Plastic Instability And Deformation Behavior Of Tube Hydro-forging For Irregular-section Component

Structural lightweight is one of the main means to save fuel,reduce pollutant emission and improve mobility of vehicles such as automobiles,airplanes and rockets.Lightweight material and lightweight structure are two ways to realize lightweight.For a certain material structure,the main method of weight reduction is to design and manufacture a light body structure with reasonable geometry.Variable section hollow component is one of the main forms of lightweight structure.High pressure tube hydro-forming is an important process to form the variable cross-section hollow component.By applying high-pressure liquid and axial feeding,the tube is pressed into the die cavity to form the required shape.In high pressure tube hydro-forming,the internal pressure is the main deformation driving force,and requires a very high level.At the same time,the wall thickness reduction is inevitable in tensile deformation mode.In order to solve the problems of high pressure and hard feeding,a new tube forming process,the Tube Hydro-forging,was proposed in this paper.The section of tube blank is plastic deformed under in-plane biaxial compressive stress,which has low forming pressure,small forming corner,without axial feeding,wall thinning and fracture.However,this new deformation mode brings more serious instability problems.In this paper,the plastic instability behavior and deformation behavior of Tube hydro-forging were studied deeply.The mechanical model of wrinkle development,analytical model of tube hydro-forging,theoretical model of wrinkling and warping instability were established.The wrinkle shape characteristics,instability types,critical support internal pressure and key process parameters in tube hydro-forging were analyzed systematically,which provides theoretical basis for the development of compression deformation technology.The shape of wrinkles under internal pressure is measured.The geometry of wrinkles under pressure still accords with the characteristics of trigonometric function,but the wave height?and wave length L have changed.Based on the plastic bending theory,the mechanical model of wrinkle development under internal pressure is established,and the curve of wave height to wavelength ratio?/L with internal pressure is predicted.The effects of material properties,wavelength and wall thickness on wrinkle flattening were analyzed:when the initial wave height-wavelength ratio?₀/L₀ is the same,the smaller the initial wavelength L₀,the easier to flatten;the larger the wall thickness,the easier to flatten.Finally,the criterion condition of dead wrinkle is proposed:the difference between wrinkle length and wavelength u is less than 1.3t₀+0.63,where 1mm?t₀?3mm.Based on the plastic total theory,the analytical model of tube hydro-forging was established,and the distribution of circumferential force,equivalent stress and compression strain were obtained.The plastic deformation mode of tube hydro-forging is revealed,that is,the normal force at the die corner provides the compression deformation force(circumferential force),which makes the material flowing and compressed.The formula for calculating the die closing force was derived.The die closing force is composed of deformation force and internal pressure force.Under the critical forming pressure,the internal pressure force accounts for about half of the die closing force.The die closing force increases with the increase of width,wall thickness,strength coefficient,friction coefficient and internal pressure;the corner radius has no effect on the die closing force.The distribution function of die contact stress was derived.The contact stress at the die corner is the largest,and it increases with the increase of friction coefficient,internal pressure and wall thickness.The width has no effect on the contact stress.The increasing of the corner radius can greatly reduce the die contact stress.The reason for the relatively small wall thickness at the corner is revealed,that is,the excessive normal stress at the corner increases the hydrostatic stress and reduces the equivalent stress of the material,which makes the material at the corner not easy to deform.Increasing the corner radius can significantly increase the wall thickness at the corner.It is found that the maximum wall thickness is located at the tangent of upper and lower die,and it increases with the increasing of width,friction coefficient and internal pressure,and decreases with the increas ing of strength coefficient and hardening exponent.For the tube hydro-forging of asymmetrical cross-section,the die parting scheme which is beneficial to the uniformity of wall thickness was obtained.There are two types of instability in tube hydro-forging:wrinkling and warping.Based on the energy method,the analytical models of wrinkling suppression pressure and warping suppression pressure were established respectively.Combined with the analytical model of tube hydro-forging,the critical forming pressure was obtained.It was revealed that the wrinkle instability occurs at the tangent of upper and lower die,and the wrinkle suppression pressure increases with the increase of rectangle width and friction coefficient,and decreases with the increas ing of wall thickness and corner radius.The location and mechanism of warping instability were revealed:warping instability occurs at the straight wall of upper die,which is caused by the rotation of material under compressive stress.The critical stress of warping is smaller than that of wrinkling,so the warping is more likely to occur under the same conditions.It is pointed out that wrinkling and warping would not occur at the same time.Which kind of instability is easy to occur depends on the transformation strain.The larger the transformation strain is,the warping is more likely occur.Increasing the friction coefficient and the corner radius can reduce the transformation strain and limit the occurrence of warping instability;the smaller the angle of the upper corner or the greater the wall thickness and hardening index,the greater the transformation strain,and warping would be more obvious.