Influence of in-flight particle characteristics and substrate temperature on the formation mechanisms of hypereutectic Al-Si-Cu coatings prepared by supersonic atmospheric plasma spraying

doi:10.1016/j.jmst.2021.01.048

Abstract

Abstract:

Hypereutectic Al-Si-Cu coatings were prepared by supersonic atmospheric plasma spraying to enhance the surface performance of lightweight alloys. To find out optimum process conditions and achieve desirable coatings, this work focuses on the influence of three important parameters (in-flight particle temperature, impact velocity, and substrate temperature) on the collected splats morphology, coatings microstructure and microhardness. Results show that appropriate combinations of temperature and velocity of in-flight particles cannot only completely melt hypereutectic Al-Si-Cu particles, especially the primary Si phase, but also provide the particles with sufficient kinetic energy. Thus, the optimized coating consists of 98.6 % of fully-melted region with nanosized coupled eutectic and 0.9 % of porosity. Increasing the substrate deposition temperature promotes the transition from inhomogeneous banded microstructure to homogeneous equiaxed microstructure with a lower porosity level. The observations are further interpreted by a newly developed phase-change heat transfer model on quantitatively revealing the solidification and remelting behaviors of several splats deposited on substrate. Besides, phase evolutions including the formation of supersaturated α-Al matrix solid solution, growth of Si and Al₂Cu phases at different process conditions are elaborated. An ideal microstructure (low fractions of unmelted/partially-melted regions and defects) together with solid solution, grain refinement, and second phase strengthening effects contributes to the enhanced microhardness of coating. This integrated study not only provides a framework for optimizing Al-Si based coatings via thermal spraying but also gives valuable insights into the formation mechanisms of this class of coating materials.

Key words: Thermal sprayed coatings, Al-Si alloys, In-flight particle characteristics, Substrate temperature, Process control

Peng-fei He, Guo-zheng Ma, Hai-dou Wang, Ling Tang, Ming Liu, Yu Bai, Yu Wang, Jian-jiang Tang, Dong-yu He, Hai-chao Zhao, Tian-yang Yu. Influence of in-flight particle characteristics and substrate temperature on the formation mechanisms of hypereutectic Al-Si-Cu coatings prepared by supersonic atmospheric plasma spraying[J]. J. Mater. Sci. Technol., 2021, 87: 216-233.

Figures/Tables 22

Table 1 Process parameters for depositing four representative hypereutectic Al-Si-Cu coatings.

Coating ID at Parameter ID	Ar (splm)	H₂ (splm)	Total gas flow rate (Ar+H₂, splm)	H₂ percentage (%)	U (V)	I (A)	Substrate temperature (℃)	Spraying distance (mm)
Coat.-1 at Par.-1	145	19.3	164.3	11.75	150	450	80 ± 20	115
Coat.-2 at Par.-2	100	25.5	125.5	20.32	150	450	80 ± 20	115
Coat.-3 at Par.-3	65	30.7	95.7	32.08	150	450	80 ± 20	115
Coat.-4 at Par.-4	100	25.5	125.5	20.32	150	450	230 ± 30	115

Fig. 1. (a) OM image and (b) corresponding SEM images of the cross-sectional of Al-25Si-4Cu-0.9 Mg powder after Keller’s etching for 15 s. (c) Enlarged view of the rectangle in (b). (d) Size distribution of powder. (e) Size distribution of Si phase (including primary Si and short-fibrous eutectic Si) in the powder. (f) EDS spectrum of the spot in (c).

Table 2 The measured particles surface temperature, velocity, and M.I. at three difference process conditions.

Parameter ID	Surface temperature (℃)	Velocity (m/s)	Estimated M.I.	Description^(b)
Par.-1	2112.87 ± 30.21	626.61 ± 14.90	2.16*W	L-T & H-V
Par.-2^(a)	2479.58 ± 12.16	482.35 ± 33.55	3.56*W	M-T & M-V
Par.-3	2586.45 ± 10.98	370.50 ± 30.82	4.93*W	H-T & L-V

Fig. 2. OM images of the cross-sectional of the four typical coatings: (a1) Coat.-1, (a2) enlarged view of rectangle in (a1), (a3) etched morphology of (a2); (b1) Coat.-2; (b2) enlarged view of rectangle in (b1); (b3) etched morphology of (b2); (c1) Coat.-3; (c2) enlarged view of rectangle in (c1); (c3) etched morphology of (c2); (d1) Coat.-4; (d2) enlarged view of rectangle in (d1); (d3) etched morphology of (d2). Note that green and yellow arrows in (a2-a3) point to eutectic Al/Si and boundaries of UM particles, respectively; black arrows in (b2, b3) and (c, c3) indicate that few UM primary Si still exist in Coat.-2 and Coat.-3, respectively.

Fig. 3. SEM micrographs of three typical regions: (a1) UM particle in the coating showing rod-like eutectic Si and coarse primary Si; (a2) PM region showing sub-μm Si phase; (b1) FM region showing featureless (FL) zone; (b2) enlarged view of the rectangle in (b1) showing ultrafine (UF) zone; (c1) Overheated region; (c2) enlarged view of rectangle in (c1). Note that (a2), (b2), and (c2) were captured at the same magnification to make a visualized comparison of Si phase with different morphologies.

Fig. 4. Size distributions of Si phase in the four typical coatings. Considering that the polygonal primary Si phase in the UM region from Coat.-1 is similar to that in feedstock, and the fractions of PM regions from Coat.-2, Coat.-3 and Coat.-4 are no more than 1.0 %, the Si phase size in the above mentioned regions is not plotted in the figure. And the corresponding values in the four coatings are 1.90 ± 0.82 μm, 1.41 ± 0.54 μm, 1.53 ± 0.52 μm, and 1.67 ± 0.49 μm, respectively.

