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Additive manufacturing of an equimolar CoCrNi medium entropy alloy (MEA) by selective laser melting (SLM) was investigated, emphasizing its microstructure, properties, and metallurgical defects. It was found that SLM sample density exhibited a non-monotonic relation with their volume energy density (VED); the density first increased but then decreased while the input VED was gradually increasing. A maximal relative density of 98.9 pct was accessible at a VED of 83.3 J/mm3. X-ray diffraction indicated that the printed CoCrNi MEA exhibited an FCC single-crystal structure where the lattice constant decreased with the increasing VED owing to the evaporation of the Cr element in the SLM process. The average grain size gradually increased with increasing VED, irrespective of whether viewed from the top or the side of the printed part. Similarly, the residual stress increased with increasing VED, which produced more microcracks and deteriorated the tensile preformation of the printed samples, although the yield strength (630 MPa) showed no apparent difference at different VEDs. Owing to the ultrafine cell structure, the mechanical property of the SLM printed sample was twice that of cast or wrought CoCrNi MEA.

Dense boron carbide bearings with a central through hole were fabricated by hot-press sintering. When the sintering temperature was above the critical grain growth temperature, an increase in the applied pressure at 2100 °C caused a distinct abnormal grain size region. Two grain growth mechanisms were explored; normal grain growth (NGG) is controlled by a grain boundary diffusion mechanism whereas the abnormal grain growth (AGG) pattern abides by the ‘Bose–Einstein’ grain growth law.

Spark plasma sintering (SPS), as a novel powder consolidation method, has great potential in the production of high entropy alloys (HEAs). The heating rate is an important parameter in SPS, and understanding its effect is a critical issue for the purpose to optimize microstructure and promote mechanical properties of HEAs. The AlCoCrFeNi was adopted as a representative HEA in this study. The phase compositions, microstructural textures, and mechanical properties were investigated in this study in detail. It was found that the FCC phase formed at BCC grain boundaries while the σ phase precipitated at the interface of B2/BCC no matter of the heating rate. The texture varied at different heating rates as the texture {110} in B2/BCC was characterized at the heating rate of 150 °C/min while the complex texture {051} in FCC was observed at the heating rate of 200 °C/min. The Vickers hardness and wear resistance decreased with increasing of the heating rate. Furthermore, the compressive strength and corresponding plastic strain reached their highest 2752.0 ± 48 MPa and 26.5 ± 1.3% at the heating rate of 150 °C/min.

Whether selective laser melting (SLM) printed AlMgScZrMn alloy possesses more excellent performances than other processing routes was unknown. In the study, the microstructures and mechanical properties of AlMgScZrMn alloys fabricated by selective laser melting (SLM), spark plasma sintering (SPS) and cast were comparatively investigated. SLM printed sample shows columnar and equiaxed duplex grains with average grain size about 7 μm, which is coarser than SPS consolidated sample with finest equiaxed grain (4 μm) but smaller than casting sample with coarse equiaxed grain (25 μm). SLM printed sample shows the highest tensile strength (394 ± 5 MPa) and elongation (10.5 ± 1.2%) than SPS sample (231 ± 3 MPa, 0.59 ± 0.21%) and cast sample (188 ± 2.3 MPa, 3.7 ± 0.58%), although SPS sample has the finest equiaxed grains. For SPS sample, previous particle boundary (PPB) with oxide inclusion significantly deteriorates ductility. For cast sample, the coarse equiaxed grain with intergranular eutectic leads to inferior strength. In addition, the average dimension of Al3(Sc,Zr) particle in SLM sample is about 1–10 nm, which is much finer than SPS sample (0.5–0.8 μm) and cast sample (1–2 μm). The present study will provide a basis for the future industrial applications of the AlMgScZrMn alloy.

A densification creep model was proposed to analyze the densification mechanism involved in spark plasma sintering (SPS) of boron carbide containing low-melting-point sintering aid. This model incorporated the effect of the sintering aid on relative density as well as the actual internal stress during SPS of boron carbide ceramics. The internal stress bias caused by sintering aid was considered and a more accurate effective stress was used to investigate the kinetics during SPS. The proposed model explains why an increase of sintering aid amount decreases the sintering activation energy and promotes densification.

The traditional theory attributes the rapid densification of spark plasma sintering (SPS) to the thermal effect of electric current. However, the nonthermal effects of electric current are still not well understood, despite its proven significance. In this study, the mechanisms by which nonthermal effects contribute to the rapid densification of tungsten powder during SPS were investigated. Electromigration was considered as an important nonthermal effect of current, and an electromigration-enhanced creep model incorporating the contribution of temperature, pressure, and electric current on densification was constructed. The kinetics model agreed well with the experimental data and was applied to evaluate the contribution of electromigration during SPS quantitatively. Although the contribution of electric current is relatively minor compared with the temperature and pressure, it is a driving force to be reckoned with in densification.

