Electromagnetic-thermal coupling behaviors and thermal convection during two-phase zone continuous casting high strength aluminum alloy tube

Abstract

In this paper, the two-phase zone continuous casting (TZCC) technique was used to prepare 2D12 aluminum alloy tubes and a 3D model of TZCC was established to investigate the electromagnetic-thermal coupling as well as fluid flow behaviors under different mold temperatures. The calculated temperature in the induction heated mold agrees well with the measurement. It was found that the heat flux from the mold to the melt leads to a convex isoliquid fraction line of dendrite coherency point and circular thermal convection in front of the mushy zone. The velocity of the transverse flow increases remarkably with increasing mold temperature while the upward flow velocities at the outer wall are nearly constant. Experimental results indicate that the surface exudations with segregated solute appear at the surface of tubes when the mold temperature is higher than 883 K, and their size and quantity increase with increasing mold temperature. The serious solute redistribution and the thermal convection lead to solute segregation at the surface of tubes. Further, the residual liquid with segregated solutes flows to the surface of the tube and forms the surface exudation due to its lower freezing point than the Al matrix.

Introduction

High strength aluminum alloys tubes with high strength-to-density ratio, appropriate electrical and heat conductivities are widely used in the aerospace, transportation, electric power system and petrochemical industry applications [1,2]. Comparing with plastic processing method, continuous casting is a promising method for preparing tubes with near net shape, which has high efficiency and saves materials and energy [3,4]. However, the presence of macrosegregation in the ingot results in heterogeneous microstructures and mechanical properties and affects the subsequent processing and the final quality of the products [5].

The formation of macrosegregation results from the transportation of segregated solute at large scale due to the relative movement of melt and solid phases, as well as the rejected solute during solidification [6]. During direct chill casting, the water-cooled mold results in radial intensive heat flux and temperature difference in the melt. As a consequence, the melt solidifies from the wall to the center and a sump forms, and the radial temperature difference would lead to thermal convection in the sump [7]. Usually, negative segregation appears in the centerline and subsurface, while the positive segregation can be found at the surface and in the middle radius of the continuously casted high strength aluminum alloys ingot with wide solidification interval [8,9]. Further, serious segregation at the surface could result in surface exudation when the liquid with segregated solute penetrates through the outer shell of the ingot to the surface and finally solidifies [10]. The macrosegregation affects the quality of the ingot and can lead to edge cracking and streaking during further plastic processing. Therefore, it is much more economical to continuous cast high strength aluminum alloys ingot with good surface quality. It is believed that the formation and evolution of macrosegregation are highly dependent on the heat transfer and fluid flow during the continuous casting process [11,12]. In order to diminish the undesirable effects of the cold mold, several modified mold technologies of continuous casting have been proposed to reduce the mold (primary) cooling, such as low-head casting [13], hot top casting [14], lubrication through the mold, air pressurised mold [15], electromagnetic casting (EMC) [16,17]. The electromagnetic stirrer could induce fluid flow in the melt and result in more homogeneous distribution of the alloying elements, grain size as well as the intermetallic precipitate features [18]. However, the above techniques still use the cold mold for cooling and it is complicated to completely eliminate the effects.

The two-phase zone continuous casting (TZCC) technique first proposed by Liu et al. [19,20] is an effective method for processing alloys with wide solidification interval, which remarkably reduces the mold cooling during continuous casting, as well as promote the formation of columnar crystal and good surface quality [21]. During the TZCC process, the mold is induction heated and its temperature was controlled by an electromagnetic induction heating facility with fast heating rate [22,23]. The induction heated mold reduces the radial heat losses during continuous casting, which could reduce the radial temperature difference and the depth of sump. Moreover, it was reported that high strength aluminum alloys ingot with unidirectional columnar crystal, low concentration of defects and uniform microstructures own excellent tensile and fatigue properties [24,25].

The aim of the present work is to investigate the electromagnetic-thermal coupling process and its effects on the thermal convection and solute transfer as well as the formation of macrosegregation during the TZCC process. A verified 3D numerical model of TZCC is used for solving the problems. The numerical investigations are carried out under different controlled mold temperatures, and the 2D12 aluminum alloy tubes are continuously casted by TZCC as an example under the same conditions. Based on the simulations and experiments, our investigations could provide the prediction of macrosegregation formation during TZCC process based on heat and solute transfer behaviors.