Columnar-to-equiaxed grain transition in powder bed fusion via mimicking casting solidification and promoting in situ recrystallization

June 10, 2021

Paria Karimi (1), Esmaeil Sadeghi (1), Joakim Ålgårdh (1,2), Ali Keshavarzkermani (3), Reza Esmaeilizadeh (3), Ehsan Toyserkani (3), Joel Andersson (1)
Additive Manufacturing, 46, June 2021. DOI: 10.1016/j.addma.2021.102086


Keywords

Additive manufacturing; Electron beam-powder bed fusion; Alloy 718; In situ recrystallization; Grain structure


Abstract

Columnar grain structure typically formed along the build direction in the electron beam-powder bed fusion (EB-PBF) technique leads to anisotropic physical and mechanical properties. In this study, casting solidification condition was mimicked, and in situ recrystallization was promoted in EB-PBF to facilitate columnar-to-equiaxed grain structure transition in Alloy 718. This is achieved via a unique linear melting strategy coupled with a specific selection of process parameters in EB-PBF. It was found that site-specific melting using line order number (LON) function affected the cooling rate and temperature gradient, which controlled grain morphology and texture. A high LON resulted in a large equiaxed grain zone with a random texture, whereas a fixed LON with a high areal energy density led to a strong texture. The main driving force in the formation of cracks and shrinkage defects during the transition was investigated. A high LON at a fixed areal energy density reduced the average total shrinkage defects and crack length. The hardness was decreased through the transition, which was linked to the reduction in the size of the γ″ precipitates.


How Our Software Was Used

Dragonfly was used to analyze 3D reconstructed CT images.


Author Affiliation

(1) Department of Engineering Science, University West, 46186 Trollhättan, Sweden.
(2) GE Additive | Arcam EBM, Designvägen 2, 43533 Mölnlycke, Sweden.
(3) Multiā€‘Scale Additive Manufacturing Lab, Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada.