Deformation behaviors under quasi-static and dynamic compression and high velocity impact condition of Ti-6Al-4V ELI (extra low interstitial) alloys in two different conditions were investigated. Mill annealed (MA) alloy, consisted of equiaxed α, and thermomechanically treated (TMT) alloy, consisted of mixed structure of equiaxed α and transformed β, were prepared. Compression tests were performed in low strain rate regime using hydraulic testing machine and were performed in high strain rate regime using split Hopkinson pressure bar. High velocity impact tests were also performed by impacting the test projectiles made of these alloys against a steel target at a velocity of ~400m/s. The compression test results showed that deformation behaviors were influenced by the strain hardening exponent at low strain rate regime, and by both the strain hardening exponent and the strain-rate hardening rate at high strain rate regime. TMT alloy showed higher strength but almost similar fracture strain as MA alloy at a high strain rate of ~6000/s, due to the effect of strain-rate hardening. The high velocity impact test results showed that the projectile of TMT alloy withstood without fracture at higher impact velocity, but the maximum amounts of deformation prior to crack were nearly the same for both alloys. These results were in accord with the results of compression tests at high strain rate regime, that is, higher strength but same fracture strain of TMT alloy compared to MA alloy.
The globularization behavior of ELI grade Ti-6Al-4V alloy having different initial lamellar structures during multi-step forging under nonisothermal condition was investigated. The samples with either a thin or thick lamellar structure, which were produced from annealing followed by different cooling conditions, were upset and stretched repeatedly at forging start temperatures of 940 C and 900 C, and subsequently air cooled. The microstructural changes after non-isothermal multi-step forging were analyzed with respect to globularization of lamellae. After multi-step forging at 940 C, the initially thin lamellar structure changed to homogeneous equiaxed globules, but elongated globules with high aspect ratio were obtained from the initially thick lamellar structure. By forging at 900 C, the initially thin lamellar structure was changed to a mixed structure of both equiaxed and elongated globules, while severely deformed lamellae were preserved inside distorted colonies with little globularization for the initially thick lamellar structure. Globularization of the samples by forging at 900 C with both initially thin and thick lamellar structures was less effective compared to that by forging at 940 C. A quantitative microstructural analysis revealed that, in addition to the initial microstructure and forging start temperature, globularization of lamellae was significantly affected by the instantaneous microstructural development by thermal fluctuation during non-isothermal multi-step forging.
Prediction of final microstructures after high temperature forming of Ti-6Al-4V alloy was´attempted in this study. Using two typical microstructures, i.e., equiaxed and Widmanstätten microstructures, compression test was carried out up to the strain level of 0.6 at various temperatures (700 ~ 1100°C) and strain rates (10-4 ~ 102/s). From the flow stress-strain data, parameters such as strain rate sensitivity (m) and activation energy (Q) were calculated and used to establish constitutive equations for both microstructures. Then, finite element analysis was performed to predict the final microstructure of the deformed body, which was well accorded with the experimental results.
The hot backward extrusion process of Ti-6Al-4V tubes was designed by finite element (FE) simulation and experimental validations. In the hot backward extrusion of Ti-6Al-4V alloy, the main problems to emerge were surface cracks and sticking between the punch and the workpiece. The punch designs and back draft angles were investigated with a common hot backward extrusion condition. Then, based on a relatively good die design, the optimum process conditions were suggested. The design criteria were to achieve uniform distributions of strain and temperature and to produce a defect-free final product. As a model for predicting the forming defects, the continuum instability criterion developed by Ziegler was coupled to the internal variables generated from FEM simulation. Experimental observations were carried out on the extruded Ti-6Al-4V tubes to validate the model for predicting the forming defects. The die-chilling and friction greatly influenced the deformation mode of the tube and the formation of surface cracks. The formation of defects in the extruded tube was attributed to the non-uniform distribution of strain, strain rate, and temperature in the extruded tubes at the given experiment conditions.
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