Magnesium alloys have advantages such as low density, high specific strength and stiffness, good damping performance, strong electromagnetic shielding, and easy recycling. They are currently widely used in fields such as automobiles and communications. Currently, almost all magnesium alloy products are formed by die-casting. Compared to other forming processes, die casting has advantages such as high precision, high efficiency, and low cost.
AZ91D magnesium alloy has excellent corrosion resistance and casting performance, and is currently a widely used die-casting magnesium alloy. In the field of communication, more and more products are pursuing refinement and lightweight, and the demand for magnesium alloys is constantly increasing. However, AZ91D magnesium alloy has disadvantages such as low melting point, active chemical properties, multiple defects during die casting, and severe gas entrapment, which restrict its wider use. Especially in the production of ultra-thin parts, there are still many problems, such as low mechanical properties, poor accuracy, and high defect rate. At present, the thickness of 3C magnesium alloy electronic products is generally within 0.8mm. If the thickness continues to be reduced, the defect rate will significantly increase. The injection speed is too low, the shrinkage is not timely, the internal defects increase, and even there is a shortage of material near the exhaust groove position; If the injection speed is too high, the wall thickness of the ultra-thin notebook shell will increase, affecting product quality and installation requirements. At the same time, the grain will coarsen, resulting in poor mechanical properties. However, for thin-walled parts, especially 3C thin-walled die-casting parts, the die-casting process requirements are more stringent. The height of the mold cavity is even below 1mm, and the frictional resistance generated by the upper and lower surfaces of the mold cavity during the flow of the melt is significant. Therefore, it is necessary to improve the fluidity of the melt, that is, the pouring temperature. There are currently no reports on the die-casting parameters for such thin-walled parts.
To solve the problem of poor formability of magnesium alloy ultra-thin shell parts during die-casting, the effects of pouring temperature, mold temperature, and injection speed on the mechanical properties of castings were studied through orthogonal experiments.
Graphic and textual results
The equipment used in the experiment was an 8000kN horizontal die-casting machine produced by Izumi Company. The mold was made of 8418 hot work mold steel and did not have a vacuum system. The mold temperature was controlled by a 300L (D) mold temperature machine, with a maximum output port temperature of 300 ℃. Spray MK-TF environmentally friendly magnesium alloy release agent, with a release agent to water ratio of 1:80. The die-casting part of AZ91D magnesium alloy ultra-thin notebook shell is shown in Figure 1, with dimensions of 31mm × 21mm × 0.65mm. The thinnest part in the middle has a thickness of only 0.5mm. The sampling positions for tensile specimens are marked as 1, 2, and 3 in Figure 1.
conclusion
(1) Through orthogonal experiments, it was found that the pouring temperature has a greater impact on the AZ91D ultra-thin laptop shell parts than the mold temperature and injection speed. The order of the three is pouring temperature, injection speed, and mold temperature. The A4C3B2 process condition has the optimal tensile strength, and the A4C1B2 process condition has the optimal yield strength.
(2) When the mold temperature is 210 ℃ and the injection speed is 5m/s, the mechanical properties of the casting show a gradually increasing trend with the increase of pouring temperature, while the number of fracture shrinkage and shrinkage holes shows a gradually decreasing trend. The optimal mechanical properties are achieved when the pouring temperature reaches 700 ℃, with a tensile strength of 242.6 MPa and a yield strength of 206.8 MPa.
(3) Compared with general thick magnesium alloy die-casting parts, ultra-thin AZ91D magnesium alloy castings require higher pouring temperature, mold temperature, and injection speed.
Author of this article:
Luo Zhemin, Li Liejun, Ni Donghui, Peng Jihua
School of Mechanical and Automotive Engineering, South China University of Technology
Li Yangde, Li Weirong
Dongguan Yian Technology Co., Ltd
This article comes from: "Special Casting and Nonferrous Alloys" magazine, "Die Casting Weekly" strategic partner
