Excessive P content (≥ 10 wt%) easily leads to cracks on the surface of electrodeposited NiP coatings, causing rapid deterioration of corrosion performance. This study uses picosecond laser remelting ...(LR) technology to close cracks and improve corrosion performance. The influence of the laser parameters on the quality of laser-remelted (LRed) coatings in ambient air is discussed in detail, and the corrosion performance is evaluated through electrochemical corrosion tests. The results indicated that the scanning rate, laser power, and repetition frequency significantly affect the degree of crack closure and LRed defects. For cracks of similar sizes, both multiple-LR with a lower power (10.4 W, five times) and single-LR with a higher power (19.2 W, once) achieved an excellent crack closure. After single-LR, the surface roughness increased from 3.9 ± 0.3 nm to 33.4 ± 3.2 nm. In addition, the LRed layer crystallized from the initial amorphous state. Although the energy-dispersive spectroscopy results indicated that the oxidation behavior during the LR process was insignificant, X-ray photoelectron spectroscopy showed that the surface composition evolved from Ni/Ni–P compounds to Ni–POx after LR. Electrochemical corrosion tests indicated that the corrosion current density decreased approximately three times from 9.19 × 10−6 to 2.91 × 10−6 A/cm2 after LR. Moreover, the corrosion mechanism shifted from the original stress corrosion to pitting corrosion. Thus, the LR technology can effectively improve the corrosion performance and reduce the issues caused by cracks.
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•Picosecond laser remelting removes surface cracks in NiP coating.•Laser remelted quality mainly affected by laser power and spot overlap rate.•The corrosion rate after laser remelting reduces by approximately three times.•The corrosion mechanism evolved from stress to pitting corrosion.
Here, a hypoeutectic, Fe-modified Al–Ce–Ni alloy (Al–6Ce–3Ni-0.7Fe, wt.%) is studied in terms of microstructure, thermal stability, ambient temperature strengthening, and creep resistance. The ...as-cast microstructure consists of primary Al dendrites and interdendritic binary eutectic regions (Al–Al11Ce3 and/or Al–Al9(Fe,Ni)2), with micron/submicron lamellar spacing, depending on the location along the height of the ingot. The cast alloy exhibits excellent coarsening resistance at 400 °C, with mostly unchanged microstructure and microhardness after 6 weeks of aging, indicating good thermal stability of Al11Ce3 and Al9(Fe,Ni)2. Orowan strengthening and load transfer are identified as strengthening mechanisms at ambient and elevated temperature. A high volume fraction of the intermetallic phases (providing load transfer) and relatively coarse eutectic spacing (for modest Orowan strengthening) result in a moderate as-cast microhardness of 566 ± 32 MPa. Creep resistance at 300 and 350 °C is similar to a binary Al-12.5Ce eutectic alloy (with twice the Ce content) because of two countering effects: Al–6Ce–3Ni-0.7Fe shows a higher volume fraction of strengthening intermetallic phases, but it also exhibits a large fraction of primary Al dendrites which weaken the alloy. By contrast, the alloy, when laser-remelted at the surface, has a fully eutectic microstructure without primary aluminum dendrites achieved by high undercooling on solidification, with a refined network of eutectic phases that doubles the microhardness as compared to the cast alloy. Whereas coarsening is faster due to the shorter diffusion distances between the eutectic phases, hardness remains ~30% higher than the as-cast alloy after ~6 weeks aging at 400 °C.
