含稀土鎂鋁合金潛變行為與銲接性質之研究

Abstract

本研究是利用富鑭系稀土元素(La-rich Mischmetal，RE)的添加以改善Mg-8Al鎂合金的顯微結構、潛變行為、銲接與腐蝕性質，藉以開發耐高溫的鎂合金。主要研究的合金成份是Mg-8Al-xRE(x代表0，1，2或3 wt.%)，以真空感應熔煉爐(Vacuum induction melting furnace，VIM)配製成含有RE之Mg-8Al-xRE合金鑄錠。合金成份以感應耦合電漿原子發射光譜儀(Inductively coupled plasma-atomic emission spectrometer，ICP-AES)作定量分析。由於鑄造合金中容易生成化學成份偏析、組織不均勻與鑄造缺陷等，因而以間接熱擠型法(Indirect extrusion)擠製成3 mm厚之薄板，試圖藉由擠型減少鑄造缺陷與成份偏析。顯微結構分別以光學顯微鏡(Optical microscope，OM)、掃描式電子顯微鏡(Scanning electron microscope，SEM)及穿透式電子顯微鏡(Transmission electron microscope，TEM)進行分析，相組成則以X-光繞射(X-ray diffraction，XRD)進行分析。潛變測試後的試片以SEM觀察其破斷面及破斷型態。腐蝕試驗則分別以電化學腐蝕法與浸泡試驗法進行評估。銲接性質研究主要是利用工業界常用的惰性氣體鎢極電弧銲接(Tungsten-arc inert gas，TIG)與二氧化碳雷射銲接(CO2 laser beam welding，LBW)兩種方式，施以平面堆積銲(Bead-on-plate)的方式進行銲接。銲後試樣再進行銲道之顯微結構觀察、機械性質測試與腐蝕試驗。 由實驗結果可知，Mg-8Al合金中主要是α-Mg基地及β (Mg17Al12)析出相。當RE添加入Mg-8Al合金時，可形成高溫熱穩定性較佳的Al11RE3介金屬析出相，且同時抑制高溫熱穩定性較差的β相生成，進而改善Mg-8Al鎂合金的高溫抗潛變性質。RE的添加對高溫高荷重(200℃/80或100 MPa)條件下的潛變性質僅有輕微的改善效果；但是，在高溫及中低荷重(200℃/60或40 MPa)條件下，RE的添加即具有較顯著的效果。其中尤以中低溫及中低荷重(175或150℃/60或40 MPa)條件下的抗潛變性提升最為顯著。Mg-8Al-xRE合金之應力指數大小約為2，故可判定其潛變控制機構為晶界滑移(Grain boundary sliding)。Mg-8Al及Mg-8Al-2RE合金之潛變活化能分別為114與104 kJ mol-1。而Al含量對於Mg-Al合金抗潛變性亦有相當影響；以AZ31-1RE與Mg-8Al-1RE合金作比較，則不論於高溫(200℃)或低溫(150℃)下，當施以高荷重(100 MPa)時AZ31-1RE合金之抗潛變性較Mg-8Al-1RE合金差。相反地，當施以低荷重(40 MPa)時，AZ31-1RE合金之抗潛變性則優於Mg-8Al-1RE合金。因而Al含量對低荷重之長時間潛變性質影響甚巨。 由高溫潛變與高溫熱穩定性試驗後的顯微結構觀察；發現Mg-8Al鎂合金經高溫長時間時效後，於晶界附近可產生大量的層狀析出物(β相)，此等析出物即是造成Mg-8Al鎂合金高溫熱穩定性與抗高溫潛變性較差的主因。相反地，含有RE的合金經高溫長時間後，於晶界附近僅有少量的β相形成，且可有效地提升Mg-8Al鎂合金的抗高溫潛變性。故RE的添加可以形成高溫熱穩定性甚佳的Al11RE3化合物，以抑制層狀析出物的大量形成，因而其顯微組織之熱穩定性頗佳，此即改善抗高溫潛變的主因。 實驗研究之TIG熔融銲接的最佳參數為電流110 A與銲接速率7 m/min。由於TIG銲接的能量密度低、熱輸入量高，故Mg-8Al-xRE鎂合金銲後的巨觀組織可明顯區分出銲道(Weld metal)、熱影響區(Heat-affected zone，HAZ)及母材(Parent metal)三個區域。銲道的深寬比(Aspect ratio)較小(約0.5)。銲道顯微組織隨RE含量增加而呈現細化的趨勢，且亦有大量的長針狀(Needle-like)或桿狀(Rod-like)富含RE的化合物形成。銲接的接合率 (Joint efficiency)約85 %，銲接試片的接合強度隨RE含量增加而增高，但是RE含量超過2wt.%時，強度反而下降。銲道的硬度較母材稍低一點，與熱影響區相近。Mg-8Al合金之抗蝕性因RE含量增加而改善。銲道的抗蝕性較擠製狀態的母材差。而添加1wt.%RE的Mg-8Al合金具有最佳的銲後抗蝕性。當RE添加過量時，過多的介金屬化合物會產生嚴重的伽凡尼腐蝕，對銲後的抗蝕性有負面的影響。 雷射銲接的最佳參數則為功率2.0 kW與銲接速率2500 mm/s。由於雷射銲接的高能量密度與低熱輸入量，銲道金屬的冷卻速率極快。因而Mg-8Al-xRE合金銲後的顯微組織皆較TIG銲接為細，並且銲道與母材間並未有存在明顯的熱影響區。因雷射銲接的顯微組織較細，所以硬度亦較TIG銲接為高；但銲道硬度較擠製母材稍低；銲道深寬比較TIG銲接大(約1.5)。

