EFFECTS OF PHASE TRANSFORMATIONS ON LASER FORMING OF Ti-6Al-4V ALLOY

In laser forming, phase transformations in the heated affected zone (HAZ) take place under steep thermal cycles, and have a significant effect on the flow behavior of Ti-6Al-4V alloy and the laser forming process. In the present work, the transformation during heating is modeled based on the Johnson-Mehl-Avrami (JMA) theory, and the decomposition of phase, producing martensite ' or lamellae dependent on cooling rate, is also numerically investigated. The volume fractions of phases are obtained. Consequently, the flow stress of Ti-6Al-4V alloy is calculated by the rule of mixtures based on the phase ratio and the flow stress of each single phase, which is also a function of temperature, strain and strain rate. According to the obtained flow stress data, the laser forming process of Ti-6Al-4V alloy is modeled by finite element method (FEM), and the deformation is predicted. A series of carefully controlled experiments are also conducted to validate the theoretically predicted results. INTRODUCTION In the past decades, considerable research has been carried out on the computer modeling of laser forming, however, the application of numerical modeling is limited by lack of precise data of the temperature and strain rate dependent flow stresses of materials. In general, the flow stress data were generally obtained directly from experiments or the constitutive equation based on limited experimental data. The experiment is cumbersome and time consuming, and flow stress data must be obtained under steady temperatures, that is, the experimentally obtained flow stress data are valid only under phase equilibria, and are not accurate under transient phase transformations like in laser forming. With very fast heating and cooling, laser forming is significantly different from other hot working processes, in which steep temperature gradients and thermal cycles lead to severe microstructural changes in heat affected zone (HAZ) within very short time. A methodology (Fan et al., 2004) has been introduced to model flow behavior under transient phase transformations in laser forming, in which flow the contribution of each phase to flow stresses was calculated by the mixture rule based on its volume fraction, and the temperature, strain and strain rate dependent flow stress data of every single phase can be obtained from experiments. The results show that the method predicts flow behavior of low carbon steel under transient phase transformations very well. However, the prerequisite to apply the methodology is that the kinetic details about the microstructural evolution are Transactions of NAMRI/SME 235 Volume 33, 2005 well known. With recent developments in computer simulation of phase transformations based on the fundamental kinetic and thermodynamic theories, it is possible to introduce those techniques to describe the particular laser forming process. In this work we seek to quantitatively understand the kinetics of phase transformation during laser forming of Ti-6Al-4V alloy and their influence on flow behavior and deformation. In particular, the phase transformation during rapid heating, the decomposition of the phase during rapid cooling. During rapid heating, the kinetics of phase transformation have been extensively investigated by other researchers, and the process can be modeled by the modified Johnson-Mehl-Avrami (JMA) equation for non-isothermal process. The decomposition of the phase during cooling is much more complicated, and the product is martensitic ’ or different morphology secondary phase depending on cooling rates. Ahmed and Rack (1998) investigated the phase transformations during cooling in Ti-6Al-4V alloy, and concluded that the martensitic transformation takes place as cooling rate above 410 K/s. Slower cooling rates lead to diffusion controlled nucleation and growth process of secondary lamellae into the grains. The diffusion controlled phase transformation during cooling is also able to be modeled by JMA equation (Malinov et al., 2001). The objective of this work is therefore to investigate the phase transformations during laser forming of Ti-6Al-4V alloy and their effects on the alloy flow behavior and forming process, and present a thermal-mechanical-microstructure model for the complex laser forming process with rapid heating and cooling. To validate the theoretically predicted results, a series of carefully controlled experiments are also conducted, and the experimental and numerical results are in close agreement. MATERIALS AND EXPERIMENTAL PROCESUDRES