The work has started with a deep work of literature review of the state of the art of metal AM processes. Due to the high cooling rates during the metal deposition, the release of latent heat during solidification has been found to have minor importance to the overall thermal and microstructural behaviour of the alloy during the entire process of deposition of the part. Then, the activities have focused mainly on the overall thermal behaviour of the part during the entire printing, considering the thermal cycles imposed by the successive depositions and their influence on microstructural evolution.
As regards the thermal modelling of the AM process, the welding path has been modelled by means of an ad-hoc activation methodology that switches on the FEM elements according to the scanning sequence considering the energy given by the specific process to melt the alloy and calculate the overall thermal behaviour of the part. Several methods of modelling simplification have been implemented such as hatch by hatch (activate at the same time an entire line of the scanning sequence) and layer by layer (activate at the same time an entire horizontal layer of the part). These simplified models are not capable of predicting with high fidelity the local complex thermal history. However, they are able to capture the accurate global thermal response that have the main influence on the microstructural transformation that take place at the temperature relative to the solid state transformations (improvement of the state of the art). Several activities of calibration and validation of the thermal modelling approaches have been carried out using thermal data recorded from real AM cases. The temperature measure has been performed mainly by means of thermocouples located in key points of the alloy during the AM fabrication of real parts mainly involved in activities of ongoing CIMNE R&D.
The numerical results of temperature evolution during the AM process provide the thermal inputs to predict the microstructure in each point of the part. Eventually, a model for the prediction of microstructure evolution of Ti6Al4V during AM processes has been defined and its formulation is presented in paper [1]. The main phase evolutions that take place during the solid state transformations of Ti6Al4V are considered, both for cooling and re-heating conditions. A phenomenological approach has been employed, allowing to directly correlate the local temperature-time curves with the evolution of the phase fractions. Medium-slow diffusional phase transformations are modelled by means of JMAK evolution laws and considering the Temperature-Time-Transformation (TTT) curves as material data input. In the case of fast diffusion-less transformations (such as martensite formation), empirical evolution laws are employed. For each single type of metallurgical transformation, calibration and validation of the model have been carried out mainly using data recollected from literature.
The microstructural model is specifically designed in order to take into account the typical conditions of AM processes. During these processes, heating and cooling transformations take place several times due to the thermal cycles generated by the layer-by-layer metal deposition sequence. Most of the times, these cycles are so fast that the transformations remain incomplete. The developed model is specifically designed to deal with AM processes where fast cooling and re-heating cycles occur. Innovative features are introduced allowing the model for consider incomplete transformations, varying initial content of phases and simultaneous transformations (improvement of the state of the art).
The overall thermal-microstructural models have been implemented in COMET, a Finite Element (FE)-based framework used for the numerical simulation of metal deposition processes. Eventually the overall microstructural model has been validated using the data recollected in literature of final Ti6Al4V microstructure of parts fabricated by powder bed AM process during previous experimental campaign available in literature.
The main results are reported in the paper:
E. Salsi, M. Chiumenti and M. Cervera. Modeling of microstructure evolution of Ti6Al4V for Additive Manufacturing (2018) Metals, 8(8), 633.
The developed FEM software for the simulation of AM processes have been employed in R&D activities of European Projects (Caxman, Emusic) or in industrial collaborations (ArcelorMittal)
