Abstract:(Fe_63.3Mn₁₄Si_9.1Cr_9.8C_3.8)_99.5?xCu_xAg_0.5 (x=1, 2, 3, 4, 5, at%) alloys were prepared by water-cooled copper crucible magnetic levitation vacuum melting furnace. The effects of Cu contents on microstructure, corrosion resistance, and antibacterial performance of the alloys were investigated. The results show that the medium entropy alloys possess fcc phase after solid solution and aging treatment. With the increase in Cu content, the Cu-enriched and Ag-enriched fcc2 phase is precipitated on the fcc1 Fe-rich matrix. The corrosion resistance of the alloys in 3.5wt% NaCl solution is superior to that of AISI304. The corrosion current density first decreases and then increases, and the impedance arc radius first increases and then decreases, indicating an initial enhancement and subsequent weakening of the corrosion resistance as the Cu content increases. Moreover, the corrosion rate of the alloys in Escherichia coli suspension shows a trend of increasing first and then decreasing. When x=2 the alloy exhibits the best corrosion resistance, and there is a trade-off effect between the corrosion resistance and antibacterial performance. The fcc2 phase effectively enhances the antibacterial performance of the alloy, and the alloy of x=5 shows the optimal antibacterial rate of 99.94%.

Abstract:The corrugated cold rolling bonding (CCRB) process, as a new rolling technique, has gained widespread attention in the preparation of metal composite plates. However, the mechanical properties of corrugated composite plates and the microstructure of the interface at different reduction levels are not yet clear. Numerical simulation and experimental methods were employed to investigate the preparation of Cu/Al corrugated composite plates under reduction levels of 55%, 60%, 65%, and 70%. A three-dimensional model was established by finite element simulation software ABAQUS to simulate the normal stress and strain curves during the rolling process. The interface morphology of the composite plate was characterized by scanning electron microscopy, electron backscatter diffraction, and X-ray energy dispersive spectroscopy. Results show that the ultimate tensile strength and shear strength reach the maximum values at a reduction level of 65%, measuring 221.08 and 79 MPa, respectively; while they reach the minimum values at a reduction level of 55%, measuring 169.34 and 45 MPa, respectively. Particularly, at reduction levels of 65% and 70%, the composite plate exhibits elongated grains and fine equiaxed grains due to severe plastic deformation. At a reduction level of 70%, excessive rolling force causes microcracks in the matrix metal, leading to a decrease in tensile performance, which is consistent with the mechanical test results.

Abstract:TC17 titanium alloy was weld under different laser heat input conditions. Optical microscope, scanning electron microscope, transmission electron microscope, tensile and fatigue tests were used to compare the macroscopic morphologies, microstructures, and mechanical properties of the welded joints. The results show that with the increase in heat input, the morphology of weld changes from Y- to X-shaped. The number of pore defects in the weld increases first and then decreases. The pore defects are mainly distributed in the middle and lower part of the weld zone. The weld is composed of coarse columnar grains with strip dendrites inside, and the spacing of dendrite increases gradually with the increase in heat input. The heat affected zone comprises finer equiaxed grains, and the increase in heat input leads to the refinement of α phase and coarsening of β phase. Moreover, the TC17 laser welded joints all fracture at the weld zone in the tensile and fatigue tests. Under the influence of dendrite size, the tensile strength decreases with the increase in heat input. The welding pore is the main reason for the fatigue fracture, and the fatigue life peaks when the number of pore defect is the lowest.

Abstract:The microstructure of TC18 titanium alloy die forging shows delamination. The brighter microstructure has lower performance and is often called cold die microstructure (CDM). Decreasing the cooling rate can hinder the generation of CDM, but it may also aggravate the die wear. The balance relation between microstructure delamination of TC18 frame forging and the die wear in different parameters was studied by simulation and experiment. The programs to predict the CDM and wear depth were built and realized by secondary development. Continuous forging production process was simulated by DEFORM software and the characteristics of CDM and wear were researched. The balance relationship between the die wear and the CDM content in different parameters was discussed by the response surface method and the optimal parameters. Results show that the preheating temperature of die plays a dominant role in the variation of the wear depth. The most influential factor of CDM content is the contact condition. Applying glass fiber can reduce the CDM content without increasing the wear depth.

Abstract:With the rapid depletion of fossil fuels and a series of environmental problems, it is urgent to develop and to utilize new electrochemical energy storage devices, and the design, preparation and optimization of electrode materials are key factors to determine the performance of supercapacitors. Hydrothermal method was used to convert hollyhock stalks into porous carbon matrix with MnO and Co nanocrystals anchored on it. Results show that the prepared biocarbon has porous structure and good electron transport properties, and the nanosrystal MnO-Co on it has high capacitance. Due to the unique nanostructure of carbon skeleton and large specific surface area (345.9 m²·g^-1), MnO-Co nanocrystal/porous carbon shows excellent electrochemical capacitance (146 F·g^-1 at 1 A·g^-1) and cycle stability. After 1000 cycles, the specific capacity still remains 99.4%.

