The Mutual Effects Between Heat Treatment & Microarc Oxidation Of AZ91D Magnesium Alloy With Permanent Mold Cast

Combining the heat treatment process and microarc oxidation(MAO) process, this paper mainly studied the mutual effects between heat treatment and microarc oxidation of AZ91D magnesium alloy by permanent mold castFor permanent mold casting(PMC) AZ91D magnesium alloy,β-phase dissolved in the matrix after treated 24h under 415℃, and then the single phase formed. This solid-solution treatment brings the lower hardness, higher Ultimate Tensile Strength(UTS), and higher elongation to the material. After aging 16h for 200℃, theβ-phase precipitate again. It brings the material higher hardness, higher UTS, but a little shorter elongation. Partial solid solution is a kind of incomplete process of traditional heat treatment process. Its properties are between state untreated and state treated by traditional process. When the material properties required not very rigorous, the partial solid solution can low the energy spending and short the cycle, so it lows the cost and increases the efficiency. When partial solid solute samples 2h between 380℃to 395℃, there will be a good outcome.In the same MAO parameters, the surface morphology, element and phase components of MAO coatings can not change on samples in different heat-treatment states. The MAO process of AZ91D magnesium alloy is a process contained ejecting and depositing. During this process, different kinds of inner stress accumulated, then more and more cracks grown. Solid solution treatment makes the elements of AZ91D magnesium alloy distributing uniformly. The stress accumulation decreased. So it brings good effects of MAO coating forming. It reduces the energy spending of MAO process, too. The eject deposition can not changed after aging. But the aging process can release the inner stress slowly. It expands the cracks slightly. Under the same parameters and the same time, the thickness of coatings on samples treated by T61 heat treatment always less than that on samples treated by solid-solution heat treatment or no. The roughness of coatings on samples treated by T61 heat treatment is between samples treated by solid-solution heat treatment and samples with no heat treatment.The MAO process can not changes the microstructure of the material. The MAO coatings can strengthen the as-cast AZ91D magnesium alloy, but the effect is not very big. Treating by the MAO process, the UTS of solid-solution samples can be improved to the level of samples which treated by T61. But the cracks of coatings in some thickness can reduce the strengthen effect, then the UTS decreases. And the elongation has the same trend which increases in the beginning and decreases in the later. When the samples treated by MAO process in the middle of T61 treatment, that is after the solid solution and before aging, the UTS decreases as the thickness increasing. The elongation fluctuates slightly. The MAO process has few effects on the UTS and elongation of AZ91D magnesium alloy which treated by T61. The MAO coatings have different delaminations when they rupture under the pull stress: the dense layers have some toughness and link well with the substrates; the porous layers have little toughness and flaking away.