均匀铝微滴沉积成形微观组织演化机理研究

发布时间：2018-05-19 09:44

本文选题：铝合金 + 微滴　；参考：《西北工业大学》2015年博士论文

【摘要】：均匀微滴喷射沉积技术是一种新兴的快速成形技术,它基于离散-堆积成形原理,以喷射产生的均匀微滴为基本制造单元,按照CAD模型分层数据在基板上逐点、逐层地沉积,从而实现三维实体的直接成形。该技术具有短周期、高柔性、适用材料广泛、无需专用工具等优点,被认为是一种极具潜力的微小复杂金属零件快速成形方法。然而,均匀微滴喷射沉积成形金属零件是个受多因素综合影响、多场耦合作用的复杂过程,涉及碰撞金属微熔滴的铺展、回缩、震荡与冷凝等瞬时热、动力学行为,以及连续沉积熔滴间的交互作用,使得沉积态金属微观组织结构的形成与演化有别于传统凝固成形过程,且研究难度较大,目前尚未针对此类问题开展系统而深入的研究。本文采用理论分析、数值模拟和实验研究相结合的方法,依托本课题组自行设计开发的均匀金属微熔滴按需喷射沉积成形实验平台,以7075高强度铝合金为实验材料,对均匀熔滴受控沉积成形行为、沉积态金属微观组织形成演化过程、接触熔滴界面结合方式以及沉积体内部常见缺陷等关键问题进行深入研究,以期为该技术在微小复杂金属零件快速制造领域的实际应用提供一定的理论与实验依据。本文主要研究内容和结果有:(1)利用改进的水平集法模拟研究了铝合金微熔滴的气动按需喷射产生过程,并探讨了飞行下落熔滴的热力学和动力学状态变化规律,进而确定了熔滴初始尺寸、碰撞速度和沉积温度等参数取值范围。在此基础上分析了金属熔滴/固态表面间的常见交互作用类型及其发生条件,并获得了熔滴非等温沉积过程中的热流耦合作用规律,涉及铺展、回缩和震荡等瞬时现象以及伴随发生的高温熔体冷凝行为,最终揭示了受控沉积铝熔滴的典型表面形貌(如波纹)及内部微观组织(包括粗大晶粒、微细定向枝晶组织及粗大非定向枝晶)的形成机理;(2)借助高速摄像技术,观察研究了均匀铝微滴连续沉积过程涉及的两种基本模式(层间垂直堆叠和层内水平搭接),明确了垂直柱和水平线条等典型一维结构组织形貌的形成与演化过程,掌握了周期性热循环效应和竞争生长机制对各类典型结构凝固组织的形成与演化过程的影响机理。进而分析了层间热累积效应的产生机理、影响因素、及其对垂直柱组织形貌与力学性能等的作用规律,为掌握不同模式下沉积态金属微观组织的形成与演化机理奠定了理论基础;(3)分析了均匀铝微滴喷射沉积三维实体的成形原理,探讨了基本沉积模式的结合方式以及扫描轨迹对熔滴沉积成形行为的影响规律,并观察研究了铝合金沉积体的组织形貌与物相分布特点,包括层状组织结构特征、晶粒形貌尺寸、相组成及其分布、以及元素偏析状况等,有助于进一步确定沉积态金属微观组织的强化机理;(4)确定了接触金属微熔滴间的主要界面结合方式(机械结合和冶金结合),并通过分析局部重熔温度条件,提出了熔滴界面结合状态的数值预测模型,并探讨了熔滴温度、沉积表面温度和界面接触热阻等主要因素对界面结合状态的影响规律。进而结合拉伸试验结果确定了熔滴界面结合状态对铝合金沉积体力学性能及其断裂方式的影响规律,为沉积态金属零件成形工艺参数的优化和组织性能的改善提供了依据;(5)观察了铝合金沉积体内部常见缺陷形貌特征,包括孔洞缺陷(间隙孔洞、气孔和凝固收缩孔洞等)和裂纹缺陷(熔合线裂纹和热裂纹等),并确定了上述各类缺陷的形成机理及主要影响因素,进而提出了抑制或消除不同缺陷的有效措施,为实现喷射沉积成形金属零件的应用奠定了基础。
[Abstract]:Uniform micro drop spray deposition technology is a new rapid forming technology. Based on the principle of discrete accumulation forming, it uses the uniform droplet produced by the injection as the basic manufacturing unit. The CAD model is layered on the base plate and deposited by layer by layer on the base plate. This technology has a short period, high flexibility and suitable application. It is considered to be a very promising method for rapid prototyping of small and complex metal parts without special tools. However, homogeneous micro drop spray forming metal parts is a complex process affected by multiple factors and multi field coupling, involving the spreading, retracting, concussion and condensation of micro droplets of collision. The interaction of heat, dynamics, and continuous deposition of droplets makes the formation and evolution of the microstructure of the deposited metal different from the traditional solidification process, and the research is difficult. At present, there has not been a systematic and in-depth study of such problems. This paper combines theoretical analysis, numerical simulation and experimental research. In this method, based on the experimental platform of the uniform metal micro droplet designed and developed by our group, 7075 high strength aluminum alloy is used as the experimental material. The controlled deposition forming behavior of uniform droplets, the formation and evolution process of the microstructure of the deposited metal, the contact mode of contact droplet interface and the common defects in the sedimentary body are taken as the experimental materials. In order to provide some theoretical and experimental basis for the practical application of the technology in the rapid manufacturing of small and complex metal parts, the main contents and results of this paper are as follows: (1) using the improved horizontal set method to simulate the gas dynamic injection production process of aluminum alloy micro droplets, and to discuss the flying process. The thermodynamic and dynamic state changes of droplet drop are carried out, and then