【作者】 马凯；

【导师】 孙大千；

【作者基本信息】 吉林大学 ， 材料加工工程， 2010， 博士

【摘要】 镁合金是工程结构应用中最轻的金属材料之一,具有比强度和比刚度高、导热性与导电性好、无磁性与电磁屏蔽特性、良好的阻尼特性和切割加工性等优点,在航空航天、汽车制造、电子电器、国防科技等领域都有广泛的应用,是继钢铁材料和铝合金材料之后发展起来的第三类金属结构材料。但是,镁合金的电极电位较低,化学性质活泼,在空气中形成的氧化膜疏松多孔,耐腐蚀性能和耐磨损性能较差,在一定程度上限制了其更广泛的应用。为提高镁合金的综合性能,特别是表面性能,扩大其应用范围,本文首先采用电弧喷涂技术,在AZ91D镁合金表面制备纯Al涂层。涂层组织均匀,晶粒细小,部分晶粒尺寸已达到纳米级。涂层与基材之间存在界面,界面结合良好,发生原子之间的相互扩散,有新组织Mg17Al12生成。涂层中几乎没有氧化物或杂质颗粒,但存在一定量光滑近圆形的孔隙。涂层断口中有大量塑性韧窝状断裂。喷涂电压和喷涂气压两个参数是影响涂层结合强度以及致密性的重要因素。本试验条件下,喷涂电压为30V,喷涂气压为0.6MPa时,涂层的致密性最好,结合强度最高,孔隙率为5.18%,结合强度为12.84MPa。盐雾腐蚀试验表明,涂层及其封孔涂层的耐腐蚀性能明显优于镁合金基材。采用等离子粉末喷涂技术在AZ31镁合金表面制备纯Al涂层和Al/Al₂O_3p复合材料涂层。用不同配比的复合粉末（Al+Al₂O₃）喷涂涂层,观察各涂层表面3D形貌、SEM形貌,进行XRD分析和划痕试验,优化出的最佳复合粉末配比为:Al₂O₃的质量分数20wt%,Al的质量分数为80wt%。设计六因素五水平正交实验表,用极差分析方法确定的Al/Al₂O_3p复合材料涂层最佳工艺参数为:喷涂电流700A、喷涂距离90mm、主气压力0.4827MPa、次气压力0.2069MPa、喷涂速度400mm/s、送粉量30.3g/min。Al/Al₂O_3p复合材料涂层成型良好,涂层没有明显的层状组织产生,Al₂O₃颗粒增强相均匀弥散地镶嵌在涂层的Al基体中,二者之间的相容性和润湿性较好。涂层与基材之间存在着界面,界面两侧互有一定程度的扩散,并产生AlMg和Al12Mg17两种新组织。涂层呈现韧性断口形貌,平均孔隙率仅为3.80%。Al₂O₃颗粒在等离子焰流中的位置不同,熔化程度不同,冷却凝固后的形状也不相同,有的呈圆形和椭圆形,有的呈多棱角形。等离子弧热源的平均温度高,可使Al₂O₃颗粒熔化充分,基材获得的热量充足。熔融粒子在等离子焰流中的飞行速度比普通电弧高,撞击基材时具有的动能和撞击力大,可使扁平粒子层与基材之间以及层与层之间结合得更加紧密,片层被压得更薄更扁,孔隙很少。处在Ar气氛围中的等离子喷涂比普通电弧喷涂的保护条件好,熔融粒子和涂层不容易被氧化。一般等离子喷涂涂层的氧增量在0.1-1%左右,而普通电弧喷涂涂层的氧增量达到0.5-3%左右。而氧增量高,涂层的氧化物杂质含量就会增多,导致涂层孔隙增多。Al/Al₂O_3p复合材料涂层与基材的平均结合强度为16.04MPa。涂层显微硬度由涂层表面至基材呈逐渐增大趋势,基本分布在HV45-HV55之间。通过快速磨损和环块磨损对比试验可以看出,Al/Al₂O_3p复合材料涂层的磨损体积和磨损质量失重量比纯Al涂层和基材都要小很多,而摩擦系数却最大。Al₂O₃颗粒增强相减小和阻碍了涂层中Al基体的塑性变形,改变了摩擦副与基材之间的接触特性,充分显示出Al₂O₃颗粒增强相在涂层中的强化效果。从极化曲线的测试结果可以看出,电弧喷涂纯Al涂层、等离子喷涂纯Al涂层及Al/Al₂O_3p复合材料涂层的电极电位都比镁合金低,而腐蚀电流又都比镁合金高,他们都可作为阳极与镁合金基材形成原电池,对其起到牺牲阳极的阴极保护作用。Al表面容易形成致密的Al₂O₃膜层,可阻止腐蚀介质浸入基材内部,但溶液中的Cl-离子穿透力强,容易穿透Al₂O₃膜层,破坏Al的钝化状态。同时,Cl-离子还容易吸附在涂层表面,取代Al₂O₃膜层中的O而破坏涂层。溶液中的O还能够引起金属表面的去极化过程,加速阳极溶解,使渗入涂层缺陷里的盐溶液体积膨胀,引起应力腐蚀。Al/Al₂O_3p复合材料涂层与其热处理涂层的耐腐蚀性能差别很小,在盐雾腐蚀试验环境中的耐腐蚀性能优于其在浸泡腐蚀试验环境中的耐腐蚀性能。在盐雾腐蚀试验中,NaCl溶液沉降量比较小,腐蚀主要发生在涂层表层,其腐蚀行为主要是点蚀。在浸泡腐蚀试验中,涂层被完全浸泡在流动的NaCl溶液中,涂层产生的腐蚀不仅是表面的点蚀,更主要的是发生在Al与Mg接触面之间的电偶腐蚀,导致涂层的腐蚀失重量远大于基材和封孔处理的Al/Al₂O_3p复合材料涂层。封孔涂层在浸泡腐蚀试验和中性盐雾腐蚀试验中,腐蚀失重量都很小,几乎没有被腐蚀,对镁合金基材有很好的保护作用,大幅提高了镁合金的耐腐蚀性能。基于Al、Al₂O₃涂层的特点,Al₂O₃颗粒增强Al基复合材料(Al/Al₂O_3p)涂层是改善镁合金耐腐蚀性能和耐磨损性能更为有利的选择,本文的研究工作为改善镁合金的耐腐蚀性能和耐磨损性能提供了必要的理论依据。更多还原

【Abstract】 Magnesium alloy is one of the lightest metals on the earth, which has the characteristics of high specific strength, high specific stiffness, excellent thermal conductivity, superior electrical conductivity, non-magnetic, electromagnetic shielding, good damping and excellent cutting, and so on. As the third type of metallic structural material except for the steels and aluminum alloys, magnesium alloy is widely used in industries such as aviation and aircraft, automobile manufacturing, electronics, national defense technology, and etc. due to these superior properties. However, the applications of magnesium alloy is restricted by its low electrode potential and active chemical properties, forming oxide film with porous structure in the air, the poor corrosion resistance and wear resistance.In this paper, aluminum coating prepared on the surface of the magnesium alloy AZ91D by wired arc spray was investigated firstly to improve the comprehensive properties of