The effects of ultrasonic impact peening (UIP) and laser shock peening (LSP) on 316L stainless steel were compared in terms of surface morphologies, microstructural evolutions and mechanical ...properties. The grain refinement mechanisms by mechanical and laser shock wave were subsequently analyzed. Experimental results showed that both UIP and LSP produced micro-grooves with the same depth (~48 μm) at the surface of 316L. The nano-grain size induced by double UIP treatment (10–90 nm) was much smaller than that by triple LSP treatment (>70 nm) because the impact numbers and total impact energy of UIP were much higher. The mechanical twinning was almost complete absence in the sample by UIP. On the contrary, the mechanical twinning was frequently observed in samples by LSP. The magnitude of peak pressure determined the transition from dislocation-dominated mechanism (~680 MPa for UIP) to twinning-dominated mechanism (~2200 MPa for LSP). The resultant dislocation cell size by UIP was much smaller than that by LSP due to the difference of dislocation density caused by different shock wave speed and impact numbers. Additionally, the compressive residual stress on the surface by UIP was higher than that by LSP in both measuring direction. Furthermore, both grain refinement and high dislocation density induced by UIP contributed to a significant increase in the hardness (~433 HV) and yield strength (~447 MPa). By contrast, the LSP induced mechanical twins which can act as dislocation blockers significantly improved the yield strength (~423 MPa).
•Surface work hardening takes place after implementation of single UIP/LSP treatment.•Large scale mechanical twins are not observed in the surface of 316L treated by UIP.•Compared with a shallow nano-grain layer (5-10 μm) in samples by LSP, UIP produced a well-defined layer (60-80 μm).•The surface compressive residual stress by UIP was much higher than that by LSP.•Compared with LSP, UIP shows a better strengthening effect on mechanical properties.
•The coatings showed a low roughness and a large thickness without defects when the overlap ratio was 70%.•Formation mechanism of the adhered powders during the EHLA cladding was proposed.•The ...response surface analysis was conducted to investigate the effect of parameters on the width of the single-track to keep the overlap ratio constant.•High-temperature wear properties of the H13 coatings and the 5CrNiMo substrate were compared.
Extreme high-speed laser (EHLA) cladding, which refers to metal deposition at speeds > 20 m/min, is the optimization for conventional laser cladding. Improving the surface properties of 5CrNiMo die steel has always been a research hotspot during engineering application. However, few research reported the enhancement of 5CrNiMo properties by EHLA cladding. The present study aims to optimize the surface roughness and analyze the high-temperature wear performance of the coatings produced by EHLA cladding. Results indicated that a laser power (4000 W), a reasonable scanning speed (376.8 mm/s) and a proper powder feeding rate (20.64 g/min) were essential for achieving the coatings with a low roughness and a large thickness without defects when the overlap ratio is 70%. In addition, the formation mechanism of the adhered powders during the EHLA cladding was discussed. The increase of the velocity and radius of the powders could reduce the adhered powders. Furthermore, the response surface analysis was conducted to investigate the effect of parameters on the width of the single-track to keep the overlap ratio constant. Additionally, friction and wear tests were carried out to discuss the tribological properties of the coatings and the 5CrNiMo substrate. The oxide film on the surface of the H13 coating enhanced the tribological property.
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.
It is challenging to apply laser metal deposition (LMD) in underwater environment to realize on-site repair of marine equipment due to the potential impact of water. In the present work, we report an ...innovative underwater repair technique termed underwater laser metal deposition (ULMD) which can overcome the challenges. This new technique renders both technical and theoretical advancements from the following aspects: (1) A special in-house designed drainage nozzle was integrated with the laser cladding head to create local dry cavity which ensured the successful manufacturing of titanium alloy Ti-6Al-4V in underwater environment; (2) Unique microstructure formation/evolution mechanisms have been revealed for the ULMD process, which significantly differ from those of the in-air LMD process; (3) The hydrogen content has been well controlled during ULMD, which can effectively prevent the formation of hydrogen-induced cracks; (4) The mechanical properties of the ULMD Ti-6Al-4V parts were equal or even better than that fabricated by in-air LMD or SLM technique. In the meantime, a systematically parametric study was performed for this new technique and the experimental results showed that a high laser power (1600 W), a reasonable scanning velocity (800 mm/min) and a cross-hatching strategy were essential for achieving minimum metallurgical defects and full densification, which can provide very helpful guidance for the future research in this area. This research work opens a new avenue which makes underwater 3D-printing an applicable tool when coping with the fabrication and repair of customized components or complex shape parts in underwater environment.
•Powder feeding underwater laser metal deposition was firstly carried out on Ti-6Al-4V.•The quality of Ti-6Al-4V alloy by ULMD was equal or even better than that by in-air LMD.•How the underwater environment affects metallurgical behavior of the melt pool was studied.•Unique microstructure formation/evolution mechanisms were related with ULMD thermal process.•Influence mechanisms of the ULMD microstructure on tensile properties were revealed.
•LMD can repair grooves on 316L SS under proper processing parameters.•Phases formed with different powders were different.•Mechanical properties of the specimens were inferior to that of the ...substrate.•Powder components close to the substrate showed better mechanical properties.
