Hydrogen metallurgy is a technology that applies hydrogen instead of carbon as a reduction agent to reduce CO₂ emission, and the use of hydrogen is beneficial to promoting the sustainable development of the steel industry. Hydrogen metallurgy has numerous applications, such as H₂ reduction ironmaking in Japan, ULCORED and hydrogen-based steelmaking in Europe; hydrogen flash ironmaking technology in the US; HYBRIT in the Nordics; Midrex H₂^TM by Midrex Technologies, Inc. (United States); H₂FUTURE by Voestalpine (Austria); and SALCOS by Salzgitter AG (Germany). Hydrogen-rich blast furnaces (BFs) with COG injection are common in China. Running BFs have been industrially tested by AnSteel, XuSteel, and BenSteel. In a currently under construction pilot plant of a coal gasification–gas-based shaft furnace with an annual output of 10000 t direct reduction iron (DRI), a reducing gas composed of 57vol% H₂ and 38vol% CO is prepared via the Ende method. The life cycle of the coal gasification–gas-based shaft furnace–electric furnace short process (30wt% DRI + 70wt% scrap) is assessed with 1 t of molten steel as a functional unit. This plant has a total energy consumption per ton of steel of 263.67 kg standard coal and a CO₂ emission per ton of steel of 829.89 kg, which are superior to those of a traditional BF converter process. Considering domestic materials and fuels, hydrogen production and storage, and hydrogen reduction characteristics, we believe that a hydrogen-rich shaft furnace will be suitable in China. Hydrogen production and storage with an economic and large-scale industrialization will promote the further development of a full hydrogen shaft furnace.

Biodegradable magnesium alloys as new biomedical implant materials have been extensively studied because of their notable biodegradability over traditional bio-inert metals. However, the extreme degradation rate of pure magnesium leads to the loss of its mechanical integrity before the tissue recovers completely. The solutions to this challenge are as follows: (1) purification, (2) alloying, (3) surface modification, and (4) biodegradable magnesium-matrix composites (BMMCs) synthesis. Owing to the tunability of mechanical properties, the adjustability of degradation rate, and the improvement of biocompatibility, BMMCs reinforced with bioactive reinforcements have promising applications as a new generation of biomedical implants. In this review, the processing methods, Mg matrix, and reinforcement phases of BMMCs are discussed. Moreover, the review comprehensively discusses various BMMCs synthesized thus far, aiming to show the governing aspects of the achieved mechanical properties, corrosion behavior, and biocompatibility. Finally, this paper also discusses the research direction and further development areas for these materials.

The formation of calcium titanate in the carbothermic reduction of vanadium titanomagnetite concentrate (VTC) by adding CaCO₃ was investigated. Thermodynamic analysis was employed to show the feasibility of calcium titanate formation by the reaction of ilmenite and CaCO₃ in a reductive atmosphere, where ilmenite is more easily reduced by CO or carbon in the presence of CaCO₃. The effects of CaCO₃ dosage and reduction temperature on the phase transformation and metallization degree were also investigated in an actual roasting test. Appropriate increase of CaCO₃ dosages and reduction temperatures were found to be conducive to the formation of calcium titanate, and the optimum conditions were a CaCO₃ dosage of 18wt% and a reduction temperature of 1400°C. Additionally, scanning electron microscopy–energy dispersive spectrometry (SEM–EDS) analysis shows that calcium titanate produced via the carbothermic reduction of VTC by CaCO₃ addition was of higher purity with particle size approximately 50 μm. Hence, the separation of calcium titanate and metallic iron will be the focus in the future study.

The evolution of oxide inclusions during isothermal heating of 18Cr–8Ni stainless steel with yttrium addition at temperatures of 1273 to 1573 K was investigated systematically. Homogeneous spherical Al–Y–Si(–Mn–Cr) oxide inclusions were observed in as-cast steel. After heating, most of the homogeneous inclusions were transformed into heterogeneous inclusions with Y-rich and Al-rich parts, even though some homogeneous oxide particles were still observed at 1273 and 1573 K. With the increase in heating temperature, more large-sized inclusions were formed. The shape of the inclusions also changed from spherical to irregular. The maximum transformation temperature of inclusions was determined to be 1373 K. The evolution mechanism of inclusions during heating was proposed to be the combined effect of the (i) internal transformation of inclusions due to the crystallization of glassy oxide and (ii) interfacial reaction between inclusions and steel matrix. Meanwhile, the internal transformation of inclusions was considered to be the main factor at heating temperatures less than 1473 K.

