Forming limit of 6061 aluminum alloy tube at cryogenic temperatures

https://doi.org/10.1016/j.jmatprotec.2022.117649Get rights and content

Highlights

  • Tube cryogenic bulging method was proposed to test the cryogenic forming limit in the tension-tension zone.

  • The established modified M-K model can predict the cryogenic forming limit with high accuracy.

  • Dramatically improved forming limit can be obtained at cryogenic temperature, facilitating the forming of complex components.

  • Improvement mechanism of cryogenic forming limit was revealed by theoretical analysis and fracture characterization.

Abstract

Cryogenic medium pressure forming has been proposed to fabricate aluminum alloy complex tubular components. It is urgently needed to evaluate and predict the cryogenic forming limit of aluminum alloy tubes for the novel process. Cryogenic bulging test using different length-diameter ratios was proposed to test the forming limit in the tension-tension zone. A modified M-K model was established to rapidly predict the cryogenic forming limit, based on the dynamic hardening with deformation. Theoretical analysis and fracture characterization were implemented to reveal the improvement mechanism of cryogenic forming limit from the macroscopic and microscopic perspectives, respectively. The forming limit of AA6061 tube at −196 °C is approximately double that at room temperature, especially in the tension-tension zone, which contributes significantly to the forming of complex features under biaxial tension. The cryogenic limiting deformation ability increases gradually as the strain state changes from plane strain to biaxial tension. Under the biaxial tension condition of λ=1.2, the limiting strain can reach 0.52 at −196 °C. The cryogenic forming limit can be successfully predicted by the modified M-K model with a prediction deviation of only 4.35% at −196 °C. The enhanced cryogenic hardening ability promotes the improvement of deformation uniformity and delays the occurrence of uncoordinated deformation. Ductile fracture is main fracture mechanism at cryogenic temperature. The significantly increased ductile fracture characteristics contribute to the improved cryogenic forming limit. This research can provide important guidance for the cryogenic forming of aluminum alloy tubular components.

Introduction

Aluminum alloy tubular components have gained increasing applications in automobile and aerospace industries due to their high strength to weight ratio (Zhai et al., 2020). As the increasing requirements for lightweight and high reliability in late-model transportation equipments, the use of integral components to replace traditional tailor-welded components has become an important approach (Yuan and Fan, 2019). Integration brings high accuracy requirement and complex structures, such as irregular sections, large cross-sectional differences, small local fillets, etc (Hu, 2013). In the forming process of these features, great challenges are caused by the poor formability of aluminum alloy (Hua et al., 2021). Heating can effectively improve the plasticity, so various hot forming processes have been developed, such as hot metal gas forming, superplastic forming, hot stamping and so on, as reviewed by Zheng et al. (2018). However, the lower hardening ability at high temperature results in the severe localized deformation and excessive thinning (El Fakir et al., 2014). Moreover, microstructure deterioration often occurs due to the complex microstructure evolution, which brings potential dangers to the service safety of components (Talebi-Anaraki et al., 2020).

In order to improve the plasticity and avoid the troubles caused by hot forming, new forming processes need to be developed to fabricate aluminum alloy complex tubular components. Gratifyingly, the cooperative enhancements in the elongation and hardening ability of aluminum alloy have been found at cryogenic temperatures. Xu et al. (2015) reported that the fracture elongation of AA6060 increases from 14.5% at room temperature to 21.5% at − 196 °C, and the work hardening rate also improves significantly at cryogenic temperatures. Liu and Hao (2021) tested the mechanical properties of AA7075 sheet at cryogenic temperature and found the uniform elongation and strain hardening exponent increase by 52.0% and 15.4%, respectively, compared to room temperature. Gruber et al. (2020) found that dynamic recovery is significantly suppressed at cryogenic temperature, based on an extended Kocks-Mecking method. This microscopic phenomenon results in an obviously increased cryogenic strain hardening rate. In addition, due to the obviously inhibited PLC effect at cryogenic temperature, the post-form surface quality significantly improves as reported by Schneider et al. (2014) and Wang et al. (2021). Cryogenic forming has been proved to have tremendous potential for forming complex-shaped plate components. Grabner et al. (2019) fabricated a miniaturized engine hood with superior surface quality by deep-drawing at approximately − 180 °C. Kumar et al. (2017) manufactured a B-pillar by cryogenic stamping. The maximum side depth increases from 6 mm at room temperature to 8 mm at − 150 °C. Recently, cryogenic medium pressure forming has been proposed to form aluminum alloy tubular components with complex shape and uniform wall thickness (Wang et al., 2022). During deformation, the tube is usually in a state between plane strain and biaxial tension. The formability of aluminum alloy tubes needs to be evaluated under different cryogenic loading conditions.