Fig. 5. OM images of the four typical coatings after Keller’s etching and the resulting color coded overlay image for the identifications of FM region, UM/PM Si phase, and pores: (a) Coat.-1; (b) Coat.-2; (c) Coat.-3; (d) Coat.-4.

Fig. 6. (a) Cross-sectional OM image of original powder shown here as a reference. Overall morphology of splats collected at (b) Par.-1 and (c) Par.-2. Schematic formation mechanism of (d) Splat-A, (e) Splat-B, and (f) Splat-C.

Fig. 7. OM image, corresponding 3D morphology, and cross-sectional profile along the center line of (a) Splat-A; (b) Splat-B; and (c) Splat-C. (d) SEM image containing Splat-B and Splat-C, (e) EDS mapping results of the square on the surface of Splat-C in (d), and (f) corresponding EDS results of spot 1-3 in.(d).

Fig. 8. Schematic of in-flight droplets with four types of melting states when exposed to the supersonic plasma jet at (a) Par.-1 and (b) Par.-2.

Fig. 9. Schematic illustration of the mathematical modeling geometry of three droplets deposition on a substrate with a moving boundary condition.

Table 3 Physical properties and other parameters used in the simulation calculations.

Parameter symbol	Parameter name	Unit	Parameter value
ρ_sub	Density of substrate	kg/m³	2620
ρ_splat	Density of splat	kg/m³	2540
k_sub	Thermal conductivity of substrate	W/(m K)	160
k_splat	Thermal conductivity of splat	W/(m K)	125
C_{p_sub}	Thermal capacity of substrate	J/(kg K)	1200
C_{p_splat}	Thermal capacity of splat	J/(kg K)	1100
L_sub	latent heat of substrate	J/kg	499,200
L_splat	latent heat of splat	J/kg	644,300
T_{m_sub}	Melting point of substrate	K	853
T_{solidus_spla}_t	Solidus temperature of splat	K	853
T_{liquidus_splat}	Liquidus temperature of splat	K	1099
h_{ref_sub}	Reference enthalpy of substrate at its reference temperature	J/kg	0
T_{ref_sub}		K	853
h_{ref_splat}	Reference enthalpy of splat at its reference temperature	J/kg	0
T_{ref_splat}	Reference enthalpy of splat at its reference temperature	K	853
th_sub	Thickness of substrate in the computational domain	m	6 × 10^-6
th_splat	Thickness of each splat in the computational domain	m	0.6 × 10^-6

Fig. 10. Temperature (T), liquid fraction (β), and enthalpy (H) distributions of part of the substrate and three splats arriving one after the other at a constant time interval at (a) Par.-1, (b) Par.-2, (c) Par.-3, and (d) Par.-4.

Fig. 11. Time evolutions of the average (a) temperature (T), (b) liquid fraction (β), and (c) enthalpy (H) of Splat-1 at different process conditions.

Fig. 12. (a) XRD patterns of feedstock powder and the four coatings; enlarged pattern near peak of (b) Al (311) from 77.5° to 79.5° and (c) Si (111) from 26° to 31°. The insert two tables are the corresponding peaks information of Al (311) and (c) Si (111), respectively.

Fig. 13. α-Al lattice parameters (determined by Al (311) reflections) and corresponding Si solubilities of Al matrix from feedstock powder and coatings at different processing parameter.

Fig. 14. Williamson-Hall plots exhibiting peaks broadening as function of Bragg angle for the samples (The four most intensive reflection peaks of Al were utilized in the line broadening analysis).

Fig. 15. (a) Typical bright-field TEM image of the overheated Coat.-4. (b) SADP taken from the red-circle area in (a) confirming the presence of Al2Cu phase, the insert is the tetragonal crystal structure of Al2Cu phase; (c) corresponding HRTEM lattice image of Al2Cu phase, the inserts are the FFT and IFFT (after auto-correlation) images of the box area in (c). (d) Enlarged view of (a); (e) HRTEM image of the multiple twinned Si growing along <112 > Si, the insert is the IFFT image of the red square; (f) FFT image of the square in (e) and simulated diffraction patterns showing twins in Si.

Fig. 16. Representative HAADF-STEM image and corresponding EDX-mapping of the constituent elements Al, Si, and Cu for (a) Coat.-4 and (b) Coat.-2.

Fig. 17. (a) Enlarged view of the white square in Fig. 17(a), and corresponding EDX-mapping of the constituent elements (b) Si, (c) Al, and (d) Cu. (e) HRTEM image of the white square in (a) showing precipitated nano-Al2Cu phase, the insert is FFT image of the red square in (e). (f) HRTEM image of the white-dashed square in (a), the insert is FFT image of the red square in (f) showing precipitation free Al matrix. (The supersaturated solid solution is abbreviated to SSSS).

Fig. 18. (a) Vickers hardness of the four coatings (Note that the error bar is plotted based on the standard deviation of the statistical results of microhardness for each coating). Representative OM image and corresponding 3D morphology of the 25g-load indentation at (b) FM region in Coat.-2/Coat.-3; (c) Pore-rich region in Coat.-3; (d) UM + PM regions in Coat.-1; (e) PM + FM regions in Coat.-1; (f) homogeneous region in Coat.-4. The dashed line in the respective OM image help to identify the indentation. Scale bars, 6.7 μm.

Fig. 19. Radar map of relationship between microstructural characteristics and microhardness of the four coatings.

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