Research On Hydroforming Of 1Cr18Ni9Ti Stainless Steel Deep Cavity Parts With Half Closed Spherical Geometry

Deep cavity parts with half closed spherical geometry are widely used in aerospace,petrochemical industry and transportation.This kind of components have characteristics such as thin-walled,deep cavity,surface with negative angle and high accuracy.Its service environment is some extreme conditions including high pressure,very low temperature.Traditional forming method of these parts faces unavoidable disadvantages such as high cost of the mould,long manufacturing cycle,low forming precision and reliability.In order to solve the mentioned problems,integral hydroforming method is presented in this paper.The reliability andintegrity of the component can be improved significantly.However,because of the deep depth and highrequirement of thickness uniformity,the whole hydroforming process is difficult to carry out.The half closed spherical deep cavity part with 1Cr18Ni9 Ti stainless steel was studied as an object,and the hydroforming process was researched on this kind of components.The thickness distribution on the half closed spherical deep cavity part with 1Cr18Ni9 Ti stainless steel was analyzed by FEM,the preformed part of cylinder cup with inner diameters of 350 mm,375mm and 400 mm were studied,and the results shown that the thickness improvement with increasing of diameter.The radius on bottom corner for the cylinder cup with radius of 50 mm,100mm,and 150 mm were studied,and the results shown that the most uniform thickness distribution can be obtained with radius of 100 mm.Moreover,the spherical bottom and planar bottom of cylinder cups were studied on the thickness and the planar bottom show more uniform thickness than the other.A forming process combining hydromechanical deep drawing and hydrobulging was proposed in this research,as well as corresponding experimental setup.A 1Cr18Ni9 Ti stainless steel sheet with 2.4mm thickness was prepared to form a complex deep cavity parts with: 300 mm open radius,375 inner radius of the cylindrical part,100 mm corner radius and the height of the parts is 445 mm.A proper heat treatment standard was obtained by measuring the mechanical properties of the sheet in solid state and the blank after plastic forming process.In order to form this complex part,the forming process consists of two steps including: pre-bulging and reshaping under high pressure.Results show that the thickness distribution satisfies a shape as “S”,the thickness is decreased first,and then this value is increased.The most thinning ratio is about 17.89%,locating at the equatorial zone of the parts.The sheet flowing process during the pre-bulging process is like the deep drawing process of cylindrical shells.The in-flowing of blank at the open hole position can participate in the pre-bulging period,thus the deformation level in bulging process.When the pre-bulging pressure is lower than 7MPa,the thickness distribution is better than that when the pressure is higher than 7MPa.The parts surface can be formed completely without any defects and same as the inner surface of the molds under a preforming pressure of 5MPa and reshaping pressure of 35 MPa.The microstructure analysis of the formed parts indicates that with the improvement of plastic deformation,the percentage of martensite is obviously improved.After measuring the mechanical properties of the formed parts,it can be seen that the strain hardening phenomenon is remarkable,and this hardening effect can be eliminated well after solid treatment process.