It is known that the bleeding phenomenon during the paste filling of the low concentration full tailings may result in the blocking of pipes, afterwards, the inhomogeneous distribution of strength and volume shrinkage of the filling body. For solving the above problems, orthogonal tests were designed for revealing the effect of tailing gradation, amount of binding material, slurry concentration and content of anti-bleeding material on bleeding rate by means of electrical resistivity test (ER), SEM-EDS and XRD. The results show that the bleeding rate of filling materials is inversely proportional to the tailing gradation, amount of binding material, slurry concentration and the content of anti-bleeding material. The concentration of slurry and the content of water solidifier had a great influence on the bleeding rate. By adding 5% anti-bleeding material equivalent to the total amount of glue-powder, the initial viscosity reduced by 30%, the 2 h viscosity reduced by 53%, the bleeding rate can be reduced by 10% and the 28 d compressive strength increased by 20% for the filling body. The anti-bleeding material improves the porosity of the, increases the concentration of SO42- and pH value of the hydration system of the filling body, as well as generates ettringite, trapezium, C-S-H gel and silicoaluminate gel, which consumed a lot of free water, improves system stability by entangling molecules so that improves the bleeding situation for the filling body.
In order to simplify the complex process of preparing TiO2 nano bowl array by traditional method, the basic principle of two-step anodizing method was adopted. Namely, during the first and second anodizing process, oxidation voltage and electrolyte concentration all keep as before, but varying the anodizing time. The first anodizing time is 1 hour, and the second anodizing time changes from 0 s to 140 s. After the first anodizing, as the initial oxide layer for the second anodization, the residual bowl like pits on the surface of Ti-sheet may undergo three stages of growth. Firstly, the TiO2 barrier began to grow longitudinally at the bottom of the pit. Then the dissolution of the electrolyte at the edge of the bowl took effect, and nano pores appeared in the bowl. Secondly, the longitudinal growth rate of the barrier layer and the dissolution rate of the electrolyte reached a relative balance. The bottom center of the TiO2 barrier layer was first corroded, then the corrosion position changed around the bowl edge, and other nanopores were corroded laterally. Thirdly, the relationship between the longitudinal corrosion rate and the transverse corrosion rate was constantly changing, which showed that the depth of the nanopore in the bowl increased all the time, but the diameter of the bowl and the pore first increased and then decreased. Under the condition of the growth rate of TiO2 barrier layer was in relative equilibrium with the chemical dissolution rate of electrolyte, and the longitudinal corrosion rate was relatively consistent with the transverse corrosion rate, TiO2 nano bowl array with large pore diameter was synthesized. The optimal second anodizing time was 110 s, and the diameter of bowl was 133 nm.
The effect of post-weld artificial aging on microstructure and mechanical properties of friction stir welded (FSW) joint of 7003-7046 dissimilar Al-alloys was investigated by means of differential scanning calorimetry, hardness test, room temperature tensile test, backscattered electron diffraction and transmission electron microscopy. The results show that the hardness is significantly higher on the retreating side (7046 Al-alloy side) than that of the advancing side (7003 Al-alloy side), and the average hardness difference between the two sides is about 30HV; After artificial aging, the hardness increases, the yield strength increases significantly, the tensile strength increases slightly, and the elongation has little change of the joint, while the hardness difference increases to about 50HV for the two sides. The reason has been discussed based on the microstructure of different regions of the FSW joint before and after artificial aging treatment.
The microstructure and tensile properties of laser butt welding joints for cold-rolled plates of metastable austenitic stainless steel with four grades of strength were investigated, namely 301L-DLT, 301L-ST, 301L-MT and 301L-HT. The laser molten pool solidifies as primary ferrites and the formed weld bead presents low thermal cracking susceptibility, which is composed of columnar grains that grew vertically inward from the fusion boundary, but a central equiaxed grain region is absent. Microstructure of the weld seam consists of austenite and lathy, skeleton- and vermicular-ferrite, while no impurities, hot cracking and precipitates were detected. The average spacing of the primary ferrite dendritic arms is approximately 17.5 μm, and the average ferrite amount is 5.7% (volume fraction). The hardness of weld seam is 208~241HV, which is lower than the hardness of 301L-ST, 301L-MT and 301L-HT plates. The tensile fracture of laser weld joints of 301L-DLT and 301L-ST occurs within the base metal, and that of 301L-MT and 301L-HT takes place in the weld seam, correspondingly their fracture strength is 886 MPa and 921 MPa respectively. Except for the lower plasticity of the 301L-HT weld joint, the tensile properties of weld joint of the other three steels all meet the requirements of mechanical properties in JIS G 4305 standard for the cold-rolled 301L plates of the relevant strength grade.
