Study On Rheo-Diecasting Process Of Semi-solid Magnesium Alloy

Magnesium alloy, which fulfills the requirements of lightweight and friendly to environment for materials, has very vast development foreground as a new engineering materials because of its low density, high strength-to-weight ratio, high modulus, good heat conductivity, superior damping characteristic, good machinability, and so on. Semi-solid metal processing technology, synthesized the merits of liquid-casting technology and solid-press forming technology, is catching many researchers'attention in various fields, and regarded as one of the near net-shape forming technologies with wide application in 21 century. However, research on semi-solid rheomoulding for magnesium alloys is seldom uptodate worldwide. In this paper, the rheo-diecasting process of semi-solid magnesium alloy has been studied with a self-made twin-screen slurry mixer, cooperated with cold-chamber die casting machine. Therefore, this advanced technique opens a new route for application of magnesium alloy products and development of semi-solid metal processing, and it is of great importance for academic research and industrial application. In order to solve the problem that magnesium alloy is easy to be oxidizd and combust at elevated temperature, a delivery pump has been developed with capacity dosing and gas-pressure transferring methods, and it is a originated technology in China. Delivery error can be controlled within ±5%. The characteristic of magnesium semi-solid slurry has been analyzed. A die casting mold was designed with assembly multi wall thickness cavities. A measure device was set up for shot speed, which was simple in structure and reliable in accuracy. The least length can be measured was 3cm, and the measure time error was in 1ms. By analysis of injection pressure curve and theoretical calculation, the influence of the die temperature, shot chamber temperature, slow shot speed, first fast shot speed on the rheological flow of semi-solid slurry of magnesium alloy in shot chamber were investigated. The constitutive equation of the slurry temperature reduction was established in relation with shot chamber temperature, solid fraction, shot chamber coating and stay time after thermodynamic analysis. The effects of the ingate velocity and ingate dimension upon mould filling process of liquid magnesium and semi-solid slurry were systematically studied. The results showed that: liquid flows in strong turbulent flow mode; and the slurry at solid fraction of 15 % flow in weak turbulent flow mode; and the slurry at solid fraction of 40 % or 50 % flows in laminar current obviously, but also has feature of expansion fluid. The relation between the macrograph structure of the semi-solid magnesium alloy formed by rheo-diecasting and mould filling route was comparatively surveyed. The results showed that: the correlation between the structure and solid is great, the texture of the cross-section presents non-homogeneous feature, the surface appears chilling layer, and the center appears aggregation of the solid grains. At the meantime, the thicker the wall and the bigger the solid fraction, the more non-homogeneous structure, and the more the aggregation in the center area. When the thickness reaches 5 mm , the chilling layer appears in multi-layers, and fluid transition region arises. The surface of the primary crystal grains was studied. In the process of making slurry by the twin-screen mixer, a turbulent boundary layer emerges on the surface of the grain with constitutional undercooling, the surface of the grains has cellular structure, which promotes the primary crystal as granule. The groove of the surface increases surface energy, and accelerates aggregation and agglomeration of the grains. The grooves enlarge the meshing system, increase the transient shear stress. The mechanical properties of the rheo-diecasting were tested. The hardness and density increase with increasing of the solid fraction, and when the solid fraction was 40 % approximately, the tensile strength and the elongation were best, and the solid fraction was up to 58 % or so, the tensile strength and the elongation were decreased to low level. After heat treatment, the mechanical properties can be increased, overcoming the nonheat-treatable shortage of normal die casting.The microscopic fractographs of the rheo-diecasting specimen were carefully analyzed. The fractographs of the liquid diecasting sample were typical quasi-cleavage fracture, and as the solid fraction increased, the equiaxed dimple increased in the fractograph of the rheo-diecasting on the SEM. The liquid quantity around the primary crystal grains affected the mechanical properties greatly. When the solid fraction is low, the liquid quantity around the primary crystal grains is large, the mechanical properties are decided by the liquid mainly; as the solid fraction increases, the liquid quantity decreases, the primary crystal grains impact, the properties improve. As the solid fraction increases to some degree, the liquid film of the grains can not be integrate, and create holes, the properties decrease. The aggregation of the grains aggravate the tendency of the decreasing. When the solid fraction is 50 % or so, there are many holes among the grains, and the properties decrease abruptly.