Research on hydroforming through combination of internal and external pressures for manufacturing the structure of double-layer tube with gap

According to the current development of commercial vehicles, a novel structure of the double-layer exhaust pipe with gap was designed. Hydroforming through the combination of internal and external pressures (HIEP) was proposed to form the complex structure of the new exhaust pipe. The double-layer tube without gap was used as the initial tube blank. High-pressure liquid was introduced into the inner tube and between the two tubes. The outer tube would bulge outward and fit the die cavity as its inner surface was subjected to hydraulic pressure. Meanwhile, the inner tube would not deform due to both the inner and outer surfaces were subjected to hydraulic pressure. In addition, before the HIEP, it is necessary to pre-bend the double-layer tube. The feasibility of the process was analyzed by numerical simulation. The process parameters were optimized by response surface methodology (RSM). The objective functions such as the maximum thinning rate, unevenness of wall thickness and gap dispersion were established in RSM models, whose accuracy was verified by a statistical method of analysis of variance. Subsequently, the die of HIEP was designed and developed. The verification experiments were carried out and the double-layer exhaust pipe with gap was successfully manufactured.