## 7 Conclusions and Prospect  
Based on the digital twin concept, this paper studies the integrated framework and key technologies of complex product design, manufacturing and O&M. The design, manufacturing and O&M integration of product is the key point of achieving intelligent manufacturing. The digital twin reacts to the true status of the product at different stages by constructing the multidimensional simulation model of a complex product, and achieves the deep integration of design, manufacturing and O&M of a complex product through data driving. The design, manufacturing and O&M integration framework of a complex product based on digital twin solves the information disconnection at each stage of design, manufacturing and O&M in traditional manufacturing mode, and the poor real-time feedback ability of data, to form a kind of good closed-loop manufacturing mode. Such framework comprises the design-manufacturing-O&M physical entity module, virtual model module and twin data platform. At the twin data platform, the design, manufacturing and O&M process of products are connected to achieve the optimization design, accurate manufacturing and maintenance of the product and to enhance the management quality of the product's whole life cycle.  
Finally, this paper applies the framework in the O&M of bogie key parts of certain EMU. Based on the fault prediction results, the data are fed back to the O&M, manufacturing, and design departments. The design department achieves the optimization design and synchronous simulation of the product in virtual space as per the feedback results of manufacturing data and O&M data. The manufacturing department promptly avoids the problems generated in the manufacturing process as per feedback results of design data and actual O&M data and achieves the high-precision manufacturing of products via visual interaction and virtual verification. The O&M Department promptly formulates the O&M plan, configures the O&M resources as per the obtained data, and evaluates and optimizes the scheme at the virtual model layer to improve the actual O&M efficiency. The design, manufacturing and O&M integration of EMU bogie key parts are achieved via the interaction and iteration of the physical and virtual layers.  
Digital twin technology is in its initial stage, so many key technologies are not yet mature. This paper also discusses some key technologies in the design-manufacturing-O&M integration of digital twin. With the development of communication technology and information technology, we believe the specific achievement of digital twin technology will be further advanced to encourage further development of intelligent manufacturing.  
Conceptualization, C.Z. and L.D.; methodology, C.Z. and L.D.; formal analysis, L.D.; investigation, C.Z.; resources, C.Z.; data curation, L.D.; writing--original draft preparation, L.D.; writing--review and editing, Y.W. and L.D.; visualization, C.Z.; supervision, C.Z.; project administration, C.Z.; funding acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the General Project 51974229 of the National Natural Science Foundation of China, and the Shaanxi Province Innovation Capability Support Program 2021 TD-27.  
Not applicable.  
The authors declare no conflict of interest.