Laser powder bed fusion of Ti–6Al–4V: In-situ rolling for microstructure modification and grain refinement

Laser additive manufacturing is known for high geometrical freedom, high cooling rates and fine microstructures with high performance properties. Regarding Ti-6Al-4V, often a strong microstructural texture results with mostly undesired anisotropic mechanical characteristics. By adding a deep rolling module in-situ layer-wise and locally adjustable severe plastic deformation can be applied during the build-up phase in additive manufacturing. Scanning of subsequent layers allow a recrystallization and refinement of the prior β-grains in the ambient heat affected zone of the melt tracks due to deformation induced high dislocation densities after applied in-situ deep rolling. Rapid heating to and cooling from the β-phase field limits the β-grain growing leading in ultra-fine β-grains. Additionally, the β-grain morphology transitions from columnar to globular reducing the texture and allowing isotropic mechanical properties. By deep rolling every fifth layer in additive manufacturing, a technical periodic graded microstructure results, consisting of fine globular and ultra-fine β-grains. Post hot-isostatic pressing is applied to decompose the initial α’-martensite into a α+β microstructure. While the standard laser additive manufacturing process leads in long α-lamellas surrounded by β-phase, the in-situ rolling leads in a refined periodic graded microstructure. This consists of layers of shorter α-lamellas and layers of globular α. This approach allows local microstructure modification by targeted severe plastic deformation and recrystallization processes in-situ during laser additive manufacturing. Combining the potential of ultra-fine-grained Ti-6Al-4V with the high design flexibility and near-net-shape manufacturing capabilities of laser additive manufacturing offers new possibilities. This combination enables tailored and locally precise microstructure modifications of Ti-6Al-4V.