【作者】 玄伟东；

【导师】 任忠鸣；

【作者基本信息】 上海大学 ， 钢铁冶金， 2013， 博士

【摘要】 高温合金涡轮叶片作为航空发动机和工业燃气轮机关键部件，其性能与凝固组织密切相关。在高温合金叶片定向凝固常存在一些问题，如：叶片的缘板处形成杂晶，晶体取向偏差过大，成分偏析，粗大枝晶和粗大强化相等有害相的生成等，严重影响了高温合金叶片的性能。因此，控制高温合金定向凝固过程中的凝固缺陷，改善其凝固组织性能，对于高温合金叶片制造具有非常重要的意义。本文围绕高温合金叶片制备过程中存在一些凝固缺陷问题进行研究。以镍基高温合金为研究对象，采用模拟叶片的变截面试样，实验研究了镍基高温合金单个试样和多组试样定向凝固过程中杂晶的形成规律，探讨了磁场对高温合金凝固组织以及变截面处杂晶的影响；利用Procast软件模拟了实际熔模组铸件凝固过程中变截面处杂晶的形成规律。论文主要内容和结果如下：1．变截面试样定向凝固过程中杂晶的形成规律在镍基高温合金DZ417G变截面试样（变截面平台尺寸为2mm）的定向凝固过程中，发现在拉速小于100μm/s时，在截面变化前后，均可获得生长取向一致的枝晶组织；当抽拉速度大于150μm/s时，截面变化后的平台拐角处有杂晶形成，且杂晶区域随着拉速的增大而增大。同时测量了平台拐角处的内外温差，发现该温差达到8.5℃后，将出现杂晶，表明8.5℃温差是出现杂晶的临界值。当温度梯度由70℃/cm提高到120℃/cm时，变截面处出现杂晶的抽拉速度为175μm/s，表明温度梯度的增大能够有效的抑制变截面处杂晶的形成。进行了强磁场控制杂晶的探索，发现在变截面试样高温合金DZ417G定向凝固过程中施加12T磁场后，在150μm/s的拉速下仍可获得生长取向一致的枝晶组织，没有杂晶形成，测温曲线表明，此时变截面处温差达到10℃，表明磁场具有抑制杂晶的作用。其原因可归结为强磁场增大了固/液界面能，引起形核温度的降低（临界形核过冷度变大），从而抑制了变截面处杂晶的形成。2．磁场对高温合金定向凝固组织的影响进而考察了磁场对凝固组织的影响。在磁场下高温合金DZ417G的定向凝固过程中，发现施加磁场后，一次枝晶间距随着磁场的增大先减小后增大，当磁场强度达到6T时，一次枝晶间距达到最小；随着拉速的增大，磁场对一次枝晶间距的影响程度减弱。但在较低的拉速下施加磁场（≤10μm/s），可使试样边缘的柱状晶组织破碎形成等轴晶组织，并随着磁场的增加，等轴晶组织逐渐由试样边缘向中心扩展，最终全部转变为等轴晶组织；当试样尺寸增大时，试样不仅边缘处柱状枝晶遭到破坏，且中心区域的枝晶也出现破碎，形成等轴晶组织，且凝固组织中出现“斑状”组织。这主要是因为磁场诱发的液相中热电磁对流和磁阻尼，及固相中热电磁力三种效应共同所致。在高拉速下（≥40μm/s），磁场对凝固组织作用时间减小，进而对凝固组织作用减弱，柱状枝晶组织保持规则形态。3．截面突变试样熔模组铸件定向凝固过程中杂晶的形成规律对于单晶高温合金DD483对称变截面试样熔模组铸件定向凝固过程，在2mm/min的拉速下，发现当变截面尺寸在10mm至15mm之间时，杂晶仅出现在背阴侧；而当变截面尺寸大于20mm时，在受热和背阴侧，截面变化后的平台拐角处均有杂晶形成。加入随形挡板后，受热侧变截面区域为单晶组织，而背阴侧变截面区域依然有杂晶。其原因为随形挡板可减少加热侧保温炉热区向冷区的辐射传热，提高凝固界面处温度梯度，但对背阴侧的辐射较小影响的缘故。说明利用挡板技术可有效提高温度梯度，减少杂晶。根据以上结果，设计了非对称变截面试样，获得了完整的单晶试棒。因此，需综合控制拉速、温度梯度、挡板、变截面尺寸等参数来消除杂晶。4．熔模组铸件定向凝固过程中变截面处杂晶形成规律的数值模拟结合以上实验结果，利用ProCAST模拟软件，按照试样实际尺寸的大小建模，对实际熔模组试样定向凝固过程中温度场进行了模拟，模拟结果与实测结果误差在5%以内，说明模拟结果可靠。利用该模型对变截面试样（Φ40mm/Φ60mm）定向凝固过程中拉速、保温炉加热温度、模壳厚度对温度分布和杂晶形成倾向的影响进行了分析。计算结果表明降低拉速有利于避免杂晶，在受热侧，仅当拉速大于4mm/min时，平台拐角处熔体过冷度超过形核过冷度，导致该区域有杂晶生成；在背阴侧，几种拉速下背阴侧平台拐角处熔体过冷度始终大于其形核过冷度，因此变截面区域总有杂晶形成。提高加热温度有利于减轻杂晶倾向，当保温炉加热温度为1450℃时，受热侧和背阴侧变截面区域平台拐角处熔体过冷度均大于形核过冷度，因而该处将有杂晶形成；当加热温度大于1500℃时，仅有背阴侧平台拐角处熔体过冷度大于形核过冷度，出现杂晶；增大模壳厚度将增大杂晶的倾向，本研究条件下，仅当模壳厚度大于8mm时，受热侧平台拐角处熔体过冷度将大于形核过冷度，可形成杂晶，而几种模壳厚度下背阴侧变截面平台拐角处熔体过冷度始终大于形核过冷度，因而该区域总有杂晶形成。更多还原

【Abstract】 Performance of superalloy turbine blades as key components for aircraftengines and industrial gas turbines is closely related to solidification microstructures.However, a number of problems occur during directional solidification such as straygrains formation, misorientation, elements segregation, coarse dendrite and coarseprecipitated phase, which greatly influence performance of the superalloy blades.Therefore, it is of great importance for manufacturing the supealloy blade to controlthe solidification defects and improve the microstructures and mechanical propertiesof superalloys.The work is devoted to solidification defects in the process of manufacturingNi-based superalloy blades. The formation of stray grains in cross-section area ofsingle sample and multiple samples during directional solidification was investigated.The effect of high magnetic field on microstructures and stray grains was explored.In addition, the formation mechanism of stray grains in large-scale production wassimulated using the commercial software ProCAST during conventional directionalsolidification. The main results are summarized as follows:1．