Fatigue properties of rolled magnesium alloy (AZ31) sheet: Influence of specimen orientation

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Abstract

High-cycle fatigue (HCF) and low-cycle fatigue (LCF) properties of a rolled AZ31-O alloy along different directions were evaluated at room temperature. Two types of samples denoted as RD (rolling direction) and TD (transverse direction) were compared because the samples along the two typical directions show an obvious anisotropy. By evaluating the fatigue parameters following the Manson–Coffin and Basquin’s equations, it is found that the fatigue lives of TD samples are longer than those of RD samples under both stress-controlled and strain-controlled cyclic loadings. Finally, the microscopic and macroscopic fracture features under tensile and fatigue loadings are observed and compared with the available literatures.

Introduction

A recent push by the automotive industry to lower the fuel consumption and cost of automobile production is providing enhanced motivation for the study of lightweight materials for structural components. Magnesium and its alloys have the lowest density among the practical metals and alloys, with high strength or stiffness-to-weight ratio, good machinability and recyclability [1], [2]. So they are being considered for automotive and aerospace applications [3], [4]. As structural materials in service, magnesium alloys always involve cyclic deformation, therefore, the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) properties of these materials need to be systemically investigated for safety reasons.

Currently, the majority of the magnesium alloys used in automotive applications is cast ones because of their high productivity and complex shape; at the same time, the defects such as casting porosity and cavity [5] made the structural component’s mechanical properties impaired, especially for the fatigue resistance. Wrought magnesium alloys can avoid the casting defects and improve the mechanical properties [6], especially the fatigue endurance. There are many processing technologies for magnesium alloys, such as extrusion, equal-channel angular pressing (ECAP) and rolling [7]. For the rolled magnesium alloys, the sheet can be made as large as for the requirement. Another feature of the rolled magnesium alloys is that the basal texture is easy to form, leading to the obvious anisotropy in its mechanical properties [8], [9], [10]. Therefore, it is necessary to systemically compare its mechanical properties, especially its fatigue properties along different directions.

There are a lot of fatigue data on magnesium alloys under stress-controlled condition [3], [7], [11], [12], [13], [14], [15], [16], some studies on magnesium alloys under strain-controlled condition [17], [18], [19], [20], [21], [22], [23], [24], but only very limited work about rolled AZ31 alloy sheet under strain-controlled condition [9], [18], [24]. Park et al. [9] recently reported that the LCF life of ND samples is longer than that of RD samples. In their study, the rolled AZ31 magnesium alloy sheets were found to have obviously anisotropic mechanical properties along RD and ND due to the texture and microstructure formed during rolling procedure. In this investigation, the tensile and compressive yield strengths of TD samples are greater than that of RD samples. Due to the obvious difference in the mechanical properties along RD and TD as stated above, further investigation on the fatigue behaviors of the AZ31 magnesium alloy sheet along the two typical directions is necessary under both strain-controlled and stress-controlled conditions. The most important is that the fatigue properties or resistance of the AZ31 magnesium alloy can be optimized based on the LCF and HCF tests.

Section snippets

Materials and experimental procedures

The material used in the present study was a rolled AZ31 magnesium alloy sheet. The chemical composition of this alloy is Al 2.8%, Zn 1.1%, Mn 0.4% and balance of Mg by wt%. The material was rolled for the first pass at 623 K with reduction of 10–25%, total reduction controlled under 60%; then at 573 K with reduction of 6–20%, total reduction under 50%; the last pass was rolled at room temperature with a reduction less than 5% and total reduction less than 15%. Finally, the sheet was rolled to a

Microstructure and texture

The pole figures of the rolled AZ31 samples on the sheet surface are shown in Fig. 2, there is a strong {0 0 0 2} texture in the alloy sheet with a maximum intensity of 14.61, while the {101¯0} and {101¯2} pole figures are rather weak. This strong basal texture reveals that the basal slip plane {0 0 0 1} in each grain is almost parallel to the sheet surface, however, the orientation distribution is not uniform along RD and TD, and more inclined to RD, which is one of the important factors influencing

Conclusions

Mechanical properties of TD and RD samples of rolled AZ31 alloy, including tensile, compressive and fatigue properties, were investigated in the current study and the main conclusions can be drawn as follows:

  • 1.

    The rolled AZ31 magnesium alloy shows strong anisotropic mechanical properties along RD and TD, both the yield strength and elongation of TD sample are better than those of RD sample.

  • 2.

    Tensile fracture modes along RD and TD are quite different, and the tensile fracture of the RD sample shows

Acknowledgements

The authors would like to thank Mrs. W. Gao and Dr. H.F. Zou for their help of the SEM-EBSD observations. National Natural Science Foundation of China (NSFC) under Grant Nos. 50625103, 50890173, 50931005 and the National Basic Research Program of China under Grant No. 2010CB631006.

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