Monitoring residual strain relaxation and preferred grain orientation of additively manufactured Inconel 625 by in-situ neutron imaging

Abstract

Microstructures produced by Additive Manufacturing (AM) techniques determine many characteristics of components produced by this particular manufacturing technique. Residual stress and texture are among those characteristics, which need to be optimized to meet dimensional and strength requirements. Post-build heat treatments are necessary to relief residual stresses, homogenize the microstructure and to develop the necessary microstructures (precipitation or solution hardening) for strength. Our focus in this study is the measurement of residual stresses during a vacuum stress relief (VSR) heat treatment. We employ the energy-resolved neutron imaging to investigate, in-situ, the uniformity of the as-built texture and to map the residual strain distributions within samples of Inconel 625 (IN625). The samples used in this study were printed using the powder-bed metal laser melting additive manufacturing technique. Strain and texture variations were measured at room temperature for the as-built samples as well as their changes during stress relief annealing at 700 °C and 875 °C in a vacuum furnace. The uniformity of crystalline plane distribution, from which texture can be inferred, was imaged with sub-mm spatial resolution for the entire sample area exceeding several square centimeters. Despite the limited accuracy of in-situ lattice strain reconstruction during sample annealing, the results indicate that most of the strain relaxation occurs at 700 °C within the first 2–3 h. Relaxation of the strain at 875 °C happened at a much faster rate. In addition, energy-resolved neutron imaging demonstrates the possibility to correlate the uniformity of grain orientation and the degree of texture variations with printing parameters, in particular the laser translation speed. Simulations of the AM build process were carried out to predict the residual stress distributions within the samples as a function of the build process parameters. Comparisons of the simulation results to the room temperature experimental measurements show good agreements when the simulation data are averaged over the volume of the part confirming the usefulness of the experiments for validating simulation results. The neutron imaging technique described in this study can be helpful for the evaluation of bulk crystallographic characteristics in AM printed materials and can be used as a guide for developing the heat treatment cycle and for validations of VSR simulation results.