- 學位論文
摩擦攪拌點銲Mg-Al/Mg-Li異質鎂合金之接合強度與破壞行為探討
A Study on the Welding Strength and Failure Behaviors of Friction Stir Spot Welded Mg-Al/Mg-Li Dissimilar Magnesium Alloys
摘要
摩擦攪拌點銲(FSSW)是以摩擦攪拌銲接(FSW)為基礎所開發出來的固態銲接法，因接合的工作溫度低於熔點，適合應用於鎂、鋁合金等低熔點金屬。鎂合金屬於輕金屬材料，具有高比強度、比剛性、電磁波遮蔽效應與可回收再利用等優點，本研究採用LZ91鎂鋰合金、AZ61、AZ91鎂鋁合金等材料進行異質金屬之摩擦攪拌點銲，並觀察銲接後的微觀組織變化、相組成分析、銲點的硬度變化、拉伸機械性質及破斷面形貌觀察。微觀組織變化顯示LZ91分別與AZ61、AZ91搭接後，材料除了在銲點區有明顯的塑性流動與晶粒細化之現象外，Mg17Al12析出相亦受到攪拌效應影響而分散細化，微硬度量測結果顯示在銲點區之硬度有明顯提升的現象，表示攪拌過後銲點區的組織型態與晶粒大小改變而造成強化。FSSW點銲試片之拉伸機械性質測試結果顯示A9-L9平均抗拉強度為63.7 MPa，然而A6-L9接合之平均抗拉強度皆超過100 MPa，經SEM觀察發現材料破斷從試片上下板接合界面之交界處開始。韋伯分佈函數分析顯示A9-L9、A6-L9異質鎂合金接合皆屬於破壞率遞增型之損耗破壞。
並列摘要
Friction stir spot welding (FSSW) is a kind of solid state welding method developed on the basis of the friction stir welding (FSW) process. The working temperature of FSSW is generally lower than melting temperature of metals, and this process is quite suitable for joining magnesium (Mg) and aluminum (Al) alloys. Light-weight Mg alloys have some advantages, such as high specific strength,high specific stiffness, well recyclability and radiation absorption of electromagnetic waves. The aim of present study is to investigate the microstructure variation, phase composition, microhardness, welding strength and fracture morphologies of FSSW butt-joined AZ91/LZ91 and AZ61/LZ91 dissimilar Mg alloys. Experimental results show that grain size and Mg17Al12 precipitate particles are significantly refined within the welding zone (WZ) due to the intense plastic flow after FSSW joining process. The microhardness with WZ region is obviously higher than the base metals. The increasing microhardness is resulted from the grain refining effect during the FSSW joining process. Tensile testing results show that the average welding strength of A9-L9 joints is about 63.7 MPa. However, the average welding strength of A6-L9 joints is generaly higher than 100 MPa. SEM observations for fracture morphologies show that the fracture of dissimilar joints is occurred from the butt-joining interface. Through the statistical analysis of the Weibull distribution function, FSSW-joined A9-L9 and A6-L9 dissimilar Mg alloys with a wear-out failure model are recognized as reliable joints for further engineering application.