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In this study, a laser remelting process was applied to the FeCrCoNiTiAl0.6 high entropy alloy coating in order to improve the density and the surface quality of the coating. The coating was fabricated by high-velocity-oxygen-fuel (HVOF) technology. The microstructure and phase composition of the coating were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), X-ray diffractometer (XRD) and confocal scanning laser microscope (CSLM). Moreover, the wear behavior of the coating was evaluated by use of a ball-on-disc test. The coating was denser after laser remelting treatment by eliminating the previous lamellar structure. The microstructure of the laser-remelted coating exhibits two body-centered cubic (BCC) phases, which is different from the HVOF coating. In addition, aluminum oxide formed during laser remelting. Different from the wear mechanism of the HVOF coating, which comprised abrasion and fatigue, the major wear of the laser remelted coating was abrasion. [ABSTRACT FROM AUTHOR]

• A new method of non-contact ultrasonic-assisted laser metal deposition was used to perform laser deposition of Fe-based powder with 42CrMoA substrate. • The effects of ultrasonic amplitude (UA) and excitation distance (d) on the depth width of deposition layer, columnar crystal length, grain size and distribution, and mechanical properties were studied. • Due to the cavitation effect of ultrasonic, it effectively refines the grains and promotes the columnar to equiaxed transition (CET) of grains at the interface. Meanwhile, the cracks generated in the deposition layer can be effectively inhibited and the mechanical properties can be improved. • The application of ultrasonic-assisted can effectively improve the mechanical properties of the deposition layer, and the uniformity of mechanical properties is improved. In order to study the effects of ultrasonic parameters on the microstructure and mechanical properties of non-contact ultrasonic assisted laser deposition, Fe-based powder was deposited on 42CrMoA steels with different ultrasonic amplitudes (UA) and excitation distances (d). The results show that the ultrasonic-assisted laser metal deposition (LMD) of Fe-based powder increases depth and width of the deposition layers. Due to the cavitation effect of ultrasonic, it effectively refines the grains and promotes the columnar to equiaxed transition (CET) of grains at the interface. When d = 50 mm, the grain size greater than 8 μm accounts for only 3.1 %, but the ultrasonic vibrations haven't changed the phase type of the structures. With ultrasonic-assisted, the cracks generated in the deposition layer can be effectively inhibited and the mechanical properties can be improved. When d = 80 mm and UA = 30 μm, the average microhardness of the deposition layer reaches 772.9 HV, which is 95.8 HV higher than that without ultrasonic-assisted deposition. When UA = 40 μm, the tensile strength is 1237.9 MPa, which is increased by 26.8 % compared to without ultrasonic-assisted deposition. Compared to without ultrasonic-assisted, the friction coefficient of deposition layers is significantly decreased by 25.5 % with the application of UA = 30 μm. [ABSTRACT FROM AUTHOR]

Volcanic ash dispersed in the air adheres to thermal barrier coatings (TBCs) on the hot section components of gas turbines, posing a severe threat to aviation safety. Here, we report a novel strategy inspired by lotus leaf for mitigating the molten volcanic ash adhesion and corrosion. A hierarchical micro-nano structure composed of arrayed micro-conoids and numerous nanoparticles were constructed on the surface of NdYbZr 2 O 7 ceramic pellet by ultrafast laser direct writing technology, whose morphology could be changed by optimizing the processing parameters. The laser-ablated pellet exhibits larger contact angle of molten volcanic ash than the original one, revealing an enhanced resistance repelling volcanic ash. This phenomenon is primarily attributed to the air trapped in the micro-grooves, together with these nanoparticles. Note that the evolution of viscous force and capillary force with time accelerates the transition of molten volcanic ash from the Cassie state to the Wenzel state and reduces its contact angle. Also, this structure effectively inhibits the infiltration of molten volcanic ash by rapidly developing a reaction layer. These findings are an important step toward the design and development of next-generation silicate ash-repellent TBCs. [ABSTRACT FROM AUTHOR]

