Wear Resistance Of AZ91D Magnesium Alloy & Fabrication And Properties Of Its Micro-arc Oxidation Coating

Magnesium alloys present a great potential as structural materials in the aerospace and automobile industries because of several advantages, such as low densities, high specific strength and good machinability. However, AZ91D alloy, as one of the most common commercial magnesium alloys, is suffering the challenge for meeting the requirements of strength, ductility, fatigue, creep and wear resistance.Firstly, the effects of mould temperature, reheating temperature and reheating duration on sliding wear and friction properties of thixoformed AZ91D alloy were investigated. The results indicated that the wear rate increased gradually with the increasing of the mould temperature and the heating time of the feedstock. The wear rate firstly decreased and then increased with the increasing temperature of the feedstock. The thixoforming parameters had no obvious influence on the friction coefficient.Secondly, the friction and wear properties of thixoformed (TF) and permanent mould cast (PMC) AZ91D magnesium alloys were investigated under dry reciprocating sliding conditions. The results showed that the wear resistance of the TF alloy was significantly superior to that of the PMC alloy. The wear mechanism of both alloys presented similar tendency to change from mild wear regime (oxidation wear and/or micro-machine wear) to severe wear regime (plastic deformation induced wear or/and severe plastic deformation accompanied by severe adhesive wear) with reciprocating frequency or load increased. The effect of wear mechanism on the friction coefficient was larger than that of the wear rate.Thirdly, the effects of current density and treatment time on the growth of the micro-arc oxidation (MAO) coating on the thixoformed AZ91D magnesium alloy by adopting a electrolytic solution maining including Na₂SO₃ were investigated. Surface morphology and hardness of MAO coatings were discussed under different treatment time, and mechanical properties of TF magnesium alloy with or without MAO coating were compared. The results indicated that the growth velocity of MAO coating increased faster with the current density increased, but it grew slowing after the current density reached up to 5.0A/dm². Thickness and hardness of MAO coatings were increased obviously with the treatment time increased, but its thickness increased slower and its hardness descended after the treatment time was 10min. The tensile strength and bending strength of the thixoformed AZ91D magnesium alloywith MAO coating were descended, compared with its substrate, but the impact toughness was increased.Lastly, the friction and wear behaviors of MAO coating on thixoformed (TF) AZ91D magnesium alloys were investigated under dry reciprocating sliding conditions with two different counterparts, GCr15 and Si₃N₄. The wear rate and the friction coefficient of MAO coating, when adopting the counterpart as Si₃N₄, 'were larger than that of GCrl5. With the reciprocating frequency increased, the wear mechanism of the MAO coating has presented as micr-machining wear type under GCrl5 counterpart, whereas it has changed from severe micro-machining wear to fatigue wear with micro-fracture, when adopting Si₃N₄ counterpart. However, with the loads increased, the near mechanism has shown a transformation from micro-machining wear to severe plastic deformation induced wear for both counterparts.