藉由析出處理提升AZ91D 鎂合金壓鑄件之高溫機械性質

Improving the Elevated Temperature Mechanical Properties of AZ91D Die Castings Mg alloy by Using a Precipitation Treatment

朱育志(Yu-Jhih Ju)

指導教授 ： 楊智富

大同大學/工程學院/材料工程學系所/碩士(2008年)

摘要

本研究主要探討(a)AZ91D壓鑄件高溫軟化之原因、(b)Mg17Al12之高溫析出反應、(c)析出物對AZ91D壓鑄件高溫強度之影響、及(d)藉由時效處理來提升AZ91D壓鑄件之高溫強度。藉由DSC、OM、SEM/EDS及XRD，探討Mg17Al12之析出反應及金相組織觀察，並藉由硬度測試及拉伸試驗探討時效處理對機械性質之影響。研究結果顯示導致AZ91D壓鑄件高溫軟化之原因有兩種：一為AZ91D壓鑄件在鑄造成型時，由於凝固收縮造成內部有殘留應力存在，此強化效果在150℃環境以上之使用時發生退火消除應力造成材料軟化；另一個為AZ91D壓鑄件超過200℃環境下，因Mg17Al12相在此溫度下易粗化/球狀化成顆粒狀，造成過時效軟化。基於此，對AZ91D壓鑄件進行預時效處理，一方面可利用預時效之高溫對壓鑄件先行消除殘留應力，避免日後在高溫使用時發生軟化；另一方面還可藉由高溫時效處理析出Mg17Al12來提升AZ91D壓鑄件在150~180℃之強度。

關鍵字

高溫機械性質

並列摘要

This research aims to investigate the causes for the softening of AZ91D Mg alloy die castings and the aging treatment to improve the elevated temperature strength of the AZ91D die castings. The precipitation of Mg17Al12 intermetallic compound in the AZ91D Mg alloy was examined by using DSC, OM, SEM/EDS and XRD, and the effects of Mg17Al12 precipitation upon the mechanical properties of the Mg alloy was studied. The results showed that there are two causes for the softening of the AZ91D Mg alloy die castings; one due to the occurrence of elevated temperature annealing to soften the tension strengthened Mg die castings by releasing the residue stress, and the other caused by the coarsening of Mg17Al12 precipitates which somewhat like the effect of overaging. This study also concludes that, by applying an aging treatment to the AZ91D Mg alloy die castings, one may achieve the goal to avoid the elevated temperature softening in the alloy at temperatures up to 180℃.

並列關鍵字

Elevated Temperature Mechanical Properties

參考文獻

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