【作者】 李伟；

【导师】 陈传忠；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 由于具有比强度高、耐热、耐腐蚀以及良好的低温性能等优点,钛及钛合金被广泛应用于航空航天、石油化工、机械、冶金、医疗等领域。但是钛合金硬度低、耐磨性差,不宜用于制造机械产品中的传动件,限制了钛合金更广泛的应用。因此,如何提高钛合金的表面硬度和耐磨性能,扩大其应用范围,引起了国内外材料领域研究人员的广泛关注。激光合金化技术因为处理效率高、工件变形小、制备出的涂层可与基体达到冶金结合,已成为钛合金表面强化与改性领域的研究热点之一。本课题采用激光合金化技术对在工业领域中应用最广泛的Ti-6A1-4V钛合金进行表面强化,在开放的氮气环境下采用B4C和石墨粉实现B-C-N多元复合激光合金化,制备出以TiC、TiN、TiB、TiB2、Ti(CN)等陶瓷硬质相为主要强化相,以TiAl、Ti3Al和Al3Ti等金属间化合物为辅助强化相的高硬度、耐磨陶瓷复合涂层。课题将粉末合金化与气体合金化相结合,通过对涂层材料的设计和制备工艺的调整来控制涂层中陶瓷相的种类、含量和分布特征,减轻涂层的脆化倾向；通过对涂层的化学成分、物相结构、微观组织、硬度及耐磨性的研究,揭示出激光合金化过程中陶瓷相的原位形成机制、合金化层的界面结构以及涂层的强化机制,阐明Ti粉和稀土氧化物Y203在基体相的凝固过程和陶瓷相的形成过程中的作用机制,揭示涂层成分-制备工艺-微观组织-性能之间的内在规律。研究表明,采用B4C陶瓷粉与石墨粉,在开放的氮气环境下对Ti-6A1-4V钛合金表面进行多元复合激光合金化,能够获得与基体具有牢固冶金结合的高硬度复合陶瓷涂层；涂层中含有TiC、TiN、TiB、TiB2、Ti(CN)和Ti3Al等多种原位生成的硬质强化相,这些强化相在熔池凝固过程中相间生长、相互牵制,有利于获得组织细小、致密的激光合金化层；涂层粉末成分、氮气压力及激光工艺参数均会影响合金化层的表面宏观质量和微观组织,本课题试验条件下,氮气压力值为0.4MPa,B4C与石墨粉的配比为2：1和1：1,激光比能E范围为3.8-5.7kJ-cm-2时,能够获得宏观表面平整,微观组织致密,无明显气孔及裂纹的合金化层。将Ti粉引入合金化粉末中,有利于合金化过程中熔池中原位反应的发生,生成更多TiC、TiN、TiB、TiB2、Ti(CN)等陶瓷强化相,使得合金化层硬度及耐磨性得到提高。但是当Ti含量过高时,过量的Ti残留在合金化层中,不利于合金化层整体硬度和耐磨性能的提高。研究表明,当Ti的添加量为30wt.%时,获得的合金化层硬度最高、耐磨性最好。稀土氧化物Y203对合金化层的微观组织具有明显细化的作用,激光合金化过程中,部分未熔化Y203可以作为凝固过程中的异质形核核心,部分Y203会分解为Y与02。Y作为表面活性元素,容易吸附在晶界或相界,阻碍晶界移动；Y还可以减小液态金属的表面张力和临界形核半径,提高形核率,有利于细化合金化层的组织。但是,过多的Y203会增加合金化层的脆性,不利于合金化层耐磨性能的提高。研究表明,添加1wt.%Y2O3时,可以获得耐磨性能最好的激光合金化层。在合金化材料中同时引入适量的Ti粉和Y203,在适宜的激光工艺参数下,可以制备出组织细小致密、硬度高、耐磨性优异的合金化层。研究表明,同时添加30wt.%Ti粉与1wt.%Y2O3的合金化层硬度约为Ti-6Al-4V合金基体硬度的4倍,并表现出较好的耐磨性。本课题利用激光合金化技术在Ti-6Al-4V合金表面制备出多元强化的复合陶瓷涂层,揭示了合金化过程中陶瓷相的原位形成机制、合金化层与基体相的界面结构,阐明了金属Ti粉与稀土氧化物Y203在合金化层中的作用机理,为激光合金化技术在钛合金机械传动件制备领域的应用提供理论依据。本课题研究可应用于航空航天、石油化工、汽车机械、海洋工业等领域钛合金零部件的制造及修复,延长零件的使用寿命,具有广阔的应用前景,一经应用,将会产生巨大的经济效益和社会效益。更多还原

【Abstract】 Titanium and its alloys are extensively used in aerospace, metallurgy, mechanical and petrochemical industries owing to their special properties such as high strength, excellent corrosion and high-temperature resistance, perfect low-temperature property, etc.. Nevertheless, the application of titanium alloys under severe wear and friction conditions is highly restricted due to their poor tribological properties such as high friction coefficient and low hardness. Titanium and its alloys have been the focus materials of governments in the world. Therefore, how to improve the hardness and wear resistance of titanium alloys has caused extensive concern of the material research field. Laser alloying technique has become one of the focus studies in the surface strengthening and modification field of titanium alloys due to its high efficiency, few deformation, the metallurgical bonding between substrate and the alloying coating, etc..In this paper, laser alloying technique was carried out for surface strengthening of Ti-6A1-4V titanium alloy, which is widely used in industry. During the laser alloying process, B4C and graphite powder were preset on the surface of titanium alloy under open nitrogen atmosphere to realize B-C-N multi-component alloying. Laser powder alloying and laser gas alloying techniques were combined to form composite ceramic laser alloying coatings including ceramic hard phases TiC, TiN, TiB, TiB2, Ti(CN) and intermetallic compound such as TiAl, TisAl and Al3Ti, etc., which is beneficial to apparently increase the hardness and wear resistance of the coating. The type, content and distribution characteristics of ceramic phases in the coating were controlled through designing of coating materials and adjustment of preparation technology to improve the comprehensive properties of the coating. Through the research on the chemical composition, phase structure, microstructure, hardness and wear resistance of laser alloying coatings, the in situ formation mechanism of ceramic phases, the interface structure and the coating’s strengthening mechanism were revealed in the paper. The strengthening mechanism of titanium and rear earth oxide