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[Objective] The impacts of precipitation on the groundwater preservation capacity for agricultural regions in North China were studied in order to provide a theoretical basis for the sustainable use of groundwater resources in this region. [Methods] We established a theoretical system for evaluating groundwater preservation capacity (F) and determined the influence of precipitation changes on F value in agricultural regions of North China using methods such as geoscience statistics and correlation analysis. [Results] ① The spatial distribution of F value changed dramatically as precipitation changed. The multi-year average and wet years were dominated by the “basic guarantee” distribution, while the dry years were dominated by the “difficult to guarantee” distribution; ② As precipitation changed, the frequency of F value also varied in different locations. In the Yanshan Plain and the Taihang-Hebei Plain, when the precipitation frequency was greater than 75% and 50%, respectively, the F value was less than 0.5; in the Taihang-Henan Plain, when the precipitation frequency was less than 75%, the F value was greater than 1.0; in the Central Henan Plain, when the precipitation frequency was less than 50%, the F value increased to 1.0; in the Central Shandong Plain and Littoral Plain, when the precipitation frequency was less than 25%, the F value was greater than 0.5; in the Central Hebei Plain, there were only a few years where the F value was greater than 0.5. [Conclusions] Precipitation is an important factor that affects groundwater preservation capacity. For areas that can be “highly preserved” even during normal and dry years, the scale of agricultural planting can be appropriately expanded to ensure ample food production. However, for areas that are “difficult to preserve” and have “lower preservation” capacity during wet years, the scale of agricultural planting should be appropriately reduced to alleviate the pressure of groundwater overexploitation.

Proton exchange membrane fuel cell (PEMFC) usually need an activation process to obtain its best electrochemical performance.Compared with the traditional activation methods, electrochemical hydrogen pump has the advantages of saving time and hydrogen cost. The electrochemical hydrogen pump is a method in which hydrogen is oxidized into protons at the anode, and the protons migrate to the cathode with an applied electric field, and then are reduced to hydrogen again. The performance, internal impedance and electrochemical specific area(ECSA) changes of PEMFC after electrochemical hydrogen pump activation were studied with the help of polarization curves, electrochemical impedance spectroscopy(EIS) and cyclic voltammetry(CV) tests. Then the mechanism of the hydrogen pump activation was analyzed. Moreover, the influence of different current densities, inlet humidities and activation temperatures were investigated. The results show that after the hydrogen pump activation, the fuel cell performance is improved obviously, the slope of Tafel curve is decreased, the charge transfer resistance and mass transfer resistance get lower values, while the ohmic resistance is basically unchanged, and the ECSA is increased. Therefore, the mechanism of the hydrogen pump activation is related to the quantity of active species and microstructure of catalyst layer. The performance of hydrogen pump activation under 200 mA·cm-2 is better than that of under 100 mA·cm-2. The performance of hydrogen pump activation under the inlet humidity of 150%RH is better than that of 100%RH and 200%RH. In addition, the activation temperature has little impact on the PEMFC performance after hydrogen pump activation, and the fuel cell can be fully activated by hydrogen pump activation for 30 min at room temperature.

Al–Li alloy has been widely used in the aerospace field owing to its high strength and low density. In this study, alternating current cold metal transfer (AC CMT) along with a high-frequency pulse current technique was used to weld a 2060 Al–Li alloy using an ER5356 wire. The effect of pulse frequency on the arc shape, microstructure, and mechanical properties of the welded joints was examined, and mechanical performance testing was conducted. The results revealed that the arc diameter, arc length, and arc volume showed a trend of increasing first and then decreasing with an increase in the pulse frequency and reached their peak values when the pulse frequency was 50 kHz. Coupling the welding process with a high-frequency pulse resulted in grain refinement, which was attributed to the stirring action of the arc force. Both the porosity levels and grain size decreased with increasing frequency. When the pulse frequency was 70 kHz, the porosity level was the lowest, and the grain size was refined to 24.1 μm. The tensile strength of the welded joints also increased with the pulse frequency, and a maximum tensile strength of 249 MPa was observed at 70 kHz.

6082-T6 aluminum alloy plate with thickness of 42mm was butt welded by friction stir welding (FSW) from two sides. The microstructures of the joints exhibited different grain sizes because of unequal frictional heating and plastic flow during FSW process. The transition from the heat affected zone (HAZ) to the nugget zone (NZ) in thermos-mechanical affected zone of advancing side (AS-TMAZ) was more sudden than thermos-mechanical affected zone of retreating side (RS-TMAZ). Kissing bond (KB) defect throughout the entire FSW joint was displayed both at the grain boundary and in the interior of the grain with semi-continuous bands. KB had no direct effect on tensile properties. Vickers hardness of the FSW joint was lower than the BM because its high heat input, dissolved and coarsened precipitates and little to the grain size after FSW. Hardness distribution of double-sided welding joint showed X-shaped area softening characteristics, that is to say the lowest hardness was the junction of two welding joint of NZ and the junction of TMAZ and HAZ. The tensile fracture position occurred in the lowest hardness region of the FSW joint, and it did not occur in the KB defect position.

The tunnel defect formed in friction stir weld would dramatically push the mechanical properties of joints into deterioration. In this study, friction stir welding process was adopted to repair the joints of 7N01 aluminum alloy with tunnel defect. The effects of friction stir repair welding process on the microstructure and mechanical properties were comprehensively investigated. Microstructure of the repaired joints shows that the grain size in nugget zone decreases slightly while the recrystallization in the retreating side of thermo-mechanically affected zone is intensified as the joints are repaired. The microhardness of the repaired joints declined slightly compared with the defective joint. However, the yield strength and tensile strength increase and recover to the values of the joints free of defect. The longitudinal residual stress in weld zone increased remarkably as the repair times increase. Compared with the once repaired joint, yield strength and tensile strength of the twice repaired joint reduced slightly, and the throat thickness also reduced during the repeated repair welding process. Therefore, the times of repair welding applied should be limited actually.