In this study, high-velocity arc-spraying (HVAS) was employed to produce Fe-based amorphous coatings, and subsequent to that, laser remelting (LR) technology was applied on the as-sprayed coatings ...(ASC) to prepare laser remelted coatings (LRC). The corrosion resistance of both coatings was tested in 3.5 wt% NaCl solutions containing varying Na2S concentrations (0, 20, 50, 100, and 200 ppm) to accurately simulate the corrosion environment of offshore components. The corrosion behavior was investigated using electrochemical workstation, scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The experimental results revealed that LR resulted in the elimination of defects in Fe-based amorphous coatings, a substantial reduction in porosity, the disappearance of the laminar structure, and the achievement of a homogeneous composition. The corrosion tendency of ASC and LRC both exhibited a pattern of initially decreasing, then increasing and finally decreasing with increasing sulfide concentration. This was attributed to the deposition of corrosion products on the surface and the dissolution of the corrosion products. When the sulfide concentration exceeded 50 ppm, the corrosion current density of both coatings slowly and was lower than that at 20 ppm, indicating that both coatings possessed strong corrosion resistance at higher sulfide concentrations. Highly polarized anions in solution (e.g. HS−, S2−) occupied vacancies in the passivation film, destabilizing the coating's passivation film and consequently diminishing its corrosion resistance. From the electrochemical analysis, it was evident that LRC exhibited greater corrosion resistance than ASC at all sulfide-containing solution concentrations, which was attributed to its smoother surface, denser structure and more stable passivation film.
•Laser remelting decreased the porosity of arc-sprayed Fe-based amorphous coating.•Laser remelting increased stability of passivation film on the coating's surface.•Laser remelting improved corrosion resistance in all NaCl media containing sulfide.
Al
85
Ni
8
Y
4
Ce
3
amorphous coatings were prepared on Q235 steel by laser thermal spraying and laser remelting. The structure and thermodynamic behaviour of the coating were analysed by scanning ...electron microscope, energy-dispersive spectrometer, X-ray diffractometer and differential scanning calorimeter. The microhardness distribution and wear properties of the coating were investigated by microhardness tester and wear tester. The results revealed that a small number of pores and AlFeNi crystal phases presented on the surface of spraying coating, and the remelting layer presented a dense structure with a smooth surface. The volume fraction of the amorphous phase in the remelting layer was 89.9% that indicated a better glass-forming ability. The surface microhardness of the spraying coating was 754.2 HV
0.2
, and the hardness of the remelted layer was 847.6 HV
0.2
. After laser remelting treatment, the friction coefficient of the coating decreases by 20% and the wear resistance increases by 31%.
Electrodeposited amorphous Ni-P alloys have enormous internal stress and high brittleness, resulting in cracks and poor wear resistance. In this study, the surface cracks of an amorphous Ni-P ...coating, prepared by laser-assisted electrodeposition were removed by laser remelting. The surface quality was characterized and the wear mechanism was analyzed by evaluating its microhardness and wear resistance. The results showed that the remelted layer had a corrugated structure, and the surface roughness increased. The remelted layer is transformed from amorphous to crystalline, consisting of stable phases of Ni, and Ni-P compounds. Compared with the non-remelted coating, the microhardness and wear resistance of remelted coating are improved. The main wear mechanisms of remelted coatings include abrasive wear, oxidation wear, and plastic deformation.
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Laser-assisted metal processing has received great attention for repair, manufacturing, and surface modification in various industries. However, the mesoscopic segregation and oxidation phenomena ...during these processes require further investigations. In present work, the H13 tool steel is remelted by a laser beam. The generation of oxide layer, the profile and the mesoscopic segregation of the molten pool, Marangoni convection, the and hardening effect are studied by an experimentation combined with a multiphysics coupled simulation. An oxide layer was formed on the remelted surface with a thickness of ∼ 60 µm in the middle and ∼10 µm in the edges. The lightweight elements (Si, V, Cr, and Mn) float upon the molten pool driven by both buoyancy and Marangoni convection. In addition, due to their higher redox activity, they tend to have a higher proportion in the oxide layer with respect to the base metal. As a consequence, elemental mesoscopic segregation appeared in the remelted zone. The oxide layer shows an average hardness of 11.8 GPa, which is similar to that of the remelted zone (11.7 GPa). However, the brittleness of the oxide layer leads to cracks, hence deteriorating its mechanical properties. Together, these results bring a further fundamental understanding of the mesoscopic segregation that is attributed to the Marangoni convection and oxidation phenomenon during the laser processing of alloys, which might be a critical factor to optimize the mechanical properties.
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•During the layer-by-layer laser remelting process, the cooling rate is increased.•The surface quality is improved after layer-by-layer laser remelting.•The microhardness and tensile ...performance are enhanced due to grain refinement.
