Search

Studies regarding the wettability transition of micro- and nano-structured metal surfaces over time are frequently reported, but there seems to be no generally accepted theory that explains this phenomenon. In this paper, we aim to clarify the mechanism underlying the transition of picosecond laser microstructured aluminum surfaces from a superhydrophilic nature to a superhydrophobic one under ambient conditions. The aluminum surface studied exhibited superhydrophilicity immediately after being irradiated by a picosecond laser. However, the contact angles on the surface increased over time, eventually becoming large enough to classify the surface as superhydrophobic. The storage conditions significantly affected this process. When the samples were stored in CO 2 , O 2 and N 2 atmospheres, the wettability transition was restrained. However, the transition was accelerated in atmosphere that was rich with organic compounds. Moreover, the superhydrophobic surface could recover their original superhydrophilicity by low temperature annealing. A detailed XPS analysis indicated that this wettability transition process was mainly caused by the adsorption of organic compounds from the surrounding atmosphere onto the oxide surface. [ABSTRACT FROM AUTHOR]

Abstract: WC/Ni coating was formed by laser cladding of a W/C/Ni powder blend. The formed WC crystals have rectangular or quadrangle cross-section shapes with size of 2–30μm. Step, twist and cross growth morphologies of WC formation were observed. The coating contains WC, CW3, WNi, FeW3C, Fe6W6C, W3O, W, C, and (Fe,Ni) phases. [Copyright &y& Elsevier]

Abstract: To reduce the mixed fuel induced excessive wear of the cast iron engine cylinder liners, research on laser alloying of NiCr alloy with powder feeding was performed to locally change both the composition and the microstructure of the liner. The research indicated that laser alloying of 75Ni25Cr on grey cast iron liner demonstrates sound alloying layers free of cracks and porosities. The microstructure of the alloyed layer is composed of pre-eutectic austenite and ledeburite. The alloying element Ni is mainly located in the austenite, while Cr is mainly in cementite. The average hardness is HV0.2500. The corrosion resistance of the alloyed layers in diluted H2SO4 solution and NaOH solution is dramatically improved compared to the grey cast iron. The relative wear resistance of the laser-alloyed 75Ni25Cr layer is 4.34 times of that of the grey cast iron. The improvements on the corrosion and wear resistance of the cast iron are attributed to the composition and microstructure change by laser alloying of 75Ni25Cr. Laser alloying can be a good solution to improve wear and corrosion resistance of the grey iron liners in mixed fuel environment. [Copyright &y& Elsevier]

The microstructural evolution in high power CO2 laser cladding of steel with Stellite 6+WC powder mixture was investigated, among which the WC vol.% was, respectively, 0, 9, 18, 27, 36, 45, 54, 72 and 100% by dual powder feeding Stellite 6 and WC powders with different feeding rates. Two significantly different solidification characteristics were found in the microstructural evolution in laser clads of Stellite 6+WC. The first one was characterized by the dendrites and interdendritic eutectics from 0 to 36% WC, in which the added WC was completely melted into the melt pool and the re-solidified structure contained α-Co,σ-CoCr and various carbides such as M7C3. The second one was characterized by various faceted dendrites in block, flower, butterfly, star shapes and the matrix from 45 to 100% WC, in which most of the WC was melted, the microstructure contained re-solidified WC, Co and various Co–W–C/Fe–W–C complex carbides. Maximum 26 and 64 wt.% W were, respectively, dissolved into the matrix and the faceted dendrites. When laser cladding WC powder with severe dilution from the substrate, a microstructure with different composition appeared within the faceted dendrites with interior approximately 90 wt.% of W and exterior approximately 70 wt.% of W. Such a structure was probably the frozen result of the peritectic reaction during laser cladding rapid solidification. [Copyright &y& Elsevier]

Abstract: Nano-particulate reinforced metal matrix composites (nPRMMCs) exhibit excellent comprehensive properties unmatched by conventional micro-particulate reinforced metal matrix composites (μPRMMCs). However, current techniques for fabricating nPRMMCs usually use nano-powders as raw materials, which are not preferred due to their agglomeration trend and harmful size. In this paper, we developed a technique to fabricate nano-TiCp reinforced Inconel 625 composite coatings by laser cladding of an Inconel 625+5wt.% TiC powder mixture, particle size of the raw powders both in micrometer range. By controlling the specific energy input, the micro-TiCp partially dissolved into nanometer scale. The influence of specific energy input on particle size, morphology and the microstructure, phase constitution and mechanical properties of the composite coatings were investigated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and nano-indentation test. Nano-TiCp reinforced Inconel 625 composite coatings were achieved at the specific energy input of 25.3kJ/g. The hardness and modulus of the nPRMMCs are 3.36GPa and 190.91GPa, increased by 10.33% and 12.39% respectively compared to laser cladded Inconel 625 substrate. The nPRMMCs show potential in applications such as the fabrication of turbine blades and engine components with improved performance. [Copyright &y& Elsevier]