To predict the failure and formulate a reasonable manufacturing process, the forming limit curve has been widely used in industrial production to evaluate the formability under various forming paths, as reviewed by Zhang et al. (2018). For sheets, numerous methods can be used to test the forming limit simply and effectively, such as Nakazima test, Marciniak test, Hasek test and so on (Banabic et al., 2010). Due to the cross-section shape, these traditional tests are ineffective for tubes. At present, some methodologies have been developed to obtain the forming limit of tubes. Yoshida et al. (2005) adopted the combined tension-internal pressure testing, being capable of achieving arbitrary loading path for tubes, to research the forming limit under linear and combined stress paths. Based on a similar method, Korkolis and Kyriakides (2009) studied the path dependence of failure of Al-6260-T4 tube and they reported that failure stress is path-dependent under significant prestrain. Zhu et al. (2020) proposed a new forming limit diagram combining the geometric parameter of tubes, based on hydro-bulging with different end conditions. In order to evaluate the formability in tube hydroforming, Li et al. (2012) put forward a novel elliptical bulging method for determining the right hand side of forming limit curve, in which the strain state is controlled via different elliptical die inserts. Lateral compression-internal pressure cooperative control was used to establish the forming limit diagram of the seamed tube under different loading paths (Chen et al., 2011). At present, these methods are mainly adopted at room temperature, and there is no report on cryogenic forming limit test of tubes due to the numerous challenges caused by cryogenic environments, including equipment establishment, cryogenic pressurization, cooling, thermal insulation, etc. Therefore, novel cryogenic test method urgently needs to be established by addressing a series of problems to evaluate the cryogenic forming limit, especially in the tension-tension zone where tubular parts are often formed.

At present, many theoretical models have been proposed to predict the forming limit. Hill (1952) and Swift (1952) put forward the localized necking theory and diffuse necking theory, respectively. However, prediction results with larger deviations are often obtained. Based on the assumption of initial defects, Marciniak and Kuczyński (1967) proposed the groove theory, which has been most widely used to predict the forming limit (Li et al., 2020). In order to improve the applicability under various conditions including temperatures, strain rates, deformation paths, etc, M-K model has been rapidly developed, as summarized by Banabic et al. (2021). In view of the model's connatural defects, a lot of efforts have also been made to improve the prediction accuracy. Ahmadi et al. (2009) took into account the yield surface evolution and used the advanced BBC 2003 yield criterion to establish M-K model. Du et al. (2011) developed an M-K model with physical meaning, based on the relation among initial inhomogeneity coefficient, microstructure and strain. To reveal the influence of non-planar stress components on forming limit, an extended M-K model under non-planar stress state was established by Xiang et al. (2018), and they concluded that through-thickness shear stress can improve the formability. Yang et al. (2015) introduced the ductile fracture criterion into M-K model to improve the accuracy of instability judgment. An M-K localization model coupled with the evolving non-associated plasticity model was proposed by Shen et al. (2021) for forming limit prediction of AISI 439 steel with improved accuracy. However, these theoretical models are not universal under various conditions. And the models may require additional parameter testing or calibration, which brings enormous challenges due to the closed circumferential section characteristic of tubes and limitation of harsh cryogenic environments. Forming limit has been proved to be strongly correlated with hardening ability (Paul, 2021). So, accurate hardening ability evaluation during loading is the key to forming limit prediction, especially at cryogenic temperatures with high strain hardening exponent. Considering that existing models are invalid at cryogenic temperatures and forming may be performed in a wide cryogenic temperature range, it is urgent to establish an accurate theoretical model for rapidly predicting the forming limit of aluminum alloy tubes at different cryogenic temperatures.

