Dynamic frictional characteristics for the pulsating hydroforming of tubes

Abstract

In tube hydroforming (THF), the friction between the tube and dies plays an important role. The coefficient of friction (COF) in THF is generally determined by assuming that the contact pressure between the tube and dies is equivalent to the hydraulic pressure acting on the internal surface of the tube so that the COF can be calculated by multiplying the hydraulic pressure by the contact area. For the pulsating hydroforming process of a tube, the pulsating amplitude and frequency of the hydraulic pressure may impact the friction and result in complicated frictional characteristics. This paper presents a method for determining the COF in the THF guiding zone by a self-developed measurement system. THF experiments were performed under pulsating and non-pulsating pressures on SUS304 stainless steel tubes to investigate the relationship between the contact and hydraulic pressures and the effects of fluctuations in the hydraulic pressure on the COF. The contact pressure increased periodically with fluctuations in the hydraulic pressure and was always less than the hydraulic pressure with a maximum pressure difference of 20 %. The pulsating amplitude and frequency of the hydraulic pressure impacted the fluctuations of the contact pressure and pressure difference. The COF decreased faster with deformation; the average COF was less under pulsating pressure than under non-pulsating pressure. Increasing the pulsating amplitude and frequency decreased the average COF but had little effect on the decreasing rate of the COF. The pulsating amplitude had a clearer influence than the pulsating frequency on the amplitudes of the COF curves.