【作者】 刘宁；

【导师】 李廷举；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2013， 博士

【摘要】 双金属复层管是利用复合技术将两种具有不同性能的金属管材以层状方式在界面处实现牢固结合的新型结构和功能材料。其既能保持原管材的性能优势,又能通过“相补效应”弥补两种金属的缺点从而获得比单一管材更加优越的物理化学力学性能,被广泛应用于石油化工、航空航天、交通运输等领域。例如用作汽车散热器的3003/4045复层铝合金管,就有效结合了两种合金的性能优势,保证芯层具有良好耐蚀性的同时又提高了管材表面焊接性。目前制备双金属复层管的方法主要有：(1)塑性成形法,包括拉拔、胀接、旋压、滚压等冷成型法以及热轧、热挤压等热成型法；(2)焊接法,包括堆焊、卷焊、钎焊、爆炸焊接法等;(3)铸造法,包括离心铸造法和消失模铸造法等；(4)其它方法,如粉末冶金、激光熔覆、喷射成形等。但是传统制备技术存在工艺复杂,界面结合质量不稳定以及受到设备尺寸约束等缺点；采用连续铸造法生产双金属复层材料具有生产成本低、环境污染小、结合界面易实现冶金结合、可连续生产等优点,因此受到越来越多的重视。但是目前仅应用于复层板和复层棒的生产,还未见制备复层管材的报道。本文结合连续铸造的技术优势和复层管材的性能特点,以3003/4045复层铝合金管为实验对象,突破传统制备工艺,研究新型的铝合金复层管水平连续铸造技术。通过数值模拟计算电磁场对管坯凝固时温度场、速度场和液相率分布的影响,并结合具体实验确定最佳工艺参数,研究复层铝合金管坯组织、成分和性能特点,从传热学角度探讨结晶器内冷却区的传热行为,建立复层管坯的界面结合机理,同时探索复层铝合金管轧制过程中组织和性能的变化规律。本文主要研究内容包括：研究整体结晶器液—液电磁复合法制备复层管坯的新技术。采用ANSYS软件模拟旋转磁场对管坯凝固过程的影响。数值模拟结果表明,磁感应强度在石墨模具内的分布规律为边缘大中间小。电磁场均匀了内外层合金熔体的温度场,扩大了固液两相区,提高了内外层合金在环形石墨挡板出口处的温度和液相率,有利于在界面处形成良好的冶金结合。同时电磁场改变了石墨模具内合金熔体的运动方式,促使熔体沿复层管坯横截面产生强迫定向运动。在数值模拟的基础上优化设计旋转磁场发生器位置、石墨挡板长度及浇注温度等参数,并结合实验确定最佳工艺参数。当3003和4045合金的浇注温度分别为720℃和650℃,一次冷却水水量1m3/h,输入电流强度100A,铸造速度为120mm/min和140mm/min时,可制备出外径86mm,内外层壁厚分别为16mm和7mm的复层铝合金管坯。此时石墨模具内壁和中心处的磁感应强度分别为25.6mT和7.4mT。电磁场显著细化晶粒,消除复层管坯凝固组织的各向异性,促进了内外层合金之间形成冶金结合。由于实际铸造过程中复层管坯周向散热的不均匀性,管坯界面处同时存在机械附着,扩散结合和混熔三种结合形式。研究液—固复合法制备复层管坯的新技术,分别提出整体结晶器法和分体结晶器法。分析结晶器冷却区的传热行为,建立结晶器一维稳态传热模型,得出铸造速度v与管坯表面温度TX之间热力学关系,引入修正系数K,并通过实际测量计算出K的具体数值,从而就可以根据冷却水进出水口温差理论计算出不同拉速下管坯表面温度。对于整体结晶器法,当3003和4045合金的浇注温度分别为730℃和650℃,铸造速度100mm/min,冷却水量120L/h,可制备出直径60mm,内外层壁厚分别为9mm和3mm的复层铝合金管坯,在复层管结合界面附近存在一层以Al12(FeMn)3Si相为主的弥散颗粒层。对于分体结晶器法,采用ANSYS软件对中间包内的外层合金熔体的凝固行为进行模拟计算,探讨了内层管坯表面温度和外层合金浇注温度对其凝固过程的影响；当3003和4045合金的浇注温度分别为730℃和690℃,铸造速度160mm/min,内层冷却水量1.5m3/h,外层冷却水量0.12m3/h,保护气体0.2MPa时,成功制备出直径80mm,内外层壁厚分别为12mm和3mm的复层铝合金管坯。建立了复层管液—固复合的界面结合机理,分析认为在界面处4045合金熔体以3003合金表面作为基底发生非均匀形核,初生a-A1与基底之间形成共格或半共格界面,并以平面状、胞状和树枝状的生长顺序变化。凝固速率对晶体生长有明显影响,较快的凝固速率抑制了a-A1的生长；最终在界面形成良好的冶金结合。拉伸剪切实验中变形与断裂均发生在界面附近强度较低的3003合金侧,界面剪切强度大于3003合金的剪切强度极限,复层管顶部和底部界面剪切强度值相近且均匀。热轧试验结果表明,3003合金和4045合金在轧制过程中发生不均匀变形,样品从15.2mm轧制至1mm时包覆率从19.7%增加至33%；铸态组织中粗大针状共晶硅在轧制过程中发生破碎细化。轧制过程中4045合金侧显微硬度没有明显变化,而界面处和3003合金侧显微硬度值随样品变形量的增加而增加。更多还原

