G. Giuliano

A posteriori optimisation of the forming pressure in superplastic forming processes by the finite element method

In order to optimise the superplastic forming processes, it is necessary to control the strain-rate induced in the material by the pressure gas. This control ensures high material deformability. In this paper, to determine the optimum pressure-time profile in superplastic forming processes, the authors will show an original technique based on finite element method. The proposed technique was tested with reference to the superplastic forming of a thin circular plate clamped in a rigid die and formed by a pressure differential. The experimental activity was carried out using a Pb/Sn-based metallic alloy. Final results of the finite element modelling agree with the experimental ones.

On the optimisation of superplastic forming processes by the finite-element method

Abstract This paper presents an original algorithm, with a finite-element interface capability, that can calculate, for superplastic forming processes, the load curve to be applied during forming. The algorithm makes it possible to maintain the maximum strain rate as near as possible to the optimum characteristic value of the material thereby reducing forming time. The proposed algorithm was validated by calculating the optimum pressure curve to be assigned in the case of both a conical-bulging process and a cap-forming one. Parallel experimental activity was carried out using a Pb/Sn-based metallic alloy that is superplastic at room temperature. For both geometries examined, comparison between numerical and experimental results proved to be acceptable. In particular, the relative error was less than 6% for measurements of both the strain–time relationship and the thickness distribution. Different measurements were carried out during each process: the shift in the sheet centre-point for the conical-bulging process and the thickness distribution corresponding to a particular sheet configuration for the cap-forming one.

Modelling of superplastic blow forming

Abstract Some metals and metallic alloys, when deformed in particular conditions, manifest exceptional ductility giving tensile elongations of up to 1000%. This behaviour, known as “superplasticity”, could undergo considerable development in sheet metal production. The aerospace industry has already taken the opportunity of producing complex-shaped objects in a limited number of mechanical operations. It makes use of superplastic characteristics to noticeably reduce the weight and cost entailed in manufacturing some components, including structural ones. To better take advantage of the superplastic characteristics of the material, it is necessary to control the temperature and the strain rate during the manufacturing process. Since the material undergoes significant elongation, it also, necessarily, undergoes extreme thinning. The latter can prove not to be uniformly distributed because of (i) the particular geometry of the manufactured product, (ii) the characteristics of the material used, (iii) the lubrification and (iv) the process parameters adopted. The design stage should take account of the real thickness distribution in order to avoid critical areas. At this stage, thus, it is necessary, not only to design the product, but also to design the process in order to establish the optimum production parameters and to foresee the real geometry of the product. Numerical modelling is used since “in the field” analysis could prove to be expensive, and, analytical modelling would be limited only to some forms and to the use of largely approximated assumptions. The finite element method can be considered to be the most dependable both for analysing complex geometries, and for taking into consideration all the phenomena involved in the manufacturing process. One of the most delicate operations in this method is sub-dividing the continuum into elements since this discretization can have an influence both on the reliability of the results, and on computational requirements. The objective of this paper is to verify the approximation of the results that can be obtained compared to the different options possible both in terms of element type and number. The production of an axisymmetric cup in commercial aluminium based alloy Al 7475 using blow forming technology was taken as a reference case. Comparison between the results of the different simulations showed a substantial equivalence and a good correspondence to the measured thickness values. Since the computational resources required are very different for the cases examined, it can be stated that the best solution is discretization of the start-off sheet with a row of 55 axisymmetrical four node elements.

A method to characterise superplastic materials in comparison with alternative methods

Abstract Superplastic materials show a very high ductility, i.e. maximum elongation of about 5000%, even if they are lowly stressed. This is due to both peculiar process conditions and material intrinsic characteristics. The aerospace industry has shown that, in order to produce complex parts requiring large tensile elongations that cannot be formed by conventional processes, superplastic forming can be used. A detailed design of technological process is necessary so as to exploit at best the peculiar potentialities of superplastic forming. The aim of the present work is to show a method to characterise superplastic materials. This method is based on the approximate analysis of a superplastic forming process in a triangular indefinite prismatic-shaped die. It has been experimentally validated through laboratory samples on material formed by room temperature; moreover, it has been compared to several methods proposed by other authors.

