Selective Laser Melting of Carbon-Free Mar-M509 Co-Based Superalloy: Microstructure, Micro-Cracks, and Mechanical Anisotropy

doi:10.1007/s40195-021-01348-5

Abstract

Abstract:

In this work, the microstructural evolution, micro-crack formation, and mechanical anisotropy of the selective laser melted (SLM) carbon-free Mar-M509 Co-based superalloy were systematically studied under different linear energy densities (LED). Observation shows that the SLM Mar-M509 superalloy possesses a fully dense structure, whereas some micro-cracks exist along the building direction. The electron backscatter diffraction results reveal that dominant columnar grains tend to elongate along the building direction parallel to the XZ plane. Meanwhile, both a < 001 > near fiber texture and a {100} < 001 > near sheet texture are observed in different specimens. For the specimen with fiber texture, a high misorientation angle exists among different columnar grains, which aggravated the generation of micro-cracks under thermal stress. Higher LED results in higher micro-crack density in the SLM specimen due to higher thermal stress. Mar-M509 specimen fabricated under lower LED exhibits higher tensile strength due to more significant grain refinement. More prominent anisotropy of tensile performance was found in the high LED specimen, which can be attributed to the higher density of micro-cracks and crystallographic texture. Furthermore, the SLM Mar-M509 superalloy exhibits better mechanical properties than the traditional cast technique. In summary, this work can contribute to the development and the future application of SLM-fabricated Co-based superalloy.

Key words: Selective laser melting, Mar-M509 Co-based superalloy, Linear energy density, Anisotropy, Mechanical properties

Xiaodong Wang, Chaoyue Chen, Ruixin Zhao, Longtao Liu, Sansan Shuai, Tao Hu, Jiang Wang, Zhongming Ren. Selective Laser Melting of Carbon-Free Mar-M509 Co-Based Superalloy: Microstructure, Micro-Cracks, and Mechanical Anisotropy[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(3): 501-516.

Figures/Tables 19

Table 1 Chemical composition of carbon-free Mar-M509 powder (wt%)

Co	Cr	Ni	W	Ta	Zr	Ti	Al
Bal.	23.09	9.04	6.75	3.02	0.26	0.18	0.15

Fig. 1 a Surface morphology; b size distribution of the gas-atomized Mar-M509 superalloy powder

Table 2 Annotation of SLM specimens with the corresponding processing parameters and linear energy density (LED)

Annotation	Processing parameters		Linear energy density (J/m)
Annotation	Power (W)	Scanning speed (mm/s)	Linear energy density (J/m)
#L	252	2400	105
#H	252	1700	148

Fig. 2 a Schematic diagram of the scanning strategy; b location of extracted specimens for mechanical tests and microscopy

Fig. 3 a Cross-sectional microstructure of the gas-atomized Mar-M509 powder; b XRD spectrums of the powder and original master-alloy

Fig. 4 3D SEM images showing the overview observation of SLM-fabricated Mar-M509 with different LEDs, with the red arrows indicating the micro-cracks: a specimen #L with an LED value of 105 J/m; b specimen #H with an LED value of 148 J/m

Table 3 Density of micro-cracks in SLM Mar-M509 specimens under different LEDs

Annotation	Crack density (mm/mm²)
#L (105 J/m)	0.97
#H (148 J/m)	2.04

Fig. 5 a XRD spectra of SLM-fabricated Mar-M509 specimens; b enlarge of red dotted portion in a

Fig. 6 Phase map of SLM Mar-M509 specimen fabricated under different LED values: a specimen #L with an LED value of 105 J/m; b specimen #H with an LED value of 148 J/m, the red area represents γ phase, while the blue area represents ε phase

Fig. 7 SEM images of SLM-fabricated specimens which are enlarged of the areas in the red dotted boxes in Fig. 4: a specimen #L on the XY plane; b specimen #H on the XY plane; c specimen #L on the XZ plane; d specimen #H on the XZ plane

Fig. 8 EPMA mappings of specimen #H and the element content distributions

Fig. 9 Inverse pole figure (IPF) maps (IPF color map along the Y-axis) of SLM-fabricated Mar-M509 specimens obtained by EBSD on different planes: a specimen #L with an LED value of 105 J/m; b specimen #H with an LED value of 148 J/m

Table 4 Average grain size of the SLM Mar-M509 superalloy under different LED conditions

Annotation	Average grain size (μm)
#L (105 J/m)	23.47
#H (148 J/m)	35.47

Fig. 10 Pole figures (PF) of SLM-fabricated Mar-M509 specimens showing the evolution of texture: a XY plane; b XZ plane

Fig. 11 Stress-strain curves of SLM-fabricated Mar-M509 specimens on different building planes

Fig. 12 Mechanical property comparisons of SLM Mar-M509 specimens on different planes: a hardness; b yield strength; c ultimate tensile strength; d elongation to fracture. The dotted line indicates the mechanical properties of cast Mar-M509 superalloy from ASM Specialty Handbook

Fig. 13 Fracture surface of specimens: a specimen #L on the XY plane; b specimen #H on the XY plane; c specimen #L on the XZ plane; d specimen #H on the XZ plane

Fig. 14 Characterization of the micro-cracks in specimen #H: a SEM image, b EBSD micrograph, c, d misorientation variation across each micro-crack indicated in b

Fig. 15 Distribution of micro-cracks near the tensile fracture of specimen #H and the diagram of influence of micro-cracks in the tensile tests: a loading direction (LD) perpendicular to the building direction; b loading direction parallel to the building direction

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