【Abstract】 Bimetallic tubes which consist of layers of two different materials have been widely used in many industrial fields due to their excellent mechanical and functional properties relative to that obtained in monolithic alloy parts. For example,3003/4045clad aluminum tube which has been used in the heat exchanger system of automobile can combine the advantages of individual raw materials, and the inner interface of the clad tube has excellent corrosion resistance while the outer interface has good weldability.Up to now, several conventional preparation methods have been developed for bimetallic tubes. These methods include:(1) Plastic forming method, such as drawing, expansion joint, spin forming method, rolling method and hot extrusion.(2) Welding process, such as bead weld, braze and explosive welding.(3) Casting, such as expendable pattern casting and centrifugal casting.(4) Other methods, such as powder metallurgy, spray forming process, et al. However, the main disadvantage of these methods is that the metallurgical bonding without discontinuities along the interface can not be achieved. Recently, much attention has been directed to continuous casting. By this method, excellent metallurgical bonding between the two different alloys can be obtained. Meanwhile, it can offer the advantages of low energy consumption, low costs and a simple production procedure, compared with other methods.This paper combines the technological superiority of continuous casting and the outstanding characteristic of clad tube, and researches on the new methods to prepare clad hollow billet by horizontal continuous casting. The main contents are as follows:The method of preparing clad hollow billet by horizontal electromagnetic continuous casting with liquid-liquid state has been studied. ANSYS software is used to compute the influence of rotate magnetic filed on the solidification of clad hollow billet. The results show that the distribution of magnetic flux density near the inner interface of graphite mold is much higher than that in the center of graphite mold. The electromagnetic field fiercely promotes the temperature field uniform, enlarges the mushy zone, and increases the temperature and liquid fraction of both internal and external alloys in the end of graphite plate to improve the combination of the two alloys. The electromagnetic field obviously changes the flow pattern of the melt, and the melt circumrotates in cross section of clad hollow billet.According to the results of numerical simulation, some experimental parameters, such as the location of coil, the length of graphite plate and the casting temperature, were optimally designed. When the casting temperatures of3003alloy and4045alloy are720℃and650℃, respectively, the cooling water flow rate is1m3/h, the input current intensity is100A, the casting speed are120mm/min and140mm/min, the clad hollow billet with internal layer of3003and external layer of4045was prepared. The outer diameter of the billet is86mm, while the thicknesses of the external and internal layer are7mm and16mm, respectively. The electromagnetic filed obviously refine the grains, change the inhomogeneous columnar grains into homogeneous equiaxed grains, and improve the formation of excellent metallurgical bonding between the two alloys. Since the heat transfer is nonuniform around the billet during casting processes, the bonding interface of the clad hollow billet consists of mechanical attachment, diffusion bonding and mixture at the same time.The method of preparing clad hollow billet by horizontal continuous casting with liquid-solid state has been studied. Single crystallizer and divided crystallizer methods were systemic investigated. The heat transfer in crystallizer was analyzed. The relationship between the casting speed v and the surface temperature of inter layer Tx was founded. The correction factor K was presented, and the value of K for this experiment was ensured through measurement and calculated. Then, the surface temperature of internal layer with the certain casting speed can be calculated through measure the temperature of inflow and outflow cooling water. For single crystallizer method, when the casting temperatures of3003alloy and4045alloy are730℃and650℃, respectively, the cooling water flow rate is120L/h, the input current intensity is30A, the casting speed is100mm/min, the clad hollow billet was prepared. The outer diameter of the billet is60mm, while the thicknesses of the external and internal layer are3mm and9mm, respectively. A lot of tiny particles which are identified to be the intermetallic compound Al12(FeMn)3Si are observed near the interface. For divided crystallizer method, ANSYS software was used to compute the solidification of external layer in the tundish, and the influence of the surface temperature of internal layer and the casting temperature of external melt on the solidification was discussed. When the casting temperatures of3003alloy and4045alloy are730℃and690℃, respectively, the cooling water rate for internal and external layers are1.5m3/h and0.12m3/h, respectively, the casting speed is160mm/min, the clad hollow billet was prepared. The outer diameter of the billet is80mm, while the thicknesses of the external and internal layer are3mm and12mm, respectively. The bonding mechanism of the two alloys combined by solid-liquid state was discussed.4045alloy attached on the surface of the3003alloy to nucleate and to grow. As the emergence and enlargement of constitutional super cooling zone on solidification front, the morphologies of the crystals of4045alloy changed according to following order:planar, cellular and dendritic. The solidification rate has great influence on the crystal grows. The failure and fracture always occurred in the3003side of the samples in the tensile-sheer test, indicating that the strength of the interface is higher than that of the3003alloy. The tensile-shear strengths in the different regions of the clad hollow billet are nearly uniform. Incompatible deformation between3003and4045layers took place during the rolling processes. The needle-like Si phase transformed into the dispersive particles when the thickness of the clad sample reduced from15mm to1mm, whereas no defects appeared in the interface. With the increase of the deformation, the values of microhardness increased in the3003side and interfacial region, while that stayed nearly constant in the4045side. The gradient distribution of hardness in the interfacial region was retained after the rolling deformation.更多还原