• A novel composite interlayer prepared with V multi-layers and a CrZrCu plate is introduced to join steel/Ti joint. • A sound steel/Ti joint is successfully obtained using the prepared composite interlayer. • The steel/Ti joint with 1.2 mm-thick V layer possesses the maximum UTS of 418 MPa and an EI of 15 %. • It can be found that Cu solid solution and V solid solution are generated in the FZ and FZ/V interface at TC4 alloy side. Vanadium (V) interlayers of 0.4 mm, 0.8 mm, and 1.2 mm thickness were firstly deposited on the TC4 alloy side using a laser metal deposition (LMD) process, which was then combined with a CuCrZr plate as a composite intermediate layer, for dissimilar laser welding of TC4 titanium alloys to 304 stainless steels (SS) successfully. The effects of different thicknesses of V interlayers on the weld shape, microstructure and mechanical properties of TC4/304SS joints were investigated. The results show that a sound TC4/304SS butt joint with full penetration is successfully obtained using the prepared composite interlayer. The TC4/304SS joint exhibits a maximum ultimate tensile strength (UTS) of 418 MPa and an elongation index (EI) of 15 % with a V layer thickness of 1.2 mm. With the V-layer thickness increasing, the size and content of the V-Cu solid solution in the fusion zone (FZ) on the TC4 alloy side are gradually increased without the formation of intermetallic compounds (IMCs) generated. [ABSTRACT FROM AUTHOR]

Silicide coatings and B-modified silicide coatings were prepared on Nb-Si based alloys using halide activated pack cementation. The hot corrosion behavior of the coatings exposed to Na 2 SO 4 and Na 2 SO 4 + NaCl mixture salts at 900 °C was investigated. The results show that B-modified silicide coatings possess better hot corrosion resistance than the silicide coatings. The hot corrosion products of silicide coatings show a regional microstructure, including uplift regions of AlNaSiO 4 imbedded with Al 2 O 3 phases, and depressed silica regions with NaNbO 3 and TiO 2 particles. The addition of B to the silicide coatings promotes the formation of the uniform and dense boroaluminosilicate scale with better fluidity during hot corrosion, which impedes the self-sustaining cyclic chlorination/sulfidation reactions, and thus, improves the hot corrosion resistance of the silicide coatings. • The hot corrosive scale of silicide coatings possesses high viscosity, resulting in a regional fluctuating microstructure. • The addition of B improves the hot corrosion resistance of the silicide coatings. • B-modified silicide coatings have a flatter hot corrosive scale than silicide coatings due to the better fluidity. • The boroaluminosilicate scale on B-modified silicide coating suppresses the chlorination/sulfidation reactions. [ABSTRACT FROM AUTHOR]

Wire-arc sprayed Al-based composite coatings attracted great attentions due to their prospective application in offshore engineering. However, the difficulty in regulating ceramic reinforcements limited the performance of these coatings. In this work, we proposed a novel rotary swaging (RS) process to improve the deposition of ceramic particles, thus the Al/Al 2 O 3 composite coating was prepared and evaluated in comparison with the traditional coating by spraying the drawing cored wires. As expected, the RS process promotes the melt of Al 2 O 3 particles, leading to a significant increase of their deposition fraction with the simultaneous improvement in coating compactness and the interfacial bonding of heterogeneous splats. The sufficient melt of Al 2 O 3 particles is attributed to the enhanced cohesion strength of filled powders to bear the vibration of arc force and facilitate heat transport. The explored composite coating displays a superior corrosion resistance in 3.5 wt% NaCl aqueous solution, due to the reduced active area on electrode surface and the lower sensitivity to pitting. Benefiting from a dynamic self-reinforcing effect, the explored composite coating presents lower coefficient of friction and significantly enhanced wear resistance under dry frictional condition. The present results demonstrate a great potential of the developed Al/Al 2 O 3 composite coating in offshore application and provide an inspiration for future works aiming to exploit the high-performance metal-matrix composite coatings synthesized by low-cost wire-arc spraying. • The rotary swaging process improved the melt and deposition of Al 2 O 3 particles. • The improved oxide film enhanced the corrosion resistance of composite coating. • The dynamic self-reinforcing effect enhanced the wear resistance of composite coating. [ABSTRACT FROM AUTHOR]

Thermal sprayed high entropy alloy coatings have garnered substantial attention for their remarkable mechanical properties and wear resistance. In this study, we explored the wear behavior and mechanisms of Al 0.6 TiCrFeCoNi coatings produced through atmospheric plasma spray (APS) at both room temperature and 500 °C. The microstructural and mechnical characterization of the APS-sprayed coating was performed using XRD, OM, SEM, CSLM and hardness testing techniques. The dry sliding wear test of the coating against Al 2 O 3 ball were conducted on a pin-on-disc tribometer at room temperature and 500 °C. The coating exhibited a single body-centered cubic (BCC) phase, with a typical lamellar structure marked by oxides and pores. Wear tests showed that the APS-sprayed coating exhibits superior wear resistance at elevated temperatures compared to room temperature due to the formation of compact oxide layer on the wear track. Detailed investigations on wear track reveal that abrasive wear, oxidation wear and fatigue wear are the prominent wear mechanisms on both conditions, while tribo-oxidation is dominated at 500 °C. Furthermore, We demonstrated that the looser, more porous microstructure of the APS-sprayed coating contributes to its enhanced wear resistance, preventing crack propagation and increasing strain tolerance. This provides a promising strategy for outstanding wear resistant coatings by designing looser lamellar coating structure. [Display omitted] • APS-sprayed Al 0.6 TiCrFeCoNi HEA coating exhibited superior wear resistance at elevated temperatures. • The superior wear resistance is attributed to the formation of a compact oxide layer. • At elevated temperature, in-situ oxidation and debris gathering and compaction devoted to the formation of oxide layer. • Loose lamellar structure in thermal sprayed coating reduces fatigue effect by inhibiting crack propagation. [ABSTRACT FROM AUTHOR]

