Influence of secondary phases of AlSi9Cu3 alloy on the plasma electrolytic oxidation coating formation process

doi:10.1016/j.jmst.2019.12.031

Abstract

Abstract:

To understand the formation and growth of plasma electrolytic oxidation (PEO) coatings in presence of different secondary phases in a substrate, PEO treatment was carried out on AlSi9Cu3 alloy at different treatment times ranging from 15?s to 480?s. The coating formation and evolution process was traced by surface and cross-sectional observation of the layers on the different phases of the alloy. The results demonstrated a sequential involvement of the different phases in the plasma discharges: firstly, discharges start on the α-Al matrix, then on the intermetallic Al₂Cu and β-Al₅FeSi phases at the same time and finally on the eutectic Si. The presence of intermetallic Al₂Cu remarkably affects the initial dissolution, the deposition of conversion products and the ignition of discharges at the early stages of processing. Eutectic Si in the substrate exhibits the highest electrochemical stability at all stages and contributes in the beginning to a distinct coating morphology eventually. The resultant PEO coating tends to be uniform if processing times are longer and a double-layer structure appears in the coating.

Key words: Plasma electrolytic oxidation, Aluminum alloy, Secondary phases

Ting Wu, Carsten Blawert, Mikhail L.Zheludkevich. Influence of secondary phases of AlSi9Cu3 alloy on the plasma electrolytic oxidation coating formation process[J]. J. Mater. Sci. Technol., 2020, 50: 75-85.

Figures/Tables 11

Fig. 1. X-ray diffraction patterns of the specimens before (substrate) and after PEO treatment for different times.

Fig. 2. Microstructures of AlSi9Cu3 alloy with four different phases: (a, c) BSE images; (b, d) SE images.

Fig. 3. Voltage-time response during 480?s of PEO processing.

Fig. 4. Surface micro-morphologies on specimens after PEO treatment: (a) 15?s; (b) 30?s; (c) 60?s; (d) 120?s; (e) 240?s; (f) 480?s.

Fig. 5. EDS mapping results of local micro-areas on AlSi9Cu3 after different PEO treatment times: (a) 15?s; (b) 30?s; (c) 60?s.

Fig. 6. Surface morphologies and local EDS mapping results on different secondary phases after 120?s PEO treatment: (a) Al2Cu; (b) β-Al5FeSi; (c) eutectic Si.

Fig. 7. Surface evolution of PEO coating after 240?s treatment: (a) overview of PEO coating on AlSi9Cu3 with EDS mapping results; surface morphologies on (b) eutectic Si, (c) α-Al matrix and (d) the overlapping location of Al2Cu and β-Al5FeSi.

Fig. 8. Surface evolution of PEO coating after 480?s treatment: (a) overview of PEO coating on AlSi9Cu3. Surface morphologies on different phases with EDS mapping results: (b) on eutectic Si; (c) on α-Al matrix; (d) on the overlapping location of Al2Cu and β-Al5FeSi; (e) characteristic element contents at different selected areas in (a).

Fig. 9. Cross-sectional morphologies and EDS mapping results of PEO coating after different treatment time: (a) 60?s; (b) 120?s; (c) 240?s; (d) 480?s.

Fig. 10. Cross-sectional evolution of PEO coatings at 240?s and 480?s: (a, b, c) cross-sectional morphologies of PEO coatings on different secondary phases at 240?s; (d, e, f) cross-sectional morphologies of PEO coatings on different secondary phases at 480?s; (g, h) characteristic element contents on different phases at 240?s and 480?s, respectively.

Fig. 11. Schematic diagram of coating formation before and during PEO process: (a) air-formed oxide layer prior PEO processing; (b) initial deposition and accumulation of conversion products around Al2Cu intermetallics; (c) initial plasma breakdown discharges on all phases except eutectic Si; (d) non-uniform growth of PEO coating with single layer morphology; (e) uniform growth of PEO coating with inner layer and outer layer.

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