Large Thin-wall Magnesium Bracketlow Pressure Casting Technology Research

In recent years, with the development of aerospace and ordnance industry, the demand of lightweight and structural function integration is higher and higher. Therefore, structure of the castings developed to the direction of integration and thin wall. Large complicated thin-walled magnesium alloy castings are widely used. And not only require high metallurgical quality of castings, but also high precision, and near net shape. Traditional green sand mold casting is difficult to satisfy the high accuracy requirement of dimension. Therefore, resin sand anti-gravity precision forming technology gets more and more attention. The large casting magnesium alloy stent was chosen as the research object, by using numerical simulation software, to simulate the low pressure casting process analysis, under the conditions of different casting process of casting filling and solidification temperature field, and predict casting defects. The reasons for the formation of defects were analyzed and the technology was optimized. Finally qualified castings are obtained. The main results were as follows:Firstly, the casting structure characteristics were analyzed, and in view of the casting structure, the gating system was designed for four pouring position respectively. And numerical simulation of low pressure casting process results show that because there are a several big flat structure in the casting, and a thinner wall thickness, plane position easily involved in oxide inclusions during the filling process. The choice of pouring position should avoid this kind of situation. Therefore, horizontal and upward type pouring position is reasonable. Slot gating system should ensure the casting filling process flow smoothly.Because of thin wall and complex structure, it is difficult to achieve strict top-down directional solidification through cold iron and subsidies for the four kinds of pouring position.Combined with the four pouring process of the simulation results, the technology was optimized, selecting horizontal pouring position with the slot gating system, and riser was used to feed position of thin wall positions. Numerical simulation results of the optimized process, show that the casting smooth during the filling process, solidification temperature field is reasonable, to avoid the defect of solidification.The stress field of casting solidification process was simulated, to analyze the actual crack formation in the pouring of the casting. The simulation results show that, the casting internal triangular reinforced bar at the bottom of the solidification time is short, linear shrinkage stress is big, on both ends of the beam and the casting contraction by casting block wall connection position, the formation of cracks, the crack defects as a typical hot crack.The castings pouring process is optimized further, in the casting internal thin steel plate location increases process brace and the feeding runner. Simulation results show that at the same time adding process brace and the feeding runner, stiffened plates can delay line contraction rate, expanding the scope of the contraction stress, the original maximum effective thermal stress decreased to less than 120 MPa, avoid the happening of cracking. And the optimized process was experimentally certificated.Finally, in this paper, specimens were cut from the casting large magnesium alloy, and mechanical properties were tested. Test results show that after heat treatment, the average tensile strength of casting was 262 MPa. Different positions have bigger difference, the performance of casting side slope and vertical cylinder surface of tensile strength, the highest average about 280 MPa, the vertical tube side post and casting tensile strength at about 270 MPa. This is different from the casting position related to the content of oxide inclusions. Taking sample fracture mode for casting was the cleavage fracture mainly mixed fracture.