As one of the key additive manufacturing methods, Wire and Arc Additive Manufacturing (WAAM) is particularly well-suited for producing complex, large-scale steel components due to its high efficiency, cost-effectiveness, adaptability to complex environments and versatility. 921A steel (10CrNi3MoV) is a domestically produced low-alloy, tempered high-strength steel in China, renowned for its excellent combination of mechanical properties, toughness, weldability, and corrosion resistance. These characteristics make it a preferred material for manufacturing critical components in ocean engineering, particularly in shipbuilding. Consequently, WAAM technology holds significant potential for the production and repair of critical ship components fabricated from 921A steel.
The WAAM process is inherently a multi-scale and multi-physics coupling process, encompassing macroscopic phenomena such as heat and mass transfer and microscopic mechanisms like grain growth kinetics within the melt pool. The interplay between grain growth and nucleation strongly influences the final microstructure and mechanical properties, as they depend on the unique thermal history experienced during manufacturing. Utilizing microstructure simulation technology is crucial for understanding the complex dynamics of the additive process. This knowledge is key to ensuring uniform internal grain structures and achieving superior mechanical properties in the fabricated components.
Recently, a team of material scientists led by Lei Shi from Shandong University, China developed a multi-scale model of the heat transfer flow and microstructure evolution of the molten pool during the WAAM process of 921A steel, combining computational fluid dynamics (CFD) and cellular automata (CA) method, and utilizing the open-source ExaCA code for the microstructure simulation. The model successfully predicted the temperature field, flow field, and microstructural evolution within the sedimentary layer of 921A steel WAAM.
The team published their work in Materials and Solidifications on January 22, 2025.
"In this report, we carried out a systematic study of single-layer single-pass deposition process experiments, FLOW-3D temperature field and CA microstructure multiscale simulation for 921A steel WAAM. The macroscopic metallographic of the deposited layer is columnar grains consistent with the direction of heat flow, organized as granular bainite and ferrite." said Lei Shi, professor at School of Materials Science and Engineering at Shandong University (China), a senior expert whose research interests focus on the field of friction stir welding and additive manufacturing.
"We used FLOW-3D software to simulate the macroscopic temperature and flow fields of the 921A steel WAAM. The combined surface and internal flow shows that the molten pool metal mobility is weakened when the melting speed is higher. The results of the simulation and the experimental forming dimensions are basically in agreement." said Lei Shi.
Accurate modeling of the macro-scale thermal history provides critical input information for subsequent microstructure simulation models, and the solidification data from the thermal history model is used for subsequent CA simulations. "We used the ExaCA model to simulate the solidification process in the cross-section of the melt pool under the effect of the welding transient temperature field. In this case, the columnar crystals of the melt pool grow toward the center of the melt pool normal to the fusion line, and the nucleated equiaxed grains blocking the original columnar grains are formed in the center of the melt pool at low temperature gradient." said Lei Shi.
However, more detailed research work is needed to explore the microstructure evolution during the WAAM process of 921A steel. In this regard, Shi also suggests possible research directions to be pursued in future work, including more quantitative grain size, grain orientation versus actual, and larger-scale microstructure simulation analysis.
Other contributors include Xiaohui Lyu, Ji Chen, Chuansong Wu, Ashish Kumar from the School of Materials Science and Engineering at Shandong University in Jinan, China; Ming Zhai from the Metals and Chemistry Research Institute, China Academy of Railway Sciences Co. Ltd. in Beijing, China; Wenjian Ren from Shandong Aotai Electric Co., Ltd. in Jinan, China.
This work was supported by the National Key Research and Development Program of China (Grant No. 2022YFB4600902), the National Natural Science Foundation of China (Grant Nos. 52275349 and 52035005), the Shandong Provincial Science Foundation for Outstanding Young Scholars(Grant No. ZR2024YQ020), the Excellent Young Team Project of Central Universities (No. 2023QNTD002) and the Key Research and Development Program of Shandong Province (Grant No. 2021ZLGX01).
About Author
Lei Shi is currently a professor at School of Materials Science and Engineering, Shandong University, China. He received his doctoral degree from Shandong University, China, in 2016. His research interests include friction stir welding and additive manufacturing.
About Materials and Solidification
Materials and Solidification is a single-blind peer-reviewed, fully open access international journal published by Tsinghua University Press, with academic support provided by the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University. The Journal aims to publish cutting-edge research results in solidification theory and solidification technologies for metal, semiconductor, organic, inorganic, and polymer materials in bulk or as thin films. It includes, but is not limited to, casting, welding, and additive manufacturing related to solidification processing, and is also involved in nonequilibrium solidification phenomena in multiphysical fields, such as electricity, ultrasonication, magnetism, and microgravity.
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