magnesium alloy, especially surface properties, and expand its application scope. Homogeneous microstructure and fine grains of the coating were gained. Part of the grain size has reached the nanometer level. The bonding of interface between coating and Mg alloy substrate was excellent. The study showed that there were new phases Mg17Al12 evolved due to the interdiffusion of atoms. There were no oxides and impurities in the coating except for some smooth and nearly circular pores. A large number of plastic and dimple-like fractures were observed. In this experiment, arc voltage and spray gas pressure were two important factors which have obvious effects on the bonding strength of Mg alloy substrate and coatings. The compactness of the coating and bonding strength were satisfied under spray voltage of 30V, spray gas pressure of 0.6MPa, which were 5.18% and 12.84MPa, respectively. The coating without sealing agent and the coating with sealing agent had better corrosion resistance than that of the Mg alloy substrate.Pure Al coating and Al/Al₂O_3p composite coating were prepared on the surface of Mg alloy AZ31 by plasma powder spraying. The optimum powder mixing ratio, obtained by observing surface 3D morphology, SEM morphology, XRD analysis and scratch test of the different ratio mixed sprayed coatings, was 80wt.% Al and 20wt.% Al₂O₃, respectively. Based on the orthogonal experiment of six factors and five levels, the proper parameters of arc spray for preparation of the Al/Al₂O_3p coating were spray current of 700A, spray distance of 90mm, main gas pressure of 0.4827MPa, secondary gas pressure of 0.2069Mpa, spray speed of 400mm/s and powder feed rate of 30.3g/min.The formability of the Al/Al₂O_3p coating was good and there was no obvious lamellar structure in the composite coating. As reinforced phase, Al/Al₂O_3p was uniformly dispersed in the coatings. The matrix Al and reinforced phase Al₂O₃ showed good compatibility and wettability. The interface between the coating and the substrate could be easily observed and the new phases AlMg and Al12Mg17 were found nearby the interface, which indicated that the diffusion of atoms occurred. The mean porosity of the coating was only about 3.80% and plastic fracture surface could be observed. The study revealed that the particles of Al₂O₃ exhibited different shape in the coating such as round, oval or multi-angular shape, which resulted from different melting degree of the Al₂O₃ in the plasma.Due to high thermal density of the plasma, the Al₂O₃ was melted fully and plenty of thermal was gained for the substrate. Flight speed of molten particles in plasma flame was faster than that in arc spray. So greater kinetic energy and percussive force were obtained when molten particles striking the substrate. At the same time, the bonding was more closely between the plat particle layer and substrate, layer and layer, and lamella was getting thinner and flatter with lower porosity. The oxides in the coatings were decreased greatly resulting from the better protection of the inert gas Ar. Generally, the oxygen increase was about 0.1-1% for plasma spray process and 0.5-3% for arc