On the role of melt flow into the surface structure and porosity development during selective laser melting

Abstract

a b s t r a c tIn this study, the development of surface structure and porosity of Ti-6Al-4V samples fabricated by selective laser melting under different laser scanning speeds and powder layer thicknesses has been studied and correlated with the melt flow behaviour through both experimental and modelling approaches. The as-fabricated samples were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The interaction between laser beam and powder particles was studied by both high speed imaging observation and computational fluid dynamics (CFD) calculation. It was found that at a high laser power and a fixed powder layer thickness (20 lm), the samples contain particularly low porosity when the laser scanning speeds are below 2700 mm/s. Further increase of scanning speed led to increase of porosity but not significantly. The porosity is even more sensitive to powder layer thickness with the use of thick powder layers (above 40 lm) leading to significant porosity. The increase of porosity with laser scanning speed and powder layer thickness is not inconsistent with the observed increase in surface roughness complicated by increasingly irregular-shaped laser scanned tracks and an increased number of discontinuity and cave-like pores on the top surfaces. The formation of pores and development of rough surfaces were found by both high speed imaging and modelling, to be strongly associated with unstable melt flow and splashing of molten material.

Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones

“…Recently, Qiu et al [18] performed an experimental parameter study, whereby the surface roughness and area fraction of porosity were measured as a function of laser scan speed. They noted that the unstable melt flow, especially at high laser scan speed, increases porosity and surface defects.…”

Densification Behavior of 316L Stainless Steel Parts Fabricated by Selective Laser Melting by Variation in Laser Energy Density

“…This excessive energy input could result in a large molten pool with high liquid phase content affecting the formation of the elongated and more irregular melt pool, which causes the quality of nal products to deteriorate by formation of large pores 29,30) . Melt ow behavior is another factor that may have the tendency to in uence the porosity development during SLM process and further affect the surface quality 31,32) . Figure 6 elucidates the representative top surface morphologies of SLM cubic samples manufactured using different laser energy densities of 29.2, 116.9, and 233.8 J/mm 3 , respectively, which contains the laser scanned tracks indicating the traces of melt ow during the SLM process.…”

Experimental analysis of spatter generation and melt-pool behavior during the powder bed laser beam melting process

“…However, a significant part of metal ejections either contaminate the powder bed surface or directly fall on the as-built metal, thus promoting metallurgical defects on final parts as indicated by Qiu et al (2015) on Ti-6Al-4V titanium alloy and Liu et al (2015) on 316L stainless steel. For such reasons, the characterization of spatter generation and the understanding of underlying physics are of the highest importance to globally improve and stabilize laser melting in powder-bed based ALM processes.…”