Die casting method of magnesium alloy in die casting die warm machine: magnesium alloy can be die-cast by cold chamber or hot chamber die casting machine. At present, the improvement of the hot-chamber die-casting machine mainly includes: using the accumulator to pressurize, the injection speed of the injection plunger can reach 6 m/s; Induction heating gooseneck and nozzle to maintain the optimum temperature; Double furnaces are used for melting and heat preservation, and thermal insulation devices and recirculation pipes are used to accurately maintain the temperature of the molten pool. When the common cold chamber die casting machine is used for die casting of magnesium alloy, the injection system and automatic feeding system of the die casting machine must be modified to meet the requirements of magnesium alloy die casting.
Like other die-casting alloys, the traditional die-casting technology makes the magnesium alloy liquid fill the die-casting cavity with high-speed turbulence and dispersion, and the gas is dissolved in the die-casting alloy under high pressure, or forms many high-pressure micro-pores dispersed in the die-casting parts. Therefore, the magnesium alloy die castings produced by the traditional die casting method cannot be strengthened by heat treatment, nor can they be used at higher temperatures. In order to eliminate this defect, improve the internal quality of die castings, and expand the application range of die castings, some new die casting methods have been developed in the past 20 years, including oxygen-filled die casting, semi-solid metal rheological or thixotropic die casting and squeeze casting, as well as several undulating vacuum die casting.
The contents of transformation include:
(1) The fast injection speed of the injection system is increased from 4~5m/s to 6~10m/s for die-casting aluminum alloy;
(2) Shorten the pressure build-up time in the pressurization process;
(3) Improve the injection force;
(4) Adopt electromagnetic automatic quantitative feeding device to prevent magnesium alloy from oxidation during pouring;
(5) If special die-casting processes such as vacuum die-casting are used, necessary supporting equipment shall be equipped.
Oxygen-filled die casting is also called pore-free die casting process (P.F method). In this method, oxygen or other active gases are filled into the mold cavity before filling the molten metal to replace the air in the mold cavity. When the molten metal is filled, the active gases react with the molten metal to form metal oxide particles which are dispersed in the die casting, thus eliminating the gas in the die casting and making the die casting heat treatable and strengthened. The AZ91 magnesium alloy integral magnetic head bracket of the computer produced by Japan Light Metal (Japan) by oxygen filling die casting method, instead of the original multi-layer laminated bracket, not only reduces the weight of the bracket, but also achieves great economic benefits. The company also produced AM60 magnesium alloy automobile wheel hub and motorcycle wheel hub in batches by oxygen-filled die-casting method, which is 15% lighter than aluminum wheel hub.
Vacuum die casting can eliminate or significantly reduce the porosity and dissolved gas in the die casting by removing the gas in the mold cavity during the die casting process, and improve the mechanical properties and surface quality of the die casting. At present, AM60B magnesium alloy automobile wheel has been successfully produced by vacuum die-casting on the cold chamber die-casting machine, and AM60B magnesium alloy automobile steering wheel parts have been produced on the hot chamber die-casting machine with a locking force of 2940kN, with the casting elongation increased from 8% to 16%.
In recent years, companies in the United States, Japan and the United Kingdom have successfully developed semi-solid thixotropic injection casting machines for magnesium alloys. The semi-solid magnesium alloy thixotropic injection die casting machine injects the semi-solid magnesium alloy into the die casting mold under a certain pressure to form it. Its working principle is similar to the injection molding machine. It sends the prefabricated non-dendritic magnesium particles into the screw feeding mechanism, heats the magnesium particles to semi-solid state in the screw feeding mechanism, and sends the semi-solid magnesium alloy slurry into the injection chamber through the magnesium alloy slurry collection chamber at the other end of the screw feeding mechanism for injection molding. This casting method represents a development direction of magnesium alloy casting production.
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