4.8 Article

Refinement of Ti-6Al-4V prior- ß grain structure in the as-deposited condition via process control during wire-direct energy deposition

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ADDITIVE MANUFACTURING
卷 74, 期 -, 页码 -

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ELSEVIER
DOI: 10.1016/j.addma.2023.103712

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Ti-6Al-4V; Additive manufacturing; Grain refinement; Columnar-to-equiaxed transition; As-deposited

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This study demonstrates the refinement of Ti-6Al-4V alloy's prior-β grains through process control alone, without the need for post-processing or alloy modification. By increasing the deposition rate and reducing the specific energy density, the grain structure can be transformed from columnar to equiaxed morphology.
This study demonstrates that refinement of the prior- ss grains generated during the wire-direct energy deposition of Ti-6Al-4V is achievable whilst maintaining a stable processing condition in the as-deposited condition, not reliant on penetration of the feedstock to the melt pool. Previous studies on prior- ss grain refinement of Ti-6Al-4V have been reliant on either post-processing, alloy modification or agitation of the melt pool to achieve refinement. Through process control alone, it was identified that an increase in the deposition rate for a fixed energy input led to a reduction in the specific energy density of deposited material. Utilising pyrometry, it was determined that the reduction in the specific energy density led to a decrease in the thermal gradient, measured using a linear thermal gradient approximation, encouraging apparent homogenous nucleation within the melt pool. Whilst maintaining a stable 'droplet' transfer mode, the gradual reduction in specific energy density transformed the grain structure from the typical columnar morphology through a mixed morphology to an equiaxed grain morphology; confirmed via prior- ss grain reconstructions and inverse pole figure analysis made possible by electron back scattered diffraction microscopy of the deposited samples. Microhardness measurements demonstrated larger variations in the refined prior- ss grain specimens as compared to the columnar as a result of more complex a morphologies generated.

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