Investigation On Plastic Hardening Of Thin-walled Tubes In Pulsating Hydroforming

Tube hydroforming(THF) technology is the significant approach to realize lightweight of auto parts, owing to the low cost in manufacturing, low weight and high strength of products. Tube pulsating hydroforming technology is a novel technique that applies the pulsating hydraulic pressure with varying frequency and amplitude to deform tube metal. As a hot research spot in THF, it is expected becoming a new technology to improve the formability of the tube. The plastic hardening model of tubular material is essential for the analysis of deformation mechanism, and the performance of precision finite element(FE) simulations of tube hydroforming. Therefore, it has the important theoretical significance for the research on plastic hardening of thin-walled tubes in pulsating hydroforming.In present study,(1) a method, based on the mechanical model of tube hydraulic forming, the plastic incremental theory and principle of plastic deformation work, was proposed to determine the plastic hardening model of a tube in THF.(2) The pulsating hydroforming tests(a variety of frequency and amplitude) and the non-pulsating hydroforming test(frequency and amplitude are equal to zero) of a SS304 stainless steel tube were carried out continuously. And the deformation data on the bulged region of tube were obtained.(3) The plastic hardening relationships of tube were determined according to the deformation data obtained from experiment.(4) The simulation model of tube hydroforming was established. Then the plastic hardening relationships of tube, obtained from the pulsating hydroforming tests of tube, the non-pulsating hydroforming test of tube and the uniaxial tensile test, applied to perform the FE simulation of tube pulsating and non-pulsating hydroforming. Finally, the plastic hardening relationships obtained by the proposed method were validated by comparing the results between the FE simulation and the experiments.(5) According to the deformation data obtained from the experiments, the formability and deformation law of tube in pulsating hydroforming were analyzed. The effect of pulsating frequency and amplitude on the deformational behavior was analyzed. The deformation at a different location of bulged region was compared. And the possible reasons for caused differences of tube deformation in the pulsating hydroforming process of tube and the hydroforming process of tube were analyzed.The main results of present study can be summarized as following.(1) With the proposed method the pre-assumption of a bulged profile shape is eliminated. The plastic hardening relationship of tubular material can be conveniently determined based on the deformation data obtained by the strain measurement system from experiment. By comparing the results from FE simulation and experiments, the results show that the plastic hardening relationship of tube determined by the proposed method in this paper has higher accuracy.(2) By comparing the values of thickness, circumferential strain, axial strain, maximum bulge high(before burst) et al. obtained from the tube pulsating hydroforming test and the tube non-pulsating hydroforming test, the results show that the pulsating loading can improve the formability of tube.(3) When the frequency is less than or equal to 1.7 c/s, or the amplitude is less than or equal to 3.34 MPa. Larger the frequency or amplitude is, the smaller wall thickness is, and the larger the circumferential strain is, and the smaller for the absolute valve of the axial strain is, and the higher the biggest bulging height is. When the frequency is greater than or equal to 2.1 c/s, or the amplitude is greater than or equal to 3.92 MPa. The deformation degree of tube is best under some combination of frequency and amplitude, rather than with the increase of the frequency or amplitude, the formability of tube is better too.(4) There is a fluctuation of the equivalent stress-strain curve. With the increase of frequency and amplitude, the equivalent stress-strain curves increase.(5) By comparing the distribution of thickness, strain and stress et al. at different location on the bulged region obtained from tube pulsating hydroforming tests and tube non-pulsating hydroforming test. The results show that distribution of wall thickness is more uniform, the wall thickness thinning and circumferential strain are bigger and the axial strain is smaller at different location on the bulged region in pulsating hydroforming.(6) In the tube pulsating pressure, internal hydraulic pressure is in condition of cyclic loading and unloading, which promotes the contraction of materials at the tube end. The material on the bugled region gets timely supplement, thus improves formability of the tube.