Materials World Network: Developing a New Mg Alloy with Optimized Texture for Enhanced Formability
Final Report Abstract
Magnesium sheet alloys are in a worldwide spotlight of industrial interest with a view towards enhanced lightweight applications. Commercial magnesium sheets, however, suffer from poor formability at room temperature and from anisotropic mechanical properties. In order to form magnesium sheets into the shapes needed for, e.g., appropriate automotive applications, sheets and forming tools have to be heated up, thus increasing energy consumption and costs of the whole production process. Therefore, this project aimed to deepen the knowledge of the forming mechanisms of magnesium sheets and to provide knowledge, which finally will enable the design of magnesium alloys with increased room temperature formability. Sheet formability depends on the crystallographic structure of the sheets, which is usually described by means of textures. In commercial magnesium sheets the hexagonal crystals are usually oriented in a way that most of their basal planes are parallel to the sheet surface. The sheets exhibit a strong basal texture, which is responsible for limited formability and leads to anisotropic mechanical properties. It has often been observed that adding rare earth (RE) elements and/or yttrium to magnesium alloys could weaken the strong texture and, thus, improve formability as well as reduce the anisotropy of mechanical properties. For these alloys, the weakening of the texture is generally attributed to the recrystallisation. At project start, however, the responsible mechanisms were not understood, thus preventing the design of magnesium alloys with weak randomised textures, good formability at room temperature and isotropic mechanical properties. In this project the effect of RE-elements on the texture and properties of magnesium sheets was studied in detail and led to a better understanding of the deformation mechanisms during rolling of wrought magnesium alloys and forming of magnesium sheets. A number of rolling experimenls were performed with RE-elements containing magnesium alloys at various rolling temperatures leading to a number of conclusions. So it was found that PSN is an active mechanism contributing to the development of some RE-containing Mg-alloys. However, this mechanism does not appear to dominate the final texture. It was shown that the addition of a single RE-element like cerium, neodymium or yttrium within element dependent concentration limits significantly weakens the texture. The weakening of the texture is connected with the appearance of deformation bands containing twins and the restriction of grain growth by boundary pinning effects like, for example, Zener drag. Further addition of zinc, however, causes a part of the crystals to change their orientation towards the transverse direction of the sheets. Thus, the combination of RE elements and zinc within magnesium alloys should increase the formability and reduce the anisotropy of mechanical properties. This anticipation was clearly confirmed by forming experiments showing that 2E10 sheets (containing up to 1% zinc and less than 0.5% RE-elements) can be successfully deep-drawn at lower temperatures than the commercial magnesium alloy AZ31. The experimental work performed was closely linked with crystal plasticity modelling of the influence of texture on the formability by the US-partner project. So it became possible to model the forming limits of RE- and Al-containing magnesium sheets. Also themiodynamic modelling was used to analyse phase formation in the investigated alloys. The project generated new knowledge regarding the development of microstructure and texture of RE-containing Mg-alloy sheets. It also developed new insight into the recrystallisation mechanisms during rolling of these sheets. These results allow the development of magnesium sheets with a well recrystallised microstructure, more randomised texture, reduced anisotropy, and, thus, increased formability at lower temperatures compared to currently commercially available sheets (150-200°C vs. 200-250°C). The ability to form Mg-sheets at lower temperatures will reduce the energy consumption during magnesium sheet forming and, thus, increase the competitiveness of the final Mg-sheet products. Potential products are, for example, dash boards, inner roof parts, inner door sheets and outer roof parts of cars. Also inner sheet parts of trucks, light trucks and buses are realistic applications. On the longer term outer parts of car bodies may also be produced from Mg-sheets. Further potential applications are interior parts of aircrafts and helicopters like seat elements or inner door sheets. There are some findings in this project that implicitly raise the need for further research. It has, for example, been shown that the presence of RE-elements alone in magnesium is not sufficient for texture randomisation. The comparison of rolling experiments using binary alloys with experiments performed with the ZEK100 alloy showed that the interaction with further elements like, e.g., zinc is necessary to form additional texture components. Future research should clarify to what extent phase formation and particles in ternary and quaternary alloys influence recrystallisation and growth direction. Other research showed that the texture component towards TD appears also in other alloys, e.g. in modified AZ31 after the first rolling pass. During the subsequent rolling procedure, however, this component vanishes. Thus, it would be worthwhile to study in detail how the alloy composition controls the recrystallisation process and selects certain grain orientations for further growth. In this context it is worthwhile to investigate the influence of small amounts of alloying elements - even in concentrations less than 0.01 wt% - on the formation of phases and particles which in turn might influence recrystallisation behaviour and texture development. Thermodynamic simulations performed in this project showed, however, that these small amounts of impurities might remarkably influence thermodynamic/physical properties like solubility. This might explain, why in several samples of binary alloys (in as-cast, heat treated and rolled states) investigated in this project precipitates were found although the content of the alloying element was below the solubility limit given by binary phase diagrams. As information extracted from computed phase diagrams practically play an essential role in alloy design. These diagrams should be consequently re-computed taking into consideration small amounts of impurities.
