【作者】 闫华；

【导师】 黄早文； 王爱华；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2010， 博士

【摘要】 铜和铜合金因具有优异的导电性、高的导热率及良好的塑性而日益广泛地应用于电力、冶金、机械装备、航空航天等领域。然而,铜及铜合金作为苛刻工况条件下的工件如连铸结晶器等要求低的变形量、高耐磨、耐热腐蚀等,其难以直接满足工程要求。激光熔覆技术具有可提高零件特定工作面服役性能及整体工件寿命的特点。尽管激光熔覆技术在金属表面强化方面具有巨大的优势,但铜合金良好的导热性及对激光低的吸收率导致铜表面激光熔覆成为的难点。本文利用Nd：YAG固体激光器作为热源,以预置粉末方法提高铜对激光能量的吸收率,在Cu-Cr-Zr合金表面成功制备了耐磨熔覆层。利用扫描电子显微镜(SEM)、x射线衍射(xRD)、透射电子显微镜(TEM)、差示扫描量热法(DSC)及干滑动摩擦磨损测试对熔覆层形貌、微观组织、界面结构、增强相生长机理及摩擦磨损行为进行了全面研究。针对Ni与Cu能够无限互溶的特性,在铜合金表面分别进行了4种非自熔性Ni基合金与2种自熔性Ni基合金熔覆层的制备。结果表明,纯Ni、Ni-30Cu、GH-01及Ni30、Ni45与铜基材浸润性较差,熔覆层及界面区存在大量缺陷；B和Si等元素形成硬脆性化合物是导致自熔性Ni基合金裂纹萌生和扩展的原因之一；即使合金粉末中含有适量Cu也不能改善熔覆层与Cu基材界面相容性。含适量Al的KF-6合金在激光与反应放热耦合下形成了良好冶金结合界面,熔覆层组织致密呈细小枝晶状；(Ni,Cu)固溶体是良好冶金结合界面的基础,马兰戈尼对流和非平衡快速凝固综合作用使界面组织局部表现为层状结构。为提高铜合金抗磨、耐热性能,在铜合金表面分别进行了非自熔性Co基合金、自熔性Co基合金及Ni/Co复合熔覆层的制备。结果表明,工艺参数对Co基合金与Cu基材润湿性有明显影响；自熔性Co基合金中形成裂纹,将限制其工程应用。非自熔性Co基合金HG-Co01致密而无缺陷,熔覆层山α-Co固溶体和Cr23C6组成；其合金粉末中适量Ni使界面形成了富Ni的(Ni,Cu)固溶体和富Co的(α-Co,Ni)固溶体,使冶金结合界面形成。为显著改善Co基合金／铜基材界面性能,以KF-6镍基合金为中间过渡层,实现了Co/Ni/Cu的梯度过渡,Co/Ni、Ni/Cu界面结构为固溶体,均实现冶金结合；复合层组织细小,无气孔、裂纹等缺陷。以KF-6合金为中间过渡层,采用非自熔性Co基合金HG-Co01为金属粘接相,在铜合金表面分别采用直接引入和原位合成的方法成功制备了Co基金属陶瓷复合层。研究证实,Ni包WC增强Co基复合层与中间过渡层形成良好冶金结合,WC颗粒分布均匀,未发生严重烧损,基本保持原有多角形态,部分WC边缘发生扩散形成合金化层。以Ni包WC、HG-Co01、Ti粉和石墨粉为原料,采用激光原位合成的方法成功制备了TiC+WC增强Co基合金复合层。复合层中TiC以枝晶、细小颗粒形态析出；随着(Ti+C)含量的增加,TiC析出量增加。原位合成复合熔覆层形成机理为：复合粉末在激光作用下首先形成共熔体,并发生Ni包WC的表层分解和扩散形成TiWC2合金化层；同时,Ti、C元素在熔体中发生扩散形成亚稳定的TiXCy增强体,持续激光热作用和反应放热的耦合及浓度补偿最终形成稳定的TiC增强相,TiC的长大机制主要为扩散机制和溶解—析出机制铜合金表面熔覆层硬度较铜基材明显提高, KF-6合金中间过渡使复合熔覆层硬度在界面区域呈梯度下降。室温干滑动摩擦磨损试验结果表明,复合材料熔覆层摩擦系数均小于0.25,耐磨性大幅提高。铜基材粘着磨损严重,相同条件下,复合熔覆层耐磨性均较Cu基材显著提高。以典型的铜合金零部件——连铸结晶器铜板为工程实际应用对象,分析了镀层失效的主要形式为磨损和疲劳裂纹,将结晶器铜板表面激光复合Ni/Co熔覆层与镀层比较发现,Ni/Co复合熔覆层高温(400℃)耐磨性是电镀Ni-Co层的3.15倍,Ni/Co复合层摩擦系数呈下降趋势并稳定。目前,已成功将Ni/Co复合激光熔覆层应用于结晶器铜板,使用寿命比Ni-Co镀层的提高一倍以上。更多还原