Laser metal deposition was used to repair grooves on 20 mm thickness 316L stainless steel plates using two different 316L stainless steel commercial powders – Fe-0.15C-11.8Cr-0.15Mn-0.2Ni-0.031P-0.56Si-0.05S (wt.%) powder and Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder. Good comprehensive performance of fine metallurgical bonding with were successfully achieved under optimized processing parameters. The microstructure and mechanical properties (micro-hardness, ultimate tensile strength, bending strength, low-temperature impact toughness) of the repaired specimens were investigated. Results indicated that chemical composition (different element contents) of the powders played an important role in determining the microstructure, phases and properties of the specimens. It was found that the microstructure of the specimens repaired with Fe-0.15C-11.8Cr-0.15Mn-0.2Ni-0.031P-0.56Si-0.05S (wt.%) powder was homogeneous and consisted of Cr-rich martensite while a number of cellular dendrite were presented in microstructure of specimens repaired with Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder and the repaired specimens consisted of ferrite and austenite. Due to solid solution strengthening, the average hardness of the specimens repaired with former powder was higher than that of the specimens with later powder. However, in the aspect of mechanical properties, LMD with Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder had better performance than the specimens repaired with another kind of powder. The relationship between the microstructure characteristics and mechanical performances of the repaired specimens was also discussed.
Underwater laser directed energy deposition (UDED) can be employed to repair and maintain the offshore engineering structures due to its advantages of flexible adjustment of feedstock materials and ...controllable heat input to the structures. For the first time, preprepared HSLA-100 steel plates were successfully remanufactured by UDED at an ambient pressure of 0.3 MPa (water depth of 30 m). The relationships between the hyperbaric underwater environment, solidification process, microstructures and mechanical properties of the HSLA-100 steel were clarified. The results show that the cooperation of surrounding water, central gases and gas curtain gas with large flow rates increased the cooling rates of the underwater melt pool. A lath martensitic microstructure with high dislocation densities and a number of inclusions was formed in the sample remanufactured by UDED. The in-situ precipitation of Cu-enriched nanoparticles was caused by the unique intrinsic heat treatment involved in the underwater deposition process. The average diameter of the Cu-enriched nanoparticles increased with increasing laser energy density. The microstructure of the sample remanufactured by UDED was harder than that of the sample remanufactured by in-air DED. The Charpy impact toughness and tensile properties of the samples remanufactured by UDED were close to those of the sample remanufactured by in-air DED. This work demonstrates the feasibility of high-quality remanufacturing of HSLA-100 steel via UDED in a hyperbaric underwater environment. The results obtained in this study could provide useful guidance for the application of UDED to offshore engineering structures.
•The HSLA-100 is firstly repaired by UDED in an underwater hyperbaric environment.•The high ambient pressure influences the metallurgical processes and microstructures.•In-situ precipitation of Cu is determined by the unique underwater thermal cycles.•HSLA-100 steel repaired by UDED possesses high strength and excellent toughness.•This work expands the applications of UDED to the hyperbaric underwater environments.
Quasi-Continuous-Wave laser direct energy deposition offers a number of advantages, such as a lower heat accumulate, a higher cooling rate and improved mechanical properties over Continuous-Wave ...counterpart. However, how pulse frequency of Quasi-Continuous-Wave affects sample's morphology, microstructure and corrosion resistance is not clear. In this work, high‑nitrogen steel samples were fabricated by Quasi-Continuous-Wave laser directed energy deposition with optimized process parameters to investigate the effect of pulse frequency on coating's surface morphology, secondary dendrite arm spacing, micro-hardness, and corrosion resistance based on experiments and temperature field simulation by Finite Element Method. The results indicated that as the pulse frequency increased (5–20 Hz), the surface profile curve of the sample tended to be stable and the surface roughness decreased. Meanwhile, when the pulse frequency increased from 20 Hz to 80 Hz, the maximum peak temperature in the melt pool decreased from 2860 °C to 2690 °C. Correspondingly, the minimum peak temperature increased from 1210 °C to 1480 °C. Once the pulse frequency exceeded 60 Hz, the melt pool can keep continuous during the laser-off period. Furthermore, a lower pulse frequency resulted in a refined equiaxed dendrite microstructure in samples and a more even distribution of nitrogen in the melt pool due to an improved cooling rate and reduced shape control factor. As a consequence, the samples fabricated at a lower pulse frequency exhibited higher micro-hardness and corrosion resistance.
•Effects of pulse frequency on microstructure evolution and corrosion resistance of HNS are investigated.•The thermal behavior and solidification parameters distribution pattern are investigated.•Microstructure refinement and decreased micro-porosity content lead to the enhancement of corrosion resistance.
•The temperature and the size of the molten is affected by the heat accumulation.•The center of the molten pool shifts slightly toward back of the laser beam.•The simulated and experimental ...morphology in repaired area shows a good agreement.•The relationships among the thermal cycle, the grain size and the micro-hardness.
316L stainless steel plate with a trapezoidal groove was repaired by laser metal deposition (LMD) with 316L stainless steel powder. Finite element method (FEM) is adopted to predict the thermal behavior in the molten pool during the LMD. There is an overlapping between the adjacent tracks and adjacent layers during the LMD, forming a three dimensional structure. The influence of the second track in the first layer and the first track in the second layer on the first track in the first layer was investigated by the simulation process and experimental method. The numerical results indicated that the maximum temperature and the dimensions of the molten pool at the second track in the first layer and the first track in the second layer are larger than those at the first track in the first layer. The temperature of the first track in the first layer increases after the deposition of the second track in the first layer and the first track in the second layer. The microstructure evolution of the first track in the first layer was examined by the optical microscope (OM) and scanning electron microscope (SEM). The experimental results show that the thermal cycle caused by adjacent tracks brings about a significant effect on the microstructure and micro-hardness of the first track in the first layer.
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