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Laminated ZrB2–SiC ultra-high temperature ceramics (UHTCs) with different layer thickness ratios and strong interfaces were fabricated by hot-pressing. Residual stresses developed on the surfaces of laminated ZrB2-SiC ceramics were evaluated by X-ray diffraction (XRD) and indentation techniques. Results showed that the characteristic XRD peaks of the ZrB2 phase on the surface of the laminated ceramics presented a shift due to the residual surface stress existence. Both XRD and indentation tested results suggested that the surface residual stress increases with the increasing layer thickness ratio confirming the theoretical analysis and calculation reported by our previous work. The presence of a compressive residual stress in the external layer of the heterogeneous material is expected to improve the mechanical properties of laminated ZrB2-SiC ceramics.

Many acoustic metamaterials suffer from a narrow bandwidth transmission because of the impedance mismatch at the air-metamaterial interface. In this paper, a two-dimensional impedance-matched metamaterial with broadband transmission performance is investigated. The impedance matching layer is introduced for a gradient variation of effective impedance from the inlet of the unit to the outlet. The effective medium theory and corresponding effective model are used to explain the underlying mechanism. The improved energy transmission of our designs is demonstrated by experiment and numerical simulation within a broad frequency bandwidth over 6 kHz. Our impedance-matched design can be used to enhance sound absorption, which is expected to present improved acoustic performance in the applications of acoustic damper and muffler.

This study presents a novel approach for the fabrication of commercially pure titanium (CP–Ti) components. The approach conferred superb strength to CP-Ti without sacrificing its ductility. A yield strength of 807 MPa combined with 19.15% elongation was realized through selective laser melting (SLM) by using a high-power laser and incorporating solute atoms from the Ar−N 2 reactive atmosphere. Mechanical properties and microstructures of the as-printed CP-Ti were systematically investigated. Transmission electron microscopy, electron backscatter diffraction, and atom probe tomography were employed to reveal the mechanism underlying the in-situ reaction between CP-Ti and the reactive atmosphere. Results suggest that nitrogen generally dissolved in the α′-Ti matrix as interstitial solute atoms. The beneficial N content has a critical limit of ∼0.43 wt%. The ductility of CP-Ti will decrease drastically if its N content exceeds this limit. A constitutive model was developed for describing the tensile deformation behavior of the in-situ strengthened CP-Ti over various solute contents and grain sizes. This work demonstrates a promising methodology for the fabrication of high-performance metallic components and extends the fundamental understanding of SLM process under the reactive atmosphere.

The influences of processing parameters such as volumetric energy density (&epsilon, ) and, particularly, defocusing amount (DA) on densification, microstructure, tensile property, and hardness of the as-printed dense AlSi10Mg alloy by selective laser melting (SLM) were studied systematically. The molten pool boundaries (MPBs) were found overwhelmingly at regular and complex spatial topological structures affected by DA value to exist in two forms, while the &ldquo, layer&ndash, layer&rdquo, MPB overlay mutually and the &ldquo, track&ndash, track&rdquo, MPBs intersect to form acute angles with each other. The microstructure of MPBs exhibits a coarse grain zone near the MPBs and the characteristics of segregation of nonmetallic elements (O, Si) where the crack easily happened. The DA value (&minus, 2 to 2 mm) affected both the density and the tensile mechanical properties. High tensile strength (456 &plusmn, 14 MPa) and good tensile ductility (9.5 &plusmn, 1.4%) were achieved in the as-printed condition corresponding to DA = 0.5 mm. The tensile fracture surface features were analyzed and correlated to the influence of the DA values.

Ti-6Al-4V is the single most important Ti alloy, accounting for use in almost 60% of all the applications of Ti materials. Additive manufacturing (AM) offers design freedom with regard to Ti-6Al-4V for creation of high-quality, customized products. However, the large-scale development of this technology is constrained by the high raw material costs. In this study, a method based on ball milling powder modification was proposed to convert low cost, non-spherical hydrogenated-dehydrogenated Ti (HDH-Ti) powder into spherical, printable Ti powder. Following mechanical mixing, the ball-milled HDH-Ti powder was further blended with elemental powders of aluminum and vanadium to develop low-cost HDH Ti-6Al-4V. Simultaneously, the issue pertaining to high oxygen associated with HDH Ti-6Al-4V was addressed via introduction of yttrium. A cost-affordable, high-performance AM Ti-6Al-4V alloy was finally developed via laser-based powder bed fusion of metals (PBF-LB/M) after printing parameter optimization and heat treatment. The AM-prepared Ti-6Al-4V demonstrated a relative density of 99.3%, an ultimate tensile strength of ~1083 MPa, and an elongation of 9%, comparable to those obtained using costly pre-alloyed powders. Further, numerical simulation and detailed microstructural characterization were performed to reveal the underlying mechanism. Powder modification, compositional modulation, and laser in-situ alloying were the three essential techniques used as part of this approach for optimizing the mechanical properties of the Ti-6Al-4V alloy. Overall, this method demonstrates excellent potential in terms of mitigating the high cost. Moreover, it may further promote research on AM of a variety of Ti alloys besides Ti-6Al-4V.

