At present, the qualified rate of large thin-walled magnesium alloy castings is low. In this study, the effects of mold structure and process parameters were investigated to improve the production qualification rate of castings. The filling process of die castings was simulated by numerical simulation technology to optimize their structure. On the basis of an optimized mold structure, the process parameters of die castings were optimized using a response surface model, and a group of optimal process combinations were obtained: pouring temperature—660 °C; mold preheating temperature—200 °C; injection speed—6.5 m/s. The rationality of the optimized mold structure and process parameters is verified by die-casting experiments. The results show that the optimized mold structure and process parameters can effectively reduce the internal shrinkage cavity casting defects of automotive CCB castings, and effectively improve the production qualification rate of magnesium alloy CCB castings. This research has important guiding significance for the production of large thin-walled magnesium alloy parts.

Hydrogen energy is a clean, carbon-free, flexible, efficient, and widely used secondary energy source, which is an ideal alternative to promote the clean and efficient use of traditional fossil fuels. Hydrogen fuel cell bus has the advantages of a high-energy conversion rate, absolute pollution-free, sufficient raw materials, and convenient filling. The hybrid power system, composed of fuel cell and auxiliary energy source, is one of the key technologies to promote the development of hydrogen fuel cell vehicle. This study aims to propose an energy management strategy by analyzing the output characteristics and power allocation of fuel cell and power battery in the hybrid power mode with fuel cell as the main and battery as the auxiliary. A GM (1, N) power prediction strategy was proposed and compared with other strategies as an on-off control strategy and logical threshold value strategy in this study. The variation curves of the battery SOC and fuel cell output power under two working conditions of CCBC and real vehicle conditions were analyzed by using these three strategies, when the initial SOC of power battery is 30, 70, and 90, respectively. Results showed that the power prediction strategy based on GM (1, N) has a better performance in output efficiency and fuel economy when compared to the other two strategies by analyzing the aspects of the battery in the SOC variation and equivalent hydrogen consumption and the fuel cell in the output power variation and hydrogen consumption. This research can be helpful to provide the suggested solution for energy management of the hybrid power system for hydrogen fuel cell buses.

Magnesium alloy materials are often affected by cyclic stress, resulting in fatigue fracture due to changes in mechanical properties. In this study, external cyclic loads were introduced to simulate the loading of AZ91D (Mg-9Al-0.5Zn) parts. Tensile tests under static and external circulation loads were performed on the same specimens, and the effect of loading conditions on the mechanical properties of the specimens (tensile strength, yield strength and elongation) were studied. The internal microstructure of the test specimens was analyzed, and the influence of cyclic loading on the microstructure change of the specimens was discussed. Compared with static load, under the condition of cyclic loading, the tensile strength and yield strength of the specimens are increased by more than 15%, and the elongation rate is increased by 3 times. In addition, the microstructure images showed that many twins and the second phase β-Mg17Al12 were generated inside the AZ91D microstructure under cyclic loading. The former results in specimens crack initiation and it is the root cause of fatigue fracture. And the latter accounts for the strengthening of the mechanical properties of specimens.