Abstract: Friction plays an important role in tube hydroforming (THF)process, and then accurate determination of the coefficient of friction (COF) becomes a paramount task to promote the application of the process. After a detail discussion of the method in existence, a new measuring model of determining COF in expansion zone of THF is proposed on the basis of the principle of tube hydroforming with radial crushing. The measuring model features a round tube hydro-formed into a square part under both a constant internal hydraulic pressure and a radial compression, with the difference of the two diagonal lengths in the square section of deformed part as a friction force or COF indicator. The COF can be determined through matching experimental and FEM simulation results by comparison of the variation of the indicator. The finite element method simulation results of low carbon steel and stainless steel tubes show that the indicator is conveniently measurable and extremely sensitive to the friction force or the COF, therefore can be used as a index to evaluate lubricant performance under realistic friction conditions occurring in the expansion zones of THF process. The results also indicate the measuring model is simple and feasible.

Key words: Finite element method, Friction coefficient, Radial crushing, Tube hydroforming