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

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.

Automated summary

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.