The CVD nano-diamond film and Ti-6Al-4V alloy were successfully brazed with AgCuTi active brazing filler. The interfacial microstructure was investigated by SEM, EDS, XRD and TEM. Typical interfacial microstructure of brazed joint was conformed as Ti-6Al-4V/diffusion layer/Ti 2 Cu + TiCu + Ti 3 Cu 4 /Ag(s,s) + Cu(s,s) + Ti 2 Cu 3 /TiCu + TiC/nano-diamond film. The effects of brazing temperature on interfacial microstructure and mechanical properties of the brazed joints were analyzed. With the increasing brazing temperature, the thickness of reaction layers adjacent to Ti-6Al-4V substrate and nano-diamond film increased obviously. Moreover, the Ti 2 Cu 3 phase coarsened and aggregated in brazing seam at higher temperature. The joint was formed by the diffusion and reactions between atoms, and the microstructure evolution of brazed joint was discussed. In addition, a slight graphitization of nano-diamond film occurred during brazing process, and the highest shear strength can reach 25 MPa when the joint was brazed at 880 °C for 10 min. Finally, the fracture positions and fractographs of brazed joints were also discussed.

Large and hard inclusions often deteriorate the service performance and reduce the fatigue lifetime of drill rods. In this paper, the main reasons of the rupture of drill rods were analyzed by the examination of their fracture and it is found that the large inclusions were the main reason of breakage of rod drill. The inclusions were high of Ca content or Al2O3 rich. Smaller and better deformability inclusions were obtained by the optimization of refining slag, calcium treatment process and the flow control devices of tundish. Results of industrial experiment after optimization show that total oxygen content of drill rods decreased by more than 50%, macro-inclusions weight fraction decreased from about 4 mg/10 kg to about 0.3 mg/10 kg and the micro-inclusions average size decreased from 6 to 3.6 μm. The average using times of drill rods after optimization were increased by about 60%.

Vacuum brazing of C/C composite to TC4 alloy was performed using nano-Al2O3 strengthened AgCuTi composite filler. The microstructure and main properties of the composite filler were characterized. The typical interfacial microstructure of brazed joint is TC4/diffusion layer/Ti–Cu intermetallic layers/Ag(s,s) + Cu(s,s) + TiCu/TiC/C/C composite. The effects of brazing temperature and addition of nano-Al2O3 on microstructure of brazed joints were investigated by SEM, EDS, TEM and XRD. The diffusion layer, Ti–Cu intermetallic layers and TiC layer thicken with the increasing temperature while the thickness of brazing seam decreases. The TiCu phase in brazing seam excessively develops and almost replaces the Ag(s,s) and Cu(s,s) when brazing temperature reaches 920 °C. The addition of nano-Al2O3 improves the performance of brazed joints by inhibiting the growth of Ti–Cu layers adjacent to TC4 side and offering dispersive nucleation sites for TiCu phase in brazing seam. The joint brazed at 880 °C for 10 min exhibits the highest shear strength of 27.8 MPa. The fracture positions of brazed joints are also discussed in this paper. Keywords: C/C composite, TC4 alloy, Brazing, Composite filler, Interfacial microstructure, Fracture positions

Different heat treatments, including quenching and hot isostatic pressing, were conducted on the Ti–24Nb–4Zr–8Sn alloy fabricated by powder metallurgy, in order to improve its mechanical properties. As-sintered samples have high oxygen content over 3000 ppm, resulting in α phase precipitates and low ductility. But the mechanical properties can be improved by solution treatment at a suitable temperature and subsequent quenching. Although the treatment is only effective within a narrow temperature window ranging from 950 to 1000 °C, the treatment can completely eliminate the detrimental α phase and substantially improve the ductility of the alloy from 2% to 19.33%. In comparison, although the hot isostatic pressing can improve the density of the as-sintered alloy to nearly fully dense, it proves unable to remove the α phase from the material or improve its ductility alone.

