WEB 2D and 3D EBSD studies of alpha variant selection during electron powder bed fusion of Ti-6Al-4V
Ti-6Al-4V is one of the most commonly used alloys for additive manufacturing (AM) due to its applications as orthopaedic implants and aerospace components which can uniquely benefit from the advantages of AM. However, the inherent inhomogeneity and cyclic thermal loading in additive manufacturing processes significantly complicate the microstructural evolution of this alloy. The current understanding of the microstructural evolution in Ti-6Al-4V is based on traditional processing and cannot adequately predict its behaviour during AM.
Analysis of different interfaces (prior beta grain boundaries, alpha inter-variant boundaries) formed during AM of Ti-6Al-4V can clarify the competition between different modes of alpha variant selection in determining the final microstructure. Here, traditional 2D characterization methods are often insufficient for uncovering complex, interconnected microstructural features that arise from solid-solid phase transformations during AM.
In this work, Ti-6Al-4V blocks were produced using different scanning strategies during electron beam powder bed fusion and characterized using 2D and 3D electron backscatter diffraction. This revealed that the microstructure is more complex, and that alpha laths are more interconnected than previously believed. For example, inter-variant boundary character distributions showed significant evidence of different variant selection mechanisms as a function of the position in the build and the scanning strategy. Some of these variant selection mechanisms are more desirable than others. Understanding the complexity of the microstructure in 3D provides new insights into the fundamental phenomena behind the microstructural evolution of titanium alloys in AM, and will help to give recommendations for optimising process design.