Superplasticity: Microstructural Refinement and Superplastic Roll Forming
by Oscar A. Kaibyshev and Farid Z. Utyashev
Volume 3 of the ISTC Science and Technology Series
ISTC, Science & Technology Series, Vol. 3, Futurepast, Arlington, Virginia, USA
The book is based on the work of two Russian scientists who have devoted a great deal of their professional life to the subject of superplasticity in metal base materials. Kaibyshev and Utyashev have collaborated over a 25-year period, with Kaibyshev emphasizing the science base and Utyashev emphasizing the processing and mechanics base for successful manufacture of superplastic materials.
The modern study of superplasticity has its origin in Russia. Bochvar and Sviderskaya in a 1945 publication, coined the term “sverhplastichnost” [ultrahigh plasticity] based on their research studies on aluminum-zinc alloys. They showed that a metallic material, when processed properly, could be stretched like molten glass much below the metal’s melting temperature. After the end of the Second World War, Academician A. A. Bochvar was selected to head a large institute in the field of nuclear materials (that institute now bears his name), and new objectives were pursued. The superplastic Al-Zn experiments fell into oblivion until E. E. Underwood in the United States publicized Bochvar’s original work in a review paper in 1962. Researchers at MIT repeated the Bochvar experiments and demonstrated the ease of superplastic forming spherical objects. A superplastically formed prototype component, a typewriter head, demonstrated by IBM technologists attracted a great deal of world attention. The beginning of intense research on superplasticity and on superplastic forming was started. Dr. Kaibyshev, as a young researcher at the Aviation Institute in Ufa, published one of the earliest books on superplasticity (1975). A decade later Kaibyshev completed yet another book on Superplasticity of Commercial Alloys. His many accomplishments and his leadership skills led to his selection as the opening speaker at the Third International Conference (1988) on Superplasticity and Superplastic Forming in Blaine, Washington. At about this time, he became director of a newly created institute known as the Institute for Metals Superplasticity Problems. The Russian Academy of Science created the Institute in recognition of the pioneering work of Academician Bochvar and of subsequent work on superplasticity at universities and industrial laboratories in Russia.
The title of the book “Superplasticity: Microstructural Refinement and Superplastic Roll Forming” describes the contents quite well. The first portion of the book covers the phenomenology of superplasticity and the requirements that are needed to make a given material superplastic. The second part of the book describes the various processing procedures and heat-treating methods that are now being used to develop ultra-fine grain structures, a prime requirement to achieve superplasticity. The third part of the book describes novel roll-forming procedures that show promise for achieving complex shaped components in the automotive and aerospace industry.
The attributes of superplastic materials at warm and high temperatures cannot be overstated. These materials can be processed to make complex shapes without fracturing, and the forces needed to make these shapes are low. This means that only low pressure is needed to manufacture the structural components. Therefore the massive presses and forging hammers that are currently used can be replaced by small manufacturing assemblies. An additional benefit is that the fine structure inherent in superplastic materials contributes to obtaining a very strong material at ambient temperature in the end product.
The book by Kaibyshev and Utyashev covers material that is useful for those whose interests are in superplasticity and superplastic forming. A wealth of new information is described in detail that is only available in Russian publications. Specifically, of the 454 references quoted in the book, over 350 refer to publications in Russian journals. The authors have provided the titles to all Russian references giving valuable information to the serious reader of their book. The book contains 150 figures that is a welcome addition for better understanding of the text. The book centers on the processing, microstructure and properties of superplastic materials based on aluminum, nickel and titanium. The emphasis is on bulk superplastic forming, with a minimum of emphasis on sheet superplastic forming. Superplastic forming of ceramic materials is not emphasized.
The first section of the book is devoted to understanding the requirements for achieving superplasticity in metal-base materials. The major requirement is developing a structure consisting of very fine grains. Since the grain boundary is a region of weakness, deformation takes place by grain boundary sliding rather than by slip mechanisms. This gives the metal a viscous-like behavior similar to that of semi-molten glass. The precise mechanism of grain boundary sliding remains a debated subject in superplasticity. Kaibyshev points out that it probably occurs by a cooperative action of a number of grains following the direction of the maximum shear stress. It is a very attractive model that has not yet been fully developed. Appropriately, the model is called co-operative grain boundary sliding (CGBS). The authors present a constitutive relation that predicts the creep rate as a function of the applied stress, the rate of diffusion of atoms in the grain boundary, the grain size, and a threshold stress.
