【摘要】：鎂合金密度較小,只有1.7 g/cm3。因此,它作為最輕的金屬之一,在航空航天以及汽車、電子等對(duì)材料輕量化要求較高的領(lǐng)域具有日趨廣泛的應(yīng)用。然而,耐腐蝕性差作為瓶頸一直制約其發(fā)展以及應(yīng)用。目前,為了提高鎂合金耐腐蝕性能,設(shè)計(jì)新型鎂合金以及表面處理是近年來(lái)采用的主要手段。添加了適量鋁的鎂合金在壓鑄性、疲勞性能、壓縮性和強(qiáng)度等方面性能獲得提高。然而鋁元素作為鎂合金中主要的合金元素,其對(duì)鎂合金及其等離子體電解氧化(PEO)膜的顯微結(jié)構(gòu)及耐蝕性能影響的相關(guān)研究很少。本論文系統(tǒng)地研究了鋁含量對(duì)鎂合金及其PEO膜顯微結(jié)構(gòu)及耐蝕性能的影響,主要內(nèi)容如下:(一)研究了共晶點(diǎn)附近的三種鎂合金AZ91,AZ111和AZ141(亞共晶和過(guò)共晶結(jié)構(gòu))在3.5wt.%NaCl水溶液中的腐蝕行為。顯微結(jié)構(gòu)分析表明,鋁含量影響了β相和共晶相(α+β)的含量和分布。不同鋁含量的鎂合金顯微結(jié)構(gòu)對(duì)耐蝕性能影響表現(xiàn)為:1、共晶相的增加促進(jìn)了α基相的腐蝕。2、β相的含量和分布是影響鎂合金耐蝕性能的主要因素,其含量影響腐蝕程度,其分布影響腐蝕進(jìn)程;隨著鋁含量的增加,β相的體積分?jǐn)?shù)增大,電偶腐蝕加速效應(yīng)起主導(dǎo)作用。在腐蝕過(guò)程中,β相的網(wǎng)狀分布,表現(xiàn)出一定的腐蝕阻擋作用;但是由于β相分布的不連續(xù)性,腐蝕總會(huì)沿著β相分布不連續(xù)處向α相繼續(xù)腐蝕。(二)以不同鋁含量的鎂合金為基體,進(jìn)行PEO膜層制備,并系統(tǒng)地研究了基體的顯微結(jié)構(gòu)、膜層的顯微結(jié)構(gòu)、膜層的耐蝕性能三者之間的關(guān)系。結(jié)果表明,在PEO過(guò)程中,基體的顯微結(jié)構(gòu)對(duì)其膜層顯微結(jié)構(gòu)影響顯著,主要原因?yàn)榛w中的鋁含量影響了β相及共晶相(α+β)的含量和分布,不同相的含量以及分布對(duì)PEO放電過(guò)程影響顯著,從而影響了膜層的顯微結(jié)構(gòu)包括致密內(nèi)層和疏松外層的成分、含量以及膜層的孔隙率。腐蝕發(fā)生在膜層顯微結(jié)構(gòu)缺陷的位置。(三)前面的研究結(jié)果表明,在兩相合金中,不同相的成分差異會(huì)導(dǎo)致合金表面電化學(xué)性質(zhì)不同,在PEO過(guò)程中放電不穩(wěn)定,導(dǎo)致介質(zhì)阻擋層的顯微結(jié)構(gòu)缺陷。而介質(zhì)阻擋層的形成是PEO過(guò)程中擊穿放電進(jìn)行的必要條件,其質(zhì)量的好壞對(duì)放電的質(zhì)量有直接影響,影響PEO過(guò)程中等離子體放電的穩(wěn)定性及后續(xù)成膜能力。因此,考慮采用前處理的方法使鎂合金的表面活性均勻。采用HF超聲活化AZ91鎂合金,在鎂合金表面形成一層低溶解性的薄膜,使鎂合金表面活性均勻一致。結(jié)果表明表面狀態(tài)均勻的鎂合金形成的致密層質(zhì)量好,在PEO過(guò)程中放電穩(wěn)定,后續(xù)成膜質(zhì)量好:致密,均勻,沒(méi)有顯微結(jié)構(gòu)缺陷。因此耐蝕性能顯著提高。
[Abstract]:The density of magnesium alloy is small, only 1.7 g / cm ~ 3. Therefore, as one of the lightest metals, it has been widely used in the fields of aerospace, automobile, electronics and so on. However, poor corrosion resistance as a bottleneck has been restricting its development and application. At present, in order to improve the corrosion resistance of magnesium alloys, the design of new magnesium alloys and surface treatment is the main means used in recent years. The properties of magnesium alloy with proper amount of aluminum in die casting, fatigue, compressibility and strength were improved. However, as the main alloy elements in magnesium alloys, the effect of aluminum on the microstructure and corrosion resistance of magnesium alloys and their plasma electrolytic oxidation (PEO) films is seldom studied. The effect of aluminum content on microstructure and corrosion resistance of magnesium alloy and its PEO film was studied systematically in this paper. The main contents are as follows: (1) the corrosion behavior of three kinds of magnesium alloys AZ91, AZ111 and AZ141 (hypoeutectic and hypereutectic structure) near eutectic point in 3.5wt.