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Effects of Water Cooling on the Microstructure of Electron Beam Additive-Manufactured Ti-6Al-4V

The inferior mechanical properties of EBAM Ti-6Al-4V samples are due to the coarse columnar grains containing coarse lamellar structures. One can expect that water cooling of the build platform will increase the cooling rate of the molten pool during the build-up process, causing microstructure refinement. In the present work, the substrate cooling effects on the microstructure and phase composition of EBAM Ti-6Al-4V samples are studied using optical, scanning electron, and scanning transmission microscopy, as well as X-ray diffraction analysis. It is shown that the microstructure of the EBAM Ti-6Al-4V samples built on the substrate without water cooling consists predominantly of columnar prior β grains with lateral sizes ranging up to 2000 µm, while cooling of the build platform causes the appearance of equiaxed prior β grains measuring 1000 µm. Moreover, the refinement of the martensite structure and the precipitation of α′′ martensite platelets within α laths occur in the EBAM Ti-6Al-4V samples built on the water-cooled build platform. An explanation of the mechanisms underlying the α′→α + β and α′→α + α′′ + β transformations during the building process is provided based upon ab initio calculations. The fragmentation of the α laths under the residual compressive stresses is discussed.