Korean Journal of Materials Research (한국재료학회지)

Materials Research Society of Korea (한국재료학회)
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Effect of Plasma Electrolytic Oxidation Conditions on Oxide Coatings Properties of Die-Cast AZ91D Mg Alloy

플라즈마 전해 산화 처리조건에 따른 다이캐스트 AZ91D Mg 합금 위에 제조된 산화피막 특성

  • Park, Seong-Jun (Dept. of Materials Engineering, Graduate School of PaiChai University) ;
  • Lim, Dae-Young (Dept. of Materials Science and Engineering, PaiChai University) ;
  • Song, Jeong-Hwan (Dept. of Materials Science and Engineering, PaiChai University)
  • 박성준 (배재대학교 대학원 재료공학과) ;
  • 임대영 (배재대학교 신소재공학과) ;
  • 송정환 (배재대학교 신소재공학과)
  • Received : 2019.07.22
  • Accepted : 2019.09.10
  • Published : 2019.10.27
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Abstract

Oxide coatings are formed on die-cast AZ91D Mg alloy through an environmentally friendly plasma electrolytic oxidation(PEO) process using an electrolytic solution of $NaAlO_2$, KOH, and KF. The effects of PEO condition with different duty cycles (10 %, 20 %, and 40 %) and frequencies(500 Hz, 1,000 Hz, and 2,000 Hz) on the crystal phase, composition, microstructure, and micro-hardness properties of the oxide coatings are investigated. The oxide coatings on die-cast AZ91D Mg alloy mainly consist of MgO and $MgAl_2O_4$ phases. The proportion of each crystalline phase depends on various electrical parameters, such as duty cycle and frequency. The surfaces of oxide coatings exhibit as craters of pancake-shaped oxide melting and solidification particles. The pore size and surface roughness of the oxide coating increase considerably with increase in the number of duty cycles, while the densification and thickness of oxide coatings increase progressively. Differences in the growth mechanism may be attributed to differences in oxide growth during PEO treatment that occur because the applied operating voltage is insufficient to reach breakdown voltage at higher frequencies. PEO treatment also results in the oxide coating having strong adhesion properties on the Mg alloy. The micro-hardness at the cross-section of oxide coatings is much higher not only compared to that on the surface but also compared to that of the conventional anodizing oxide coatings. The oxide coatings are found to improve the micro-hardness with the increase in the number of duty cycles, which suggests that various electrical parameters, such as duty cycle and frequency, are among the key factors controlling the structural and physical properties of the oxide coating.

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References

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