The main aim of this study was to investigate liquation cracking in the heat-affected zone (HAZ) of the IN939 superalloy upon tungsten inert gas welding. A solid solution and age-hardenable filler metals were further studied. On the pre-weld heat-treated samples, upon solving the secondary γ′ particles in the matrix, primary γ′ particles in the base metal grew to “ogdoadically diced cubes” of about 2 μm in side lengths. The pre-weld heat treatment reduced the hardness of the base metal to about HV 310. Microstructural studies using optical and field-emission scanning electron microscopy revealed that the IN939 alloy was susceptible to liquation cracking in the HAZ. The constitutional melting of the secondary, eutectic, and Zr-rich phases promoted the liquation cracking in the HAZ. The microstructure of the weld fusion zones showed the presence of fine spheroidal γ′ particles with sizes of about 0.2 μm after the post-weld heat treatment, which increased the hardness of the weld pools to about HV 350 and 380 for the Hastelloy X and IN718 filler metals, respectively. Application of a suitable solid solution filler metal could partially reduce the liquation cracking in the HAZ of IN939 alloy.

To better understand the stress-corrosion behavior of friction stir welding (FSW), the effects of the microstructure on the stress-corrosion behavior of the FSW in a 2198-T34 aluminum alloy were investigated. The experimental results show that the low-angle grain boundary (LABs) of the stir zone (SZ) of FSW is significantly less than that of heated affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and parent materials (PM), but the grain boundary precipitates (GBPs) T1 (Al₂CuLi) were less, which has a slight effect on the stress corrosion. The dislocation density in SZ was greater than that in other regions. The residual stress in SZ was +67 MPa, which is greater than that in the TMAZ. The residual stress in the HAZ and PM is −8 MPa and −32 MPa, respectively, and both compressive stresses. The corrosion potential in SZ is obviously less than that in other regions. However, micro-cracks were formed in the SZ at low strain rate, which indicates that the grain boundary characters and GBPs have no significant effect on the crack initiation in the stress-corrosion process of the AA2198-T34. Nevertheless, the residual tensile stress has significant effect on the crack initiation during the stress-corrosion process.

Cold metal transfer plus pulse (C + P) arc was applied in the additive manufacturing of 4043 Al alloy parts. Parameters in the manufacturing of the parts were investigated. The properties and microstructure of the parts were also characterized. Experimental results showed that welding at a speed of 8 mm/s and a wire feeding speed of 4.0 m/min was suitable to manufacture thin-walled parts, and the reciprocating scanning method could be adopted to manufacture thick-walled parts. The thin-walled parts of the C + P mode had fewer pores than those of the cold metal transfer (CMT) mode. The thin- and thick-walled parts of the C + P mode showed maximum tensile strengths of 172 and 178 MPa, respectively. Hardness decreased at the interface and in the coarse dendrite and increased in the refined grain area.

6061 aluminum alloy semisolid billet was prepared by the equal-channel angular processing (ECAP)−recrystallization and partial (RAP) process (a combination of equal-channel angular processing and recrystallization and partial remelting). The effects of different process parameters on the alloy microstructure were studied and the quantitative relationship between the process parameters and microstructure was established by response surface methodology (RSM) to optimize the process parameters. According to the orthogonal test, the holding temperature and holding time of the four ECAP−RAP process parameters were found to have the greatest impact on the microstructural characteristics, including average grain size and average shape factor. Through RSM, it was also found that when the average grain size or the average shape factor is optimized separately, another will be degraded. When the two indexes were simultaneously considered, the optimal process parameters were found to be a holding temperature of 623°C and holding time of 13 min, and the corresponding average grain size and average shape factor were 35.97 μm and 0.8535, respectively. Moreover, comparing the experimental and predicted values, the reliability of the established response surface model was verified.

To produce a highly refined microstructure, several metals or alloys have been processed via equal-channel angular pressing (ECAP). In this work, the mechanical and microstructural changes of the 5083 aluminum alloy in H11 condition after processed by two ECAP passes were investigated. An ECAP H13 steel die with an inner angle (α) of 120° and outer curvature (β) of 20° was used. The microstructural changes were associated with the loss of texture symmetry. The morphologies of the Mg₂Si and α-Al(Mn,Fe)Si precipitates for the sample at the initial condition were similar to those subjected to two ECAP passes. The peak broadening measured by X-ray diffraction revealed an increment of both grain refinement and microstrain. After the second extrusion pass, the hardness increased by 62% compared with the initial condition. Moreover, the heterogeneous hardness behavior was compatible with a highly localized dislocation density. After two ECAP passes, shear parallel bands were observed to be at nearly 45° to the extrusion direction. The evaluation of first-order residual stress as a function of the depth of the analyzed sample displayed compressive or tensile values, depending on the measured face. With the plastic deformation applied, the first and second-order residual stresses exhibited significant increment. Williamson-Hall plots showed positive slopes, indicating an increment in the microstrain.

