Formation of residual stresses during quenching of Ti17 and Ti–6Al–4V alloys: Influence of phase transformations

Abstract

The formation of internal stresses during quenching of titanium alloys from the β phase field are investigated both experimentally and by simulation, in order to show the effects of phase transformations. Two titanium alloys are considered: the β-metastable Ti17 alloy and the α+β Ti–6Al–4V alloy. During the quench into water of laboratory scale samples (40 mm diameter cylinders), no phase transformations occurred in the Ti17 alloy because of its β-metastable character and the fast cooling. However, β→α+β and β→α′ phase transformations occurred in the Ti–6Al–4V sample. Both alloys are compared in order to highlight the effects of the phase transformations. Residual stresses were determined by neutron diffraction, by the contour method and at the surface by the hole drilling method. A model for the coupled thermal, mechanical and metallurgical evolutions is established in order to simulate the quenching operations. The material model for the Ti17 alloy was established in a previous study. Regarding the Ti–6Al–4V alloy, modeling approaches and experimental data from literature are utilized to build the material model. From both experiment and simulation, it is found that the internal stress evolutions are governed by the thermal gradients. The phase transformations have a weak impact because of the small deformation strains induced by the phase change. Nevertheless, good prediction of the phase transformation kinetics is necessary for accurate simulations, because the α and α’ phases strengthen the alloy, thereby limiting the plastic straining at the origin of the residual stresses. As most plastic strains are cumulated at high temperature, the thermomechanical model should be established accurately over the temperature ranges in which there is a significant proportion of β phase.