| Due to the wrinkle and rupture defects appear in the unsupported area is hard to eliminate when using the traditional sheet hydroforming process to form the aluminum alloy thinner curved sheet parts represented by the ellipsoidal rocket tank bottom,which has become a bottleneck problem.In order to solve this problem and improve the thickness distribution of formed sheet parts,by introducing an upper aided sheet to the single sheet hydroforming process,a new hydroforming process-double-layer sheets hydroforming was proposed in this paper,focusing on a certain type of ellipsoidal rocket tank bottom,selecting the 2198 Al-Li alloy and5A06 aluminum alloy sheets as the experimental materials,the deformation behavior of double-layer sheets hydroforming was investigated,the wrinkling mechanism and improvement mechanism of deformation uniformity for double-layer sheets hydroforming were revealed.It provides a theoretical guidance for weight reduction and reliability improvement of new ellipsoidal rocket tank bottom.In order to determine the minimum thickness of the upper sheet in double-layer sheets hydroforming process,the mechanical analysis of thinner curved sheet parts during deep drawing process was carried out.Combining with the geometrical model,stress and numerical analysis during forming process,the calculation model of the critical thickness of upper sheet was obtained.The effect of material and experimental parameters on the critical thickness of upper sheet were analyzed.Results show that by increasing the axial length ratio,work-hardening exponent,blank holding force and initial radius of the sheet,decreasing the Dimensionless standardized modulus of elasticity,the value of critical thickness of upper sheet is reduced.The selection method of the thickness of upper sheet is obtained by combining the critical thickness of upper sheet and the practical engineering.In order to obtain the establishment method of forming window in double-layer sheets hydroforming process,the mechanical analysis of thinner curved sheet parts during double-layer sheets hydroforming process was carried out.Combining with the geometrical model and stress analysis during forming process,the calculation model of the rupture liquid pressure was obtained(rupture liquid pressure under different punch stroke was the upper limit of forming window),the critical wrinkling stress of formed sheet parts was calculated through analytical method and the lower limit of forming window was obtained,in which the magnitude of the circumferential stress and critical wrinkling stress is the same.In order to reveal the wrinkling inhibition mechanism of double-layer sheets hydroforming process,the finite element analysis of thinner curved sheet parts during double-layer sheets hydroforming process was carried out.The stress distribution and position of stress boundary were compared and the influence of the thickness of upper sheet,hydraulic pressure,interfacial lubrication condition and axial length ratio on wrinkle behavior of formed sheet parts were studied.Results show that by introducing the upper sheet,wrinkle defects were effectively controlled as the critical wrinkling stress of the lower sheet is increased and the circumferential compressive stress is reduced.By increasing the thickness of upper sheet and axial length ratio,the critical wrinkling stress of the lower sheet is increased,by increasing the hydraulic pressure and the interfacial friction coefficient,the critical wrinkling stress of the lower sheet is increased and the circumferential compressive stress is reduced.Experiments of aluminum alloy double-layer sheets hydroforming were conducted to illustrate the wrinkle behavior of formed lower sheet parts under the effect of the thickness of upper sheet,hydraulic pressure,interfacial lubrication condition and axial length ratio.Increasing the thickness of upper sheet,hydraulic pressure,interfacial friction coefficient and axial length ratio can effectively control the wrinkle defects with average height of wrinkling decreased by 98.33%,91.43%,90% and 99.56%,respectively.Through theoretical analysis,finite element simulation and experimental research,forming window of 2198 Al-Li alloy thinner curved sheet part with axial length ratio 1.4 and radius-thickness ratio 660(ratio of thickness to diameter 1.52‰)for single and double-layer sheets hydroforming process was built.Wrinkle defects are difficult to be eliminated as there is overlapping area in the forming window of single sheet hydroforming,while for double-layer sheets hydroforming,as the forming window is wide between the upper and lower limits,wrinkle defects are easy to be controlled and eliminated.In order to obtain the improvement mechanism of deformation uniformity of double-layer sheets hydroforming,hydroforming of single and double-layer sheets were conducted through finite element simulation and experimental research.The shape and strain distribution of unsupported area of formed sheet parts under the same loading path and the same punch stroke were compared and ana lyzed.After that,combining with the distribution of shear frictional stress at the interface,the improvement mechanism of deformation uniformity of double-layer sheets hydroforming was obtained.By introducing the upper sheet,on the one hand,the radial and circumferential strain of formed lower sheet is reduced,on the other hand,the strain distribution uniformity of formed lower sheet is improved.Thus,the uniform deformation ability of the lower sheet is improved.Experiments of aluminum alloy double-layer sheets hydroforming were conducted to illustrate the deformation behavior of formed lower sheet parts under the effect of the thickness of upper sheet,hydraulic pressure,interfacial lubrication condition and axial length ratio.On the one hand,by increasing the thickness of upper sheet and fluid pressure,the wall thickness reduction of lower sheet decreases and the thickness and strain distribution uniformity are improved.On the other hand,as the thickness of upper sheet and fluid pressure increases to a certain value,the weakening of interfacial shear friction effect,increasing of the reverse bulging effect in the unsupported area and the increasing of friction between sheet and blank holder leads to the thickness and strain distribution uniformity improvement slightly reduced.By increasing the interfacial friction coefficient and axial length ratio,the wall thickness reduction of lower sheet decreases and the thickness and strain distribution uniformity are improved. |
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