本文選題：鋁合金 + 微滴��； 參考：《西北工業(yè)大學》2015年博士論文

【摘要】：均勻微滴噴射沉積技術是一種新興的快速成形技術,它基于離散-堆積成形原理,以噴射產(chǎn)生的均勻微滴為基本制造單元,按照CAD模型分層數(shù)據(jù)在基板上逐點、逐層地沉積,從而實現(xiàn)三維實體的直接成形。該技術具有短周期、高柔性、適用材料廣泛、無需專用工具等優(yōu)點,被認為是一種極具潛力的微小復雜金屬零件快速成形方法。然而,均勻微滴噴射沉積成形金屬零件是個受多因素綜合影響、多場耦合作用的復雜過程,涉及碰撞金屬微熔滴的鋪展、回縮、震蕩與冷凝等瞬時熱、動力學行為,以及連續(xù)沉積熔滴間的交互作用,使得沉積態(tài)金屬微觀組織結構的形成與演化有別于傳統(tǒng)凝固成形過程,且研究難度較大,目前尚未針對此類問題開展系統(tǒng)而深入的研究。本文采用理論分析、數(shù)值模擬和實驗研究相結合的方法,依托本課題組自行設計開發(fā)的均勻金屬微熔滴按需噴射沉積成形實驗平臺,以7075高強度鋁合金為實驗材料,對均勻熔滴受控沉積成形行為、沉積態(tài)金屬微觀組織形成演化過程、接觸熔滴界面結合方式以及沉積體內部常見缺陷等關鍵問題進行深入研究,以期為該技術在微小復雜金屬零件快速制造領域的實際應用提供一定的理論與實驗依據(jù)。本文主要研究內容和結果有:(1)利用改進的水平集法模擬研究了鋁合金微熔滴的氣動按需噴射產(chǎn)生過程,并探討了飛行下落熔滴的熱力學和動力學狀態(tài)變化規(guī)律,進而確定了熔滴初始尺寸、碰撞速度和沉積溫度等參數(shù)取值范圍。在此基礎上分析了金屬熔滴/固態(tài)表面間的常見交互作用類型及其發(fā)生條件,并獲得了熔滴非等溫沉積過程中的熱流耦合作用規(guī)律,涉及鋪展、回縮和震蕩等瞬時現(xiàn)象以及伴隨發(fā)生的高溫熔體冷凝行為,最終揭示了受控沉積鋁熔滴的典型表面形貌(如波紋)及內部微觀組織(包括粗大晶粒、微細定向枝晶組織及粗大非定向枝晶)的形成機理;(2)借助高速攝像技術,觀察研究了均勻鋁微滴連續(xù)沉積過程涉及的兩種基本模式(層間垂直堆疊和層內水平搭接),明確了垂直柱和水平線條等典型一維結構組織形貌的形成與演化過程,掌握了周期性熱循環(huán)效應和競爭生長機制對各類典型結構凝固組織的形成與演化過程的影響機理。進而分析了層間熱累積效應的產(chǎn)生機理、影響因素、及其對垂直柱組織形貌與力學性能等的作用規(guī)律,為掌握不同模式下沉積態(tài)金屬微觀組織的形成與演化機理奠定了理論基礎;(3)分析了均勻鋁微滴噴射沉積三維實體的成形原理,探討了基本沉積模式的結合方式以及掃描軌跡對熔滴沉積成形行為的影響規(guī)律,并觀察研究了鋁合金沉積體的組織形貌與物相分布特點,包括層狀組織結構特征、晶粒形貌尺寸、相組成及其分布、以及元素偏析狀況等,有助于進一步確定沉積態(tài)金屬微觀組織的強化機理;(4)確定了接觸金屬微熔滴間的主要界面結合方式(機械結合和冶金結合),并通過分析局部重熔溫度條件,提出了熔滴界面結合狀態(tài)的數(shù)值預測模型,并探討了熔滴溫度、沉積表面溫度和界面接觸熱阻等主要因素對界面結合狀態(tài)的影響規(guī)律。進而結合拉伸試驗結果確定了熔滴界面結合狀態(tài)對鋁合金沉積體力學性能及其斷裂方式的影響規(guī)律,為沉積態(tài)金屬零件成形工藝參數(shù)的優(yōu)化和組織性能的改善提供了依據(jù);(5)觀察了鋁合金沉積體內部常見缺陷形貌特征,包括孔洞缺陷(間隙孔洞、氣孔和凝固收縮孔洞等)和裂紋缺陷(熔合線裂紋和熱裂紋等),并確定了上述各類缺陷的形成機理及主要影響因素,進而提出了抑制或消除不同缺陷的有效措施,為實現(xiàn)噴射沉積成形金屬零件的應用奠定了基礎。
[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.
【學位授予單位】：西北工業(yè)大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG146.21

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