【作者】 徐富家；

【导师】 徐滨士； 何鹏；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 弧焊快速成形技术作为金属零件直接成形的一种典型工艺，同其它成形技术相比，具有成形零件致密度高、成本低和效率高等优点，是最有希望实现高质量金属零件经济、快速制造的方法之一。但在连续的热循环作用下，零件内部组织转变复杂，主要体现在组织粗大、不均匀以及严重的成分偏析。这不仅大大增加了控制零件性能的难度，也是制约该技术广泛应用的一个主要原因。镍基合金具有良好的组织稳定性、高温强度、高温疲劳蠕变性能等优异的机械性能，在航空航天、核工业、能源动力等众多领域应用广泛。然而这类合金零件往往形状复杂，制造工序繁琐，生产成本高。因此，利用弧焊快速成形技术直接成形镍基高温合金零件具有突出的技术优越性和广阔的应用前景。本文以脉冲等离子弧快速成形技术为研究对象，以Inconel625合金为成形材料，分析了工艺参数、沉积方式、沉积路径以及热处理方式对薄壁和块体试样的快速成形组织及性能的影响，旨在提高快速成形零件的质量。该问题的研究将为镍基合金的广泛应用提供一种比较成熟优异的加工方法。研究了等离子弧快速成形Inconel625合金典型的沉积态组织特征，利用成分过冷理论并结合快速成形热过程，分析了组织中枝晶生长和相析出机制。分析了工艺参数对沉积态组织及力学性能的影响规律，最终在试验参数范围内获得了最优的工艺参数。结果发现沉积态组织为柱状枝晶形态，沿着沉积高度方向，具有较强生长取向性的外延枝晶组织特征；析出相为γ相、Laves相和MC碳化物；低的线能量密度有利于细化组织，减弱元素的显微偏析，保持柱状枝晶组织生长的连续性，从而有效提高力学性能。利用红外测温和有限元模拟方法研究了连续沉积和间隔冷却沉积方式条件下薄壁和块体试样由底部到顶部的组织特征及性能变化规律。重点总结了两种条件下试样底部到顶部成形过程中温度梯度和冷却速率的演变规律，揭示了温度场分布特征对两种试样沉积态组织的影响机制。通过对沉积态试样的显微硬度和拉伸性能进行表征，分析了两种试样整体力学性能的均匀性以及两种沉积方式对力学性能的影响规律，从而建立组织特征和力学性能的对应关系。结果表明，在连续沉积过程中，热量的不断积累导致温度梯度和冷却速率不断下降，试样从底部到顶部，组织逐渐粗化，析出相数量增多且尺寸变大，力学性能严重不均。间隔冷却方式可降低热量积累，提高组织和性能的均匀性，从而获得更优异的力学性能。两种试样的组织性能对比结果发现，连续沉积块体试样时，更加复杂的热循环导致了大量针状δ相的生成，从而严重影响了力学性能。在相同沉积方式条件下，薄壁试样的力学性能要优于块体试样。采用KGT模型预测了温度梯度、枝晶生长速率同枝晶间隙的关系；利用Clyne-Kurz公式计算了两种沉积方式下凝固过程中Nb的偏析，并进一步分析了Inconel625合金快速成形的凝固过程。结果表明，随着枝晶生长速率和温度梯度的降低，枝晶尖端半径不断增加，而枝晶间距也随之增加，枝晶间距接近2~3R，计算结果和测量结果得到了较好的吻合，而Nb的偏析量随着凝固过程的进行呈现逐渐增大的趋势，在凝固开始阶段，冷却速率对Nb的偏析影响较小，Nb偏析量的增加主要集中在凝固的结束阶段。研究了四种沉积路径（包括长光栅、短光栅、正反交替以及交叉路径）对块体试样组织及性能的影响规律，利用建立的组织生长模型，揭示了不同沉积路径的组织生长机制。结果表明，在四种沉积路径中，正反交替路径可获得最佳的成形质量和力学性能，交叉路径虽然促进了层内区域细小枝晶的生成和Laves相的弥散析出，但在层与层边界处，严重的元素偏析，大量脆性相的析出以及枝晶的粗化是力学性能没有大幅度提高的根本原因。此外，沉积路径对组织特征的影响机理主要表现为不同的热输入和散热方向，导致了温度梯度的变化，从而影响组织生长特征。进一步研究了直接时效、固溶时效、均匀化固溶时效处理以及固溶温度和时间对薄壁和块体试样组织和力学性能的影响，以及两种试样在相同热处理条件下组织和性能的差别，从而获得了最佳的热处理工艺。重点研究了在不同热处理过程中析出相的转变机理，包括Laves相的溶解行为，强化相γ′，γ″以及δ相的析出机制。最后，对优化后的热处理态和沉积态试样高温拉伸性能进行了表征。结果表明，980℃/2h固溶时效处理为最佳热处理工艺；Laves相的溶解过程主要由Nb原子的扩散过程控制，热处理温度的提高为原子扩散提供了更多的能量；扩散系数以及溶质原子在基体中饱和固溶度的增加，有利于加快Laves相的溶解速度；δ相在晶界处的析出机制比晶内更复杂。更多还原

【Abstract】 As a typical forming process for metal parts, the welding rapid prototypingtechnique has many advantages such as high density, low cost and high formingefficiency over other processing techniques. Thus, it is a favourite way to realizeeconomic and rapid forming for metallic components. However, continues thermalcycle leads to complex internal structural transformation like coarse andnon-homogeneous structure and severe microsegregation. This increases thedifficulty of property control of the parts, and is also a main cause to restrict theapplication of the process. It is well-known that the Ni-based refractory alloy hasgood structural stabilization, and high temperature strength, fatigue and creep, andthus has wide applications in the areas of aerospace, nuclear industry, energy andpower. However, this kind of alloy components often has complex shape, so that themanufacturing process is intricated, and the cost is expensive. Hence, using thewelding rapid prototyping technology to derectly form Ni-based refractory alloyparts has outstanding technical superiority and broad application prospects.This thesis aims at studying pulsed plasma arc rapid prototyping, where theni-based refractory alloy is the key object. The influence of processing parameters,depositing strategy, depositing paths and heat treatment on the microstructure andproperties of thin-wall and block components was analyzed, in order to improve thequality of the forming parts. The desired results will find important applications invarious fields.The as-deposited structure of the pulsed rapid forming Ni-based refractoryalloy was studied. The constitution undercooling theory and thermal process ofrapid prototyping were used to analize dendritic growth and phase precipitation.The influence of processing parameters on as-deposited structure and mechanicalproperties was investigated. The optimum processing parameters were finallyobtained in the range of experimental parameters. It turns out that the depositedmicrostructures are mainly columnar crystal with strong growth oriented epitaxialdendrites along the deposition height direction. The precipitated phases are γ-phase,Laves and MC carbides. Low energy density is conducive to refine microstructure,weaken elements‘microsegregation and maintain the continuity growth of columnar dendrite, and thus improve mechanical properties.By using infrared measuring temperature and finite element analogue, thestructural transformation and mechanical properties of the thin-wall and blocksamples from the bottom to the top under the condition