Experimental and numerical study on formability of friction stir welded TWB sheets based on hemispherical dome stretch tests
Introduction
In recent years, demand for light-weight and/or high-strength sheet metals such as aluminum, magnesium alloys and (advanced) high-strength steels has steadily increased in automotive applications associated with the need to reduce the weight of vehicles. Also, in a parallel effort to reduce the cost of raw materials and improve structural performance of automotives, automotive companies are increasing the use of tailor welded blanks (TWB) made of these new sheets. However, there are several difficulties to develop TWB particularly for aluminum and magnesium alloys because of their less familiar weldability requirements associated with conventional welding methods such as laser welding (Stasik and Wagoner, 1998, Pastor et al., 1999). Therefore, as a newly emerging welding technology for TWB, friction stir welding (FSW) was developed primarily for aluminum alloys in 1991 by The Welding Institute (TWI), in Cambridge, UK (Thomas et al., 1991). The FSW has various advantages over conventional fusion welding techniques such as its low capital investment, extremely low energy use and its capability to weld very thick plates with little or no porosity. In the FSW, work pieces are butted together and firmly clamped as schematically shown in Fig. 1 and then joining is achieved by heat and material flow generated by the FSW tool, which rotates as it moves along the butt line (London et al., 2003).
Considering its various advantages, many studies have been performed on FSW (Liu et al., 1997, Mahoney et al., 2001, John et al., 2001, Lockwood et al., 2002, London et al., 2003, Lee et al., 2003a, Cabibbo et al., 2007, Hirata et al., 2007, Afrin et al., 2008) and numerical simulations (Goetz et al., 2001, Ulysse, 2002, Colegrove et al., 2003, Soundararajan et al., 2005, Nandan et al., 2007, Zhang and Chen, 2008). However, they are mainly to understand the process itself, especially the effect of process parameters (tool geometry, tool materials, rotation speed, moving speed, tool angle, base materials and their arrangement in the advancing or retreating sides, base material thickness) on the quality of welding (temperature distribution, material flow or deformation, microstructure, residual stress, defects, texture) and studies on the macroscopic performance of friction stir welded TWB automotive sheets are rare. Therefore, the formability performance of friction stir welded TWB automotive sheets were experimentally and numerically investigated in this work, in particular based on the hemispherical dome stretch test. As for materials, five automotive sheets were considered in this work covering light-weight and high-strength sheets: 6111-T4, 5083-H18, 5083-O aluminum alloy, DP590 steel and AZ31 magnesium alloy sheets. Only the same base sheets were friction stir welded together for TWB samples.
Regarding measured mechanical properties within and around the FSW zone, many researches have been performed. For aluminum alloys, Sato and Kokawa (2001) reported that the FSW zone of 6063-T5 aluminum alloys had reduced strength and ductility compared to the base material. The reduction in strength and ductility of the FSW zone of 6XXX series aluminum alloys was also observed for 6056 aluminum alloys by Cabibbo et al. (2007). However, Hirata et al. (2007) found that formability in FSW 5083 aluminum alloys was improved by grain refinement in the stir zone while the flow stress was lowered (Johannes et al., 2007). As for dual-phase steels, Park et al. (2007) examined that the weld zone showed higher strength with less ductility, alike conventional mild steels (Lienert et al., 2003). As for magnesium alloys, the evolution of bigger grain size has been observed in the weld zone, resulting in lower hardness in the stir zone (Lee et al., 2003b, Cao et al., 2007, Afrin et al., 2008).
Note that the aluminum alloy, DP steel and magnesium alloy sheets studied in this work have FCC, BCC and HCP crystal structures, respectively. These samples commonly have a certain amount of anisotropic mechanical properties, even though they are not so significant. Also, they behave similarly under tension and compression because their plastic deformation involves mainly dislocation sliding (symmetric behavior), except the magnesium alloy sheet, which behaves differently under tension and compression (asymmetric behavior) because its plastic deformation is mainly subjected to dislocation sliding and twinning under tension and compression, respectively. Because of its added complexity of the asymmetric behavior, many studies have been performed recently on the mechanical properties of magnesium alloy sheets associated with twinning (Yoo et al., 2001, Agnew et al., 2003, Brown et al., 2005, Wu et al., 2008).
Since the distinctive asymmetric property of the magnesium alloy sheet is related to its crystal structure, many numerical modeling techniques have been recently developed based on polycrystal viscoplasticity. Staroselsky and Anand (2003) developed a crystal-mechanics-based constitutive model accounting for both slip and twinning. Agnew and Duygulu (2005) proposed a polycrystal plasticity model to describe the anisotropy and texture evolution. Agnew et al. (2006) also simulated macroscopic flow curves and internal strains in the textured magnesium alloy using the elasto-plastic self-consistent polycrystal model proposed by Turner and Tomé (1994). In order to model the temperature dependent twinning behavior as well as the flow stress profile, the strain anisotropy and texture evolution, Jain and Agnew (2007) introduced a viscoplastic self-consistent polycrystal model developed by Lebensohn and Tome (1993).
