Study On The Effects Of Surface Pretreatment And Electronic Parameters On Micro-arc Oxidation Film On Thixoformed AZ91D Magnesium Alloy, And Chemical Plating Technique

Based on our previous investigations, the effects of surface pretreatment, voltage and adding methods of voltage on the microstructure, constitute and properties of micro-arc oxidation (MAO) film on thixoformed (TF) magnesium alloy AZ91D were studied by using MAO equipment made by Chang-An university. All of these had been compared with those of the permanent mould casting (PMC) AZ91D alloy. Simultaneously, the technique and the microstrcutre about chemical plating Ni-P alloy coating on MAO film were also investigated. The results are as followings:1. The alloy's surface properties have a significant effect on MAO process. There is a great difference in the phenomena occurred during initial stage of MAO process of the TF and PMC alloys due to their large difference in microstructure. The breakdown voltage of the TF alloy is 16V higher than that of the PMC alloy. The surface pretreatment can change the elements' concentrations on the surface, then affect the compositions and properties of the resulting MAO film. The surface pretreatment technique used in this work can increase Al concentration in the MAO film and obviously enhance its hardness.2. The anodic initial voltage has a remarkable influence on the MAO process and properties of MAO film. As the initial voltage increased, the film growth rate increased and decline magnitude of current density with the MAO time also increased. The thickness, roughness, bonding strength with the matrix and hardness of the film all also increased. Corrosion resistance of the film declined firstly, and then increased and was up to maximum. The initial voltage also affected the phase constitutes of the film. Some new phases could be formed with the voltage.3. The voltage must be constantly increased in order to keep the oxidation reaction proceeded. According to the effects of initial voltage and the role of current density on the properties of the film, a new method, increasing the voltage by several times, was invent. This method has good operability and high efficiency.4. Chemical plating Ni-P alloy coating could seal the holes in MAO film. The coating could cover any place where the solution could infiltrate in. The resulting Ni-P coating was so chemically active that it was passivated as soon as being immersed in H2SO4 acid solution. The acid corrosion resistance of MAO film was improved. In the alkaline solution prepared as sequence of main salt, complexing agents D, buffer B, buffer A, complexing agents C, reducing agent and surface active agent, a coating with low P concentration of 1.8%-4%P (Wt%) was obtained. Thiscoating was an amorphous Ni-P alloy.