Die casting of magnesium alloy

Die casting method of magnesium alloy

Magnesium alloys can be die-cast by cold chamber or hot chamber die casting machine. At present, the improvement of hot chamber die casting machine mainly includes: using energy accumulator to boost, the injection speed of injection plunger can reach 6M/S; Induction heating of the gooseneck tube and nozzle to maintain the optimum temperature; Double furnace melting heat preservation, and the use of thermal insulation device and recirculation pipe, precisely maintain the molten pool temperature. The injection system and the automatic feeding system of the die casting machine must be reformed to make it suitable for the magnesium alloy die casting. The modification includes :(1) the rapid injection speed of the injection system is increased from 4 ~ 5M/S to 6 ~ 10M/S in the die-casting of aluminum alloy; (2) Shorten the pressure building time during the pressurization process; (3) Improve the injection force; (4) The electromagnetic automatic quantitative feeding device is adopted to prevent the oxidation of magnesium alloy in the pouring process; (5) When vacuum die casting and other special die casting processes are used, necessary supporting equipment shall be configured.

As with other die casting alloys, the traditional die casting technique causes the magnesium alloy liquid to fill the die casting cavity in a high speed turbulent and diffuse state. The gas at high pressure either dissolves in the die casting alloy or forms many high-pressure micropores dispersed throughout the die casting. Therefore, the magnesium alloy die castings produced by traditional die casting method can not be strengthened by heat treatment and can not be used at higher temperature. 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 researched and developed in recent 20 years, including oxygenated die casting, semi-solid metal rheological or thixotropic die casting and squeeze casting, as well as several ups and down vacuum die casting.

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

Oxygenated die casting is also known as pore-free die casting process (P. F method). Before filling the mold with liquid metal, oxygen or other active gas is filled into the cavity to replace the air in the cavity. When filling the mold with liquid metal, the active gas reacts with the liquid metal to generate metal oxide particles which are distributed in the die casting, so that the gas in the die casting can be eliminated and the die casting can be strengthened by heat treatment. The AZ91 magnesium alloy monolithic magnetic head bracket for computer is produced by the oxygenated die casting method in Nippon Light Metal Co., LTD., instead of the original multilayer laminated bracket, the weight of the bracket is reduced and great economic benefits are obtained. The company also manufactured AM60 magnesium alloy automotive and motorcycle wheels in batches using an oxygenated die-casting process that reduces weight by 15 percent compared to aluminum wheels.

In recent years, American, Japanese and British companies have successfully developed semi - solid thixotropic injection casting machines for magnesium alloys. The semi-solid thixotropic injection casting machine for magnesium alloys is used to shoot the semi-solid magnesium alloy into a die cast mold at a certain pressure and to form it, which is similar to an injection molding machine. It will prefabricated non-dendritic magnesium particles into the spiral feeding mechanism, in the spiral feeding mechanism will be heated to semi-solid magnesium particles, and through the spiral feeding mechanism at the other end of the magnesium alloy slurry collection chamber will be semi-solid magnesium