| 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. |
PDF Full Text Request