【作者】 王传琦；

【导师】 周荣； 刘洪喜；

【作者基本信息】 昆明理工大学 ， 材料加工工程， 2013， 博士

【摘要】 激光熔覆能使金属材料快速熔凝,但高裂纹敏感性镍基合金涂层制备过程中一直受到涂层开裂的影响,阻碍了该技术在表面工程领域的发展进程。为解决此问题,本文首次提出了机械振动辅助激光熔覆新技术,拓展了机械振动作用下激光熔覆镍基合金涂层凝固组织形成机理及应力控制理论研究。涵盖复合技术所制备涂层的组织凝固特征、宏观形貌、开裂机理、开裂行为、应力控制和性能等主要内容。采用了有限元数值模拟、实际测试结果和理论分析相结合的方式,探讨了机械振动对涂层表面、结合界面、基材热影响区等部位应力分布的影响,对激光熔覆镍基合金涂层的应力控制、改善和消除裂纹具有理论指导和现实意义。采用ANSYS有限元软件,以无和有机械振动辅助Ni60合金涂层的最终凝固尺寸为基础,模拟了材料热物性参数随时间变化的涂层沿激光扫描方向(Z轴)的应力场,讨论了机械振动对其应力-应变场分布规律的影响,预测最优的机械振动工艺参数。结果表明,优化的振动参数使得残余应变峰值在涂层表面由4.36×10-4降至3.17×10-4,结合界面边缘处由2.88×10-4降至2.1×10-4,在基材热影响区由4.24×10-5降至3.09×10-5。表面残余拉应力峰值在涂层表面由114MPa降至19.7MPa,在结合界面处压应力峰值由197MPa降至165MPa,在基材热影响区拉应力峰值由74.8MPa降至50.5MPa。证明机械振动能改善激光熔覆Ni60合金涂层及其周边的应力-应变分布状态,起到应力控制的作用。利用X射线衍射法进行涂层表面、结合界面、基材热影响区的Z轴方向残余应力分布测量。结果表明,有和无机械振动辅助试样相比在涂层表面的残余拉应力值,结合界面到基材热影响区的残余压应力值呈下降趋势。比较有限元数值模拟与实际测量值之间的关联性发现,应力性质在不同区域的模拟和实测结果是几乎相同的,而且模拟数值反映出的应力变化规律与实测值较为符合,证明了数值模拟的可行性。最优振动参数下Z轴方向测量值在涂层表面残余拉应力为251.1±60MPa,结合界面处残余压应力为121.6MPa,基材热影响区残余压应力为13.5MPa。通过SEM、EDS、XRD等观察和表征手段,研究了机械振动辅助激光熔覆镍基合金涂层的凝固组织特征。发现机械振动作用下Ni60/TiC复合涂层的组织更加均匀,基体相由树枝晶向近等轴晶转变、细化,碳化物数量及尺寸有明显增加；晶间网状相的分离使得基体相的连续性增强,且晶体结构完整性得到改善。Ni60CuMoW合金涂层基体相向短小枝晶和近等轴晶转变,硬质相分布均匀性增强,残留气孔减少。机械振动作用下激光熔覆过程受激振能量的影响,凝固过程转变为固-液前沿多游离核心,推进速度放缓的胞状晶、等轴晶的方式。熔池微区内存在的“平流层”影响了涂层液相的凝固机理,表现在冲断了基底胞状初生枝晶,增加了熔池内固-液界面前沿游离核心数量；削弱了结晶前沿与液相的成分过冷,抑制了胞状晶向树枝晶的转变进程；减缓了液相中形核长大的颗粒随固-液界面推进的上浮速率和熔池内各元素的浓度起伏。起到的效果体现在改善了硬质相颗粒尺寸并优化了其分布状态；增强了基体相的连续性,降低了镍基合金涂层的裂纹敏感性。在涂层表面宏观质量影响方面,“平流层”缓解了熔池内的对流翻转,涂层表面的波纹状不再明显,整体分布也较为平滑。考察定振幅机械振动辅助Ni60CuMoW合金涂层的宏观裂纹分布时发现,涂层表面裂纹交叉减少,单位长度内裂纹数由0.17条/mm下降至0.08条/mm。而定频率的涂层在振幅为0.17mm时宏观裂纹数为零。激光熔覆镍基合金涂层开裂的发展源于在结合界面处的萌生,后以穿晶开裂和沿晶开裂方式向涂层顶部扩展,且存在内部裂纹交叉现象。而有机械振动辅助的涂层内部已无裂纹交叉,换以单个贯穿方式存在,削弱了裂纹对涂层的破坏作用。有和无机械振动Ni60CuMoW合金涂层相比,自腐蚀电位正移了1134.9mVSCE,腐蚀电流密度从0.076到0.008μA·cm-2,下降了近一个数量级,耐蚀性能显著提高。熔覆区平均显微硬度由720上升到835HVo.5,提高了近16%,且显微硬度值波动明显减缓。热影响区到基材远端的显微硬度值从468迅速且无波动地下降到182HV0.5。合金涂层的平均磨损质量损失由9.2mg降至7.6mg,下降了17%；平均摩擦系数由0.081降至0.068,下降了16%,且曲线波动减弱。磨损表面无深沟,呈现出均匀的浅犁沟状分布,只有少量黏着磨损发生,归因于基体相与颗粒之间结合强度的增强：优化了涂层的抗磨损性能和磨损过程的稳定性。更多还原

