【摘要】：三維焊接是增材制造中的一類,直接從零件的數字化模型(例如CAD文件)中制造金屬部件。特別適用于生產形狀復雜,高價值和定制化零件,以及修復報廢零件。目前,三維焊接的主要挑戰(zhàn)包括成形缺陷(如氣孔),表面光潔度低,材料性質在空間上不均勻分布。這些挑戰(zhàn)主要來自于對增材制造復雜的物理過程認識不足,尤其是熔池動態(tài),比如熔池液態(tài)金屬流動,熱傳導,合金元素的蒸發(fā)以及固化。由于熔池面積小,同時瞬態(tài)變化較大,采用實驗的方法直接測量比較困難。而且,目前的光學和紅外攝像機在觀察熔池表面時也受到限制。所以,很難通過實驗研究熔池中熔融金屬的流動、熔池動態(tài)和宏觀缺陷。另一方面,基于嚴格求解質量、動量和能量守恒方程的數值模擬可以提供發(fā)生在增材制造過程中復雜傳輸現(xiàn)象的重要定量認識。首先,通過分析電弧增材制造中遇到的種種問題,發(fā)現(xiàn)電弧增材制造精度不高,組織性能不好的部分原因是對工件的熱輸入太大,因此提出了旁路耦合微束等離子弧焊(Double Electrode Micro Plasma Arc We l d i n g,D E-M PAW)。搭建了D E-M PAW三維焊接平臺,整個平臺由三維運動系統(tǒng)、DE-MPAW焊接系統(tǒng)、信號采集系統(tǒng)和焊接控制系統(tǒng)組成。運用CCD攝像機實時跟蹤熔滴過渡,采用熱電偶測量對母材的熱輸入并且實時采集焊接過程中的電流電壓。對DE-MPAW焊進行了一系列的基礎研究。分析了旁路電流對焊接過程中熔滴過渡、母材熱輸入和焊縫形貌的影響并采用優(yōu)化的工藝參數進行了單道多層堆積試驗。其次,采用FLOW-3D軟件,使用有限差分算法,開發(fā)了熔滴、熱源和熔池受力子模型,建立了考慮自由表面的DE-MPAW焊三維瞬態(tài)熔池模型。計算了熔池溫度分布和流動狀況,深入理解熔池中熔融金屬的傳熱過程和流動模式,為獲得高性能的SUS304不銹鋼DE-MPAW三維焊接成形件打下良好的基礎。在該模型中,將熔滴處理為在熔池上方固定高度,恒定溫度和初始速度的球形液滴。對模型中主路,旁路和熔滴之間的能量分配做出分析,假設熱源和電流密度符合高斯分布模式,考慮了外界與工件之間的蒸發(fā)、輻射和對流熱損失,以及馬蘭戈尼效應、電磁力、熔滴沖擊力、電磁力、浮力和等離子流力等對熔池對流的影響。然后,從模擬的角度分析了DE-MPAW三維焊接中單一熔滴定點動態(tài)堆積過程。熔滴由重力,電磁力和電弧等離子流力驅動,攜帶質量、動量和能量周期性的沖擊基板,形成一個液態(tài)熔池。計算了主要由熔滴沖擊動量,馬蘭戈尼效應、電磁力等綜合作用下的瞬態(tài)熔池形狀和熔池中復雜的流動。并且分析了旁路電流和冷卻時間對熔池溫度分布、流動模式和形貌的影響。最后,從理論角度分析了熔池中的各種作用力對流場流動方向、最大流速及熔池形貌的影響。在較全面的考慮了眾多因素后,研究D E-M PAW焊熔池的流場,并重點分析熔池中液態(tài)金屬的流動模式及固化后的焊縫組織。
[Abstract]:Three-dimensional welding is a kind of material augmentation manufacturing, which manufactures metal parts directly from the digital model of parts (such as CAD files). It is especially suitable for producing complex, high-value and customized parts, as well as repairing end-of-life parts. At present, the main challenges of 3D welding include forming defects (such as porosity), low surface finish and uneven distribution of material properties in space. These challenges stem mainly from a lack of understanding of the complex physical processes of fabricating materials, especially pool dynamics, such as molten pool metal flow, heat conduction, evaporation of alloying elements, and solidification. Because of the small area of the molten pool and the large transient change, it is difficult to measure directly by the experimental method. Moreover, current optical and infrared cameras are limited in their observation of molten pool surfaces. Therefore, it is difficult to study the flow, dynamics and macroscopic defects of molten metal in molten pool. On the other hand, the numerical simulation based on the strict solution of mass, momentum and energy conservation equations can provide an important quantitative understanding of complex transmission phenomena in the process of material augmentation manufacturing. First of all, by analyzing the problems encountered in the arc material increasing manufacturing, it is found that the low precision and poor structure and properties of the arc material increasing manufacturing are partly due to the excessive heat input to the workpiece. Therefore, a by-pass coupled microbeam plasma arc welding (Double Electrode Micro Plasma Arc We l d i n) D E-M PAW). Is proposed. The 3D welding platform of D E-M PAW is built. The platform consists of three dimensional motion system, DE-MPAW welding system, signal acquisition system and welding control system. The CCD camera is used to track the droplet transfer in real time, the thermocouple is used to measure the heat input of the base metal and the current and voltage in the welding process are collected in real time. A series of basic research on DE-MPAW welding is carried out. The effects of by-pass current on droplet transfer, heat input of base metal and weld morphology were analyzed. Single pass multilayer stacking test was carried out with optimized process parameters. Secondly, by using FLOW-3D software and finite difference algorithm, the submodels of droplet, heat source and molten pool force are developed, and the 3D transient molten pool model of DE-MPAW welding considering free surface is established. The temperature distribution and flow state of molten pool were calculated, and the heat transfer process and flow mode of molten metal in molten pool were deeply understood, which laid a good foundation for obtaining high performance SUS304 stainless steel DE-MPAW 3D welding forming parts. In this model, the droplet is treated as a spherical droplet with fixed height, constant temperature and initial velocity over the molten pool. The energy distribution between main circuit, bypass and droplet in the model is analyzed. Assuming that the heat source and current density conform to Gao Si distribution mode, the evaporation, radiation and convection heat loss between the outside and the workpiece, as well as Ma Lan Goni effect, are considered. The influence of electromagnetic force, droplet impact force, electromagnetic force, buoyancy force and plasma flow force on the convection of molten pool. Then, the dynamic stacking process of a single fusion titration point in DE-MPAW three-dimensional welding is analyzed from the point of view of simulation. Droplets are driven by gravity, electromagnetic force and arc plasma flow, carrying mass, momentum and energy periodically to form a liquid melting pool. The shape of the transient molten pool and the complex flow in the molten pool under the combined action of droplet impact momentum Ma Lan Goni effect and electromagnetic force are calculated. The effects of bypass current and cooling time on the temperature distribution, flow mode and morphology of the molten pool were analyzed. Finally, the effects of flow direction, maximum velocity and shape of molten pool on the flow direction of various forces in the molten pool are analyzed theoretically. The flow field of the molten pool in D E-M PAW welding is studied, and the flow mode of liquid metal in the molten pool and the weld microstructure after solidification are analyzed.
【學位授予單位】：蘭州理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG44

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