Key Takeaways & Executive Findings
- •• Cryogenic treatment combined with cross-rolling significantly enhances the mechanical properties of AZ31 Mg alloy, with the 3-pass sample achieving the highest tensile strength (351 MPa) and hardness (76.1 HV) with excellent uniformity. • The 2-pass sample exhibits superior ductility but suffers from poor hardness evenness, indicating a trade-off between strength and ductility depending on the number of rolling passes. • The strengthening mechanism for the 3-pass sample is attributed to fine grains, bimodal structure, high dislocation density, and precipitation strengthening, providing a comprehensive understanding of the microstructural evolution. • Increasing the number of passes to 4 or 5 leads to a decrease in overall performance due to repeated heat preservation, highlighting the importance of optimizing the number of passes in the process.
Abstract
In this paper, the multi cross-rolling and cryogenic treatment were adopted to process the AZ31 Mg alloy to study the influence of passes and cryogenic treatment on cross-rolled AZ31 Mg alloy. The tensile properties and hardness were tested. The microstructure was characterized using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) in order to elucidate the influencing mechanism. The results indicate that the treatment method can significantly improve the mechanical properties of AZ31 Mg alloy. The 3-pass sample processed by cryogenic treatment shows the highest strength (351 MPa) and has the highest hardness (76.1HV) and best hardness uniformity (standard deviation=0.9HV). The 2-pass sample has the highest ductility among all the samples but poor hardness evenness. The strengthening mechanism of 3-pass sample can be attributed to the fine grains, bimodal structure, high dislocation density, and precipitation strengthening. Due to repeated heat preservation of 4-pass and 5-pass, their comprehensive performances decrease.
1. Introduction
As a light-weight structural material, Mg alloy has broad applications because of its advantages, such as high specific strength and stiffness, high conductivity and convenience of recovery [1, 2]. However, due to the hexagonal close-packed (HCP) structure, its low absolute strength and low-temperature processability limit its development and application [3−6]. So, improving the mechanical properties of Mg alloy, broadening its applied range is greatly significant. Developing new processing technique is a practical method.
Compared with traditional rolling, cross-rolling can reduce the intensity of basal texture and improve anisotropy [7−9]. Moreover, PEI et al [10] reported that multi-pass high speed rolling can promote twinning-induced dynamic recrystallization (TDRX) mechanism and disperse grain orientations. MA et al [11] used multi-pass rolling in the second stage of rolling and found that coarse grain and twinning decreased at 3-pass. Due to the superiority of cryogenic treatment in the field of steel and iron [12−14], many researchers pay attention to the application of cryogenic treatment in Mg alloy [15]. KUMAR et al [16] stated that after cryogenic treatment, the rolled AZ31B Mg alloy presents higher micro-hardness, tensile stress (UTS), and yield strength (YS) than the rolled specimens due to grain refinement and the formation of Mg17Al12. CHEN et al [17] reported that the AZ31B Mg alloy gains greater strength though cryogenic rotary swaging than that at room temperature. DONG et al [18] indicated that the grain size of Mg-2Nd-4Zn Mg alloy was significantly refined and the number of second phase was increased by the cryogenic treatment. The main reasons of these situations can be attributed to that cryogenic treatment can make grains contract due to the characteristics of metal expanding when heated and contracting when cooled, changing the orientation of grain and introducing more dislocations. Meanwhile, cryogenic treatment has the function of promoting the precipitation of the second phases, which can improve the strength [19, 20]. However, the research of influence mechanism of cryogenic treatment combination of hot treatment is still limited.
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LU Li-wei, PANG Hao-ran, SHEN Tian-yuan, XI Yu-ze, WU Yu-juan, WANG Wen, JING Lei, LIU Gang (2025). Effect of rolling passes on AZ31 Mg alloy subjected to cross-rolling and cryogenic treatment. Journal of Central South University. https://doi.org/10.1007/s11771-025-6095-z
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Frequently Asked Questions
What is the effect of cryogenic treatment on AZ31 Mg alloy?
Cryogenic treatment significantly improves the mechanical properties of AZ31 Mg alloy by refining grains, increasing dislocation density, and promoting precipitation of second phases, leading to enhanced strength and hardness.
How does the number of rolling passes affect the properties of AZ31 Mg alloy?
The number of rolling passes influences the balance between strength and ductility. The 3-pass sample achieves the highest strength and hardness with good uniformity, while the 2-pass sample offers the best ductility but poorer hardness uniformity. Increasing to 4 or 5 passes reduces overall performance due to repeated heat preservation.
What are the main strengthening mechanisms observed in the 3-pass sample?
The strengthening mechanisms include fine grain size, bimodal structure, high dislocation density, and precipitation strengthening, which collectively contribute to the superior mechanical properties of the 3-pass sample.
Why does the 2-pass sample have poor hardness evenness?
The 2-pass sample exhibits poor hardness evenness likely due to less uniform microstructure and insufficient recrystallization compared to the 3-pass sample, which benefits from more complete grain refinement and precipitation.
What is the significance of combining cross-rolling and cryogenic treatment?
Combining cross-rolling and cryogenic treatment offers a practical method to enhance the mechanical properties of Mg alloys, addressing their low strength and processability limitations, and broadening their application range in lightweight structural components.
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