The influence of two-step homogenization on microstructures and properties of AZ40M magnesium alloy were investigated by the scanning electronic microscopy (SEM), X-ray diffraction analysis (XRD) and tensile experiment. Through comparing the microstructure and mechanical properties of the alloy before and after homogenization treatment, a two-step homogenization process of the semi-continuous casting AZ40M magnesium alloy ingot was determined. The result shows that the as-cast AZ40M alloy mainly contained two kinds of second phase, β-Mg17Al12 and Al8Mn5. Treating at temperature of 380 °C for 8 h as the first step homogenization treatment and then treating at temperature of 420 °C for 20 h as the second step homogenization treatment made the eutectic phase (α-Mg/β-Mg17Al12) completely eliminated and the alloying elements such as Al, Zn distributed uniformly. Al8Mn5 is the residual second phase in the homogenized AZ40M alloy. During the homogenization process, the β-phase dissolved which made the content of Al increased in the matrix. The decomposition of β-phase benefits the tensile strength and elongation of the alloy. The tensile fractures of as-cast and homogenization alloys are characteristic of ductile fracture.
In view of complex and fuzziness of geological adaptive cutterhead selection for earth pressure balance (EPB) shield, a cutterhead selection method based on BP neural network is put forward. Considering the structure characteristics of EPB shield cutterhead, typical cutterhead types are classified and summarized based on cutterhead topology structure and number of spokes. After analyzing the determinants of cutterhead selection, one-to-many mapping relation between cutterhead type and geological parameters is put forward, and then core geologic parameters related to cutterhead selection are concluded. The feasibility of using neural network method to choose the cutterhead type is analyzed, and a BP neural network training model for cutterhead selection is set up and tested in testing sample data. The result shows that the selected cutterhead and the construction cutterhead are basically consistent. The feasibility of this method is proved and it can be theoretical basis for the cutterhead structure design which will improve scientific of cutterhead selection.
The reason for the distinct difference in the thermal conductivities of different series of Mg alloys was investigated.The crystallographic lattice parameter and the thermal conductivity of Mg–Zn, Mg–Al, and Mg–Gd binary alloys, which all contain the same atomic percentage of the solutes were measured. The Mg–Zn alloys exhibited the highest thermal conductivity and the smallest lattice distortion, and Mg–Gd alloys exhibited lowest thermal conductivity and largest lattice distortion, respectively. Results indicate that the thermal conductivity of the Mg alloys depends on the difference in atomic radius of the solute and matrix atoms. Therefore, the reason for the Mg-7Gd-5Y-0.5Nd-0.5Zr alloy components have excellent thermal resistance is the serious lattice distortion caused by the significant difference in the atomic radius between the solute and matrix atoms.
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