spray, respectively. The high oxygen increase meaned the high oxides content in the coating, which resulted in high porosity.The mean bonding strength of the Al/Al₂O_3p coating and Mg alloy substrate was 16.04MPa. The microhardness increased gradually from the coating surface to the substrate, ranging from HV45-HV55. Based on the rapid wear test and ring block wear test, wear volume and weight loss of the Al/Al₂O_3p coating were lower than those of the Al coating and AZ31 alloy but with highest friction coefficient. This could be explained by that the reinforced phase Al₂O₃ restricted the plastic deformation of Al and changed the interface state. This was also contributed to the effect of the reinforced phase.From the Tafel curves of the pure Al coatings prepared with arc spray and plasma spray, the composite coatings of Al/Al₂O_3p, the electrode potentials of three kinds of coatings were lower than that of the substrate but with higher corrosion currents. The three kinds of coatings mentioned above could be regarded as anodes of the primary battery and aimed to sacrifice the anodes to protect the Mg alloy substrate, which was cathode in the primary battery. Though Al₂O₃ oxides film formed easily on the Al surface, which can protect the substrate from attack of the corrosive media, the Cl- could penetrate the film and attack the Al substrate. Further, the Cl- can adsorb onto the coating surface and substitute O of the Al₂O₃, thus destroy the coating. Meanwhile, the atom O in the solution could cause the depolarization process of metal surface, accelerate anode to dissolve, make volume expansion for infiltration coating salt solution and led to stress corrosion.The corrosion resistance of the composite coating Al/ Al₂O_3p before and after heat treatment showed no obvious difference, which was better in the salt spray corrosion test than that in the soak corrosion test. During the salt spray corrosion test, NaCl hardly settled down in the solution. The corrosion mainly occurred at the surface of the coating, which belonged to the pit corrosion. During the soaking corrosion test, the coating was surrounded by the NaCl solution. The main corrosion, galvanic corrosion, occurred at the interface of the Al and Mg alloy except for the pit corrosion at the surface. In this case, the weight loss due to the corrosion was larger than those of the Mg alloy substrate and the composite coating Al/Al₂O_3p with sealing treatment. The weight loss of the composite coating Al/Al₂O_3p with sealing treatment was very small in either soak corrosion test or salt spray corrosion test, which indicated the coating provided good protection to the Mg alloy substrate and greatly improved performance of the Mg alloy in corrosive environment.According to the characteristics of Al and Al₂O₃ coating, the composite coating Al/Al₂O_3p, based on the Al substrate reinfored with Al₂O₃ particles, became the better choice to increase the wear resistance and corrosion resistance of the coating. The invetigation in this work offered some theoretical foundation for improving the wear resistance and corrosion resistance of the Mg alloy coating.更多还原