Research On Instability Behavior Of Magnesium Alloy Tube In Warm Hydroforming

Warm tube hydroforming is an advanced process for forming a tubular part by a designed loading path in the warm condition. Due to the improvement of formability at elvated temperature, light-weight metal tubes with high specific strength but low formability can be formed by this method. Therefore, the part weight can be reduced further. In this dissertation, the constitutive equation of AZ31B magnesium alloy tube among the temperature rangs of 150℃and 300℃was built based on experimental results. For forecasting the critical wrinkling stress of a tube, the theoretical model was proposed. The principles of stress states and strain states, wrinkles'shape and wall thickness of an AZ31B magnesium alloy tube during its wrinkling process were investigated. Influences of temperature, cross-section preforming and axial feeding preforming on the bursting of an AZ31B magnesium alloy tube were researched. Moreover, the feasibility about taking wrinkled parts as preforms and the feasibility about forming a tube with a nonuniform initial wall thickness in differential temperature fields were also analyzed.Axial tensile tests for a seamless AZ31B alloy tube were conducted when the processing temperature changed from 20℃to 300℃, and strain rate changed from 0.001s-1 to 0.1s-1. Results showed that the yielding strength and the tensile strength decreased as the processing temperature increased or the strain rate decreased. The total elongation increased, but the uniform elongation first increased and then decreased. The maximum uniform elongation was obtained at the condition of 175℃or 0.01s-1. Contitutive equation of the AZ31B magnesium alloy tube was obtained by fitting multiple regression Beckofen equation. The phenonman of work hardening can be reflected by this equation.Critical stress for the onset of axisymmetric wrinkling in tube hydroforming was derived. Influences of internal pressure and stress ratio, the geometries and properties of a tube and temperature field on the critical wrinkling stress were analyzed. Results showed that the critical wrinkling stress increased as the stress ratio increased when the partial derivative of the model function was positive. The critical wrinkling stress first deceased and then increased as the stress ratio increased when the partial derivative of the model function was negative. The critical wrinkling stress would decrease if the relative value between the yielding strength and the reduced modulus was negative. Otherwise, it would increase. Variety of wrinkles'shape and demension, wrinkling location and quantity, wall tickness distribution and stress and strain states during wirnkling were researched at different internal pressures, temperatures and feedings. The feasibility about taking wrinkled parts as preforms was analyzed. Results showed that the wall thickness in the initial yielding state would increase if the ratio of internal pressure to yielding strength was less than 1 3 times the thickness radius ratio. Otherwise, the wall thickness at initial yielding state would decrease. The increasing magnitude of the tube was lower if the processing temperature was higher. As feeding increased, the circumferential strain increased, but the axial strain decreased. The thickness strain first increased and then decreased. The outer radius of the wrinkles increased but the width decreased. The wrinkles moved vis-a-vis. As internal pressure or temperature was elevated, both the outer radius and the width of the wrinkles increased and the wrinkles also move vis-a-vis. In addition, it was a reasonable method to form parts with large expansion ratio by taking wrinkled parts as preforms.Influences of temperature, cross-section preforming and axial feeding preforming on the bursting of an AZ31B magnesium alloy tube were investigated. The feasibility about forming a tube with a nonuniform initial wall thickness in a differential temperature field was analyzed. Results showed that the internal pressure for initial yielding and bursting decreased as the processing temperature increased. The maximum expansion ratio first increased and then decreased. As the flatten displacement increased, both the maximum expansion ratio and the bursting internal pressure decreased. As the axial feeding increased before bulging, the maximum expansion ratio increased which was good for forming parts with large expansion ratio. Besides, the tube wall thickness could be changed more uniform if the tube section with higher initial wall thickness was at higher temperature.