The joint surface of the 1060 aluminum and AZ31 magnesium alloy was prepared through friction stir lap welding (FSLW) under different welding process parameters. The joint surface was characterized three-dimensionally using a three-dimensional (3D) optical profiler, and the coordinate data were obtained. The fractal dimension of the joint surface was calculated by the box-width transformation method using a MATLAB program. Furthermore, the influence of the welding process parameters on the fractal dimension of the joint surface was studied. The response surface model was established based on the principle of central composite design (CCD), and analysis of variance (ANOVA) was carried out to test the accuracy of the response surface. The results showed that the joint surface morphology had fractal characteristics, and the fractal dimension could be used as an index to characterize the quality of the joint surface. The change of the welding process parameters had a great impact on the fractal dimension of the joint surface, the interaction between the parameters was small, and the fitting accuracy of the response surface model was high. The fractal dimension of the joint surface decreased with the increase in the welding and rotational speeds and the effect of the rotational speed was more significant.

Addressing the issue of shallow-buried, closely spaced multiple coal seam repeat mining, where the development of overlying rock fractures causes interlayer airflow disturbances, leading to spontaneous combustion hazards in leftover coal and affecting the safe and efficient mining of the working face. Taking Zhangjiamao Coal Mine as the research object, a discrete element numerical model of shallow-buried, closely spaced overlying rock structure particle flow is established to study the development patterns of overlying rock fractures and the evolution of porosity in the working faces during the repeated mining processes of the 2⁻² and 4⁻² coal seams. Based on simulated data, we establish a fitting formula for the relationship between mining height, advance distance, and fracture development height. The research results indicate the following: as the working face continues to advance, the number of fractures in the overlying roof rock increases, and the fracture zones exhibit a horizontal and vertical intersecting distribution pattern, with the range continuously expanding until it extends to the surface, forming a moderately flat subsidence basin in the middle. The porosity range of the roof rock increases progressively with the mining of the working face, and the porosity of the roof rock when mining the lower coal seam is greater than that when mining the upper coal seam. The comparison between the research results and the on-site measured results verified the reliability of the simulation results.