The structure and morphology of carbides and the concentration of Cr near various type of grain boundaries of Inconel alloy 600 after aging treatment at 715℃ for different time were investigated by means of TEM, EDS and EBSD. The results show that there are obvious differences in structure and morphology of carbides precipitated at grain boundaries. Few carbides precipitated at Σ3c grain boundary, irregular shape M23C6 carbides precipitated at Σ3i grain boundary, and lager M23C6 carbide particles precipitated at Σ9 grain boundary. The coarse M7C3 carbide particles precipitated randomly at Σ27 grain boundaries. The precipitation of Cr-rich carbides causes Cr-depletion near the grain boundaries. Under the same aging conditions, the Cr-depletion is more severe near the grain boundaries of higher Σ value. The width of Cr-depletion zone near the grain boundary increases with the increase of aging time. However, the depth of Cr-depletion zone near the grain boundary reaches the maximum value after aging for 15 h, and then decreases to different degrees after aging for 50 h. Based on the experimental results the effect of grain boundary character on the carbide precipitation and Cr-depletion is discussed.
A TC4-DT Ti-alloy of two tracks and three layers was manufactured via arc additive manufacturing (CMT WAAM) coupled with cold metal transfer mode coaxial wire feeding, while the TC4-DT Ti-alloy wire of 1.2 mm in diameter was adoped as feeding wire. The microstructure of the acquired alloy was then characterized. Results show that fine equiaxed prior β-grains were found in the cambered heat affected zone; The bottom layer of the deposition zone consisted of thin columnar grains; The middle and top layers were composed of equiaxed grains and short columnar grains. Which was quite different from the coarse columnar grains produced by processes of EBRM and TIG WAAM. The microstructure of deposition zone presents basket weave α-phase laths, similar with that of EBRM and TIG WAAM. The 3D-Rosenthal solution was used to investigate the formation of the microstructure of the deposition zone. The maximum temperature gradient of the molten pool boundary calculated is about 12652.6 K/cm, and the maximum solidification speed is about 1.5 cm/s. The calculated solidification conditions just located in the mixed zone in the columnar-equiaxed-transformation (CET) model, consistent with the experiment results. The calculation results demonstrated that with the increasing input power P and the welding gun traveling speed V, the formation of equiaxed grains was promoted, while the grain size would gradually decrease with the increase of V. The mixed macrostructure would form when P>153 W and V>3.2 mm/s.
The Ti/ZrN/PbO2 electrodes were prepared via an arc spraying ZrN-interlayer on the Ti-substrate and then followed by anodic electrodeposition of β-PbO2 surface coating. By taking the plain Ti/PbO2 electrode as comparison, the prepared electrodes were characterized in terms of microstructure, surface roughness, coating adhesion, accelerated life assessment, electrochemical performance and electro-oxidation of phenol. The results show that the accelerated life-time for Ti/ZrN/PbO2 anodes was obviously prolonged to 8 times of that for the plain Ti/PbO2 electrode. Meanwhile, the electrocatalytic degradation activity of this anode for the organic pollutants was enhanced. This is directly related to the improvement of the conductivity of the electrode brought by the arc sprayed ZrN interlayer and which then enables the layer of electrodeposited PbO2 to be significantly refined and thicker with much flat surface and better adhesive to the substrate, as well as more active sites caused by the rough surface characteristics of this arc spraying intermediate layer.
A new type of dual-phase high-entropy alloy (FeCoNiTi) was designed by means of thermodynamic software and then block material of FeCoNiTi high-entropy alloy was prepared via vacuum arc smelting and then heat treatment. Characterization results demonstrate that the as-homogenized FeCoNiTi alloy presents dual-phase microstructure composed of the lamellar structure (hexagonal close packed (Laves) phase) and the Widmanstätten laths (face-centered cubic (FCC) phase). The FeCoNiTi alloy shows excellent comprehensive property at room temperature with compressive strength σb=2.08 GPa and compression strain ε=20.3%. The high strength can mainly be attributed to the hard Laves phase (lamellar structure) strengthening; while dislocation slip and deformation twin in the soft FCC phase (Widmanstätten laths) provide the ductility.
The carbon steel corrosion behavior at defects of epoxy coating in the effect of different AC current densities was investigated by means of in situ micro-area electrochemistry and traditional macro-electrochemical techniques, namely electrochemical scanning microscopy (SECM) and electrochemical impedance spectroscopy techniques. SECM observation can directly reveal the change of electrochemical active points during the local corrosion process of carbon steel at coating defects. The results show that when AC current is present, the number of corrosion active points at the coating defects is significantly more than that in the absence of AC current, correspondingly, the inhibitory effect of the corrosion products is significantly weaker than that in the case without AC current. The corrosion at the coating defect at the initial stage of immersion is an electron transfer control process, and it is converted to a diffusion control process after soaking for 10 hours. With the increase of AC current intensity and immersion time the degree of coating peeling increases, and the depth and width of the pits increase.
The effect of Al content on the microstructure and mechanical properties of directionally solidified Ti-(43-48)Al-2Cr-2Nb alloy prepared by cold crucible was investigated. The results show that the directional solidification structure is mainly composed of α2-phase and γ-phase. With the increase of Al content, the α2-phase decreases and the γ-phase content increases, while the direction of microstructural lamellae changed from parallel lamellae and 45° lamellae to vertical lamellae; For the alloy with 48%Al (atomic fraction), the orientation of lamellae is perpendicular to the direction of stress, its compressive strength is high, but plasticity is low; For the alloy with 45%Al (atomic fraction), the orientation of lamellae is parallel to the direction of stress, while maintaining high strength, the room temperature elongation is also high, in other word, the comprehensive properties are good.