Magnesium alloys have advantages of low density, high specific strength and specific stiffness, good damping performance, strong electromagnetic shielding and easy recycling. They are currently widely used in automobiles, communications and other fields. At present, almost all magnesium alloy products are die-cast. Compared with other forming processes, die-casting has advantages of high precision, high efficiency and low cost.
AZ91D magnesium alloy, with good corrosion resistance and casting performance, is a very widely used die-cast magnesium alloy. In the field of communications, more and more products are pursuing refinement and lightweight, and demand for magnesium alloys is increasing. However, disadvantages of AZ91D magnesium alloy, such as low melting point, active chemical properties, many defects in die-casting process, and severe air entrainment, restrict wider use of magnesium alloys. In particular, there are still many problems in production of ultra-thin parts, such as low mechanical properties, poor precision, and high defective rate. At present, thickness of 3C magnesium alloy electronic products is generally within 0.8mm. If thickness continues to be reduced, defective rate will increase significantly. If injection speed is too low, shrinkage is not timely, internal defects will increase, and even material shortage will occur near exhaust groove; if injection speed is too high, wall thickness of ultra-thin notebook shell will increase, affecting product quality and installation requirements, and grains will coarsen, resulting in poor mechanical properties. However, for thin-walled parts, especially 3C thin-walled die-casting parts, die-casting process requirements are more stringent. Cavity height is even less than 1mm. Friction resistance generated by upper and lower surfaces of cavity during melt flow is very large, and melt fluidity, that is, pouring temperature, needs to be increased. There is currently no report on die-casting parameters for such thin-walled parts.
In order to solve problem of poor die-casting formability of magnesium alloy ultra-thin shell parts, effects of pouring temperature, mold temperature and injection speed on mechanical properties of castings were studied through orthogonal experiments.
Graphical results
Equipment used in test process is an 8000kN horizontal die-casting machine produced by Yizumi. Mold is made of 8418 hot-working die steel. Mold has no vacuum system. Mold temperature is controlled by a 300L (D) type mold temperature controller, and maximum output port temperature is 300℃. Spray MK-TF environmentally friendly magnesium alloy release agent, and ratio of release agent to water is 1:80. AZ91D magnesium alloy ultra-thin notebook shell die-casting is shown in Figure 1. The overall dimensions are 31mm×21mm×0.65mm, and thickness of the thinnest part in the middle is only 0.5mm. 1, 2, and 3 marked in Figure 1 are sampling positions of tensile specimens.

 

Figure 1 Ultra-thin notebook shell casting

Table 1 Chemical composition of AZ91D magnesium alloy (%)

Table 2 Experimental factor level table

 

Figure 2 Microstructure and main phase composition of castings

 

Table 3 Orthogonal test results

 

Table 4 Orthogonal test level effect value

 

Figure 3 Effect of pouring temperature on casting strength

 

Figure 4 Tensile fracture morphology at different pouring temperatures
Conclusion
(1) Through orthogonal test, it was found that influence of pouring temperature on AZ91D ultra-thin notebook shell is greater than that of mold temperature and injection speed. Order of the three is pouring temperature, injection speed and mold temperature. A4C3B2 process has the best tensile strength and A4C1B2 process has the best yield strength.
(2) When mold temperature is 210℃ and injection speed is 5m/s, as pouring temperature increases, mechanical properties of casting show a gradual increase, while fracture shrinkage and shrinkage number show a gradual decrease. When pouring temperature reaches 700℃, optimal mechanical properties are achieved, with a tensile strength of 242.6MPa and a yield strength of 206.8MPa.
(3) Compared with general thick magnesium alloy die castings, ultra-thin AZ91D magnesium alloy castings require higher pouring temperature, mold temperature and injection speed.