Deciphering the transformation pathway in laser powder-bed fusion additive manufacturing of Ti-6Al-4V alloy

The nature of rapid cyclic heating and cooling in metal additive manufacturing poses a great challenge in the control of microstructure while a metallic part is being built. With metastable alpha' martensites commonly present in a columnar prior-beta grain structure, Ti-6Al-4V alloy made by laser powder-bed fusion additive manufacturing (L-PBF AM) is strong but often suffers from anisotropic mechanical behavior, inferior ductility and low fracture toughness. This drives the recent development in L-PBF process optimisation to produce ultrafine lamellar alpha + beta microstructures directly in the as-built state of Ti-6Al-4V. Currently, in-situ martensite decomposition is deemed as the transformation pathway responsible for the formation of such lamellar microstructures. However, without solid experimental evidence this consensus cannot be reached and is still in question. Here we show that, instead of martensite decomposition, a pathway of slow cooling from the beta phase field at much reduced cooling rates (below 5 degrees C s(-1)) is proven to give rise to the observed lamellar alpha + beta microstructure. This is underpinned by several microstructural "fingerprints" such as grain-boundary alpha (GB-alpha), a colony and a lath width, and crystallographic orientations of the constituent phases. The finding deepens our established wisdom in L-PBF AM and opens a new avenue for microstructural control in metal additive manufacturing.