Effect Of Surface Chilling On The Microstructure, Mechanical Properties And Corrosion Properties Of Mg-Gd-Y Alloy

In this paper, the step-like permanent mould and sand mould with a copper chilling block are used to get a Mg-10Gd-3Y(-Zr) magnesium alloy cast with surface fine-grained layer. Then the main work is: the difference of microstructure and hardness between surface and inside in different cooling rates supplied by step-like mould; the characteristics of microstructure and composition distribution after chilling using copper block; the effect of surface layer (copper block) on hardness, tensile and impact properties; the corrosion properties of surface layer and inside; and the thermodynamic stability of surface layer and the influence of mechanical properties after different heat treatment including T5(aging treatment after casting without solution treatment) and T6 (aging treatment after solution treatment ). The results show that:1. After Mg-10Gd-3Y magnesium alloy ingot is casted using step-like mould, its surface layer is directional dendrite, and the inside is equiaxed dendrite. There is a mixed zone between them. The dendrite arm spacing (DAS) decreases, the hardness and the mechanical properties increase with the increase of cooling rate (supplied by different step). When the cooling rate is lower, the hardness of mixed zone is the least. After the inside can form equiaxed dendrite with the increase of cooling rate, the inside hardness is the highest.. At last, the thickness of chilling layer is measured through the analysis of the standard deviation of hardness.2. Through using sand mould with a copper chilling block, a surface fine-grained layer in Mg-10Gd-3Y-Zr ingot forms, which thickness is about 5.5mm. Comparing to the inside, this layer has a good mechanical properties. In as-cast condition, the yield strength, tensile strength and elongation is improved from 150MPa,210MPa and 1.2% to 170MPa,240MPa and 3%. After the observation of microstructure, the grain size increases from 80μm in surface to 130μm in inside. And there is no ternary phase in the surface layer. The ternary phase appears and increases to the normal level of sand casting gradually with the increase of the distance from the chilling surface. It can be found that there is no obvious composition segregation through EDAX. It indicates that the above microstructure contribution is due to the cooling rate but not the composition segregation. So it can be concluded that the alloy elements Gd and Y are dissolved into the grain. And this microstructure can be aged without solution treatment.3. Microstructure contribution and mechanical properties change a lot after T5 or T6 heat treatments. After T5 treatment the grain size contribution characteristics is the same to as-cast condition, the yield strength, tensile strength and elongation is improved to 250MPa,335MPa and 2.0%, because of the effective aging strengthen. After T6 treatment the characteristics of surface fine-grained layer is totally destroyed. The grain size decreases from 180μm in surface to 140μm in inside. And the yield strength, tensile strength and elongation becomes 220MPa,310MPa and 1.8%. It is obvious that the effect of T5 treatment is stronger than that of T6 in surface. But T6 treatment is better than T5 treatment in inside.4. There is great effect of surface layer on impact properties. If the surface layer locates in the crack initiation zone, the impact absorption work will improve from 3~5J to 45J. It also will be 20J in the worst condition. The macrostructure of the fracture shows that the specimens with the surface chilling layer have the biggest reduction of crosssection area (10%) and strain (35°). It can be found that there are a lot of gliding fracture characteristics in SEM graphs.5. There are some research works about corrosion properties, too. The corrosion rate is improved from 2.00mg*cm-2*d-1(sand cast,metal cast is 1.5mg*cm-2*d-1) to 0.87 mg*cm-2*d-1。For one thing, the effect of dipole corrosion is weakened greatly, because there is no ternary phase along the boundaries in the surface layer. For another, the alloy elements dissolved into the grain can change the electrode potential, which can reduce the corrosion rate. Moreover, the uniform corrosion is the main corrosion way of surface layer, but the pitting is weak, which can elongate the life of the products. According to the SEM analysis, the uniform corrosion is very serious on surface, and some zones are helcoidly corroded because of micro composition segregation. But there is no corrosion hole on surface. However, the phase interface is corroded firstly due to the dipole corrosion in sand casting specimens. After the boundaries is totally corroded, a grain will fall from the matrix (the ternary phase distributes on the boundaries), and there will be a hole. It can make the corrosion more serious.