Superplasticity in Advanced Materials - ICSAM 2009

Volume 433

doi: 10.4028/www.scientific.net/KEM.433

Paper Title Page

Authors: Terence G. Langdon
Abstract: The origin of true laboratory-scale superplasticity may be traced to a publication appearing seventy-five years ago in 1934. This overview examines the subsequent major developments in obtaining a fundamental understanding of superplastic flow and then looks to the future to summarize very new developments that provide the potential for invigorating the field of superplastic research.
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Authors: A.J. Barnes, M.J. Stowell, Roger Grimes, D.B. Laycock, B.M. Watts
Abstract: Prior to 1969 the pioneering work carried out by Backofen and Fields in the USA and Johnson and Hundy in the UK demonstrated the 'promise' of Superplastic Forming. Using fine grained dual phase alloys, typically of eutectic or eutectoid compositions, they produced some of the very first superplastically formed prototype components. Although not always 'practical', these dual phase alloys were stable when heated and if appropriately processed often proved to be very superplastic. At that time 'dilute' alloys, including the majority of commercial aluminum alloys, having only a small volume fraction of alloying additions, were thought not to be capable of superplastic behavior due to their propensity to grain coarsen when heated. Breakthrough came in 1969 when at the research labs of Tube Investments, Hinxton Hall Nr Cambridge UK; the first 'SUPRAL' type dilute superplastic aluminum alloys were created. This paper describes the events and 'science' that led up to this development and the remarkable technology that has emerged since the authors began their superplasticity careers more than forty years ago. The future direction that this intriguing technology is likely to take is also explored.
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Authors: Richard Curtis, R. Omar, J. Bahra, M. Ditta, A. Chotai, Lucy DiSilvio
Abstract: Steinemann, 1998 [1] reported an observation made several decades earlier in 1951, by Leventhal [2] in which ‘bone reaction was studied by the insertion of up to 80 titanium screws into the femora of rats. At the end of sixteen weeks the screws were so tight that in one specimen the femur was fractured when an attempt was made to remove the screw’. Consequently, the main reasons given for the suitability of titanium for surgical implantation are its strength, its failure to cause tissue reaction, and the fact that bone becomes attached to titanium. Now, we call this attachment osseointegration which is considered to be the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant. However, osseointegration is not considered to be a chemical bond between titanium and bone. Implant materials that actually bond to bone are considered to be bioactive. Materials for clinical use can be classified into three categories: resorbable, bioactive and nearly inert materials. A bioactive material is defined as a material that elicits a specific biological response at the interface of the material, which results in the formation of a bond between the tissue and that material. Whereas specific bioceramics are considered to be bioactive, titanium alloys are not normally considered to be so. However, recent surface modification of titanium alloys provide evidence that titanium alloys can become bioactive after treatment with NaOH and the ensuing development of a titanate gel on the metal surface.
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Authors: Bill Swale, Mauro Pizzingrilli, E. McCullagh
Abstract: When Superplastic Forming (SPF) was offered as a production process in the mid 70’s, it became the panacea of all processes for sheet metal products designed to be made from Titanium and Aluminium materials. The claims were (1) reduced part count (2) reduced assembly time (3) weight reduction (4) monolithic parts and (5) stronger structures. Following Pearson’s work in the mid 30’s with Lead-Tin and Bismuth-Tin alloys [1], showing higher than 1000% elongation without failure, the Aluminium industry developed SPF alloys and launched into numerous commercial applications. Other research facilities focused on the potential of achieving superplasticity in Titanium alloys. This was demonstrated in the late 60’s using the now well established Ti6Al4V alloy. Considerable funding was allocated, both in the USA & UK, specifically for the development of the process. The USA focused on the military programmes and the UK on the civil (Concord) and some military aircraft. Success in these programmes and the claims made, resulted with a production process. Companies invested in suitable plant and equipment, and designers grasped the process potential and applied SPF to their sheet metal designs expecting to reap the claimed benefits. The claims are valid if applied to correctly chosen components. All too often, the SPF manufacturing choice did not deliver its claims. In many cases cost of material, need to chemical mill and higher energy costs, were either not envisaged or taken into account. Today all processes, material cost and alternative material types have to be assessed before the manufacturing method is chosen. The aerospace industry is attacking the Buy-Fly ratio. Energy and labour cost are at a premium and these have caused the SPF and Hot Forming community to examine ways of producing products (a) from less material (b) by Hot Forming (eliminating the need to apply chemical milling to remove the alpha case) (c) questioning the material choice (CP instead of Ti6Al4V) and (d) by applying modern fabrication methods. The paper will illustrate this change in philosophy; shows today’s choices and demonstrate how the SPF process can be cost effective, and in fact still does have a major role to play in producing airframe and engine structures.
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Authors: Larry D. Hefti
