钛合金薄板低阈值背反射激光焊接数值分析与特性的研究

发布时间：2018-06-13 00:15

本文选题：钛合金薄板 + 激光焊接　；参考：《江苏大学》2017年硕士论文

【摘要】：针对钛合金薄板背反射激光焊接现有研究存在的问题,本文以1.5mm厚TC4钛合金薄板为对象,围绕“钛合金薄板低阈值背反射激光焊接数值分析与特性”这一主题,开展相关理论及应用基础研究。研究中,通过对背反射激光焊接过程温度场和应力场数值模拟与分析的研究,阐明背反射激光焊接过程温度场和应力场的变化及其分布规律,进而为揭示背反射激光焊接成形机理奠定理论基础;同时,通过钛合金薄板背反射激光焊接成形工艺特性和组织性能特性的研究,验证这一高效低阈值激光焊接新方法对厚度相对于现有研究增大了约1倍的较厚薄板的适应性,从而为进一步拓展其应用提供技术支撑和应用基础。研究过程中采用“理论分析与实验研究相结合、数值模拟和试验优化并重”的研究方法,所开展的研究工作和取得的主要成果如下。(1)钛合金薄板背反射激光焊接过程的温度场数值模拟与分析的研究。以经实验验证的0.8mm厚钛合金薄板背反射激光焊接过程的“双热源”温度场模型为基础,通过设定1.5mm的板厚、1400W的功率和850mm/min的扫描速度建立新的温度场模型,对1.5mm厚钛合金薄板背反射激光焊接过程的温度场进行了数值模拟与分析。研究表明:此模型能够较好地模拟背反射激光焊接成形过程中的温度场分布情况,薄板正面和背面均在很短时间内形成了类似“泪滴状”的稳定温度场,热源前端等温线密集、温度梯度大,后端等温线相对稀疏、温度梯度相对较小。熔池逐渐从V形过渡到X形的温度场时程变化,较好地描述了背反射激光焊接过程中所形成的背面辅助能场对工件背面进行二次加热的物理过程;同时,随着功率的增大或者扫描速度的降低,熔池熔宽和熔深逐渐增大,温度梯度与功率呈正比,与速度呈反比规律。(2)钛合金薄板背反射激光焊接过程应力场的数值模拟与分析研究。以温度场为基础,采用间接耦合法对钛合金薄板背反射激光焊接过程的应力场进行了数值模拟与分析。研究表明:钛合金薄板背反射激光焊接过程中,随着焊接时间的延长其热应力随之增大;而在其冷却过程中,钛合金薄板的应力值分布在两端位置处最大,表现出比较明显的边界效应。同时,常规激光焊即无反射垫板状态下,焊缝横切面上表面残余应力整体大于下表面且残余应力分布区域较宽;而背反射激光焊的焊缝横切面上表面与下表面的残余应力曲线重合度较高,上下表面残余应力分布也基本相当。此外,不同线能量下,背反射激光焊接钛合金薄板的纵向残余应力变化很小,这表明线能量对其焊接残余应力影响的敏感度较低。(3)钛合金薄板背反射激光焊接成形工艺特性研究。以接头焊缝的背宽比、堆高和变形角这三个指标对成形性进行定性和定量的表征,进而探究激光工艺参数对背反射低阈值激光焊接接头成形性的影响及其规律。研究表明:在本文实验条件下,焊接正背面均呈现亮蓝色或金黄色,焊接宏观形貌较好,焊缝形貌均呈现典型的X形焊缝;激光工艺参数对背宽比的影响次序为:焊接速度离焦量激光功率,实现焊缝最优背宽比数据下的激光工艺参数为功率1400W,扫描速度800mm/min,离焦量为1mm;激光工艺参数对焊缝的堆高的影响次序为:焊接速度激光功率离焦量,实现焊缝最小堆高数据下的激光工艺参数为功率1400W,扫描速度900mm/min,离焦量为0;激光工艺参数对焊缝的变形角的影响次序为:激光功率离焦量焊接速度,实现焊缝最小变形数据下的激光工艺参数为功率1400W,扫描速度1000mm/min,离焦量为-1mm。(4)钛合金薄板背反射激光焊接接头组织和性能特性研究。以本文实验条件下优化工艺参数所获得的钛合金薄板背反射激光焊接接头为研究对象,较为系统地研究了焊接接头的横切面宏观形貌、焊缝区和热影响区的微观组织、硬度沿焊缝横切面的分布情况和拉伸强度及其断口形貌,并与相同激光工艺参数下常规激光焊接同种钛合金薄板试样进行对比。研究表明:相对于常规激光焊接,采用背反射激光焊接钛合金薄板所获得的焊缝呈现较为标准的“X形”,且正背面焊缝均较为平整;而且,背反射激光焊接焊缝中心区域晶粒略大,热影响区中的过热区和部分相变区要小,而正火区则要大。两种焊接方式测得硬度分布均呈现出相似分布规律,但背反射焊缝区硬度低于常规激光焊,而其他区域则略高于常规激光焊,母材基本保持一致;两种焊接方法下拉伸试件断裂位置均处于近焊缝基体处,断口均呈现出典型的韧窝特征即均属于塑性断裂。
[Abstract]:In view of the existing problems existing in the research on the back reflection laser welding of titanium alloy sheet, this paper takes the 1.5mm thick TC4 titanium alloy sheet as the subject and focuses on the theme of "the numerical analysis and characteristics of the low threshold back reflection laser welding of the titanium alloy sheet", and carries out the relevant theory and application basic research. The study of numerical simulation and analysis of field and stress field illustrates the change and distribution of the temperature field and stress field in the process of back reflection laser welding, and then lays a theoretical foundation for revealing the mechanism of back reflection laser welding forming. Meanwhile, the research on the forming process characteristics and the characteristics of the microstructure and properties through the laser welding of the back reflection laser welding of the titanium alloy sheet is verified. This new method of high efficiency and low threshold laser welding is suitable for thicker thin plates with a thickness of about 1 times larger than the existing research, which provides a technical support and application basis for further expansion of its application. The research work and the main achievements are as follows. (1) the numerical simulation and analysis of the temperature field of the laser welding process of the back reflection of the titanium alloy sheet. Based on the experimental verification of the "double heat source" temperature field model of the 0.8mm thick titanium alloy sheet back reflection laser welding process, the power and the power of the 1400W are set by setting the thickness of the 1.5mm. A new temperature field model is established by the scanning speed of 50mm/min. The temperature field of the back reflection laser welding process of 1.5mm thick titanium alloy sheet is simulated and analyzed. The study shows that the model can well simulate the distribution of temperature field in the process of back reflection laser welding, and the front and back of the thin plate are formed in a very short time. The stable temperature field similar to "teardrop", the front isotherm of the heat source is dense, the temperature gradient is large, the back end isotherm is relatively sparse and the temperature gradient is relatively small. The melting pool gradually transition from V shape to X shape temperature field time history, which describes the back auxiliary field formed in the back reflection laser welding process to the back of the workpiece two. The physical process of secondary heating; at the same time, with the increase of power or the decrease of scanning speed, the melting width and depth of molten pool gradually increase, the temperature gradient is proportional to the