of continuous and intervalcooling deposition were investigated. The distribution characteristics of rapidforming temperature field under two kinds of deposited modes were presented, andthe influence mechanism of distribution characteristics of temperature field on theas-deposited microstructure of two samples was revealed. The as-depositedspecimen microhardness and tensile properties were characterized, the uniformity ofmechanical properties was evaluated and the effects of two deposited methods onthe mechanical properties were analysed, and thus the corresponding relations of thestructure characteristics and mechanical properties could be revealed. The resultsshowed that, in the process of continuous deposition, the temperature gradient andcooling rate decreased gradually because of the heat accumulation. The structure ofthe sample from the bottom to the top gradually coarsened, precipitated phasesincreased, the size became bigger, and mechanical properties were seriusly unequal.However, interval cooling reduced heat accumulation, and improved the uniformityof microstructure and properties, and thus more excellent mechanical propertieswere obtained. The comparison results of structure and mechanical properties oftwo samples showed that the more complex heat cycle of continuous deposition ofblock sample led to a large number of needle-like δ-phases, which seriously affectedthe mechanical properties. The mechanical properties of the thin-wall sample arebetter than the block sample in the same deposited condition.The KGT model was used to predict the relationship between the temperaturegradient and dendrite spacing as well as dendrite growth rate, while the Clyne-Kurzformula was used to calculate Nb segregation in the solidification process. Thesolidification process of Inconel625alloy rapid prototyping was further analized.The results revealed that the dendrite spacing and dendrite tip radius increased withthe dendritic growth rate and temperature gradient decreased. The dendritic spacingis two to three times of dendrite tip radius. The measuring results and computingresults are consistent. The segregation of Nb became gradually larger during thesolidification process. At the beginning of solidification process, the effect ofcooling rate on segregation of Nb was small. The increasing of segregation of Nb mainly took place at the end of solidification process.The effects of four deposited paths (including long raster, short raster, reversepath and cross path) on the microstructure and mechanical properties of blocksample were investigated. From structure growth model, the growth mechanism ofstructure with different deposited paths was revealed. The results presented that thesample was equiped with good quality and mechanical properties, when the reversepath was used. The cross path promoted the growth of finer dendrites andsegregation of Laves phase in the layer. However, the reasons of un-improvement ofmechanical properties were the segregation of elements, the precipitation of brittlephases and the coarse dendrites at the boundary of layer. Meanwhile, the effect ofdeposited paths on structure revealed that different heat input and radiatingdirections resulted in the change of temperature gradient, affecting the growthcharacteristic of microstructure.The influences of direct aging, solid aging and uniform heat treatment, as wellas solid solution temperature and time, on the structure and mechanical propertiesof the thin-walled and block samples, and differences of microstructure andmechanical properties between two kinds of samples under the same heat treatmentwere further investigated. The transformation mechanism of the precipitated phasesincluding dissolution behavior of Laves phases, and precipitation mechanism ofstrengthening phases consisting of γ′, γ″and δ-phases was studied. Finally, theoptimized heat treated state and high temperature tensile properties of theas-deposited samples were characterized. The results showed that980℃/2h solidsolution ageing was the best heat treatment process. The solution process of Lavesphases were mainly controlled by diffusion process of Nb atoms. Thus, improvingheat treatment temperature could provide more energy for atom diffusion. Also,increasing the diffusion coefficients and saturation solid solubility of the soluteatoms in the matrix could help accelerate the dissolution rate of Laves phases. Theprecipitation mechanism of δ phases at the grain boundaries was more complex thanthat in the intragranular.更多还原