Although laser solid forming (LSF) has a wide range of applications in material manufacturing, the technique has been severely limited due to the solidification cracks during rapid laser melting/solidification. In this paper, an LSF technology assisted by friction stir processing (FSP) is proposed to eliminate the LSF crack through the FSP thermomechanical coupling effect, with the Ni–16Cr–8Fe alloy as a representative material. By FSP-assisted LSF, the cracks at the top surface of the Ni–16Cr–8Fe alloy layer were eliminated. Meanwhile, the severe plastic deformation layers can be observed and gradient grains with tens of nanometers to tens of micrometers from the top surface to the inner. The LSF-printed dendrite microstructure was transformed into nanocrystals or nanotwins in the deformation zone with twining as the main deformation mechanism. The grain refinement results in the increase of hardness about 30%.

Theoretical understanding in the innate function of electric current on the spark plasma sintering of amorphous powder is very limited. We proposed an improved isothermal model considering particle size evolution in this work to study the effect of current mode on shrinkage of amorphous Fe 41 Co 7 Cr 15 Mo 14 Y 2 C 15 B 6 powder. It revealed a reduction in the activation energy from 71.1 ± 1.6 kJ·mol −1 under direct current to 55 ± 2.0 kJ·mol −1 under pulse current, representing a resultant acceleration of densification by adjusting current mode. The enhancement of consolidation is attributed to increase of electric density induced by pulse current.

Micron-sized boron carbide (B 4 C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime ( n = 2, Q d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb ( n = 6.11).

Although laser deposition has found its widespread applications in materials manufacture, this technology has been significantly limited by the associated metallurgical defects, such as pores, brittle phase precipitation along grain boundary during the laser melting/solidification process. In this paper, we used a novel laser deposition technology assisted by friction stir processing (FSP) to eliminate these common metallurgical defects. A Fe–Cr–Si alloy was selected as a representative material. Severe plastic deformation caused by FSP has refined the coarse solidification microstructure into ultrafine grained or even nano-grained microstructure on the top surface of the Fe-Cr-Si samples. Meanwhile, the large and interconnected Fe-Cr intermetallics precipitates were broken down to nanoparticles. The refinement of both primary grains and intermetallic particles results in an increase of microhardness from 640 HV to above 700 HV and a decrease of wear rate from 4.8 × 10−14 to 3.2 × 10−14 mm3/m.

Although laser cladding has find its widespread application in surface hardening, this technology has been significantly limited by the solidification crack, which usually initiates along grain boundary due to the brittle precipitation in grain boundary and networks formation during the laser rapid melting/solidification process. This paper proposed a novel laser cladding technology assisted by friction stir processing (FSP) to eliminate the usual metallurgical defects by the thermomechanical coupling effect of FSP with the Ni-Cr-Fe as representative coating material. By the FSP assisted laser cladding, the crack in laser cladding Ni-Cr-Fe coating was eliminated and the coarse networks of laser cladding coating was transformed into dispersed nanoparticles. Moreover, the plastic layers with thicknesses 47–140 μm can be observed, with gradient grain refinement from substrate to the top surface in which grain size reached 300 nm and laser photocoagulation net second phase crushed in the layer. In addition, cracks closed in the plastic zone. The refinement of grain resulted the hardness increased to over 400 HV, much higher than the 300 HV of the laser cladding structure. After FSP, the friction coefficient decreased from 0.6167 to 0.5645 which promoted the wear resistance.

Rapid densification associated with spark plasma sintering has been well documented; however theoretical understanding is still very limited. In this work we proposed a densification creep model to explicitly elucidate the effect of current density on densification kinetics, by taking electromigration into account. This new model was then validated experimentally. It is found that an increased current density decreases densification activation energy (energy barrier), thus accelerating powder densification. In essence, the rapid densification is caused by direct current that accelerates mass transport and increases dislocation mobility.

The strong electric current in spark plasma sintering (SPS) often gives sintered materials unique microstructures. In the present study, it is found that the Co3W grains formed at the W/Co bonding interface preferentially grow along [2 1 ¯ 1 ¯0] (or its equivalent orientations) in the current direction during SPS, which is significantly different from that without current. We propose that the resistance anisotropy of grains under electromigration leads to anisotropic atomic diffusion, inducing the preferential growth of grains under current. This work may provide an approach and a theoretical foundation for the preparation of materials exhibiting directional growth.

Al0.7CoCrFeNi high-entropy alloy (HEA), as a promising material, has attracted growing interest. This study aimed to investigate the effect of phase transformation in non-equilibrium Al0.7CoCrFeNi HEA on its properties and densification kinetics during spark plasma sintering (SPS). We found that the BCC-to-FCC phase transformation causes a grain size reduction from 5.66 to 3.10 μm, while decreases the Vickers hardness from 372.5 ± 4 to 328.4 ± 3 HV. The decreased Vickers hardness reduces the wear resistance with the volume wear rate increasing from 1.583 × 10−4 to 2.108 × 10−4 mm3·N−1·m−1. Besides, the BCC-to-FCC phase transformation improves the ductility from 2.5 ± 0.4 to 8.4 ± 0.3%. The further precipitation of a θ phase observed in the sample sintered at 1100 °C improves the ultimate tensile strength from 856.1 ± 13 to 1053.7 ± 8 MPa, although the ductility decreases from 8.4 ± 0.3 to 7.3 ± 0.3%. Interestingly, we also observed that the BCC-to-FCC phase transformation reduces densification kinetics, because it decreases the diffusion coefficient.