Publications
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Magnesium sheet alloys for structural applications. Proceedings of the Light Metals Technology Conference 2007, ed. K. Sadayappan and M. Sahoo (Saint-Sauveur. Quebec: LMT, 2007), pp.189-200
K. Hantzsche, G. Kurz, J. Bohlen, K. U. Kainer, D. Letzig
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The texture and anisotropy of magnesium-zinc-rare earth alloy sheets. Acta Materialia, 55 (2007),2101-2112
J. Bohlen, M. R. Nürnberg, J. W. Senn, D. Letzig, S.R. Agnew
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Crystal plasticity-based forming limit prediction for noncubic metals: Application to Mg alloy AZ31B. International Journal of Plasticity 25 (2009) 379-398
C. John Neil and Sean R. Agnew
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Influence of process temperature on the formability of AZ31 magnesium alloy sheets. In: Proceedings of the 8th International Conference on Magnesium Alloys and their Applications, Wiley-VCH, 2009, pp. 571-576
L. Stutz, D. Letzig, K.U. Kainer
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Mg Sheet: The Effect of Process Parameters and Alloy Composition on Texture and Mechanical Properties. JOM, VOLUME 61, NO.08, August 2009, 38-42
K. Hantzsche, J. Wendt, K.U. Kainer, J. Bohlen, D. Letzig
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Microstructure and Texture Development in Binary Magnesium Alloys during Rolling. In: Proceedings of the 8th International Conference on Magnesium Alloys and their Applications, Wiley-VCH, 2009,627-632
Hantzsche, K.; Yi, S.B; Bohlen, J.; Yi, S.; Letzig, D.; Wendt, J.; Kainer, K.U.
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On the Microstructure and Texture Development of Magnesium Alloy ZEK100 During Rolling. Magnesium Technology 2009, ed. S. Agnew et al. (Warrendale, PA: TMS, 2009), pp. 289-294
J. Wendt, K. Kainer, G. Arruebarrena, K. Hantzsche, J. Bohlen, and D. Letzig
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Development of the Microstructure and Texture of RE containing Magnesium Alloys during Hot Rolling. Materials Science Forum Vols. 654-656 (2010) pp 580-585
K.U. Kainer, J. Wendt, K. Hantzsche, J. Bohlen, S.B. Yi, D. Letzig
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Effectof rare earth additions on microstructure and texture development of magnesium alloy sheets. Scripta Materialia, 63 (2010), 725-730
K. Hantzsche, J. Bohlen, J. Wendt, K.U. Kainer, S.B. Yi, D. Letzig
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Formability ofa more randomly textured magnesium alloy sheet: Application of an improved warm sheet formability test. Journal of Materials Processing Technology 210 (2010) 37-47
CE. Dreyer, W.V. Chiu, R.H. Wagoner, S.R. Agnew
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Mechanical anisotropy and deep drawing behaviour of AZ31 and ZE10 magnesium alloy sheets. Acta Materialia 58 (2010) 592-605
S.B. Yi, J. Bohlen, F. Heinemann, D. Letzig
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Texture weakening mechanisms of Rare Earth containing Mg-alloys. Dissertation, University of Virginia, December, 2010
J.P. Hadorn
