钛合金激光熔覆钴基涂层的制备及数值模拟

发布时间：2018-03-01 13:08

本文关键词： 激光熔覆钴基合金钛合金数值模拟显微组织　出处：《大连理工大学》2015年硕士论文　论文类型：学位论文

【摘要】：随着我国航空航天事业的发展,钛及钛合金因其密度小、比强度高等优点得到了广泛应用,但硬度低、耐磨性差等自身缺陷又严重限制了钛合金的应用领域。近年来,材料表面改性领域研究的热点之一——激光熔覆技术,可以为改善钛合金表面缺陷提供极具发展前景的途径。利用激光熔覆技术可在钛合金表面熔覆一层高硬度、耐磨损的复合涂层,在保证基体良好性能不变的同时有效改善其表面缺陷。在激光熔覆过程中,尺寸很小的熔池内存在极其复杂且反应迅速的传热现象和化学变化,这直接影响材料成形熔覆层的质量和力学性能,因此对熔池内温度场的控制显得尤为重要。利用计算机软件对熔池内温度场进行数值模拟,用以指导熔覆工艺参数的选择及涂层质量缺陷的预测,从而达到改善熔覆层质量的目的,这在实际应用中具有重要的意义。本文利用激光熔覆技术在TC4钛合金上制备了钴基复合涂层。首先利用ANSYS有限元软件对激光熔覆过程中熔池内温度场分布情况进行了研究,并对激光功率参数进行了优化选择。然后利用优化的工艺参数进行激光熔覆实验,熔覆材料有两种,纯KF-Co50钴基自熔性粉末和掺杂10%Zr02的混合粉末,并对两组试样熔覆层的显微组织形貌和力学性能进行了分析。具体研究内容如下：利用ANSYS有限元软件建立了预置式激光熔覆几何模型,并利用参数化设计语言APDL实现了移动激光热源的施加。温度场模拟结果表明,激光熔覆熔池呈椭球形,熔池内温度等温线呈勺状,并且光斑前沿温度梯度大(等温线密集),而光斑后的熔覆层温度梯度小(等温线稀疏)。不同激光输出功率下熔池内温度场分布的研究结果表明,随着激光功率的增大,熔池内最高温度也逐渐增大,熔深增加,为保证熔覆层较小的稀释率,激光功率选择在1000W—1100W之间比较合适。利用Laserline LDF 4000-100型号半导体激光器在TC4合金上进行预置式激光熔覆实验,实验参数为：激光输出功率P=1000W,扫描速度V=5mm/s,激光光斑直径D=3mm。随后利用XRD、EPMA、SEM等实验设备对熔覆层进行形貌及相组成分析。分析结果表明,A1和A2(添加10%部分稳定Zr02)两组试样的熔覆层物相大体相同,主要是在7-Co和少量β-Ti固溶体上分布着长条或块状的TiB2和WB,原位生成的呈颗粒状的TiC,以及CoTi、Cr23C6、CrB硬质增强相等,A2试样整个熔覆层内弥散分布着大量细小的白色Zr02颗粒。采用型号为DHV-1000的维氏硬度计测量熔覆层的显微硬度值。熔覆层横截面显微硬度分布曲线表明,钴基复合涂层的显微硬度值与钛合金基体相比得到显著提高,约为基体硬度的3倍,显微硬度值由熔覆层表面至基体呈梯度平缓下降趋势。横向对比两组试样的显微硬度分布曲线可知,陶瓷颗粒氧化锆的添加并没有显著提高熔覆层的硬度,主要是为了减少熔覆层内裂纹的产生及扩展,提高熔覆层的强度和韧性。
[Abstract]:With the development of aerospace industry in China, titanium and titanium alloys have been widely used because of their low density and high specific strength. However, their own defects, such as low hardness and poor wear resistance, have seriously restricted the application fields of titanium alloys. Laser cladding technology, one of the hot research topics in the field of material surface modification, can provide a promising way to improve the surface defects of titanium alloy. Laser cladding technology can be used to cladding a layer of high hardness on titanium alloy surface. The wear-resistant composite coating can effectively improve the surface defects while keeping the good performance of the substrate. In the laser cladding process, the heat transfer phenomena and chemical changes are extremely complex and react rapidly in the very small melting pool. This directly affects the quality and mechanical properties of the cladding layer, so it is very important to control the temperature field in the molten pool. It can be used to guide the selection of cladding process parameters and predict the quality defects of the coating, so as to improve the quality of the cladding layer. This is of great significance in practical application. In this paper, cobalt based composite coatings were prepared on TC4 titanium alloy by laser cladding technique. Firstly, the distribution of temperature field in the molten pool during laser cladding was studied by ANSYS finite element software. The laser power parameters were optimized, and the laser cladding experiments were carried out using the optimized process parameters. There were two kinds of cladding materials, pure KF-Co50 cobalt-based self-fluxing powder and mixed powder doped with 10Zr02. The microstructure and mechanical properties of the cladding layer of two groups of specimens are analyzed. The main contents are as follows: a prefabricated laser cladding geometry model is established by using ANSYS finite element software. The temperature field simulation results show that the laser cladding pool is ellipsoid, and the temperature isotherm in the molten pool is spoonlike. And the temperature gradient at the front of the spot is large (the isotherm is dense, but the temperature gradient of the cladding layer behind the spot is small (the isotherm is sparse). The results of the study on the temperature field distribution in the molten pool with different laser output power show that, with the increase of the laser power, The maximum temperature in the molten pool also increased gradually, and the penetration increased. In order to ensure the lower dilution rate of the cladding layer, the laser power was chosen in the range of 1000W-1100W. A preset laser cladding experiment was carried out on the TC4 alloy by using the Laserline LDF 4000-100 semiconductor laser. The experimental parameters are as follows: laser output power PQ 1000W, scanning speed V = 5mm / s, laser spot diameter DX 3mm. then the morphology and phase composition of the cladding layer are analyzed by means of XRDX EPMA-SEM. The results show that there are two groups of samples, I. e. A1 and A _ 2 (adding 10% partially stabilized Zr02). The cladding material of the sample is roughly the same. TiB2 and WB were mainly distributed on 7-Co and a small amount of 尾 -Ti solid solution, and in situ formed granular tic, and CoTiC23C6CrB hard reinforced A2-samples distributed a large number of fine white Zr02 particles in the whole cladding layer. The microhardness of the cladding layer is measured by Vickers hardness meter of model DHV-1000. The microhardness distribution curve of the cross section of the cladding layer shows that, The microhardness value of cobalt-based composite coating was significantly higher than that of titanium alloy substrate, which was about 3 times of that of the substrate. From the surface of the cladding layer to the substrate, the microhardness value decreased gradually. Comparing the microhardness distribution curves of the two groups of samples, it can be seen that the addition of ceramic particle zirconia does not significantly improve the hardness of the cladding layer. The main purpose is to reduce the generation and propagation of cracks in the cladding and to improve the strength and toughness of the cladding.
【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG174.4

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