Giuliano Superplastic Forming of Advanced Metallic Materials

Methods and Applications
1. Auflage 2011
ISBN: 978-0-85709-277-9
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 384 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Ultra fine-grained metals can show exceptional ductility, known as superplasticity, during sheet forming. The higher ductility of superplastic metals makes it possible to form large and complex components in a single operation without joints or rivets. The result is less waste, lower weight and manufacturing costs, high precision and lack of residual stress associated with welding which makes components ideal for aerospace, automotive and other applications. Superplastic forming of advanced metallic materials summarises key recent research on this important process.Part one reviews types of superplastic metals, standards for superplastic forming, processes and equipment. Part two discusses ways of modelling superplastic forming processes whilst the final part of the book considers applications, including superplastic forming of titanium, aluminium and magnesium alloys.With its distinguished editor and international team of contributors, Superplastic forming of advanced metallic materials is a valuable reference for metallurgists and engineers in such sectors as aerospace and automotive engineering.Note: The Publishers wish to point out an error in the authorship of Chapter 3 which was originally listed as: G. Bernhart, Clément Ader Institute, France. The correct authorship is: G Bernhart, P. Lours, T. Cutard, V. Velay, Ecole des Mines Albi, France and F. Nazaret, Aurock, France. The Publishers apologise to the authors for this error. - Reviews types of superplastic metals and standards for superplastic forming - Discusses the modelling of superplastic forming, including mathematical and finite element modelling - Examines various applications, including superplastic forming of titanium, aluminiun and magnesium alloys

Autoren/Hrsg.

Giuliano, G.

