| Summary: | 碩士 === 龍華科技大學 === 工程技術研究所 === 96 === The products of Mg alloys are mainly manufactured by using the die-casting and semi-solid forming, because of their poor formability of rolling, pressing and forging. However, the high manufacturing cost and high ratio of defect products in the die-casting or semi-solid forming of Mg alloys will reduce their manufacturing efficiency. Therefore, the developments of new Mg alloys with high formability of rolling, pressing and forging are important issues to improve the manufacturing ability of thin plates of Mg alloys. The AZ80 is a commercial forging-type Mg alloy, due to its high strength and low price. However, the AZ80 Mg alloy has much difficulty in forging process because of its poor ductility. Hence, in this study, 2wt% Li is added into AZ80 Mg alloy to improve its ductility. The effects of 2wt%Li addition on its crystal structures, mechanical properties, corrosion resistance and precipitation hardening are investigated. Meanwhile, the precipitation hardening of this AZ80+2%Li Mg alloy is also studied.
Experimental results show that AZ80+2%Li Mg alloy still exhibits the fine two-phase microstructures of α and β phases. The density of AZ80 Mg alloy can be reduced to be 1.71 g/cm3 by addition of 2wt%Li. The AZ80+2wt%Li Mg alloy has a slightly lower tensile strength, but an obviously higher fracture strain, than AZ80 Mg alloy. This indicates that the addition of 2wt%Li can indeed improve the ductility of AZ80 Mg alloy. The AZ80+2%Li Mg alloy exhibits higher impact toughness than AZ80 Mg alloy. This feature verifies that the addition of 2wt%Li can significantly increase the formability of forging and the resistance of impact fracture. The tensile strength of both AZ80+2%Li and AZ80 Mg alloy increases with increasing strain rate, but their fracture strain decrease with increasing strain rate. Both AZ80+2%Li and AZ80 Mg alloy can have excellent corrosion resistance in pure water. However, the AZ80+2%Li Mg alloy exhibits a worse corrosion resistance than AZ80 Mg alloy, when in a 3.5 wt% NaCl solution. The 400℃ solution treated AZ80+2%Li Mg alloy will produce the AlLi precipitate after aging at 170℃, and produce the Mg17Al12 precipitate after aging at 250℃. The AZ80+2%Li Mg alloy with aging at both 170℃ and 250℃ will exhibit the phenomenon of precipitation hardening.
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