Study On The Forming Law Of 304 Stainless Steel Stiffened Tube Hydro-Assisted Forming

With the development of science and technology,lightweight materials are more and more important in areas of aerospace and weapons which definitely improves energy saving,environmental protection and mobility.Thin-walled stiffended tube is a common lightweight structure with a series of features such as shape of closed cross section,thin wall thickness,complex section,large size etc.Currently there is no easy way to form thin-walled stiffened tube perfectly,especially for the integral forming.In this paper,hydro-assisted forming was applied to the integral forming of the stiffended tube.The forming law was studied through experients and numerical simulation based on ABAQUS.The defects and deficiencies of the forming parts were analyzed and solved by various methods.Hydro-assisted forming was proved to be a feasible way to the forming of the stiffended tube.It is known to us that the essence of the deformation of the stiffended tube is actually the change of its curvature.This paper has designed three types of cross section shape including elliptical cross section,half elliptical cross section and rectangular cross section.Stiffended tubes with all these three kinds of cross section are proved to be able to be formed using the technology of hydr o-assisted forming.Furthermore,stiffended tubes with more complex shapes of cross section are able to be formed based on the technology too.The range and mode of the internal pressure loading during the process of the hydroforming were analyzed.Under constant pressure loading path,the filling of the tube material is good,the wall thickness distribution is not uniform,the stress fluctuates greatly.It is the opposite under linear loading path.Thence,linear loading path is a better way for the forming of the ribbed tube.The defects of stiffended tube were investigated.There are three types of instable failure: dumping of the ribs in the position of the increased curvature,wrinkle of the ribs in the position of reduced curvature,collapse of the tube in the position of reduced curvature.The causes of the wrinkle and collepse of the ribs are that the hoop stress of the rib and the tube is greater than the critical buckling stress.The cause of the dumping is that the hoop stress of the rib in the position of rounded corners is uneven,which results in bending moments.The reasons of the deformation uncoordinated phenomenon were investigated.The wall thickness of outmost ribs is thicker in the position of the straight segment and thinner in the position of rounded corners.The wall thickness of innermost ribs is thinner in the position of the straight segment and thicker in the position of rounded corners.The wall thickness of the tube does not change.The perimeter of the inner ribs in the position of the rounded corner is smaller than the tube,which results in the “waist” phenomenon.The simulation results are in good agreement with the theoretical analysis.The influence of the factors including friction coefficient,high-thickness ratio internal pressure and springback on the forming limit of the tube is analyzed.The stiffended tube can be formed perfectly when the range of high-thickness ratio is from 0 to 6.The filling speed of the upper fillet is larger than the lower fillet because of the friction.The larger the friction coefficient is,the worse the forming limit is.The loading of the internal pressure significantly increases the forming limit of the ribbed tube.The springback has little influence on the forming limit.