| Summary: | 碩士 === 大葉大學 === 醫療器材設計與材料碩士學位學程 === 104 === Consumer demand on products has risen with today’s technological improvement. To adhere to the varying needs, products have evolved from the thick and heavy materials to thin and light materials that are fashionably appealing. To account for environmental protection, material selection and use must exhibit the characteristics of lower pollution and emission, lower power consumption, higher energy efficiency, and high recycle and renewability. Thus, magnesium alloys have become an essential and emergent material.
According to industrial needs for fatigue-resistance and light-weight materials. This study applied solid-solution aging treatment in AZ80A extruded magnesium sheet with yttrium (0.25 wt.%) addition, and explored the influence on mechanical and fatigue-resistance properties of this alloy. By applying gas tungsten arc welding, adjusting the aging treatment parameters, and using the S-N curve obtained from fatigue tests, this study determined the optimal aging parameters for predicting the change in durability of such magnesium welds under various stress states.
From experimental results show that administering 32 ampere current will completely penetrate the 2.6 mm sheet, and exhibit the optimal welding heat input control. The weld also exist favorable performance in mechanical properties. Thus, this current value was used as the welding current value in subsequent welding for investigating the aging treatment.
To examine the microstructure of aged welding sheets revealed that the amount of precipitate increased with increasing aging time, which in turn of enhanced the hardness. The types of precipitate from the aging-treated welds were quite similar. However, differences were observed in shape at different aging times. Under short aging treatment period, the continuous layer-shaped precipitate assisted more evidently in enhancing the tensile strength and hardness of weld. However, it had a detrimental influence on elongation. Rod-shaped precipitates were uniformly precipitated inside the grains with increasing aging time, but these precipitates did not enhance weld strength. In addition, the continuous increase in precipitate negatively affected the elongation.
Properties of fatigue resistance were evidently enhanced with increasing the aging time. After aging, the β phase particles exhibited delayed crack growth and hindered dislocation movement. Especially, precipitates obtained with 8 hours treatment evidently enhanced the weld fatigue strength, but those obtained after 16 hours did not and instead exhibited a decreasing trend.
Although the mechanical properties and S-N fatigue curve performance were less favorable than those of the original extruded material. However, for applying this type of rare-earth AZ80A alloys to industrials, recommended to use gas tungsten arc welding processes with AC current output should obtain favorable mechanical properties. Regarding post-weld aging treatment, one could apply quenching after 400°C、1 hours solid solution, and followed by aging treatment at 200°C for 8 hours, may effectively enhance weld strength.
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