The magnet/dielectric composites with tunable structure and composition have drawn much attention because of their particular merits in magnetoelectric properties compared with the sole dielectric or magnetic composites. In addition, porous materials at the nanoscale can satisfy the growing requirements in many industries. Therefore, constructing porous metal alloy/carbon nanocomposites is to be an admirable option. Unfortunately, traditional synthesis methods involve multistep routes and complicated insert-and-remove templates approaches. Here we report a facile process to synthesize CoxNiy/C composites via a spontaneous cross-linking reaction and subsequent calcination process, during which multiple processes, including reducing polyvalent metal ions, forming alloy, and encapsulating alloy nanoparticles into porous carbon matrix, are achieved almost simultaneously. By adjusting the feed ratio of Co2+ to Ni2+ ions, controllable composition of CoxNiy/C composites can be gained. It should be noted that the...

Cross-linking-derived Synthesis of Porous CoxNiy/C Nanocomposites for Excellent Electromagnetic Behaviors

A review of ultrasonic testing applications in additive manufacturing: Defect evaluation, material characterization, and process control.

Laser powder bed fusion (LPBF) is a 3D printing technology that can print parts with complex geometries that are unachievable by conventional manufacturing technologies. However, pores formed during the printing process impair the mechanical performance of the printed parts, severely hindering their widespread application. Here, we report six pore formation mechanisms that were observed during the LPBF process. Our results reconfirm three pore formation mechanisms - keyhole induced pores, pore formation from feedstock powder and pore formation along the melting boundary during laser melting from vaporization of a volatile substance or an expansion of a tiny trapped gas. We also observe three new pore formation mechanisms: (1) pore trapped by surface fluctuation, (2) pore formation due to depression zone fluctuation when the depression zone is shallow and (3) pore formation from a crack. The results presented here provide direct evidence and insight into pore formation mechanisms during the LPBF process, which may guide the development of pore elimination/mitigation approaches. Since certain laser processing conditions studied here are similar to the situations in high energy density laser welding, the results presented here also have implications for laser welding.

Direct Observation of Pore Formation Mechanisms during LPBF Additive Manufacturing Process and High Energy Density Laser Welding

During the additive manufacturing (AM) process, energy is transferred from the energy beam to the processed material. The high-energy input and uneven temperature distribution result in the high-temperature gradient, large thermal stress, and warping deformation. The scanning strategy, one of the representative AM processing parameters, plays an important role in the microstructures, mechanical properties, and residual stresses of 3D printed parts. It is necessary to review the current state of research about scanning strategy in additive manufacturing, and this paper seeks to address this need. This review mainly focuses on the scanning strategies in selective laser melting process. Various scanning strategies and their effects on mechanical properties, microstructures, and residual stresses of selective laser melted parts are summarized. Finally, some suggestions on the optimization of scanning strategy for better performance are provided based on the above analysis.

https://link.springer.com/content/pdf/10.1007/s00170-021-06810-3.pdf

Scanning strategy in selective laser melting (SLM): a review

Advanced in situ characterization is essential for determining the underlying dynamics of laser-material interactions central to both laser welding and the rapidly expanding field of additive manufacturing. Traditional characterization techniques leave a critical experimental gap in understanding the complex subsurface fluid flow and metal evaporation dynamics inherent in laser-induced heating of the metal. Herein, in situ ultra-high-speed transmission X-ray imaging is revealed to be essential for bridging this information gap, particularly via comparison with and validation of advanced multiphysics simulations. Imaging on submicrosecond timescales enables correlation between dynamics of the laser-generated vapor–liquid interface and melt pool surface instabilities in industrially relevant alloys. X-ray imaging and complimentary simulations reveal vapor depression oscillations and rapid expansion due to reflection of the processing laser from the front surface of the vapor depression. Pore formation studies at steady state and during prompt removal of laser heating at the end of track reveal that the rapidly solidifying melt pool traps pores near the base of the vapor-filled depression. Moreover, pores within the melt pool are entrained by Marangoni convection which overcomes the force of buoyancy and forces the pores downward from the surface immediately before solidification. Observed solidification kinetics, consistent with previous results, give insight into surface morphology and porosity in the processed material. The information presented here is key for defining the physical models that describe laser-material interaction and ultimately increases our understanding of the emerging field of laser-based metal additive manufacturing.

Ultrafast dynamics of laser-metal interactions in additive manufacturing alloys captured by in situ X-ray imaging

The effects of crack growth direction on the fatigue crack growth of wire arc additive manufactured Ti-6Al-4V were studied. The fatigue crack growth rate (FCGR) of horizontal and vertical samples has different stress intensity factor transition point, ΔKT, which is 11.3 MPa m1/2 and 10.3 MPa m1/2, respectively. The FCGR of vertical sample is 5% higher than that of horizontal sample when the stress intensity factor, ΔK is smaller than ΔKT. The difference in FCGR is resulted from the microstructure characterization and the fatigue crack growth direction.

Anisotropy of fatigue crack growth in wire arc additive manufactured Ti-6Al-4V

A blue diode laser has a higher absorption rate than a traditional laser, while the maximum power is limited We report the structure and laser beam profile of a 250 W high-power blue laser (445 nm) for material processing The absorption rate of the blue laser system for the steel was 275 times that of a single-mode fiber laser system (1070 nm) The characteristics of the steel after laser irradiation were determined, validating the potential of this high-power blue laser for material processing, such as heat treatment and cladding The cost of the developed laser system was lower than that of the existing one To the best of our knowledge, this is the first blue laser with a power as high as 250 W

Development of a high-power blue laser (445 nm) for material processing

Microscale interaction between laser and metal powder in powder-bed additive manufacturing: Conduction mode versus keyhole mode

In this manuscript, weld pool dynamics in laser welding of various series of aluminum alloys were investigated by the in situ X-ray phase contrast imaging system. The experimental results showed that metal irradiated by laser was evaporated immediately, which generated the keyhole. Then metal surrounding the keyhole was melted gradually with the heat from keyhole. The growth rate of keyhole depth had a positive linear correlation with the total content of low boiling temperature elements (TCE), so did the keyhole depth and diameter at the stable stage. Longitudinal view area of the molten pool had a negative linear correlation with the thermal conductivity of aluminum alloy. The measured laser absorption rate had the same variation trend with the ratio of keyhole depth to diameter, and the highest absorption rate of 58% appeared in laser welding of aluminum alloy with TCE equal to 2.1%. Violent fluctuation in keyhole shape was avoided in aluminum alloy with TCE lower than 2.1%, where the surface tension and recoil pressure of metal vapor were balanced. To sum up, the effect of alloy element on weld pool dynamics in laser welding of aluminum alloys was firstly quantified in this manuscript.

Hongze Wang

Papers

Scanning strategy in selective laser melting (SLM): a review

Anisotropy of fatigue crack growth in wire arc additive manufactured Ti-6Al-4V

Development of a high-power blue laser (445 nm) for material processing

Microscale interaction between laser and metal powder in powder-bed additive manufacturing: Conduction mode versus keyhole mode

Effect of alloy element on weld pool dynamics in laser welding of aluminum alloys