Formation of stray grains in directionally solidified Ni-based superalloywith cross-section change regionsDuring directional solidification of superalloy DZ417G samples withcross-section change size of2mm, it was found that the growth direction of dendritesdid not change and no stray grain appeared in the reentrant corner at the withdrawalvelocities lower than100μm/s. When the withdrawal velocity was beyond150μm/s,the stray grain formed in the cross-section region. Additionally, when the horizontaltemperature difference was8.5℃, stray grain formed in cross-section change region,which meant that the temperature was the critical value of formation of stray grains.With increasing withdrawal velocity, the size of stray grain increased obviously. It isindicated that the temperature difference of8.5℃is the critical condition for the formation of stray grains. When the temperature gradient increased from70℃/cm to120℃/cm, the withdrawal velocity of formation of stray grains corresponded to175μm/s. It was shown that the high temperature gradient suppressed the formationof stray grain in cross-section change region during directional solidification.The effect of high magnetic field on stray grains was explored. It was found thatat the withdrawal velocity of150μm/s, the growth direction of dendrites did notchange and no stray grain appeared in the reentrant corner in the magnetic field of12T. In the case, the temperature difference was10℃. It was indicated that the highmagnetic field suppressed the formation of stray grains in cross-section changeregion during directional solidification. The suppression of stray grain may beattributed to increase of solid-liquid interfacial energy in the magnetic field whichmade the nucleation temperature decrease and nucleation of stray grains on thecross-section change region more difficult.2．Effect of the magnetic field on microstructures of Ni-based superalloyduring directional solidificationThe effect of magnetic field on microstructures in directionally solidifiedDZ417G alloy was explored. When the magnetic field was applied, the primarydendrite arm spacing decreased and then increased with increasing the magnetic fieldand reached a minimum in6T. With increase of withdraw velocity, the effect ofmagnetic field on primary dendrite arm spacing became weak.At low withdrawal velocities (≤10μm/s), a few columnar dendrites on the edgeof sample were fractured and changed into equiaxed grains. With the increase ofmagnetic field, the number of equiaxed grains increased and they gradually extendedfrom the edge to the center of sample and finally changed into equiaxed grains.When the size of samples increased, columnar dendrites at the edge and centersimultaneously were fractured and equiaxed grains formed. Some frecklemacrosegregation appeared. The phenomena could be attributed to thermoelectricmagnetic convection and magnetic damping in liquid phase and the thermoelectric magnetic force in solid phase induced by the magnetic field. At a high withdrawalvelocity (≥40μm/s), the magnetic field effect became weak and the well-developedcolumnar dendrite structures were obtained.3．