power, and the velocity is inversely proportional to the velocity. (2) the numerical simulation and analysis of the stress field in the process of laser welding of the back reflection of titanium alloy sheet. Based on the temperature field, the indirect coupling method is adopted. The stress field of the back reflection laser welding of titanium alloy sheet is simulated and analyzed. It is shown that the thermal stress increases with the time of welding in the process of back reflection laser welding of titanium alloy sheet, and the stress value of the titanium alloy sheet is most distributed at both ends and shows the comparison during the process of cooling. There are obvious boundary effects. At the same time, the residual stress on the upper surface of the transverse section of the weld is larger than the lower surface and the residual stress distribution is wider in the conventional laser welding, that is, the non reflective plate, and the residual stress curve of the upper and lower surfaces is higher and the residual stress distribution on the upper and lower surfaces is also basic. In addition, under different lines of energy, the longitudinal residual stress of the titanium alloy sheet with back reflection laser welding is very small, and the sensitivity of the energy of this surface to its welding residual stress is low. (3) study on the process characteristics of laser welding forming of the back reflection of the titanium alloy sheet. The three indexes of the back width ratio, the heap height and the deformation angle of the joint weld seam are used. The effect of laser technological parameters on the formability of low threshold laser welded joint with back reflection and its regularities were investigated. The results showed that under the experimental conditions, both the front and the back of the welding showed bright blue or golden yellow, the welding macroscopic morphology was better, the weld appearance had a typical X shaped weld; laser engineering was a laser worker. The influence sequence of the parameters on the back width ratio is: the laser power of the welding speed defocusing quantity, the laser process parameters under the optimal back width ratio of the weld are 1400W, the scanning speed is 800mm/min and the defocus amount is 1mm; the order of the influence of the laser process parameters to the heap height of the weld is the laser power defocuse of the welding speed, and the minimum heap height of the weld is realized. The laser process parameters of the data are 1400W, the scanning speed is 900mm/min and the defocus is 0. The order of the influence of the laser process parameters to the weld deformation angle is the laser power defocusing speed, the laser process parameters under the minimum deformation data of the weld are 1400W, the scanning speed is 1000mm/min, the defocus volume is -1mm. (4) titanium alloy thin. Study on the microstructure and properties of laser welded joint of plate back reflection. The laser welded joint of titanium alloy sheet, obtained by optimizing the technological parameters under the experimental conditions, was studied. The macroscopic morphology of the transverse section of the welded joint, the microstructure of the weld zone and the thermal shadow zone, and the hardness along the transverse section of the weld seam were systematically studied. The distribution and tensile strength and its fracture morphology are compared with the conventional laser welding of the same titanium alloy sheet under the same laser process parameters. The results show that compared with the conventional laser welding, the welding seam obtained by the back reflection laser welding of the titanium alloy sheet presents a more standard "X shape", and the weld on the front and back is more than that of the normal laser welding. In addition, the grain in the center area of the back reflection laser welding seam is slightly larger, the superheated area and the partial phase transition zone in the heat affected zone are small and the normalizing area is larger. The hardness distribution of the two welding methods shows a similar distribution law, but the hardness of the back reflection weld zone is lower than the conventional laser welding, while the other regions are slightly higher than the conventional laser welding. The parent material is basically the same. The fracture positions of the tensile specimens under the two welding methods are in the near weld base, and the fracture surfaces are typical of the dimples, that is, the plastic fracture.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG456.7

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