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Abstract The low retainability of reinforcing particles in functional composite coatings fabricated by cold spray (CS) is a major problem in maximizing coating properties. In this work, the Ni-coated FeSiAl composite powders were used to fabricate soft magnetic composite (SMC) coatings via CS. The feedstock particles were preheated prior to the entrance of nozzle to improve the retainability of FeSiAl particles and the coating properties. Our results showed that the dense Ni/FeSiAl coatings containing the uniformly-dispersed and crack-free FeSiAl particles were produced. The thermal softening effect of Ni layer, as a result of particle preheating, can effectively increase the volumetric content of FeSiAl particle (from 24.3% to 46.2%) in the composite coating. Particle preheating also conducted to the significant improvement of coating thickness due to enhanced deposition efficiency. Dry-sliding test showed that the tribological performance of SMC coating with particle preheating weakens, owing to lowered microhardness. Enhanced magnetic property was achieved with higher soft ferromagnetic characteristic and decreasing coercivity value at higher particle preheating temperatures. The study on coating formation mechanism highlighted that thermal softening effect can give rise to adiabatic shear instability of Ni layer. This results in the significantly improved retainability of FeSiAl particles and soft magnetic performance. Graphical abstract Unlabelled Image Highlights • Ni-coated FeSiAl powder was used to fabricate soft magnetic composite coating by cold spraying. • Higher retainability of FeSiAl particles was achieved by particle preheating. • Tribological performance of the Ni/FeSiAl composite coatings with particle preheating was studied. • Soft magnetic performance was enhanced at higher particle preheating temperatures. • The thermal softening effect of preheated composite particles was discussed. [ABSTRACT FROM AUTHOR]

Remarkable residual stress is commonly present in metal components produced by laser additive manufacturing due to the inherent rapid heating/cooling rate and high temperature gradient. This is especially true in the laser powder bed fusion (LPBF) process, whereas solid-state phase transformation further adds to its complexities. This work presents the strategies for in-situ controlling the magnitude and distribution of residual stress by regulating layer thickness, preheating temperature, and scanning strategies in LPBF of Ti-6Al-4V alloy. A combination of characterization techniques was used for revealing the residual stress from mesoscale to microscale, measured in millimeters and micrometers, respectively. By manipulating the thermal gradient and heat accumulation in the LPBF process, the residual stress was altered from an inhomogeneous distribution with a local value of up to 600 MPa, to a relatively flat distribution below 100 MPa. This in-situ control method significantly reduced the stress concentration to a level equivalent to the effect of stress-relieving treatment. The findings from this work provide an effective approach for in-situ stress reduction during LPBF, which is critical for the fabrication of large-scale or complicated-shape metal components. • Inhomogeneous distribution of residual stress was found in the as-built Ti-6Al-4 V. • Effective residual stress reduction was achieved during the LPBF process. • The inhomogeneous distribution of residual stress was also successfully altered. • In-situ stress control was achieved by managing thermal gradient and accumulation. [ABSTRACT FROM AUTHOR]

The premature failure of components due to poor inter-particle bonding is the most critical issue in cold spray (CS) additive manufacturing. Herein, a hybrid strategy combining gas-atomization (involving in-situ reaction), CS, and post-friction stir processing was proposed to design a nano-TiB 2 /AlSi10Mg composite. Multiscale characterization in terms of X-ray diffraction and scanning and transmission electron microscopy was conducted to track microstructure evolution for better understanding the mechanisms determining mechanical performance of the produced composites. The results showed simultaneous improvement in both ultimate tensile strength (365 ± 35 MPa) and ductility (16.0 ± 1.2%), which represents a breakthrough. The strengthening and toughing mechanisms were attributed to the fine matrix grains with the significantly improved metallurgical inter-particle bonding, and the uniformly distributed TiB 2 nanoparticles as reinforcement that was strongly bonded with the matrix (i.e. the formation of semi-coherent TiB 2 /Al interface). This study provides new guidance for hybrid additive manufacturing of metal matrix composites with high performance. Image 1 • A hybrid CSAM strategy combining composite powder design was used to produce MMC. • Post-FSP was applied to reduce defects and improve TiB 2 nanoparticle distribution. • Simultaneous improvement in tensile strength and ductility was achieved for MMCs. • The strengthening mechanisms related to interfacial feature were discussed. [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]