Melt Pool and Microstructure Characterization for AM Model Development

Abstract

Additive manufacturing (AM) technologies represent a significant advancement in the ability to create parts with unique geometries and functionalities. However, AM of metals results in parts with large residual stresses and dramatic changes in microstructure compared to conventionally processed materials, leading to significant differences in the constitutive responses of the materials. The development of models that capture the evolution of these complex microstructures and their constitutive response is therefore extremely important. In addition, the evaluation and validation of these models to ensure that they adequately describe AM materials and their performance characteristics is required. Here, we present both in situ and ex situ characterization results of rapid solidification (RS) aimed at developing a mesoscale phase-field model to describe microstructure evolution under RS conditions relevant to metals-based AM. In addition, SEM and TEM microstructure characterization results will be presented that are aimed at development of a microstructurally aware strength model for AM metals. Microstructure evolution during AM of metals begins in the melt pool, where laser processing involves rapid melting and subsequent rapid solidification (RS) under conditions involving large thermal gradients, high cooling rates, and high solidification front velocities. These conditions present significant challenges for both in situ characterization and modeling efforts. Our in situ characterization work has focused on using time-resolved imaging of solidification fronts with dynamic transmission electron microscopy (DTEM) to capture solid-liquid interface evolution during laser-induced rapid alloy solidification under processing conditions relevant to metals-based AM [1]. RS results will be presented from experiments with Al-based alloys, as shown in Figure 1 for an Al-4at%Cu alloy, and Ni-Cu alloys. Complementary ex situ solidification and characterization results involving single-track laser melting experiments with Ti-Nb alloys will also be shown. These in situ [2] and ex situ [3] results have been used to benchmark and calibrate a mesoscale phase-field model, in terms of solidification velocity, solid-liquid interface morphology, and non-equilibrium partitioning during RS. This model is coupled to thermodynamic and kinetic databases within the CALPHAD methodology. Phase-field simulations and comparisons with experimental results will be shown. To begin developing a microstructurally aware strength model, Ti-6Al-4V was chosen as a test case material due to its common use in the AM community and its sensitivity to thermal processing conditions, which can lead to a large variation in the available microstructures. The mechanical response of these 2562