Abstract
AM processes are characterized by complex thermal cycles that have a deep influence on the microstructural transformations of the deposited alloy. In this work, a general model for the prediction of microstructure evolution during solid state transformations of Ti6Al4V is presented. Several formulations have been developed and employed for modeling phase transformations in other manufacturing processes and, particularly, in casting. The proposed model is mainly based on the combination and modification of some of these existing formulations, leading to a new overall model specifically dedicated to AM. The accuracy and suitability of the integrated model is enhanced, introducing new dedicated features. In fact the model is designed to deal with fast cooling and re-heating cycles typical of AM processes because: (a) it is able to consider incomplete transformations and varying initial content of phases and (b) it can take into account simultaneous transformations.The model is implemented in COMET, an in-house Finite Element (FE)-based framework for the solution of thermo-mechanical engineering problems. The validation of the microstructural model is performed by comparing the simulation results with the data available in the literature. The sensitivity of the model to the variation of material parameters is also discussed.
Highlights
Compared to traditional forming processes, AM metal deposition is characterized by successive thermal cycles with unusual ranges of cooling and heating conditions
The local thermal cycles are strongly influenced by the distance from the heat source, which is continuously moving according to the metal deposition sequence
The base plate was preheated at 200 ◦ C in order to promote martensite dissolution
Summary
Compared to traditional forming processes, AM metal deposition is characterized by successive thermal cycles with unusual ranges of cooling and heating conditions. According to the process parameters and to the deposition strategy, these thermal cycles present different ranges of temperatures in each point of the part. The local thermal cycles are strongly influenced by the distance from the heat source, which is continuously moving according to the metal deposition sequence. The complex temperature evolution directly influences the kinetics of microstructure formations during the solidification and the solid state transformations. Peripheral areas of the deposited metal present different thermal conditions if compared with massive areas, causing differences in the grain orientation and size. The high cooling rates, typical of fast AM thermal cycles, can lead to the formation of metastable martensitic structures
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