Highly ductile hypereutectic Al-Si alloys fabricated by selective laser melting

doi:10.1016/j.jmst.2021.07.050

Abstract

Abstract:

In the endeavor to maximize the refinement effect of primary Si and alleviate the inherent brittleness of hypereutectic Al-Si alloy, the approach of coating P as a modifier on powder was adopted. The ultimate aim was to create more heterogeneous fine AlP nucleus and enhance the nucleation efficiency of primary Si on AlP to refine the coarse primary Si to nano-scale during 3D printing. In the combination of large undercooling and high density of nucleation sites, the size of primary Si was successfully refined to 200-300 nm and the divorced eutectic was also induced to modify the microstructure of matrix. In the presence of nano-scale primary Si, the melting pool boundary (MPB) feature disappeared and the fracture mechanism also changed from load transfer to interfacial fracture. Compared with the pristine alloy, the ductility was increased four times without significantly changing the ultimate tensile strength (UTS) and wear resistance. The improvement of ductility is attributed to the refinement of primary Si, the disappearance of MPB features and the formation of divorced eutectic. The optimal tensile properties were: UTS-482 MPa, yield strength-320 MPa and ductility of 8.1% at 0.05 wt.% P. These are comparable to those for high-strength Al alloys.

Key words: Al-Si alloys, Heterogeneous nucleation, Ductility, Powder processing, Selective laser melting

Yafeng Yang, Kang Geng, Shaofu Li, Michael Bermingham, R.D.K. Misra. Highly ductile hypereutectic Al-Si alloys fabricated by selective laser melting[J]. J. Mater. Sci. Technol., 2022, 110: 84-95.

Figures/Tables 16

Table 1. Summary of the refinement treatment of primary Si in hypereutectic Al-Si alloys with similar compositions.

Cooling condition	Modifier addition	Size of primary Si (μm)	Refs.
Spray atomization	-	3-10	[12]
Melt-spinning	-	0.5-5	[10]
Rapid quenching	-	∼33	[13]
Cu mold casting	Ce	50-75	[14]
Steel mold casting	P	30-100	[15]
Serpentine pouring channel process	Sr	26-29	[16]

Fig. 1. SEM images of powder morphologies: (a) original AlSi20 powder; (b-d) coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%. (a’-d’) are the magnified BSE images of a single powder in (a-d). (e) Microstructure in the original AlSi20 powder and (f) XRD patterns of the original and coated powders.

Fig. 2. Optical images of the printed samples from (a) original AlSi20 powder and coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%. BD means building direction.

Fig. 3. BSE images of microstructure in the printed samples from (a) original AlSi20 powder and coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%. The melt pool boundary was abbreviated as MPB, which was pointed by yellow dotted line.

Fig. 4. The enlarged views of microstructure at the MPB in the printed samples from (a) original powder and coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%.

Fig. 5. Microstructure inside the melting pools of the printed samples from (a) original AlSi20 powder and coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%.

Table 2. The number density and area fraction of primary Si.

P content (wt.%)	Number density (/μm²)	Area fraction (%)
0	0.11	4.06
0.05	1.42	18.18
0.14	2.14	17.04
0.22	4.94	26.52

Fig. 6. The enlarged views of the marked area in the melting pool shown in Fig. 5. The samples were printed from (a) original AlSi20 powder and coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%.

Fig. 7. EBSD results of the printed sample from (a) to (c) original AlSi20 powder, (b) and (d) coated powder of P content at 0.05 wt.%. (e) and (f) are the corresponding pole figures and inverse pole figures of printed sample from original AlSi20 powder and coated powder, respectively.

Fig. 8. Tensile stress-strain curves of the printed samples from original AlSi20 powder and coated powders with different P contents. The UTS values are given in the brackets.

Table 3. The average tensile properties of the printed samples with different P-content.

P content (%)	0	0.05	0.14	0.22
UTS (MPa)	507 (± 12)	490 (± 10)	463 (± 22)	433 (± 62)
YS (MPa)	450 (± 15)	343 (± 23)	318 (± 12)	315 (± 0)
Elongation (%)	2.2 (± 0.3)	7.0 (± 1.1)	6.3 (± 0.7)	5.7 (± 3.4)

Fig. 9. Fractographs of printed tensile bars from (a) original AlSi20 powder and coated powders with different P contents at (b) 0.05 wt.%, (c) 0.14 wt.% and (d) 0.22 wt.%. (a1-d1) The low-magnification images and (a2-d2) high-magnification images of the horizontal section of tensile bars. (a3-d3) The high-magnification images of fracture surface of tensile bars.

Fig. 10. Coefficient of friction of the printed sample from pristine AlSi20 powder and coated powders.

Fig. 11. (a), (c) and (d) TEM images of primary Si in the printed sample from the coated powders with P-content at 0.05 wt.%. (b) The EDS spot analysis results of the spherical particle marked in (a).

Fig. 12. The estimated relationship between the critical nucleation size of primary Si and the degree of undercooling. The inset is the enlarged view when undercooling degree is from 0 to 100 K.

Fig. 13. TEM images of microstructure in the vicinity area of crack in the tensile failure samples: (a, b) unmodified alloy, (c, d) modified alloys with the P content at 0.05 wt.%. (b) is the high-magnification view of the primary Si marked in (a).

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