Understanding size-dependent thermal, microstructural, mechanical behaviors of additively manufactured Ti-6Al-4V from experiments and thermo-metallurgical simulation

Thanks to manufacturing flexibility provided by the laser powder bed fusion process (L-PBF), functional metal components could be designed and built with topological and complex structures. However, microstructures and mechanical properties of L-PBF parts are known to exhibit strong size-dependency. Therefore, the understanding of process-structure-property relationships from bulk samples may not fully translate to samples with small features. As a result, the present study developed a multiscale thermo-metallurgical model to reveal thermal, microstructural, and mechanical behaviors in the L-PBF Ti-6Al-4V struts with various diameters. The thermal simulation showed that struts with small diameters exhibit more rapid cooling rates than those with large diameters. Subsequently, the rapid cooling rate led to samples with smaller lath width where the lath width varied between 0.56 and 0.85 μm for struts with diameters between 0.3 and 2 mm, respectively. However, the solid phase fraction seems to be negligibly influenced by the feature size. Followingly, the Hall-Petch model was used to predict the yield strength based on the solid phase fraction and the lath size, where the grain boundary strengthening was recognized as the primary mechanism, dictating size-dependent mechanical behaviors. Both predicted lath width and yield strength were compared with experimental results from EBSD maps and tensile testing, where numerical prediction exhibited reasonable agreement with the experiments. Ultimately, the present study experimentally and numerically quantified the influence of the sample's size on physical behaviors. This understanding is an important aspect which could assist the consideration of the size effect in a design process.