Made superplástico: approach to emerging technologies

Many industrial processes of forming, as rolling or extrusion produces large plastic deformations, and sometimes, can form submicrométricos grains (100 nm to 1 µm). However, through these techniques of deformation or made, one or more dimensions of the material is reduced in such a way that, if they are required to accumulate large deformations, what you get are very thin slices (lamination) or filaments (extrusion) that have little practical useIf one thinks in structural applications.

However, there are methods of intense deformation in which the material suffers minimal changes in their dimensions, so there is not a defined geometric limit to deformation that can be achieved, assuming that the material has the sufficient ductility.

The superplasticidad term introduced in metallurgy Bochvar and Sviderskaya, in 1945, is the property that has a polycrystalline material experience (without high applied efforts) large stretching, so isotropic, without fracture, when it was subject to efforts of mechanical traction (see Figure 1).

Although there is a limit of lengthening that defines the superplástico behavior that is not, the maximum stretching to fracture are obtained with superplásticos materials range from several hundred to several thousand percent. In alloys metal superplásticas have reported deformations of up to 8,000% alloys of bronze, aluminum and other materials.

Three types of superplasticidad are recognized in the literature: the superplasticidad of transformations, which is on display in polycrystalline materials undergoing changes dimensional anisotropic against the implementation of certain physical changes. Secondly, the superplasticidad microstructural or fine grain, which is the one that is on display in metal, intermetallic or ceramic, polycrystalline materials whose grain size does not exceed the 10 µm, when they are subjected to low velocities of deformation and absolute temperatures of the order of, or greater than half of the temperature of fusion, without reaching it. Thirdly, the superplasticidad by internal efforts.

What interested many researchers was the great ductility, defined as the potential benefit of the superplasticidad in the area of metal, which led to the development of this technique (Superplastic Forming, SPF) as a means to form single class superplásticos materials forming. The technique of SPF is considered as a process of forming almost finished forms (near-net shape) that requires a single surface array, instead of pairs of matrices that are typically used in operations of formed sheet metal. A sheet of material is formed into a step inside the cavity of the matrix (which is heated prior to the desired temperature) who usually take the final form to the dimensions of the desired part, using gas pressurethe process is schematically illustrated in Figure 2.

The microstructural or fine grain superplasticidad is used for the production of parts for forming of sheet metal and the basic requirements for superplástica deformation are, essentially, three:

1. Microstructure with fine-grained, uniform and less than 10 µm equiaxial
2. Speed controlled deformation, usually in the range 1 x 10^-5 - 1 x 10^-1 s^{- 1}
3. Temperature of experimental work more or equal 0.5 t_m (where T_{is the absolute temperature of fusion of material} )

This last requirement (shown by the fact that the superplasticidad is a process controlled by diffusion), tends to be incompatible with the retention of a small grain, as high temperatures favor the growth of grain (need to some metal alloys in the presence of 'dispersoides' distributed evenly to prevent the growth of grain and) (that favour the superplástico behavior).

In addition to conventional forming, by the technique of SPF can get additional benefits through the improvement of the technique, driven mainly by the great flexibility of the process. SPF can be combined with other manufacturing processes for a better and more efficient process of forming; for example with processes of conventional Union, Union for dissemination (Difussion Bonding, DB) and deep drawing (Deep Drawing, DD).

The SPF is combined with the Union by diffusion (Diffusion Bonding, DB) to make more complex shapes (type honeycomb). The DB is the Union two components by applying load at high temperatures where the resulting molecular Union is completely homogeneous. With SPF/DB technique parties are manufactured by binding by diffusion of number plates or sheets with a specific pattern, and then expand superplásticamente plates to produce an integral rigid structure (Figure 3).

Modifications, alterations or even additional features can be incorporated in order to produce advanced concepts that enhance the capacity of the conventional technique SPF, in various ways and aspects. There are several examples of such concepts including the SPF of double-sided, multipart SPF and SPF with back pressure. The latter concept is schematically illustrated in Figure 4, where it represents the presence of pressure on the back of the sheet metal forming. Do not increase aspects such as cost or efficiency, what we want is to avoid or minimize the cavitation in the shaped piece and thus improve their mechanical properties, which is believed are associated with the presence of back pressure.

The combination SPF/DD, for example, is a new concept in which a part conforms partially through a fast deep drawing, followed by a phase of SPF, which is responsible for the complex details of the form that was created. In this way, the formation time is reduced even more, while maintaining the importance of the SPF. An outline of the process is shown in Figure 5.

The formed commercial superplástico preferentially includes automotive, aerospace, medical alloys of aluminium, nickel, zinc and titanium for the production of components in the industry. Several major industrial and commercial light alloys exhibit superplasticidad, such as Ti6Al4V titanium alloy and aluminium 5083 alloy AZ31 magnesium alloy. In fact, for these and many other light alloys, the conforming with any other technique would be impractical, due to its limited formability in conditions of received. Figure 6 shows some parts manufactured by this method.

Properties materials obtained with this technology, thermomechanical make their industrial application irreplaceable to obtain pieces of complicated forms that require high demands in forming or the service, as the structural parts of supersonic aircraft. Currently, this property is present in ceramic with grain size < 1 µm and has located the study of ceramic superplásticos as a field of research of great interest. In these last found deformations of 1.040% in a policristal of tetragonal zirconium stabilised with yttria (YTZP).

The forming superplástico (SPF) offers many advantages with respect to the operations of formed conventional:

• The ability to form components with very complex shapes, which can not be produced by conventional techniques, or can be obtained only by multiple shaped successive followed by Union or welding of several parties
• The ability to give shape to materials very difficult shape, with relative ease as they are alloys of titanium and magnesium, which is known to have lower ductility in the condition of arrival due to their crystalline structures HCP
• Low cost matrix, since that is an array with a single cavity to form the component, regardless of the complexity of the shape and dimensional aspect ratio
• The process is carried out in a single step, producing a finished good or almost finished component
• Reduction of the total number of parties and, consequently, the number of fixations and/or unions, which leads to the improvement of security in certain applications (e.g., aerospace)
• Greater flexibility of design and dimensional control

The technique of SPF has faced a series of challenges and problems that prevent its widespread on a larger scale use. Expensive operations of preconformado, as the preparation of materials with fine grain structure and heating to the temperature of forming desired, represent one of these issues. In addition, because the speed is controlled and limited, low-speed, makes the process slow and relatively unfavourable to implement it in the production of a high volume of automotive components. However, the technique provides a unique tool for made of light alloys, and, despite the obstacles and challenges along the way, still offers significant advantages and merits on the techniques of forming conventional.

References

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• Peter Anderton. Superplastic Forming / Diffusion Bonding (SPF/DB) of Titanium and Aluminium Alloys, ENIMEP Novotel São Paulo Center, São Paulo, Brazil, 26th & 27th November 2007
• http://www.formtech.de/en/en-spfdb.htm