Finite Element Modelling and the Experimental Verification of Superplastic Forming

FEM analysis has proved to be a powerful investigative tool capable of encompassing all the aspects that characterise an SPF process. However, despite the high potential of FEM programs they do not allow one to directly and suitably obtain the thickness of a sheet product for high deformation values, as commonly occurs in SPF processes. Many papers have been published on finite element analysis of S.P.F. process but the question of calculus accuracy in thicknesses of a sheet product has not been directly investigated. This problem has been already considered by the authors in a previous study which proposed an algorithm to determine thicknesses for a specific application. The software set up starts out with the results of the FEM modelling, keeps track of the “deformation” undergone by each element of the mesh and calculates to a good approximation the thicknesses at the end of the forming. Although the original version of the algorithm could only be used for the application studied an updated version is introduced in this study that can be used for any case. In other words, the software generates the thickness profile at the end of the analysis independently of technological set up, item shape and type of simulation (3D and 2D). The proposed algorithm was tested with reference to the superplastic forming of an item of simple geometry beginning with a thin circular plate blocked at the edges and put under constant hydrostatic pressure on one side. The test material, made superplastic by means of a series of repeated laminations, was characterised using an alternative method to the traditional tension test. The results of the experiments are in good accordance with the numerical predictions both in terms of thickness distribution and forming times.

The Effect of the Punch Radius in Dieless Incremental Forming

Publisher Summary The dieless incremental forming process is an innovative sheet metal working technology where a considerable amount of knowledge and intelligence is required, in order to obtain accurate and efficient operations. In conventional forming process, the final shape is mostly determined by the die shape. In dieless incremental forming, however, the final shape must be predicted and controlled only by means of a proper process design. For this reason, several issues of this process must still be investigated on a scientific base. This chapter discusses the dieless forming process as an aid towards an intelligent process planning. In most sheet metal incremental forming processes (shear spinning, flow forming, and dieless forming), the deformation occurs by pure shear. The main process parameters are the feed rate, the part conicity, and the punch radius r. Formability in incremental forming has been often investigated and it is well recognized that severe strain may occur before fracture. Thinning and fracture largely depend on the part conicity. Besides, decreasing the feed rate has a positive effect. On the contrary, the effect of the punch radius r has been seldom explored in quantitative terms. The chapter investigates the effect of r on the formability of thin sheet metals, when plastically deformed by incremental forming.

Forming Apparatus to Investigate the Effect of Temperature on the Superplastic Behaviour of Alloys

In this paper the authors show an experimental apparatus designed and made up to investigate the effect of temperature on the deformation of superp lastic alloys up to 473 K. The authors used the 3D finite volume software FLUENT ® to design the experimental apparatus. The numerical results have been employed for the system optimisation and metrological characterisation. The system was tested through bulge tests on a Pb‐Sn alloy, showing its good versatility.

Analysis Of Superplastic Bulge Forming By The Finite Element Method

his article is a finite element method (FEM) study of hemispherical shell superplastic forming. The scope of this numerical simulation is to obtain the more detailed information about the deformation process that cannot be obtained hy means of analytical and experimental studies. Two original calculation techniques proposed hy the authors were used to investigate several industrially interesting parameters: using a subroutine presented in [I] it was possible to calculate the thickness distribution in a deformed sheet; an original techniquc presented herein provides the optimum pressure-time load curve needed to maintain a constant strain rate. Furthermore, on the basis of results obtained through a numerical simulation, the authors propose an accurate and alternative method to the tension test to characterise superplastic materials. Unlike the known methods present in the literature, this method does not require any hypothesis regarding deformation geometry. The experimental activity, carried out to support the numerical activity, showed good agreement bctween the numerical predictions and the experimental data.

Effect of Lubrication on the Erichsen Test

This study analyzes experimentally the influence of the friction between the sheet metal and the die surfaces on the results of the Erichsen test in terms of load-displacement curve of the punch, the normalized thickness measured at the specimen apex and the distance measured between the thinnest area of the specimen and the lateral surface of the blankholder. Two types of aluminium alloys, AA 2017 Al-Cu alloy (Al-4.5%Cu-1.0%Mn-1.0%Mg) and AA 5083 Al-Mg alloy (Al-4.5%Mg-1.0%Mn-0.15%Cr), with thickness of 1.0 mm are selected as the experimental materials for Erichsen test.

Evaluation of the Coulomb Friction Coefficient by the Erichsen Test

In this study, the Erichsen test is used to identify the friction coefficient of the Coulomb friction model coupling experimental tests results with numerical ones. The evaluation of the Coulomb friction coefficient is based on the distance measured between the thinnest area of the specimen and the lateral surface of the blankholder. Two types of aluminium alloys, AA 2017 Al-Cu alloy and AA 5083 Al-Mg alloy, with thickness of 1.0 mm are selected as the experimental materials for Erichsen test. Specimens are tested in unlubricated condition as well as using two different lubricants, namely Grease LB4 and Mexmoly.