Ti6Al4V functionally graded material via high power and high speed laser surface modification

This study investigates the fabrication of Ti6Al4V functionally graded material via high power and high speed laser surface modification (LSM). The original sample microstructures consisted of elongated equiaxed α phase with a grain boundary of β phase. Nine different LSM process parameter sets were applied to these samples. Scanning electron microscopy showed a fine acicular martensitic phase next to the surface of the laser treated samples in all cases. A transition microstructure zone beneath the martensitic zone was observed, with larger, equiaxed grains and some martensitic α phase growth. The interior of the sample contained the original microstructure. The surface roughness was found to increase after the surface modification for all process parameter sets. Nanoindentation tests were performed in order to obtain the hardness and modulus of the three phases, i.e. martensitic α, equiaxed α and the grain boundary β. A dual phase crystal plasticity finite element model was developed to investigate the three zone functionally graded microstructure under uniaxial tensile loading. The hardened surface zone prevented the propagation of continuous slip bands, while the transition zone prevented excessively sharp stress concentrations between the outer surface and interior of the samples.