The effect of powder oxidation on defect formation in laser additive manufacturing

Abstract

Understanding defect formation during laser additive manufacturing (LAM) of virgin, stored, and reused powders is crucial for the production of high quality additively manufactured parts. We investigate the effects of powder oxidation on the molten pool dynamics and defect formation during LAM. We compare virgin and oxidised Invar 36 powder under overhang and layer-by-layer build conditions using in situ and operando X-ray Imaging. The oxygen content of the oxidised powder was found to be ca. 6 times greater (0.343 wt.%) than the virgin powder (0.057 wt.%). During LAM, the powder oxide is entrained into the molten pool, altering the Marangoni convection from an inward centrifugal to an outward centripetal flow. We hypothesise that the oxide promotes pore nucleation, stabilisation, and growth. We observe that spatter occurs more frequently under overhang conditions compared to layer-by-layer conditions. Droplet spatter can be formed by indirect laser-driven gas expansion and by the laser-induced metal vapour at the melt surface. In layer-by-layer build conditions, laser re-melting reduces the pore size distribution and number density either by promoting gas release from keyholing or by inducing liquid flow, partially or completely filling pre-existing pores. We also observe that pores residing at the track surface can burst during laser re-melting, resulting in either 2 formation of droplet spatter and an open pore or healing of the pore via Marangoni flow. This study confirms that excessive oxygen in the powder feedstock may cause defect formation in LAM.

Bulk-Explosion-Induced Metal Spattering During Laser Processing

“…When they collide with each other or with the cold raw powders, those blown away by the metal vapor as shown in the Supplemental Material Video S1 [6], larger and irregular agglomerations can then form via particle-particle coalescence or sintering [7]. These particles tend to have different compositions, microstructures, and morphologies from the original feedstock [8][9][10], creating problems for powder recycling. Also, when they fall back onto the powder bed, they will negatively affect the powder recoating, resulting in the formation of structural defects (e.g., lack-of-fusion porosity) and the degradation of mechanical properties (e.g., fatigue life) in the end products [11,12].…”

A detailed microstructural and corrosion analysis of an additively manufactured magnesium alloy produced by selective laser melting

“…In the LPBF-based methods, the formation of defects depends on the wetting of molten pool, which is governed by the two critical process parameters: (i) laser power and (ii) scan speed. As such, increasing laser power provides more energy for powder consolidation whilst improving molten pool wetting and decreasing scan speed increases the laser-matter interaction time, making the movement of the liquid metal less violent [42,43]. Thus, a trade-off between the scan speed and laser power is required to achieve the appropriate range of molten pool temperature/wetting, i.e., the "Forming zone" in Fig.…”

“…9c). Note that oxides (and hydroxides) affect the wettability of the molten pool during SLM ( [43]) and thus such an investigation is crucial. In the area viewed in Fig.…”

Near Net Shape Manufacture of Titanium Alloy Components from Powder and Wire: A Review of State-of-the-Art Process Routes

“…The high affinity for oxygen is another concern for metal AM with titanium alloys. Both the pick-up of oxygen during the build process as well as the the oxygen content in the starting feedstock material can result in nucleation sites for pore formation during the build process [37]. The formation of such pores can have detrimental effects to the ductility and fatigue properties of the component produced [23].…”