| Summary: | 碩士 === 國立中興大學 === 材料工程學研究所 === 91 === The process of surface remelting for AC8A Al-Si-Ni-Mg alloy by arc remelting method and the characterization of the remelted layer were in-vestigated. The number of voids in remelted region and the uniformity of the layer strongly depend on experimental working parameters such as peak current, pulse duration and pulse-off time, etc. Two insulations kerosene, distilled water were used in this study. Aging treatments were performed, focusing on the improvement of hardness for remelting layer.
The experimental results show that the AC8A alloy which was melted and casted in vacuum contains a little less voids in the remelted layer, as compared to the layer of the alloy being casted in air. Hence, vacuum casting process, which can apparently reduce the hydrogen con-tent in AC8A, does not have significant effect on voids reduction in remelted layer. In this work, by using suitable working parameters such as peak current 21A, pulse duration 400µs, pulse-off time 15µs and main power supply voltage 50V, the number of voids in the remelted layer can be reduced to a much lower limit. Aging treatment performed at 170oC for 10 hours can effectively improve the hardness of remelted layer from Hv 138 to Hv 168, though the hardness of AC8A substrate has only Hv 90. Results of TEM examinations show cell structure is very fine with cell size about 100~200nm. After aging, nano-size precipitates distrib-uting in the interior of cells as well as in the cell boundary were observed. The precipitation which occurs not only in cell but cell boundary causes the hardness of remelted layer increased.
|
|---|