Abstract: Hydrophobic Cu surfaces with tunable regular microstructure and random nanostructures were fabricated by nanosecond pulsed laser texturing and chemical etching. The regular micro-scale roughness can be tuned from 5μm to 30μm Rz by nanosecond laser texturing while the ligament width and spacing of the random nano porosity about 10–50nm can be realized by selective chemical etching. Wettability tests demonstrate that the Cu surface with this micro/nanostructural hierarchy, analogous to that of lotus leaves found in nature, is superhydrophobic with a water contact angle around 153°. The random nanostructure by selective etching plays an important role for hydrophobicity in addition to the regular microstructure by nanosecond laser texturing. [Copyright &y& Elsevier]

Laser metal deposition (LMD) additive manufacturing process was applied to produce ultrafine TiC particle reinforced Inconel 625 composite parts. The effects of laser energy input per unit length (LEIPUL) on microstructure development, densification response, and mechanical performance including wear performance and tensile properties were comprehensively studied. A relationship of processing conditions, microstructural characteristics, mechanical performance, and underlying strengthening mechanisms was proposed for a successful LMD of high-performance Inconel based composite parts. It revealed that using an insufficient LEIPUL of 33 kJ/m lowered the densification behavior of LMD-processed parts, due to the appearance of residual large-sized pores in inter-layer areas of the parts. An increase in LEIPUL above 100 kJ/m yielded the near fully dense composite parts after LMD. On increasing LEIPUL, the TiC reinforcing particles became significantly refined and smoothened via the elevated melting of particle surfaces and the dispersion state of ultra-fine reinforcing particles was homogenized due to the efficient action of Marangoni flow within the molten pool. The dendrites of Ni–Cr γ matrix underwent a successive change from an insufficiently developed, disordered microstructure to a refined, ordered microstructure with the increase of LEIPUL. However, the columnar dendrites of the matrix were coarsened apparently at an excessive LEIPUL of 160 kJ/m because of the elevated thermalization of the input laser energy. The formation of the refined columnar dendrites of Ni–Cr γ matrix combined with the homogeneously distributed ultra-fine reinforcing particles contributed to the enhancement of wear performance of LMD-processed composites with a considerably low coefficient of friction (COF) of 0.30 and reduced wear rate of 1.3×10 −4 mm 3 /N m. The optimally prepared TiC/Inconel 625 composite parts demonstrated a ductile fracture mode with a sufficiently high tensile strength of 1077.3 MPa, yield strength of 659.3 MPa, and elongation of 20.7%. The superior tensile properties of LMD-processed parts were attributed to the significant grain refinement effect of the matrix during laser processing and the efficient prohibition of ultrafine reinforcing particles on the mobility of dislocations. [ABSTRACT FROM AUTHOR]

Abstract: Laser metal deposition (LMD) was applied to deposit Inconel 718 metal matrix composites reinforced with TiC particles. The influence of laser energy input per unit length on constitution phases, microstructures, hardness, and wear performance of LMD-processed TiC/Inconel 718 composites was studied. It revealed that the LMD-processed composites consisted of γ Ni–Cr solid solution matrix, the intermetallic precipitation phase γ′, and the TiC reinforcing phase. For the laser energy input per unit length of 80–120kJ/m, a coherent interfacial layer with the thickness of 0.8–1.4μm was formed between TiC reinforcing particles and the matrix, which was identified as (Ti,M)C (M=Nb and Mo) layer. Its formation was due to the reaction of the strong carbide-forming elements Nb and Mo of the matrix with the dissolved Ti and C on the surface of TiC particles. The microstructures of the TiC reinforcing phase experienced a successive change as laser energy input per unit length increased: Relatively coarsened poly-angular particles (80kJ/m) – surface melted, smoothened TiC particles (≥100kJ/m) – fully melted/precipitated, significantly refined TiC dendrites/particles (160kJ/m). Using the laser energy input per unit length ≥100kJ/m produced the fully dense composites having the uniformly dispersed TiC reinforcing particles. Either the formation of reinforcement/matrix interfacial layer or the refinement in TiC dendrites/particles microstructures enhanced the microhardness and wear performance of TiC/Inconel 718 composites. [Copyright &y& Elsevier]