In this study, a modified M-K model was proposed to theoretically predict the forming limit at different cryogenic temperatures. Cryogenic bulging method was put forward to test the forming limit of aluminum alloy tubes at cryogenic temperatures. The forming limit curve of 6061 aluminum alloy tube at − 196 °C was experimentally tested via a newly-established device. The validity of the established model for predicting cryogenic forming limit was verified by accuracy analysis. Furthermore, the improvement mechanism of cryogenic forming limit was revealed by theoretical analysis and fracture characterization. The research provides important guidance for the cryogenic forming of aluminum alloy tubular components.

Section snippets

Modified M-K model for predicting tube cryogenic forming limit

In this section, the shortcomings of traditional theoretical prediction are firstly analyzed, including the imprecise extrapolation of hardening curve and neglected dynamic change in hardening ability. Based on these analyses, a modified M-K model is established by introducing more accurate dynamic hardening exponent (n value) with strain under different loading conditions. The different cryogenic temperatures are taken into account in the theoretical model to achieve the rapid forming limit

Principle and device of cryogenic forming limit test

Cryogenic tensile and tube bulging were adopted to obtain the forming limit in the tension-compression and tension-tension zones, respectively, as illustrated in Fig. 5. The dimensions of the tensile specimens were shown in detail. In the tube bulging tests, the length-diameter ratio λ of deformation zone changes from 3.0 to 1.2 to achieve different strain paths. Considering that liquid nitrogen temperature (−196 °C) is most commonly used in cryogenic medium pressure forming process, the

Material

An extruded 6061 aluminum alloy tube with a thickness of 1.27 ± 2.4% mm and outer diameter of 46.04 ± 0.4% mm was used for the forming limit test. Before testing, all specimens were subjected to a solution heat treatment of 520 °C × 60 min in a resistance furnace with a temperature control accuracy of ± 1.0 °C and then quickly quenched by water. The axial mechanical properties of the material at different temperatures were obtained by uniaxial tensile test. The stress-strain relations at

Cryogenic forming limit

The bulging limit is often used to evaluate the formability of materials (Wang et al., 2019a). Fig. 8 shows bulging specimens with different length-diameter ratios λ. The strong correlation between the bulging limit and length-diameter ratio is observed. The expansion rate of the tube increases from 25.5% to 40.4% as λ decreases from 3.0 to 1.2 at − 196 °C. The cryogenic bulging limit is significantly improved. The expansion rate at − 196 °C can reach approximately 2 times that at room

Conclusions

The forming limit evaluation of aluminum alloy tubes at cryogenic temperature is an enormous challenge in metal forming field. In this study, cryogenic bulging method was proposed to test the cryogenic forming limit of tubes. A modified M-K model was put forward to theoretically predict the cryogenic forming limit. Theoretical analysis and fracture characterization were implemented to reveal the improvement mechanism of cryogenic forming limit from the macroscopic and microscopic perspectives,

Outlook

This paper focuses on the cryogenic forming limit test and prediction of aluminum alloy tubes and the analysis of the improvement mechanism. The proposed cryogenic bulging method can successfully measure the cryogenic forming limit of tubes. The established modified M-K model can accurately and rapidly predict the forming limit of tubes at cryogenic temperatures, which can be applied in actual industrial production due to its high efficiency and accuracy. Further studies regarding the cryogenic

CRediT authorship contribution statement

Xugang Wang: Data curation, Formal analysis, Writing − original draft. Xiaobo Fan: Methodology, Writing − review & editing. Xianshuo Chen: Visualization. Investigation. Shijian Yuan: Conceptualization, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was financially supported by National Key Research and Development Program of China (No. 2019YFA0708804) and the Fundamental Research Funds for the Central Universities (No. DUT20ZD101). The authors sincerely express appreciation to the funds.

References (42)

Cited by (14)

View all citing articles on Scopus
View full text