Ti–22Al–25Nb (at.%) intermetallic is a light weight, high performance, high temperature material with density of merely ˜60% of Ni-based superalloys. The advent and rapid development of selective laser melting (SLM) enable direct fabrication of the Ti–22Al–25Nb intermetallic into complex geometry parts, promising for various critical applications. This paper is dedicated to better understanding the effects of hatch distance, an important but often underestimated processing factor, on microstructure and mechanical properties of the SLM-prepared Ti–22Al–25Nb. Along with analytical means such as transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), finite element simulation has also been employed to explore the underlying mechanisms. It is determined that the highest mechanical properties are achieved at a hatch distance of 0.16 mm due to the following factors: (a) High density dislocation, (2) favorable phase features, and (3) high relative density. As-fabricated parts of micro-turbine engine using the optimized parameters are provided in the end of the study to demonstrate the capability of SLM to manufacture high quality and delicate structural parts of the Ti–22Al–25Nb intermetallic.

The enthalpy sensitivity of liquid fraction has been proposed as a novel criterion for assessing the semi-solid processability. Via experimental analysis and thermodynamic prediction, the enthalpy sensitivity turns out to be more reasonable than previous criterions. Based on criterion of enthalpy sensitivity, the effects of elements Si and Cu on semi-solid aluminium processability were analysed, indicating the approach to develop new alloys. Furthermore, the influences of heat exchange rate on semi-solid processability were investigated. It is suggested that increasing heat exchange rate would benefit the processability but with a sacrifice of the process time window in practice.

To realize near net-shaping of hard-to-process intermetallics is an often challenging but critical issue to their wider industrial applications. In this work, we report that an intermetallic Ti-22Al-25Nb has been successfully fabricated by selective laser melting (SLM). The as-printed samples show a high room-temperature ultimate tensile strength ~1090 MPa and excellent ductility ~22.7%; both values are higher than most conventionally fabricated Ti-22Al-25Nb intermetallic. We clarify the mechanical performance achieved by detailed microstructure analysis, including dislocation analysis and phase constitution analysis. High-density dislocation networks significantly contribute to the strength and ductility, which are further enhanced by the favorable phase constitution, including the nano-scale O phase precipitates within the disordered β phase and disappearance of the brittle α2 phase in the microstructure. Phase evolution during SLM has also been clarified using in situ heating, high-temperature synchrotron X-ray diffraction and Scheil simulation, particularly regarding the O phase's formation. It is demonstrated that O phase is formed by shearing primary cubic B2 phase along (110)[111] direction into an orthorhombic structure under high residual stress. Furthermore, a demonstrative part of turbine blade has been fabricated to highlight the importance of SLM in fabricating critical structural part like the hard-to-process intermetallics.

Herein, we present a systematic investigation of oil-in-water Pickering emulsions stabilized by the complex of polystyrene (PS) particles and chitosan (CS). The presence of both PS particles and CS resulted in a stable emulsion, in contrary to those prepared with PS particles or CS alone, and the adsorption of PS particles and CS on oil droplet surface was observed using SEM and fluorescence microscopy. In addition, the mechanism of the emulsifying ability of the complex has been studied by determining the wettability of PS particles modified by CS, as well as the flocculation of PS particles. Specifically, at the low concentration of CS, the emulsion was stabilized by the flocculated PS particles induced by the adsorption of CS molecules, whereas at the high concentration of CS, the emulsion was stabilized mainly by free CS.

One of the bottleneck issues for commercial scale-up of Ti additive manufacturing lies in high cost of raw material, i.e. the spherical Ti powder that is often made by gas atomization. In this study, we address this significant issue by way of powder modification & ball milling processing, which shows that it is possible to produce printable Ti powders based on ultra-low cost, originally unprintable hydrogenation-dehydrogenation (HDH) Ti powder. It is also presented that the as-printed Ti using the modified powder exhibits outstanding mechanical properties, showing a combination of excellent fracture strength (~895 MPa) and high ductility (~19.0% elongation).

The narrow bandwidth and low amplification performance of a subwavelength acoustic resonator greatly limit its applications. In this study, broadband acoustic amplification using an array of two-dimensional quarter-wavelength resonators is investigated. A new acoustic resonator unit is designed, which contains one open end with an impedance-gradient meta-structure and another closed end to confine the acoustic transmission and achieve sound amplification. The sound pressure level (SPL) gain and bandwidth of the resonator can be controlled by changing its geometric parameters. In an energy-harvesting experiment, the broadband amplification resonator provides a 17.2-dB SPL gain and enhanced output voltages.