Abstract: Fe-based composite coatings reinforced by in situ synthesized particles have been produced by laser melting a precursor powder mixture of Fe-based alloy powder, Ti, Zr and C. The microstructure of the coating is typically hypo-eutectic, which consists of martensite, residual austenite, ledeburite and dispersed in situ carbide particles. The experimental results show that laser melting a precursor containing both Zr and Ti is favorable to form particles in the maximum volume fraction compared with those containing either of them. The particles are investigated to be multiple carbides. [Copyright &y& Elsevier]

Various weight ratios of TiC particulate nickel-based composite coatings were produced on carbon steel substrates by laser cladding. The experimental results showed that the coating was uniform, continuous and free of cracks when a CaF2 flux was used. Excellent bonding between the coating and the medium carbon steel substrate was ensured by the strong metallurgical interface. The microstructures of the coating were mainly composed of γ-Ni dendrites, M23C6, a small amount of CrB, and dispersed TiC particles. The morphologies of TiC particles changed from the globular, cluster to flower-like shape. The volume fraction of TiC particles and the microhardness gradually increased from the bottom to the top of the coating, and the volume fraction of TiC particles increased with increasing of volume fraction of Ti and C. The maximum microhardness of the coatings with 10, 15 and 20 wt.% (Ti+C) in the preplaced alloy powder was approximately HV0.2750, HV0.2850 and HV0.2920, respectively, which was much larger than that of the steel substrate, HV0.2240. [Copyright &y& Elsevier]

In order to improve resistance to high temperature and wear resistance of pure aluminum, laser-cladding experiments were conducted on a 3 kW continuous wave CO2 laser to produce in situ synthesized molybdenum silicide (MoSi2)/SiC composite coatings on pure aluminum. The laser consolidified microstructure and phase evolution were investigated using scanning electron micrograph and X-ray diffraction (XRD). The in situ formation of MoSi2/SiC was carried out by adding a mixture of Mo, Si and C precursor powder layer followed by melting the layer and substrate under a laser beam. The experimental results showed that these compounds, MoSi2 and SiC, can be formed from three distinct powders of Mo, Si and C on Al alloy, and, provided that supplied energy is sufficient to start the reaction between the initial products, a good metallurgical bond between coating and the substrate can be achieved. The microstructures of the coating were composed of MoSi2, Mo5Si3 and SiC phases. The microhardness of coating was about 12 times that of the aluminum substrate, the maximum value reached HV0.2 1200. [Copyright &y& Elsevier]

Detailed experiments were carried out to investigate the effects of crystallography on the microstructure evolution during the solidification of the laser molten pool with a DD2 single crystal specimen using a 5 kW CO2 laser. The experimental results showed that crystallographic effects can influence dendrite growth by favoring growth along a preferred crystallographic direction, and crystallographic effects are greater than that of heat flux during dendrite growth. On the same crystal face, the microstructures in the molten pool are very different when laser beam scanning along different crystal orientation. The same conclusions can be obtained by laser beam scanning along a fixed direction on different crystallographic surface. A growth velocity analysis was carried out during laser surface remelting processing based on a minimum velocity or minimum undercooling criterion and the relationships between dendrite growth velocities in the three <100> directions, moving velocities of the solid/liquid interface and laser scanning velocities for the different crystallographic orientations were also set up. According to this analysis, the reconstructed and predicted molten pools concurred with the experimental results very well. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports an investigation into the mechanisms of three-dimensional manganese-based nanoporous structure (3D-Mn-NPS) fabricated by laser deposition of Mn–Cu precursor followed by selective dealloying of Cu component. The micro/nano-structures of both the precursor alloy and the post-dealloying 3D-Mn-NPS are characterized. It has been found that compositional inhomogeneity in the precursor alloy is crucial to the final structure of 3D-Mn-NPS. Additionally, the formation of 3D-Mn-NPS is a coupled process of nanopore/nanoligament development and Mn oxidation. The insights from the study on the formation of our 3D-Mn-NPS can be extended to guide the design and fabrication of other nanoporous metals. [Copyright &y& Elsevier]