Abstract: In the past, engine aft fairing heat shields have typically been titanium castings. With a current single aisle airplane, these components were converted to sheet metal titanium 6Al-4V details fabricated by hot forming or Superplastic Forming (SPF). This conversion saved approximately 20% in both cost and weight per airplane. When heat shields for a twin aisle airplane were being developed, the engineers were interested in a sheet metal version of their heat shields hoping to achieve similar savings. However, the twin aisle configuration was different from the single aisle and did not allow the details to be simple pieces of formed sheet metal. Instead, these twin aisle heat shields are assemblies of details containing SPF components as well as Superplastically Formed and Diffusion Bonded (SPF/DB) panels. Some of the heat shield components are fabricated using the world's first applications of fine grain 6Al-4V titanium, which was developed to SPF at 775°C, covered by a U.S. patent [1], instead of 900°C, which is used for standard grain material. The SPF/DB technology being used contains innovative process developments that are covered by several patent applications [2-4]. The twin aisle heat shield assemblies were estimated to save approximately 15% in both cost and weight per airplane. Actual weight measurements of the first assembly showed an additional 5% savings over the calculated weight per engine resulting in a total weight savings of approximately 20% per airplane compared to titanium castings.
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Authors: Ho Sung Lee, Jong Hoon Yoon, Yeong Moo Yi
Abstract: Titanium alloys have been widely used in aeronautics and aerospace industries due to their high strength, good corrosion resistance and low density. Since many aerospace vehicle systems require high performance lightweight pressurized vessel for storage of propellant, nitrogen, oxygen, or other medium, the titanium alloy is one of the excellent candidates for this purpose. Conventionally spin forming and TIG welding process have been applied to manufacture titanium spherical vessel. In this work, an innovational method of blow forming and solid state bonding technology has been developed to save manufacturing cost and reduce weight of titanium vessel. High temperature behavior of titanium alloy was characterized and according to this result, solid state bonding process was established with demonstration of manufacturing spherical and hollow cylinder pressure vessel. The optimum condition for solid state bonding of this alloy was obtained by applying hydrostatic pressure of 4MPa at 1148K for 1 hour. For blow forming, the pressure profile was developed using MARC software and the maximum pressure of 30MPa was applied. The structural integrity of the vessel was demonstrated by performing a hydraulic pressurization test.
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Authors: Alan Jocelyn, Allan Keevil, Jerome Way, Terry Flower, Alexander Fanourakis, Mike Ackerman, Aravinda Kar
Abstract: Superplastic forming (SPF) has traditionally relied on hot platen presses and furnaces as the principal heat sources to raise materials to superplastic forming temperatures. However, recent research, in the UK and the US, has concluded that such indirect heating methods are slow, expensive, and can only provide a single temperature to the work piece, which can be undesirable. In contrast, LISTechnology Limited (laser induced superplasticity technology) has been created to provide an alternative technology that can directly heat materials to be superplastically formed fast, at low cost and with the potential to control thickness distribution during forming through differential heating. The first ‘laser cell’ for components formed from single sheet titanium is currently being built to demonstrate how direct heating of SPF materials with a laser will significantly increase material heat up rates, compared with current methods, whilst the low thermal mass of the cell will allow rapid cooling to below oxidation temperatures, thus significantly reducing manufacturing cycle times. Furthermore, the cell will only utilise thermally stable, inert ceramic dies within which the titanium will be formed, these being contained within a sealed argon environment, thereby offering the possibility of alpha case free, rapid forming at high temperatures.
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Authors: Vincent Velay, Gérard Bernhart, A. Martinier, Jean Yves Moraux
Abstract: Superplastic forming of titanium alloy sheets requests long time operating conditions in the range of 900-950°C. Moreover, in a classical press-furnace process environment, die surface temperature drops during sheet unloading and induces high temperature thermo-mechanical fatigue. In order to withstand such extreme conditions in oxidative atmosphere, cast heat resistant nickel chromium steel grades have been developed. The high chromium content (close to 25%) aims to protect against the oxidizing environment, whereas the nickel content is selected with respect to the expected in service loads. The 50% nickel grades are in general used for heating plates, huge casings and cover-plates; whereas 40% nickel grades are selected for inserts and medium size self-standing dies. Cost considerations (Nickel and machining) are also taken into account by the end users for making their choice. An extensive testing program has been performed, in the range of 20 to 950°C, to understand the high temperature fatigue behaviour of these grades and to identify material behaviour models for simulation purposes. This paper presents the major results of these research works and highlights the impact of the nickel content in terms of stress level and life time. Nevertheless, when looking on behaviour, test results show that a unified elasto-visco-plastic cyclic behaviour model is well suited for thermo-mechanical cyclic modelling whatever the grade is. Isothermal identification strategy and out of phase SPF die representative anisothermal fatigue validation are presented.
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