Research On The Influence Of Forming Process On The Forming Quality And Performance Of 316L Bellows

Metal bellows are widely used in aerospace,shipbuilding,petrochemical,electric power and other fields as elastic components because of their sealing,flexible compensation,energy storage and other functions.Bellows formed by hydroforming and mechanical bulging usually have different wall thinning degrees and contour elastic deformation,leading to problems such as insufficient stiffness,strength and other mechanical properties divergence in the subsequent use,which affect the long-term service performance of the bellows.With the continuous demand of high precision and high performance bellows in engineering,it is urgent to investigate the differences in forming quality and mechanical properties of bellows caused by different forming processes.Therefore,a systematic investigation on the deformation characteristics of 316 L bellows during hydraulic and mechanical bulging process is carried out,and axial stiffness and stress distribution of bellows are analyzed by employing finite element numerical simulation,theoretical analysis and experimental study.The main achievements are as follows:A 3D finite element model for the whole process of hydroforming and mechanical bulging of 316 L bellows is established based on ABAQUS platform.Some key technologies are solved in modeling,including friction between interfaces,element selection,boundary conditions and predefined field setting.The reliability of the finite element model of hydroforming and mechanical bulging is verified by experiment from two aspects: the typical structure size and contour characteristics.The deformation behavior of bellows in hydroforming is revealed.It is found that the wall thinning degree at the crest is the largest,while the one at the trough is the smallest,and springback occurs after the mold is removed.The effect of process parameters on the forming quality of hydroformed bellows is studied.The results show that: the increase of forming pressure and die spacing will lead to the increase of convolution height and wall thinning rate,but the extrusion speed has no obvious effect on the forming quality of bellows(wall thinning rate,wave height).Through orthogonal test,the forming process window of bellows hydroforming which meets the different forming requirements is obtained.And the optimal combination of the main parameters of bellows hydroforming is determined as follows: forming pressure is 2MPa,and die spacing is 30 mm.The convolution height deviation of bellows is 0.4%,and the maximum wall thinning rate is5.2% under this set of parameters.The deformation behavior of bellows in mechanical bulging and rolling calibration process is revealed.It is found that: the convolution height is less than the die feeding because of springback;obvious inner diameter deviation and pitch deviation is observed;the convolution height does not change significantly during the calibration,and the convolution radius and pitch gradually decrease.The influence of die feeding and die spacing on the forming quality of bellows is studied.The results show that with the increase of die feeding,the convolution height and wall thickness decrease,and the decrease of trough diameter becomes more serious after the calibration,so the die feeding should not be too large.The inner diameter of the bellow after calibration is easy to increase with a small die spacing.A large die spacing will lead to serious concave at the trough of the bellow after calibration,as well as an increase of the inner diameter deviation.Through the orthogonal test,the process window of the mechanical bulging bellows is obtained,and the optimized process parameters are determined as: the feed of the die is20.5mm,and the convolution spacing of the die is 52 mm.Under these forming conditions,the convolution height deviation of bellows is 0.3%,the inner diameter deviation is 0.11%,and the maximum wall thinning rate is 4.37%.Based on the simulation and experiment of bellows hydroforming and mechanical bulging,the forming characteristics of bellows are compared and analyzed.The results show that the equivalent stress-strain distribution of hydroformed bellows is more uniform,and the maximum equivalent stress and strain of hydroformed bellows is smaller than that of mechanical bulging bellows.The wall thinning rate of mechanical bulging bellows is smaller than that of hydroforming bellows,but the crest profile of hydroformed bellows is smoother and more closed to the design profile.Based on the hardness test,it is found that the hardness of mechanical bulging bellows is greater than that of hydroformed bellows.The axial stiffness and stress test of bellows are carried out.The results show that the deviation between stiffness obtained by EJMA calculation,finite element modeling and experiment is not large.It is feasible to calculate the axial stiffness of bellows through the established simulation model.The stiffness of hydroforming bellows is smaller than that of mechanical bulging bellows.Under the axial compression condition,the axial stress of hydroformed bellows is smaller than that of mechanical bulging bellows,and the hydroformed bellows can meet greater displacement requirement with better service performance.