Abstract:It is difficult to achieve Al/Cu dissimilar welds with good mechanical properties for T-lap joints, due to the low heat input and poor plastic flow of the inner corner of the T-joint in friction stir welding (FSW), which leads to easy occurrence of wormholes, tunnel, bonding line defects, etc, and thus further causes stress concentration. Therefore, pre-set welding wires at the fillet were innovatively applied to 6061-T6 aluminum alloy (4 mm in thickness) and pure copper dissimilar plate FSW T-lap joints, in order to improve the internal plastic flow of T-joints, reduce defects, and obtain joints with good microstructure and properties. The effect of three types of pre-set wires on the microstructure and mechanical properties of Al/Cu dissimilar FSW T-lap joints was analyzed. Results reveal that three types of pre-set wire joints exhibit onion ring-like pattern in the large pin stirring zone at a constant travel speed of 35 mm/min and a rotation speed of 700?800 r/min. The progressive tool at all rotation speeds effectively inhibits migration of large amounts of stringer material to the skin and avoids base materials mixing. Small amounts of Cu particles are mechanically stirred and have a long flow path in the large pin stirring regions, which inhibits the formation of brittle Al/Cu intermetallic compound (IMC) phases during welding. Al/Cu forms effective metallurgical bonding, and the IMC thickness of the Al/Cu interface is less than 1 μm. The Al/Cu T-joints with pre-set Cu are similar to butt joints of the same material in the skin direction, showing a typical ductile fracture. In Al/Cu T-joints with pre-set Al, the direction of the bonding line defects is changed, a certain height of Al/Cu mixing zone is obtained in the direction of the stringer, achieving optimal mechanical interlocking bonding, and break mostly occurs at the intersection, with a tensile strength of 157 MPa, showing hybrid fracture. The pre-set welding wire is proved to be a good method for Al/Cu dissimilar FSW T-lap joints.

Abstract:The fatigue crack propagation behavior of DD6 nickel-based single-crystal superalloy was investigated at temperatures ranging from 530 °C to 850 °C. The fatigue properties were assessed along the [001] direction, parallel to the loading axis in tension. After the fatigue crack propagation test, the fracture morphology was examined by scanning electron microscope and classified into four zones, including source zone, prefabricated crack zone, stable extension zone, and rapid extension zone. Electron backscatter diffraction was utilized to observe the profiles of plastic deformation perpendicular to the fracture. Additionally, the dislocation motion mechanism near the fracture was studied by transmission electron microscope. Results show that oxidation occurs at 650 °C under combined influences of the temperature field, stress field, and exposure time. Furthermore, due to weakened γ′ phase, a significant number of consecutive dislocations form in the γ and γ′ phases between 650 and 760 °C, resulting in increased oxidation of alloy. Besides, a notable decrease in fatigue propagation life can be observed at 760 °C.

Abstract:Regional microstructure characteristic always appears in shear-compression deformed GH4169 superalloy, which is detrimental to subsequent cold-rolling process in engineering. Recrystallization annealing treatments within temperature range of 1000?1080 °C and holding time range of 1?3 h were carried out to investigate the microstructure evolution behavior, and the cold-forming property of GH4169 superalloy was optimized by regulating the grain size. Results show that static recrystallization (SRX) grains are fully nucleated at 1000 °C and the original coarse grains are completely replaced by fine recrystallized grains. Bulges of high angle grain boundaries are the preferred nucleation points of SRX. At 1020?1060 °C, grain annexation takes place among adjacent SRX grains, causing partial grains to increase, while the original dynamic recrystallization (DRX) grains keeps tiny in the strain con-centration region. Recrystallized grains (both SRX and DRX) uniformly grow up, with an average grain size of 87.89 μm at 1080 °C, at which the regional characteristic completely disappears, and the microstructure is significantly homogenized. Step twins appear at 1080 °C due to the SRX growth accidents, and the length fraction of twin boundaries (Σ3) reaches 35.8%, which can effectively improve the high temperature resistance of GH4169 superalloy. Ultimately, the optimal recrystallization annealing of shear-compression deformed GH4169 superalloy is determined as 1080 °C-1 h, followed by water cooling.

Abstract:The forging load of super large turbine disc with a diameter over 2 m may approach or even surpass the limit of 800 MN of the largest press machine in China, which is the extreme manufacturing. Thus, maintaining good mechanical properties and controlling forging load are two key factors during the forging process of super large turbine disc. 25 groups of forging parameters was designed based on Taguchi method. The multi-objective optimization of finite element method simulation results was conducted by SNR and ANOVA methods. Results show that the most uniform and refined recrystallization microstructures are obtained under optimal forging load. The optimal combination of process parameters is determined under extreme manufacturing condition: temperature=1120 °C, strain rate=0.06 s^-1, pre-forging size=985/610/475 mm, and die temperature=280 °C. The order of importance of each parameter to the simulation results is as follows: temperature>strain rate>billet shape>>die temperature. The experimental results obtained under the optimal parameters combination show good agreement with the simulated results, which demonstrates that this approach may be used to manage the load and microstructure of super large forgings while avoiding a significant number of experiments and numerical simulations.

Abstract:Ni-based superalloys exhibit exceptional mechanical properties and high-temperature resistance, making them suitable for use in aggressive environments. The oxidation behavior of Ni-based superalloys is primarily influenced by the intrinsic material properties and oxide scale properties, which are largely dependent on the complex composition and content of alloying elements. Various environmental parameters, including atmosphere composition, temperature, stress and molten salts, directly impact the oxidation behavior of materials. The effects of alloying elements and the service environment on the oxidation behavior of Ni-based superalloys were comprehensively reviewed. Aluminium, chromium and cobalt are considered as favorable elements to form compact and adherent scales that protect the matrix. The addition of titanium, molybdenum, niobium, tungsten and tantalum is traditionally believed to be detrimental. However, recent researches have presented different opinions, which were discussed. The oxidation mechanism was also explored and an insight into the future developments of Ni-based superalloys was provided.