【Abstract】 The excellent thermal conductivity, outstanding electric capability, good elasticity of copper and its alloys make them interesting materials for applications involving electrical apparatus, machinery, metallurgical equipment and aerospace components, etc. However, due to the poor wear resistance, resistance to deformation and hot corrosion resistance, the service life of copper components such as chill mould for continuous casting is decreased seriously in extreme conditions without surface treatment. Laser surface cladding (LSC) can raise the reliability of working surface and increased service life of components.Though LSC can be easily achieved for many nonferrous metals it is difficult to prepare crack-free and nonporous coatings on copper, due to the good thermal conductivity, poor wettability with many other materials and reflectivity to laser beam during laser processing.In the present study, Wear-resistant coatings were fabricated onto a Cu-Cr-Zr alloy by LSC using a pulsed Nd:YAG laser with uniformly powder preplacement beds to increase the laser energy absorptivity of Cu surface. Surface and cross-section topography, microstructures, phase constitutions, interfacial structure, formation mechanism of reinforcement and wear properties of coatings wear investigated by means of scanning electronic microscopy (SEM) with X-ray energy dispersive microanalysis (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), as well as dry sliding wear test.Based on Ni and Cu has complete miscibility with each other, which forms an ultimate mutual solution, four Ni-based non self-fluxing alloy and two Ni based self-fluxing alloy were deposited by LSC. It is found that pure Ni, Ni-30Cu, GH-01, Ni30 and Ni45 alloys have lower wettability on copper, which induces cracks or pores in coatings and interface zone. One reason of cracks is borides or silicides formation in coatings and interface zone. Even though an amount of Cu is in coating, it still does not improve the wettability with copper. The KF-6 alloy coating was generated metallurgical bonding with copper substrate due to chemical heat release between Ni and Al induced by laser. The coupling interaction between marangoni effect and non-equilibrium rapid solidification resulted layer structure in interface zone.Co-based alloy coatings have been fabricated on copper substrate to improve heat and wear resistance of copper components. The results showed that technological parameters influence the wettability of HG-Co01 alloy with copper. Cracks were formed in HG-Co02 alloy coating due to borides and silicides. Microstructure of HG-Co01 coating was composed ofα-Co solution and Cr23C6. The Ni-based solid solutions (a-Co, Ni) and (Ni, Cu) were formed at interface, which generate metallurgical bonding by diffusion between Co-based coating and copper substrate. To obtain better interface between HG-Co01 coating and copper, the KF-6 alloy as an interlauer was prepared by LSC on copper firt and then HG-Co 01 multilayer coatings were cladded. By this mean, Co/Ni gradient coating was prepared on copper which dramatically improve the compatibility between Co and Cu. The compact-grainstructure was obtained without defects in gradient coating.With KF-6 alloy as an interlauer and HG-Co01 alloy as metal matrix, metal matrix composite (MMC) coatings were successfully fabricated on copper by LSC using introducing Ni-coated WC reinforcement phase and in-situ synthesis of TiC, respectively. The results indicated that composite coatings and copper substrate were bonded metallurgically. WC particles were dispersively distributed in MMC coatings with slightly heat damage. WC particles kept their original multiangular shape. The alloying elements such as Co, Cr and Ni diffused into WC particles rapidly during laser processing, thus forming an alloyed layer with limited thickness. In situ synthesized TiC+Ni/WC reinforced Co-based coatings were fabricated on copper substrate by Nd:YAG LSC using preplaced powder. Reindorcements dispersed uniformly in the matrix, and TiC shows the morphology of dendritic and particle. The atomic ratio for formating reinforcements is of importance to the constituent of raw components. During LSC, the laser-materials interaction induces the heating effect and formating melt first, then Ti element diffused into WC particles forming TiWC2 alloyed layer. In the meantime, heating effect results the Ti atom and C atom in interdiffusion action and formation of TixCy. The coupling of lsaer and exothermic reaction with concentration compensation generated stable TiC finally. The nucleation and growth mechanism of TiC is diffusion and dissolution-reprecipitation mechanismThe hardness and wear resistance of copper were significantly improved by LSC Co-based alloy coatings. The hardness of interfacial zone in composite coatings decreased gradually. The results of wear test at room temperature showed that the friction coefficients of MMC coatings were less than 0.25. The copper adhesive wear severely while after LSC composite coatings, its wear resistance was improved significantly.In engineering application, chill mould plate for continuous casting was taken as typical practical application copper components to treat by LSC. The main failure forms of electroplating coating on chill mould plate are wearing and fatigue crack. The results of wear test under 400℃indicated that the wear resistance of Ni/Co composite coating was 3.15 times of that of electroplating coating. The Ni/Co composite coating on chill mould plate for continuous casting by LSC has successfully fabricated for practical application. The service life of LSC Ni/Co composite coating is more than two times of that of Ni-Co electroplating coating.更多还原