Formation mechanism of stray grains in large-scale production withcasting clusters in directionally solidified Ni-based superalloy with cross-sectionchange regionsDuring directional solidification of superalloy DD483with symmetricalcross-section change regions, it was found that at the withdrawal velocities of2mm/min, the formation of stray grains only appeared on the shadow side when thecross-section change of sample was between10mm to15mm. When thecross-section change increased above20mm, the stray grains formed on two sides ofsample in cross-section change region. Applying arbitrary shape baffle, well-ordereddendrite structures were obtained on the heating side and stray grains still existed onthe shadow side. The suppression of stray grains could be attributed to reduction ofradiation heat transfer. The temperature gradient on the heating side was improvedby arbitrary shape baffle, which made the nucleation of stray grains on cross-sectionchange region more difficult. But the effect of baffle on radiation heat transfer on theshadow side became weak. It was indicated that the baffle technique could obviouslyimprove the temperature gradient and reduce the formation of stray grains.According to above results, the single crystal can be obtained by properly adjustingthe size of cross-section change region on the shadow side. Therefore，it is necessaryto comprehensively control solidification parameters like withdrawal velocity,temperature gradient, baffles, variable cross-section size to eliminate stray crystals.4． Numerical simulation of formation mechanism of stray grains inlarge-scale production with casting clusters of multiple blades using ProCASTAccording to above experimental results, ProCAST software was used tosimulate the temperature field during directional solidification of single superalloyDD483. The difference between simulated cooling curves and experimental ones was less than5%and thus the simulated results were rather reliable. Using thismodel, analysis on effect of the withdrawal velocity, heating temperature and shellthickness on the temperature distribution and stray grain during directionalsolidification of the sample (Φ40mm/Φ60mm) with cross-section change region wascarried out. The results indicated that reduction of the withdrawal velocity will helpto eliminate stray grains. On the heating side, when the withdrawal velocity reachesto4mm/min, the undercooling was larger than nucleation undercooling and straygrains appeared. The stray grains appeared on the shadow side because undercoolingin the corner of cross-section was always larger than nucleation undercooling atvarious withdrawal velocities. The increase of the heating temperature reducedformation tendency of stray grains. When the heating temperature was1450℃，theundercooling on two sides of the corner of cross-section change regions was largerthan nucleation undercooling, the stray grains formed on two sides of sample withcross-section change region. When the heating temperature was larger than1500℃,the formation of stray grains only appear on the shadow side, because theundercooling on the shadow side was larger than nucleation undercooling. Formationtendency of stray grains increased with increasing the shell thickness. On the heatingside, only when the shell thickness was larger than8mm, stray grains could form incross-section change region. On the shadow side, the undercooling in the corner ofcross-section at different shell thickness was always larger than nucleationundercooling and thus stray grains always formed.更多还原