The second section of the book is dedicated to methods that are used for the creation of fine-grained materials. This is a required first step to create the desired structure in a given material for use in subsequent superplastic forming operations. The authors treat this section thoroughly. It represents a very important contribution in a field that Russian scientists and technologists have pursued extensively. In fact, this type of work was pursued much before superplastic forming studies were started. The transliterated Russian word for this discipline is “ter.mo.me.chess.kaya-o.bra.bot.ka”. It is now universally called thermal-mechanical- processing and the acronym for the process is TMP. A well-known international symposium is held once every three years on this subject (Thermec 2003 in Madrid, Spain and Thermec 2006 to be held in Vancouver, Canada). The authors describe many approaches that include thermal-mechanical processing of cast materials and of powder metallurgy materials. Approximately two hundred pages are devoted to this general subject. The object, of course, is to describe the use of TMP for creating ultra-fine grain size materials. The authors point out that the finer the ultra-fine grained material, the lower the temperature for successful superplastic forming. A lower temperature of superplastic forming means economy in fuel consumption in heating the material for manufacturing the product. In addition, the finer-grain size will result in a higher strength in the superplastic component for ambient temperature use. Another advantage of creating ultra-fine grains is that the rate of forming the material in the superplastic state can be increased, thus increasing the production rate of components. This is an important factor in the decision for using superplastic materials versus conventional materials and procedures in the production of structural components.
The nature, that is the structure, of the grain boundary is of paramount importance in obtaining ideal superplastic behavior. The boundary structure must be fully disordered, that is, have an amorphous, structure-less feature. These are called high angle boundaries. If the grain boundary consists of defects in the form of dislocations, then the materials will not be superplastic even though the grain size is very fine. These boundaries are called subgrain boundaries. The authors point out that if unidirectional deformation is used in the TMP step, the formation of a fine grain size with high angle boundaries is not readily achieved. The result is a fine subgrain-size material that will not show superplastic behavior, although the strength at ambient temperature will be attractive. The authors point out that high angle grain boundaries are more readily achieved if the TMP involves multi-directional deformation. Two processing methods are described where such a state is created and the amorphous-type grain boundary is achieved. One method is by applying unidirectional deformation simultaneously with torsion deformation. Another method is by deformation through equal angle channel pressing. In the latter case, the material is pressed through a sharp turn in a curved die that results in a shearing deformation with no change in the shape of the sample. This pressing operation, usually done at warm temperature, is repeated several times. The deformation is multi-directional. In both cases, it is shown that high angle grain boundaries are achieved, and the grain sizes can be made as small as 100 nm in dimensions. Such sizes indicate that high strain rate superplasticity is achievable, and that low temperature superplastic deformation is possible.
Kaibyshev and Utyashev have published many papers, sometimes together and more often separately. The authors, however, appear equally motivated to work together in promoting superplastic forming of materials for the aerospace industry. This is amply evident in their work on nickel-base superalloys that have application in gas turbine engines. Similarly, the authors’ activities with titanium alloys indicate their interaction with the titanium industry in developing fine structures in large-scale billets.
The third section of the book is dedicated to roll forming under superplastic conditions. Approximately 100 pages are devoted to this subject. The authors point out that the manufacture of axisymmetric components with superplastic materials has considerable promise in many applications. An example is that of roll-forming of wheels for automobiles. A major benefit of making a wheel by superplastic forming is that the final product will contain a uniform fine-grained structure. Automobile wheels are made of aluminum because of its low density. The steps in making the wheel are described in the following three sequences, all done isothermally at 450°C. A commercial aluminum alloy is extruded to a selected diameter. A round disk is cut from the extruded bar and is isothermally forged superplastically into a simple wheel preform containing a rim. The operation takes about 10 minutes. The final phase involves roll-forming the preform on a converted lathe-type machine. This operation also takes about 10 minutes. A heat-treatment is given to the wheel to achieve the specified hardness, followed by surface machining. This work has stimulated the interest of Kaiser Aluminum and Chemical Corporation and researchers at Lawrence Livermore National Laboratory. A report on the properties of the wheel and its microstructure has been published (C. K. Syn, D. R. Lesuer, T. G. Nieh, H. S. Yang, K. R. Brown, R. O. Kaibyshev, and E. N. Petrov, Automobile Alloys II, Ed. S. K. Das, The Minerals, Metals and Materials Society, Warrendale, PA, 1998, 173-183). Adoption of the isothermal forming of automobile wheels would represent a big advance for the emerging field of superplastic bulk forming of materials.
Oleg D. Sherby
Dept. of Materials Science and Engineering
Stanford, CA 94305