【作者】 宫新勇；

【导师】 张永忠；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2014， 博士

【摘要】 相较于常规修复技术,激光熔覆沉积技术具有修复精度高、对基体的热影响和热输入小、沉积材料组织性能优良、界面结合强度高等优点,近年来逐渐成为零件再制造领域研究及发展的重点。对受损叶片盘外形及使用性能的恢复是实现对其高质量修复的必要条件,结合某一型号TC11钛合金压气机整体叶片盘的修复需要,论文对激光熔覆沉积过程中熔覆层外形尺寸控制、沉积材料及界面热影响区组织性能控制等基础问题展开研究。为提高压气机叶盘的气动效率,叶片普遍被设计成扭转的自由曲面轮廓,为精确恢复此类断裂叶片的几何外形,需严格依据待修复区域的高度来制定熔覆扫描轨迹,因此对熔覆层的形状特征参数的控制及预测成为恢复叶片外形亟需解决的基础问题。论文将侧向送粉式激光熔覆沉积过程中单层单道熔覆层的横截面面积作为研究的目标控制参量,在此方面完成的工作及结论简述如下：(1)通过定义粉末输送效率(即粉末流场中送入熔池的粉末粒子占所有输送粉末的质量比例),依据质量守恒定律,分别建立“圆柱”及高斯型流场下的粉末输送模型,推导粉末输送效率的解析式,并据此计算了不同工艺参数(激光功率、扫描速度、送粉率)组合下该目标参量的理论预测值。修正后的理论值虽然较实验测量结果偏大,但能够真实反映出工艺参数对熔覆层横截面面积的影响规律,且高斯型流场的粉末输送模型更接近于真实物理模型。(2)基于高斯型粉末流场,利用平面向量的儿何关系进一步推导数学模型,可计算任意送粉条件参数(离焦距离、送粉角度、熔池宽度)调整后的粉末输送效率。影响零件使用性能的一个重要因素是材料内部的组织特征。激光熔覆沉积修复后,TC11钛合金叶片的组织分布为：沉积仿形区的魏氏组织、热影响区呈过渡演变的混合组织及叶片基材的等轴组织。为提高修复叶片的服役寿命,论文针对薄壁零件在沉积区和热影响区组织特征的控制问题展开了实验及理论研究,分别将初生p晶粒尺寸、晶内单个片层α相厚度(针对仿形区激光熔覆沉积材料),和等轴α相的分布(针对热影响区材料)作为目标控制参量,在此方面获得的结论简述如下：(1)实验研究表明降低激光功率或增加扫描速度可以减小初生β晶粒尺寸；增大激光功率、扫描速度、层间停留时间可以细化晶内片层α相。在理论研究方面,对比分析了不同工艺条件下的温度场特征参数(峰值温度、β相变点以上的持续时间、固液界面处的温度梯度、β相变点以下的冷却速度),工艺参数对目标参量影响规律的理论预测与实验结果能够很好吻合,表明依据该理论分析方法调整工艺参数,可以实现对TC11钛合金激光熔覆沉积组织的控制。(2)实验研究表明降低激光功率、增加扫描速度或层间停留时间能够减弱基材等轴α相的扩散转变,减小热影响区面积。在理论研究方面,基于修正后的JMA方程,递推出描述激光熔覆沉积复杂热历史过程的扩散相变模型,计算了不同工艺条件下基材热影响区等轴α相的分布情况；依据此模型,认为温升阶段的升温时间、峰值温度均与等轴相的转变量呈正相关,并基于对热影响区温度场的分析,获得了工艺参数对基材目标控制参量的影响规律,该理论预测能与实验结果相一致,因此可将此模型作为锻态薄壁基材热影响区组织控制的理论依据。结合上述基础研究成果,确定出优化的激光熔覆沉积修复工艺,成功实现对某受损TC11钛合金整体叶片盘的高质量修复。更多还原

【Abstract】 Compared with other conventional repair technology, laser cladding deposition presents advantages of high precision, small heat input and influence to substrate, superior performance of deposited materials, high bonding strength and so on. This process has become a key method of research and development in remanufacturing field recently. The recovery of shape and performance for damaged blade is a necessary condition in high quality repair. Based on the background of repairing a blisk in TC11titanium alloy, fundamental researches including controls of cladding size and microstructure in repaired part are studied in this paper.In order to improve the aerodynamic efficiency of compressor blisk, the blade is generally designed in an irregular twist surface contour. To accurately restore the shape of such broken blade, track of laser scanning should be designed strictly according to the height of the area to be repaired. Therefore, control and prediction on the size of cladding layer has become an urgent basic problem for blade repairing. The cross-sectional area of single laser cladding layer by lateral powder feeding is set as the target control parameter in this study. The main work and conclusions in this regard are as follows:(1) By defining the powder transport ratio as the mass ratio of powder particles fed into the molten pool to all powders transported, based on the conservation of mass, powder transport models for flow field with cylindrical and Gaussian distribution, and corresponding analytical formula of powder transport ratio are proposed. Then, the target parameter for different process parameters (laser power, scanning speed, powder feed rate) are calculated. Although modified theoretical predictions are still larger than experimental results, above two models could reflect the effects of process parameters on the cross-sectional area and the Gaussian model is more reasonable. (2) Based on the Gaussian powder flow field, powder transport ratios according to arbitrary adjustments of injection condition parameters (de focusing distance, powder-feeding angle, pool width) could be calculated by geometrical relationship of plane vectors.Microstructure of material is one of important factors affecting parts performance. Micro structure of TC11titanium alloy blades after repaired by laser cladding deposition shows widmanstatten structure in profiling zone, mixed structure with gradual transition in heat affected zone (HAZ), and equiaxed structure in initially forged substrate. To improve service lives of repaired blades, controls on the first two regional microstructures in thin-walled parts are investigated from experimental and theoretical aspects. Size of initial β grain, thickness of intragranular single a phase (for materials as deposited), and distribution of equiaxed a phase (for materials with mixed structure in HAZ) are set as target control parameters. In this aspect, main conclusions are summarized as follows:(1) In experimental studies, size of initial β grain could be reduced by decreasing laser power or increasing scanning speed, and intragranular lamellar a phase could be refined with the rise of laser power, scanning speed, and interlayer stay time. In theoretical research, characteristic parameters of temperature field for different process parameters are compared, such as peak temperature, duration time above P phase transition, temperature gradient at the solid-liquid interface, cooling rate below β phase transition. For effects on the target parameter, predictions and experimental results could be in good agreement. According to above theoretical analysis, microstructure control of deposited materials in TCI1titanium alloy can be realized by adjustment of process parameters.(2) Experimental results indicate that transition of equiaxed to lamellar phase and thermal influence on forged substrate are weakened with the reduction of laser power, increase of scanning speed and interlayer stay time. In theoretical research, based on the modified JMA equation, diffusion transformation model for the complicate thermal history during laser cladding deposition is established. Distributions of equiaxed a phase in HAZ corresponding to different process parameters are calculated. A positive correlation between transition of equiaxed phase and duration time, peak temperature in heating stage can be deduced. After comparing the temperature field parameters in HAZ, influences of process parameters on the target parameter are obtained. The theoretical prediction is in accordance with experimental results, so the model could be used as basis for microstructure control in H AZ of forged thin-walled parts.Combined with above fundamental researchs, a damaged blisk in TC11titanium alloy is repaired in high-quality by using an optimized technology program.更多还原