Microstructures of laser-deposited Ti–6Al–4V

Abstract

Direct laser fabrication (or LENS) has been used to build up solid samples from powders of the alloy Ti–6Al–4V. The effects of processing conditions, such as laser power, scan speed, powder feed rate, etc. on the microstructure of the build have been assessed. It has been found that Ti–6Al–4V is very susceptible to the formation of columnar grains during laser deposition. Long columnar grains dominate the microstructure of the laser-deposited Ti–6Al–4V alloy for a large range of laser powers and are formed for all but very high laser powers. The scale of columnar grains also increases with decrease of the laser scan speed and when other parameters are maintained constant. For the lowest powers used a change from a near fully dense product to a very porous structure is found. The laser-fabricated Ti–6Al–4V has a basket weave microstructure and the size of the α and β laths increases with increase of laser power and decrease of scan speed. During the addition of the new layers, there is a tendency for coarsening of the α and β laths in the re-heated region near the top of the previous layer. This effect is most obvious in the area where heat extraction is rapid, such as the region near the substrate and is not obvious when the whole region remains hot, as occurs towards the top of the sample. In this region, the α/β laths coarsen throughout the whole of each layer. It is shown that the influence of the substrate in acting as a variable heat sink makes prediction of the effects of altering process variables on the microstructure very difficult.