Formability testing of AZ31B magnesium alloy tube at elevated temperature

https://doi.org/10.1016/j.jmatprotec.2010.01.020Get rights and content

Abstract

Magnesium alloy is a promising material for reaching the goal of weight reduction in automotive and aerospace industry. The mechanical properties of AZ31B seamless tube were tested by ring hoop tension test and compared with the results by axial tension test. Hydro-bulge test and newly proposed flatten-hydro-bulge test were also carried out to evaluate the formability of the tube. The effect of forming temperature and flattening displacement on the maximum expansion ratio and bursting pressure were measured and discussed. A tubular part with curved axis and different cross-sections was then manufactured successfully. It was found that the changing tendency of total elongation in hoop direction is quite different with that along axial direction. As temperature increased from RT. to 300 °C, two peaks of total elongation in hoop direction occurred at about 150 °C and 300 °C. In hydro-bulge test, the maximum expansion ratio increased as temperature increasing and reached the maximum value about 30% at 170 °C, then decreased quickly at higher temperature until 230 °C. In flatten-hydro-bulge test, flattening deformation has significant effect on the subsequent hydro-bulging process, and both maximum expansion ratio and bursting pressure decreased as flattening displacement increased. This should be taken into account during part design and process determining.

Introduction

Tubular components manufactured by hydroforming have been widely used in automotive and aerospace industries to reach the goal of weight reduction and energy saving. Light-weight materials, such as aluminum and magnesium alloys are becoming very promising materials in realizing further fuel cost reduction and engineering design flexibility (Merklein and Geiger, 2002). However, the traditional tube hydroforming technique for steel tubes cannot be applied for light-weight materials because of their poor formability at room temperature. In addition, aluminum and magnesium tube are generally manufactured by extrusion, which will result in considerable property differences along extrusion direction and hoop direction. This anisotropic characteristic of these tubes will decrease the whole formability for hydroforming process (Groche and Breitenbach, 2005).

As for welded tube, tensile test properties obtained using specimen taken from flat sheet prior to roll forming and welding will change during the tube manufacturing process. Uniaxial tension test using flat or arc-shaped specimen taken from the tube blank is often used to test the mechanical properties of tube, especially along axial direction. As for extruded tubes with considerable property difference along axial direction and hoop direction, tension test should be carried out separately along both directions. Murai et al. (2003) investigated the effect of extrusion conditions on microstructure and mechanical properties of AZ31B magnesium alloy extrusions. However, no accurate data in hoop direction can be obtained if traditional uniaxial tension test is used, because work hardening will cause considerable errors in characterizing the mechanical properties, especially for tube with small radius (Kleiner and Uggowitzer, 2004). Ring stretch tensile test was first proposed by Price (1972) to analysis the failure of Zircaloy cladding under transverse plane-strain deformation. In this test, two D-blocks were inserted in the ring specimen, and then were separated reversely to cause deformation of the specimen. Later, Cohen et al. (1999) and Wang et al. (2002) developed this testing method in order to better describe the anisotropic characteristic of extruded tubes. In the analytical model they developed, friction between the D-blocks and specimen was neglected, which may be invalid when testing at elevated temperature. Later, He et al. (2008) investigated the effect of friction between specimen and D-blocks. It was shown that uneven deformation will happen and necking and fracturing tends to occur near the edge of gauge section. Therefore, special attention should be paid to measuring and analyzing the strain distribution.

Hydro-bulge test (also called free expansion test) is a new method to evaluate the property of tube instead of tension test, because no additional deformation will be caused during specimen preparation (Fuchizawa and Narazaki, 1993). In additional, the stress state in hydro-bulge test is quite similar to hydroforming process, which can reveal the characteristic of tube better.

Many results have been reported up to now about this testing method. A tube bulge tests system was developed and analytical method for the stress–strain characteristics of thin tubes was proposed by Fuchizawa and Narazaki (1993). Axial force and internal pressure can be controlled separately in order to change the forming condition, i.e., the stress state of the tube. Using this testing system, the forming limit diagram of aluminum alloy tube was determined (Fuchizawa et al., 2002). Sokolowski et al. (2000) described a practical test method for determining the flow stress of tube materials, which can also be used to evaluate the formability and quality of the tubes. Later, one simple testing die-set was developed, which can change the stress state of tube by using tube specimen with different expansion length (Altan, 2002). However, only the right side of FLD can be determined by this method, because the ends of tube were fixed during hydro-bulging and the tube will be expanded under biaxial tension stress state.

