Spheroidisation and oxide disruption phenomena in direct selective laser melting (SLM) of pre-alloyed Al-Mg and Al-Si powders

Spheroidisation and oxide disruption phenomena in the direct laser melting of pre-alloyed Al-Mg and Al-Si powders had been explored and interpreted in terms of Raleigh's instability, Marangoni convection, laser absorptivity, heat conductivity, fluidity of the melt volume and the powder's oxide content. Balling occurred only in the single layer parts due to the lower thermal conductivity of the bed and possibly the difference in wettability of the powder bed and sintered layers. The existence of a high degree of thermal expansion mismatch between the oxide film and the parent metal and a uniform oxide layer thickness were found to favour the disruption of the oxide shell promoting inter-particulate melting across the layers. This was the case for Al-Si parts, whereas SLM of pure aluminium and pre-alloyed Al-Mg powders gave rise to high incidence of randomly distributed pores.