Development of gradient microstructure in the lattice structure of AlSi10Mg alloy fabricated by selective laser melting

doi:10.1016/j.jmst.2019.06.015

Abstract

Abstract:

To identify the microstructural features of the lattice structures of Al alloys built via the selective laser melting (SLM) process, AlSi10Mg alloy with a body-centered cubic (BCC)-type lattice structure was prepared. Characteristic microstructures comprising melt pools with several columnar α-Al phases with <001 > orientations along the elongation direction and surrounded by eutectic Si particles were observed at all portions of the built lattice structure. In the node portions of the lattice structure, a gradient microstructure (continuous change in microstructure) was observed. The columnar α-Al phases were observed near the top surface of the node portion, whereas they became coarser and more equiaxed near the bottom surface, resulting in softening localized near the bottom surface. In the strut portions of the lattice structure, the columnar α-Al phases were elongated along the inclined direction of struts. This trend was more prevalent near the bottom surface. The α-Al phases became coarser and more equiaxed near the bottom surface as well. The aforementioned results were the basis of a discussion of the development of the gradient microstructure in lattice-structured Al alloys during the SLM process in terms of thermal conductivities at the boundaries between the manufactured (locally melted and rapidly solidified) portions and adjacent (unmelted) alloy powder.

Key words: Additive manufacturing, Lattice structure, Aluminum alloy, Microstructure, Thermal conductivity

Mulin Liu, Naoki Takata, Asuka Suzuki, Makoto Kobashi. Development of gradient microstructure in the lattice structure of AlSi10Mg alloy fabricated by selective laser melting[J]. J. Mater. Sci. Technol., 2020, 36: 106-117.

Figures/Tables 18

Table 1 Chemical composition of the SLM-fabricated AlSi10Mg alloy samples (wt%).

Sample	Si	Fe	Cu	Mn	Mg	Ni	Zn	Pb	Sn	Ti
Nominal	9.0-11.0	≤0.55	≤0.05	≤0.45	0.20-0.45	≤0.05	≤0.10	≤0.05	≤0.05	≤0.15
ICP analysis	10.23	0.12	-	-	0.37	-	-	-	-	-

Fig. 1. CAD models of (a) the BCC-type lattice structure, (b) the corresponding unit cell, (c) the cross-section of the BCC-type lattice structure and (d) optical micrograph of the BCC-type lattice structure showing node portions and strut portions.

Fig. 2. Schematics showing (a) the node portion and (b) the strut portion of the lattice structure. The centroids of the node and strut portion were defined as the original points. The directions along and against the building direction were defined as positive (+) and negative (-), respectively. The distances from the original points of the node portion and strut portion were defined as dN and dS, respectively.

Fig. 3. SEM images showing the cross-sections of AlSi10Mg powder particles at (a) low magnification and (b) high magnification and (c) orientation color map of the AlSi10Mg powder particles in (a).

Fig. 4. Relative densities as functions of the distances from the centroids of (a) the node portion (dN) and (b) the strut portion (dS) of the lattice structure.

Fig. 5. Optical micrographs showing the microstructures in (a) the node portion and (b) the strut portion of the lattice structure.

Fig. 6. (a) Schematic showing corresponding locations observed by SEM and (b-d) SEM images showing the microstructures in the node portion of the lattice structure with different distances from the centroid: (b) dN = 0.8 mm; (c) dN = 0 mm; (d) dN = -0.8 mm.

Fig. 7. Mean width of columnar α-Al phases in the node portion of the lattice structure as a function of the distance from the centroid (dN).

Fig. 8. (a) Schematic showing the corresponding locations in node portions of the lattice structure and (b-d) size distribution of Si particles in the node portion of the lattice structure with different distances from the centroid: (b) dN = 0.8 mm; (c) dN = 0 mm; (d) dN = -0.8 mm.

Fig. 9. (a) Orientation color map of node portion of the lattice structure, (b) the corresponding mean intercept length of high-angle boundaries (equivalent to the grain size) and (c) the area fraction of the (001)-oriented grains as functions of the distances from the centroids (dN) of node portions of the lattice structure.

Fig. 10. (a) TEM bright-field image showing the microstructure in the node portion of the lattice structure with the distance of dN = 0.8 mm from the centroid, (b) selected area electron diffraction (SAED) patterns of the circled area in (a), (c) STEM-HAADF image showing the microstructure and (d-f) the corresponding EDS element maps of Al, Si and Mg.

Fig. 11. (a) Schematic showing the corresponding locations observed by SEM, (b-d) SEM images showing the microstructures in strut portions of the lattice structure with different distances from the centroid: (b) dS = 0.2 mm; (c) dS = 0 mm; (d) dS = -0.3 mm.

Fig. 12. Mean width of columnar α-Al phases in strut portions of the lattice structure as a function of the distance from the centroid (dS).

Fig. 13. (a) Schematic showing the corresponding areas in a strut region of the lattice structure and (b-d) size distribution of Si particles in strut portions of the lattice structure with different distances from the centroid: (b) dS = 0.2 mm; (c) dS = 0 mm; (d) dS = -0.3 mm.

Fig. 14. (a) Orientation color map of the strut portion of the lattice structure, (b) the corresponding mean intercept length of high-angle boundaries (equivalent to the grain size) and (c) the area fraction of the (001)-oriented grains as functions of the distances from the centroids (dS) of the strut portions of the lattice structure.

Fig. 15. Average hardness values as functions of the distances from the centroids of (a) the node portion (dN) and (b) the strut portion (dS) of the lattice structure.

Fig. 16. Schematics of the microstructural changes in node portions of the lattice structure with different distances from the centroid: (a) dN = 0.8 mm; (b) dN = -0.8 mm. The red arrows indicate the direction of heat flow.

Fig. 17. Schematics of the microstructural changes in strut portions of the lattice structure with different distances from the centroid: (a) dS = 0.2 mm; (b) dS = -0.3 mm. The red arrows indicate the direction of the heat flow.

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