While there have been various studies towards Android apps and their development, there is limited discussion of the broader class of apps that fall in the fake area. Fake apps and their development are distinct from official apps and belong to the mobile underground industry. Due to the lack of knowledge of the mobile underground industry, fake apps, their ecosystem and nature still remain in mystery.To fill the blank, we conduct the first systematic and comprehensive empirical study on a large-scale set of fake apps. Over 150,000 samples related to the top 50 popular apps are collected for extensive measurement. In this paper, we present discoveries from three different perspectives, namely fake sample characteristics, quantitative study on fake samples and fake authors' developing trend. Moreover, valuable domain knowledge, like fake apps' naming tendency and fake developers' evasive strategies, is then presented and confirmed with case studies, demonstrating a clear vision of fake apps and their ecosystem.

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Abstract-Aluminum laminated composite (ALC) is a new type of structural material with a broad application space and development prospects. Due to the laminated structure, the mechanical properties and failure patterns of ALCs differ significantly from those of uniform materials. The friction stir welding (FSW) of Al alloy can effectively avoid many defects caused by fusion welding, but the FSW of ALCs is in its infancy stage. Friction stir welding experiment of 7B52 ALC was carried out in this study. The Vickers hardness test, tensile test and impact test were performed on the specimens of the joint and base metal to analyze the effect of friction stir welding on microhardness, tensile properties and impact properties. The fracture surfaces were observed by SEM to investigate the fracture mechanism. A defect free weld was obtained. However, the laminated structure was destroyed, resulting in the decrease of microhardness and tensile strength. The microstructure of the weld was improved, significantly increasing the impact toughness.