FINITE-ELEMENT MODELING OF SUPER-PLASTIC FORMING PROCESSES

The present work is devoted to the development of the production of
three-layered hollows structures made of VT6 titanium alloy by means of superplastic forming (SPF) and pressure welding. Finite- element modeling can be successfully applied to optimize the  forming process, if the adequate constitutive relations would be  defined and the friction at the contact surface of the material with  the die would be specified. To find the friction coefficient and the  parameters of the constitutive relations for metal forming process,  test experiments are conducted to the forming of sheet into dies of  various shapes. In such test experiments, a biaxial loading is  realized, as in the actual processes of fabricating complicated  structures from sheet by SPF. To this end, Finite-element modeling of the SPF process of sheet forming into dies of two types is performed: (i) into wedge die having cross section in the form of equilateral triangle, and (ii) cone die. Recommendations are given for the choice of the optimum angle at the vertex, determining the geometry of the dies, which results in the constancy of the stresses during forming at constant pressure. The methodology for estimating the coefficient of  friction on the contact surface between sheet and die is given. Finite- element modeling of the SPF process of three-layer hollow structures is carried out using the parameters of the constitutive relations  obtained by the proposed methods. Technological constraints on the geometric parameters of structures, such as the angle of inclination  of the stiffening ribs and the thickness ratio of outer to inner sheet  thicknesses are established, which provides forming without the  formation of folds on the shell and the minimum variability of ribs thickness.

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