Effectively mitigated macro-segregation and improved tensile properties of twin-roll casting Al-Cu strips via the addition of TiC nanoparticles

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Abstract

In this work, Al-5Cu and TiC/Al-5Cu strips were fabricated by vertical-type twin-roll casting (TRC), and the effects of TiC nanoparticles on the Al-Cu strips were investigated. The minor addition of TiC nanoparticles refined the grain structure, increased the Cu concentration in the α-Al dendrites and effectively mitigated the center macro-segregation of the as-cast strip, which could be due to the heterogeneous nucleation effect and the solute diffusion control of the TiC nanoparticles. The yield strength, ultimate tensile strength, and uniform elongation of the as-cast, as well as T6 treated Al-5Cu strip with the addition of TiC nanoparticles were simultaneously improved owing to fewer residual Cu-rich phases, refined θ′ precipitates, and finer grain structure. This work provided a strategy for the effective control of macro-segregation and producing high-formability alloy strips by the TRC process.

Introduction

Twin-roll strip casting (TRC) is supposed to make a profound impact on the automotive manufacturing owing to its increased cost-effectiveness, energy efficiency and environmental sustainability, as compared to conventional direct-chill and hot/cold rolling processes (Zapuskalov, 2003). So far, most mass-produced aluminum sheets fabricated by TRC are limited to the low- and medium-strength alloy systems with low contents of alloying elements. Recently, the burgeoning requirements of practical high-strength aluminum sheets for industrial manufacturing have led to development on the preparation of aluminum products with high contents of alloying elements, such as the 2xxx and 7xxx series. However, Birol (2009) and Kim et al. (2018) found that depending on alloy types and casting conditions, various segregation-related defects (mainly center macro-segregation) can form in the TRC process, particularly in a high solute alloy with a wide freezing range. These defects are generally harmful to mechanical properties, resulting in reduced strength and ductility. Hence, it is of great significance to well control the macro-segregation in the TRC process.

Various methods have been essayed to alleviate the macro-segregation in the TRC process. Chen et al. (2017) and Rodrigues et al. (2020) have reported that the optimization of casting parameters (rolling speed, heat transfer coefficient, roll gap, etc.) might control the macro-segregation for a specific alloy, but the optimization is time-consuming. Besides, external physical fields including pulse electric current field (Sun et al., 2017), oscillating field (Su et al., 2014a, 2014b), electro and static magnetic field (Li et al., 2019), etc., have been applied to the TRC process. The applied external physical fields were more or less effective in reducing the macro-segregation in alloys with a low content of alloying elements through the called electromagnetic braking effect, the shock wave effect, and forced oscillation effect, but they made the TRC process more complex and costly. In the case of high solute aluminum alloys like 7075 Al alloy sheets (Su et al., 2014a,b), the macro-segregation remained after applying external physical fields. Generally, the amount of center macro-segregation rises with the increased alloying content. Hence, it requires economical and feasible methods to solve this problem in high solute alloy systems.

Birol (2007) investigated the effects of grain refiner Al-Ti-B/C containing micron/submicron-sized nucleants (TiB2/TiC) on the Al-Fe-Si thin foils and found that the Al-Ti-B/C showed grain refinement in TRC Al-Fe-Si thin foils and the center macro-segregation was reduced limitedly. Compared with commercial Al-Ti-B (mainly micron-sized TiB2) particles, TiC nanoparticles in Al-Ti-C master alloys are relatively more favorable than the TiB2 particles and are less apt to agglomeration. Geng et al. (2020) prepared the TiC/6061 Al composites and TiB2/6061 Al composites, concluding that the TiC nanoparticles reduced the density of second phases on the grain boundary, while the TiB2 particles caused denser second phases on the grain boundary. Moreover, the addition of TiC nanoparticles into Al alloys could reduce the micro-segregation and the mechanical properties were greatly improved (Liu et al., 2020). Hence, it is supposed that the effects of TiC nanoparticles may be effective on the grain refinement and mitigation of center macro-segregation in TRC. In this work, Al-5Cu and TiC/Al-5Cu strips were fabricated by vertical-type twin-roll casting, and the effects of TiC nanoparticles on the Al-Cu strips were investigated. This paper aims to provide a strategy for the effective control of macro-segregation and producing high-formability alloy strips by the TRC process.

Section snippets

Materials and methods

The material used in this experiment was Al-5Cu (wt%) binary alloy. The alloy was heated and melted in an electrical resistance furnace at 1073 K, and then de-slagged prior to casting. The nano-sized TiC particles were added into the Al-Cu melt through an Al-30 wt%TiC master alloy and the nominal content of TiC nanoparticles was controlled to be 0.5 wt%. The preparation of Al-TiC master alloy and the specific addition procedures to the melt have been reported by Liu et al. (2021), and the TiC

Results

The alloys prepared by TRC exhibited the inhomogeneous layered structure, as shown in Fig. 1. The grains around the strip surface were elongated grains inclining towards the rolling direction, and the grains around the mid-thickness region of the strip were near-equiaxed. The relative percentage of elongated grains in Al-Cu strip was higher than that in the TiC/Al-Cu strip. The addition of TiC nanoparticles showed a positive effect on the grain refinement. Besides, the EBSD maps shown in Fig. 1

Discussion

The solidification during TRC generally proceeds by dendritic growth from the roll surface. Recently, Zhang et al. (2020) have established a thorough thermo-kinetic model for the dendrite growth in TRC, connecting the dendrite growth and practical processing. The solute-rich liquid is pushed ahead of the solid/liquid (S/L) mushy zone during solidification with the presence of roll separation force, generating center macro-segregation, as illustrated in Fig. 9(a–b). Besides, in the study of

Conclusions

In this paper, Al-5Cu and TiC/Al-5Cu strips were fabricated by TRC, and the effects of TiC nanoparticles on TRC Al-Cu strips were investigated. The minor addition of TiC nanoparticles refined the grain structure, increased the Cu concentration in the α-Al dendrites and effectively mitigated the center macro-segregation of the as-cast strip, which could be due to the heterogeneous nucleation effect and the solute diffusion control of the TiC nanoparticles. The yield strength, ultimate tensile

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Xiao Liu: Investigation, Methodology, Writing - original draft, Formal analysis, Data curation. Qinglong Zhao: Conceptualization, Writing - review & editing, Funding acquisition, Supervision. Zhenwei Ji: Investigation, Methodology. Bao Wang: Methodology. Yunlong Zhu: Formal analysis. Qichuan Jiang: Funding acquisition, Supervision.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This work was funded by the National Natural Science Foundation of China (Grant No. 51790483) and the Graduate Innovation Fund of Jilin University (Grant No. 101832020CX143).

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