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Fully dense and thick Ni/FeSiAl soft magnetic composite deposits were successfully fabricated by cold spraying (CS) using the Ni-coated FeSiAl composite powder. To improve magnetic properties, the post-annealing treatments were conducted on CS deposits to eliminate the undesired internal defects, like strain stress and dislocations, and fine grains. Characterization of the composite coating was carried out in terms of X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) analyses. XRD patterns displayed that no newly formed phases were observed at different annealing temperatures, even though EDS analysis showed obvious element diffusion between Ni matrix and FeSiAl particles at the temperature of 800 °C for 2 h. Grain growth and stress relief after annealing treatment were proved by both XRD and EBSD analyses. The magnetic property measurements showed that the coercivity was decreased by around two times (from 62 to 30 Oe) and the magnetization saturation was increased by 15% (from 68 to 80 emu/g). The relevant mechanisms were discussed in terms of microstructural evolution. This work shows that the post heat treatment can be a potential method to improve the soft magnetic properties of CS coating. • Ni-coated FeSiAl powder was used to fabricate Ni/FeSiAl SMCs by cold spraying. • Generation of defects during cold spraying deteriorates the magnetic performance. • Significant improvement of magnetic properties was achieved by post-annealing. • The strain stress and grain size evolution were studied by XRD and EBSD. [ABSTRACT FROM AUTHOR]

The residual stress and distortion caused by non-uniform temperature distribution has limited the application of cold spray (CS) technique as an effective additive manufacturing and damage repair method for large-scale components. In order to investigate the influence of heat accumulation on coating quality, two types of CS Ni coatings with different building dimensions were deposited onto substrates. The type II Ni coatings with smaller fabrication size possess a fully dense microstructure without cracks, while the type I coatings with larger size show a significantly higher porosity value. The type II coatings show relatively higher microhardness. The tribological tests show that the type II Ni coatings present a better wear resistance with lower coefficient of friction and lower wear rate. The distinct worn morphologies show that the lower microhardness and porous microstructure of type I coatings lead to numerous delamination and exposure of interior coating. According to the temperature history recorded by infrared imaging camera, higher average temperature was found at the type II coatings due to enhanced heat accumulation of coating/substrate system at the short nozzle movement. The increased particle plastic deformation can further promote the coating densification rate and further ameliorate the coating quality. It can be summarized that the nozzle trajectory and deposition strategy can affect the temperature distribution and determine the coating qualities. This work provides a potential guidance for the optimization of coating quality by cold spraying additive manufacturing. • Effect of heat accumulation on coating properties in cold spray is studied. • Ni coatings with distinct building dimensions were deposited on substrates. • Ni coatings with smaller size possess lower porosity and better wear resistance. • The higher temperature at smaller building dimension brings better coating quality. [ABSTRACT FROM AUTHOR]

The effect of a high magnetic field on the microstructure in directionally solidified NiAl-Cr(Mo)-Hf eutectic alloy was investigated experimentally. The obtained results indicated that the application of the magnetic field caused the degeneration of the regular lamellar eutectic structure and induced the columnar-to-equiaxed transition (CET) of eutectic dendrites. In addition, the magnetic field modified the macroscopic solid-liquid interface shape and decreased the volume fraction of the Heusler phase. Furthermore, the amplitude and distribution of the thermoelectric magnetic force (TEMF) and thermoelectric magnetic convection (TEMC) in NiAl-Cr(Mo) eutectic alloy during directional solidification under a high magnetic field were studied by numerical simulation at different scales. It was found that the abovementioned results should be attributed to the coupling effects between the TEMC and TEMF. Besides, present work may provide a new method for modification of eutectic morphology during directional solidification under a high magnetic field. [ABSTRACT FROM AUTHOR]

Effect of a high magnetic field on the microstructure in directionally solidified Ni-rich and Al-rich Ni 3 Al alloys with composition of Ni-21.5Al-0.4Zr-0.1B and Ni-26.5Al-0.4Zr-0.1B (at%), named as Ni-22Al and Ni-27Al hereafter, was investigated experimentally. Results revealed that the primary dendrites, γ-Ni dendrites in Ni-22Al and β-NiAl dendrites in Ni-27Al, were both broken by the magnetic field. However, the γ′ phase transformed into equiaxed grains along with primary dendrites in Ni-22Al, while the γ′ phase in Ni-27Al kept directional growth. The above results should be attributed to the combined action of the thermoelectric magnetic force (TEMF) acting on the dendrites and the different solidification processes of γ′ phase in these two alloys. Further, this study may initiate a method to fabricate a novel two-phase composite consisting of single crystal matrix and equiaxed dendrites. [ABSTRACT FROM AUTHOR]