To obtain compatible properties of low thermal conductivity and high thermal stability, Al 0.6 CoCrFeNiTi high-entropy alloy was designed as a novel candidate of metal-based thermal barrier coatings (MBTBCs). The corresponding high-entropy alloy coatings were fabricated by both high-velocity oxygen-fuel spraying (HVOF) and atmospheric plasma spraying (APS), and then the dependence of thermal insulation properties on microstructure was investigated. The both coatings exhibit a simple body-centered cubic (BCC) structure, but present obvious difference in microstructure and defect characters which relates to the evolution of in-flight particles. Benefit from the extremely low thermal conductivity, the APS-deposited coating can increase 13.24 °C of the surface temperature of piston crown and yield a temperature reduction of 19.00 °C along the thickness direction, which mean a positivity on enhancing the power efficiency of vehicle engines without sacrificing the strength of aluminum alloy components. In virtue of a decoupling method, the crucial effect of microstructure on thermal conductivity is disclosed, thus interpreting the excellent thermal insulation property of APS-deposited coating dominated by grain refinement and disordered BCC structure. The present results demonstrate a great potential of high-entropy alloy coatings as thermal barrier application and provide an inspiration for future works aiming to design these coatings to meet specific engineering needs. • High-entropy alloys were firstly proposed to perform a candidate of metal-based thermal barrier coatings. • Low thermal conductivity and high thermal stability were balanced in APS-deposited Al 0.6 CoCrFeNiTi coating. • The application of Al 0.6 CoCrFeNiTi coating enhanced power efficiency and protected aluminum alloy pistons. • The thermal insulation properties depended on microstructure including grain size and atomic arrangement. [ABSTRACT FROM AUTHOR]

In the present research, spherical W Mo alloy powder for additive manufacturing was successfully synthesized by spray granulation (SG) in combination with thermal plasma spheroidization (TPS). Firstly, W and Mo particles (1–3 μm) were banded together through SG and formed porous agglomerated powder with desired particle size distribution and flowability. The porous agglomerated W Mo powder was subsequently melted through thermal plasma, W and Mo dissolved together and formed solid solution accompanied with powder densification during TPS. Through optimization of the process conditions, the spheroidized W Mo alloy powder exhibits homogeneous element distribution, low oxygen content (~0.08%), and good flowability (10 s/50 g), which meet the demand for laser powder bed fusion (LPBF). Remarkably, nearly full dense W Mo alloy with few pores could be fabricated by LPBF based on the spheroidized W Mo alloy powder. Unlabelled Image • Spherical W Mo alloy powder for AM was synthesized combined SG with TPS. • W and Mo formed solid solution accompanied with powder densification during TPS. • Nearly full density W Mo alloy could be fabricated by LPBF based on W Mo alloy powder. [ABSTRACT FROM AUTHOR]

Autogenous laser welding of 5A90 Al⁻Li alloy sheets in a butt-joint configuration was carried out in this study. The microstructure characteristics of the weld metal and base metal in the horizontal surface and the transverse section of the welded joints were examined quantitatively using electron back scattered diffraction (EBSD) technique. The results show that the weld metal in the horizontal surface and the transverse section exhibits similar grain structural features including the grain orientations, grain shapes, and grain sizes, whereas distinct differences in the texture intensity and misorientation distributions are observed. However, the base metal in the horizontal surface and the transverse section of the joints reveals the obvious different texture characteristics in terms of the grain orientation, grain morphology, predominate texture ingredients, distribution intensities of textures, and grain boundary misorientation distribution, resulting in the diversity of the microhardness in the base metal and the softening of the weld metal. However, the degree of the drop in the hardness of the weld metal is highly correlated to the microtexture developed in the base metal.