Comparison between continuous and modulated wave laser emission modes for the selective laser melting of Ti-6Al-4V: Dimensional accuracy, microstructure and mechanical behaviour

Abstract

This study investigated the difference between the properties of Ti-6Al-4 V alloy parts printed using continuous wave (CW) and modulated wave (MW) laser emission modes, with equivalent input energy densities. Laser powers in this powder bed fusion process, were systematically altered to evaluate the effect of different input energies on the as-built parts. Printed alloy parts were obtained across five equivalent laser energy densities, in the range of 31–51 J/mm3, investigated using the two laser emission modes. The impact of changes in laser emission mode and energy density was examined based on printed part dimensions, microstructure, porosity, hardness and tensile properties. It was demonstrated that microstructural refinement occurs as a function of melt pool size, which was associated with the penetration depth of the laser energy. The microstructure of samples produced using CW laser emission mode were characterised by thinner laths and narrower prior β grains. Tensile testing according to ASTM E8M, obtained at the different energy levels, demonstrated that over the energy density range investigated there was up to a 17.4% increase in yield strength (YS) and a 2.5% increase in Ultimate Tensile Strength (UTS) for the CW printed samples, when compared with those obtained using the MW laser wave emission mode. Comparatively, MW laser emission mode resulted in thicker laths with slightly wider prior β grains, which exhibited an elongation at break of 13.6% during tensile testing at high energy densities. The influence of the laser emission mode on dimensional accuracy was also investigated. Sample printed using the MW laser emission mode showed up to a 50% improvement in dimensional accuracy when compared with that obtained for CW wave laser emission mode.