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Currently, resistance spot welding (RSW) is a common welding method used in automobile and other industries, and 7000 series aluminum (Al) alloys have become a very common industrial material because of their high strength and easy machinability. Scandium (Sc) is often used for alloy strengthening; however, there are few studies on the RSW joints of aluminum alloys containing scandium. Here, 7075-T6 alloys containing a small amount of scandium were obtained to investigate the joint of resistance spot weldability. The simulation shows that the effective contact area between the aluminum sheet surface is about 19.5%, which is an important factor in nucleation. It was found that the average grain size of the nugget decreased from 51.51 ± 5.74 to 8.15 ± 1.09 μm when scandium was added from 0 to 0.4 wt%, which is mainly due to Al3Sc particles acting as a heterogeneous nucleus in the melt. The test results show that the tensile shear strength of welded joints with 0.4 wt% scandium addition is 29.2% higher than that of the joints without scandium because of grain refinement strengthening.

To study the occurrence mechanism of rock burst during mining the irregular working face, the study took irregular panel 7447 near fault tectonic as an engineering background. The spatial fracture characteristic of overlying strata was analyzed by Winkler elastic foundation beam theory. Furthermore, the influence law of panel width to suspended width and limit breaking span of key strata were also analyzed by thin plate theory. Through micro-seismic monitoring, theoretical analysis, numerical simulation and working resistance of support of field measurement, this study investigated the fracture characteristic of overlying strata and mechanism of rock burst in irregular working face. The results show that the fracture characteristic of overlying strata shows a spatial trapezoid structure, with the main roof being as an undersurface. The fracture form changes from vertical “O-X” type to transverse “O-X” type with the increase of trapezoidal height. From the narrow mining face to the wide mining face, the suspended width of key strata is greater than its limit breaking width, and a strong dynamic load is produced by the fracture of key strata. The numerical simulation and micro-seismic monitoring results show that the initial fracture position of key strata is close to tailgate 7447. Also there is a high static load caused by fault tectonic. The dynamic and static combined load induce rock burst. Accordingly, a cooperative control technology was proposed, which can weaken dynamic load by hard roof directional hydraulic fracture and enhance surrounding rock by supporting system. Keywords: Rock burst, Irregular working face, Key strata, Dynamic and static combined load, Cooperative control

AZ31 magnesium alloy sheets were A-TIG-welded through a coating of flux, which contained different ratios of Ce powder and nano-sized SiC as reinforcement particles and equal mass of TiO2 as activating fluxes. The microscopic analysis results illustrated that relatively low content of Ce in the reinforcement particles caused the formation of Al3Ce precipitates and refined the grains of α-Mg phase together with β-Mg17Al12 and SiC particles. The increase in microhardness and ultimate tensile strength of the joints was 6.2% and 19.2%, respectively, when reinforcement particles contain 20 wt% Ce compared to the joints coated without Ce. By studying the electrochemical test results, when using 20 wt% Ce + 80 wt% SiC as reinforcement particles, the corrosion current density was the lowest and the corrosion resistance reached the largest value, reflecting the improvement of corrosion property of the joint affected by Ce element.

This paper presents a new numerical modeling procedure and design principle for longwall pillar design with the assistance of numerical simulation of FLAC3D. A coal mine located in Yanzhou city, Shandong Province, China, was selected for this case study. A meticulously validated numerical model was developed to investigate the stress changes across the longwall pillar with various sizes. In order to improve the reliability of the numerical modeling, a calibration procedure is undertaken to match the Salamon and Munro pillar strength formula for the coal pillar, while a similar calibration procedure is used to estimate the stress–strain response of a gob. The model results demonstrated that when the coal pillar width was 7–8 m, most of the vertical load was carried by the panel rib, whilst the gateroad was overall in a relatively low stress environment and could keep its stability with proper supports. Thus, the rational longwall pillar width was set as 8 m and the field monitoring results confirmed the feasibility of this pillar size. The proposed numerical simulation procedure and design principle presented in this study could be a viable alternative approach for longwall pillar design for other similar projects.

Cu-Cu joints bonded by nano-copper pastes with different amounts of phenolic resin were developed at a low temperature (240°C) and a low pressure (1 MPa). The shear strengths of the joints were measured and the bonding layers were observed by scanning electron microscopy. The electrical conductivity of the joints was tested by a parameter analyzer. The results indicated that 2% phenolic resin in the pastes doubled the shear strength of Cu-Cu joints compared to joints made with 0%, 5% and 8% phenolic, because it filled the voids and coated sintered Cu nanoparticles in the joints without affecting the electrical conductivity of the Cu-Cu joint. This was because 2% phenolic paste prevented Cu nanoparticles from oxidation. However, 8% phenolic resin led to an increase of electrical resistivity due to the insulating property of phenolic resin, and a decrease of shear strength due to the hindrance to sintering between the nanoparticles.

Dopamine (DA), with strong adhesion to metal substrate via self-polymerization reaction, was introduced as a versatile template layer for subsequent polyether imide (PEI) deposition to confer a highly robust coating on AZ31 magnesium alloy. The complicated corrosion behaviour of the coating was disclosed and investigated by employing electrochemical impedance spectroscopy (EIS) coupled with equivalent electrical circuits (EECs) analysis. The results showed that the hybrid PEI/PDA coating exhibited evidently higher retardation performance (up to 35 days) after immersing in 3.5 wt.% NaCl solution compared to PDA or PEI coated Mg.

This work mainly studies on Sc microalloying in Al-7075 alloys. The microstructure characteristics and mechanical properties of the Sc-contained Al-7075 alloys were investigated after homogenization at 460 ℃ for 10 h. The corrosion behavior of the 7075 alloys after aging was examined using a three-electrode system. The results showed that significant reduction in grain sizes from 129.92 μm without Sc alloy to 32.17 μm with 0.6% Sc. The microhardness increased with the increase of Sc content in the alloy from 71.7 ± 1.3 HV without Sc alloy to 89.2 ± 4.7 HV. The tensile strength also increased from 151.82 ± 1.07 MPa to 212.85 ± 7.35 MPa. The results also revealed that the addition of Sc element in 7075 aluminum alloys not only reduced the corrosion current density of the alloy, but also increased the corrosion resistance of the alloy in the corrosive environment.