Leseproben

2 Standards for superplastic forming of metals
F. Abu-Farha,     Penn State Erie, USA R. Curtis,     King’s College London, UK Abstract:
In this chapter, a review of the three main standards that describe the proper method for testing superplastic materials is presented, focusing on the critical issues that significantly impact testing results, and pointing out the points of agreement and disagreement among the three standards. In an attempt to resolve those issues, related investigations in the literature are reviewed and combined in an integrated testing methodology that covers the test specimen, clamping device and proper testing procedure. The methodology is hoped to provoke the development of a more universally accepted standard method for testing superplastic materials at elevated temperatures. Key words testing standards superplastic metallic sheets uniaxial tensile testing specimen geometry quick-mount grips heating and holding time 2.1 Introduction
The uniaxial tensile test is quite possibly the most common and the easiest testing procedure for characterising the mechanical behaviour of the various engineering materials. This is mirrored by the standardisation and versatility of the test’s elements (specimen geometry, clamping device, test parameters) to fit the different material classes, in addition to the widespread use of uniaxial load frames by industrial, academic and research facilities where such standards are primarily adopted. Not surprisingly, the tensile test is the test of choice for studying, characterising and modelling the unique class of superplastic materials. For a start, the accepted definition for superplastic behaviour dictates the material achieving 200% elongation in simple tension (Pilling and Ridley 1989); therefore, the tensile test is essential for determining whether a material is superplastic or not, and then the range of conditions over which the material is expected to behave superplastically. Then, the flow stress/strain rate data derived from several tensile tests are plotted in the form of a sigmoidal (S-shaped) curve, uniquely associated with the class of superplastic materials. Also, there is the strain rate jump test, which is simply a uniaxial tensile test with multiple jumps in the applied strain rate (Comley 2008). The test is essential for the accurate determination of the strain rate sensitivity index of the material (known as m); the latter is a particularly important parameter for superplastic materials. While the sensitivity index of conventional metals does not exceed 0.3, even at elevated temperatures, superplastic materials are expected to have 0.3 < m < 0.7 (Pilling and Ridley 1989). Finally, the efforts on constitutive modelling of superplastic deformation are predominantly based on the uniaxial loading case, hence requiring an abundance of tensile stress/strain data to calibrate and verify the proposed models. The latter are the mathematical relations necessary to describe and predict the deformation behaviours of superplastic materials. Yet, more importantly, they are the backbones of the finite element simulations that we completely rely on to produce the pressure–time profiles needed for regulating and controlling superplastic forming operations. 2.2 Need for standards
Superplastic materials have been studied for several decades now, yet they seem to have gained a greater attention recently due to the growing interest in lightweight alloys (titanium, aluminium and magnesium alloys) for energy-saving potentials; and the superplastic forming technique is known to go hand-in-glove with these particular alloys. While these materials are deformed pneumatically via the superplastic forming technique, resulting in predominant biaxial loading conditions, the fact is ‘we are still largely dependent on the uniaxial tensile test’ to study, characterise and model them, and then control their process deformation. Naturally, this dependence, combined with the growing interest, bring the need to regulate the multitude of testing activities in the field. Yet with superplastic materials in particular, the need is further accentuated from several perspectives. Superplasticity in metallic materials is associated with warm/elevated temperatures, making standards like the ASTM E8/E8M (2009), ISO 6892-1 (2009), DIN EN 10002-1 (2009) and DIN 50125 (2004), not suitable for testing superplastic materials, from both specimen geometry and testing procedure points of view. There are several standards, on the other hand, that describe the proper method for testing metallic materials at higher-than-ambient temperatures, such as the ASTM E21 (2009), ISO 783 (1999) and the DIN EN 10002-5 (1991). However, there are several reasons why they are not particularly suitable or applicable to superplastic materials. The prime one is the viscoplastic nature of superplastic materials, which is rather different to that of other non-superplastic materials, even at equivalent temperatures and stretching conditions. This viscoplasticity requires special attention with regard to the grip design, specimen geometry and heating path prior to testing. These items will be further elaborated in the next sections; yet it is worth mentioning at this point that none of the abovementioned standards describes a specimen geometry, nor a gripping device particularly tailored for higher-than-ambient temperature tensile testing. Disparities in testing procedures and methodologies are clearly observed among the various efforts on superplastic tensile testing, despite the introduction of superplastic testing standards (covered next). Such disparities could be detrimental to the test results, since superplastic materials are very sensitive to the conditions at which they are tested. Heating time, for example, is often not reported in superplastic studies; reported ones vary between 10 minutes (Abu-Farha et al. 2010), 20 minutes (Kim et al. 2001) and 30 minutes (Jäger et al. 2004), for the same alloy and testing temperature (Mg AZ31 at 400 °C). The resulting stress/strain curves have been shown to be different, thanks to the microstructural evolution in the material (Abu-Farha et al. 2007b). Of all the testing facets, adopting a variety of test specimen geometries is one of the clearest and most detrimental forms of discrepancy. When reviewing the efforts on tensile testing of superplastic materials, one need not look for long before realising the disagreements among researchers on the issue. Selected examples of recent efforts in the field show that investigators adopt test specimens with different gauge lengths, varying from as long as 25.4 mm to as short as 4 mm (Verma et al. 1996, Lee and Huang 2004, Chino et al. 2004, Watanabe et al. 2005, Liu et al. 2009, Abu-Farha and Khraisheh 2007a, Chang et al. 2009, Hong et al. 2009, Ma and Langdon 1994). Moreover, the discrepancies are not limited to gauge length; they cover gauge width, gauge length-to-width ratio, fillet radius, and even the size of the grip region. All things considered, it is rather hard to find two efforts where the same specimen geometry is used. The aforementioned discrepancy is very crucial in testing superplastic materials, as specimen geometry has been shown to have great impact on the results of the tensile test (Johnson et al. 1994, Khaleel et al. 1996, Bate et al. 2008, Abu-Farha et al. 2010a, Nazzal et al. 2010). It was shown that specimens with smaller gauge length tend to produce higher values for maximum elongation before failure (Bate et al. 2008, Nazzal et al. 2010). The implications of this are detrimental in regard to our ability to cross-reference the results obtained by different investigators, provided the great variation in specimen geometry found in the literature. These points highlight the need for standardising the method for tensile testing of superplastic materials; this is to unify current efforts and provide sufficient guidelines for prospective efforts in the field. 2.3 Existing standards
The highlighted need for standardisation was tackled recently, and the reader can find three major testing standards in the field. The first standard method for the tensile testing of superplastic materials was issued by the Japanese Standards Association in 2002, and was reaffirmed in 2007 (JIS H7501 2002). The American Society for Testing and Materials followed with a similar standard in 2005, which was reapproved in 2008 (ASTM E2448 2008). Finally, the International Organization for Standardization issued its standard in 2007 (ISO 20032 2007). Generally speaking, the ISO and JIS standards are quite similar in their contents, the items they cover, and the depth in which those items are covered. The ASTM one, on the other hand, is rather different and distinguishes itself from the others. It does cover the testing methodology...