Numerical Simulation Study On Suppressing The Forming Defects Of Multi-Point Stretch-forming For Skin Parts

Multi -point forming (MPF) is advantaged flexible manufacture technology for sheet metal forming. Its main idea is dividing the curved surface of the traditional solid stamping dies into a series of matrices of elements. The position and movement of each element is controlled real-time or independently by the computer system .Adjusting the relative positions of the elements, the tooling can easily shape the desired tool surface according to the design model, and therefore realize the rapid and flexible sheet metal forming. Compared with traditional fixed die forming, the MPF has the advantages of saving lots of time for die designing, die manufacturing and testing, by which we exploiting cycle and costs can be shortened. Currently At present the MPF technology has already been applied extensively in many fields, such as manufacturing skin parts of high speed train, ship plate hulls, automobile panels, and large pieces of urban sculpture. And ,multi-point stretch forming is an important direction of the MPF technology. The implementation of multi-point stretch-forming has a great significance on the realization of digitally manufacturing of aircraft, improvment the capacity and level of aircraft manufacture in China.MPF process for the sheet metal panels is a complex mechanics process of simultaneity concerning nonlinear fields on geometry ,material and boundary. Various forming defects happen on the process of MPF, and effect geometry precision , surface quality and mechanical properties of formed panels . Numerical analysis method is an essential tool to optimize die design, predict forming defects and analyze forming process ,but also predict dimples and cracks , calculate the magnitude of spring-back ,and improve the blank design, and so on.In the process of MPF,the areas of sheet metal which contacted with the punches are easily to be dimpled because of concentrated load of punches. Meanwhile the other areas which correspond to the interspaces among punches, are easy to cause depressions, for the reason of special stress state, discontinuity contact, and lack of displacement constraints. And along the direction parallel to the stretch forming direction, these depressions will form wavy gap-shaped groove. The existence of stretch-forming groove has critical effects on the surface quality and the precision of sheet. Therefore, it is studied in this paper that the arrangement of punches-array is changed with dislocation, by which the formation of stretch-forming groove is effectively suppressed, and the surface quality of shaped panel is improved. In order to suppress stretch-forming groove, the elastic cushion is adopted to fill gap and multi-point tooling loads on the sheet to accomplish the forming through the elastic cushion. The cushion supports maintaining force for the areas which are easy to form depressions, enlarges force-subjected areas, and thus suppress stretch-forming groove. An dynamic explicit finite element software is introduced to simulate the multi-point stretch-forming, to study how the arrangement of punch-array, and the suppressing affect of elastic cushion thickness on the formation of depressions, analyse the influence of friction coefficient on the quality of sheet panels during forming process. The main contents and results in this paper are as follows:(1) The numerical simulation for effect of punch-array arrangements on suppressing stretch forming groove and the quality of sheet panels.The finite element models of multi-point stretching forming for aircraft skin panels are built to study the cause of stretch-forming groove and the effect of punch-array arrangements on suppressing stretch-forming groove and the quality of sheet panels. The results indicate that dislocating the punch-array can change the distribution of stretch-forming grooves, and can suppress stretch-forming grooves to some extent. While the normal arrangement of punch-array applied, stretch-forming grooves are parallel with each other, uniformly distributed in the sheet, the groove geometry are almost the same. With arrangement of dislocating punch-array, the inhibitory effect of stretch-forming groove is not very obvious, but to change the direction of stretch-forming grooves and geometry are changed, and in some small areas of sheet stretch-forming grooves are disappear. While with use closely packed arrangement, the stretch-forming groove is significantly weakened, and the inhibition is more effective.(2) The study on the effect of the elastic cushion on suppressing stretch-forming groove and influencing the quality of sheet panels.One-quarter finite element models are built for numerical simulation of spherical skin parts and saddle skin parts to study the effect of elastic cushion to suppressing groove of stretch forming grooves by changing the thickness. The results show that the implementation of elastic cushion is effective to suppressing stretch-forming grooves. With 10mm thickness cushion, there are still some stretch-forming grooves, and the obvious. effect With the 20mm thickness,and stretch-forming grooves are slight. With the 30mm thickness, the surface of parts are smooth, the stretch-forming grooves are eliminated thoroughly. (3) Simulation study on friction coefficient of multi-point stretch forming tooling to the quality of sheet panels and the stress distribution..Different friction coefficient between the elastic cushion and the sheet are simulated for the spherical skin and saddle skin parts. The simulation result indicate that friction coefficient has great influence on the fluidity of the material plastic flow. Under the same multi-point stretch forming conditions, while the value of friction coefficients are 0.25 and 0.35, the sheet thinning serious, the stress distribution is very uneven, even some of the region does not yield which is not benefit in reducing the panel springback..(4) Simulation research on the effect of friction coefficient on the springback of of stretch forming skin partsIn the process of multi-point stretch forming, friction coefficient is one of the important factors related to panel springback. The friction and tension between sheet and tooling are variable, the impact of friction coefficient on panel springback is relative to the shape of the skin panels, which is that the different panel shape results in different stress distribution in the panel. The impact of friction coefficient worked on saddle parts is more significant than spherical skin parts.