鋁基碳化矽複材之時效硬化與表面防護

Abstract

鋁基碳化矽複材(AMC)為鋁合金發展的重要項目之一，能夠在維持一定延展性的同時大幅提升材料的機械強度，因而廣受多方學者關注，本實驗聚焦於兩個部分，分別在於機械性質與表面抗腐蝕防護。

6061雖然作為析出強化型鋁合金在各種領域皆有其應用，但近年來有許多學者發現在固溶處理後若至於室溫下不立即進行人工時效，會在室溫下產生微小析出物(Cluster)，減少後續人工時效所能提升的硬度，因此本研究之前半聚焦在此，首先利用光學顯微鏡與EPMA了解SiC顆粒大小、在AMC當中的分佈，確認顆粒大小最大不超過5微米並平均分散在基地當中，並透過不同檢測方式，包含維氏硬度試驗、ASTM拉伸試驗、示差掃描熱分析儀(DSC)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)等，以了解6061鋁合金及AMC之析出強化行為，熱處理包含前期自然時效、人工時效以了解6061及AMC兩者之硬化行為，以及後續探討自然時效對於人工時效(T6)以及烤漆製程(短時間人工時效)的負面影響，最後利用70^o C與90^o C兩種不同溫度的預時效減少6061當中的自然時效負面效應；結果顯示自然時效負面效應對於長時間人工時效(T6)影響不大，析出之cluster會在持溫時重新溶回基地並析出成強化相 β''但對於烤漆製程影響較大，而在自然時效前添加預時效能有效減少自然時效對於烤漆製程的負面效應；此外亦發現AMC具和6061不同的析出硬化行為，並提出相對應假說。

實驗後半部分則針對AMC的抗腐蝕性質進行改善，首先確認添加在AMC內部的SiC顆粒對於微弧氧化反應的影響，利用電壓改變的趨勢搭配SEM得到的表面、橫截面形貌以及XPS確立不同反應階段，分別為(i)傳統陽極氧化, (ii)SiC顆粒影響放電行為使電壓上升減緩, (iii)鋁合金表面產生輝光放電與微小電弧, (iv)微弧氧化反應加劇，使表面SiC顆粒熔融混合於膜層中, (v)進入一般微弧氧化平坦區，產生劇烈放電；確立反應機制後測量不同反應佔空比、頻率下的微弧氧化膜層之動電位極化曲線，找到最佳電參數，後續再藉由改變反應時間已找到最佳微弧氧化參數為佔空比0.15、反應頻率100Hz、反應時間15分鐘，能有效降低腐蝕電流密度約兩個數量級；此外也同時對AMC進行不同反應時間之陽極處理以進行比較，在經過SEM橫截面影像及動電位極化曲線測量後，發現陽極處理可以有效提升材料的腐蝕電位，並且能夠減少腐蝕電流密度約三個數量級。

The Aluminum Matrix Composite (AMC) has been a significant topic of interest within the field of aluminum research due to its ability to achieve enhanced strength while maintaining relatively good ductility. This thesis primarily addresses two key aspects of AMC. The first area of focus is the mechanical property, while the second is corrosion resistance.

Although 6061, a type of precipitation-hardening aluminum alloy, has been widely used in numerous fields, some scholars have recently proposed that natural aging may have a negative effect on the alloy if artificial aging is not conducted immediately after solution heat treatment. This could reduce the ability of the 6061 alloy to undergo precipitation hardening. Some scholars have indicated that the phenomenon may be caused by the formation of Si-rich clusters during natural aging. The initial section of the thesis addresses the adverse effects associated with the aforementioned issues. Initially, an optical microscope and an electron probe microanalyzer (EPMA) technique were employed to ascertain the particle size and distribution in AMC. This analysis revealed that the SiC particles exhibited uniform distribution and a particle size that was generally smaller than 5 microns. Subsequently, the precipitation hardening effect, encompassing both natural and artificial aging, was investigated through a series of tests and instrumentation, including Vickers hardness testing, ASTM tensile testing, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Following an understanding of the fundamentals of precipitation hardening, further experiments were conducted to investigate the impact of natural aging on artificial aging and paint baking (a form of short-term artificial aging). The results indicate that the negative effect is not significant in the context of long-term artificial aging, as the Si-rich cluster would remelt into the matrix and form β'' precipitates. However, the effect is more pronounced in the case of paint baking. Lastly, two kinds of pre-aging (at 70°C and 90°C for 30 minutes) were added to the 6061 alloy immediately following solution treatment, prior to the natural aging process. The results demonstrated that this approach can effectively mitigate the negative effects observed. Furthermore, the investigation revealed that AMC undergoes a two-step hardening mechanism during the artificial aging stage. A potential hypothesis for this mechanism was proposed.

The latter portion of the thesis is dedicated to enhancing the corrosion resistance of AMC. In this section, the same MAO (micro-arc oxidation) process is conducted on both the 6061 alloy and AMC. This experiment serves to confirm that SiC particles will indeed exert an influence on the MAO process. By dividing the stages of MAO for AMC based on voltage change during the process and using SEM top view, cross-section morphologies, and XPS, it was proposed that there are five stages in the MAO process for AMC, which differs from that of pure 6061 alloy. The five stages are as follows: (i) Traditional anodization, (ii) Voltage growth slow down owing to SiCp presented in AMC, (iii) Fine arc arise on Al surface, (iv) Arc getting more intense and melting the SiC particles into MAO film, (v) Maintaining high, stable voltage with severe arc hitting the MAO film. Once the mechanism had been elucidated, a potentiostat was employed to conduct potentiodynamic polarization (PDP) on specimens with varying MAO duty ratio and frequency, with the objective of identifying the optimal electro parameter. Subsequently, the process time was modified, resulting in the optimal parameter set of a duty ratio of 0.15 and a frequency of 100 Hz, with a process time of 15 minutes. The MAO film is capable of reducing the corrosion current density by approximately two orders of magnitude when the aforementioned parameters are employed. Furthermore, anodization was conducted with varying process times on AMC. The SEM cross-section images and PDP results demonstrate that anodization effectively increases the corrosion potential of the material and reduces the corrosion current density by approximately three orders of magnitude.