Physics-based thermal-chemical-fluid-microstructure modelling of in-situ alloying using additive manufacturing: Composition-microstructure control

Nucleation and grain growth during metal additive manufacturing (AM) still remain debatable since the nature of rapid melting and solidification induced by laser-powder interaction during AM may cause particular liquid metal behaviour especially when in-situ alloying is used. The rationalisation of the role of heterogeneous mixing in the microstructure development is of importance for composition control and hence process optimisation. In this work, coupled thermal-chemical-fluid-microstructure modelling is developed for simulating in-situ AM to understand the chemistry-induced solidification, re-melting and microstructure development. The results indicate that thermal fluid flow and chemical mixing play an important role in the rapidly solidified microstructure. The heterogeneous nucleation resulting from the undercooling due to the large thermal gradient and the large cooling rate initiates grain nuclei forming equiaxed grains, and with the extent of the thermal gradient, more anisotropic columnar grain growth occurs in AM. The keyhole serves as a strong stirrer to enhance chemical species mixing by inducing convective flow motion, which determines the local chemical composition and microstructure in in-situ alloying while melting newly fed powders and re-melting part of the previous layer. This work enhances a fundamental and comprehensive understanding and sets a guideline in tailoring the grain structure in as-fabricated components, which is difficult to observe directly in experiment.