Under the laser directed energy deposition (LDED) process, the rapid melting and solidification usually lead to the emergence of pores and coarse columnar dendrites, which in turn compromise the properties of the deposited alloys. This study introduced in-situ ultrasonic rolling (UR) as an innovative method to enhance the corrosion resistance of LDED specimens, and the microstructural characteristics and their correlation with corrosion resistance were deeply investigated. The findings reveal that the LDED-UR specimen exhibits a reduction in both the fraction and size of pores. Under the influence of severe plastic deformation generated by LDED-UR process, fully equiaxed grains appear with a reduced average size of 28.61 µm (compared to 63.98 µm for the LDED specimen with columnar grains). The electrochemical corrosion resistance of the LDED-UR specimen is significantly enhanced compared to the LDED specimen. This enhanced corrosion resistance can be attributed to the low fraction of small-sized pores, the fine and uniformly distributed Cr-enriched ferrite phase, and the formation of a compact and thick passive film due to dense grain boundaries. The insight of the correlation between microstructure and corrosion behavior opens up a new pathway to enhance the corrosion resistance of LDED specimens.

In this study, to improve the fatigue strength of the LDED (laser-directed energy deposition) 316L stainless steel, an in situ ultrasonic rolling technology is developed to assist the laser-directed energy deposition process (LDED-UR). The microstructural characteristics and fatigue behavior are comprehensively discussed. The results show that the average size of pores of the LDED-UR alloy is about 10.2 μm, which is much smaller than that of the LDED alloy (34.1 μm). Meanwhile, the density of the LDED alloy is also enhanced from 98.26% to 99.27% via the in situ ultrasonic rolling. With the application of the in situ ultrasonic rolling, the grains are transformed into fully equiaxed grains, and their average grain size is greatly reduced from 84.56 μm to 26.93 μm. The fatigue limit of the LDED-UR alloy is increased by 29% from 210 MPa (LDED alloy) to 270 MPa, which can be ascribed to the decreased porosity and the fine grains. In particular, the crack initiation site of the LDED alloy is located at the surfaces, while it is nucleated from the sub-surface for the LDED-UR alloy. This is mainly attributed to the compression residual stress induced by the in situ ultrasonic rolling. This research offers a valuable understanding of the failure mechanisms in additively manufactured metals, guiding the development of effective strategies to improve their fatigue threshold under severe operating conditions.

Isothermal deformation experiments of the Hastelloy C276 alloy were executed using the Gleeble-3500 hot simulator at a temperature range of 1000–1150 °C and a strain rate range of 0.01–10 s⁻¹. Microstructural evolution mechanisms were analyzed via transmission electron microscope (TEM) and electron backscatter diffraction (EBSD). Results reveal that the influences of hot compression parameters on the microstructure variation features and flow behaviors of the Hastelloy C276 alloy were significant. The intense strain hardening (SH) effects caused by the accumulation of substructures were promoted when the strain rates were increased, and true stresses exhibited a notable increasing tendency. However, the apparent DRV effects caused by the annihilation of substructures and the increasingly dynamic recrystallization (DRX) behaviors occurred at high compressed temperature, inducing the reduction in true stresses. In addition, a physical-based (PB) constitutive model and a long short-term memory (LSTM) model optimized using the particle swarm optimization (PSO) algorithm were established to predict the flow behavior of Hastelloy C276 alloy. The smaller average absolute relative error and greater relation coefficient suggest that the LSTM model possesses a higher forecasting accuracy than the PB model.

In order to improve the hardness and wear resistance of titanium alloys, an equimolar ratio high-entropy alloy (HEA) FeCuNiTiAl coating was fabricated on the surface of titanium alloy Ti6Al4V by means of laser metal deposition for the first time. The microstructure and composition of the HEA coating and the transition zone were observed by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results show that HEA coating and Ti6Al4V have suitable metallurgical bonding, and no defects, such as cracks, are found at the interface. The hardness of the HEA coating is between 450 and 500 HV_0.5, which is about 1.5 times that of the Ti6Al4V substrate. Wear tests show that the wear rate of HEA coating is 0.89 × 10⁻⁵ mm³/(N·m), while that of Ti6Al4V reaches 53.97 × 10⁻⁵ mm³/(N·m), and the wear resistance of substrate is increased 60 times by the HEA coating. The wear mechanism of the Ti6Al4V substrate is mainly abrasive wear, and the wear mechanism of FeCuNiTiAl HEA coating is mainly adhesive wear, accompanied by slight oxidation wear and abrasive wear.

Stainless steel parts with complex shape can be fabricated using additive manufacturing, which do not rely on molds and dies. However, coarse dendrites induced by repeated heating of additive manufacturing result in weak properties, which limits its application. In this study, an in-situ ultrasonic rolling (UR) device was developed to assist the laser directed energy deposition (LDED) process. The microstructural characteristics, as well as the microhardness and wear behavior, were studied for the 316L stainless steel manufactured by in-situ ultrasonic rolling assisted LDED. It is found that austenite, ferrite, and small Si oxides are the main constituents of both the LDED and LDED-UR alloy samples. Under the severe plastic deformation of ultrasonic rolling, the long-branched ferrites by LDED are transformed into the rod-like phases by LDED-UR. Meanwhile, the ferrite is more uniformly distributed in the LDED-UR alloy sample compared with that in LDED alloy sample. Columnar grains with the size of 97.85 µm appear in the LDED alloy sample, which is larger than the fully equiaxed grains (22.35 µm) of the LDED-UR alloy. The hardness of the LDED-UR alloy sample is about 266.13±13.62 HV_0.2, which is larger than that of the LDED alloy sample (212.93±12.85 HV_0.2). Meanwhile, the wear resistance is also greatly enhanced by applying the assisted in-situ ultrasonic rolling. The achieved high wear resistance can be ascribed to the uniformly distributed hard matter (ferrites) and the impedance of dislocations by high fraction of grain boundaries. Abrasive wear and adhesive wear are identified as the primary wear mechanisms of the studied alloy. Gaining an in-depth understanding of the relationship between wear mechanisms and microstructures offers an effective approach in manufacturing high wear resistant alloys suitable for use in harsh working environments.

Directionally solidified (DS) nickel-based superalloys are widely used in manufacturing turbine blades, which may fail due to wear and/or material loss during service. Laser metal deposition (LMD) has been considered to be a promising technology in repairing the damaged components thanks to the high temperature gradient formed, which is conducive to the growth of directional microstructure. Intergranular liquation cracking in the heat-affected zone (HAZ) has been one of the major problems in LMD of the DS superalloys. In this paper, the influences of two cooling conditions (conventional cooling and forced cooling) on the microstructure development and liquation cracks were studied for the laser deposition of a DS superalloy IC10. The experimental results showed that, as compared to the conventional cooling, both number and length of the liquation cracks in HAZ were notably reduced under the forced cooling condition. The effects of cooling conditions on temperature and stress fields were analyzed through a thermo-elastoplastic finite element analysis. It was revealed that the maximum tensile stress and high tensile stress region in the substrate were effectively minimized while using the forced cooling measure. The forced cooling on the substrates is a promising method for mitigating the liquation cracking in LMD of DS superalloys.