Evolution of Microstructure and Residual Stress in Hot Rolled Ti-6Al-4V Plates Subjected to Different Heat Treatment Conditions

Abstract

Abstract. Hot rolled Ti-6Al-4V plate samples were taken from three different stages of an industrial heat treatment process; one as-rolled and two heat treated. This was followed by microstructure characterization using optical microscopy. Surface and through-thickness residual stress was determined using a combination of X-ray diffraction (XRD) and the contour method. Measured residual stress distributions showed similarities in distribution with that obtained for rolled Al-7050 alloy; including compressive troughs near the outer thickness on both sides, leading towards a tensile zone around the center with a local minima at the plate center thickness. Microstructure and residual stress data was then used to draw comparisons between the investigated conditions. IntroductionTitanium alloys are often selected as critical components in the aerospace sector due to their exceptionally high strength-to-weight ratio and ability to produce different microstructures, with tailored mechanical properties, by thermo-mechanical processing at elevated temperatures. Thermo-mechanical processing may generate residual stress which can lead to distortion out of dimensional tolerance during subsequent manufacturing processes; such as during machining [1], and can also affect material performance in service; such as reducing fatigue life [1]. The difficulty arises because Ti alloys usually have a complex, multi-phase microstructure, which varies within a component depending on process history [1]. This makes the final residual stress distribution highly dependent on microstructure and deformation history. Residual stress arises from heterogeneity in plastic deformation and during thermo-mechanical processing, phase transformation, and the differential gradient between colder outer surface and hot inner interior induced during cooling. The latter would be even more intensified by non-uniform heat exchange with the cooling environment. The deformation in Ti alloys is complex due to the inherent thermal and mechanical anisotropy of the constituent phases. Furthermore, in the temperature range of interest (600°C -950°C), the material undergoes a reversible -phase transformation, which can contribute to stress relaxation [2].Thus, it is imperative that the evolution of residual stress is understood when designing and modifying microstructure for high value, critical aerospace components. The aim of this work was to evaluate microstructure and the through-thickness residual stress distribution induced by thermo-mechanical and subsequent industrial heat treatment processing of 38 mm Ti-6Al-4V plate, using multiple residual stress measurement techniques.