Study On Mold Filling And Solidification Of Magnesium Alloy In Lost Foam Casting Process Under Controllable Pressure

Magnesium alloy could be used in automobile, vehicle, electric appliances, communication, space and aviation and military industries because of its lower density, high strength-to-weight ratio, high modulus, good heat conductivity, well electromagnetic shielding, superior damping characteristic, good machinability and rich in natural resources. It was entitled as new metal materials friendly to environment in the 21st century following iron and aluminum alloy, but high oxidative combustion and absence of formation technology holdback its widely application. In this paper, a new precision casting process being known as lost foam casting (LFC) process under controllable pressure was presented, which takes advantages of LFC process, low-pressure casing process, and vacuum suction casing process, so as to solve the oxidative combustion and lower flowability of magnesium alloy. It is very important of the investigation on LFC process under controllable pressure for academic sense and applications of magnesium alloy. First of all, the processing principle and facility design guideline of LFC process under controllable pressure had been investigated, and first facility for experiments which includes system of SF6 gaseous mixture was developed. Contrasted with die-cast process, this new technology for the formation of magnesium castings possess multiplicate advantages such as low investment and productive cost, good adaptability, and castings were able to be strengthened by heat treatment. The casting of Mg-alloy produced by the new process would possess high precision, smooth surface, good mold-filling and feeding ability. The rate of filling mold can reach 80～150mm/s which is much higher than the rate of 50 mm/s in gravitational LFC process, so that the production of complicated thin-walled Mg-alloy castings with 3mm or more thinner section thickness would be made possible. The effects of gas flux, vacuum degree, coating thickness, pouring temperature, pattern density, and critical gate area on the mold-filling ability of magnesium alloy were investigated. According to the data obtained by orthogonal experiments, a nonlinear regression equation about the influences of processing parameters on flowability of magnesium alloy was created shown below. L1=-5.107+2.736Q1.6+55.645T0.01-1.4322δ3.64-1.9685ρ1.69 It was indicated that the filling length of Mg-alloy increased with the increase of gas flux and pouring temperature, and decreased with increase of coating thickness and pattern density. The flowability, front pattern and filling velocity of Mg-alloy in LFC process under controllable pressure were studied by electrode contact test method and photogrammetric procedure. The physic model with convex filling front was posed. The solidification characteristics of magnesium alloy was studied using electrical probes connected to data acquisition controlled by computer. The maximal temperature and front actual temperature of Mg-alloy melts along the filling course were detected. Based on the cooling curse of Mg-alloy melts, the mode and range of solidification, solidification time, and heat transfer in the dry sand mould were investigated. A thermal equilibration equation about filling length of Mg-alloy has been deduced, as shown below. dtdvAHHTTCTTfHgL QkEEEmLm= δ(1 .62?E )?ρL(0?+ 2K )+(s?Mg)? The results shown that the temperature range of solidification is 160~180℃, and the range of solidification time is 66~338s. The melting processing and heat treatment processing were opted. The microstructure, mechanical properties and fraction characteristics were measured. The results shown that the tensile strength reached 180.8 MPa, yield strength reached 113.2 MPa, elongation was 4.4% for as-cast Mg-alloy, and after heat-treating, the tensile strength can rise to 258.70MPa and elongation to 10.3 percent in T4 heat-treating process, yieldstrength rose to 161.21 MPa in T6 heat-treating process,which are much higher compared with other magnesium alloy produced in other casting process. Lastly in the paper, pouring practices of three kind of Mg-alloy castings with medium complicated shape were tried. The pouring application shown that low-pressure EPC process was much applicable for the complicated thin-walled and common casting defects and its formation mechanics were analyzed.