Study On The Wrinkle Instability Of A Thin Walled Tube In Hydroforming With Radial Crushing

The main advantages of tube hydroforming (THF) over conventional manufacturingtechniques are enhanced component strength and rigidity, reduced weight and assemblycost by using common material more efficiently, so it has enjoyed increasingly widespreadapplication in the manufacture. Tube hydroforming with radial crushing (THFRC) hasadvantages in forming large diameters and longer lengths tubes, such as car chassis, enginecradles, etc. It is beneficial to reduce the friction between the tube and dies, avoid thedifficulty of material flowing caused by the die geometry constraint, which is the mainreason for over thinning, poor corner-filling ability or cracking. Wrinkling instability is oneof the main failure modes of THFRC. The researches of instability wrinkle status,wrinkling features of THFRC are the premise of putting this technology into productionand application. It has great significance for the spread and exploitation of THFRC.In this paper, the formability and wrinkling characteristics of a thin walled tube indifferent stages of hydroforming with radial crushing are investigated on the combinationof theoretical analysis, finite element simulation and experimental studies. The initial yieldpressure of the initial tube, the thickness distribution, circumferential and axial radius ofthe bulging area during the free bulging stage, the initial yield load of the tube during thecombined forming stage are discussed, at the same time, and the erected mechanical modelare checked via finite element simulations and experiments. A indicator is proposed tocharacterize the form and extent of wrinkling, and the wrinkling critical value that thenaked eye can identify is pointed out at the same time. The wrinkle distribution of1Cr13Mn9Ni1N stainless steel tube in hydroforming with radial crushing under linear andconstant hydraulic pressures is analyzed. A criterion of useful wrinkling of THFRC isproposed on the base of the wrinkling peculiarity, and the critical value is deduced too.Furthermore, the influence of the radius, thickness and length of the tube, coefficient offriction (COF) between the tube and the dies, material strength coefficient and hardeningexponent on the wrinkling are deliberated, and the relative importance of every processingvariable for wrinkling has been investigated on the base of virtual orthogonal experiment.The results show that:(1) The deduced formulas (3-17),(3-16) and (3-19), of thethickness distributions circumferential and axial radius of the bulging area during the freebulging stage, the initial yield load, formulas (4-6) and (4-19), of the tube during thecombined forming stage are credible, which can be used for the analysis of the formabilityof the free bulging stage; and the mechanical model of the combined forming stage arereliable.(2) concave-wrinkle status appears along the circumferential direction at the tube bulging area, which contacted with the moving die; the curvature Kminof the bugledworkpiece cross-section can be used to identify wrinkling, the value of Kminindicates theextent of wrinkle, and the plus-minus of Kminshows the forms of wrinkle.(3) The wrinkleis useful to improve the formability of the tube when the wrinkle height meet certainrequirements.(4) Bulging methods has significant effects on wrinkling. Wrinkle inTHFRC happens with a higher probability and greater extent comparing with free bulgingprocess. The most serious region of wrinkle distributes at the side edge of bias section ofTHFRC. Loading path has too much effect on wrinkling in THFRC too. Under the constantload, wrinkling on the cross-section away from the middle cross-section is more distinctand the wrinkling on the side edge is more obvious than that on the bottom of the bugledworkpiece.(5) Material strength coefficient, hardening exponent, the radius and the lengthof bulging area of the tube have too much effect on wrinkling, while the thickness andcoefficient of friction have little, the relative importance of every processing variables forwrinkling from the largest to the smallest can be expressed as strength coefficient, thelength of bulging area, coefficient of friction, hardening exponent, the radius and thicknessof the tube.The results achieved by the present research work are useful for further study andwide application of tube hydroforming with radial crushing.