Study On Ductile Damage Constitutive Model And Forming Limit Of TA15 Titanium Alloy Sheet At Elevated Temperatures

TA15 titanium alloy, a typical near a titanium alloy, has been widely used in manufacturing structure parts in aerospace industry due to its excellent performance in specific strength, good corrosion resistance, thermal stability etc. It has a high strength, low elastic modulus, and poor plasticity at room temperature, so hot forming is generally used to manufacture complex components of TA15 alloy. Superplastic forming is the main technology for complex sheet part of TA15 alloy. Although, a higher forming limit can be obtained using superplastic forming, the productivity is low and the forming temperature is high.With the development of energy problem, forming technology using lower temperature and higher forming speed than the superplastic forming becomes a popular topic. Rupture defects would occur due to lower forming temperature and higher forming speed which is affected by forming temperature, speed, stress state, strain path, and micro structure. It is not only high cost but also time consuming to determine the forming limit by experiments. Therefore, it will be of great significance to develop a ductile damage constitutive model based on the continuum damage mechanics (CDM) and basic mechanical tests, and predict the forming limit of the TA15 alloy sheet at elevated temperatures using the damage constitutive model.Firstly, uniaxial hot tensile tests of TA15 alloy were conducted at a temperature range of 750℃ to 850℃ and a strain rate range of 0.001 s-1 to 0.1 s-1. Effects of deformation temperature, strain rate, and initial microstructure on the deformation behavior were analyzed. The results indicate that the resistance to deformation and ductility are both moderate for plastic forming, and the TA15 alloy with initial equiaxed microstructure has a better ductility. The flow softening mechanism has been analyzed by microstructure observation using SEM and EBSD. The main flow softening mechanism of the TA15 alloy with initial bimodal microstructure is the transformation of slip mode from hard slip mode to softening slip mode caused by the kink, rotation and globularization of lamellar microstructure. The flow softening of TA15 with initial equiaxed microstructure, is mainly caused by dynamic recrystallization.According to the observations of the cracked specimens using SEM, the mechanism of ductile damage is attributed to the breakdown of the compatibility requirements at the a/(3 interface. In order to analyze the effect of micro structure on ductile fracture, six different initial microstructures were obtained by vacuum annealing, and hot tensile tests were conducted. The effects of microstructure on the ductile fracture were analyzed. The influence of stress states on ductile fracture was investigated using hot tensile tests of notched specimens at 800℃. The experiments results show that the fracture strain increases as the stress triaxiality increase from 0.33 to 0.45, and the fracture strain decreases as the stress triaxiality increase from 0.45 to 1.59.Based on the flow softening and ductile damage mechanism revealed by the experiments, a set of mechanism-based unified elastic-viscoplastic constitutive equations were formulated to model the flow behavior and the damage evolution of TA15 alloy in hot forming conditions. The model constants were determined using a Genetic Algorithm (GA)-based optimization method. Furthermore, the proposed constitutive equations were evaluated. The results indicate that the calibrated predictions, including flow stress, volume fraction of each phase, and fracture strain, are in good agreement with experimental results.The forming limit at 800℃ and 30 mm/min was determined by conducting Nakajima tests. Considering the effects of the max principal stress, equivalent stress and hydro stress on the damage evolution, the uniaxial damage constitutive model was extended to plane stress state, and the model parameters were determined using the experimental FLC data. The FLCs at different temperatures, strain rates and strain paths were predicted using the proposed plane stress state damage constitutive model. The results indicate that the limit stain under plane strain state increased by 67%～100% as the temperature increased from 750℃ to 850℃, and the limit strain under plane strain state increased by 88%～125% as the strain rate dropped from 0.1 s-1 to 0.001 s-1. Strain path has significant effect on the forming limit. Uniaxial tensile pre-strain can promote the forming limit in the biaxial tension zone, and biaxial tensile pre-strain can promote the uniaxial tension zone of the forming limit curve.An implicit integral algorithm and elastic prediction and plastic iterative integration algorithm were used to program the VUMAT subroutine of damage constitutive model. Single element simulation was used to prove the accuracy and robust of the subroutine. Uniaxial hot tensile tests of TA15 alloy at high temperatures were simulated, and the load-displacement curves were compared with the experimental results, indicating that the error of the predicted limit displacement and peak load were less than 3.24% and 7.14%. Hot bulge test of square blank using rigid die was simulated by VUMAT of shell element. The simulated bulge limit and rupture location and mode agree well with the experiments. The predicted rupture location agrees well with the experimental results, and the error of limit displacement of the die between the computed and experimental data is 1.6%. Finite element model of hot gas bulge forming of TA15 tube was developed using axisymmetric element. The effects of temperature, pressure, and load path on bulge limit were analyzed. The results indicate that the optimal forming temperature, pressure, and the range of pressure rising rate are 800℃,15 MPa, and 0.1-0.6 MPa/s, respectively. The forming limit can be further promoted by reducing the pressure after the pressure reaches the forming pressure.In a word, uniaxial hot tensile tests, hot bulge tests and microstructure observation were used to investigate the hot deformation behavior and ductile damage mechanism of the TA15 titanium alloy sheet. Uniaxial hot tensile constitutive model coupled damage was developed, and the model was extended to plane stress state by considering the effects of stress state on the damage evolution. The forming limit curves were predicted using the plane stress state damage constitutive model. Numerical simulations on thermal-mechanical-damage-microstructure of TA15 alloy were realized, which provides theoretical guidance for forming limit prediction and process optimization.