Magnesium Alloy Micro-arc Oxidation Process And Ceramic Coatings

Micro-arc oxidation(MAO) is new type of surface treating technology that occurs at Al, Mg, Ti and their alloys, grows oxidation coating on original position of metal surface which can improve wear resistant, corrosion resistant and bonging strength.Therefore, MAO is one of the most prospective method of surface treatment for magnesium alloys. A gray oxidation ceramic film was prepaped by micro-arc oxidation on Mg-4Gd-3Ymm-0.5Zr magnesium alloy in the paper.The coating surface has many pores in the center of volcano-like heap, the microarc oxidation treatment obviously improved the corrosive resistance of Mg alloy. The coating surfaces and cross-sections morphology were characterized by scanning electron microscopy, phase composition and microstructure are studied by XRD and EDS analysis. The results show that there are three layers in oxidation coating, of which inner layer is dense, outer layer is loose and porous, transition layer between compact layer and porous layer in structure. The coating is mainly composed of MgO.The effect of different technical parameters on the microstructure and corrosion resistance of microarc oxidation film of Mg-4Gd-3Ymm-0.5Zr alloy treated by micro-arc technique was investigated by SEM, XRD, coating thickness gauge and electrochemical workstation etc. By one factor experiment in silicate system, thickness and properties of prepared coating of different electrolyte (include Na₂SiO₃, KOH, NaF and glycerol)content were studied.The results indicated coating thickness,hardness and corrosion resistance show certain orderwith electrolyte content. When Na₂SiO₃ 6g/L, KOH 2g/L, NaF2g/L, glycerol 10ml/L, coating quality was optimize.Then on the foundation of silicate system, the effect of positive&negative pulse voltage, pulse frequency, duty cycle, treatment time on oxidation film thickness, morpholog and properties were studied.The experiment results show the coating thickness increased, fewer pores with larger diameters with the prolonging of MAO process, film thickness growth slowly and rougher. Hard- ness and corrosion resistance of the film increases first and thereafter decreases as oxidation time prolonging.The positive&negative pulse voltage greatly influences the microstructure and properties of micro-arc oxidation film in micro-arc oxidation for magnesium alloy. With the increasing of pulse positive, the film thickness increased, fewer pores with larger diameters, more strength sintering and larger crack were found on the film. corrosion resistance of the film increases first and thereafter decreases as positive pulse raise, The optimal positive pulse voltage was 450 V.The film thickness firstly increases and then decreases a little followed by a vital increasing when the negative pulse voltage varies from zero to 100V. The pores diameter of microarc oxidation film surface change from small to large and then small again, whereas the pore amount goes in the opposite way, and the best corrosion resistance of the film was achieved when negative pulse voltage was 40V.The duty cycle and frequency have little effect on growth thickness. More pores with smaller diameters and rougher surface were found on the film when the frequency increased while opposite changes occurred with the increase of duty cycle. Corrosion resistance of the film increases first and thereafter decreases as frequency and duty cycle raise gradually and the optimal conditions were 600Hz and 10%.In this paper through a lot of tests, studies about composition ,positive&negative pulse voltage,frequency, duty cycle and oxidation time on the Mg-4Gd-3Ymm-0.5Zr Mg alloy ceramic coating. The result of experiment shows , best technical parameters is : 6g/L Na₂SiO₃, 2g/L KOH, 2g/L NaF, 10ml/L glycerol, positive&negative pulse voltage 450&40V, frequency 600 Hz, duty cycle 10 %, oxidation time 30min.