Selective Laser Melting of an Al-Fe-V-Si Alloy: Microstructural Evolution and Thermal Stability

doi:10.1016/j.jmst.2016.09.015

Abstract

Abstract:

Selective laser melting was used to produce an aluminum alloy Al-8.5Fe-1.3V-1.7Si (wt%). The effects of heat treatment on microstructure evolution and phase stability during long-term thermal exposure of the deposits were investigated. Results show that the microquasi-crystalline phase, Al₁₂(Fe,V)₃Si and Al_mFe metastable phases coexisted with α-Al in the as-produced alloy. Annealing at 400 °C resulted in decomposition of microquasi-crystalline phase and supersaturated α-Al into Al₁₂(Fe, V)₃Si phase in the fusion zone, accompanied by the decrease in alloy hardness. The activation energy of this decomposition process was 115 kJ/mol. A more homogenous microstructure was obtained after annealing at 400 °C for 60 min, which was resistant to coarsening exposed at 425 °C up to 500 h. The Al₁₂(Fe,V)₃Si and Al_mFe phases were coarsened at 475 and 525 °C with increasing the exposure time. Coarsening of Al₁₂(Fe,V)₃Si phase was attributed to a combination of volume diffusion and grain boundary diffusion mechanism of Fe. Heat treatment at 600 °C resulted in accelerated microstructure coarsening and formation of large-sized equilibrium phases, which significantly degraded the room temperature microhardness.

Key words: Selective laser melting, Al-Fe-V-Si alloy, Microstructural development, Thermal stability, Microhardness

Sun Shao-Bo, Zheng Li-Jing, Liu Jin-Hui, Zhang Hu. Selective Laser Melting of an Al-Fe-V-Si Alloy: Microstructural Evolution and Thermal Stability[J]. J. Mater. Sci. Technol., 2017, 33(4): 389-396.

Figures/Tables 15

Fig. 1. (a) SEM image of the as-built sample, (b) a magnified view at location A showing the detail of two adjacent deposited layers, (c) TEM micrographs showing three different zones across the sample section, (d) TEM image of the FZ and diffraction pattern taken from the nanocellular region.

Table 1 Chemical compositions of α-Al cells in FZ of SLM built sample and spherical dispersoids precipitating at 400 °C for 60 min (in at%)

Phase	Al	Fe	V	Si
α-Al cells	96.4	2.3	0.7	0.6
Spherical dispersoids	83.3	9.2	1.5	6.0

Table 2 The d-spacing (nm) measured from SAD ring patterns obtained from the intercellular region in FZ. The values are compared with earlier reported values of microquasi-crystalline phase in different alloy systems

Ring	Current SLM alloy	Al-Fe^[27]	Al-Fe-V^[18]	Al-Fe-V-Si ^[17]
No.1	0.384	0.386	0.382	0.374
No.2	0.214	0.214	0.217	0.213
No.3	0.203	0.202	0.206	0.203
No.4	0.145	0.144	0.145	0.147
No.5	0.124	0.125	0.126	0.125
No.6	0.107	0.107	0.108	0.107

Fig. 2. (a) HRTEM image shows matrix/intercellular phase interface and (b) EDS spectra of the microquasi-crystalline phase.

Fig. 3. DSC curve of the as-built Al-Fe-V-Si alloy.

Fig. 4. Evolution of microstructure in the FZ during annealing at (a) 350 °C for 30 min, (b) 400 °C for 30 min and (c) 400 °C for 60 min.

Fig. 5. (a) XRD patterns of and (b) lattice parameters of as-built and annealed Al-Fe-V-Si samples, (c) TEM image showing the overall microstructure after annealing at 400 °C for 60 min.

Fig. 6. The changes of room temperature microhardness annealed at 350 and 400 °C.

Fig. 7. TEM images showing the morphologies of (a, b, c) Al12(Fe,V)3Si and (d, e, f) AlmFe after thermal exposure at (a, d) 425 °C for 500 h, (b, e) 475 °C and (c, f) 525 °C for 200 h.

Table 3 Average sizes of Al12(Fe,V)3Si and AlmFe particles after thermal exposure (in nm)

Time (h)	Al₁₂(Fe,V)₃Si			Al_mFe
Time (h)	425 °C	475 °C	525 °C	425 °C	475 °C	525 °C
0	80 ± 22	80 ± 22	80 ± 22	286 ± 60	286 ± 60	286 ± 60
50	82 ± 20	86 ± 25	108 ± 28	290 ± 58	294 ± 64	320 ± 68
100	82 ± 24	92 ± 30	134 ± 34	290 ± 51	315 ± 68	350 ± 72
200	83.4 ± 26	100 ± 34	156 ± 39	293 ± 62	320 ± 73	367 ± 78
500	84 ± 22	—	—	298 ± 68	—	—

Fig. 8. The representative microstructure of the samples exposed at 600 °C for (a, b) 10 h, and (c, d) 100 h.

Table 4 Chemical compositions of Al8Fe2Si and Al13(Fe,Si)4 phases precipitating at 600 °C for 100 h (in wt.%)

Phase	Al	Fe	V	Si
Al₁₃(Fe,Si)₄	69.9	25.3	—	4.8
Al₈Fe₂Si	68.6	24.6	—	6.8

Fig. 9. Variation of the microhardness of SLM alloy with increasing annealing time. The annealing temperature was fixed to 425, 475, 525 and 600 °C.

Fig. 10. (a) The DSC profiles of as-built samples at heating rates of 5, 10, 20 and 40 K/min, and (b) Kissinger plot from different DSC scans. R2 = 0.991, is the correlation coefficient of the least squares method.

Fig. 11. (a) A plot of Al12(Fe,V)3Si average radius versus exposure time at different temperatures, and (b) the relationship between temperature (T) and the coarsening rate (K).

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