Residual Stress Measurement of Ti-Metal Samples by Means of XRD with Ti and Cu Radiation

Abstract

Abstract:The use of titanium in structural components has been growing for years, especially in highly demanding applications in the aerospace industry where quality control is essential. One of the critical properties is fatigue strength, which is strongly affected by residual stresses. Residual stresses may be an unavoidable by-product of the manufacturing process or intentionally imparted through processes like shot peening. X-ray diffraction (XRD) is commonly used for residual stress measurement. Yet titanium alloys are more difficult to measure than other metals like steels and aluminium alloys. Many titanium alloys have a two-phase microstructure, one of them being hexagonal alpha titanium. Also, the commonly used Cu-radiation generates very strong fluorescence, which results in a low penetration depth. Both reduces measurement quality. Ti-radiation is much less frequently used. It benefits from very low fluorescence and a higher penetration depth. This paper compares XRD residual stress measurements using Cu-and Ti-radiation on two titanium grades: Grade 2 and Grade 5 (Ti6Al4V). The samples from both are in the rolled condition and have been shot peened. The x-ray elastic constants were determined by XRD with Cu-and Ti-radiation and samples and residual stress depth distributions were measured up to 0.5 mm depth. IntroductionTitanium alloys are widely used in aerospace structures due to their excellent strength to weight ratio. For aerospace applications reliability is critical from which follows that all factors affecting the performance of the components have to be studied and controlled extremely well. One important factor is residual stress, which is always present due to the manufacturing processes and which affects fatigue strength significantly. Titanium alloys are challenging because their machining is difficult due to their high chemical reactivity, poor thermal conductivity, high strength that is maintained at elevated temperatures and low modulus of elasticity [1]. The same factors make welding challenging. Since productivity is always an important issue it is always attempted to minimize the need for machining and welding [2,3].For the evaluation of titanium components, reliable, fast and easy residual stress measurement is a key issue. There are several methods for measuring residual stresses nondestructively and destructively. This paper deals with X-ray diffraction, using X-ray instruments specifically designed for residual stress measurement. The measurement accuracy depends on a number of factors, such as grain size, preferred orientation, accessibility of measurement location etc. The X-ray wavelength is given by the X-ray tube chosen. It is preferable to select it so that the 2θ -angle of the diffraction peak analyzed is the highest possible that has sufficient intensity. However, some combinations of material and radiation cause significant fluorescence, which affects the net intensity level and also the effective measurement depth [3]. X-ray diffraction does not measure stress directl...