Abstract
In 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 (20μm), the samples contain particularly low porosity when the laser scanning speeds are below 2700mm/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 40μm) 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.
Highlights
Selective laser melting (SLM), due to its capacity to fabricate complex freeform geometries directly from computer-aided design (CAD) models, has been hailed as one of the most promising manufacturing technologies for net shape industrial scale production
We perform a systematic parametric study to investigate the influence of laser scanning speed and powder layer thickness on porosity development and correlate the porosity development with the top sample surface structures and to the melt pool and flow behaviour studied by both high speed imaging and computational fluid dynamics (CFD) simulation
Given that the laser scanned tracks are the traces of melt flow during SLM, the results suggest that the melt flow was extremely unstable at the high laser scanning speed
Summary
Selective laser melting (SLM), due to its capacity to fabricate complex freeform geometries directly from computer-aided design (CAD) models, has been hailed as one of the most promising manufacturing technologies for net shape industrial scale production. It is clear from this work that there are several concerns associated with this process that need to be addressed These include residual stress development [11], cracking ( for certain materials such as nickel-based superalloys) [2], porosity [5,7] and mechanical anisotropy [5]. Qiu et al [5] directly observed open pores on the top surfaces of selectively laser melted samples and argued that the spherical pores could be due to incomplete re-melting of some localised sites on the previous layer and to the insufficient feeding of molten material to these sites All these suggested mechanisms for the development of porosity, are based on unsubstantiated assumptions concerning the detailed mechanisms occurring during melting and solidification during SLM. We perform a systematic parametric study to investigate the influence of laser scanning speed and powder layer thickness on porosity development and correlate the porosity development with the top sample surface structures (which could be considered as the traces of melt flow in SLM) and to the melt pool and flow behaviour studied by both high speed imaging and CFD simulation
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