relatively poor ductility and low toughness due to the presence of acicular martensite α′. This work can provide an important foundation and useful guidance for tailoring the microstructure and properties of titanium alloys repaired by UDED in underwater environments.

Investigation of the microstructure and mechanical properties of Ti–6Al–4V repaired by the powder-blown underwater directed energy deposition technique

Cheng Zhou

Papers

Investigation on in-situ laser cladding coating of the 304 stainless steel in water environment