%NaCl aqueous solution has been studied. Microstructure analysis shows that aluminum content affects the content and distribution of 尾 phase and eutectic phase (偽 尾). The effect of microstructure of magnesium alloy with different aluminum content on corrosion resistance is as follows: 1. The increase of eutectic phase promotes the corrosion of 偽 -base phase. The content and distribution of 尾 phase are the main factors affecting the corrosion resistance of magnesium alloy, and the content of 尾 phase affects the corrosion degree of magnesium alloy. With the increase of aluminum content, the volume fraction of 尾 phase increases, and the accelerated effect of galvanic corrosion plays a leading role. In the process of corrosion, the net distribution of 尾 phase shows a certain corrosion blocking effect, but because of the discontinuity of 尾 phase distribution, the corrosion will continue to corrode the 偽 phase along the discontinuous distribution of 尾 phase. (2) the PEO film was prepared on magnesium alloy with different aluminum content, and the relationship between the microstructure of the substrate and the corrosion resistance of the film was systematically studied. The results show that in the PEO process, the microstructure of the substrate has a significant effect on the microstructure of the film. The main reason is that the content of aluminum in the matrix affects the content and distribution of 尾 phase and eutectic phase (偽 尾). The content and distribution of different phases have a significant effect on the discharge process of PEO, thus affecting the microstructure of the film, including the composition of the dense inner layer and the loose outer layer, the content of the film and the porosity of the film. Corrosion occurs at the location of the microstructural defects in the film. (3) the results of the previous study show that in the two-phase alloy, the different phase composition will lead to different electrochemical properties of the alloy surface, discharge instability in the PEO process, leading to the microstructure defects of the dielectric barrier layer. The formation of dielectric barrier layer is a necessary condition for breakdown discharge in PEO process. The quality of dielectric barrier layer has a direct impact on the quality of discharge. It affects the stability of plasma discharge and the ability of subsequent film formation in the process of PEO. Therefore, the pretreatment method is considered to make the surface activity of magnesium alloy uniform. A low solubility film was formed on the surface of AZ91 magnesium alloy by HF ultrasonic activation, which made the surface activity of magnesium alloy uniform. The results show that the dense layer formed by magnesium alloy with uniform surface state is of good quality, the discharge is stable in PEO process, and the quality of subsequent film formation is good: dense, uniform and without microstructure defects. Therefore, the corrosion resistance is improved significantly.
【學(xué)位授予單位】：中國(guó)科學(xué)技術(shù)大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：TG174.4

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本文編號(hào)：2151076