| 摘要: | 摘 要 本研究針對鎂合金材料於生產過程中所可能面臨之關鍵技術來加以研究及探討。研究主要可分為三部分，包括：第一部分為鎂合金材料於不同氣氛下之燃燒行為的探討，藉以瞭解鎂材料之燃燒行為與其防制之道；第二部分為鎂合金材料之鑽削加工參數的最佳化，除避免因加工而導致燃燒災害外，亦可確保加工品質及避免機械加工後所導致的機械性質之下降；第三部分為鎂合金材料之非鉻系表面化成處理的開發與研究，開發環保型之非鉻系表面防蝕化成處理法，使表面化成處理法除可以達到防蝕效果外，亦可兼顧環保及處理後工件之機械性質。以下針對各主題再加以說明： 第一部份：實驗中以瓦斯火炬加熱鎂合金AZ61A之塊狀材料，並加以觀察及記錄其燃燒反應過程。鎂合金塊於加熱初期生成Mg(OH)2於試塊表面，因此可以保護試塊並避免其氧化。隨著溫度上升及試塊的熔化，Mg(OH)2 -> MgO + H2O及2Mg + CO2 -> 2MgO + C 等反應產生。隨著鎂熔液的溢出，反應產物 MgO 逐漸生成，並伴隨釋出大量的熱量。當溫度足夠高時，逆反應 MgO + C -> Mg + CO 產生，消耗了MgO、碳層及部分先前所釋出之熱焓。而所產生的Mg則繼續與外界氣氛反應生成MgO，而形成連續且循環之氧化燃燒反應。最後之燃燒產物為燃燒殆盡之鎂試塊所轉化成的氧化鎂及些許為反應完之碳。 在固定容積的氣氛下時，利用實驗與對照組所獲得之溫度差可以得知鎂合金燃燒放熱所釋出之熱焓量。實驗中所使用的氣氛種類包括：氬氣（Ar）、空氣及不同比例之二氧化碳/氬氣（CO2/Ar）混和氣氛。根據實驗的結果可推論出每材料於不同氣氛下的燃燒機制與放熱特性。而可能進行之相關化學反應亦於研究中加以整理匯集並討論。 第二部份：對鎂合金材料來說，加工參數的選擇及進行加工時必須避免引起鎂切削屑的燃燒與破壞加工後之表面狀態與被加工件的機械性質。本研究中，首先針對鑽削參數，如：鑽頭刃口角角度、加工速度等參數對加工後表面品質及切削抵抗力的影響。由結果可知，表面粗糙度與切削抵抗力隨著偏離最佳值越遠而增加，且隨著材料移除速率的提高而增加。實驗中並探討加工過程的壓應力對被加工材料AZ61A微結構之影響。結果顯示，當鑽頭之刃口角角度由標準的118度降至55度時，被加工件可獲得最佳之表面品質及最小的變形雙晶層（Deformation twinning）產生。此外，若配合使用5％氫氧化鈉水溶液（5% NaOH solution）來當作鑽削加工時的介質時，被加工件的表面粗糙度及微結構型態可再進一步地被改善。 第三部分：本研究主要為開發鎂及鎂合金材料所適用之非鉻系防蝕化成處理，利用過錳酸鹽及磷酸氫鹽溶液來作為鎂合金材料的表面化成溶液。研究中利用SEM來觀察化成後之表面層型態；並且利用XRD及XPS-ESCA來探討非鉻系化成皮膜之組成。XRD的結果顯示，該化成皮膜層為非晶質之結構組成，並且化成處理前、後，鎂材料中的b相（Mg17Al12）之繞射強度有些許改變。而XPS-ESCA的結果顯示，AZ系列之鎂合金材料中之非鉻系化成皮膜存在有MgO、Mg(OH)2、MgAl2O4、Al2O3、Al(OH)3、MnO2或Mn2O3、多氧氫氧化物（oxy-hydroxides）及磷酸物等化合物；而純鎂材料的非鉻系化成皮膜則存在有MgO、 Mg(OH)2、MnO2、多氧氫氧化物（oxy-hydroxides）、磷酸鹽及 等化合物。根據電化學極位測試（Polarization test）之結果顯示，AZ系列之鎂合金材料經過非鉻系過錳酸鹽及磷酸氫鹽溶液之化成處理後可以具有與常用之鉻系（JIS H 8651 MX-1）之化成處理法相當或略高之防蝕能力。 Abstract This study aimed at investigating some important issues when dealing with the material, magnesium and its alloys, which are the most popular light metal applying to many common purposes. The study could be divided into few main subjects, they are combustion behaviors, machining and improving corrosion resistance of magnesium and its alloys. Regarding to the knowing of combustion behavior of magnesium materials under different atmospheres, it could help preventing the fire hazard and developing the protective atmosphere when melting. The optimizing of machining conditions, not only help preventing fire hazard from ignition of chips but also improving the properties of magnesium work-pieces after machining. Non-chrome conversion treatment is a substitute of dichromic treatment. It is expecting to provide less environmental impacts and maintain good corrosion resistance as well. The individual briefs are as following: Part I: AZ61A cakes were heated by the gas torch and combusted. The progressive development of heated cake was recorded and described. The product of Mg(OH)2 covers on the surface of heated AZ61A cake and protects it from oxidation. Reactions of Mg(OH)2 → MgO + H2O