Die Casting Method of Magnesium Alloy

Magnesium alloy can be die-casted by cold chamber or hot chamber die casting machine. The current improvements to the hot chamber die-casting machine mainly include: the use of accumulators for pressurization, the injection speed of the injection plunger can reach 6M/S; the induction heating of the gooseneck and the nozzle to maintain the most suitable temperature; the use of double furnaces Melting heat preservation, and adopting heat insulation device and recirculation pipeline to accurately maintain the temperature of the molten pool. When using the ordinary cold chamber die casting machine to die-cast magnesium alloy, the injection system and automatic feeding system of the die-casting machine must be modified to make it suitable for the requirements of magnesium alloy die-casting. 

The content of the transformation includes: 

(1) Increase the fast injection speed of the injection system from 4～5M/S when die-casting aluminum alloy to 6～10M/S; 

(2) Shorten the pressure building time of the pressurization process; 

(3) Improve the injection force; 

(4) Use electromagnetic automatic dosing device to prevent magnesium alloy from oxidizing during the casting process; 

(5) Configure necessary supporting equipment if special die casting processes such as vacuum die casting are used.

Like other die-casting alloys, traditional die-casting technology makes the magnesium alloy liquid fill the die-casting cavity in a high-speed turbulent and dispersed state. The gas is either dissolved in the die-casting alloy under high pressure, or forms many high-pressure microstructures dispersed in the die-casting. Therefore, magnesium alloy die-castings produced by traditional die-casting methods cannot be strengthened by heat treatment, nor can they be used at higher temperatures. In order to eliminate this defect, improve the inherent quality of die castings, and expand the application range of die castings, some new die casting methods have been researched and developed in the past 20 years, including oxygenated die casting, semi-solid metal rheological or thixotropic die casting and extrusion casting, and vacuum die-casting that has experienced ups and downs.

Vacuum die casting eliminates or significantly reduces the pores and dissolved gases in the die casting by removing the gas in the cavity during the die casting process, and improves the mechanical properties and surface quality of the die casting. At present, AM60B magnesium alloy automobile wheels have been successfully produced by vacuum die casting on cold chamber die casting machines, and AM60B magnesium alloy automobile steering wheel parts have been produced on hot chamber die casting machines with a locking force of 2940KN. The elongation of the castings has increased from 8% to 16%.

Oxygen-filled die-casting is also called non-porous die-casting (PORE-FREEDIECASTINGPROCESS, that is, P.F method). In this method, oxygen or other active gases are filled into the cavity before the molten metal is filled, and the air in the cavity is replaced. When the molten metal is filled, the active gas reacts with the molten metal to form metal oxide particles dispersed in the die casting In order to eliminate the gas in the die casting, the die casting can be heat treated and strengthened. Nippon Light Metal Co., Ltd. uses the oxygen-filled die-casting method to produce the AZ91 magnesium alloy monolithic magnetic head bracket for the computer, instead of the original multi-layer laminated bracket, which not only reduces the weight of the bracket, but also achieves great economic benefits. The company also produced AM60 magnesium alloy automobile wheels and motorcycle wheels in batches by means of oxygen-filled die-casting, which reduced the weight by 15% compared with aluminum wheels.

In recent years, companies in the United States, Japan and the United Kingdom have successively successfully developed magnesium alloy semi-solid thixotropic injection casting machines. The magnesium alloy semi-solid thixotropic injection die casting machine uses a certain pressure to inject the semi-solid magnesium alloy into the die-casting mold to form it. Its working principle is similar to that of an injection molding machine. It feeds the prefabricated non-dendritic magnesium particles into the screw feeding mechanism. The magnesium particles are heated to semi-solid in the screw feeding mechanism, and the semi-solid magnesium is collected through the magnesium alloy slurry collection chamber at the other end of the screw feeding mechanism. The alloy slurry is sent into the injection chamber for injection molding. This casting method represents a development direction in the production of magnesium alloy castings.