Despite the existence of conventional methods for recycling chips, solid-state techniques have become popular, whereby waste metals are directly recycled into consolidated products with the desired shapes and designs. We investigated the feasibility of recycling phosphor bronze chips through a hot extrusion process using aluminum powder as a metal binder for the fabrication of a metal-fiber-reinforced aluminum matrix composite. To do so, mixtures containing 20vol%–50vol% of chips were prepared, cold-compacted, and extruded. The quality of the consolidated samples was evaluated by determining the density of the fabricated composites and studying their microstructures. In addition, we performed tensile and hardness tests to evaluate the mechanical properties of the fabricated composites. We also analyzed the fracture surfaces of the samples to study the fracture mechanism as a function of the volume fraction of phosphor bronze chips in the fabricated composite. The results indicated that the most effective consolidation occurred in the sample containing 20vol% of chips extruded at 465°C in which the chips serve as ideal fibers for improving the mechanical properties, especially the ultimate tensile strength, in comparison with those of Al matrixes that contain no chips but are produced under the same conditions.

Multi-layered functionally graded (FG) structure Ni−W/Er₂O₃ nanocomposite films were prepared by continuously changing the deposition parameters, in which the Er₂O₃ and W contents varied with thickness. The microstructure and chemical composition of the electrodeposited Ni−W/Er₂O₃ films were determined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The anti-corrosion and wear properties of the electrodeposition films were investigated by electrochemical measurement and ball-on-disk friction test. The microhardness distribution of the cross section of nanocomposites was measured by nanoindentation. The results showed that with decreasing agitation rate or increasing average current density, the contents of Er₂O₃ nanoparticles and tungsten were distributed in a gradient along the thickness, and the contents on the surface were larger. By comparison, FG Ni−W/Er₂O₃ films had better anti-corrosion and wear properties than the uniform Ni−W/Er₂O₃ films. Atomic force microscopy (AFM) and profilometry measurements indicated that Er₂O₃ nanoparticles had an effect on the surface roughness.

(GO/TiO₂)_N (GO represents graphene oxide, and N represents the period number of alternate superposition of two dielectrics) one-dimensional photonic crystal with different lattice constants was prepared via the sol–gel technique, and its transmission characteristics for photocatalysis were tested. The results show that the lattice constant, filling ratio, number of periodic layers, and incident angle had effects on the band gap. When the lattice constant, filling ratio, number of periodic layers, and incident angle were set to 125 nm, 0.45, 21, and 0°, respectively, a gap width of 53 nm appeared at the central wavelength (322 nm). The absorption peak of the photocatalyst at 357 nm overlapped the blue edge of the photonic band gap. A slow photon effect region above 96% reflectivity appeared. The degradation rate of tetracycline in (GO/TiO₂)_N photonic crystal was enhanced to 64% within 60 min. Meanwhile, the degradation efficiency of (GO/TiO₂)_N one-dimensional photonic crystal was effectively improved compared with those of the GO/TiO₂ composite film and GO/TiO₂ powder.

A novel method for exfoliating graphite oxide (GrO) was implemented through the mass water absorption of a GrO–poly(ethylene glycol) (GrO–PEG) composite. The GrO–PEG composite was prepared by intercalating PEG into the lamellae of GrO, and the variation of the basal spacing was measured by X-ray diffraction analysis. The yield of graphene was measured with an ultraviolet–visible spectrophotometer, and the properties of graphene oxide (GO) were characterized by atomic force microscopy, transmission electron microscopy (TEM), Raman spectrometry, and Fourier transform infrared spectroscopy. Increasing intercalation time was found to improve the yield of GO, whereas increasing the PEG molecular weight had the opposite effect. The GO sheets produced from the intercalation–absorption–exfoliation process were found to be a four-layer structure. TEM and Raman analyses indicate that the graphitized structure and oxygen groups of GO were preserved during the exfoliation process. Most importantly, the results show that good-quality GO could be prepared via a mild method involving water absorption of a GrO–PEG composite.

A simple and novel technique for the preparation of anatase TiO₂ nanopowders using natural ilmenite (FeTiO₃) as the starting material is reported. Digesting ilmenite with concentrated H₃PO₄ under refluxing conditions yields a white α-titanium bismonohydrogen orthophosphate monohydrate (TOP), Ti(HPO₄)₂·H₂O, which can be easily isolated via gravity separation from unreacted ilmenite. The addition of ammonia to the separated TOP followed by calcination at 500°C completes the preparation of anatase TiO₂. Calcination at temperatures above 800°C converts the anatase form of TiO₂ to the stable rutile phase. The removal of iron from ilmenite during the commercial production of synthetic TiO₂ is problematic and environmentally unfriendly. In the present study, the removal of iron was found to be markedly simple due to the high solubility of iron phosphate species in concentrated H₃PO₄ with the precipitation of TOP. The titanium content of the prepared samples on metal basis with silica and phosphorous as major impurities was over 90%. Prepared TiO₂ samples were characterized using X-ray fluorescence, Fourier-transform infrared spectroscopy, Raman spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, and X-ray diffraction analyses. The photocatalytic potentials of the commercial and as-prepared TiO₂ samples were assessed by the photodegradation of rhodamine B dye.