occurred during heating by flame and after AZ61A melted. Molten metal continuously oozed out and produced MgO accompanied with great amount of heat to react with carbon film (2Mg + CO2→ 2MgO + C). This led to reaction of MgO + C → Mg + CO and the product of Mg carried out oxidation and formed MgO. The residual showed that all AZ61A completely burned out and became a mess of white powder. The specimens of AZ61A were also set on the specimen holder in the designed containers filled with several atmosphere gases (Ar, air, CO2 and different ratios of CO2/Ar). The responded temperatures from specimen holders were recorded and analyzed. Mechanisms for illustrating the reaction of Mg with different gases were schematically illustrated. Possible reactions of Mg with O2, CO2 and CO were also discussed in this study. Part II: For magnesium alloys, optimizing the machining conditions is necessary to prevent ignition of chips. In this study, effects of point angles of drill bits and drilling parameters on surface roughness and cutting resistance forces were measured and studied. Surface roughness and cutting resistance forces are increased following the increase of point angle and material removal rate. Point angle (2p) descends from 118° to 55° producing the smoothest machined surface and minimum variance in the measured roughness. In addition, effect of drilling operation on varying microstructure of AZ61A was also investigated in this study. The drilled sample showed a minimum extent of deformation twinning layer, when the drill bit adopted a point angle of 55°. The drilled sample developed a superior surface roughness and a short extent of twinning layer generated on the matrix of machined sample, if a 5% NaOH solution was used as lubricant and a 55° point angle was used. Part III: A chrome-free conversion coating treatment, mainly permanganate phosphate solution, for magnesium and its alloys was developed in this study. The morphology of conversion-coated layer was observed by using SEM; the crystal structures and compositions was analyzed and determined by using XRD and XPS. The XRD results indicate an existence of amorphous structure on the coated specimen and only show a greater intensity of Mg17Al12 than as received sample. The XPS results show that the coated layer includes products of MgO, Mg(OH)2, MgAl2O4, Al2O3, Al(OH)3, MnO2 or Mn2O3 and amorphous oxy-hydroxides for Mg-Al-Zn alloys but MgO, Mg(OH)2, MnO2, amorphous oxy-hydroxides and MgMn2O8 for pure Mg. The electrochemical polarization test results demonstrate that the presented conversion treatment for series of AZ alloys develop an equivalent capability in corrosion potential to JIS H 8651 MX-1 (similar to Dow NO. 1), chrome-based method. |