In this study, the effect of RE content on the microstructure, creep behavior, weld and corrosion properties of Mg-8Al-xRE (x=0, 1, 2 or 3 wt.%) alloys were investigated. The alloys were prepared by melting and casting in a vacuum induction melting furnace in an atmosphere of argon gas. Chemical analysis of the alloys was performed by inductively coupled plasma-atomic emission spectrometer (ICP-AES). To avoid the possible casting defects, such as micro-pore or micro-segregation, the cast ingots were indirect hot extrusion to remove the cast-defects. The microstructural analysis and phase characterizations of alloys were performed by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). The fracture surfaces of crept specimens were examined by SEM. Corrosion tests were carried out by potentiodynamic polarization and immersion tests. The study of weld property was accomplished by tungsten-arc inert gas and CO2 laser beam welding. The microstructure analysis, mechanical properties, and corrosion properties of welded specimens were investigated. The microstructure of Mg-8Al alloy with 1-3 wt.% RE additions were conducted: (i) The as-extruded Mg-8Al-xRE alloys consisted of α-Mg matrix, with β (Mg17Al12) and Al11RE3 compounds. (ii) Raising the extent of RE in the alloy also increases the amount and coarsening of the Al11RE3 compounds, but the amount of β phase diminishes and turns into the fine particles. The creep rupture life increment measured at 150℃ is around 40-100 MPa, and the creep rupture life over 150℃ is also prolonged. The marked improvement of the high-temperature tensile creep properties is attributed to the fine rod-like Al11RE3 compound having high thermal stability in the alloys. The stress exponent of the Mg-8Al-xRE alloys is approximately 2, which suggested the creep mechanism of the alloys is controlled by the grain boundary sliding. The creep activation energy of Mg-8Al and Mg-8Al-2RE alloys are 114 and 104 kJ mol-1, respectively. In addition, the aluminum content also has great effects on the creep property of the Mg-Al alloy; when comparing the AZ31-1RE with the Mg-8Al-1RE alloys. The creep resistance of the AZ31-1RE is inferior to that of the Mg-8Al-1RE, as the applied load is high; at either higher or lower test temperature. On the contrary, the creep resistance of the AZ31-1RE is superior to that of the Mg-8Al-1RE, as the applied load is low at any test temperature. Therefore, the effect of aluminum content on the creep properties of Mg-Al alloy is great. The close-grain boundary microstructure of pose-crept Mg-8Al shows that high volume fraction of lamellar β precipitates close-grain boundaries during the creeping. Since the lamellar precipitation occurring during the creeping may effectively multiply the grain boundary area available for easy deformation by grain boundary sliding through the elevated temperature creep. Therefore the creep resistance of Mg-8Al is poor at elevated temperature. Oppositely, no significant change in the microstructure morphology after creep exposure has been observed in the RE-containing alloys. The addition of RE to the Mg-8Al alloy forms the Al11RE3 intermetallic phase, which may suppress the precipitation of lamellar β phase during the creep test. Consequently, the sliding of grain boundaries and the slip of dislocations in the matrix are effectively prevented at elevated temperature, improving the creep resistance of Mg-8Al base alloy. The optimization welding parameters of TIG welding are current 110 A and welding speed 7 m/min. Because the TIG welding has low energy density and high heat input characteristics, the macrostructure of welded Mg-8Al-xRE alloys possess three clearly distinguished regions, including weld metal, heat-affected zone, and parent metal. The aspect ratio of the weld pool is low, about 0.5. Microstructure of weld metal is refined with the increased RE content; and a lot of needle-like or rod-like RE-containing compounds precipitated. The join efficiency is approximate 85%, and the bonding strength is increased with the increasing RE content. However, when the RE content over 2 wt.%, the strength is decreased. The hardness of weld metal is lower than the parent metal, and close to the heat-affected zone. Corrosion rate of the alloy may slightly decrease with the increase of the added RE contents. The corrosion resistance of welded specimens is poor, comparing with the as-extruded alloys. The optimized welding parameters of CO2 laser beam welding are power 2.0 kW and welding speed 2,500 mm/s. The CO2 laser beam welding has high energy density and low heat input characteristics, so the cooling rate of the weld metal is high. The microstructure of the welds is finer than that of TIG welding; and no obviously heat-affected zone is observed, which is due to the laser welded alloys owns fine structure, thus the hardness is higher than that of the TIG welded alloy. In this study, the hardness of the welded metal is slightly lower than that of the parent metal. The aspect ratio of the laser welds is higher than that of the TIG welding, which is about 1.5.