Failure analysis of fusion clad alloy system AA3003/AA6xxx sheet under bending
Introduction
Bending is an important forming property in a range of applications. For a monolithic sheet under bending, Datsko and Yang [1] have shown that the area reduction (or macroscopic fracture strain) and bendability are related, with the assumption that failure occurs in the outer surface of a material being bent when the true strain there equals the true strain at the instant of fracture in a tensile test specimen. As for monolithic automotive aluminum alloys e.g. AA5754 and AA6111, bendability has been shown by some experiments such as the cantilever bend test (CBT) and wrap-bend test to be strongly correlated with the material microstructures, especially, the statistics of second phase particles such as volume fractions and distributions [2], [3]. The onset of sheet failure during bending starts with outer surface rumpling followed by micro-cracking as the maximum tensile stress increases. As the bend angle increases, these micro-cracks further develop and propagate inward to the neutral axial of the sheet until the entire sheet fails.
A new co-casting technology, the Fusion technology [4], [5], has been developed by Novelis to produce clad materials by direct chill (DC) casting. The improvement of sheet formability in this clad system by the Fusion technology has been intensively investigated. Previous works by Lloyd et al. [6], Jin and Lloyd [7] show that Al–Si–Mg–Cu alloy AA6111 clad with ductile Al–Mn alloy AA3003 through the Fusion technology results in a significant improvement in bendability. It is generally accepted that the strain distribution at the outer surface is very inhomogeneous during bending. The soft clad layer usually demonstrates higher bendability than the hard core alloy. The increase in bendability, observed in previous studies of Fusion AA3003/AA6111 type material, relies upon the softer, more ductile clad layer accommodating the strain at the outer edge of the bend that results from its superior elongation to fracture. Fusion material with a high bendability clad layer tends to minimize the localized shear band evolution and delay the outer surface crack as the bend angle increases. This improvement of bendability through cladding of a more ductile layer relies on the assumption of perfect interface bonding and no sub-surface cracking. Previous studies [6], [7], [8] have pointed out that second-phase particles in the core, fine or coarse, play an important role in bending performance. When fine particles are precipitated in the grain boundaries, the bendability is significantly reduced due to the decreasing grain boundary strength. On the other hand, when many particles with particle size over 1 μm are present, cracks may initiate around the particles. If the particles are coarse and have a high aspect ratio, extensive particle fracture may occur during rolling or bending processes, and the cracked coarse particles are often the nuclei for failure.
Computational modeling, e.g. finite element (FE) analysis, provides a useful tool to study the bending behaviors in terms of predicting the bendability and understanding the failure mechanism at a microstructural level. Kuroda and Tvergaard [9] used a unit-cell model with simplified microstructure information to study the texture components in the rolled aluminum alloy sheet on shear band formation in plane tension/compression and bending. Besides the study of texture and material properties on the bending performance, Dao and Li [10] investigated the second-phase particle position, distribution and its influence on pure bending behavior. Hu et al. [11], [12] studied the particle shape and volume fraction and distribution on strain localization and post-necking deformation/fracture in uniaxial tension of AA5754 alloy. Hu et al. [13] further performed a macro–micro-multi-scale modeling analysis to investigate the influence of particles distributed primarily in stringers on bending performance in DC casting AA5754 sheet.
In the current work, an experimental study is conducted to investigate potential failure of fusion product of AA6xxx cladded with a more ductile AA3003 layer. We focus on the crack initiation and propagation in this laminate system during bending. To further assist the understanding of bending failure mechanism in this laminate system, we extended Hu׳s approach [13] and numerically study the effect of second phase particles on this clad alloy׳s bending; and investigate the possible factors which could influence the crack growth leading to the failure of bent sheet.
Section snippets
Experiments and results
An ingot of AA6xxx Al–Si–Mg–Cu alloy clad on both sides with AA3003 Al–Mn alloy was co-casted by Fusion technology. The chemical compositions are listed in Table 1. The ingot was sawed and machined into four 50 mm thick blocks. The blocks were homogenized for 8 h at 560 °C, hot rolled to 5 mm, then cold rolled to the final gauge of 0.914 mm with an average thickness of 8% clad layer of AA3003 alloy. Test samples were solutionized for 2 min at 560 °C, force air quenched for 30 s and then naturally aged
The multi-level modeling scheme
The multi-level modeling technique applied to the CBT simulation was explained by Hu et al. [13]. The purpose of the macro–micro-multi-scale finite element modeling is to capture the sheet deformation behavior at the microstructural level with a reasonable computational scheme. Firstly, a sample sized plane strain macro-model simulation for CBT is performed and the location of the highest strain is identified. Then a microstructurally based sub-model (plane strain) of this location is developed
Modeling results
The intention of this bending simulation is to investigate the particle induced failure in a laminate system such as AA6xxx clad with AA3003 during Cantilever bending. The global model is based on the experimental setup. In bending, there are two regions of large deformation when the sheet is bent over the mandrel, which are the outer (top) and inner (bottom) surfaces. The outer surface experiences hydrostatic tensile stresses which usually causes material damage, while the hydrostatic stresses
Discussion and conclusions
Bendability is an important characteristic for aluminum sheet. The basic mechanics of bending are well established involving plane strain tension, which is a maximum at the outer surface, and decreases to zero at the neutral axis close to the center thickness of the sheet, and where after it changes to compressive strain. Several approaches have attempted to relate bendability to other mechanical properties at the continuum level, such as tensile elongation, but the most successful has been the
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Cited by (15)
Evolution of microstructure and properties of AA 7072/7075/7072 composite sheets during various finish rolling temperature
2023, Journal of Materials Research and TechnologyBendability enhancement of an age-hardenable aluminum alloy: Part II — multiscale numerical modeling of shear banding and fracture
2019, Materials Science and Engineering: ACitation Excerpt :This is contrary to actual bending operations which are rather complex, and the pure bending condition is rarely achieved in practice due to surface contacts between tooling and the sheet part being bent [11,37]. Lastly, studies on micro-cracks and failure during bending are even scarcer and are primarily focused on understanding the effects of particle distribution on failure using phenomenological plasticity [37,38]. In a companion paper, hereafter referred to as Part I [39], a comprehensive experimental investigation is conducted to study the fracture behavior of an age-hardenable aluminum alloy AA6016 during wrap-bending and it is shown that its bendability can be improved significantly by cladding it with a lower strength and higher ductility aluminum alloy AA8079.
Bendability enhancement of an age-hardenable aluminum alloy: Part I — relationship between microstructure, plastic deformation and fracture
2019, Materials Science and Engineering: ACitation Excerpt :Several studies have been performed to relate the bend performance of precipitation hardened aluminum alloys to microstructural changes occurring during bending [12–16]. It is suggested that bendability in these alloys is limited by surface induced cracking due to intense strain localization, preceded by the development of surface roughness and progressively increasing surface undulations [3,8,9,17–22]. In other words, the development of surface roughness during bending is a key component that controls the overall bending performance.
Bending properties of functionally graded 300M steels
2016, Materials Science and Engineering: ACitation Excerpt :In the graded steels, the presence of ductile outer layers improves the fracture resistance in bending. Similar observations were reported in a study by Shi et al. on bendability of a clad AA3003–AA6xxx alloy system processed by fusion technology [15]. They found out that the clad alloy system exhibits high bendability due to the introduction of a softer layer at the outer edge.
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