【Abstract】 Laser cladding can make metal materials rapid melting-coagulation, but the influence of coating cracking in the process of preparing high crack sensitivity nickel-based alloy coatings that has hindered the development of the technology in surface engineering field. To overcome this difficulty, a new technology of mechanical vibration assisted laser cladding is proposed in this dissertation, which extends the microstructure forming mechanism of nickel-based alloy coating and the stress control theory. The research contents will cover the organization solidification characteristics, macro-appearance, cracking mechanism and behavior, stress control, mechanical properties. The method of finite element numerical simulation combining with the theoretical analysis and actual test results was used. The stress distribution of the coating surface, interface, substrate heat-affected zone and influence of mechanical vibration were discussed. The research has theoretical innovation and practical significance to the stress control, improve and eliminate of Ni-based alloy coating crack.Build the ANSYS finite element model in the basis of the solidification alloy coating final size without and with mechanical vibration Ni60coating. The stress field of coating along the laser scanning direction (Z-axis) with the material thermal physical parameters those changes over time was simulated. The distributing characteristic of stress and strain field with the mechanical vibration was discussed. The optimum mechanical vibration process parameters are predicted. Results show that the strain peak value dropped from4.36×10-4to3.17×10-4in coating surface,2.88×10-4to2.1×10-4in interface and from4.24×10-5to3.09×10-5in the heat affected zone. The stress peak value of Z axis direction (parallel to the laser scanning direction) dropped from114MPa to81.3MPa (residual tensile stress) in the surface of coating, from197MPa to165MPa (residual compressive stress) in the interface and from74.8MPa to50.5MPa (residual tensile stress) in heat affected zone. The improvement of the stress distribution showed that the mechanical vibration in cladding process can play a role of stress control and improve the stress and strain distribution in and around the Ni60alloy laser cladding coating effectively. The Z axis residual stress distribution in coating surface, interface and substrate heat affected zone was measured by X-ray diffraction method. Results show that residual tensile stress value in coating surface and residual compressive stress in the interface and substrate heat affected zone showed a decreasing trend. By comparison finite element numerical simulation and actual measurements show that the simulation and measured results have almost the same property of residual stresses in different regions. The stress change rules of simulation numerical conform better to measured values. The feasibility of numerical simulation was proved. Residual stress is251.1+60MPa (tensile stress) in the centre of coating surface,121.6MPa (compressive stress) in the interface,13.5MPa (compressive stress) in the substrate heat affect zone. It will contribute to prevent the new crack formation and improve the stability of coating in the use process. The reasonable difference of analysis results between numerical simulation and the ideal simulation conditions was more accordant with the actual situation. And this has instructional significance.The solidification characteristics of nickel-based alloy coating with mechanical vibration were analyzed by OM, SEM, EDS, XRD technology synthetically. The results reveal that mechanical vibration can improve the distributed uniformity of solidification structure and refine the grain size, the dendrite-to-equiaxed grains transition in matrix phase; and the quantity and size of carbides has increased significantly; the continuity of matrix phase and the integrity of crystal structure are improved under the separation of intercrystalline network phase in Ni60/TiC composite coating. The gross dendrites are converted into dendrite and equiaxed. hard phase uniform distribution, the pores is reduced in Ni60CuMoW alloy coating.The solidification process of laser cladding is affected by vibration energy from the mechanical vibration. The solidification mechanism of the coating liquid was affected. The ionized core is increased in the front of the solid-liquid interface; the advancing velocity slowed down and the cellular and equiaxed grains solidification mode. The flow of the molten pool impels the formation of "stratosphere". The primary dendrite was swept away that increases free core number in the front of the solid-liquid interface; the composition undercooling was alleviated on the front edge of crystallization; the transition of cellular to dendrites was inhibited; the floatation speed of nucleation particles with the solid-liquid interface was slow down; the concentration fluctuation of element in molten pool was alleviatied. The effects on the article size of hard phase and distribution was improved; the continuity of matrix phase was enhanced; the crack sensitivity of the Ni-based alloy coating was reduced.The "stratosphere" alleviates convective overturn in molten pool, the surface corrugation are replaced by smooth distribution on coating. The macroscopic crack distribution of Ni60CuMoW composite coating with mechanical vibration (limited amplitude) is investigated. The observations show that crack cross is reduced on surface coating, and the crack number of the unit length decreased from0.17crack/mm to0.08crack/mm. There is no crack appears in the alloy coating when the vibration of frequency150Hz and amplitude0.17mm. The cracking behavior of nickel-based alloy coating from interface cracks initiation and expands to the surface of coating by the transgranular and intergranular cracking ways, and crack cross exists in coating. However, the crack cross is instead of a single through cracks in coating with mechanical vibration, the disruptive impact of cracks was weakened.The corrosion potential of mechanical vibration Ni60CuMoW alloy coatings compared with no mechanical vibration coating moves about1134.9mVSCE, and corrosion current density dropped from0.076to0.008μm·cm-2, almost an order of magnitude. The corrosion resistant performance was improved significantly. The average microhardness value rose from720to835HV0.5on cladding zone, increased by nearly16%, and the microhardness value curve was slowed down. The microhardness value drops rapidly from468on heat affected zone to182HV0.5on substrate with no fluctuation. The average wear mass loss of alloy coating dropped from9.2to7.6mg in wear test, declined by17%; the average friction coefficient dropped from0.081to0.068, decreased by16%; the fluctuation of curve was weakened. The wear surface show a uniform distribution of shallow furrow shaped without deep ditch, but a small amount of adhesive wear; the bonding strength between matrix phase and particle was reinforced; the wearing resistance and stability of Ni60CuMoW alloy coating has been optimized with mechanical vibration.更多还原