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Formation of Plasma Electrolytic Oxidation Coating on Aa5052 And Az31 Alloys Joint with Friction Stir Welding

Safari, Saeed | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51846 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ghorbani, Mohammad
  7. Abstract:
  8. Plasma Electrolytic oxidation has recently been known as a new technology in surface engineering. It is one of the low-cost techniques used for improving corrosion and wear resistance in Al-Mg alloys. The purpose of this study first is to attach aluminum and magnesium alloys safely and without any defects, and then finding the optimum coating condition in connection line to achieve most possible corrosion resistance. For this purpose, first of all optimization of Al-Mg alloys friction stir welding (FSW) condition is required. After required investigations, it was shown that the weld created between Aluminum and Magnesium sheets by the cylindrical pin moving in a rotational speed of 550 rpm and traverse speed of 20 mm/min, while Aluminum sheet is forehead, has the best possible quality. But it also shows that any changes in FSW’s parameters in a way which leads to an increase in generated heat, can create metal compound such as Al3Mg2 and Al12Mg17. These compounds may act as a micro-cathode in weld zone and so increase corrosion rate. This increased heat can raise corrosion rate in the sample which has been welded in optimized condition from 2.48 ×a10-3A/cm2 to 6.34×a10-3 A/cm2 in not optimized sample. In this regards, after optimization of FSW parameters, effect of coating process variables such as electrolyte type, electrolyte containment concentration, current density and coating time on coat properties are investigated. Fore this purpose, welded samples will be coated in two silica base and alumina base electrolytes in different concentrations and current densities after surface preparation. After required investigations, it is was found that applied PEO coat on Al-Mg couple can decrease corrosion rate considerably, as applying Plasma Electrolytic coat in silicate electrolyte under optimized condition (sodium silicate concentration of 10 g/lit and current density of 350 mA/cm2) can decrease corrosion rate of weld zone from 2.84×a10-3 A/cm2 in uncoated sample to 6.62×a10-5 A/cm2 in Plasma Electrolytic coated sample. In other words, corrosion rate in weld zone through PEO coating process has been reduced more than 40 times. The main reason for this corrosion rate cut-down is the production of a robust oxidation coat consisting phases like MgSiO4, MgO on magnesium part and Al2O3 on Aluminum part of the sample. Finally, it is observed that the quality of PEO coat produced in aluminate electrolyte is much higher than the PEO plasma coated in silicate electrolyte whereas plasma electrolyte coat produced in aluminate electrolyte under optimized condition (sodium Aluminate concentration of 10 g/lit and current density of a450 mA/cm2) can decrease corrosion rate of weld zone from 2.84×a10-3 A/cm2 in uncoated sample to 1.75×a10-5 A/cm2 in PEO coated sample. In other terms, corrosion rate of weld zone has declined through PEO coating process. The main reason for this corrosion rate cut-down is the production of a robust oxidation coat consisting phases like MgAl2O4, MgO on magnesium part and Al2O3 on Aluminum part of the sample. At the end it is claimed that the produced oxide coat by PEO method has high corrosion resistance and can highly protect surface in a corrosive environment
  9. Keywords:
  10. Plasma Electrolytic Oxidation ; Friction Stir Welding ; Aluminum Alloy 5052 ; AZ31 Magnesium Alloy

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