【作者】 毛怀东；

【导师】 张大卫；

【作者基本信息】 天津大学 ， 机械制造及其自动化， 2007， 博士

【摘要】 激光表面熔覆技术与其他表面加工技术相比具有应用面广、实用性强、应用灵活等优点,但高硬度耐磨熔覆层的裂纹问题限制了该技术的推广。本文针对熔覆层的裂纹问题进行了大量的实验研究。利用声发射技术检测了熔覆过程中的裂纹产生和扩展状况,开发了在基体未预热和未设计过渡层的情况下控制裂纹产生的新工艺,并在生产实践中得到应用。本论文的主要创造性工作如下:基于声发射技术,提出了一种新熔覆层裂纹的产生和发展过程的检测方法。利用该方法可检测不同材料的熔覆层裂纹,并能够精确确定裂纹发生的位置。根据裂纹发生的时间、位置、熔覆过程温度变化规律,以及有限元分析计算结果,给出了裂纹产生的温度范围。研制了多种铁基合金熔覆粉末,对该粉末熔覆层的组织和性能的进行的实验分析,结果表明,其组织主要为碳、硼、硅元素和铁、铬元素的各种化合物组成的一次、二次枝晶,该组织分布于马氏体和残余奥氏体组成的共晶组织上,其耐磨性可达Cr12淬火材料的3倍。针对熔覆层中裂纹问题,提出了熔覆层裂纹控制的新工艺。该工艺通过在熔覆层中加入起应力松弛作用的软化带,可有效地降低熔覆层的热应力、减少熔覆层裂纹。经实验验证,在熔覆层中加入不锈钢网作为应力软化带时,可有效减小熔覆层裂纹率。实验研究了陶瓷相在熔覆层组织内的作用,通过比较Ni包WC和纯WC对熔覆层裂纹不同影响,得出保持原状的WC颗粒尖角等处在熔覆层中形成应力集中区,引起熔覆层裂纹。上述研究成果成功地应用于天津石化公司化纤厂增压泵轴修复中,解决了该类轴的修复难题。修复后轴已安装运行两年多,取得了巨大的经济效益。上述研究成果丰富了激光熔覆工艺理论,发展了裂纹检测和控制新方法,对相关工艺中裂纹的研究同样具有重要的参考价值。更多还原

【Abstract】 The laser cladding technology is more available, practical and flexible than other surface working methods. But the cracks exist in cladding layer reduce the performance and make it hard to become widespread available.Plentiful experiments about the crack in laser cladding process are performed in this dissertation. The control method of cladding crack is studied under the condition that the substrate is not preliminary heated and the transient layer is not designed. Some experiment studies are made about the crack detection in cladding process, the configuration of cladding material and the development of cladding techniques. The research results reach the requirement of industrial application within a certain scope. The following contributions were made.By use of the acoustic emission detective technique, the generation and development of crack in cladding process are in-situ measured. The number of crack with different cladding material are measured, meanwhile, the position of crack is detected. Considering the generation time and the temperature of cladding process calculated by finite element method, the temperature scope of crack generation is analyzed. It can be concluded that the most cracks in cladding layer is cold-crack.Configure iron-based alloy that is utilized in laser cladding process and make a test of the structure and characteristics of the cladding layer. It is found that iron-based alloy cladding layer can be well metallurgicly combined with the substrate, and the cladding layer is mainly composed of the substrate of martensite and remaining austenite and the primary and secondary dendrite of various compounds that are made from C, B and Si or Fe and Cr. Compared with the quenching substrate, the wear characteristic is improved greatly.Considering the crack of cladding layer, the method of adding softening band to the cladding layer is put forward, and it aims at making the stress softening and reducing the thermal stress of cladding layer to reduce the cladding crack. The influence to the cladding crack caused by adding stainless steel meshwork to cladding layer as the stress absorption band is studied.The function of ceramal phase in cladding layer is analysed, and the valuable conclusion that the WC at raw condition could induce stress concentration at sharp corners,can be get by analyzing the different contributions of Ni-WC and pure WC to cracks of cladding layer.By using the research results, the booster pump shaft that is repaired has been operating for over two years in Tianjin Petrochemical Company Chemical Fiber Plant. It can steadily operate for a long time, and huge economic benefits have been achieved.The outcome of this dissertation can not only provided a new method for measuring and controlling of the cladding crack, but also be valuable for relative research.更多还原