17. A multi-position parallel pressurized casting method for large aluminum alloy castings, comprising a casting device, wherein the casting device comprises a platform; a top surface of the platform is a working surface, and a bottom surface of the platform is provided with holding furnaces; a number of the holding furnaces is two or more, and each holding furnace of the two or more holding furnaces is connected to a liquid filling port corresponding to the working surface through a lift device, and the each holding furnace achieves an independent liquid level pressurized control or a synchronization liquid level pressurized control in any combination by a lift control system; and a cover body is also provided on the working surface, the cover body and the working surface form a sealed working chamber, and an vacuum-pumping system and an inert gas replacement system for the working chamber and/or the each holding furnace are further provided; wherein the multi-position parallel pressurized casting method for large aluminum alloy castings comprises the following steps:1) preparation before pouring: transferring a refined aluminum melt to four holding furnaces through a quantitative delivery device, inserting a lift tube lower portion into a liquid lifting port of the each holding furnace, and locking a lift tube lower section with the each holding furnace with a bolt; moving the each holding furnace to a lower part of a frame platform through a furnace body walking mechanism, then, through a furnace body lifting mechanism, completing connections and sealings between an air inlet and outlet port of the each holding furnace and an inlet and outlet passage mechanism, and between a lift tube upper section and the lift tube lower section; placing a resin sand mold on the frame platform and compressing the resin sand mold with a pressure plate, using a sealing gasket to ensure that the resin sand mold and the lift tube are sealed; connecting electrode contacts, covering the working chamber, and driving a locking ring to lock the resin sand mold by four cylinder piston mechanisms;
2) synchronous negative pressure and inert gas replacement: opening an interconnection valve between the each holding furnace and the working chamber, performing a vacuuming and an inert gas replacement in the working chamber, firstly, opening a solenoid valve of a vacuum line, using a vacuum pump to perform the vacuuming, when a vacuum degree is reduced to 40-60 KPa, closing the solenoid valve to stop the vacuuming; opening a solenoid valve of an inert gas line, opening a Ar gas station, filing the each holding furnace and the working chamber with Ar gas, when a pressure rises to 120-150 KPa, closing the solenoid valve of the inert gas line to realize the inert gas replacement; finally, closing the interconnection valve between the each holding furnace and the working chamber;
3) melt quality correction: opening a magnetic homogenization device, generating a constant magnetic field in an iron core, wherein the iron core is placed in a preset structure, magnetic lines are scattered in a particular shape in space, under an action of a rotation motor, generating a rotating magnetic field to make the refined aluminum melt move under an action of an applied rotating magnetic field, achieving a purpose of a magnetic homogenization;
4) synchronous pre-mold filling: calculating pre-mold filling pressures of four lift tube devices according to a theoretical formula P=?hg firstly, and then carrying out the synchronous pre-mold filling of the four lift tube devices, firstly, opening a pressure control module of a first holding furnace, raising a liquid level of the lift tube to a position of an electrode contact mark, closing the pressure control module of the first holding furnace by a feedback signal of an A/D module; then opening pressure control modules of a second holding furnace, a third holding furnace, and a fourth holding furnace in sequence for the synchronous pre-mold filling; finally, raising liquid levels of aluminum melts of the four lift tube devices to a same level;
5) multi-position synchronous liquid lifting: according to a initially set liquid pressure pressurization process curve, opening the pressure control module for an initial pressurization, using the electrode contacts to capture liquid surface information, feeding back the liquid surface information to a multi-position synchronous mold filling control system through the A/D module, and adjusting pressurization rates of the four holding furnaces and ensuring that the castings are simultaneously lifted through the pressure control module, when the aluminum melt flows to a top of the resin sand mold, a top signal light is lighted up, and a mold filling is completed;
6) secondary pressure solidification: during a crusting pressurization stage, increasing a pressure of the aluminum melt in the each holding furnace, and a crystal holding time is 15-30 s, so that a shell of 3-5 mm is formed in a surface layer of the aluminum melt; during a crystallization pressurization stage, according to structural characteristics of the castings, the castings are continuously and fully fed through the lift tube device and a pouring system under an action of a melt pressure; a crystallization holding time is about 1500-1800 s, ensuring that the castings are fully solidified under pressure; and
7) pressure relief: after a crystallization retention time is over, closing the pressure control modules of the first holding furnace, the second holding furnace, the third holding furnace and the fourth holding furnace, opening a holding furnace exhaust valve, and directly discharging compressed air; opening a working chamber exhaust valve to discharge the Ar gas in the working chamber into a Ar gas recovery station for a recycling treatment; when pressures of the each holding furnace and the working chamber are less than 3 KPa, the locking ring is driven to open by the four cylinder piston mechanisms, the working chamber and a cast mold are removed, and the each holding furnace and the lift tube are lowered to a bottom through a furnace body lifting system; and then exiting a working area through a horizontal moving mechanism to perform a cleaning treatment.