the initial size of the droplet, the velocity of collision and the deposition temperature are determined. On this basis, the common interaction types and conditions of metal droplets / solid surfaces are analyzed and the heat flow coupling in the process of non isothermal deposition of molten droplets is obtained. The formation mechanism of the typical surface morphology (such as ripples) and the internal microstructure (including coarse grains, fine directional dendrites and coarse non directional dendrites) of controlled deposited aluminum droplets is revealed by the law, which involves the transient phenomena of spreading, retraction and concussion as well as the condensation behavior of high temperature melts. (2) the high-speed camera technique is used. Two basic modes (vertical stacking and horizontally overlapping in interlayer) were observed and studied. The formation and evolution of the typical structure of a typical one-dimensional structure, such as vertical column and horizontal line, was clarified. The periodic thermal cycle effect and the competitive growth mechanism have been grasped for the solidification structure of the typical structures. The mechanism of the formation and evolution process is analyzed, and then the mechanism of interlayer heat accumulation is analyzed, and the influencing factors, and its effect on the microstructure and mechanical properties of vertical columns, have been established to grasp the theoretical basis for the formation and evolution mechanism of the microstructures of the deposited metals in different modes. (3) the micro droplet injection of uniform aluminum is analyzed. The forming principle of three dimensional solid is deposited, and the combination mode of the basic deposition mode and the influence of the scanning trajectory on the droplet deposition forming behavior are discussed. The characteristics of the microstructure and phase distribution of the aluminum alloy deposits, including the characteristics of the layered structure, the size of the grain, the composition and distribution of the phase, and the element deviation are also observed and studied. The analysis is helpful to further determine the strengthening mechanism of the microstructure of the deposited metal. (4) the main interface mode of the contact metal droplets is determined (mechanical binding and metallurgical bonding). By analyzing the temperature conditions of the local remelting temperature, the numerical prediction model of the bonding state of the droplet interface is put forward, and the droplet temperature and deposition are discussed. The influence of surface temperature and interface contact thermal resistance on the interfacial bonding state and the effect of the interfacial bonding state on the mechanical properties and fracture mode of the aluminum alloy deposit are determined by the tensile test results, which provides the optimization of the forming process parameters and the improvement of the microstructure and properties of the deposited metal parts. (5) the characteristics of the common defects in the aluminum alloy deposits are observed, including hole defects (hole holes, pores and solidification shrinkage holes, etc.) and crack defects (fuse cracks and hot cracks), and the formation mechanism and main influencing factors of all kinds of defects are determined, and the effectiveness of reducing or eliminating different defects is proposed. The measures laid the foundation for the application of spray deposition forming metal parts.
【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.21

【相似文献】