Y2O3during the solidification process of substrate and the formation process of ceramic phases were also discussed. Through the study, the inherent connection among chemical composition, preparation technology, micro-structure and property was revealed.Series research results indicated that multi-component ceramic coating can be formed on the surface of titanium alloy during laser alloying process when B4C and graphite powder was used under open nitrogen atmosphere. The laser alloying coating has a perfect metallurgical bonding to the substrate. In situ formed hard strengthening phases such as TiC, TiN, TiB, T1B2, Ti(CN) and Ti3AI etc. grew alternately and restrained each other mutually during the solidification of the melting pool, which is conductive to obtain perfect laser alloying coatings with fine and uniform structures. The composition, nitrogen atmosphere and laser parameters apparently affect the macro surface quality and microstructure of the alloying coatings. Under the experimental condition in this paper, laser alloying coatings with smooth surface, compacted microstructures and no apparent poles or cracks were formed under the following parameters:nitrogen atmosphere was0.4MPa, the weight ratio were2:1and1:1, the laser specific energy E was3.8～5.7kJ·cm"2.In order to enhance the hardness and wear resistance of laser alloying coatings, Ti was added into the alloying powders, which was beneficial to the in situ formation of more ceramic phases such as TiC, TiN, TiB and T1B2in the melton pool during the alloying process. However, too much Ti had a negative influence on increasing the properties of alloying coatings due to the existence of solid solution of excessive Ti. Research results indicated that the addition of30wt.%Ti was the most optimal to form an alloying coating with high hardness and perfect wear resistance.The addition of rear earth oxide Y2O3showed apparent refining effect on the microstructure of laser alloying coatings. During laser alloying process, a portion of unmelted Y2O3acted as the heterogeneous nucleation cores during solidification process and the other portion of Y2O3decomposed into Y and O2. As a surface active element, Y was easy to absorb in the grain or phase boundary to restrict the movement of boundaries. Moreover, Y also reduced the surface tension and critical nucleation radius so that nucleation was stimulated, causing the refining of the microstructure. However, too much Y2O3was easy to cause the brittleness of alloying coatings, which will reduce the wear resistance. Research revealed that, the laser alloying coating showed the best wear resistance when lwt.%Y2O3was added in to alloying powder.The addition of moderate metallic Ti powder and rear earth oxide Y2O3was beneficial to form alloying coatings with compacted structures, high hardness and perfect wear resistance. Research results indicated that the hardness of alloying coating with30wt.%Ti and1wt.%Y2O3addition is three times higher than that of the substrate. In this study, laser alloying technique was used on Ti-6A1-4V titanium alloy to form multi-component strengthening ceramic coatings. The in situ reaction mechanism of ceramic phases in laser alloying process, the interface structure between the alloying coating and the substrate, the function mechanism of Ti and Y2O3in the coatings were illustrated.This research provided essential experimental and theoretical basis for the preparation field of titanium alloy mechanical transmission parts. The research is beneficial to promote the application of laser alloying technique in the fields of aerospace, military and petrochemical industries, etc.. The laser alloying technique studied in this paper has a wide application scope and enormous economic profits due to the obvious effects on improving the hardness and wear resistance of components and parts used in different fields.更多还原