Boundary dynamics in 3D printed samples

Additive manufacturing (or 3D printing) of metallic components has recently become very popular as it enables net-shape manufacturing as well as complex designs that are not easily achievable by conventional processing. 3D printed metallic components are likely to be deformed and/or exposed to high temperatures during application and therefore it is necessary to investigate the deformation and annealing behaviours of 3D printed components—the two objectives of this thesis. Stainless steel 316L is chosen as the example in this work because this material is relatively easy to print without introducing too many voids. Laser powder bed fusion (L-PBF) is used to print 316L samples according to industrial standards. Various characterization techniques are used, including electron microscopy: scanning electron microscopy/electron backscatter diffraction (SEM/EBSD) and transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray absorption contrast tomography, Vickers hardness measurement and tensile test.

Before analysis of the deformation behaviour of 3D printed 316L samples,conventionally manufactured 316L is deformed by cold rolling and the microstructureand the texture evolution are followed as a function of deformation strain—to serve as a solid basis for the subsequent analysis of the printed samples. It is found that a Taylorlattice structure forms at low strain; deformation twinning and shear banding gradually develop with increasing deformation; martensite transformation occurs mainly in theshear bands at rolling reductions larger than 30%. Deformation twinning transformsthe Copper orientation to twin Copper orientation, and texture transition from the Copper to the Brass type occurs at the onset of shear banding, suggesting an importantrole of shear banding and deformation twinning in this texture transition.
The 3D printed sample is cold rolled to 10% and 30% reduction in thickness. The printing induced cellular/columnar boundaries with dislocations of complex Burgersvectors, on one hand, act as an obstacle to dislocation motion and  deformation twinning, but on the other hand, facilitate nucleation of dislocations and deformation twinning. Consequently, multiple sets of deformation twins form and a strong interaction between dislocations and deformation twins are commonly observed in the 3D printed sample, contributing to the higher strength in printed than in conventionally manufactured 316L.

No recrystallization is observed in the as-printed sample during annealing at 850 °C.However, with an additional 30% cold rolling, recrystallization is observed to occur readily and progress faster than in 30% cold rolled conventionally manufactured 316L.Voids appear to be unavoidable in 3D printed samples. Effects of voids on the
recrystallization behaviour is thus investigated here by phase field simulations and exsitu EBSD observations. The voids and the deformed/printed microstructure are found to significantly affect the local recrystallization boundary kinetics.