An Investigation Of MAO Coating Forming Mechanisms Coated Ti–6Al–4V Alloy

In this paper,the Micro arc oxidation of Ti-6Al-4V alloy in 8g/L Na₂SiO₃·9H₂0,6g/L?NaPO₃?₆,4g/L Na₂WO₄·2H₂O,3g/L Na5P3O10 electrolyte was researched by WD-20 MAO power supply with DC pulse.Current density was 10A/dm2,frequency was 500 HZ,duty cycle was 50%.The temperature of molten pool reaction was tested by the phase transition and decomposition of second phase particles?SiC,gamma-Al₂O₃,kappa-Al₂O₃,and the single crystal of the zirconium oxide?;and the discharge channel was found by the distribution of the second phase particles;the corresponding relationship between the plasma spark and the molten pool was studied first time by long distance microscope;and the coatings formation mechanism and the model were studied.The results show that the oxidation voltage is the driving force for the growth of the MAO coatings.The voltage growth rate over time in first ten minutes is obviously higher than 10-120 min.Ten minutes before the oxidation,the voltage change with time is S Logistic type exponential,ten minutes after the voltage is linear growth of low slope,equation were:?1?V=541.2749-571.9594/?1+t/1.49761?^1.10423?0<t<10min??2?V=486.83797+0.2615t?10?t<120min?Thickness and roughness of coating increase with the increase of termination voltage.The size of the molten pool follows the quadratic polynomial as the coating thickness increases,the equation is y=-114.99725+7.85254x-0.06728x2,where x is the coating thickness,y is the molten pool size.During TC4 titanium alloy MAO process,there are one temperature range about the molten pool temperature distribution: the minimum temperature is less than 1223 K,the maximum temperature is greater than 3143 K.This can be determined by the phase transition of ?-Al₂O₃,?-Al₂O₃,m-ZrO₂,the phase transition and decomposition of ?-SiC.Sparks and molten pools are one-to-one correspondence during oxidation process.With the progress of micro-arc oxidation,sparks break through the coating to produce molten pool,park and molten pool size changes are in the same pattern,they are all constanly increasing,and their size is quite,but the size of the molten pool is slightly smaller than the size of the spark.With the increase of oxidation time,the density of sparks is decreasing and the duration of the spark continues to increase.During voltage rapid growth stage,voltage transition stage and voltage stable growth stage,spark size is 9.9?m,23.8925?m,108.46?m respectively,the diameter of the molten pool is 12.645?m,28.84?m,112.56?m respectively,the spark density is 132/cm²?18/cm²?4/cm² respectively,the sparks duration time is 0.07 s,0.55 s,3.315 s respectively.During MAO process,the interface between the coating and the substrate is joined by canine-tooth staggering.At the interface between the coating and the substrate,the existence of the discharge channel can be clearly observed,the size is about 0.6nm.Breakdown first occurs on the weak point of the coating by the interlocking way,when the spark breaks through the coating,sparks discharge directly along the discharge channel to the substrate so that the matrix melt and oxide volcanically erupt,flow along the discharge channel,and solidificate under on the effect of electrolyte cooling,so that the old and new pool constantly reorganize and refurbish,the coating grows through the way of reorganization.There are three types of discharge,which occur on the surface of the coating,the middle of the coating,the interface between the coating and the substrate.MAO coating formation model are figure 5-8e.