Discrete layer forming proposed in this study is a hydroforming process which can selectively deform the outer tube to a desired shape without any deformation of the inner tube by piercing small holes in the inner tube. A three-layered tube is assembled from inner, middle, and outer tubes, from either similar or dissimilar materials, and deforms simultaneously when internal pressure and axial feed are applied to the tube. In special working environments, multi-layered tubes with combined material properties, high strength, and corrosion resistance are required to satisfy conflicting performance requirements. The feasibility of proposed discrete layer forming process of three-layered tube was evaluated by a tube hydroforming experiment and process analysis was performed. An optimal loading path to prevent wrinkling around holes was developed by an analytical model and was experimentally verified. The results show that the proposed discrete layer forming process can be successfully applicable to hollow forming of non-axisymmetric multilayered tubes for structural purposes.

Discrete layer hydroforming of three-layered tubes

Friction conditions are very critical in metal forming because they can lead to important thinning in the final manufactured product. The value of the friction coefficient is then of particular importance for performing predictive finite element (FE) simulation of forming operations. Classically, numerical simulations are run with values of the friction coefficient found in technical database where the Coulomb’s coefficient has been evaluated with the pin-on-disk test. Moreover, the friction coefficient is often used as a set up parameter for running FE simulations. The pin-on-disk test is not automatically suitable for characterizing the friction coefficient in tube hydroforming. The “corner filling” test has been developed for tube hydroforming and several authors have developed analytical models for representing this test. Among them, the authors have chosen to exploit the Orban-Hu model. This model permits to build graphs. On those graphs, experimental data are overlaid in order to evaluate the friction coefficient. Thus, a friction coefficient (μ) of 0.4 has been found by the proposed method against the value of about 0.1 found in technical database. FE simulations with 3D shell and 3D solid models have been performed with the two values of the friction coefficients. The conclusion is that FE simulations with 3D shell model permit definitely better predictions for thickness evaluation. Moreover, running the simulations with μ = 0.4 leads to a more critical representation of the minimal thickness in the final component. Finally, an experimental method is proposed for the evaluation of the friction coefficient without advanced numerical methods.

Evaluation of the friction coefficient in tube hydroforming with the “corner filling test” in a square section die

Advanced sealing system to prevent leakage in hydroforming

In this work, a novel lubrication approach for T-shaped tube hydroforming is developed to reduce the forming defects referred to the wrinkles that called intermediate semiring differential lubrication (ISDL), which reduced the dependence of the internal pressure and axial-feeding loading paths to make forming process easily. The T-shaped tube was divided into four zones: bulging zone, guiding zone, back zone, and middle back zone, and the ISDL method was to coat the bulging zone and whole or part of guiding zone with PTFE film and others with MoS2. Experimental results show that the ISDL method greatly reduced wrinkles in middle back zone of the T-shaped tube without wasting time to adjust the loading path again and again. Also, formulas related to the length and width for lubrication area of ISDL method was derived to guide the tube hydroforming process. The modified tribological performance by employing regional lubrication method improved the T-shaped tube hydroforming ability.

A novel lubrication method for hydroforming of thin-walled aluminum alloy T-shaped tube

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.

Dynamic frictional characteristics for the pulsating hydroforming of tubes

Experimental investigation of friction coefficient in tube hydroforming

The present invention relates to an apparatus and method for manufacturing using a hydroforming process. The apparatus includes: upper and lower molds; first and second axial punches which are disposed on opposite sides of the steel substance so as to be movable towards each other and which have front surfaces configured so as to be insertable into each other; first and second pressurization cylinders which move the first and second axial punches towards each other and insert them into each other, thus forming a sealing space defined by the first and second axial punches and a pressurization space defined by the steel substance and the first and second axial punches; and a control unit which, through an oil-pressure passage formed in at least one of the first and second axial punches, supplies fluid into the pressurization space to pressurize the steel substance, thus forming the large caliber product.

Apparatus and method for manufacturing a large-caliber product using hydroforming

The present invention relates to an apparatus and a method for manufacturing a large-caliber product by using a hydroforming technology. The apparatus includes: upper and lower molds; first and second axial punches disposed so as to be forwardly movable from both sides of a steel material, respectively, the first and second axial punches having front surfaces that are insertable into each other, respectively; first and second pressurization cylinders defining a sealed space, which is surrounded by the two axial punches, and a pressurizing space, which is surrounded by the steel material and the two axial punches, by forwardly moving the first and second axial punches such that the first and second axial punches are inserted into each other; and a control part for supplying a fluid into the pressurizing space through an oil-pressure passage defined by at least one of the first and second axial punches so as to pressurize the steel material, thereby forming the large caliber product.

Gi Suk Chung

Papers

Experimental investigation of friction coefficient in tube hydroforming

Apparatus and method for manufacturing a large-caliber product using hydroforming

Apparatus and method for manufacturing large-caliber product by using hydroforming