Tube hydroforming with the characteristics of light weight and integration is a kind of advanced plastic forming technology in manufacturing hollow lightweight components with variable cross-sections. In this paper, the research works of developing tube hydroforming equipments independently and investigating new forming process were carried out under the background of applying in forming auto parts. Method of combining experiments and finite element simulations was employed to discuss the relationship between loading paths and formability of tubes in hydroforming. Significant effect of formability improvement by pulsating load was proved; mechanism of formability enhancement was explained in the two aspects of process parameters and material properties. The results have practical significance in playing advantages of tube hydroforming in manufacturing hollow components.
Hydroforming equipments were designed and constructed based on domestic general press. Full set of equipment is composed of the high pressure-creation system, the tools system, the data acquisition system and the control system. The equipment has strong universality and is easy to operate.
The loading path affects the metal formability remarkably in tube hydroforming. Experiment results show that the formability of TP2 copper tubes has not been improved in pulsating hydroforming without axial feeding; but for AISI 304 stainless steel tubes, the results showed that the formability can be improved through enhancing the deformation uniformity by fluctuant internal pressure. It is considered that the effect of pulsating load on formability should be understood from two different aspects of process parameters and material properties. In the aspect of process parameters, the appearance and disappearance of the small wrinkling under pulsating internal pressure matched with axial feeding is considered as the main reason for preventing local thinning; regarding material properties as in AISI 304, mechanism of formability enhancement is related to the nature of the material itself.
It was proved by intermittent uniaxial tensile test that to be deformed under intermittent loading is an effective way to improve the formability of austenitic stainless steel, but it is not suitable for TP2 pure copper. The elongation of AISI 304 stainless steel tubes can be enhanced significantly by about 40% through intermittent loading. Microstructural evolution in the loading and unloading processes is responsible for the improvement in the formability of tubes. Stress state induced microstructural evolution is the nature of this kind of materials themselves.
The experiment and simulation results showed that pulsating load path during hydroforming of tubes can achieve a more uniform deformation with a more homogeneous thickness distribution beneficial for preventing excessive local thinning. The simulation results reveal that the reasons for more homogenous deformation are that the contact friction force between tubes and die, namely flow resistance of metal, is reduced under pulsating load path condition. The main reason for friction force reduction under pulsating load path is that regular springback by pulsating internal pressure causes the reduction of equivalent contact normal force.
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