Correlative Synchrotron X-ray Imaging and Diffraction of Directed Energy Deposition Additive Manufacturing

TL;DR

This study combines synchrotron X-ray imaging and diffraction to reveal the microstructural evolution and stress development in nickel-base superalloy IN718 during directed energy deposition additive manufacturing, providing insights into solidification and cracking mechanisms.

Contribution

It introduces a novel in situ and operando synchrotron approach to analyze microstructure and stress evolution during DED-AM, linking real-time imaging with diffraction data.

Findings

Rapid cooling suppresses secondary phases and recrystallization.

Stress reaches yield strength during cooling, leading to liquation cracking.

Microstructure prediction based on thermal gradients and solidification dynamics.

Abstract

The governing mechanistic behaviour of Directed Energy Deposition Additive Manufacturing (DED-AM) is revealed by a combined in situ and operando synchrotron X-ray imaging and diffraction study of a nickel-base superalloy, IN718. Using a unique process replicator, real-space phase-contrast imaging enables quantification of the melt-pool boundary and flow dynamics during solidification. This imaging knowledge informed precise diffraction measurements of temporally resolved microstructural phases during transformation and stress development with a spatial resolution of 100 $\mu$m. The diffraction quantified thermal gradient enabled a dendritic solidification microstructure to be predicted and coupled to the stress orientation and magnitude. The fast cooling rate entirely suppressed the formation of secondary phases or recrystallisation in the solid-state. Upon solidification, the stresses rapidly increase to the yield strength during cooling. This insight, combined with IN718 $'$s large solidification range suggests that the accumulated plasticity exhausts the alloy$'$s ductility, causing liquation cracking. This study has revealed additional fundamental mechanisms governing the formation of highly non-equilibrium microstructures during DED-AM.