| Magnesium alloy is very attractive in such applications as automobile, aerospace and communication industries due to its low density, high specific strength, stiffness, good absorption of vibration and damping, excellent castability and machinability. It is praised as the green engineering material in21st century. Squeeze casting, an advanced forming technology with the combination of casting and forging, has been successfully used in the practical production of aluminum alloy and copper alloy etc. The process has the capability of producing castings which are fine grains, high density, pore free and excellent mechanical property. In recent years, more and more scholars and enterprises pay more attention to the study of theory and application of squeeze casting. But limit researches have been done about the forming mechanism of magnesium alloy squeeze casting. The promotion of the process is seriously restricted. Meantime, magnesium resource and production is very abundant in our country, and the using of magnesium alloy has a very important strategic meaning. Therefore, it is necessary to discuss the forming mechanism of magnesium alloy squeeze casting for the promotion of high-quality castings.The influence of pouring temperature, mould temperature and squeeze velocity on the fluidity of magnesium alloy with different wall thickness was investigated using orthogonal test. The experimental results show that the most important factor influencing the fluidity of specimens with1,2and3mm wall thickness is pouring temperature and then mould temperature and squeeze velocity, but for specimen with4mm thickness mould temperature at first, and then pouring temperature and squeeze velocity. In practical production, it is very effective to increase pouring temperature for better filling ability of squeeze cast AZ91D magnesium alloy between700to750℃, but it is not suitable above750℃. But the filling length of magnesium alloy in the thick section (3and4mm) increases remarkably because of the longer filling time. The fluidity of magnesium alloy increases with the increase of squeeze velocity. Based on the stopped-flow mechanism of magnesium alloy, semi-quantitative mathematical expression of the filling length is educed.Fluid mechanics analysis on the filling of molten metal shows that the filling characteristic is affected by liquid characteristic (viscosity and surface tension), runner characteristic (wall thickness and shape of ingate) and filling length etc. Numerical simulation and experimental study shows that low filling velocity (<186.8mm/s) can easily cause the great fluctuation of the fluid level, the instability of filling process and the decrease of fluidity for6mm thickness plate. Misrun and cold lap appear in the last filling position. The defects of air entrainment are caused by the jetting of the molten metal at high filling velocity (>933.8mm/s). The filling process is smoother using fanned gate than straight gate at same filling velocity for12mm thickness plate. The fan ingate helps accomplish a rapid fill without high velocities and avoids jetting effects.Microstructure, mechanical properties and fracture characteristics of squeeze casting AZ91D alloy under different states are measured and analyzed The results show that hardness, yield strength and tensile strength decrease but elongation increases with the increase of wall thickness under as-cast, T4heat-treating and T6heat-treating. Taking8mm thickness sample as analysis object, tensile strength reaches205MPa, and elongation reaches7.0%in T4heat-treating. Compare with as-cast, elongation increases in large amplitude, but hardness and yield strength drop slightly. Hardness and yield strength significantly increases, tensile strength increases slowly, and elongation drops slightly in T6heat-treating. Compare with T4heat-treating, yield strength increases by44.2%. The fracture mode of squeeze cast AZ91D specimens in as-cast is mainly brittle fracture. A few tearing ridges and dimples with small size are observed on fracture surfaces. The fracture mode of AZ91D specimens in T4heat-treating is a combined fracture of quasi-cleavage and a few dimples. The cleavage surfaces, river markings and dimples are observed on the fracture surfaces. The fracture mode of AZ91D specimens in T6heat-treating is mainly intergranular fracture, and a few shallow dimples and tiny tearing ridges are observed on the fracture surfaces.The forming mechanism of cold clamp in squeeze cast AZ91D is discussed by theory analysis and microstructure analysis. The casting is divided into two parts by cold clamp which is like a layer of insulation. Therefore, both sides structures of cold clamp are obviously different. EDS analysis results indicate that high light particles in the cold clamp involve O, Mg and Si three elements. The particles consist of high melting point oxides and contamination from cold clamp surface. Further analysis shows that molten metal must contain superheat, and solidification layer in the pressure chamber must be broken up and remelted when filling cavity in order to eliminate cold clamp. The calculating formula for solidification layer remelting is deduced in the paper. The thickness of solidification layer decreases with the increase of the temperature of pressure chamber and pouring temperature. Thin solidification layer is easy to break up and remelt. A layer of suitable coating is evenly coated in the inner wall of pressure chamber, which can reduce the capacity of heat transmission of pressure chamber and decrease the thickness of solidification layer. Reducing the resident time of molten metal in the pressure chamber and using mechanical method can decrease or eliminate cold clamp. |
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