[6A1] Ultrasonic characterisation of microtexture regions in titanium alloys

D Harra, E Mohseni, K Tant, A Gachagan, M Tabatabaeipour and K Inagaki
University of Strathclyde, UK 

Decades of attention has been directed towards the scattering of ultrasonic wave propagation through polycrystalline materials, which has enabled characteristics of the material’s microstructural properties, such as grain size, grain shape and texture, to be determined. However, polycrystals with preferred crystallographic orientation, in the form of texture or local anisotropy, has not been well studied. Titanium is used extensively throughout industry, with the aerospace industry accounting for around 60% in all of Europe and demands increasing around the globe. To achieve optimum mechanical properties, it is desirable to produce a fine-grained microstructure for titanium alloys that exhibits no local textures. However, due to the sensitivity of titanium alloys to the mechanical and thermal processes present in their manufacturing process, the near α-phase titanium alloy Ti-6Al-4V can develop microtexture regions (MTRs), where large regions of contiguous crystals exhibit a single preferential orientation. MTRs present potential sites for the nucleation of cracks, making the material more susceptible to fatigue and ultimately reducing the component’s lifespan. Regions that consist of MTRs with an area of 1 mm² or bigger are known to be a cause of concern for crack site generation and hence a need exists for a non-destructive method to assess the degree of microtexture present within a component.

In this work, we present a forward model that enables us to process raw electron backscatter diffraction (EBSD) data into a format suitable for finite element (FE) simulation studies. Firstly, we used the Dream.3d software to construct an algorithm that allows us to segment our material microstructure into synthetic MTRs and their corresponding crystallographic properties. The structure was then simulated in OnScale (FE software), which enables us to analyse the ultrasonic waves backscattered from the microstructure of titanium. Using the pulse-echo ultrasonic inspection method, we develop a relationship between the ultrasonic attenuation measurements and the MTRs within the inspection volume.

To investigate the validity and accuracy of the proposed model, a series of experimental tests were also conducted. To this end, several cubic pieces were extracted from a 3D-printed Ti-6Al-4V alloy manufactured by the wire + arc additive manufacturing process. The microstructure of each sample was measured along each respective coordinate axis by data collected via the electron backscatter diffraction (EBSD) technique, which was replicated using the simulation forward model.

The scattering and attenuation measurements produced by the microstructure in each respective direction was analysed and compared to the experimental pulse-echo technique undertaken on the sample in immersion mode to verify simulation results. It was observed that the microstructure of titanium produced backscatter signals that could be related to the attenuation and grain structure of the material. When there is a larger presence of MTRs within the inspection volume, it was found that the measured backscatter signal was higher than regions where no MTRs are present. The experimental EBSD and water immersion testing also produced results that were in good agreement with this observation. Backscatter signals were then related to the size of the MTRs.