Experimental study on mechanism of influence of laser energy density on surface quality of Ti-6Al-4V alloy in selective laser melting

Abstract

This experiment obtained different laser energy density (LED) by changing SLM molding process parameters. The surface morphology, surface quality, and microstructure of as-fabricated samples were studied. The effects of scanning speed, hatching space, and laser power on surface quality were analyzed, and the optimal LED range for surface quality was determined. The results show that pores and spherical particles appear on the sample’s surface when low LED is applied, while there are lamellar structures on the sides of the samples. Cracks appear on the sample’s surface, and the splash phenomenon increases when a high LED is taken. At the same time, a large amount of unmelted powder adhered to the side of the sample. The surface quality is the best when the LED is 150–170 J/mm³. The preferred hatch space is currently 0.05–0.09 mm, the laser power is 200–350 W, and the average surface roughness value is (15.1±3) µm. The average surface hardness reaches HV404±HV3, higher than the forging standard range of HV340–HV395. Increasing the LED within the experiment range can increase the surface hardness, yet an excessively high LED will not further increase the surface hardness. The microstructure is composed of needle-like α′-phases with a length of about 20 µm, in a crisscross ‘N’ shape, when the LED is low. The β-phase grain boundary is not obvious, and the secondary-phase volume fraction is high; when the LED is high, the α′-phase of the microstructure is in the form of coarse slats, and the secondary-phase is composed of a small amount of secondary α′-phase, the tertiary α′-phase and the fourth α′-phase disappear, and the volume fraction of the secondary-phase becomes low.

Keywords

laser energy density / surface quality / selective laser melting / Ti-6Al-4V / microstructure

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