The microstructure evolution and room temperature fracture toughness of directionally solidified NiAl–31Cr3Mo–0.2Si (at%) near-eutectic alloy at different withdrawal rates were investigated. The results showed that NiAl dendrites were eliminated and fully eutectic structure was achieved by applying appropriate withdrawal rates. The microstructure evolved from planar eutectic to primary NiAl dendrites + cellular eutectic and then to dendritic eutectic with the increasing withdrawal rate. Correspondingly, the interlamellar spacing and cell size decreased gradually. The relationship between eutectic interlamellar spacing with withdrawal rate can be summarized as λ = 4.74 V − 0.38 . The fracture toughness of alloys with fully eutectic structure was much higher than that of alloys with NiAl dendrites. It increased up to 2.5 times. The high fracture toughness may be attributed to some toughening mechanisms, such as crack bridging, crack deflection, interface debonding and microcrack linkage. The existence of primary dendrites would seriously deteriorate the fracture toughness. [ABSTRACT FROM AUTHOR]

A Ti-48Al-2Cr-2Nb alloy was prepared by laser directed energy deposition (DED). The microstructure of the DED-produced Ti-48Al-2Cr-2Nb alloy exhibited a coarse columnar structure along the building direction. Through a combination of experiment and numerical simulation, the temperature gradient and solidification rate of the single-track sample were calculated theoretically. The columnar to equiaxed transition (CET) behavior of molten pool solidification was quantitatively analyzed, and the microstructure transition diagram was established. The effect of TiB 2 on the microstructure and mechanical properties of the TiAl alloy was investigated. The columnar changed to equiaxed grains in the vertical cross section by adding nanometer-sized TiB 2 particles and the microstructure was significantly refined. The grain size was reduced from 48.5 μm to 4.9 μm. Some equiaxed γ grains were distributed around the banded TiB 2. The compressive properties were improved from 628.4 MPa to 698.8 MPa by adding nanometer-sized TiB 2 particles. Finally, the solidification behavior and the microstructure refinement mechanism caused by the addition of TiB 2 particles were discussed. • The crack-free TiB 2 /TiAl composite was successfully prepared using DED and induction heating equipment. • A solidification microstructure transition diagram for the single-track TiAl alloy prepared by DED was established. • The solidification behavior and microstructure refinement mechanism of TiB 2 /TiAl composites prepared by DED were elucidated. [ABSTRACT FROM AUTHOR]

Electron beam powder bed fusion (EBPBF) was used to fabricate a Ti–48Al–2Cr–2Nb alloy incorporated with 0.2 wt% Y 2 O 3 nano-ceramic powders. Spherical Y 2 O 3 nanoparticles with the particle size of 20–40 nm were uniformly distributed throughout the as-built TiAl alloy, and the average particle size was 30 nm. The effect of Y 2 O 3 nanoparticles on compressive creep properties was investigated and the creep tests were performed at 750 and 800 °C, respectively, with a stress of 250 MPa. The results showed that the compressive creep properties were significantly improved by Y 2 O 3 nanoparticles. After 50h creep test at 800 °C, the maximum creep strain decreased from 59.22% for the Y 2 O 3 -free alloy to 28.08% after the incorporation of Y 2 O 3 nanoparticles, and the minimum creep rate decreased from 2.56 × 10−6 s−1 to 1.53 × 10−6 s−1 correspondingly. The maximum creep strain and minimum creep rate of the Y 2 O 3 bearing Ti–48Al–2Cr–2Nb alloy were 110.9% and 67.3% lower than those of the Y 2 O 3 -free alloy at 800 °C, respectively. The content of high angle grain boundary (HAGB, >15°) increased from 62.1% to 92.9% with the creeping temperature increasing from 750 °C to 800 °C for free-particle TiAl alloy. Large amounts of fine recrystallized grains were observed for Ti–48Al–2Cr–2Nb alloy after 50h creep test at 800 °C, while the recrystallization was prevented by Y 2 O 3 nanoparticles addition. Finally, the effect of Y 2 O 3 nanoparticles addition on the strengthening mechanism and recrystallization behavior of TiAl alloy was discussed. This work suggested that the high-temperature performance of TiAl alloy can be tailored by the incorporation of highly thermal-stable Y 2 O 3 nanoparticles in EBPBF, which is likely to enhance the service temperature of additively manufactured TiAl alloy for high temperature critical applications. [ABSTRACT FROM AUTHOR]

Industrial robots are widely used in cold spray (CS) as well as thermal spray to produce various coatings by precisely controlling kinematic parameters during the process. However, the robot trajectory and its effect on the characteristics of CS deposits are important, but not fully studied. This article introduces four typical spray trajectories, including zigzag path, cross path, parallel path, and spiral path, to elaborate thick CS Cu deposits, and characterizes the corresponding Cu deposits, respectively. The experimental results revealed that the spray trajectories have a major influence on the associated thermal history and the residual stress distribution. However, no significant differences were found by the comparisons of microstructures and mechanical properties. In addition, due to high-velocity impact and extensive deformation of particles during the CS process, the as-sprayed Cu coating presented anisotropic characteristics that influenced the performances in different directions. The post heat treatment (PSH) cannot eliminate this characteristic but improves the mechanical performances to a certain extent. Therefore, reasonable planning of CS trajectory is an indispensable condition for obtaining a desirable coating. • Clarify the characteristics of CS coating produced by four typical paths. • As-sprayed Cu coating presented anisotropic characteristics. • CSed coating anisotropy could be attributed to the multiple parallels or cross spraying. • Heat treatment does not eliminate the anisotropy but improves the mechanical performance. [ABSTRACT FROM AUTHOR]