Study On Influence Of Voltage Rise Rate And Voltage Differentiation During Micro-arc Oxidation Process Of Aluminium And Magnesium Alloys

In this dissertation, 6061 aluminum alloy and AZ31 magnesium alloy were micro-arc oxidation(MAO) treated using a homemade WH-1A type MAO power supply unit. During the MAO process, a voltage rise stage was introduced before the potentiostatic stage by means of the convenient voltage operation of the MAO power. Effects of voltage rise rate on some experimental phenomena in the MAO process of 6061 aluminum alloy and the characteristics of the MAO coatings were investigated, such as the effects of voltage rise rate on the voltage, current, microdischarges behaviors in the MAO process and the morphology, phase composition, element distribution and the electrochemical impedance of the MAO coatings. When we investigate the effects of voltage rise rate a novel phenomenon was found, that is, an obvious voltage difference is present between the practically applied voltage and the simulatedly applied voltage during the MAO process. In order to investigate the universality of the novel phenomenon, both 6061 aluminum alloy and AZ31 magnesium alloy were used as the substrate, respectively, and meanwhile the relationship between the growth feature of the MAO coatings and the voltage differentiation phenomenon was investigated by adjusting the concentration of sodium hexametaphosphate in the MAO electrolyte.The results show that the voltage rise stage before the potentiostatic mode would produce a relatively safe current density. The safe current density can protect the power source and the primeval MAO coatings; The maximum current was increased with the voltage rise rate, and there would be some melt particles first occur on the surface of the MAO coating under higher voltage rise rate; Higher voltage rise rate made the outer layer looser and would process many performative pores and these pores greatly reduced the corrosion resistance of the MAO coatings; Cause the higher voltage rise process more pores, the elements of the substrate largely turn into the electrolyte, at the same time, elements in the electrolyte would get into the MAO coatings,this made the aluminum element in MAO coatings decreasing with the voltage rise rate and the contents of O, P, Si elements in MAO coating was on the rise. Due to the temperature of the MAO process under constant voltage rise-potentiostatic mode could not get the phase inversion temperature of γ-Al2O3 to α-Al2O3, thus there was no phase of α-Al2O3; Higher voltage rise rate promoted the MAO process but the corrosion resistance and the compactness were not promoted; The difference between the input voltage and the output voltage was generally exist in the MAO process, this phenomenon was not limited by the conditions of the substrate and had nothing to do with the microstructure of the ceramic but was closely related to the growth of the MAO coatings. When the ceramic coatings could not coated on the substrate or the thickness of the MAO coatings could not increasing by much more repetition times of MAO processes, the output voltage would synchronize to the input voltage; When the growth situation of the MAO coatings were changed by the additives of sodium hexametaphosphate, the output voltage which corresponding to the faster MAO process would be restrained little, at the same time, the output voltage of the faster MAO process would surpass more than the input voltage during the potentiostatic mode; When the coatings in the earlier stage of the MAO process were normal but then there could not observe the discharge phenomenon, the output voltage would not surpassed the input voltage until the end of the MAO process; When the coatings in the earlier stage of the MAO process were normal but then there was ablation phenomenon occur, the growth situation of the ceramic coatings would radically change and by this time the growth rate of the coatings could not catch the damage rate, thus the output voltage would surpass little than the input voltage; When the MAO process took place at the whole surface of the sample, the output voltage would surpass more than the input voltage but when the MAO process took place at local area of the sample the output voltage would surpass little than the input voltage, because of the discharge process in the local area of the surface was similar to the ablation phenomenon. Between the two processes above there would be a sudden drop in the output voltage.