Laser Powder Bed Fusion (LPBF) is an innovative additive manufacturing technology. But it is also limited by the defects and surface quality. In this work, the layer-by-layer laser remelting (LR) method is applied to LPBF AlSi10Mg to improve the surface quality and mechanical performance. To account for the physical mechanism of the laser remelting, a three-dimension multi-physics coupled transient model is established. The numerical results indicate that the molten pool during the LR process is significantly expanded. The larger molten pool plays a great role in removing the defects. Moreover, the temperature gradient and cooling rate are simultaneously increased during the LR process, which has a considerable impact on the microstructure transformation. The densification, surface quality, including roughness, wettability, and residual stress, microstructure, and mechanical property are investigated after LR treatment based on experiments. The experimental results show that after LR treatment, the densification can be up to 99.4%. The surface hydrophilicity is limited due to roughness reduction. The average grain size of top and side surface can be decreased by 6.74% and 28.79% due to the increasement of cooling rate. The average microhardness and ductility can be improved due to grain refinement and defect elimination.
Microstructural inhomogeneity in additively manufactured materials affects the material properties. The present study aims in minimizing such microstructural inhomogeneity in Ti6Al4V alloy fabricated ...using selective laser melting (SLM) from the gas atomized powder. A detailed and systematic study of the effect of remelting on the microstructure and mechanical properties of Ti6Al4V was undertaken. Acicular α′ martensite was present in all the samples (both in the as-built SLM and remelted) and the dimensions of the α′ phase change with the number of melting steps. The hardness of the as-built SLM sample increased and the material got homogenized with an increasing number of meltings. The ultimate tensile strength was higher in the double- and triple-melted samples while the ductility was lower than the single-melted sample. The present results clearly prove that the number of remeltings play a significant role in determining the microstructure (homogenization of the microstructure) and the mechanical properties of the SLM-built materials.
Selective laser melting (SLM) was used to manufacture fully martensitic (β′1) samples of the shape-memory alloy 81.95Cu-11.85Al-3.2Ni-3Mn (wt%). Crack-free specimens with a high relative density of ...about 98.9 ± 0.1% were produced. Immediate remelting of already processed layers during SLM enhances the relative density (99.5 ± 0.3%). Primarily by varying the scanning speed in the remelting step, the thickness of the remelted zone can be adjusted. Moreover, remelting alters the microstructure as well as the transformation temperatures, which tend to rise with the volumetric energy input. In this way, the shape-memory properties can be modified without compromising the relative density and the considerable plasticity of the samples. Thus, the remelting procedure proves to be an interesting tool for 81.95Cu-11.85Al-3.2Ni-3Mn and related alloys in order to optimize and tailor their performance already during SLM processing and without applying additional post-processing steps.
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•Selective laser remelting enhances the relative density of Cu-11.85Al-3.2Ni-3Mn shape-memory parts.•The grain size can be modified during remelting.•The transformation temperatures can be adjusted in a broad range via remelting.•Selective laser remelting is a promising approach for optimizing the properties of shape-memory alloys.
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•Amorphous/crystalline coatings were prepared by laser cladding with rectangular spot.•Laser remelting can decrease cracks, and improve the surface quality of cladding coating.•Laser ...remelting can refine the microstructure and induce the formation of amorphous phases.•Laser remelting can enhance the corrosion resistant and microhardness of cladding coating.
The WC reinforced Fe-based amorphous composite coatings were prepared by laser cladding with rectangular spot. The effect of laser remelting on the microstructure and properties of composite coatings was investigated. The results showed that laser remelting can reduce the cracks and porosities of the cladding coating and improve its surface quality. Large amounts of crystalline phases were precipitated at the top of the cladding and remelting coatings. However, the microstructure at the top of the remelting coating was finer compared to that at the top of the cladding coating. With increasing distance from the surface of substrate, the amorphous phase appeared within the remelting coating and large amounts of carbides rich in Fe and Mo, Fe23B6, γ-Fe and Cr9.1Si0.9 phases were also precipitated in the remelting coating. As a result, the corrosion resistance of the remelting coating was higher than that of the cladding coating. The microhardness of the remelting coating was approximately 1.13 times higher than that of the cladding coating.
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