Tailored grain morphology via a unique melting strategy in electron beam-powder bed fusion

September 10, 2021

Paria Karimi (1), Esmaeil Sadeghi (1), Joakim Ålgårdh (2), Jonas Olsson (1), Magnus Hornqvist Colliander (3), Peter Harlin (4), Ehsan Toyserkani (5), Joel Andersson (1)
Materials Science and Engineering: A, 824, September 2021. DOI: 10.1016/j.msea.2021.141820


Keywords

Additive manufacturing; Electron beam-powder-bed fusion; Melting strategy; Grain structure; Alloy 718


Abstract

This study presents a unique melting strategy in electron beam-powder bed fusion of Alloy 718 to tailor the grain morphology from the typical columnar to equiaxed morphology. For this transition, a specific combination of certain process parameters, including low scanning speeds (400–800 mm/s), wide line offsets (300–500 μm) and a high number of line order (#10) was selected to control local solidification conditions in each melt pool during the process. In addition, secondary melting of each layer with a 90° rotation with respect to primary melting induced more vigorous motions within the melt pools and extensive changes in thermal gradient direction, facilitating grain morphology tailoring. Four different types of microstructures were classified according to the produced grain morphology depending on the overlap zone between two adjacent melt pools, i.e., fully-columnar (overlap above 40 %), fully-equiaxed (overlap below 15 %), mixed columnar-equiaxed grains, and hemispherical melt pools containing mixed columnar-equiaxed grains (overlap ~20–25 %). The typical texture was <001>; however, the texture was reduced significantly through the transition from the columnar to equiaxed grain morphology. Along with all four different microstructures, shrinkage defects and cracks were also identified which amount of them reduced by a reduction in areal energy input. The hardness was increased through the transition, which was linked to the growth of the γสบ precipitates and high grain boundary density in the fully-equiaxed grain morphology.


How Our Software Was Used

Dragonfly was used to perform 3D reconstruction and image analysis.


Author Affiliation

(1) Department of Engineering Science, University West, 461 86, Trollhättan, Sweden.
(2) Arcam-EBM (a GE Additive), 435 33, Mölnlycke, Sweden.
(3) Department of Applied Physics, Chalmers University of Technology, 412 96, Gothenburg, Sweden.
(4) Sandvik Additive Manufacturing, 811 81, Sandviken, Sweden.
(5)Department of Mechanical and Mechatronic Engineering, Waterloo University, Waterloo, Canada.