Even though the polycrystal model is effective to study the asymmetric/anisotropic property of the magnesium alloy sheet, it is not so to analyze its forming applications, for which the macroscopic approach is more cost-effective. Compared to the polycrystal modeling, however, the macroscopic constitutive modeling is still rare. Recently, macroscopic orthotropic yield functions were proposed by Cazacu and Barlat, 2004, Cazacu et al., 2006, which account for both the anisotropic and asymmetric yielding behavior of HCP metals. As for the continuum based numerical formulation, Lee et al., 2008, Lee et al., 2009 applied the two-surface model (Lee et al., 2007) to depict the anisotropic/asymmetric hardening as well as the un-twinning behavior Lou et al. (2007) observed during reverse stretching after twinning in compression.
The objectives of this study are twofold: one is to experimentally evaluate the formability performance of friction stir welded TWB automotive sheets, particularly regarding performance changes after welding, and also to develop a numerical simulation technique to better understand the performance changes experimentally observed. Note that the formability study of welded sheets is an issue of the forming performance evaluation of whole blanks to understand the combined effect of the mechanical properties of base materials and weld zones as well as the relative directions of weld lines with respect to loading directions. Therefore, the combined effect was studied here by mainly evaluating the forming performances of welded blanks compared to those of base blanks, utilizing the hemispherical dome stretch test. Cylindrical cup drawing tests were also performed in addition to HLD tests but results were similar; therefore, only the results of the HLD tests are discussed here.
As for the numerical method, the isotropic hardening law with the non-quadratic orthotropic anisotropic yield function, Yld2000-2d (Barlat et al., 2003), was implemented into the ABAQUS implicit codes (2002) using the user-subroutine in order to describe the anisotropic (but symmetric) properties of the aluminum alloy and DP590 sheets. For the AZ31 magnesium alloy, which showed asymmetry with different hardening between tension (or thinning associated with dislocation sliding) and compression (or thickening associated with twinning), the anisotropic/asymmetric yield function proposed by Cazacu et al. (2006) was applied along with asymmetric hardening laws for tension and compression.
Section snippets
Anisotropic yield function: Yld2000-2d
The orthotropic yield function for the plane stress condition, Yld2000–2d, is defined by two linear transformations; i.e.,where is the effective stress to represent the size of the yield locus. In Eq. (1), and are the principal values of the symmetric tensor , while is defined as, by linear transformations of s,Here, and (therefore, and ) contain
Materials and welding
In order to evaluate the formability performance of friction stir welded sheets, five automotive sheets were considered: AA6111-T4 sheets (with 1.5 mm in thickness), AA5083-H18 sheets (with 1.6 mm), AA5083-O sheets (with 1.6 mm), dual-phase high-strength steel (DP590) sheets (with 2.0 mm) and AZ31 sheets (with 2.0 mm). All the aluminum alloy sheets were supplied by ALCOA, while the DP590 and AZ31 sheets were provided by US Steel and Salzgitter, respectively. The chemical compositions of the sheets
Mechanical properties of base materials and weld zones
The (assumed) isotropic elastic properties and hardening behaviors of the base sheets were obtained by tensile testing following the standard procedure KS B 0801 with Instron 8516 series machine at a strain rate of 5.0 × 10−4 1/s. The gage sections for the standard specimens measured 50.0 mm long × 25.0 mm wide. For the hardening behavior of the weld zone, sub-sized tensile samples were tested at a strain rate of 1.6 × 10−3 1/s with the gage sections of 25.4 mm × 6.4 mm at the Ohio State University (Saunders
Experiments and FE simulations
In order to investigate the overall formability performance of the FSW sheets in the stretching deformation and also to analyze the effects of weld zone properties, hemispherical dome stretching (HDS) tests were performed experimentally and numerically for the welded sheets as well as for the base sheets. In order to consider two strain modes (near balanced biaxial and near-plane strain modes), two different initial rectangular blank sheets having 200 mm × 200 mm and 200 mm × 120 mm dimensions (with
Conclusions
In order to evaluate the formability performance of automotive friction stir welded TWB sheets, experiments were performed for the limit dome height (LDH) using the hemispherical dome stretching test. In order to better understand the LDH test results (and also to build up the prediction capability somewhat), a numerical technique was developed to simulate the test. To cover a wide variety of materials commonly applied to reduce the weight of vehicles, friction stir welding of five automotive
Acknowledgements
The work has been performed under the joint project between GM and SNU. The work has been also supported through the SRC/ERC Program of MOST/KOSEF (R11-2005-065), which is greatly appreciated. Appreciation also goes to John Brem at ATC and Frederic Barlat at Postech for performing the disc compression and bulge tests.
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