In situ recrystallization of pure Cu during electron beam powder bed fusion

• Variations in hatch spacing and scanning speed result in a change in grain morphology in pure Cu processed by EB-PBF. • At large hatch spacing (> 100 µm) and slow scanning speed (< 100 mm/s), the microstructure is fully columnar. • At reduced hatch spacing (≤ 60 µm) and high scanning speed (>150 mm/s), the microstructure is nearly fully equiaxed. • In situ recrystallization during EB-PBF is responsible for the change in grain morphology. • In situ recrystallization can be exploited to tailor spatially the grain structure of pure Cu in EB-PBF. Additively manufactured parts often exhibit an anisotropic microstructure characterized by columnar grains resulting from epitaxial growth from the underlying layers in the direction of the thermal gradient, i.e. parallel to the build direction. Herein we report that the columnar grains of pure Cu produced by electron beam powder bed fusion can be transformed into equiaxed grains by reducing the hatch spacing and increasing the scanning speed. Columnar grains are observed for hatch spacings ≥ 100 µm and a scanning speed = 100 mm/s, whereas equiaxed grains are observed when the hatch spacing is ≤ 60µm and the scanning speed ≥ 150 mm/s. The underlying mechanism is discussed and identified on the basis of EBSD analyses. It is concluded that in situ recrystallization during electron beam powder bed fusion can be locally activated by varying the processing parameters. Further insight into the underlying mechanism is gained from the characterization of layered samples produced with varying processing parameters along the build direction. We show that in situ recrystallization offers an interesting pathway to produce site-specific microstructures in electron beam powder bed fusion.