Hwang and Lin (2006) developed an analytical model for hydraulic bulge test, and the anisotropic effects of tube during bulge forming in an open die were investigated. The effect of anisotropic values on the forming pressure and maximum bulge height were discussed. Based on hydro-bulge test, the formability of AZ31 magnesium alloy tubes at elevated temperatures were tested and evaluated (He et al., 2007). Liu and Wu (2007) investigated the microstructure evaluation of AZ31 magnesium alloy tube after hot metal gas forming, with special attention on the anisotropic characteristic of extruded tubes. Using a servo-controlled tension-internal pressure testing machine, the anisotropic plastic deformation behavior of extruded A5154-H112 aluminum alloy tubes was investigated (Kuwabara et al., 2005). The performance of different anisotropic yield functions in predicting the response and burst of tubes loaded under combined internal pressure and axial load was assessed, considering the observation that localized wall thinning and burst can be very sensitive to the constitutive description employed for the material (Korkolis and Kyriakides, 2008a, Korkolis and Kyriakides, 2008b). It was found that for some loading paths the computed deformations did not agree with the experimental ones, whereas rupture was generally overpredicted. These discrepancies between experimental and predicted results may be caused by inadequate representation of the anisotropy by the constitutive models adopted, and by the omission of deformation-induced anisotropy. Later, this problem was tackled using a more advanced yield function (Korkolis and Kyriakides, 2008a, Korkolis and Kyriakides, 2008b). Three different calibration schemes of this function were employed, in two of which the experimentally observed deformation-induced anisotropy was taken into account. It was demonstrated that both deformation and failure can be predicted successfully, using a hybrid procedure they developed. The effect of loading path traced on the formability of Al alloy tubes was investigated later (Korkolis and Kyriakides, 2009). A set of Al-6260-T4 tubes were loaded along orthogonal stress paths to failure and the results were compared to those of the corresponding radial paths. The results confirmed that failure strains are path-dependent, and failure stresses become path-dependent if the prestrain is significant in either hoop direction or axial direction. Based on a rate-dependent crystal plasticity theory, the large strain deformation phenomena of magnesium alloy tubes were simulated under various stress states. Forming limit diagrams (FLDs) were also simulated using the Marciniak–Kuczynski (M–K) approach (Lévesque et al., 2009).

As well known, bending and preforming are often needed for hydroforming of tubular part with complex shapes and different cross-sections. During bending or preforming, the tube often undergoes considerable deformation, which will have a big effect on the subsequent hydroforming process. Therefore, the effect of former deformation on later hydroforming should also be tested and evaluated. However, there are few research results available on this subject.

In this paper, tension test in hoop direction (Ring Hoop Tension Test) will be used to characterize the material properties of AZ31B magnesium tube. Hydro-bulge test will be used to investigate the formability of AZ31B extruded tube at different temperatures. A new test called flatten-hydro-bulge test will also be proposed and used to investigate the effect of previous deformation on formability for later hydroforming process. Based on the tested results, a tubular component with curved axis and different cross-sections will be formed.

Section snippets

Formability testing by ring hoop tension test

As mentioned above, in order to give a better evaluation of the formability of extruded tube, Wang proposed one new method called Ring Hoop Tension Test (RHTT) (Wang et al., 2002). This test uses a universal tensile testing machine to expand a ring specimen directly with a special test fixture. When the ring specimen was expanded, its original ring shape can be maintained, which can avoid the effect of work hardening during traditional flat specimen preparation, especially for tube with small

Formability testing by hydro-bulge test

As mentioned above, hydro-bulge test is now often used to test the formability of tube for hydroforming, because in hydro-bulge test the tube deforms in a stress condition similar to hydroforming process.

As well known, in hydro-bulge test the stress state of tube can be determined or controlled by different methods. One method is to control the axial force and inner pressure. The other is to change the length of bulge forming zone. If the tube can be tested in a stress state similar to ring

Formability evaluation after die-closing process

Flattening test is a standard method for determining the ability of metallic tubes of circular cross-section to undergo plastic deformation by flattening. It may also be used to reveal the defects in the tubes (ISO, 1998). However, this test does not aim to get the forming limit of tube for this kind of deformation. The tube can be accepted if it can undergo specified flattening deformation without visible cracks. What's more, flattening is just one forming step for tube hydroforming process,

Warm hydroforming of tubular part

Based on the mechanical properties and formability measured above, a tubular part with curved axis and different cross-sections will be manufactured using the warm hydroforming system developed in Engineering Research Center of Hydroforming, Harbin Institute of Technology, China. This system was developed on a tube hydroforming machine, equipped with 3150 kN closing force, up to 400 MPa high pressure system and three-axial servo-control system. The warm hydroforming system uses heat transfer oil

Discussion and conclusions

The changing tendency of total elongation in hoop direction is quite different with the result along axial direction. As temperature increased from RT. to 300 °C, two peaks of total elongation in hoop direction occurred at about 150 and 300 °C, with a minimum value at 210 °C. This result presents the considerable anisotropic characteristic of the tested AZ31B tube.

Hydro-bulge test with axially free tube ends was designed and realized, by which the tube can be expanded in a simple tension stress

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 50805033), China Postdoctoral Science Foundation (No. 200801285) and the Development Program for Outstanding Young Teachers in Harbin Institute of Technology (HITQNJS.2008.016). The authors would like to take this opportunity to express their sincere appreciation to the funding.

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