【作者】 李明喜；

【导师】 孙国雄； 何宜柱；

【作者基本信息】 东南大学 ， 材料加工工程， 2004， 博士

【摘要】 在Ni基高温合金表面,激光熔覆钴基合金及其纳米氧化物颗粒复合材料涂层。利用光学显微镜、扫描电镜、透射电镜及XRD分析了熔覆层的组织结构。对熔覆层的硬度、耐磨性和高温抗氧化性等性能进行试验。研究了激光工艺参数、粉末供给方式、多道搭接、多层熔覆、基体、预热状态以及纳米材料的加入等对熔覆层的组织与性能的影响,结果如下。Stellite 6,H?gan?s（HMSP 2528/2537）合金熔覆层主要组成相均为γ-Co,Cr₂₃C₆,HMSP 2537合金涂层中出现了亚稳相Co₃W₉C₄;CoNiCrAlY合金熔覆层的组成相为γ-Co、Ni2Y和Cr3Ni2SiC等。Ni基合金熔覆层界面区宽且不规则,未出现45钢基体与熔覆层界面的平面结晶,向中心过渡为垂直于界面胞状晶、柱状枝晶等多种形态;熔覆层的近自由表面的结晶方向与激光扫描速度方向平行。随激光输入功率增大,基体溶入较多,且部分未完全熔化的颗粒在熔池中作为现成表面,使界面凝固形态发生变化。单层熔覆层组织细小致密,预热、搭接、多层熔覆的组织较粗大;在搭接、多层熔覆的界面区,发现重熔区界面部分晶粒的结晶方向继承了第一道的结晶方向,树枝晶的生长受择优取向的影响比热流的影响大。加入纳米Al₂O₃、Y₂O₃的复合材料熔覆层中,凝固组织一次枝晶间距均得到细化;所加入的纳米增强颗粒较均匀地分布于熔覆层中,纳米颗粒的加入促进了向平衡相ε-Co的转变,且有新相形成（CoAl₂O₄）,亚结构由位错转变为层错;发现Y₂O₃、Cr₂₃C₆与γ-Co晶格错配度很小。应尽量避免纳米颗粒的长大及局部团聚现象。纳米材料的加入,熔覆层的硬度和耐磨性提高,当加入量为3%时,耐磨性反而下降。熔覆层的抗氧化性能比基体高,CoCrNiAlY合金熔覆层在1 100℃时的抗氧化性能比HMSP 2528/2537合金高,但其在1 300℃时抗氧化性能明显下降,而且表面出现少量熔化现象。建立相关模型,对熔覆层凝固组织随工艺参数变化、复合材料熔覆层中颗粒均匀分布等机理进行探讨。在滑（垫）块表面熔覆Sellite 6及HMSP 2537合金熔覆层已用于钢坯加热炉,效果良好。更多还原

【Abstract】 Co-based alloy and its nano-oxide (nano-Y2O3 / Al2O3) composite coatings, deposited on Ni-based superalloy substrate by laser cladding are introduced. Cross-section and longitudinal section of single track, multi-track, single and milti-layer of the coatings have been examined to reveal microstructure and phase composition using optical microscope, scanning electron microscope (SEM, including EDS microanalysis), transmission electron microscope (TEM) and X-ray diffraction instrument (XRD). Microhardness profile, wear and oxidation resistance of the coatings were also tested. The influences of parameters on microstructure and properties of the coatings were also studied, such as laser processing parameters, means of powder supply, multi-track and multi-layer coatings, preheating and nano-materials addition, etc.The results showed that there were primary phase (γ-Co) and eutectic (γ-Co+Cr23C6) in the coatings of Stellite 6 and H?gan?s (HMSP 2528/2537) and un-equilibrium phase (Co3W9C4) was observed in HMSP 2537 coatings.γ-Co, Ni2Y and Cr3Ni2SiC phases were shown in CoNiCrAlY coatings.A thin plane front solidification layer appeared at the interface of the coatings on 45 steel substrate, which could not be seen at the interface of the coatings on Ni-based superalloy substrate. Rapid directional solidification was found at the interface and the growth direction was perpendicular to the interface. Growth direction is found parallel to the scanning direction of laser beam at the surfaceWith the increase of laser power more substrate was fused and part of unmelted substrate in the melting pool acted as subsistent surface of solidification that resulted in changes of solidified form near interface. The fine microstructure was found in single layer coatings, whereas coarsened structures were observed at the boundary of multi-tracks or multi-layer coatings.The dendrite arm spacing (λ1) was greatly refined by adding nano-Al2O3 or nano-Y2O3 particles. The particles were well distributed. Equilibrium phase (ε-Co) and other new phases transformation appeared by adding nano particles. The sub-microstructure changes from dislocation to stacking fault. It is shown from calculation results that the degree of misfit between Y2O3, Cr23C6 andγ-Co was small. Agglomeration of nanocrystalline and its growth should be avoided by adjusting laser-processing parameters.The microhardness and wear resistance of the coating were much higher than that of the substrate, and were further improved by adding nano particles in the coatings. But the wear resistance decreased by adding two much additives (>3%). The oxidation resistance of the coatings was improved and CoCrNiAlY coatings was shown the best oxidation resistance at the temperature of 1 100℃.The mechanism of effects of parameters on microstructures in Co-based coatings was proposed and the distribution of particles in nano-composite coatings were analyzed and modelled.Pads with coatings made of Sellite 6 and HMSP 2537 alloy have been successfully used in billet heating furnace in steel plants.更多还原